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Electrochemical aspects of D.C. electroslag remelting Beynon, Gordon Thomas 1971-12-31

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THE  ELECTROCHEMICAL ASPECTS OF D.C. ELECTROSLAG REMELTING  by  GORDON BEYNON B.A.Sc, University  of B r i t i s h Columbia, 1967  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE  OF  DOCTOR OF PHILOSOPHY I i n the Department of METALLURGY  We accept t h i s  t h e s i s as conforming t o the r e q u i r e d  standards  THE UNIVERSITY OF BRITISH COLUMBIA October, 1971  In p r e s e n t i n g an  this thesis  advanced degree at  the  Library  I further for  shall  the  of  this thesis  written  University  of B r i t i s h  permission  s c h o l a r l y p u r p o s e s may his  f u l f i l m e n t of  make i t f r e e l y a v a i l a b l e  agree that  by  in partial  representatives.  be  for f i n a n c i a l gain  permission.  Department  The U n i v e r s i t y o f B r i t i s h V a n c o u v e r 8 , Canada  Date  the  I t i s understood  Columbia  shall  requirements  Columbia,  for reference  for extensive  g r a n t e d by  the  that  not  and  copying of Head o f my  be  I agree  that  study.  this  thesis  Department  copying or  for  or  publication  allowed without  my  ABSTRACT  It  i s p r e d i c t e d from t h e known i o n i c p r o p e r t i e s o f t h e s l a g s  used i n e l e c t r o s l a g m e l t i n g ,  t h a t t h e D.C. m e l t i n g  process  should be  accompanied by F a r a d a i c r e a c t i o n s on t h e s l a g / i n g o t and s l a g / e l e c t r o d e interfaces.  I n t h e p r e s e n t work we have determined the magnitude o f  the o v e r p o t e n t i a l s r e s u l t i n g from c o n c e n t r a t i o n p o l a r i z a t i o n a t t h e s e I n t e r f a c e s , f o r t h e c a s e o f pure i r o n i n c o n t a c t w i t h C a F CaF,, + CaO s l a g s . pulsing  operating  a n <  *  tatic  cell.  p o l a r i z a t i o n o v e r p o t e n t i a l e x i s t i n g on an e l e c t r o d e i n an  ESR u n i t has been measured e s s e n t i a l l y by the same  I t was found t h a t t h e p o t e n t i a l s observed on m e l t i n g agree w e l l w i t h anodic  + AljO^  T h i s has been c a r r i e d out u s i n g a galvanos  t e c h n i q u e i n an e l e c t r o l y t i c The  2  process  ESR e l e c t r o d e s  the r e s u l t s from the e l e c t r o l y t i c c e l l . i s postulated  technique.  The primary  to be the c o r r o s i o n o f i r o n , l e a d i n g to an  2+ Fe  - saturated  s l a g l a y e r on t h e anode s u r f a c e a t s u f f i c i e n t l y  current d e n s i t i e s . Faradaic  The c a t h o d i c  3 + 2 + r e d u c t i o n of A l o r Ca  process  high  i s suggested t o be the  , t o produce a c o n c e n t r a t i o n o f  [Al]_ o r (Ca) i n the cathode i n t e r f a c e r e g i o n . Fe slag n  The  concentration p o l a r i z a t i o n behaviour of other  electrode materials slags. anodic  (Cr, N i , Co) was a l s o i n v e s t i g a t e d i n C a F  F o r these m a t e r i a l s , i t has been p o s t u l a t e d process  pure 2  + A1 0 2  t h a t t h e primary  i s the c o r r o s i o n o f t h e m e t a l which a g a i n  l e a d s to i n t e r -  f a c e s a t u r a t i o n by the a p p r o p r i a t e m e t a l i o n s . The rodes  (Fe-Cr,  concentration p o l a r i z a t i o n behaviour of i r o n a l l o y Fe-C)  was a l s o i n v e s t i g a t e d .  elect-  I t was found t h a t the  more e a s i l y o x i d i z a b l e a l l o y i n g elements a r e p r e f e r e n t i a l l y removed from  iii these a l l o y s when they are a n o d i c a l l y p o l a r i z e d . At v e r y h i g h c u r r e n t p r o c e s s e s may convert The  d e n s i t i e s b o t h the a n o d i c and  to a r c s , l e a d i n g  to p r o c e s s  cathodic  instability.  c h e m i c a l and thermal phenomena a s s o c i a t e d w i t h D.C.  ESR  were s t u d i e d by making i n g o t s from pure e l e c t r o d e m a t e r i a l s and iron-alloy electrode materials. o x i d a t i o n and p o s s i b l e Faradaic  Chemical e f f e c t s which i n c l u d e m e t a l  a l l o y l o s s were e x p l a i n e d  i n terms o f the  r e a c t i o n mechanisms proposed i n the e l e c t r o l y t i c c e l l  Thermal e f f e c t s were a l s o e x p l a i n e d  on the same b a s i s .  I t was concluded t h a t e l e c t r o c h e m i c a l responsible  f o r the f o r m a t i o n  i n the pure m e t a l s .  studies.  r e a c t i o n p r o d u c t s were  o f l a r g e numbers o f s m a l l o x i d e i n c l u s i o n s  I n the case o f a l l o y m a t e r i a l s ,  electrochemically  produced o x i d a t i o n r e s u l t e d i n s i g n i f i c a n t l o s s e s o f a l l o y components during  melting.  TABLE OF CONTENTS  Page  TITLE PAGE  i  ABSTRACT  i i  TABLE OF CONTENTS  iv  LIST OF TABLES  i  LIST OF FIGURES  .  x  ACKNOWLEDGEMENTS  CHAPTER 1.  x  x  INTRODUCTION  i  1  1.1  The E l e c t r o s l a g Remelting. P r o c e s s  1.2  The problem  2  1.2.1  Chemical e f f e c t s .  3  1.2.2  Mechanical e f f e c t s  4  1.2.3  Thermal e f f e c t s  5  1.3  Background  1  to the problem  5  1.3.1  E l e c t r o n i c conduction  6  1.3.2  Ionic  7  conduction  1.3.2.1 1.3.2.2 1.3.2.3 1.3.2.4 1.3.2.5 1.3.2.6 1.3.3  Interfaces at equilibrium Polarized electrode interfaces P o l a r i z a t i o n mechanisms Mass t r a n s p o r t T r a n s i t i o n time I o n i c c h e m i c a l and thermal e f f e c t s  Arc discharge  1.4  Conclusions  1.5  Experimental  10 11 12 15 18 23 24 26  program  outline  iv  ,  27  i  i  V  Page  CHAPTER 2.  SMALL SCALE STUDIES  29  2.1  Experimental considerations  2.2  S m a l l s c a l e p o l a r i z a t i o n apparatus  32  2.3  Electrode materials  41  2.3.1  F e r r o v a c - E pure i r o n  41  2.3.2  AISI 430 s t a i n l e s s s t e e l  41  2.3.3  Fe - 1.0 wt. % Cr a l l o y  41  2.3.4  Pure chromium  41  2.3.5  Pure n i c k e l  42  2.3.6  Pure c o b a l t  42  2.3.7  Iron-carbon a l l o y .  2.4  Electroslag  2.5  Polarization  2.6  . . . . 29  .  42  r e m e l t i n g p o l a r i z a t i o n experiments results  43 47  2.5.1  F e r r o v a c - E pure i r o n  47  2.5.2  AISI 430 s t a i n l e s s s t e e l  56  2.5.3  Fe - 1.0 wt. % Cr a l l o y  56  2.5.4  Pure chromium  60  2.5.5  Pure n i c k e l  60  2.5.6  Pure c o b a l t  65  2.5.7  Iron-carbon a l l o y  65  Electroslag  CHAPTER 3.  remelting p o l a r i z a t i o n . r e s u l t s .  MELT PROGRAM .  65  72  3.1  M e l t i n g procedure  72  3.2  Electrode materials  72  3.2.1  Low carbon m i l d s t e e l :  AISI 1018  3.2.2  F e r r o v a c - E pure i r o n  73  3.2.3  Armco i r o n  74  3.2.4  Ferritic  3.2.5  Medium carbon m i l d s t e e l :  stainless steel:  73  AISI 430  74  AISI 1095  75  vi  Page  3.2.6 3.3  Pure n i c k e l  75  Slag materials  76  3.3.1  Calcium f l u o r i d e .  76  3.3.2  Alumina  3.3.3  C a l c i u m oxide  '. .  77 77  3.4  Atmospheric c o n t r o l  78  3.5  Melting  78  3.6  Melt  records  3.7  Melt  record  3.8  3.9  conditions  81 calculations  82  rate  82  3.7.1  Melt  3.7.2  S p e c i f i c coulombic d e n s i t y  3.7.3  Drop s i z e and s u r f a c e t e n s i o n  84  3.7.4  Melt  85  (Z)  program r e s u l t s  Ingot a n a l y s i s  85  3.8.1  Oxygen a n a l y s i s of i n g o t s  85  3.8.2  Aluminum a n a l y s i s o f FVE i n g o t s  87  3.8.3  A n a l y s i s of AISI 1095 s t e e l  87  3.8.4  A n a l y s i s of AISI 430 s t a i n l e s s s t e e l  87  Slag cap a n a l y s i s  CHAPTER 4.  87  DISCUSSION OF SMALL SCALE STUDIES  4.1  Introduction  4.2  Previous  4.3  A n o d i c p o l a r i z a t i o n of pure i r o n i n A ^ O ^ 4.3.1  4.4  83  99  . .  99  e l e c t r o c h e m i c a l work  99 slags  Apparent t r a n s i t i o n time .  A n o d i c p o l a r i z a t i o n of pure i r o n i n CaO s l a g s  101 115 119  vii  Page  4.5  C a t h o d i c p o l a r i z a t i o n of pure i r o n i n ESR s l a g s  120  4.6  P o l a r i z a t i o n of Fe-Cr a l l o y s  125  4.7  P o l a r i z a t i o n of pure n i c k e l .  127  4.8  A n o d i c p o l a r i z a t i o n of pure c o b a l t  129  4.9  A n o d i c p o l a r i z a t i o n of an Fe-C  130  4.10  High, c u r r e n t d e n s i t y  4.11  Electroslag  CHAPTER 5.  and pure chromium . . . . . . .  alloy.  polarization  130  process p o l a r i z a t i o n  136  DISCUSSION OF MELT PROGRAM RESULTS  137  5.1  Introduction  . . . . .  5.2  E f f e c t of e l e c t r o d e p o l a r i t y on oxygen c o n t e n t  138  5.3  Inclusion  142  5.4  E f f e c t of s p e c i f i c coulombic d e n s i t y  types  137  (Z) on the f i n a l  oxygen content  148  5.5  E f f e c t of atmosphere c o n t r o l  148  5.6  D i f f u s i o n of oxygen i n t o an anodic e l e c t r o d e  149  5.7  Drop s i z e and s u r f a c e t e n s i o n  153  5.8  Significance  154  5.9  M e l t r a t e and heat g e n e r a t i o n  5.10  Calcium oxide slags  5.11  E f f e c t of i n c r e a s i n g  5.12  E f f e c t o f aluminum a d d i t i o n  5.13  Alloy 5.13.1 5.13.2  losses  of i r o n i n the s l a g caps  and i n g o t  156 porosity  ingot/electrode  158  diameter r a t i o  158  a t the e l e c t r o d e d u r i n g m e l t i n g .  159  during remelting  A I S I 1095 s t e e l A I S I 430 s t a i n l e s s s t e e l  160 160 162  yiii Page  5.14  Remelting of n i c k e l e l e c t r o d e s .  5.15  A.C. m e l t i n g  5.16  E l e c t r o c h e m i c a l phenomena i n commercial ESR  CHAPTER 6.  REFERENCES  of pure i r o n  CONCLUSIONS. . . .  . .  . .  . . .  . . . . .  163  164  164  166  168  LIST OF TABLES  Table  I.  Page  M e l t Record R e s u l t s of AISI 1018 M i l d  II.  M e l t Record R e s u l t s of FVE Ingots  III.  M e l t Record R e s u l t s of FVE Ingots  IV.  Steel  . . . 88 89  . .  90  M e l t Record R e s u l t s of Armco I r o n Ingots  92  M e l t Record R e s u l t s of M i s c e l l a n e o u s Ingots  93  Drop S i z e and I n t e r f a c i a l T e n s i o n R e s u l t s f o r FVE . . . .  94  T o t a l Aluminum Content of FVE Ingots  95  VIII.  Composition  of AISI 1095 E l e c t r o d e and Ingots  96  IX.  Composition  of AISI 430 E l e c t r o d e and Ingots  97  V. VI. VII.  X. XI.  T o t a l I r o n Content  of FVE S l a g Caps  98  C r i t i c a l Current Density E s t i m a t i o n  132  ix  LIST OF;FIGURES  Figure  Page  1.  Cell electrical circuitry  35  2.  General c e l l design  36  3.  D e t a i l of  37  4.  Method of o b t a i n i n g  5.  ESR  6.  D e t a i l of r e f e r e n c e e l e c t r o d e c o n f i g u r a t i o n  45  7.  P o t e n t i a l decay between the m e l t i n g e l e c t r o d e and r e f e r e n c e e l e c t r o d e observed at c u r r e n t i n t e r r u p t i o n on an ESR anodic electrode  46  c e l l assembly overvoltage  39  f u r n a c e assembly.  44  8.  Anodic p o l a r i z a t i o n curves f o r pure i r o n i n CaF^  9.  Cathodic p o l a r i z a t i o n  A^O^slags  +  curves f o r pure i r o n i n CaF^  +  .  kl^O^  slags  49  10.  Anodic p o l a r i z a t i o n  11.  Cathodic p o l a r i z a t i o n  12.  Apparent t r a n s i t i o n time i n the i r o n i n C a F + 5 wt. % A l ^  curves f o r pure i r o n i n CaF^  + CaO  curves f o r pure i r o n i n CaF^  slags.  + CaO  anodic p o l a r i z a t i o n of  14.  15.  Anodic p o l a r i z a t i o n CaF + A 1 0 slags  pure  53  3  curves f o r A I S I 430  2  anodically 55  stainless steel in 57  3  Cathodic p o l a r i z a t i o n CaF + A 1 0 slags 2  17.  2  51  curves f o r pure i r o n i n  S e c t i o n s through a pure i r o n e l e c t r o d e t i p p o l a r i z e d at h i g h c u r r e n t d e n s i t y  2  16.  2  50  52  High c u r r e n t d e n s i t y p o l a r i z a t i o n C a F + 5 wt. % A 1 0 . . . 2  .  slags.  2  13.  48  curves f o r A I S I 430  stainless steel in 58  3  Cr c o n c e n t r a t i o n g r a d i e n t produced a t anodic s u r f a c e of A I S I 430 s t a i n l e s s s t e e l . .  x  59  Figure  18.  Page  Anodic p o l a r i z a t i o n curves f o r a Fe + 1 wt. i n a C a F + 1 wt. % A l ^ s l a g  % Cr  electrode 61  2  19.  Anodic p o l a r i z a t i o n curve Al Q slag 2  20.  f o r pure Cr i n a CaF_  +  1 wt.  %  7  3  S i n g l e p o l a r i z a t i o n curve f o r an i n a C a F + 2.5 wt. % A l ^ s l a g  anodic p u l s e on  62 pure n i c k e l 63  2  21.  S e c t i o n through a pure n i c k e l e l e c t r o d e c a t h o d i c a l l y i n a C a F + 8 wt. % A l ^ s l a g  polarized 64  2  22.  S i n g l e p o l a r i z a t i o n curve f o r an anodic p u l s e on pure i n a C a F + 2.5 wt. % A l ^ s l a g  cobalt 66  2  23.  A n o d i c p o l a r i z a t i o n on Armco i r o n ESR CaF + A l 0 s l a g s 2  24.  2  2  26.  67  2  2  + CaO  in 68  electrodes i n  < 69  3  C a t h o d i c p o l a r i z a t i o n on Armco i r o n ESR CaF  electrodes  3  Anodic p o l a r i z a t i o n on Armco i r o n ESR CaF + A 1 0 slags 2  in  3  C a t h o d i c p o l a r i z a t i o n on Armco i r o n ESR CaF + A l 0 s l a g s 2  25.  electrodes  electrodes  in  slags  70  27.  Argon fume hood. :  79  28.  Argon gas  cap.  80  29.  Ingot sampling scheme. .  86  30.  Schematic r e p r e s e n t a t i o n of the pure i r o n i n C a F + A l 0 s l a g s 2  31.  2  anodic p o l a r i z a t i o n curve f o r 102  3  A p p l i c a t i o n of a l i m i t i n g c u r r e n t d e n s i t y law to anodic p o l a r i z a t i o n of pure i r o n i n C a F + A l 0 s l a g s .  103  32.  P a r t i a l phase diagrams of  108  33.  Estimated l i m i t i n g current density of pure i r o n i n C a F + A 1 0 s l a g s  2  2  34.  the  2  3  2  3  system C a F  2  - CaO  f o r anodic .  - FeO polarization  IR drop as a f u n c t i o n of a p p l i e d c u r r e n t , I, f o r a n o d i c p o l a r i z a t i o n of pure i r o n i n a CaF„ +2.5 wt. % A1„0„ s l a g  I l l  . .  .112  Figure  Page  35.  Anodic p u l s e  on pure i r o n i n C a F  36.  A p p l i c a t i o n of t r a n s i t i o n time law t o anodic p o l a r i z a t i o n of pure i r o n i n C a F + 2.5 wt. % A 1 0 s l a g  117  S u c c e s s i v e anodic p u l s e s a p p l i e d to pure i r o n i n a C a F + 2.5 wt. % A 1 0 s l a g  118  2  37.  2  38.  2  Cathodic Al^^  39.  2  + 250 ppm CaO  2  114  3  3  p o l a r i z a t i o n of pure i r o n i n a C a F  slag p l o t t e d against  2  + 1 wt. %  the a p p l i e d c u r r e n t  S e c t i o n from i n t e r i o r of FVE e l e c t r o d e n e g a t i v e  122 ingot.  . . .143  40.  S e c t i o n from i n t e r i o r of A I S I 430 s t a i n l e s s s t e e l i n g o t .  41.  Alumina i n c l u s i o n s on the top of a FVE e l e c t r o d e ingot  negative  A l and 0 i n l i q u i d A 1 0 a t 1600°C.  activity  42.  2  43.  i r o n i n equilibrium with u n i t  146  147  3  V a r i o u s e l e c t r o d e t i p s showing oxide p r e s e n t iron electrode t i p :  . .144  on an  anodic 155  A C K N O W L E D G E M E N T S  The assistance  author i s indebted  t o Dr. A. M i t c h e l l f o r h i s a d v i c e and  throughout the d u r a t i o n o f t h i s work.  Thanks a r e a l s o due to my f e l l o w graduate s t u d e n t s and t o v a r i o u s f a c u l t y members f o r innumerable h e l p f u l d i s c u s s i o n s .  The nical staff  The  assistance i s greatly  of Mr. A. Thomas and other members of the t e c h appreciated.  f i n a n c i a l a s s i s t a n c e of S t e l c o i s g r a t e f u l l y acknowledged.  CHAPTER 1 INTRODUCTION 1.1  The E l e c t r o s l a g Remelting  E l e c t r o s l a g Remelting t r o d e i s consumably melted mold forming an i n g o t .  (ESR) i s a p r o c e s s i n which a m e t a l  through a s l a g to s o l i d i f y  i s Vacuum A r c Remelting  elec-  i n a water-cooled  ESR i s one of the few p r o c e s s e s s u i t a b l e f o r up-  g r a d i n g the q u a l i t y o f the more complex a l l o y s structural applications.  Process  i n use i n s p e c i a l i z e d  The most w i d e l y used p r o c e s s i n North  America  (VAR) which i s the c h i e f a l t e r n a t i v e t o ESR.  More expensive p r o c e s s e s such as E l e c t r o n Beam M e l t i n g a r e n o r m a l l y  used  to p r o c e s s only the r a r e r m e t a l s because o f the h i g h o p e r a t i n g c o s t s and the s m a l l m e l t i n g c a p a c i t y of those u n i t s . Any  r e f i n i n g a c t i o n d u r i n g VAR i s a r e s u l t of m e t a l exposure t o  the vacuum and i s t h e r e f o r e r e s t r i c t e d t o p r e s s u r e s e n s i t i v e  reactions.  In c o n t r a s t , i n ESR the m e l t i n g m e t a l spends a f i n i t e l e n g t h of time i n c o n t a c t w i t h the s l a g . solid/liquid  F o r t h i s r e a s o n c o m p o s i t i o n changes r e q u i r i n g  or l i q u i d / l i q u i d  ESR appears  r e a c t i o n s are p o s s i b l e .  to be a more s a t i s f a c t o r y secondary  than VAR mainly because of the g r e a t e r f l e x i b i l i t y  refining  process  i n melting conditions  With proper u t i l i z a t i o n o f s l a g c h e m i s t r y and m e l t i n g parameters, i t should be p o s s i b l e t o a c h i e v e r i g o r o u s c o m p o s i t i o n and s t r u c t u r e hence p r o d u c i n g an i n g o t w i t h b e t t e r o v e r a l l The r a t e o f p r o d u c t i o n of an ESR u n i t mum  tolerable freezing rate.  control  properties. i s c o n t r o l l e d by the maxi-  In g e n e r a l a lower melt r a t e produces  1  a  2  f l a t t e r p o o l p r o f i l e w h i l e a h i g h e r melt r a t e produces.a f i l e and p o s s i b l y u n f a v o u r a b l e d e n d r i t e o r i e n t a t i o n . maximum ESR  i n g o t s i z e i s 150  be i n c r e a s e d to 200  120  to 2000 kwh  Power consumption d u r i n g ESR  any  ranges  on which  can  from  parameters  to c o n t r o l , i n c o n t r a s t to a t y p i c a l 3 0-arc f u r n a c e  consumption of 550-650 kwh/ton. same c o s t per ton of m e t a l . sumption  and t h i s  diameter w i t h o u t r e q u i r i n g  per ton of m e t a l , a g a i n depending  the operator chooses  pool pro-  A t p r e s e n t the  tons w i t h a diameter of 3 m.,  tons w i t h a 3.5 m.  major t e c h n o l o g i c a l changes.  deeper  Both VAR  and ESR  add approximately  the  In t h i s c o s t s t r u c t u r e e l e c t r i c a l power con-  accounts f o r a p p r o x i m a t e l y 30-50% of the t o t a l d i r e c t and i n -  direct costs.  O p t i m i z a t i o n of t h i s power consumption would e v i d e n t l y  be  a major advantage i n f u r n a c e o p e r a t i o n .  1.2  The  problem  The m a j o r i t y of ESR u n i t s i n o p e r a t i o n today use s i n g l e - p h a s e , l i n e frequency  (60 Hz) A.C.  power.  I t i s d e s i r a b l e t h a t the power s u p p l y  c a b l e s have the s h o r t e s t p o s s i b l e r o u t e from t r a n s f o r m e r to e l e c t r o d e c o n n e c t i o n and be s i t e d on the same r o u t e as the b a s e p l a t e r e t u r n l i n e s . Such an arrangement w i l l mission e f f i c i e n c y .  reduce the i n d u c t i v e loop and m a i n t a i n  A power f a c t o r of between 0.85  and 0.92  o b t a i n e d on such equipment without c o r r e c t i o n measures.  trans-  i s normally  As l a r g e r  elec-  t r o s l a g f u r n a c e s are . b u i l t , i t i s n e c e s s a r y to determine whether they w i l l use A.C.  or D.C.  power.  A.C.  power f a c t o r s tend to decrease w i t h  increas-  i n g i n g o t s i z e , and a l t h o u g h these f u r n a c e s c o u l d o p e r a t e a t low power; f a c t o r s , the s u p p l y a u t h o r i t i e s would not permit t h i s because of waveshape d i s t o r t i o n on the incoming mains.  Power f a c t o r c o r r e c t i o n can be  3  accomplished  u s i n g v a r i a b l e c a p a c i t o r s b u t the c o s t i s e x c e e d i n g l y h i g h .  D.C. m e l t i n g , on t h e o t h e r hand, has no e l e c t r o s l a g c i r c u i t  power-factor  problems and thus has advantages f o r v e r y l a r g e i n g o t s i z e s . Strictly  from power c o n s i d e r a t i o n s then, D.C. power i s a p p a r e n t l y  p r e f e r a b l e t o A.C. power f o r v e r y l a r g e e l e c t r o s l a g f u r n a c e s , but such a d e c i s i o n must a l s o be made w i t h r e s p e c t t o p o s s i b l e e l e c t r o c h e m i c a l , and thermal phenomena which might be encountered amounts o f D.C. c u r r e n t through f o r e attempt  to answer  d u r i n g the passage o f l a r g e  the e l e c t r o s l a g system.  One must t h e r e -  the f o l l o w i n g q u e s t i o n s about D.C. e l e c t r o s l a g  melting: 1.  What e l e c t r o c h e m i c a l , c h e m i c a l , and thermal  changes w i l l occur d u r i n g r e m e l t i n g ? 2.  Why do they  occur?  3.  How important w i l l  they be i n terms of a f f e c t i n g  the f i n a l c h e m i c a l and m e c h a n i c a l  p r o p e r t i e s o f the i n g o t  metal?  1.2.1  Chemical It  effects  i s a g e n e r a l l y accepted f a c t  t h a t D.C. m e l t i n g of a l l o y mater-  i a l s produces a h i g h e r l o s s of e a s i l y o x i d i z e d a l l o y i n g elements  than  occurs d u r i n g A.C. m e l t i n g , and a l s o t h a t the main concern i s not w i t h the l o s s o f major a l l o y i n g elements,  b u t i n s t e a d w i t h changes i n minor  components such as T i , S i , A l , 0, S, e t c . l o s s e s from AISI 321 s t a i n l e s s s t e e l  Etienne  (0.5 wt. %  (1) r e p o r t e d t i t a n i u m T i i n the e l e c t r o d e ) of  40% i n the e l e c t r o d e n e g a t i v e mode and l o s s e s of 80% i n the e l e c t r o d e  4  p o s i t i v e mode. when r e m e l t i n g  Kennard (2) r e p o r t e d T i l o s s e s of a p p r o x i m a t e l y 18% N i Maraging s t e e l  the e l e c t r o d e n e g a t i v e mode.  (0.80 wt. %  T i i n electrode) i n  Other workers (3, 4) have r e p o r t e d  l o s s e s o f S i (~10%) and A l (~15%) from i r o n a l l o y s remelted power. W h i t t a k e r and  sulphur  (5) and H o l z g r u b e r  serious  with  D.C.  (6) found v a r y i n g l e v e l s of oxygen  i n i n g o t s made u s i n g e l e c t r o d e n e g a t i v e ,  and A.C. power.  30%  electrode  positive,  As mentioned p r e v i o u s l y , o x i d a t i o n l o s s e s a r e h i g h e r  d u r i n g D.C. m e l t i n g .  These c h e m i c a l  e f f e c t s must t h e r e f o r e be a r e s u l t  of e l e c t r o c h e m i c a l l y produced o x i d a t i o n .  The n a t u r e  of t h i s phenomenon  i s u n c l e a r due t o the l a c k of d e f i n i t i v e e l e c t r o c h e m i c a l s t u d i e s o f metal electrodes i n calcium f l u o r i d e - b a s e d s l a g s .  1.2.2  Mechanical General  effects  concern f o r o x i d a t i v e l o s s e s i s n o t s u r p r i s i n g , s i n c e some  alloys,  where a s m a l l c o n c e n t r a t i o n of an e s s e n t i a l a l l o y i n g element i s  present,  a r e p a r t i c u l a r l y s e n s i t i v e to composition  aging s t e e l type  I t i s apparent, then,  s e r i o u s indeed,  s t r e n g t h to drop by 10,000 p s i  t h a t these observed a l l o y l o s s e s can be v e r y  and one must be aware o f such p o t e n t i a l l o s s e s d u r i n g D.C.  remelting i n order remelting furnaces. formation  In Mar-  300, f o r i n s t a n c e , which c o n t a i n s 0.8 wt. % T i , a l o s s  of 0.1 wt. % i n T i w i l l cause the y i e l d (3%).  fluctuations.  to d e c i d e what mode of m e l t i n g  i s most f e a s i b l e i n l a r g e  E f f e c t i v e l o s s o f a l l o y i n g elements may a l s o occur by  of oxide i n c l u s i o n s i n the i n g o t .  I n t h i s case,  the elements have  not a c t u a l l y been l o s t , b u t they a r e no l o n g e r a v a i l a b l e to s e r v e t h e i r i n tended purpose i n the m a t r i x .  C o n s i d e r a t i o n must a l s o be g i v e n  to the d e t r i -  mental e f f e c t of i n c l u s i o n s on the m e c h a n i c a l p r o p e r t i e s of the m e t a l .  5  1.2.3  Thermal e f f e c t s Thermal e f f e c t s i n D.C.  if  E l e c t r o s l a g melting w i l l a r i s e  the mode of c u r r e n t passage at the m e t a l / s l a g  the r a t e o f heat g e n e r a t i o n current conduction  or heat t r a n s f e r .  directly  interfaces alters either  Depending on the type  i n the s l a g , such changes might a r i s e from a P e l t i e r  e f f e c t , from an e l e c t r o c h e m i c a l p o l a r i z a t i o n r e s i s t a n c e , o r from the mation of an a r c .  These p o s s i b i l i t i e s w i l l be d i s c u s s e d  Heat e f f e c t s a p p a r e n t l y do has been noted A.C.)  (7,8)  produce d i f f e r e n c e s i n the s p e c i f i c m e l t - r a t e different  which mode i s the most e f f i c i e n t .  subsequently.  (D.C.  +ve, further-  i n agreement as  These d i f f e r e n c e s i n s p e c i f i c  to  melt-rate  e f f e c t s a t the e l e c t r o d e , which  i n t u r n must r e s u l t from e l e c t r o c h e m i c a l and  chemical  effects  associated  the mode of c u r r e n t passage at the s l a g / m e t a l i n t e r f a c e s . S i n c e i t i s apparent t h a t c h e m i c a l , m e c h a n i c a l and  are a s s o c i a t e d w i t h u s i n g D.C. the r e s u l t s  1.3  -ve, D.C.  (kwh/ton), and,  i n v e s t i g a t o r s are not  must a r i s e , i n p a r t , from heat g e n e r a t i o n  while  for-  e x i s t d u r i n g e l e c t r o s l a g m e l t i n g because i t  t h a t the t h r e e modes of m e l t i n g  more, these f i n d i n g s by  with  of  thermal  effects  power f o r e l e c t r o s l a g r e m e l t i n g , and  to date are c o n t r a d i c t o r y r e g a r d i n g  these  i n v e s t i g a t i n g these phenomena on both s m a l l and  that  e f f e c t s , i t i s worthlarge scales.  Background to the problem Before  one  can study  v i o u s l y , i t i s necessary  the o c c u r r e n c e  s a l t system, and  i n each case to  the p o s s i b l e mechanisms w h i c h c o u l d g i v e r i s e to chemical  thermal e f f e c t s .  The  pre-  to understand f u l l y a l l the p o s s i b l e methods of  c u r r e n t passage i n a l i q u i d m e t a l / f u s e d consider  of the e f f e c t s d i s c u s s e d  and  t h r e e p o s s i b l e modes of c u r r e n t passage between a  6  l i q u i d m e t a l e l e c t r o d e and a f u s e d s a l t a r e : I. II. III.  Electronic  conduction.  Faradaic reactions. Arc discharge.  Each o f these have s p e c i f i c c h a r a c t e r i s t i c s which may or may n o t be a b l e to e x p l a i n the e f f e c t s b e i n g examined.  1.3.1  Electronic If  nic  conduction  c u r r e n t passage through  such a system takes p l a c e by e l e c t r o -  c o n d u c t i o n , i . e . , the s l a g i s an e l e c t r o n i c conductor,  t h e r e w i l l be  no F a r a d a i c r e a c t i o n s o c c u r r i n g a t the s l a g / m e t a l i n t e r f a c e s . t h e r e f o r e be no e l e c t r o c h e m i c a l l y produced try  of t h e metal and the s l a g .  m a t e r i a l to a f f e c t  D  Heat  I f any change i n the CaF^  s t o i c h i o m e t r y i s t o be produced  r e a c t i o n i t would be most l i k e l y  positions.  Hence the s l a g would behave as an n-type semiconductor.  to lead  at an i n t e r f a c e due t o the P e l t i e r  when two m a t e r i a l s have d i f f e r i n g heat so generated  comIt  l o c a l i z e d heating  e f f e c t , which i s important  electron mobilities.  The amount o f  i s g i v e n by: q =  TTI  where ir i s the P e l t i e r c o e f f i c i e n t and I i s the c u r r e n t . efficient  by h i g h  t o metal-excess  i s p o s s i b l e d u r i n g e l e c t r o n i c c o n d u c t i o n , t o generate cooling  gener-  b e i n g equal to the product of the p r o c e s s v o l t a g e and  temperature  or  from  f the s l a g w i l l o c c u r by r e s i s t a n c e h e a t i n g , the amount  of heat so generated current.  the chemis-  Any c h e m i c a l changes would r e s u l t o n l y  chemical i n t e r a c t i o n between the s l a g and the m e l t i n g m e t a l . a t i o n i n the b u l k  There w i l l  i s g i v e n by:  The P e l t i e r c o -  7  TT  CtT  =  where a i s the Seebeck c o e f f i c i e n t  and T i s the a b s o l u t e temperature.  Seebeck c o e f f i c i e n t of one of the b e s t commercial (PbTe), i s approximately  400 yV.°C \  a t u r e of 2000°K, and a c u r r e n t of 1000  comparison  thermoelectric materials  but that o f a c a l c i u m - f l u o r i d e  would l i k e l y be much lower, say approximately  such a proposed  The  A.,  10 yV.°C . 1  slag,  At a temper-  the r a t e of P e l t i e r h e a t i n g a t  i n t e r f a c e would be 20 c a l . s e c . \  a n e g l i g i b l e amount i n  to the t o t a l p r o c e s s heat g e n e r a t i o n r a t e of 6.5  Kcal.sec  A l s o , such heat g e n e r a t i o n at one e l e c t r o d e i n t e r f a c e , would be  1  .  comple-  mented by an e q u i v a l e n t amount of heat l o s s at the o t h e r i n t e r f a c e .  One  can t h e r e f o r e conclude t h a t e l e c t r o n i c c o n d u c t i o n d u r i n g ESR p r o c e s s i n g would produce no s i g n i f i c a n t g e n e r a t i o n a r i s i n g from  c h e m i c a l changes and  the P e l t i e r e f f e c t would be i n s u f f i c i e n t  the melt r a t e of the e l e c t r o d e . account f o r any  that l o c a l i z e d  heat to a f f e c t  In view of these d e d u c t i o n s , one must,  c h e m i c a l and thermal e f f e c t s observed d u r i n g e l e c t r o s l a g  m e l t i n g by means o t h e r than e l e c t r o n i c  c o n d u c t i o n , even though the  degree  of e l e c t r o n i c c o n d u c t i o n i n such a m e t a l / f u s e d s a l t system might be q u i t e substantial  1.3.2  (as w i l l be d i s c u s s e d i n Chapter  Ionic  5).  conduction  I f c u r r e n t passage between l i q u i d m e t a l e l e c t r o d e s i n c o n t a c t w i t h c a l c i u m f l u o r i d e - b a s e d s l a g s takes p l a c e by i o n i c c o n d u c t i o n , F a r a d a i c p r o c e s s e s must take p l a c e a t the e l e c t r o d e / s l a g i n t e r f a c e s . ment has  two  important  consequences.  p r o d u c t s are a v a i l a b l e i n the system and  the s l a g .  The  second  The f i r s t  This require-  i s that Faradaic r e a c t i o n  to r e a c t c h e m i c a l l y w i t h the m e t a l  i s t h a t p o l a r i z a t i o n can occur at the m e t a l /  8  s l a g i n t e r f a c e s which c o u l d r e s u l t i n l o c a l i z e d excess modify i s o - p o t e n t i a l s u r f a c e s i n the b u l k o f the s l a g . the l o c a l dynamic heat b a l a n c e  heat  g e n e r a t i o n and  T h i s would  affect  a t the m e t a l / s l a g i n t e r f a c e s , and would be  r e f l e c t e d e i t h e r i n melting e f f i c i e n c y or i n process  temperature  distri-  butions. Although are completely  i t has been s t a t e d t h a t c a l c i u m f l u o r i d e - b a s e d s l a g s  i o n i c i n nature  e l e c t r o n i c conduction  ( 9 ) , t h i s i n f a c t means t h a t t h e degree o f  i s extremely  small.  In pure l i q u i d CaF2» the t r a n s p o r t  -4 number of e l e c t r o n s i s approximately of n o n s t o i c h i o m e t r y (11).  Calcium  (12).  (10), and a r i s e s from a s m a l l degree  which i s i n h e r e n t i n a l l compounds at h i g h  temperature  f l u o r i d e w i l l have s i g n i f i c a n t e l e c t r o n i c c o n d u c t i v i t y when  calcium i s d i s s o l v e d i n i t , fluoride  10  Although  calcium being  completely  m i s c i b l e with  calcium  the e f f e c t of d i s s o l v e d c a l c i u m on t h e e l e c t r i c a l  c o n d u c t i v i t y of c a l c i u m f l u o r i d e has not been s t u d i e d , data a r e a v a i l a b l e on the e f f e c t of sodium a d d i t i o n to sodium f l u o r i d e and s i m i l a r h a l i d e tems (12).  sys-  From these data i t appears t h a t a 3O+40 i n c r e a s e i n c o n d u c t i v i t y  f o r 2->-5 mole % a d d i t i o n of c a l c i u m t o c a l c i u m f l u o r i d e i s not T h i s would r e p r e s e n t a l a r g e degree of n o n s t o i c h i o m e t r y s t a n t i a l degree of n-type e l e c t r o n i c  unreasonable.  and t h e r e f o r e a sub-  semiconduction.  F o r the moment, c a l c i u m f l u o r i d e s l a g s w i l l be presumed t o conduct  ionically,  and t h e r e f o r e d u r i n g e l e c t r o s l a g r e m e l t i n g w i t h D.C. power,  c a t h o d i c and anodic F a r a d a i c r e a c t i o n s take p l a c e c o n t i n u o u s l y a t t h e appropriate s i t e s .  It is difficult  to j u s t i f y  the presence o f substan-  t i a l n e t F a r a d a i c r e a c t i o n s i n an e l e c t r o s l a g f u r n a c e i n view o f the f a c t that the s l a g chemistry  i s e s s e n t i a l l y unchanged d u r i n g m e l t i n g .  The  9  o v e r a l l system r e a c t i o n e f f i c i e n c y must t h e r e f o r e be v e r y mately 1 % , a c o n d i t i o n t h a t can a r i s e i n two ways. ity  i s t h a t the i n d i v i d u a l t o t a l a n o d i c  operate  at v e r y  t h e r e was bility  low  and  current e f f i c i e n c i e s .  The  low,  approxi-  first  possibil-  cathodic processes T h i s would be  themselves  the case i f  a s u b s t a n t i a l amount of e l e c t r o n i c c o n d u c t i o n .  The  second p o s s i -  i s t h a t , i f the r e a c t i o n c u r r e n t e f f i c i e n c i e s are h i g h , the  v i d u a l r e a c t i o n products during melting are present  c o n t i n u o u s l y recombine d u r i n g r e m e l t i n g such t h a t  a s m a l l steady  i n the s l a g .  s t a t e amount of anodic  Observation  and  The the chemical  composition  changes i n the s l a g are  l a c k of s u b s t a n t i a l chemical changes which occur  cathodic  small.  changes i n the s l a g i m p l i e s that  i n the m e t a l are a l s o s m a l l ,  on the m e c h a n i c a l p r o p e r t i e s of the m e t a l .  The  although  serious  effects  purpose of t h i s study i s  then to f o r m u l a t e  F a r a d a i c r e a c t i o n mechanisms o c c u r r i n g at l i q u i d  f u s e d s a l t anodic  and  metal/  c a t h o d i c i n t e r f a c e s which can account f o r the  phenomena: I. II.  Chemical changes i n the s l a g are  IV.  small.  E a s i l y o x i d i z e d a l l o y i n g elements  p r e f e r e n t i a l l y removed from the m e l t i n g III.  there-  t h a t the net e l e c -  even s m a l l l o s s e s i n some a l l o y i n g elements can have v e r y  ing  products  of the system as a whole would  f o r e i n d i c a t e t h a t the r e a c t i o n s behave e f f i c i e n t l y but trochemical  indi-  are  metal.  Chemical changes i n the m e t a l a r e  small.  Thermal e f f e c t s have been observed and  a p p a r e n t l y r e l a t e d to the mode of power used d u r i n g  are  melting.  follow-  10  1.3.2.1  I n t e r f a c e s at e q u i l i b r i u m It  i s u s e f u l a t t h i s p o i n t to c o n s i d e r  the type o f m e t a l / s l a g  i n t e r f a c e t h a t would e x i s t between pure i r o n and a c a l c i u m based s l a g at e q u i l i b r i u m , (no net c u r r e n t flow a c r o s s If a s o l i d calcium  i r o n e l e c t r o d e i s immersed i n a l i q u i d  oxide  fluoride-  the i n t e r f a c e ) .  calcium  fluoride-  s l a g , e q u i l i b r i u m between the m e t a l and s l a g w i l l be  a t t a i n e d by d i s s o l u t i o n o f i r o n i o n s i n the s l a g a t the i n t e r f a c e . A l though the i r o n s u r f a c e i s i n e q u i l i b r i u m w i t h i s not a s t a t i c one. c u r r e n t equals  the s l a g , the i n t e r f a c e  An exchange c u r r e n t f l o w s , i n which the forward  the r e v e r s e  current.  Aqueous systems g e n e r a l l y have low  -12 -3 -2 exchange c u r r e n t d e n s i t i e s (10 -KL0 A.cm ) b u t some l i q u i d s a l t systems have c o m p a r a t i v e l y  high  metal/fused  exchange c u r r e n t d e n s i t i e s ( i = 210  -2 A.cm.  f o r the C d ( I I ) - C d  can be c o n s i d e r e d  reaction)  ( 1 3 ) . The exchange c u r r e n t  density  t o be a measure of the r e v e r s i b i l i t y o f an e l e c t r o c h e -  m i c a l r e a c t i o n , which i n turns  i n d i c a t e s the ease w i t h which an e l e c t r o d e  may be p o l a r i z e d . A h i g h exchange c u r r e n t d e n s i t y i m p l i e s t h a t an e l e c t r o d e i s d i f f i c u l t to p o l a r i z e .  The b e h a v i o u r o f p o l a r i z e d e l e c t r o d e i n t e r f a c e s w i l l  now be d i s c u s s e d s i n c e p o l a r i z a t i o n phenomena can be used t o answer some of the q u e s t i o n s mal  effects.  p r e v i o u s l y presented  At a simple  with  l i q u i d metal/fused  respect  to chemical  s a l t electrode interface,  p o l a r i z a t i o n phenomena have the f o l l o w i n g consequences. 1.  The presence of p o l a r i z a t i o n a t such  an i n t e r f a c e i m p l i e s t h a t excess heat w i l l be generated by  and t h e r -  c u r r e n t passage through the t h i n p o l a r i z e d s l a g l a y e r  11  r e p r e s e n t i n g p a r t of the i r r e v e r s i b i l i t y  o f the r e a c t i o n .  S i g n i f i c a n t p o l a r i z a t i o n c o u l d t h e r e f o r e account f o r any observed thermal e f f e c t s d u r i n g 2.  Polarization w i l l  r e a c t i o n s , the products important  1.3.2.2  composition  remelting. take p l a c e by F a r a d a i c  o f which may l e a d t o s m a l l b u t  changes of the m e t a l d u r i n g  melting.  Polarized electrode interfaces At e q u i l i b r i u m , t h e p o t e n t i a l , w i t h r e s p e c t to a r e f e r e n c e e l e c -  t r o d e , o f an Fe e l e c t r o d e i n c o n t a c t w i t h a c a l c i u m f l u o r i d e - c a l c i u m 2+ o x i d e s l a g i s determined by the c o n c e n t r a t i o n o f Fe metal i n t e r f a c e .  i o n s a t the s l a g /  T h i s c o n c e n t r a t i o n i s governed by the h a l f - c e l l  reac-  Fe + F e  (1-1)  tion: + 2e~  2 +  For the case of a simple m e t a l / i o n  e l e c t r o d e having  the e l e c t r o d e  r e a c t i o n v a l e n c e n = z: Me + M e  + ze  Z +  (1-2)  _  the e q u i l i b r i u m e l e c t r o d e p o t e n t i a l , E, i s w r i t t e n i n the form of a Nernst e q u a t i o n :  \ o E = E  where E° i  s  T  + ^  the. s t a n d a r d  are a t . u n i t a c t i v i t y .  P  a z +  In  e  (1-3)  e l e c t r o d e p o t e n t i a l when a l l r e a c t a n t s and products  I t must be s t r e s s e d that a l l h a l f - c e l l p o t e n t i a l s  a r e measured w i t h r e s p e c t to a r e f e r e n c e e l e c t r o d e whose p o t e n t i a l i s constant f o r a given e l e c t r o l y t e .  I d e a l l y , a r e f e r e n c e e l e c t r o d e c o n s i s t s of  an e l e c t r o d e s u r f a c e i n c o n t a c t w i t h e l e c t r o l y t e of a known and i n v a r i e n t  12  composition  such t h a t a standard  can be used.  An  example of a standard  ated c a l o m e l e l e c t r o d e . is  reference  A difficulty  e l e c t r o d e of known p o t e n t i a l  r e f e r e n c e e l e c t r o d e i s the that a r i s e s with  t h a t i t i s f r e q u e n t l y n e c e s s a r y to have two  c o n t a c t , which g i v e s r i s e  to a l i q u i d  such  satur-  electrodes  different solutions i n  junction potential.  The  value  of the l i q u i d j u n c t i o n p o t e n t i a l can be minimized by u s i n g a s a l t  bridge.  In aqueous systems where the s o l u t i o n r e s i s t a n c e s are g e n e r a l l y h i g h i t i s n e c e s s a r y to u t i l i z e  this  type of r e f e r e n c e  the IR drop between a working e l e c t r o d e and be minimized by h a v i n g electrode.  In fused  i s p o s s i b l e to use liquid  the r e f e r e n c e  Chapter 2.  electrode very  electrode  and  studied.  T h i s w i l l be d i s c u s s e d  and  alter  i n detail in  electrode  I f c u r r e n t i s passed through an e l e c t r o d e a t e q u i l i b r i u m of the  electrochemical  or products i s a l t e r e d , the e l e c t r o d e i s s a i d to be p o l a r i z e d ,  the p o t e n t i a l of the e l e c t r o d e i n t e r f a c e i s a l t e r e d from the  brium p o t e n t i a l by  1.3.2.3  bulk  important p o i n t at i s s u e i s t h a t a r e v e r s i b l e r e f e r e n c e  the c u r r e n t i s such t h a t the c o n c e n t r a t i o n  reactants  the  compositions s t u d i e d do not g r o s s l y  e l e c t r o d e must be used to measure the p o t e n t i a l of the working being  can  c l o s e to the working  a remote r e f e r e n c e e l e c t r o d e i n c o n t a c t w i t h  electrode p o t e n t i a l . The  the r e f e r e n c e  that  s a l t systems, the r e s i s t a n c e s are much lower and i t  i f the range of l i q u i d  the r e f e r e n c e  e l e c t r o d e i n order  equili-  the p o l a r i z a t i o n o v e r v o l t a g e .  P o l a r i z a t i o n mechanisms Vetter  (14)  d e f i n e s f o u r types of o v e r v o l t a g e ,  each one  i s o p e r a t i v e a t an i n t e r f a c e i f t h a t p a r t i c u l a r step of the  of which  overall  13  e l e c t r o c h e m i c a l r e a c t i o n i s the s l o w e s t o r r a t e - d e t e r m i n i n g The  process.  types of o v e r v o l t a g e are g i v e n below. 1.  n  t  The t r i c a l double 2.  charge-transfer overvoltage. t r a n s f e r of charge c a r r i e r s a c r o s s the  elec-  layer i s rate-controlling. n  - r e a c t i o n overvoltage.  A slow chemical s t e p i n the o v e r a l l e l e c t r o d e r e a c t i o n i s r a t e c o n t r o l l i n g , and definition,  independent of p o t e n t i a l . 3.  ~ diffusion Supply  products  the r a t e c o n s t a n t i s , by  overvoltage.  of r e a c t a n t s to or removal of r e a c t i o n  from the e l e c t r o d e i s r a t e c o n t r o l l i n g . 4.  n  c  - crystallization  Hindrance  overvoltage.  of the process by which atoms are  i n c o r p o r a t e d i n t o or removed from the c r y s t a l l a t t i c e to c r y s t a l l i z a t i o n  overvoltage.  In a l i q u i d m e t a l / f u s e d it  is unlikely  salt  system a t h i g h temperature (~1400°C)  that a c t i v a t i o n processes  rate c o n t r o l l i n g .  other than d i f f u s i o n would  Further d i s c u s s i o n w i l l  d i f f u s i o n a l processes  and  leads  t h e r e f o r e be concerned  be  with  a s s o c i a t e d phenomena.  D i f f u s i o n overvoltage.  D i f f u s i o n o v e r v o l t a g e , r)^,  appears when  the supply of r e a c t a n t s to the e l e c t r o d e or the removal of r e a c t i o n p r o ducts from the e l e c t r o d e i s r a t e d e t e r m i n i n g . i n c l u d i n g the c r y s t a l l i z a t i o n p r o c e s s e s  and  I f a l l chemical  processes,  a l s o the c h a r g e - t r a n s f e r  r e a c t i o n , are i n e q u i l i b r i u m , o n l y d i f f u s i o n o v e r v o l t a g e i s p r e s e n t .  14  The p o t e n t i a l of an e l e c t r o d e e x p e r i e n c i n g pure d i f f u s i o n can be c a l c u l a t e d i n terms of the Nernst  equation  polarization  i n which the  concen-  t r a t i o n of the components d i r e c t l y at the s u r f a c e must be used and those i n the i n t e r i o r of the e l e c t r o l y t e . oyervoltage  Therefore,  the  diffusion  i s e q u a l to the d i f f e r e n c e :  n  d  = E'-E  (1-4)  between the e q u i l i b r i u m p o t e n t i a l E and quired f o r current  the impressed  p o t e n t i a l E' r e -  flow.  To c o r r e l a t e the d i f f u s i o n o v e r v o l t a g e  with  concentration  changes, a g e n e r a l o v e r a l l e l e c t r o d e r e a c t i o n w i l l be (-v,)S. + 1 1 The  not  (-v„)S.•+ . . .t v.S. 1 1 11  stoichiometric factors v  n e g a t i v e f o r reduced  +  given.  . . . + v S + ne qq  a r e p o s i t i v e f o r o x i d i z e d components  components.  by  and  At e q u i l i b r i u m the o v e r a l l e l e c t r o d e  r e a c t i o n l e a d s to t h e - e q u i l i b r i u m e l e c t r o d e p o t e n t i a l E whose t i o n dependence i s expressed  (1-5)  -  the Nernst  concentra-  equation: 'f  E  In e q u a t i o n  (1-5)  =  E  °  + EL  E  v  . In  and  of the time t .  E'(i,  c  On  °f  t  n  components  e  c u r r e n t flow the  concentrations  the s u r f a c e , are f u n c t i o n s of the c u r r e n t d e n s i t y i Hence a^ = a ( i , t ) ^ a .  t) i s g i v e n i n accordance w i t h the Nernst RT E ' ( i . t ) = E° + ^ na  • Zv.  2  and by subs t r a c t i o n of e q u a t i o n voltage i s :  (1-6)  a^ are the a c t i v i t i e s a^ = f j j  of the o v e r a l l e l e c t r o d e r e a c t i o n . c_. d i r e c t l y b e f o r e  a.  Therefore, equation  the  potential  by:  • In a . ( i , t ) J (1-6)  from ( 1 - 7 ) , the d i f f u s i o n  (1-7) over-  1 5  a (i,t)  ijrp N  =  d  E'-E  =  —  nF  •  Zv.  i  «ln  -J  J  (1-8*1  a. J  D i f f u s i o n p o l a r i z a t i o n t h e r e f o r e r e s u l t s when mass t r a n s p o r t  of  r e a c t a n t s or p r o d u c t s to or away from the e l e c t r o d e i n t e r f a c e i s r a t e controlling.  T h i s produces c o n c e n t r a t i o n  i n t e r f a c e and  the r e s u l t i n g d i f f u s i o n o v e r v o l t a g e  Nernst e q u a t i o n  i n terms of c o n c e n t r a t i o n  The  i s expressed as  or a c t i v i t y  interface. face with  For  the s i m p l e s t  case,  one  a  changes.  Transport  take p l a c e only  to or away from an  assumes a p l a n a r  a d i f f u s i o n l a y e r of t h i c k n e s s  stationary.  electrode  electrode  6, which i s assumed to  of the component S_. having  x i s the p e r p e n d i c u l a r  sur-  be  the a c t i v i t y a^  i f an a c t i v i t y g r a d i e n t da./dx e x i s t s f o r t h i s  can  compo-  d i s t a n c e from the e l e c t r o d e s u r f a c e .  number of moles N^. of substance  diffusing  The  through a c r o s s - s e c t i o n of  2  per second i s g i v e n by F i c k ' s f i r s t  concentration is written  law.  For s i m p l i c i t y ,  c. w i l l be used i n p l a c e of the a c t i v i t y a., J J  and  the  the  flux  as: dc. N. = D • J j dx  All  ( 1 - 9 )  components S . of the o v e r a l l e l e c t r o d e r e a c t i o n ( 1 - 5 ) J  d i f f u s e through t h i s d i f f u s i o n l a y e r .  V  c u r r e n t d e n s i t y d i v i d e d by n • F/v_. -2  must  A t r a n s f e r of one mole of S_.  corresponds to a charge amounting to n • F / j  cm.  the  concept of a d i f f u s i o n boundary l a y e r must be used when d i s -  c u s s i n g d i f f u s i o n a l t r a n s p o r t of s p e c i e s  1 cm.  at  Mass t r a n s p o r t  1 . 3 . 2 . 4  nent,  changes i n components  coulombs.  represents  Therefore  the  mass t r a n s f e r i n mole.  -1  sec.  , which passes from the e l e c t r o d e through the l a y e r to  the  16  electrolyte.  With due  c o n s i d e r a t i o n to s i g n i*v. — = nF  i s obtained. chemical S. J  may  In the s t a t i o n a r y s t a t e and  equilibria  now  (1-10)  i n the absence of homogeneous  through the e n t i r e l a y e r , the flow of the components  must be c o n s t a n t  = constant  dc. -D. • j dx  and  a l i n e a r c o n c e n t r a t i o n g r a d i e n t dc./dx = 3  i s e s t a b l i s h e d throughout the e n t i r e l a y e r .  Equation  -(c.-c.) 3  (1-10)  be w r i t t e n a s :  n  • F  In p r i n c i p l e , w i t h  j  6  the movement o f  i o n s through the  l a y e r , the i n f l u e n c e of an e l e c t r i c f i e l d must a l s o be This f i e l d  causes an a d d i t i o n a l m i g r a t i o n  on the d i f f u s i o n c u r r e n t .  diffusion  taken i n t o account.  c u r r e n t which i s superimposed  A h i g h excess of an i n d i f f e r e n t  minimizes t h i s e l e c t r i c f i e l d  electrolyte  e f f e c t w i t h i n the d i f f u s i o n l a y e r to a  l a r g e e x t e n t , s i n c e the excess e l e c t r o l y t e causes the t r a n s p o r t number t.. of the r e a c t i n g components  to become very  small.  By  assuming t h a t  the c o n c e n t r a t i o n of ions i n v o l v e d i n the e l e c t r o c h e m i c a l r e a c t i o n i s s m a l l , the e f f e c t of the e l e c t r i c f i e l d  can be  The v a l u e of the c o n c e n t r a t i o n g r a d i e n t to  3  the c u r r e n t d e n s i t y i > [ e q u a t i o n  diffusing  neglected. (c^.-c_.)/6 i s p r o p o r t i o n a l  ( 1 - 1 1 ) ] , and  i n the case of  to the e l e c t r o d e s u r f a c e , the c o n c e n t r a t i o n g r a d i e n t  ions will  have a maximum v a l u e which i s reached when the c u r r e n t d e n s i t y i s such t h a t the i o n i c c o n c e n t r a t i o n c. of s p e c i e s S. becomes zero at the e l e c J J t r o d e s u r f a c e (c.=0). T h i s maximum c o n c e n t r a t i o n g r a d i e n t i s then c./6 3  3  17  and  the c o r r e s p o n d i n g  current density  current density  i ^ ..with r e s p e c t  is called  the l i m i t i n g  to substance  i n the o v e r a l l e l e c t r o d e r e a c t i o n e q u a t i o n  .  diffusion  For each substance  (1-5)  t h e r e i s such a  l i m i t i n g d i f f u s i o n c u r r e n t d e n s i t y which i s c h a r a c t e r i z e d by  the  index  3• i, . = - — d,j v The  e l e c t r o d e r e a c t i o n can  mined by pressed  D. • F • — £ • c. 6 j  j  take p l a c e no  this l i m i t i n g current density. on  the c e l l s u f f i c i e n t  (1-12)  f a s t e r than the v a l u e  deter-  However, i f a p o t e n t i a l i s  to produce a c u r r e n t d e n s i t y i  im-  higher  than the l i m i t i n g c u r r e n t d e n s i t y i , . , the p o t e n t i a l a t the  electrode  w i l l change to such an extent  may  t h a t a second e l e c t r o d e process  p l a c e at the c u r r e n t d e n s i t y The  (i-i^).  r a t i o of the c o n c e n t r a t i o n  c . / c . i s important f o r the c a l c u 3  l a t i o n of the d i f f u s i o n o v e r v o l t a g e  c. - c . = _J 1 = d> j  or  to e q u a t i o n  (1-11) and  i -  c. J  1  1  according  r a t i o i s e a s i l y d e r i v e d from e q u a t i o n s i  (1-8).  (1-13) i s v a l i d  e l e c t r o d e r e a c t i o n , so t h a t one t o t a l d i f f u s i o n overvoltage •*d -  § •  c. _1 c. J  ( 1  f o r every component obtains  i n the  v  j •  l  n  C ' b - ^  _  1 3 )  overall  the g e n e r a l e x p r e s s i o n  a f t e r s u b s t i t u t i o n i n t o equation Z  This  (1-12), i . e . ,  £- = i _ i  Equation  take  for  the  (1-8), i . e . , ( 1  "  1 4 )  18  The  d i f f u s i o n overvoltage  r e s u l t i n g from t r a n s p o r t of  i s w r i t t e n i n terms of the c u r r e n t d e n s i t y , i , and d e n s i t y , i , .. Q,J contains  Examination of equations  (1-I4)and  the mass t r a n s p o r t v a r i a b l e s i n h e r e n t  species  the l i m i t i n g  current  (1-12) shows t h a t i , d  in'n,.  d  1.3.2.5  T r a n s i t i o n time The  term chronopotentiometry i s a p p l i e d to the techniques  e l e c t r o l y s i s a t constant trol.  c u r r e n t under c o n d i t i o n s of l i n e a r d i f f u s i o n  con-  I t must be s t r e s s e d t h a t c h r o n o p o t e n t i o m e t r y i s a s e l e c t e d form of  unsteady s t a t e . An is  of  important  the " t r a n s i t i o n time" x.  the onset  of constant  experimental  q u a n t i t y i n chronopotentiometry  T h i s time i s the time i n t e r v a l subsequent  c u r r e n t e l e c t r o l y s i s d u r i n g which the s u r f a c e  to  concen-  t r a t i o n of d i f f u s i n g m a t e r i a l undergoing e l e c t r o d e r e a c t i o n reaches zero and rapid  i n c r e a s e of e l e c t r o d e p o t e n t i a l o c c u r s .  time can be ing  The  v a l u e of the  c a l c u l a t e d from l i n e a r d i f f u s i o n t h e o r y ,  transition  d e r i v e d from the f o l l o w -  conditions:  species  1.  Uniform i n i t i a l  concentration  c^ of  diffusing  2.  Constant c o n c e n t r a t i o n g r a d i e n t at the  S.. 3  surface equal The found by  to i * v./nFD.. 3  3  c o n c e n t r a t i o n of s p e c i e s  S. a t the i n t e r f a c e can 3  s o l v i n g the p a r t i a l d i f f e r e n t i a l e q u a t i o n  Second Law)  electrode  under the a p p r o p r i a t e  interface concentration  c. v a r i e s w i t h 3  t h a t c. = c.(x,  t ) and  initial  is written:  then  for diffusion  (Fick's  and boundary c o n d i t i o n s .  the d i s t a n c e x and  the  be  The  time t such  a  .19  9  XT -  1  /  2  2  2 N. t V*'  "  0  Z  j  ~  1/2 D  where N  i s the constant  stated i n equation  _  l/2  ,  &  "  4  x  N.x + D  Y  *  - ' ^  1  6  ~  —  y ^  J  (  1  "  1  5  )  D  t f l u x imposed by the c u r r e n t d e n s i t y i . As  (1-10) the f l u x N. i s e q u i v a l e n t  to i • v./nF a t the  J  3  e l e c t r o d e i n t e r f a c e j and t h i s can be s u b s t i t u t e d i n t o e q u a t i o n  (1-15)  f o r the boundary c o n d i t i o n x = 0 to o b t a i n 2 iv. j  c.(x = 0,t) = c. -  '  -~^'J^  (1-16)  By w r i t i n g P =  2iv =•*- • A r  +  \J TTD  nF  equation  :  (1-16) becomes c.(x = 0,t) = c. - P t 3  The e x p r e s s i o n the i n t e r f a c e w i t h but  (1-17)  1  /  (1-18)  2  3  gives  the v a r i a t i o n o f c o n c e n t r a t i o n  time, a f t e r a p p l i c a t i o n of a constant  o f s p e c i e s S^. a t  current  density,  one seeks a l s o t o know the time v a r i a t i o n o f the p o t e n t i a l d i f f e r e n c e  across  the i n t e r f a c e a t which the r e a c t i o n i s o c c u r r i n g .  r e l a t i o n s h i p , the Nernst e q u a t i o n ference  to a r e f e r e n c e E'(x  given  That i s , by s u b s t i t u t i n g (1-18)  the e l e c t r o d e p o t e n t i a l measured w i t h  respect  e l e c t r o d e i s g i v e n by:  = 0,t) = E° + ^  Before  this  can be used to r e l a t e the p o t e n t i a l d i f -  t o the i n t e r f a c e c o n c e n t r a t i o n .  i n t o the Nernst e q u a t i o n ,  To o b t a i n  In (c. - P t  1 / 2  one can develop t h i s e q u a t i o n  )  (1-19) f u r t h e r , c o n s i d e r a t i o n must be :  t o t h e m a t e r i a l produced by t h e e l e c t r o c h e m i c a l r e a c t i o n .  A  simple  20  example i s the e l e c t r o - r e d u c t i o n of f e r r i c 3+ (Fe  i o n s to form f e r r o u s  ions  2+ + e -»• Fe  one must  ) where the r e a c t a n t and product  a r e i n s o l u t i o n , and  take i n t o account the d i f f u s i o n of the e l e c t r o n donor away from  the e l e c t r o d e and the v a r i a t i o n o f i t s i n t e r f a c i a l c o n c e n t r a t i o n  with  time.  equa-  tion  I f the r e a c t i n g s p e c i e s i s  and the product  s p e c i e s S^,  (1-19) can be w r i t t e n as: E'(x-0,t) =  where c (x=0,t) = c - Pt a a  112  E  ^  RT j l  C  n  a ^  (  x  °'  =  =  Q  i  t  t )  (1-20)  )  .  (1-21)  I f 1 mole of S, i s formed from 1 mole of S and t h e i r b a c o e f f i c i e n t s a r e the same, one ^(x and  equation  diffusion  obtains = 0,t) = c  b  + Pt  1  /  (1-22)  2  (1-20) becomes:  I - P t E'(x = 0,t) = E° + ^ ~ In -2 -r-pr c + Pt n  F  1  /  2  1  /  2  (1-23)  b  I f a t zero and  equation  time, the c o n c e n t r a t i o n of S, i s n e g l i g i b l e , then c ' = 0  b  (1-23) becomes:  b •n 1/2  RT E'(x = 0,t) = E° + ^ In  Consider  the e x p r e s s i o n  °a " ^  P  t  (1-24)  a  1  /  2  (1-24) f o r the time v a r i a n t p o t e n t i a l of  the working e l e c t r o d e a t which a d i f f u s i o n c o n t r o l l e d e l e c t r o - r e d u c t i o n r e a c t i o n i s s t i m u l a t e d by a c o n s t a n t product its  Pt  1/2  logarithm.  = 0 a t t = 0. At v a l u e s  c u r r e n t switched  on at t = 0.  The  1/2 Hence, c /Pt tends to i n f i n i t y as does 9.  of time g r e a t e r than t = 0 the term P t  1/2  is  21  finite  and a t some v a l u e o f time i s e q u a l  to c . a  T h i s time has been  1/2 p r e v i o u s l y d e f i n e d as T , the t r a n s i t i o n time, and l n ( c - Pt ) tends a to minus i n f i n i t y , till  and the p o t e n t i a l changes r a p i d l y .  i t has become s u f f i c i e n t l y n e g a t i v e  of some o t h e r  i o n i c s p e c i e s can o c c u r .  In f a c t , i t sinks  so t h a t the e l e c t r o - r e d u c t i o n By d e f i n i t i o n ,  1/2 1/2 t = x and (c - P t ) = 0, c = Px and e q u a t i o n a a  t h e r e f o r e , when  (1-24) can be  w r i t t e n as: RT  E'(x = 0,t) = E°  n  Px In ^  equation  - P t ^  Pt  F  where P was d e f i n e d by e q u a t i o n  and  1 / 2  7  1  /  2  (1-25)  2  (1-17) t o be  (1-25) becomes .1/2 E'(x = 0,t) = E° + S | I n  1/2 ^ |  (1-26)  P r i o r t o c u r r e n t passage, the e q u i l i b r i u m e l e c t r o d e p o t e n t i a l determined by the c o n c e n t r a t i o n of s p e c i e s S  a  i s written as:  ' RT — E = E° + ^ In c nF a As d e f i n e d by e q u a t i o n  (1-27)  (1-4), the d i f f u s i o n o v e r v o l t a g e  can  then be w r i t t e n as> 1/2 _  " ' " E  E  "  A  +  §  l n  ~  UY-  1/2  "  (1 28)  where the c o n s t a n t A i s g i v e n by: A =  RT — - ^~ l n c nF a  (1-29)  22  By m a n i p u l a t i o n  c  of the e q u a t i o n  P T  =  a  1/2 ^  ( 1 - 3 0 )  and s u b s t i t u t i o n of e q u a t i o n  _,• IT  1 / 2  n  =  F  i t can be shown that  ( 1 - 1 7 ) ,  C  a  1 / 2  1/2 ff  -  ( 1 - 3 1 )  2  1/2 The product  iT  i s seen to be c o n s t a n t  l y s i s r e g a r d l e s s of the c u r r e n t d e n s i t y . t u t e s a simple  experimental  The p r e c e e d i n g  for diffusion controlled electroEquation  d i s c u s s i o n on i o n i c c o n d u c t i o n  overvoltage  expressions  therefore  consti-  c r i t e r i o n of d i f f u s i o n c o n t r o l .  basic information regarding d i f f u s i o n p o l a r i z a t i o n General  ( 1 - 3 1 )  were p r e s e n t e d  has p r e s e n t e d t h e processes.  to show that the p o l a r i z a t i o n -  can be d e s c r i b e d i n terms of c o n c e n t r a t i o n changes o f r e a c t a n t  or product  s p e c i e s a t the e l e c t r o d e s u r f a c e , and these  considered  i n terms o f a Nernst  changes can be  equation:  cA±, t) n = • £v. i n -2—— 'd ~ nF ' * l c.  (1-32)  J  w  J  Using  the concept of mass t r a n s p o r t a t c o n s t a n t  current density i n  u n s t i r r e d s o l u t i o n s , the d e f i n i t i o n of a l i m i t i n g d i f f u s i o n a l was d i s c u s s e d and i t was shown that the d i f f u s i o n o v e r v o l t a g e expressed density  i n terms o f the c u r r e n t d e n s i t y ( i ) and the l i m i t i n g (irf.j) '  current can a l s o be current  23  The set  t r a n s i t i o n time,  of c o n s t a n t  x, was  d e f i n e d as the time a f t e r the  c u r r e n t t h a t the c o n c e n t r a t i o n of r e a c t i n g i o n i c  becomes zero at the e l e c t r o d e i n t e r f a c e . a l s o be expressed  i n terms of x and  The  onspecies  d i f f u s i o n overvoltage  t:  . , RT . 1/2 1/2 r\, = A + — In x - t 'd nF  1.3.2.6  Ionic chemical  and  Chemical e f f e c t s .  thermal  *  the m e t a l and  The F a r a d a i c r e a c t i o n p r o d u c t s  chemical  the s l a g .  e f f e c t s observed  Thermal e f f e c t s .  produced at  Therefore i o n i c conduction  d u r i n g D.C.  electroslag  the  chemically  can account f o r  melting.  A p o l a r i z e d e l e c t r o d e i n t e r f a c e through which  h i g h c u r r e n t d e n s i t i e s pass w i l l behave i n a q u i t e d i f f e r e n t manner from an u n p o l a r i z e d i n t e r f a c e .  thermal  C u r r e n t passage through  s l a g s produces heat by J o u l e h e a t i n g a c c o r d i n g  to the  ionic  relationship:  P = Vi  (1-35)  where P i s the power g e n e r a t i o n i n w a t t s , slag.  i  (1-34)  effects  e l e c t r o d e s u r f a c e s are a v a i l a b l e i n the system to i n t e r a c t with  can  and v i s v o l t a g e drop i n the  At an n n p o l a r i z e d e l e c t r o d e , the v o l t a g e g r a d i e n t i n the s l a g at  the e l e c t r o d e i n t e r f a c e i s the same as t h a t i n the b u l k of the At a p o l a r i z e d e l e c t r o d e , the v o l t a g e g r a d i e n t i s h i g h e r the p o l a r i z e d l a y e r and heat ly higher.  The  generation  t i o n o v e r v o l t a g e and  the excess heat  P = ni  than normal i n  i n t h i s l a y e r w i l l be  i n c r e a s e d v o l t a g e g r a d i e n t i s produced by generation  i s given  slag.  the  accordingpolariza-  by: (1-36)  24  In t h i s way p o l a r i z a t i o n a t l i q u i d m e t a l / l i q u i d  slag  interfaces  can account f o r thermal e f f e c t s observed d u r i n g D.C. e l e c t r o s l a g  melt-  ing. It thermal  the p o s s i b i l i t i e s o f c h e m i c a l and  e f f e c t s when m e l t i n g w i t h A.C. power.  processes sion  i s i n t e r e s t i n g to c o n s i d e r  a r e r e s p o n s i b l e f o r D.C. m e l t i n g  I f slow d i f f u s i o n a l  e f f e c t s , and i f the d i f f u -  times a r e l o n g i n comparison t o the half-wave time of 60 Hz. A.C.  power, n e t chemical products  e f f e c t s should be z e r o .  T h i s i s to say that  reaction  produced on the anodic h a l f - c y c l e should be e n t i r e l y removed by  the c a t h o d i c h a l f - c y c l e .  However,any p o l a r i z a t i o n r e s i s t a n c e h e a t i n g  would not behave r e v e r s i b l y .  1.3.3  Arc discharge C o n s i d e r a t i o n w i l l now be g i v e n  t r a n s f e r between a m e l t i n g p l a c e by a r c d i s c h a r g e .  e l e c t r o d e and l i q u i d CaF^ based s l a g  movement through a  takes  T h i s p o s s i b i l i t y w i l l be d i s c u s s e d i n terms of  thermal e f f e c t s and chemical conducts i o n i c a l l y .  to the case i n which c u r r e n t  e f f e c t s , and i t i s assumed t h a t the s l a g  Very l i t t l e  i s known about the mechanisms o f c u r r e n t  high current arc, e s p e c i a l l y  f o r a r c s between h i g h -  temperature s l a g and m e t a l s u r f a c e s . In m e t a l e l e c t r o d e s ^ c h a r g e  i s conducted by a "degenerate g a s "  of f r e e e l e c t r o n s , b u t they remain l o o s e l y in  a s s o c i a t e d w i t h atoms.  With-  the plasma r e g i o n of. an a r c , e l e c t r o n s and i o n s e x i s t and move as  separate  e n t i t i e s but very  one type o f c o n d u c t i o n  little  i s known about the t r a n s i t i o n from  to t h e o t h e r a t the a r c t e r m i n a l s .  I n the case  25  of a r c i n g d u r i n g  e l e c t r o s l a g melting  the c o n d u c t i o n  sequence i n going  from the m e t a l to the s l a g i s e l e c t r o n i c •> e l e c t r o n i c + i o n i c •> i o n i c . At a m e t a l l i c a r c cathode, t h e r m i o n i c  emission  i s most o f t e n  proposed as the primary mechanism of e l e c t r o n l i b e r a t i o n , but  there i s  doubt as  Also,  to the s o u r c e of energy needed to s u s t a i n e m i s s i o n .  the e f f e c t of i o n c u r r e n t i n r e d u c i n g e m i s s i o n must be The  requirements f o r e l e c t r o n i c  recognized.  anodes of h i g h - c u r r e n t  m e t a l l i c a r c s appear to f u n c t i o n p r i -  m a r i l y as c o l l e c t o r s of e l e c t r o n s and vapours and  ions to the plasma.  i s e l e c t r o n bombardment.  The  s e c o n d a r i l y as c o n t r i b u t o r s of p r i n c i p a l means of anode h e a t i n g  Hence, i t i s g e n e r a l l y concluded that  the  c u r r e n t at the anode i s almost e n t i r e l y e l e c t r o n i c . With respect  to a m e t a l / s l a g  a r c , one  can propose t h a t  current  t r a n s f e r through the a r c plasma takes p l a c e by e l e c t r o n p l u s i o n i c duction.  There w i l l be F a r a d a i c r e a c t i o n p r o d u c t s d e p o s i t e d  metal/plasma i n t e r f a c e and r e a c t i o n s are  the p l a s m a / s l a g i n t e r f a c e .  t h e r e f o r e p o s s i b l e , but  phenomena are those of heat g e n e r a t i o n melting,  an a r c c o u l d be 1.  By  e v o l u t i o n of a F a r a d a i c  (e.g., C a , 1  emission  0  ).  ways.  reaction  a v e r y h i g h vapour  surface.  arc  During e l e c t r o s l a g  pressure  An a r c so formed  would be s e l f - s u s t a i n i n g by v i r t u e of continued of the m a t e r i a l s .  of  T h i s p r o d u c t xrould f a c i l i t a t e e l e c t r o n  at the i o n i c l i q u i d  the  Electrochemical  i n i t i a t e d at the e l e c t r o d e i n two  product which i s gaseous or has  at both  the most important aspect i n the a r c .  con-  production  26  2.  By passage  of s u f f i c i e n t c u r r e n t a t  the e l e c t r o d e to heat the s l a g a t t h i s p o i n t to i t s b o i l i n g p o i n t and v a p o u r i z e i t . Both p o s s i b i l i t i e s w i l l be c o n s i d e r e d i n the d i s c u s s i o n , b u t , at t h i s point, i t i s s u f f i c i e n t  to say that the most important a s p e c t  of a r c d i s c h a r g e w i l l be the h i g h r a t e of heat p r o d u c t i o n at the electrode. rates.  T h i s should produce anomalously  high electrode  consumption  Chemical e f f e c t s would be p o s s i b l e but would be secondary i n  importance  to the thermal  effects.  1.4  Conclusions  The c o n c l u s i o n s of t h i s c h a p t e r are as 1. D.C.  Chemical and  electroslag melting.  follows:  thermal e f f e c t s do e x i s t u s i n g Chemical e f f e c t s r e s u l t  d a t i o n l o s s of important a l l o y i n g elements the magnitude of which mechanical p r o p e r t i e s .  can be s u f f i c i e n t  from the m e t a l ,  to a l t e r  rate.  E l e c t r o n i c c o n d u c t i o n i n the s l a g  account f o r the observed c h e m i c a l or thermal 3.  ingot  Thermal e f f e c t s are a p p a r e n t l y  m a n i f e s t e d as d i f f e r e n c e s i n the s p e c i f i c m e l t 2.  in oxi-  cannot  effects.  I o n i c c o n d u c t i o n i n the s l a g can account  f o r the r e p o r t e d o b s e r v a t i o n s , through F a r a d a i c r e a c t i o n s and t h e i r consequent  associated polarization  effects.  27  4.  Arc d i s c h a r g e a t a m e l t i n g e l e c t r o d e  i s p o s s i b l e and i s important  i n r e g a r d to thermal  effects. 5.  D e f i n i t i v e s m a l l s c a l e and  s c a l e s t u d i e s are j u s t i f i e d the c h e m i c a l and  large  i n order to e s t a b l i s h  thermal e f f e c t s , and  to  propose  s u i t a b l e e l e c t r o c h e m i c a l mechanisms.  1.5 I t i s now  E x p e r i m e n t a l programme o u t l i n e  n e c e s s a r y to d e c i d e what e x p e r i m e n t a l r e s u l t s  needed to s a t i s f a c t o r i l y e x p l a i n the chemical and observed d u r i n g D.C.  electroslag remelting.  are  thermal phenomena  In view of the c o n t r a d i c -  t o r y n a t u r e of these phenomena as r e p o r t e d by o t h e r workers i n the field,  the f o l l o w i n g q u e s t i o n s must be A. experiments  asked.  Are the r e p o r t e d e f f e c t s r e a l i n  i n which the f o l l o w i n g parameters  are  controlled? - atmosphere - p r o c e s s v o l t a g e and - alloy - slag  amperage  composition composition B.  What i s the magnitude of these e f f e c t s ?  C.  What i s the n a t u r e of the e l e c t r o c h e m i c a l  r e a c t i o n s a t l i q u i d m e t a l / c a l c i u m - f l u o r i d e based interfaces?  slag  28  The e x p e r i m e n t a l programme designed to answer these q u e s t i o n s c o n s i s t e d of two  areas o f work, and A.  The  i n v e s t i g a t i o n of m e t a l / s l a g i n t e r -  f a c e s p o l a r i z e d at r e a l i s t i c B.  these a r e :  current d e n s i t i e s .  The p r o d u c t i o n of ESR  i n g o t from  pure  metals and a l l o y s u s i n g v a r i o u s modes of power under controlled  atmosphere and s l a g c h e m i s t r y  conditions.  CHAPTER 2  SMALL SCALE STUDIES  2.1  As was j e c t i v e was  Experimental  considerations  o u t l i n e d i n the p r e c e e d i n g  s e c t i o n , one  experimental  the study of e l e c t r o c h e m i c a l phenomena o c c u r r i n g on m e t a l  electrodes i n CaF  2  slags.  As w i l l be  shown l a t e r ,  f o r experimental  sons, c y l i n d r i c a l e l e c t r o d e s were used i n these s t u d i e s and f o r e n e c e s s a r y to c o n s i d e r a l process  The  d i f f u s i o n problem can be  (r +  of e l e c t r o l y s i s .  The  treated i n  change i n c o n c e n t r a t i o n  f l u x through a c y l i n d e r  (2-1)  of the substance b e i n g  reduced a t  dr per u n i t time:  2TT (r + d r ) q ( r + d r , t ) - 2-rrr q ( r , t )  (r,t) °  time  [ 8 q ( r , t ) / 9 r ] • dr  the e l e c t r o d e i s , i n a s h e l l of t h i c k n e s s  3C  there-  dr) : q(r,t) +  The  The  rea-  diffusion-  v a r i a b l e s , the d i s t a n c e r from the c y l i n d e r a x i s and  from the b e g i n n i n g  of r a d i u s  i t was  the e f f e c t of c y l i n d r i c a l geometry on  o c c u r r i n g i n the s l a g .  terms of two elapsed  ob-  8 t  (2-2)  2irrdr  c y l i n d e r surface area  i s 2iTr and  the c y l i n d e r h e i g h t  i s one  unit of  length. The  f l u x of the d i f f u s i n g substance through the s u r f a c e of a  der of r a d i u s r i s p r o p o r t i o n a l to the c o n c e n t r a t i o n 3 C  ^>  t ) = J )  gradient.  cylin-  Thus  (r,t)  oHfe—  (2  "  3)  30  By combining  (2-2) and (2-3) one o b t a i n s :  8C ( r , t )  Equation  D  ~2  9lr  _ 3C (r,t)  2  Q  =  9t  3 C (r,t)  r  ]  +  (2-4)  Q  r  (2-4) w i l l be s o l v e d f o r t h e case i n which the substance  i s reduced a t t h e e l e c t r o d e s u r f a c e as soon as i t reaches i t . The boundary and  initial  c o n d i t i o n s a r e as f o l l o w s :  C (r ,t) = 0 o o where r ^ i s the c y l i n d e r r a d i u s . C ( r ,o) = C° o o C ( r , t ) = C° f o r r Q  I n t e g r a t i o n of e q u a t i o n  (2-4) i s r a t h e r complicated  f o r the c u r r e n t d e n s i t y w i l l be p r e s e n t e d  i  = nFAD C° o  where <f> r e p r e s e n t s  and o n l y the expansion  i n terms of expanded B e s s e l  + 1 _ A A * +  — r  2  the d i m e n s i o n l e s s  l y s m a l l , a l l but t h e f i r s t and  °°  group  4  TT  D t/r Q  term i n the b r a c k e t  Q  .  2  2  than 0.01.  (2-5)  8  When <j> i s s u f f i c i e n t -  i n equation  the c u r r e n t i s the same as f o r l i n e a r d i f f u s i o n .  the q u a n t i t y 1/ (IT<j> ) . i s s m a l l e r  1  (2-5) v a n i s h ,  T h i s w i l l be so when  The e l e c t r o d e s used i n the s m a l l  s c a l e s t u d i e s had r ^ = 0.363 cm. and f o r an average d i f f u s i o n X) = 1 0 o  -4  cm.  2 - 1 sec. i n CaF„ s l a g s , a maximum v a l u e L  coefficient  of t can be c a l c u l a t e d  below which the c u r r e n t w i l l be the same as f o r l i n e a r d i f f u s i o n D t o  functions:  (<J> = 0.01).  31  t  so  =  so t  10  13.2  Therefore, with  the e l e c t r o d e s  -2  x 1.32  x  -1 10  seconds. i f electrolysis  considered,  times l e s s than ~10  seconds are used  c o n d i t i o n s of l i n e a r d i f f u s i o n can  be  a p p l i e d to subsequent a n a l y s i s . Galvanostatic pulsing. was  t e c h n i q u e of g a l v a n o s t a t i c p u l s i n g  used i n the s m a l l s c a l e e l e c t r o c h e m i c a l s t u d i e s r a t h e r than a p o t e n t i o -  static The  The  t e c h n i q u e because of the d e s i g n  reference  e l e c t r o d e was  aspects  of the e l e c t r o c h e m i c a l  w e l l removed from the working e l e c t r o d e  the p o t e n t i a l d i f f e r e n c e produced by s i g n i f i c a n t , and  was  t h e r e f o r e n e c e s s a r y t h a t , f o r a g i v e n s l a g composition  density,  at a g i v e n  current  the iR p r o d u c t  as the working e l e c t r o d e p o l a r i z e d and  v a l u e s would be d i f f i c u l t  It  which i s the case w i t h a galvano-  When u s i n g a p o t e n t i o s t a t i c t e c h n i q u e ,  would decrease c o n t i n u o u s l y overvoltage  flowed  o c c a s i o n a l l y as l a r g e as the measured o v e r v o l t a g e .  the i R product remain constant  s t a t i c technique.  and  the c e l l r e s i s t a n c e when c u r r e n t  was  cell.  to o b t a i n .  reliable  32  2.2  The  S m a l l s c a l e p o l a r i z a t i o n apparatus  apparatus used i n these experiments e v o l v e d  i e s of designs  i n which many problems were encountered.  experiments was  to study m e t a l / s l a g  high current d e n s i t i e s . most h i g h  The  i n t e r f a c e s at h i g h  s l a g s of i n t e r e s t are v e r y  temperature s t r u c t u r a l m a t e r i a l s .  The  through a s e r The  aim  of  the  temperatures  and  c o r r o s i v e to  c r u c i b l e s were machined  from g r a p h i t e , and  l i n e d w i t h molybdenum f o i l  to a v o i d  carbon contamina-  t i o n of the s l a g .  A d i s c of molybdenum, 0.25  i n . t h i c k , was  placed  the bottom of the c r u c i b l e to prevent molten m e t a l which had the e l e c t r o d e c o r r o d i n g s l a g to g r a p h i t e . i r o n and  calcium  Since  the g r a p h i t e  c r u c i b l e and  slag.  The  carbon f e l t  and  enclosed  purged w i t h  argon.  liquid  only with  the  tube t h e r m a l l y i n -  Induction heating  was  to e l e c t r i c a l n o i s e .  It  attempted a t a l l times to keep the temperature of the s l a g below  melting  the  p o i n t of the e l e c t r o d e m a t e r i a l to ensure that the e l e c t r o d e t i p d i d  not change shape and use  the  n e c e s s a r y to hang  i n contact  in a silica  used i n s p i t e of i t s drawbacks i n g i v i n g r i s e was  therefore exposing  f l u o r i d e s l a g s i s o t h e r m a l l y , i t was  e n t i r e assembly was  sulated with  f a l l e n from  t h e r e i s no m a t e r i a l known t h a t w i l l h o l d  the working e l e c t r o d e from above so t h a t i t was  in  therefore surface area.  o n l y s l a g s forming a primary phase w i t h  the e l e c t r o d e m a t e r i a l .  I t was  also necessary  a lower m e l t i n g  T h i s r e s t r i c t i o n arose through the  point  than  formation  of s l a g s k i n s on the e l e c t r o d e t i p which c o u l d not be removed w i t h o u t completely  melting  f i n e a l l our  the e l e c t r o d e t i p .  s t u d i e s to s l a g s w i t h  CaF  We 9  as  have t h e r e f o r e had the primary phase.  to  to conThe  33  g r a p h i t e c r u c i b l e a c t e d as the counter  e l e c t r o d e and was  supported  g r a p h i t e p e d a s t a l mounted on the water c o o l e d bottom p l a t e of the assembly.  The  top p l a t e on the s i l i c a  tube was  water c o o l e d and  to suspend the working e l e c t r o d e , the s l a g thermocouple and electrode.  The  r e f e r e n c e e l e c t r o d e was  c l o s e d end b o r o n - n i t r i d e s l e e v e and A s m a l l h o l e was  drilled  the  on a furnace  i t served reference  a s h o r t g r a p h i t e rod encased i n a  supported  by a s i n g l e bore alumina tube.  i n the s i d e of the b o r o n - n i t r i d e tube to expose  an area of the g r a p h i t e to the s l a g to a c t as the r e f e r e n c e e l e c t r o d e . molybdenum w i r e was initiating  used as the e l e c t r i c a l c o n t a c t  to the g r a p h i t e .  the g a l v a n o s t a t i c p u l s e , the r e f e r e n c e e l e c t r o d e was  p o l a r i z e d and  the r e f e r e n c e p o t e n t i a l was  C  gr  + 0  2 -  •+  CO  e s t a b l i s h e d by  g  + 2e  A  Before  anodically  the r e a c t i o n :  (2-6)  _  Reference e l e c t r o d e s of s i m i l a r d e s i g n have been used i n c r y o l i t e systems, and were found to behave r e v e r s i b l y (15, e l e c t r o d e should be  i s o l a t e d from the system by  15a).  Ideally a reference  an i o n i c membrane  such as a  b o r o n - n i t r i d e f i l a m e n t c a r r y i n g s l a g to act as a s a l t b r i d g e , but due dimensional  l i m i t a t i o n s of our system t h i s was  not f e a s i b l e and  the  to  the  absolute  r e f e r e n c e e l e c t r o d e p o t e n t i a l t h e r e f o r e c o n t a i n s an i n h e r e n t v a r i a t i o n due the o x i d e - i o n a c t i v i t y v a r i a t i o n s i n the s l a g s used. However, these v a r i a t i o n s would be s m a l l 0.5  wt.  % Al^O^  and  8 wt.  % A^O^)  e r r o r i n i n d i v i d u a l measurements.  compared w i t h In no  (~30  the  case d i d we  p o t e n t i a l between the r e f e r e n c e e l e c t r o d e and  mv  between CaF2 +  experimental f i n d that  the u n p o l a r i z e d  the  iron  to  34  e l e c t r o d e was  o u t s i d e the range 80 - 110  Finally,  the n a t u r e  mv.  of the h i g h c u r r e n t d e n s i t y experiments  t a t e s t h a t a t r a n s i e n t method must be used to a v o i d gross thermal  changes i n the system.  The method used was  t i c p u l s i n g , e s s e n t i a l l y as used by Gosh and King t h e o r e t i c a l l y by Delahay r e n t d e n s i t i e s we  (~20  t h a t of  galvanosta-  (16), and  as o u t l i n e d cur-  c o u l d not use  the more u s u a l methods of f a s t - r i s e  time  i n s t e a d used c o n s t a n t - c u r r e n t  Models 6269A and  A ) , and  and  S i n c e we wanted to i n v e s t i g a t e h i g h  w i t h i n the p o l a r i z a t i o n times we rents  chemical  (17) .  c o n s t a n t - c u r r e n t p u l s i n g , and (Hewlett-Packard  dic-  6203B) .  These have r i s e times  i n v e s t i g a t e d , except  i n these r e g i o n s we  generators well  at v e r y h i g h  used the v o l t a g e decay  cur-  rather _2  than r i s e , measurements. we would expect  to f i n d  At the h i g h e r c u r r e n t d e n s i t i e s (> 5 A. a s i g n i f i c a n t e f f e c t of the e l e c t r i c a l  superimposed on the chemical d i f f u s i o n g r a d i e n t . a l s o we  t r a n s i e n t s were r e c o r d e d  B calibrated against a  The  apparatus i s shown i n F i g u r e s 1, 2, and  KiethlyModel  House " e x t r a pure") w i t h  153  electrometric  potentiometer.  3.  calcium f l u o r i d e  ( B r i t i s h Drug  the r e q u i r e d oxide a d d i t i o n s of alumina  ton f u s e d alumina) or r e c r y s t a l l i z e d c a l c i u m o x i d e . p r e f u s e d under argon i n the e x p e r i m e n t a l  s l a g was  voltage-time  the s l a g s used i n these s m a l l s c a l e s t u d i e s were prepared, by  m i x i n g weighed q u a n t i t i e s of p r e f u s e d  was  regions  on a T e k t r o n i c s s t o r a g e o s c i l l o s c o p e model  564  All  The  ),  field  Hence i n these  have used the v o l t a g e decay measurements.  cm.  The  apparatus.  calcium  (Norfluoride  The mixed dry  then f u s e d i n the apparatus at a h i g h temperature to ensure  complete s o l u t i o n of the o x i d e .  The  to the working temperature w h i c h was  s l a g temperature was  then  lowered  always 20 to 30°C lower than  the  35  1  Tektronics Model 564 B CRO Reference electrode -Working electrode Counter electrode, crucible Slog  ballast resistor  lOk^i  Hewlett - Packard Model 6 2 6 9 A , 6203  F i g u r e 1.  Cell electrical  circuitry.  B  36  Working electrode lead Water  cooled head  Silica tube  Carbon felt  Induction coil  Water cooled base Counter electrode lead 3Re/25Re thermocouple  Figure 2 .  General  cell  design.  W control  37  Mo wire A l 0 sheath 2  3  3 Re / 25 Re-W thermocouple Fe electrode  Boron nitride Graphite Graphite Mo lifter Slag  Graphite 3Re/25Re-W thermocouple  Figure 3 .  D e t a i l of c e l l assembly.  38  melting  temperature of the e l e c t r o d e m a t e r i a l b e i n g  studied.  When the  temperature was s t a b l e , the working e l e c t r o d e was lowered s l o w l y t o wards the s l a g u n t i l i n i t i a l supply  circuit.  t r o d e was  T h i s p o s i t i o n was  by  the s l a g .  a known s u r f a c e c o n t a c t  current  (100 ma.  I t was found t h a t a s t a b l e  anodic.  was switched  tween the counter e l e c t r o d e pre-determined  and  constant  much more q u i c k l y by p a s s i n g  a  o f f , and the c u r r e n t was completed be-  ( c r u c i b l e ) and the working e l e c t r o d e a t a  current s e t t i n g .  to i t s constant  The v o l t a g e r i s e between the  t r a c e d on the o s c i l l o s c o p e and  v a l u e , u s u a l l y e s t a b l i s h e d i n l e s s than 10 s e c ,  as i t decayed when the c u r r e n t supply was stopped.  t y p i c a l low c u r r e n t  t e s t o s c i l l o s c o p e t r a c e i s shown i n F i g . 4.  shows the g r a d u a l b u i l d u p trodes.  reference  Once the system was s t a b l e , the i n -  and working e l e c t r o d e s was  also followed  stable  f o r 10 sec.) between the two e l e c t r o d e s w i t h the  electrode being  duction generator  followed  area between the e l e c t r o d e  to the working e l e c t r o d e , and e s t a b l i s h e d t o be  e l e c t r o d e p o t e n t i a l could be achieved  reference  into  W3Re/W25Re thermocouples.  and w i t h i n the range 80 - 110 mv.  reference  1.0 cm.)  t h i s p o i n t , the r e f e r e n c e e l e c t r o d e p o t e n t i a l was measured <  with respect  low  (normally  The temperature of the system was measured and c o n t r o l l e d  two s e p a r a t e At  taken as a zero p o i n t and the e l e c -  then lowered a predetermined d i s t a n c e  the s l a g to p r e s e n t and  e l e c t r i c a l c o n t a c t was made i n the c u r r e n t  A  This  of p o t e n t i a l d i f f e r e n c e between the two e l e c -  The schematic below the o s c i l l o s c o p e p i c t u r e sh ows the method  of o b t a i n i n g  the o v e r p o t e n t i a l from the t r a c e .  between the two e l e c t r o d e s  i s e s t a b l i s h e d very  The i R p o t e n t i a l drop q u i c k l y and i s not t r a c e d  Volts  Time  F i g u r e 4.  Method of o b t a i n i n g  overvoltage.  40  on the s c r e e n .  The  t o t a l charge passed through the system d u r i n g  such t e s t i s s m a l l as are the b u l k Approximately 5 minutes was  chemical  allowed  i n g e l e c t r o d e to r e - e q u i l i b r a t e w i t h s l o w l y brought back up currents  changes so produced.  between t e s t s to a l l o w the l i q u i d  to temperature.  By  one  the work-  s l a g as the s l a g  t e s t i n g at i n c r e a s e d  was applied  each time, the p o l a r i z a t i o n of a g i v e n e l e c t r o d e m a t e r i a l i n a  g i v e n s l a g can be p l o t t e d a g a i n s t  the c u r r e n t d e n s i t y to g i v e a  galvan-  o s t a t i c p o l a r i z a t i o n curve. I t was  found t h a t the l a r g e s t p a r t of the e x p e r i m e n t a l  arose from the d i f f i c u l t y  of d e f i n i n g a p r e c i s e and  a c t i v e s u r f a c e a r e a on the working e l e c t r o d e . e l e c t r o d e was  known but  chemical  The  constant  e l e c t r o d e shape.  second major s o u r c e of e x p e r i m e n t a l  e f f e c t , and Any  i s therefore modified  apparatus.  The  error  are unknown and  cannot be  by b o t h e l e c t r i c a l  and  i t was  decided  and  was  the thermal e f f e c t s produc-  s t a b i l i z e d i n t h i s type of  degree of temperature c o n t r o l i n the system was  s i d e r e d to be - 5°C  lies  shown l a t e r , t h i s i s a  e r r o r a r i s i n g from e l e c t r i c a l e f f e c t s  e l i m i n a t e d by u s i n g decay measurements, but ing convection  overheating  of the e l e c t r o d e t i p r e s u l t e d i n a change i n the  The  liquid-shear f i e l d s .  the  r e a c t i o n s at the i n t e r f a c e gave r i s e  i n the n a t u r e of the p o l a r i z a t i o n . As w i l l be concentration  electro-  immersion of  to changes i n i n t e r f a c i a l t e n s i o n which when combined w i t h and p a r t i a l m e l t i n g  error  t h a t e r r o r s a r i s i n g from  s i c thermal e f f e c t s were s m a l l i n comparison to the  others.  conintrin-  41  2.3  2.3.1  Ferrovac-E The  1.25 ing  pure  Ferrovac-E  Electrode  materials  iron pure i r o n e l e c t r o d e s were made by machining the  i n . diameter b a r down t o 0.50 i n . diameter s t o c k and then swagt h i s down to 0.285 i n . i n d i a m e t e r .  i n . lengths  and a female thread machined on one end f o r attachment t o  the e l e c t r o d e  2.3.2  T h i s rod was then c u t to 4  holder.  A I S I 430 s t a i n l e s s  steel  T h i s m a t e r i a l was found t o be u n s u i t a b l e f o r swaging i n the as r e c e i v e d c o n d i t i o n , t h e r e f o r e these e l e c t r o d e s were machined d i r e c t l y from the 1.0 i n . diameter b a r .  2.3.3  Fe-1.0 wt. % Cr a l l o y T h i s a l l o y "was produced by m e l t i n g  with wt.  a p i e c e of AISI 410 s t a i n l e s s % Cr i n i r o n .  together  s t e e l to produce a f i n a l a l l o y o f 1  The melt was d e o x i d i z e d  s o l i d i f i c a t i o n . .. The s o l i d  a p i e c e of F e r r o v a c - E  with  aluminum w i r e p r i o r t o  s l u g was then welded t o a p i e c e of m i l d  s t e e l r o d , 0.75 i n . i n diameter, machined down to the diameter o f the rod, and then swaged down to the d e s i r e d diameter of 0.285 i n .  2.3.4  Pure chromium T h i s m a t e r i a l was made by h o t e x t r u s i o n of s t e e l c l a d chromium  metal.  The purpose of the c l a d d i n g was to prevent e x c e s s i v e  oxidation  42  of the chromium d u r i n g  the hot working s t a g e .  c l a d d i n g was d i s s o l v e d i n a c i d .  A f t e r e x t r u s i o n , the  The m a t e r i a l had an i r r e g u l a r  surface  but because of i t s b r i t t l e n e s s , machining was n o t attempted, and i t was used i n the as r e c e i v e d c o n d i t i o n by s i l v e r s o l d e r i n g i t t o a threaded mild steel  2.3.5  stub.  Pure n i c k e l These e l e c t r o d e s were made by swaging the as r e c e i v e d 0.375" d i a .  n i c k e l r o d down t o the d e s i r e d s i z e , and then c u t t i n g t o the a p p r o p r i a t e length.  2.3.6  Pure  Cobalt  Because o f t h e d i f f i c u l t y  i n o b t a i n i n g pure c o b a l t i n the appro-  p r i a t e s i z e range, t h i s e l e c t r o d e was made from a deformed s i n g l e c r y s t a l o f c o b a l t which had a s l i g h t l y  irregular surface.  t a l was s i l v e r - s o l d e r e d i n t o a threaded m i l d  2.3.7  Iron-carbon  steel  The s i n g l e c r y s -  stub.  alloy  These e l e c t r o d e m a t e r i a l s were s u p p l i e d by the Babcock and W i l cox  Company.  had  only  out w i t h  They were machined t o the c o r r e c t s i z e when r e c e i v e d and  to be threaded b e f o r e use.  Only one experiment was c a r r i e d  t h i s m a t e r i a l , the purpose o f which was t o see i f an anodic  iron-carbon  a l l o y would e x p e r i e n c e  carbon l o s s .  Carbon a n a l y s e s  c a r r i e d out on the t i p a f t e r e l e c t r o l y s i s had been completed.  was  43  2.4  E l e c t r o s l a g remelting p o l a r i z a t i o n  I t was  experiments  thought, that i t would be m e a n i n g f u l to measure the degree  of p o l a r i z a t i o n of an e l e c t r o d e d u r i n g e l e c t r o s l a g r e m e l t i n g  i n order  to c o r r e l a t e the s m a l l s c a l e experiments w i t h  existing  during actual remelting. i r o n e l e c t r o d e s , and  spacers, had  attached  As shown i n F i g u r e s 5 and  6,  the  the s l a g a f t e r s t e a d y - s t a t e  taken p l a c e f o r approximately  30 minutes.  the s l a g ( F i g . 6 ) , the process  t r a c e d on the o s c i l l o s c o p e .  g i v e n i n F i g . 7.  An  melting  When the r e f e r e n c e e l e c c u r r e n t was  the v o l t a g e decay between the r e f e r e n c e e l e c t r o d e and  e l e c t r o d e was  reference  to the e l e c t r o d e by means of i r o n w i r e and m u l l i t e  such t h a t i t e n t e r e d  trode e n t e r e d and  These experiments were c a r r i e d out w i t h Armco  r e f e r e n c e e l e c t r o d e s of the same d e s i g n as used i n  the s m a l l s c a l e s t u d i e s . e l e c t r o d e was  the c o n d i t i o n s  switched the  off  melting  example of t h i s t r a c e i s  The v o l t a g e decay times observed were s h o r t compared  to the time taken f o r the s l a g to s o l i d i f y i n the e l e c t r o d e r e g i o n , which i s approximately  one minute.  I t was  e s s e n t i a l t h a t the power  be i n t e r r u p t e d immediately a f t e r the r e f e r e n c e e l e c t r o d e e n t e r e d slag.  I f t h i s was  not  r e f e r e n c e and m e l t i n g  the case the p h y s i c a l d i s t a n c e between e l e c t r o d e s was  too great and  tion  potential.  T h i s prevented  the  the ohmic p o t e n t i a l  drop f o l l o w i n g c u r r e n t i n t e r r u p t i o n exceeded the f u l l of the o s c i l l o s c o p e .  the  scale defection  any measurement of the  polariza-  44  Water - cooled copper electrode Colorlith Rubber bellows  Reference  i  electrode  Fe electrode  Gas sea I Cu mold  Water - jacket  Figure 5 ,  ESR f u r n a c e assembly.  45  Mo wire  1/  Cu mold  /-  Fe electrode Mullite spacer  I-  Graphite Boron nitride  IB  Slag  A  Liquid  •Figure 6,  D e t a i l of r e f e r e n c e  electrode  ingot  configuration,  46  J  I  I  0.5  Figure  7.  I  I  sec. /  I  I  1  L  div.  P o t e n t i a l decay between the m e l t i n g e l e c t r o d e and the r e f e r e n c e e l e c t r o d e observed at c u r r e n t i n t e r r u p t i o n on an ESR anodic e l e c t r o d e .  47  2.5  2.5.1  Ferrovac-E  pure i r o n  The  and  anodic  iron electrodes calcium  Polarization results  oxide  c a t h o d i c p o l a r i z a t i o n curves f o r F e r r o v a c - E  i n calcium  f l u o r i d e - a l u m i n a s l a g s and  s l a g s are g i v e n i n F i g u r e s  8, 9, 10 and  calcium 11.  pure  fluoride-  In c e r t a i n _2  slags  (5 wt.  % kl^O^)  at c e r t a i n c u r r e n t d e n s i t i e s (1 A.  c a l l y p o l a r i z e d Ferrovac-E  cm.  e l e c t r o d e s e x h i b i t e d apparent  times as shown i n F i g . 12.  T h i s b e h a v i o u r was  )  anodi-  transition  not observed  during  c a t h o d i c p o l a r i z a t i o n of the same e l e c t r o d e m a t e r i a l . The  usual electrode current density during melting  s l a g u n i t i s much h i g h e r during  than the c u r r e n t d e n s i t i e s n o r m a l l y  the s m a l l s c a l e t e s t s .  I t was  done f o r b o t h anodic  5 wt.  % kl^Oy  slag using  and  cathodic Ferrovac-E  a bank of 12 v o l t  to produce c u r r e n t d e n s i t i e s up  t i e s the e l e c t r o d e melted to some e x t e n t  the s l a g .  current d e n s i t i e s .  This  electrodes i n a CaF  2  At  cm.  these h i g h  during  .  The  current  each t e s t due  re-  densi-  to  local-  at the i n t e r f a c e as w e l l as r e s i s t a n c e h e a t i n g  of  These r e s u l t s , a l t h o u g h q u i t e i n a c c u r a t e , appear to agree  w e l l w i t h the lower c u r r e n t d e n s i t y r e s u l t s i n the same s l a g . noted t h a t , at c u r r e n t d e n s i t i e s above p o i n t s X i n F i g . 13, e s t a b l i s h e d a t the e l e c t r o d e .  T h i s was  r a d i a t i o n when the t e s t c u r r e n t was  +  lead-acid storage b a t t e r i e s -2  to s e v e r a l hundred A.  s u l t s of t h i s work are g i v e n i n F i g . 13.  i z e d heat p r o d u c t i o n  applied  t h e r e f o r e thought .to be u s e f u l to  c a r r y out s m a l l s c a l e t e s t s at normal r e m e l t i n g was  i n the e l e c t r o -  It  was  an a r c  was  seen as a sudden i n c r e a s e i n ,  passed through the c e l l ,  and  was  Figure 8 .  Anodic p o l a r i z a t i o n curves f o r pure i r o n i n CaF + A 1 0 slags. 2  2  3  Figure 10.  Anodic p o l a r i z a t i o n c u r v e s f o r pure i r o n i n CaF + CaO s l a g s .  51  F i g u r e 11.  C a t h o d i c p o l a r i z a t i o n curves f o r pure i n C a F + CaO s l a g s . 9  iron  J  I  I  I  I  I  I  I  L  0.2 sec./div.  F i g u r e 12.  .0  1  CaF + 5  Apparent t r a n s i t i o n time i n the anodic p o l a r i z a t i o n of pure ijjon i n 7 Al at i = A.cm  wt.%  54  o f t e n accompanied by  excessive  electrode  In t h i s s m a l l s c a l e work on CaF^ c a r r i e d out  s u c c e s s f u l l y at A ^ O ^  (18).  once a l a y e r of A ^ O ^  t i p , i t could riot be The  -'- S >  s  a  s  n  o  w o r  ^  w  a  s  T h i s a r i s e s i n the f a c t  had  been formed on the  removed w i t h o u t completely  l a r g e experimental  melting  the  electrode  s c a t t e r observed i n the "pure" CaF^ due  tent of t h i s m a t e r i a l .  data p o i n t s shown are the average of  The  to the v a r i a b l e (100  s i g n i f i c a n t concentration c a l and  low  c u r r e n t d e n s i t y i n a CaF^  electrodes  oxygen (20 ppm) +  5 wt.  CaO  con-  three  resulting  " d i r t y " microstructure,  i n a CaF^  Ferrovac-E  % A^O^  o p t i c a l p i c t u r e shows  iron-oxide.  The  high per-  the  o t h e r h a l f of  found to c o n t a i n 400  e l e c t r o d e was  ppm.  A  sig-  the remaining p o r t i o n The  concentration  c a l c u l a t e d from the probe X-ray i n t e n s i t i e s  d e t e c t f l u o r i n e or c a l c i u m  this  cathodically polarized  melted o f f , but  the "MAGIC" computer program, and was case d i d we  s l a g f o r s e v e r a l 10 s e c .  f o r A l on the e l e c t r o n microprobe.  A l i n the m a t r i x was  electrode  p o l a r i z e d at  s l a g f o r 1 hour at a c u r r e n t of 1 A.  n i f i c a n t p o r t i o n of the t i p had analyzed  F i g . 14 shows o p t i -  the probe p i c t u r e s show areas of  f o r t o t a l oxygen and  equivalent + 8 wt.  The  and  oxygen-containing m a t e r i a l , probably  An  slags r e s u l t s i n  e l e c t r o d e was  % A^O^  then removed from the s l a g .  was  ppm)  scans of a s e c t i o n through an anodic  i o d s , and  analyzed  i n A^O^  changes i n the e l e c t r o d e .  electronmicroprobe  This i n i t i a l l y  t i p was  - 500  measure-  experiments.  P o l a r i z a t i o n of F e r r o v a c - E  tip.  that  electrode  ments i n F i g . 8 i s p r o b a b l y  separate  %,  i s the primary p r e c i p i t a t e upon c o o l i n g a. hyper e u t e c t i c com-  p o s i t i o n , and  tip.  + A^O-j  c o n c e n t r a t i o n s ..greater than 10 wt.  . which i s the e u t e c t i c composition pure A ^ O g  melting.  found to be  3.0  wt.  %.  In  i n cither electrode t i p .  of  using no  Optical X 2 6 0  AE.I. X 6 5 0 Figure  14.  Sections  0 X-ray X 650  through a pure i r o n e l e c t r o d e t i p p o l a r i z e d at high current d e n s i t y f o r s e v e r a l 10 s e c . p e r i o d s i n a C a F ^ + 5 w t . % A^O^ slag.  anodically  56  2.5.2  A I S I 430 The  i n CaF  2  stainless  steel  p o l a r i z a t i o n curves  + A^O^  s l a g s are g i v e n i n F i g u r e s 15 and  these experiments, one  t e s t was  2  + 8 wt.  % A^O^  e l e c t r o d e t i p was microprobe.  e l e c t r o d e was  s l a g at a c u r r e n t of 1 A.  then cut v e r i t c a l l y  are g i v e n i n F i g u r e 17.  % Cr u s i n g  studying  anodically polarized f o r 915  sec.  The  made from the o u t s i d e edge towards These X-ray counts  the "MAGIC" program, and  from the b u l k c o n c e n t r a t i o n of  % C r , t o a v a l u e of approximately The  In a d d i t i o n t o  the  9 wt.  purpose of t h i s experiment was  approximately  % at the e l e c t r o d e - s l a g to show t h a t e a s i l y o x i -  d i z a b l e a l l o y i n g elements can be s e l e c t i v e l y o x i d i z e d at an anodic f a c e d u r i n g anodic up  of C r  polarization.  i o n s i n the s l a g .  n +  results  T h i s f i g u r e shows a d i f f u s i o n p r o f i l e i n which  the Cr c o n c e n t r a t i o n f a l l s  interface.  steel  f o r examination on the e l e c t r o n  Cr counts were taken a t 20 u s t e p s .  were c o r r e c t e d to g i v e wt.  17 wt.  An  A h o r i z o n t a l t r a v e r s e was  the c e n t e r and  16.  stainless  c a r r i e d out f o r the purpose of  chromium d e p l e t i o n of the a l l o y . i n a CaF  obtained, f o r A I S I 430  The  The  inter-  Cr l o s s so observed i n d i c a t e s a b u i l d -  c u r r e n t e f f i c i e n c y of Cr removal, assum-  3-r i n g Cr goes to Cr  i n the s l a g , was  c a l c u l a t e d by assuming the  t i o n p r o f i l e i n F i g . 17 i s l i n e a r between 9 wt. 300y depth. mately 20 mg. approximately  2.5.3  The  t o t a l Cr l o s t d u r i n g e l e c t r o l y s i s  17 wt.  i s therefore  % Cr at approxi-  w h i c h g i v e s a c u r r e n t e f f i c i e n c y f o r chromium removal of 10%.  Fe - 1 wt. The  % Cr and  concentra-  % Cr  alloy  anodic p o l a r i z a t i o n curves  f o r an Fe + 1 wt.  % Cr a l l o y i n  58  1000  -4  + 4  0 -2  In i (A.cm. ) 0  F i g u r e 16.  C a t h o d i c p o l a r i z a t i o n curves f o r AISI 430 s t a i n l e s s s t e e l i n CaF„ + A l 0 slags.  59  Distance in from surface fJL F i g u r e 17.  Cr c o n c e n t r a t i o n g r a d i e n t produced at the s u r f a c e of an AISI 430 s t a i n l e s s s t e e l e l e c t r o d e a n o d i c a l l y p o l a r i z e d f o r 915 s e c . at i = 360 ma.cm. in a C a F + 8 wt.% A 1 0 s l a g . ° 2  2  3  60  a CaF  2  + 1 wt.  % Al 0 s l a g are g i v e n 2 3  i n F i g . 18.  The  two  correspond to the r i s e o v e r p o t e n t i a l measured when the closed,  2.5.4  and  2.5.5  interruption.  a n o d i c p o l a r i z a t i o n curve f o r a pure Cr e l e c t r o d e  % A1 0  s l a g i s shown i n F i g .  2  i n a CaF  2  19.  Pure n i c k e l A pure n i c k e l e l e c t r o d e was wt.  the n i c k e l e l e c t r o d e  observed, which i s e s t a b l i s h e d r u p t i o n by  a relatively  sec.  This f i g u r e i s a p i c t u r e  of  A steady s t a t e p l a t e a u  is  i s followed  on c u r r e n t  inter-  slower p o t e n t i a l decay.  i n a CaF^  c a t h o d i c a l l y p o l a r i z e d at a current  + 8 wt.  e l e c t r o n microprobe scans are g i v e n c e n t r a t i o n of m a t r i x A l and  % A^O^  i n F i g . 21.  the  e l e c t r o d e t i p gave a s t r o n g  and  Al.  then The  of  sectioned  appropriate  These show a s t r o n g  con  areas  AEI. r e s p o n s e f o r aluminum X - r a y  t h e s e r e s u l t s were c o r r e c t e d u s i n g  give a matrix concentration  s l a g , and  a s i g n i f i c a n t number of Ca c o n t a i n i n g  c o r r e s p o n d i n g to the l i g h t e r areas on  c o u n t s , and  simiof  examined i n the e l e c t r o n microprobe f o r Ca  The  The  +.  behaviour  of 3 A.  q u i c k l y , and  A n i c k e l e l e c t r o d e was f o r 900  i n A l ^ ^ slags.  i s shown i n F i g . 20.  the o s c i l l o s c o p e t r a c e at a c u r r e n t  A.  a n o d i c a l l y p o l a r i z e d i n a CaF^  % A l ^ ^ i n order to e s t a b l i s h t h a t i t s anodic b e h a v i o u r was  l a r to t h a t of a pure i r o n e l e c t r o d e  and  is  Pure chromium  + 1 wt.  1.25  cell circuit  the decay o v e r p o t e n t i a l measured on c u r r e n t  The  2.5  curves drawn  of aluminum of 8.9  the  "MAGIC" program to  wt.  %.  However there  was  61  0.2V/div.  «  i  *  I  '  •  »  »  I sec./div.  Figure  20,  S i n g l e p o l a r i z a t i o n curve f o r an anodic g a l v a n o s t a t i c p u l s e on pure n i c k e l i n a C a F + 2.5 wt.% A 1 0 s l a g . i = 1.10 A.cm 2  2  3  Q  64  A.E.I. X 1020  Ca X-ray X 1020 F i g u r e 21.  Al X-rayXI020  S e c t i o n through a pure n i c k e l e l e c t r o d e c ^ t h o d i c a l l v p o l a r i z e d f o r 900 s e c . a t i = 450 ma.cm in a CaF + 8 wt.% A1 0 slag. ° ?  2  65  no response when Ca X - r a y s were counted i n the m a t r i x , d e s p i t e t h e f a c t that  t h e r e were Ca r i c h areas as shown i n F i g . 21.  2.5.6  Pure  cobalt  A pure c o b a l t 2.5 wt. % Al^O^ s l a g . a c u r r e n t o f 3 A.  anodically  F i g . 22 shows i t s p o l a r i z a t i o n r i s e and decay a t  ( F i g . 20).  Fe - C a l l o y An Fe + 0.83 wt. % C a l l o y was a n o d i c a l l y  rent  i n a CaF^ +  The s i m i l a r i t y i s t o be n o t e d f o r the case o f an  anodic n i c k e l e l e c t r o d e  2.5.7  e l e c t r o d e was p o l a r i z e d  polarized  at a c u r -  of 1 A. f o r 200 s e c . i n a CaF^ + 2.5 wt. % A l ^ O ^ s l a g i n order t o  study the carbon l o s s .  A p p r o x i m a t e l y o n e - h a l f o f the e l e c t r o d e t i p  melted o f f d u r i n g t h i s time. The  remaining p a r t  f o r t o t a l carbon.  2.6  The  o f the t i p was c u t from i t s base and a n a l y z e d  The carbon content had dropped to 0.31 wt. % C.  Electroslag  electroslag  remelting p o l a r i z a t i o n  results  p o l a r i z a t i o n r e s u l t s a r e g i v e n i n F i g u r e s 23->26  f o r Armco i r o n e l e c t r o d e s b e i n g r e m e l t e d i n A ^ O ^ - c o n t a i n i n g and CaO-containing slags.  Figures. 23 and 25 a r e anodic p o l a r i z a t i o n r e s u l t s w h i l e  F i g u r e s 24 and 26 a r e c a t h o d i c r e s u l t s . on  These r e s u l t s a r e superimposed  the s m a l l s c a l e r e s u l t s g i v e n i n F i g u r e s 8-KL1, and a l t h o u g h some  extra-  p o l a t i o n was n e c e s s a r y t o extend t h e s m a l l s c a l e r e s u l t s i n t o the c u r r e n t density  range e x p e r i e n c e d d u r i n g r e m e l t i n g , good agreement i s found.  The  66  0.2V/div.  •  •  i  1  *  •  I sec. /div.  Figure  22.  S i n g l e p o l a r i z a t i o n c u r v e f o r an a n o d i c g a l v a n o s t a t i c p u l s e on p u r e c o b a l t i n a C a F + 2.5 w t . % A 1 0 slag. i = 1.8 A.cm 2  2  3  r  1  r  1  w t % Al 0 2 3  •  /  Furnace Results  0  1  .  2  ,  3  4 -2  In i_ (A.cm. ) F i g u r e 23.  Anodic p o l a r i z a t i o n on Armco i r o n E.S.R. electrodes i n CaF + A l 0 slags. 9  5  68  1.5  T  wt% A l 0 2  •  V  (V.)  CD / /  3  Furnace Results  /  0/  1.0  /  /  /  /  0.5 0  4 In i F i g u r e 24,  0  (A.cm. ) 2  C a t h o d i c p o l a r i z a t i o n on Armco i r o n ESR e l e c t r o d e s i n CaF2 + A ^ O ^ s l a g s .  69  {Figure 25.  Anodic p o l a r i z a t i o n on Armco i r o n e l e c t r o d e s i n C a F + CaO slags. 9  ESR  70  F i g u r e 26,  C a t h o d i c p o l a r i z a t i o n on Armco i r o n e l e c t r o d e s i n C a F + CaO slags. 0  ESR  71  extent of p o l a r i z a t i o n was e l e c t r o d e s and AISI 1095  a l s o measured on AISI 430 s t a i n l e s s  steel electrodes.  c l u s i v e to p r e s e n t i n g r a p h i c a l 5.  The r e s u l t s  steel  are too i n c o n -  form, but w i l l be d i s c u s s e d i n Chapter  CHAPTER 3 MELT PROGRAM  3.1  All  Melting  procedure  t h e i n g o t s used i n t h i s i n v e s t i g a t i o n were made on t h e  U.B.C. E l e c t r o s l a g U n i t , t h e d e s i g n and o p e r a t i o n o f which have been d e s c r i b e d by E t i e n n e  (4).  Each r u n was s t a r t e d u s i n g a compact c o n s i s t i n g  of m e t a l t u r n i n g s and c a l c i u m - f l u o r i d e power, the m e t a l t u r n i n g s machined from t h e e l e c t r o d e m a t e r i a l . and  the melting  When the s l a g was completely  c o n d i t i o n s were s t a b l e , t h e o p e r a t o r s  the p e r t i n e n t p r o c e s s  being molten  proceeded t o r e c o r d  parameters a t r e g u l a r time i n t e r v a l s .  In t h i s manner,  a complete r e c o r d of each i n g o t was taken and used f o r subsequent a n a l y s i s .  3.2  The be  classified  Electrode materials  e l e c t r o d e m a t e r i a l s used i n t h i s program o f experiments can i n t o two types,  the f i r s t  pure metals and the second b e i n g tially  o x i d i z a b l e elements.  together with condition.  type  iron-base  c o n s i s t i n g of e s s e n t i a l l y a l l o y s containing other  poten-  The reasons f o r u s i n g each m a t e r i a l a r e g i v e n  the a n a l y s i s of t h e e l e c t r o d e m a t e r i a l s i n the as r e c e i v e d  A l l compositions  a r e g i v e n i n weight p e r cent.  72  73  3.2.1  Low carbon m i l d s t e e l :  A I S I 1 0 1 8 grade  ( S u p p l i e d by S t e l c o )  rC  KT Mn  . 1 5 - . 2 0  P  . 6 0 - . 9 0  max . 0 4 0  0  max  S  . 0 5 0  Fe  . 0 1 5 5  Bal.  T h i s e l e c t r o d e m a t e r i a l , because o f i t s low c o s t , was used i n the i n i t i a l  3.2.2  experiments where the m e l t i n g  Ferrovac-E:  vacuum m e l t e d  c o n d i t i o n s were u n c e r t a i n .  ( S u p p l i e d by the C r u c i b l e S t e e l Company, S o r e l , Quebec)  Mn  c  .010  .001  .002  Cr  Mo  N  < . 0 1  .001  . 0 0 0 2  Co  Cu  V  .006  .006  Ferrovac-E  S  P  < . 0 0 2  .004  .006  H  0  . 0 0 0 9 2  . 0 0 0 0 1 8  W .02  Fe Bal  pure i r o n was the i d e a l e l e c t r o d e m a t e r i a l on which  to study e l e c t r o c h e m i c a l phenomena d u r i n g ESR. oxygen content,  Si  Because of i t s v e r y low  any oxygen found i n the i n g o t s must have a r i s e n d i r e c t l y  as a r e s u l t o f r e m e l t i n g  the m e t a l whether by chemical  or e l e c t r o c h e m i c a l  means. A l a t e r s e r i e s o f i n g o t s were made u s i n g a second b a t c h of Ferrovac-E  which was e x c e s s i v e l y h i g h i n oxygen ( 3 1 6 ppm).  t h i s , i t was e s s e n t i a l l y pure i r o n as was the p r e v i o u s E.  S i n c e i t had been found t h a t t h i s h i g h i n i t i a l  batch  Apart  from  of Ferrovac-  oxygen content  would  74  be  l o s t during remelting,  and would have v e r y  f i n a l i n g o t oxygen c o n t e n t , was  the i n i t i a l b a t c h  second b a t c h shallow  t h i s m a t e r i a l was used i n the same way as  of F e r r o v a c - E .  of Ferrovac-E  l i t t l e e f f e c t on the  Two i n g o t s were made u s i n g  w i t h lengths  this  of aluminum w i r e h e l d i n  grooves machined on the e l e c t r o d e s u r f a c e .  The purpose of  t h i s was t o i n v e s t i g a t e whether or n o t excess aluminum would c o n t r o l the i n g o t oxygen content  i n s p i t e of electrochemical o x i d a t i o n .  The  amount o f w i r e added would produce 2000 ppm of A l i n the f i n a l i n g o t i f none were t o be l o s t d u r i n g  3.2.3  Armco i r o n :  C  Mn  .012  .017  melting.  ( S u p p l i e d by Armco S t e e l  Corporation)  P  S  Si  o  Fe  .005  .025  trace  .070  Bal.  Armco iron'was used i n p l a c e o f F e r r o v a c - E  pure i r o n f o r  s e v e r a l runs as i t was found t h a t d e s p i t e the h i g h oxygen content o f the Armco i r o n , i n g o t s made from i t had oxygen contents the 0 contents  of Ferrovac-E  Ferritic  stainless steel:  ( S u p p l i e d by A l l e g h e n y C .06  close to  i n g o t s made under the same c o n d i t i o n s .  r e s u l t e d i n s u b s t a n t i a l savings  3.2.4  very  This  i n electrode materials.  A I S I 430 grade  Ludlum I n d u s t r i e s , I n c . )  Mn  P  S  .44-  .024  .015  Si .26  Cr 17.35  0 .0115  I t was d e s i r e d t o make i n g o t s from an i r o n a l l o y c o n t a i n i n g a second major element which c o u l d b e o x i d i z e d d u r i n g  remelting.  AISI 430  stainless  s t e e l was chosen because i t i s e s s e n t i a l l y  alloy containing l i t t l e losses.  e l s e which c o u l d i n t e r f e r e w i t h Cr o x i d a t i v e  Another advantage o f t h i s a l l o y was t h a t i t s m e l t i n g p o i n t i s  v e r y c l o s e t o that of pure  3.2.5  an iron-chromium  iron.  Medium carbon m i l d s t e e l :  AISI 1095 grade  ( S u p p l i e d by S t e l c o , H a m i l t o n , O n t a r i o )  C  Mn  Si  P max  S max  0  Fe  .975  .39  .34  .040  .050  .0020  Bal.  Ingots were made from t h i s e l e c t r o d e m a t e r i a l • t o study the e f f e c t of a s u b s t a n t i a l amount o f carbon on the f i n a l oxygen c o n t e n t of  these i n g o t s .  3.2.6  Pure N i : C .01  S u p p l i e d by F a l c o n b r i d g e N i c k e l Mines v  Si .17  Mg  Fe  0  .18  1.9  .0011  Ingots were made from F a l c o n b r i d g e N i c k e l t o i n v e s t i g a t e the b e h a v i o u r of pure n i c k e l d u r i n g ESR and t o compare t h i s b e h a v i o u r t o the m e l t i n g b e h a v i o u r of pure  iron.  76  3.3  The  Slag  s l a g s used d u r i n g  materials  this melting  program were a l l based  c a l c i u m - f l u o r i d e w i t h a d d i t i o n s of aluminum-oxide (kl^O^) oxide  (CaO)  to a l t e r the m e l t i n g  v i t i e s of the v a r i o u s  3.3.1  duced by  p o i n t , the c o n d u c t i v i t y , and  the  acti-  components.  c a l c i u m - f l u o r i d e used i n these s l a g s was  a h y d r o - f l u o r i n a t i o n process.  c a l c i u m - f l u o r i d e used i n each run was  for only a very pick-up  short  pre-fused  time i n the g r a p h i t e  from the c r u c i b l e .  small p a r t i c l e s  a f i n e power  Approximately t w o - t h i r d s  i n d u c t i o n h e a t i n g under an argon c o v e r .  other  calcium-  Calcium f l u o r i d e The  by  or  on  The  of  the  i n graphite c r u c i b l e s s l a g was  h e l d as a  liquid  c r u c i b l e to m i n i m i z e carbon  A f t e r f u s i o n , the c o l d s l a g was  (approx. 1/8"  pro-  crushed i n t o  i n diameter) b e f o r e b e i n g mixed w i t h  the  s l a g components. I t was  granules  found, from e x p e r i e n c e  that i f these c a l c i u m - f l u o r i d e  were too l a r g e they could b i n d between the e l e c t r o d e and  mould w a l l thus s t o p p i n g be broken d u r i n g  e l e c t r o d e t r a v e l and  the s t a r t - u p p e r i o d of a m e l t .  s i n t e r i n g of the s o l i d p a r t i c l e s would occur, t r a v e l during  the s t a r t - u p .  c i u m - f l u o r i d e was Moisture reaction:  causing  the  the c u r r e n t path to  I f the s l a g was  again preventing  too  fine,  electrode  Another r e a s o n f o r f u s i n g p a r t of the  cal-  to d r i v e o f f the s u r f a c e m o i s t u r e on the f i n e powder.  i n the s l a g can a l t e r the s l a g composition  according  to  the  77  CaF  + HO  -> CaO + 2 HF  to form c a l c i u m - o x i d e i n the s l a g . p u r i t y and i s u s u a l l y found v a r y from melt  to melt.  (3-1)  Calcium-oxide  i s the main s l a g im-  t o be a t a l e v e l of 500 ppm a l t h o u g h i t would  T h i s i m p u r i t y was o n l y important when the s l a g  used d u r i n g r e m e l t i n g was kept low i n o x i d e c o n t e n t .  For higher oxide  content s l a g s (5 wt. % A ^ O ^ o r more) the c a l c i u m - o x i d e i m p u r i t y tent i s unimportant  con-  i n comparison to the t o t a l o x i d e content of the s l a g .  When i t was d e s i r e d t o make i n g o t s i n low o x i d e content  slags,  BDH e x t r a pure c a l c i u m - f l u o r i d e was p r e f u s e d i n g r a p h i t e c r u c i b l e s and used f o r the e n t i r e c a l c i u m - f l u o r i d e content o f the s l a g .  The as r e -  c e i v e d a n a l y s i s of t h i s c a l c i u m - f l u o r i d e i s g i v e n below as maximum im- , p u r i t y l i m i t s i n wt. %.  C h l o r i d e (CI) S u l p h a t e (SO^) I r o n (Fe) Lead (Pb) Silica (Si0 ) 2  3.3.2  .005 .01 .005 .005 .05  % % % % %  Alumina Norton g r a n u l a r alundum ( A ^ O ^ ) of 99.3% p u r i t y was used i n  the as r e c e i v e d c o n d i t i o n f o r making alumina  containing slags.  m a t e r i a l i s e l e c t r i c a l l y f u s e d from Bayer p r o c e s s  3.3.3  Calcium  This  alumina.  oxide  R e c r y s t a l l i z e d calcium oxide make c a l c i u m - o x i d e c o n t a i n i n g s l a g s . mit Nobel, Germany.  (CaO) o f 99.5% p u r i t y was used t o T h i s m a t e r i a l was s u p p l i e d by Dyna-  78  3.4  Atmospheric c o n t r o l  Ingots were made under two ment.  The  initial  types of atmospheric c o n t r o l  24 i n g o t s , as w e l l as some l a t e r ones were made u s i n g  a crude type of argon fume hood as shown i n F i g . 27.  T h i s fume hood  d i r e c t e d an argon f l o w down towards the slag/atmosphere s u r f a c e c o l l e c t e d most of the fumes g i v e n o f f d u r i n g A more s o p h i s t i c a t e d d e s i g n was the i n g o t s . bellows,  equip-  As shown i n F i g . 28,  clamped a t the lower end  and  remelting.  used to make the b a l a n c e of  i t c o n s i s t e d of a number of rubber to a f l a n g e d s e c t i o n of p i p e  and  f i x e d above to a w a t e r - c o o l e d copper e l e c t r o d e c a r r i e r , such t h a t  the  downward t r a v e l of the e l e c t r o d e caused the b e l l o w s to c o l l a p s e .  The  f l a n g e d p i p e , which was had  two  held  to the top of the i n g o t mould by  aluminum f o i l blow-out windows.  This design  provides  C-clamps, an atmos-  phere the q u a l i t y of which depends almost e x c l u s i v e l y upon the p u r i t y of the argon used.  By  c o n t r a s t , chromatographic a n a l y s i s of the argon  fume hood atmosphere i n d i c a t e s t h a t i t may argon f l o w r a t e of 100  c o n t a i n up  to 1% 0^ w i t h  an  1/hr.  3.5  Melting  Conditions  Most of the i n g o t s produced i n t h i s melt program were made under s t a b l e m e l t i n g  c o n d i t i o n s which i n t u r n were determined by  f a c t o r s as s l a g c o m p o s i t i o n , In g e n e r a l , i t was trode n e g a t i v e  was  e l e c t r o d e p o l a r i t y , and  the same v o l t a g e and  electrode materials.  found t h a t m e l t i n g u s i n g e i t h e r A.C.  q u i t e s t a b l e but m e l t i n g w i t h  such  or e l e c -  e l e c t r o d e p o s i t i v e at  c u r r e n t as used i n e l e c t r o d e n e g a t i v e  was  very  un-  79  F i g u r e 27,  Argon fume hood.  F i g u r e 28.  Argon gas cap,  81  s t a b l e , i f not i m p o s s i b l e .  T h i s i n s t a b i l i t y was the r e s u l t o f a r c i n g  between the r a d i a l s l a g segment around the e l e c t r o d e and the mould w a l l and o c c u r r e d even though the mould was i n s u l a t e d from  the base p l a t e ( 1 9 ) .  The problem of a r c i n g was overcome by p a i n t i n g the i n s i d e of the Cu mould w i t h B o r o n - N i t r i d e p a i n t (type S) which i s an e l e c t r i c a l t o r b u t has a h i g h thermal c o n d u c t i v i t y . U s i n g  insula-  t h i s t e c h n i q u e , i t was  p o s s i b l e t o make i n g o t s u s i n g the e l e c t r o d e p o s i t i v e mode a t h i g h e r o p e r a t i n g v o l t a g e s w h i l e s t i l l m a i n t a i n i n g the d e s i r e d e l e c t r o d e - s l a g ingot current path. In a number of cases, i n g o t s were made u s i n g a l i v e mould. was  done by c o n n e c t i n g s e p a r a t e l e a d s d i r e c t l y  This  to the bottom of the mould  p u t t i n g i t i n p a r a l l e l w i t h the base p l a t e thereby a l l o w i n g the p r o c e s s c u r r e n t t o f l o w through e i t h e r  the mould or the i n g o t / b a s e p l a t e on the  r e t u r n path.  3.6 During all  the m e l t i n g o f an i n g o t , d e t a i l e d accounts were kept o f  the important  Recorder  Melt records  was used  o p e r a t i n g parameters.  A Sargent Model SR M i l l i v o l t  t o keep a continuous  r e c o r d of the o p e r a t i n g c u r r e n t .  When the e l e c t r o d e was m e l t i n g i n a s t a b l e f a s h i o n (the s l a g was  complete-  l y molten) the f o l l o w i n g d a t a were r e c o r d e d i n the l o g book a t 100 s e c . intervals: V  - process v o l t a g e -> D.C. or A.C.  A  - process current  t  - time i n seconds w i t h t = 0 the b e g i n n i n g of r e c o r d i n g the d a t a  D.C. o r A.C.  82  - t o t a l coulombs  P  - t o t a l e l e c t r o d e t r a v e l i n mm. b e g i n n i n g of the r u n  M.S.  - speed of e l e c t r o d e t r a v e l d r i v e motor  AT  - temperature d i f f e r e n c e i n °C between the i n l e t and o u t l e t temperature of the mould c o o l i n g water.  Also recorded evident during  10  from the  peculiarities  melting.  Melt record c a l c u l a t i o n s  Melt rate The average m e l t r a t e of each i n g o t was  f o l l o w i n g mathematical approach. is  2 x  f o r each i n g o t were any o p e r a t i n g  3.7  3.7.1  passed x  -2  C  calculated using  The s t a r t i n g p o i n t f o r the c a l c u l a t i o n  the a c t u a l d i s t a n c e t r a v e l e d by the e l e c t r o d e , t h i s d i s t a n c e  recorded  a t r e g u l a r time i n t e r v a l s d u r i n g a r u n . e  To c o n v e r t  being  The diameter of the  e l e c t r o d e i s known (D ) and the average diameter o f the i n g o t s measured.  the  (D^) i s  the e l e c t r o d e t r a v e l i n t o i n g o t r i s e as a f u n c t i o n  of i n g o t diameter, i t i s assumed that the e l e c t r o d e i n g o t s e p a r a t i o n r e mains constant  during  a r u n and t h a t the i n g o t i s f u l l y dense.  of i n g o t r i s e r e s u l t i n g from 1 mm.  X = [1  of e l e c t r o d e t r a v e l i s g i v e n  + C^)2] * K^)2]  The weight o f i n g o t formed per mm.  Y = X • TrRjVm  *?^£V  The amount by:  n  _(3-2)  f  of e l e c t r o d e t r a v e l i s g i v e n  by  (3-3)  83  where p  i s the d e n s i t y of the m e t a l .  m  according  The melt r a t e i s then c a l c u l a t e d  to: M.R.  =  C^-)  t  where P i s the e l e c t r o d e t r a v e l i n mm. which the t r a v e l  3.7.2  A parameter was  the i n g o t .  required r e f l e c t i n g  the r a t e a t which the  electro-  the i n g o t m e t a l d u r i n g m e l t i n g .  This  then be compared to the measured f i n a l oxygen content  Such a parameter must take i n t o account t h r e e  r e n t at the s u r f a c e i n q u e s t i o n , rate  t the time i n seconds over  (Z)  r e a c t i o n p r o d u c t s entered  parameter c o u l d  and  occurred.  S p e c i f i c coulombic d e n s i t y  chemical  (3-4)  sec  terms; the  the a r e a of t h i s s u r f a c e , and  of cur-  the m e l t  (which i s a crude measure of the r a t e a t which t h i s s u r f a c e i s r e -  ceiving fresh metal).  An  the i n g o t top, which was  appropriate  a r e a was  also considered  assumed to be  to be f l a t .  The  the a r e a  of  r a t e of forma-  t i o n of e l e c t r o c h e m i c a l p r o d u c t s would be p r o p o r t i o n a l to the p r o c e s s , current  (assuming 100%  c u r r e n t e f f i c i e n c y of any  Faradaic processes)  i n v e r s e l y p r o p o r t i o n a l to the s u r f a c e a r e a as w e l l as the m e l t r a t e . parameter Z c o u l d  then be  z  c a l c u l a t e d as  _ P r o c e s s c u r r e n t (A.) Ingot top area (cm.2) x melt r a t e  c o u l . sec. 2 -1 cm. x gm. sec  Z  follows:  =  coul. cm.  2  x  gm.  _^ (gm.sec  )  and This  84  3.7.3  Drop s i z e and s u r f a c e  tension  I t i s known t h a t oxygen i s s u r f a c e a c t i v e i n l i q u i d many o t h e r elements  i r o n as a r e  (B, Se, S) and t h a t the e f f e c t of oxygen i n i r o n i s  to reduce i t s s u r f a c e t e n s i o n q u i t e markedly.  I t was t h e r e f o r e  thought  to be d e s i r a b l e t o measure the s i z e of drops produced a t the m e l t i n g electrode during  e l e c t r o s l a g melting  and to c o r r e l a t e the drop s i z e t o  the s u r f a c e t e n s i o n o f the drop, hence u l t i m a t e l y to the presence or absence o f oxygen i n the d r o p s . Campbell (20) s t u d i e d d r o p l e t f o r m a t i o n i n g and d e r i v e d  i n e l e c t r o s l a g remelt-  the r e l a t i o n g i v e n below u s i n g a d i m e n s i o n a l  r  2  argument.  V  =  (3-5)  gAp  r  -  drop  radius  Y  -  i n t e r f a c i a l t e n s i o n between two l i q u i d s  g  -  a c c e l e r a t i o n due to g r a v i t y  Ap  -  d e n s i t y d i f f . between l i q u i d s a t T  k  -  a  constant  He assumed t h a t the drop r a d i u s , r , i s independent of the e l e c trode r a d i u s f o r a s u f f i c i e n t l y l a r g e e l e c t r o d e and he s t a t e s the r e l a t i o n s h i p i s v a l i d f o r c o n i c a l e l e c t r o d e t i p s , such as i s the case d u r i n g the m e l t program.  I n order  to use t h i s r e l a t i o n s h i p i t i s f i r s t n e c e s s a r y to  c a l c u l a t e r from the melt r e c o r d s .  T h i s i s done by c o u n t i n g  r a t e on the p r o c e s s  current recorder  peak on the c h a r t .  Combining the drop weight w i t h  can f i n d  various  c h a r t , each drop c o i n c i d i n g w i t h a the m e l t r a t e , one  the drop s i z e , then r , and c a l c u l a t e y u s i n g  p h y s i c a l constants. conditions.  the drop  T h i s was done f o r F e r r o v a c - E  the a p p r o p r i a t e  e l e c t r o d e s melted under  85  3.7.4  M e l t program  results  The m e l t r e c o r d d a t a , the c a l c u l a t e d parameters the  s p e c i f i c coulombic d e n s i t y [ Z ] ) ,  the  i n g o t s are summarized i n T a b l e s I->V.  (melt r a t e and  and the average oxygen c o n t e n t o f The drop s i z e and  interfacial  t e n s i o n r e s u l t s are g i v e n i n T a b l e V I .  3.8  3.8.1  Ingot a n a l y s i s  Oxygen a n a l y s i s of i n g o t s A l l of the i n g o t s produced i n t h i s melt program were a n a l y z e d  for  t h e i r average t o t a l oxygen c o n t e n t u s i n g a Lecd Rapid Oxygen A n a l y -  ser  (No. 734-300).  in  I t must be noted t h a t the oxygen i n these i n g o t s i s  the form o f n o n - m e t a l l i c i n c l u s i o n s formed by the r e a c t i o n of d i s s o l v e d  oxygen w i t h e i t h e r metals d i s s o l v e d i n the base m e t a l or the base m e t a l itself. Ingots to be a n a l y s e d f o r t o t a l oxygen are s e c t i o n e d as shown i n F i g . 29 g i v i n g i n d i v i d u a l 1 g. specimens. t a l l y o n e - t h i r d o f the way  up from the bottom,  An i n g o t i s cut h o r i z o n and then a v e r t i c a l  one-quarter of an i n c h t h i c k i s cut from the top p o r t i o n . then cut h o r i z o n t a l l y i n t o 10 1/4" can then be c u t i n t o 1/4"  square s l i c e s  This s l i c e i s  and the d e s i r e d  slice  Normally, f i v e cubes  would  be c u t from the o u t s i d e i n t o the c e n t e r from the second, f i f t h , and  tenth  slices  cubes f o r a n a l y s i s .  slice  to g i v e 1 5 specimens  f o r a n a l y s i s from, each i n g o t .  P o r t i o n s of;  these s l i c e s were .also used f o r m e t a l l o g r a p h i c and e l e c t r o n microprobe examination of the i n c l u s i o n s i n the i n g o t s .  Ingot Top 7j8_ij|LQ - f -  r  t - T -  T  ^ -I • - l 4  -  t-  Hm-i-r  • | r t - r 4 - M ^ - -  10  ure 29.  I  J. _  Ingot sampling scheme.  87  3.8.2  Aluminum a n a l y s i s o f FVE i n g o t s F i v e Ferrovac-E  i n g o t s were a n a l y z e d  the n e u t r o n a c t i v a t i o n method  f o r t o t a l aluminum u s i n g  ( 2 1 ) . The r e s u l t s o f these a n a l y s e s a r e  given i n Table V I I .  3.8.3  A n a l y s i s o f A I S I 1095 s t e e l E l e c t r o d e m a t e r i a l and i n g o t s made from 1095 s t e e l were  analysed  s p e c t r o g r a p h i c a l l y to determine what a l l o y l o s s e s took p l a c e d u r i n g r e melting.  3.8.4  The r e s u l t s a r e g i v e n i n T a b l e  VIII.  A n a l y s i s o f AISI 430 s t a i n l e s s s t e e l E l e c t r o d e m a t e r i a l and i n g o t s made from 430 s t a i n l e s s s t e e l were  also analysed  spectrographically.  3.9  The r e s u l t s a r e g i v e n i n T a b l e IX.  S l a g cap a n a l y s i s  S e v e r a l o f the s l a g caps were observed to c o n t a i n d a r k green or brown phases, u s u a l l y near the i n g o t r e g i o n . to c o n t a i n a h i g h  concentration  of i r o n - o x i d e .  These r e g i o n s were thought Segments o f f o u r s l a g  caps from FVE runs (-, +, +(im) , A.C.) were crushed and analysed f o r t o t a l i r o n by c a u s t i c f u s i o n and a c i d d i s s o l u t i o n t o make s o l u t i o n s s u i t a b l e f o r a n a l y s i s by the atomic a b s o r p t i o n are g i v e n i n T a b l e X.  technique.  The r e s u l t s  88  TABLE I MELT RECORD RESULTS OF A I S I 1018 MILD STEEL  Atmosphere - Argon fume hood E l e c t r o d e D l a . = 3.82  cm.  Ingot D i a . ( a v . ) = 7.5 cm. E l e c t r o d e 0 content = 150 ppm. CA  INGOT NO.  c a l c i u m aluminate  SLAG ADD'N.  ELECTRODE POLARITY  VOLTS  AMPS  (wt. %)  4  MELT RATF. (gm.sec. )  OXYGEN (ppm)  Z (coul.cm.  30 CA  —  24.6  1160  2.94  481  8.9  30 CA  —  23.8  1200  2.40  500  11.3  11  30 CA  . —  23.5  1177  2.97  500  8.9  17  30 CA  —  23.2  1025  2.21  475  10.5  18  30 CA  —  23.2  1350  3.08  375  9.9  15  30 CA  (mo —- . .vn.a. insert;  14  n i l  —  22.2  1181  6  10 MgF  2  . —  20.5  1150  510  7  10 MgF  2  21.0  1100  505  5  "  0 2.50  2000 205  n.a. 10.7  1  30 CA  +  22.1  1020  . 2.45  55  9.4  2  30 CA  +  22.5  1015  2.86  50  8.0  3  30 CA  ' +  23.Q  1058  3.13  50  7.6  21  30 CA  A.C.  22  30 CA  A.C.  t r i a l runs  180 200  _^ gm. )  89  TABLE I I MELT RECORD RESULTS OF FVE INGOTS  Atmosphere - Argon fume hood E l e c t r o d e D i a . = 3.18 cm. Ingot D i a . ( a v . ) = 5.5 cm. E l e c t r o d e 0 c o n t e n t = 9 ppm. * •> new FVE 0 content = 316 ppm. CA -»• c a l c i u m a l u m i n a t e  INGOT NO.  SLAG ADD'N.  ELECTRODE POLARITY  VOLTS  AMPS  (wt. %)  MELT RATE (gm.sec )  OXYGEN (ppm)  -2 (coul.cm.  8  24 CA  -  23.5  689  1.28  1075  21.8  9  24 CA  -  23.2  862  1.63  788  21.4  10  24 CA  .-  23.5 1047  2.06  730  21.4  16  24 CA  +  20.5  782  1.32  250  23.2  23  24  A 1  2°3  -  23.3  966  2.33  425  16.7  72  25  A 1  2°3  -  22.7  892  1.49  569  22.7  24  24  A 1  2°3  +  20.2  749  1.59  150.  19.0  25  A 1  2°3  + (im)  22.5  878  1.68  174  17.9  73  25  A 1  2°3  A.C.  23.3  558  2.99  165  8.7  19  25 CaO  -  22.4 1000  2.20  800  20  30 CaO  -  22.5 1040  1.58  550,  *  * 74  *  1  Z  17.8 26.6  gm.  )  90  TABLE I I I MELT  RECORD RESULTS OF FVE INGOTS  Atmosphere - Argon gas cap. E l e c t r o d e D i a . = 3.18 cm. Ingot D i a . ( a v . ) = 5.5 cm. E l e c t r o d e 0 content = 9 ppm. * ->• new FVE 0 content = 316 ppm.  INGOT NO. SLAG ADD'N.  ELECTRODE POLARITY  VOLTS  AMPS  (wt . %) 25  25  A 1  2°3  26  25  A 1  2°3  27  25  A 1  2°3  28  25  A 1  2°3  29  25  A 1  2°3  30  25  A 1  2°3  39  25  A 1  2°3  77  25  A 1  2°3  31  25  A 1  2°3  32  25  A 1  2°3  33  25  A 1  37  2°3 25 A 1 0  38  25  A l  2°3  41  25  A 1  2°3  25  A 1  2°3  25  A 1  35 83 34  2  A  3  2°3 25 A 1 0 2  3  MELT RATE (gm.sec )  OXYGEN  Z (coul.cm. _2  -  22.6  833  1.38  450  27.3  -  22.5  949  1.75  450  22.8  -  23.3  924  1.77  470  22.8  22.2  750  1.28  450  26.6  -  24.5  859  1.64  500  23.3  -  24.0  748  1.43  680  22.0  -  23.5  991  1.83  460  23.6  -  26.0 1160  3.55  378  13.7  +  19.0  652  1.30  67  21.2  +  19.1  655  1.11  317  23.9  +  18.9  748  1.28  171  22.9  (±m)  22.0  791  1.77  175  18.1  + (im)  22.4  917  1.58  70  24.4  + (im)  22.2  957  2.50  170  16.. 7  + (lm)  21.5  857  1.15  690  32.7  22.5  933  1.76  834  22.3  24.2  655  2.22  225  12.9  4  (lm) A.C.  +  91  TABLE I I I (Continued)  INGOT NO.  SLAG ADD'N. (wt. %)  40  25  36  25 A 1 0  2°3 2  * 79  * 78  * 82  M  * *  A.C. (im) 22.9  MELT RA^ (gm.sec  OXYGEN (ppm)  (coul.cm.  819  2.62  200  13.1  A.C. (lm) 23.8 810  2.15  225  15.9  22.6 1065  2.06  342  20.8  -  25 CaO 25 CaO  + (im)  21.4  828  2.22  119  15.0  25 CaO  + (lm)  21.5  908  2.11  230  18.1  -  23.8 1150  3.07  548  8.7  24.5 1045  2.31  153  10.2  -  23.8  2.59  548  12.7  + (im)  22.0 792  1.75  526  22.1  25 A 1 0 2  76  VOLTE1 AMPS  25  A 1  3  2°3  80  25 A 1 0  3  81  25 A 1 0  3  2  2  . +(im)  779  gm  large mold (7.6  *  75  3  ELECTRODE POLARITY  o 3  K HW n  >  92  TABLE IV MELT RESULTS OF ARMCO IRON INGOTS  Atmosphere - Argon gas cap E l e c t r o d e D i a . = 3.18 cm. Ingot D i a . (av.) = 5.5 cm. E l e c t r o d e 0 content = 700 ppm.  INGOT NO.  SLAG ADD'N.  ELECTRODE POLARITY  VOLTS  AMPS  MELT RATE _ (gm.sec. )  OXYGEN  Z _  1  (wt. %)  2  (ppm)  (coul.cm.  42  25 A 1 0  3  -  22.4  908  1.49  550  26.5  49  25 A 1 0  3  -  22.6  954  2.14  470  18.7  47  10 A 1 0  3  -  23.0  886  1.70  820  21.1  22.3  896  2.37  820  15.3  - •  22.5  910  1.77  810  22.6  -  20.0  536  1.79  660  16.3  65 48  2  2  2  5  2°3 1 Al O  50 54  A 1  nil 25  A 1  2°3  57 .  25  A 1  2°3  58  25  A 1  2°3  55  10  A 1  56 59  2°3 10 A 1 0 2  5  3  44  2°3 25 A 1 0  43  25  A 1  2  3  s  +  20.0  732  1.72  300  17.9  +  18.5  757  1.80  195  17.0  +  19.0  768  1.80  290  17.3  +  18.5  808  1.53  250  22.1  +  18.8  786  1.53  260  21.5  +  19.0  827  1.58  285  21.3  + (im)  22.2  899  1.82  255  20.8  A.C.  23.0  710  2.94  260  10.9  A.C.  26.4  588  3.06  245  8.1  ;  60  2°3 25 A 1 0  66  30 CaO  -  23.0  973  2.10  505  19.1  52  25 CaO  -  23.0  1062  1.91  580  23.4  51  5 CaO  —  20.0  1104  0.98  1200  45.7  A 1  2  3  gm.  )  TABLE V MELT RECORD RESULTS OF MISCELLANEOUS  INGOTS  Atmosphere - Argon gas cap. Ingot D i a . ( a v . ) = 5.5 cm.  MATERIAL  INGOT NO.  ELECTRODE DIA.(cm.)  0 CONTENT (ppm)  AISI 430  2.54  115  AISI 1095  2.54  20  Pure N i c k e l  3.50  11  SLAG'ADD'N. (wt. %)  61  25 A 1 0  3  62  25 A 1 0  3  63  25 A 1 0  64  ELECTRODE POLARITY  VOLTS  AMPS  MELT RAT^ (gm.sec. )  OXYGEN (ppm)  _ (coul.cm. z  22.7  834  1.75  125  20.0  -  22.7  877  1.85  115  19.8  3  -  23.1  858  1.79  225  20.1  12 A 1 0  3  -  23.0  843  1.95  170  18.2  70  25 A 1 0  3  + (im)  68  25  2  2  2  2  2  very  unstable  91  -  24.0  910  2.15  145  17.8  67  2°3 25 A 1 0  + (im)  22.0  790  2.45  50  13.6  69  10  -  23.0  780  1.96  7  15.6  + (im)  22.0  628  2.23  362  11.4  71  A 1  2  3  2°3 10 A 1 0 A 1  2  3  _  2  gm.  x  )  TABLE VI DROP SIZE AND INTERFACIAL TENSION RESULTS FOR FVE  ELECTRODE POLARITY AND CONDITIONS  DROP WT. (gm)  y  (DYNES cm." ) 2  2.57  421  + ( i n c l u d e s +im and +lm)  1.11  240  A.C.  3.51  517  -  (Ingot 80- A l w i r e )  3.46  513  + (Ingot 81- A l w i r e )  1.17  250  TABLE V I I TOTAL ALUMINUM CONTENT OF FVE INGOTS  INGOT NO.  MELT CONDITIONS  ppm. A l  25  -  522  41  +(im)  55  83  +(lm)  169  80  -{Al wire]  1962  81  + ( i m ) [ A l wire].  2621  96  TABLE V I I I COMPOSITION OF A I S I 1095 ELECTRODE AND INGOTS  ELEMENT  ELECTRODE  INGOT 68 [ e l . - v e ]  INGOT 67 [ e l + v e ( i m ) ]  C  .975  .903  .914  Mn  .39  .37  .38  Si  .34  .27  .37  0  .0020  .0145  .0050  97  TABLE IX COMPOSITION OF A I S I 430 ELECTRODE AND INGOTS  ELEMENT  ELECTRODE  INGOT 63 [ e l - v e ]  INGOT 70 [ e l . + v e ( i m ) ]  Cr  17.35  17.16  16.90  Mn  0.44  0.47  0.40  Si  0.26  0.15  0.19  C  0.060  0.051  0.060  P  0.024  0.024  0.022  S  0.015  0.006  0.009  0  0.0115  0.0225  0.0091  TABLE X TOTAL IRON CONTENT OF FVE SLAG CAPS  INGOT NO.  POLARITY  (CaF  2  + 25 wt. % A l ^ ) (wt. % Fe )  39  -  0.52  31  +  1.74  37  + (lm)  1.34  34  A.C.  1.01  CHAPTER 4 DISCUSSION OF 4.1 The formulating  SMALL SCALE STUDIES Introduction  s m a l l s c a l e s t u d i e s were c a r r i e d out w i t h  the purpose of  e l e c t r o c h e m i c a l r e a c t i o n mechanisms r e s p o n s i b l e f o r c u r r e n t  t r a n s f e r across  l i q u i d m e t a l / l i q u i d s l a g i n t e r f a c e s , under ESR  The  r e a c t i o n mechanisms so proposed must be a b l e to account f o r  and  thermal phenomena which e x i s t d u r i n g D.C.  has been s t a t e d t h a t CaF^ be e s s e n t i a l l y  conditions. chemical  electroslag melting.  s l a g s conduct i o n i c a l l y and  t h i s was  It  found to  t r u e , except when t h e r e were s i g n i f i c a n t amounts of Ca  and  A l metals d i s s o l v e d i n the s l a g . The mechanism r e s p o n s i b l e f o r p o l a r i z a t i o n i n such systems i s the slow d i f f u s i o n of r e a c t i o n p r o d u c t s away from the r e a c t i o n i n t e r f a c e i n t o the s l a g and  the m e t a l .  In the f o l l o w i n g s e c t i o n we r e a c t i o n f o r pure i r o n i n CaF^ the anodic  - A^O^  s h a l l show that the anodic s l a g s i s at low  c o r r o s i o n of i r o n l e a d i n g to FeO  Faradaic  current d e n s i t i e s ,  s a t u r a t i o n i n the s l a g at  i n t e r f a c e at s u f f i c i e n t l y h i g h c u r r e n t d e n s i t i e s .  In a d d i t i o n we  the  shall  demonstrate t h a t the c a t h o d i c F a r a d a i c r e a c t i o n i n v o l v e s d e p o s i t i o n of A l and/or Ca s u c h t h a t some A l d i s s o l v e s i n the i r o n and  Ca  and  Al dis-  s o l v e i n the s l a g .  4.2  Previous  e l e c t r o c h e m i c a l work  There have been v e r y few  s t u d i e s i n which the  phenomena of l i q u i d m e t a l e l e c t r o d e s  99  electrochemical  i n slags at r e l a t i v e l y high  tempera-  100  t u r e s have been i n v e s t i g a t e d .  In the case of c h l o r i d e m e l t s (22), i t  has been shown t h a t t h e r e e x i s t s c l e a r evidence f o r e l e c t r o c h e m i c a l t i o n s i n w h i c h the slow step i s the a d s o r p t i o n T h i s i s an a c t i v a t i o n p r o c e s s sec).  At h i g h e r  and  has  of a complex  Shantarin  temperatures t h e r e are o n l y a few  (23)  ted i r o n s u r f a c e s  s t u d i e d the anodic  i n oxide melts and,  species.  a v e r y s h o r t t r a n s i t i o n time (< 1 p studies involving  the steady s t a t e p o l a r i z a t i o n of a c a r b o n - s a t u r a t e d face.  reac-  iron/oxide slag  inter-  p o l a r i z a t i o n of carbon s a t u r a -  a l t h o u g h he  d i d not f i n d  the n a t u r e  of the e l e c t r o c h e m i c a l r e a c t i o n s , he mentioned t h a t the anodic d i s s o l u t i o n 2+ of Fe  to g i v e Fe  ions i n the s l a g was  e f f i c i e n c i e s , and was interface.  assumed to l e a d to c o n c e n t r a t i o n  Other workers  (24)  saturated s t e e l s i n various cerned w i t h a l l o y content King  (16)  nique, and  f l u o r i d e - o x i d e s l a g s but  anodes.  at the e l e c t r o d e .  Gosh  They employed a g a l v a n o s t a t i c p u l s e  tech-  In another case,  found t h a t an important aspect  i n o x i d e - f r e e and  of the r e a c t i o n  evolution  the c o n c e n t r a t i o n  s t u d i e d i n the temperature range 500  n i c k e l oxide  polari-  saturated  - 600°C (25).  l a y e r formed on the metal e l e c t r o d e s u r f a c e .  Far  It  c h a r a c t e r i s t i c s i n the  the s e m i - c o n d u c t i n g n a t u r e of the p a s s i v e  temperature s t u d i e s , the aim has  and  sili-  f l u o r i d e m e l t s was  tion steps, while  carbon  i o n s from l i t h i u m  n i c k e l surfaces  c o n t a i n i n g m e l t s was  the  they were m a i n l y con-  z a t i o n of i r o n and  oxide  of  i n c r e a s e s r e s u l t i n g from e l e c t r o l y s i s . k i n e t i c s of oxide  current  p o l a r i z a t i o n at  observed e l e c t r o d e p o l a r i z a t i o n phenomena produced by  of oxygen gas  was  considerable  s t u d i e d the c a t h o d i c processes  s t u d i e d the d i s c h a r g e  c a t e m e l t s on p l a t i n u m  accompanied by  the c a s e of most  low  been to d e f i n e the e l e c t r o c h e m i c a l  e l i m i n a t i n g c o n c e n t r a t i o n p o l a r i z a t i o n from the  oxide  reac-  cell.  101  T h i s means that a l l these experiments had cm.  ) current d e n s i t i e s .  been s t u d i e d i n f u s e d s a l t s  to be  c a r r i e d out a t low  (<1  However, i n the case where p o l a r o g r a p h y  has  (26), the system has  A.  been found to f o l l o w  c l o s e l y the r u l i n g e q u a t i o n s d e r i v e d f o r analogous aqueous s i t u a t i o n s under s i m i l a r hydrodynamic c o n t r o l .  4.3  The  Anodic p o l a r i z a t i o n of pure i r o n i n A ^ O ^  anodic  shown i n F i g . 8. cally  gether w i t h  2  - A^O^  slags i s  These p o l a r i z a t i o n curves can be r e p r e s e n t e d  i n F i g . 30.  A of the anodic  b e h a v i o u r of pure i r o n i n C a F  slags  The  curve has  schemati-  t h r e e s e c t i o n s denoted A, B and C.  p o l a r i z a t i o n curves f i t s  the observed l o n g times (.5  an e x p o n e n t i a l ->• 5 sec.)  Part  form which, t o -  required  to e s t a b l i s h  steady s t a t e p o l a r i z a t i o n , i m p l i e s t h a t the mechanism i s a d i f f u s i o n t r o l l e d process, were o b t a i n e d  l e a d i n g to a l i m i t i n g c u r r e n t d e n s i t y , i n  from the curves f o r 1,  5, and  p o l a t i n g the s e c t i o n s A of the c u r v e . i n g to the e q u a t i o n  n  The  10 wt.  % A^O^  V a l u e s of i ^ s l a g s by  extra-  curves were then p l o t t e d  accord-  (1-14):  d  as shown i n F i g . 31.  =  ^  • In ^  — —  )  Values of n between 1 and  (4-1)  0.1  are o b t a i n e d  which  seems to i n d i c a t e that the e l e c t r o c h e m i c a l mechanism r e s p o n s i b l e f o r t i o n A i s indeed  con-  a diffusion limited reaction.  i n v o l v e the d e p l e t i o n of a d i l u t e i o n i c s p e c i e s near the e l e c t r o d e to such an extent  Such p r o c e s s e s  sec-  generally  i n the s o l v e n t e l e c t r o l y t e  that the r a t e of d i f f u s i o n of  this  102  F i g u r e 31.  A p p l i c a t i o n of a l i m i t i n g c u r r e n t d e n s i t y law to anodic p o l a r i z a t i o n of pure i r o n i n CaF + Al 0 slags.  104  species  towards the e l e c t r o d e equals  the e l e c t r o d e .  t h e r a t e o f removal o f the i o n s a t  Such a s i t u a t i o n i s u n l i k e l y i n t h i s fused  salt  system  because the s o l v e n t i s b e i n g e l e c t r o l y z e d ( t h e r e i s no s o l u t e t o be d e p l e ted) .  The s i m p l e s t anodic  Fe  g  Fe  process  2 +  i n t h i s case i s the r e a c t i o n  + 2e"  (4-2)  which w i l l not l e a d to s u c h a l i m i t i n g law (14).  I t i s , however, p o s s i b l e  2+ t h a t the c o r r o s i o n p r o d u c t , Fe  , could  l e a d t o s a t u r a t i o n o f the s l a g a t  the e l e c t r o d e i n t e r f a c e , and t h a t the measured e l e c t r o d e p o t e n t i a l would therefore reach  some maximum v a l u e which would be determined by the e q u i -  l i b r i u m e l e c t r o d e p o t e n t i a l of the Fe/FeO i n t e r f a c e . obtained  The v a l u e s  of n  from F i g . 31 a r e then simply a f o r t u i t o u s r e p r e s e n t a t i o n o f the 2+  exponential  d i f f u s i o n gradient  o f Fe  e x i s t i n g i n the s l a g a t the e l e c -  trode i n t e r f a c e a t c u r r e n t d e n s i t i e s too low to s a t u r a t e the i n t e r f a c e . I t i s therefore reasonable  t o expect the v a l u e s  oxide  not occur  content  which should  t i o n i n v o l v i n g oxide  i o n s was t a k i n g  o f n to v a r y w i t h  slag  i f a single diffusion limited  reac-  place.  When the c u r r e n t d e n s i t y a t the e l e c t r o d e i s s u f f i c i e n t l y s a t u r a t i o n of the i n t e r f a c e i n the anodic and  the curves enter  c o r r o s i o n product takes  s e c t i o n B on the schematic p l o t , F i g . 30.  high,  place,  It is  n e c e s s a r y to s t r e s s t h a t t h e p o t e n t i a l d i f f e r e n c e measured between the working and r e f e r e n c e  e l e c t r o d e s i s e s t a b l i s h e d by the p o t e n t i a l e x i s t i n g  a t the i r o n e l e c t r o d e s u r f a c e .  Thus a h i g h e r  current density i n section  B o f a g i v e n curve w i l l o n l y produce a t h i c k e r s a t u r a t e d not a f f e c t  l a y e r , and w i l l  t h e measured p o t e n t i a l , b u t t h i s i s t r u e o n l y i f the s a t u r a t e d  l a y e r remains i o n i c i n n a t u r e ,  and does not become an e l e c t r o n i c c o n d u c t o r .  105  I f the s l a g remains i o n i c , t h e F a r a d a i c process at the e l e c t r o d e s u r f a c e . Faradaic process  I f t h e anodic  continues  t o take p l a c e  c o r r o s i o n o f i r o n i s indeed the  r e s p o n s i b l e f o r these r e s u l t s , the r a t e at which concen-  t r a t i o n p o l a r i z a t i o n would a r i s e i n the s l a g due t o the e s t a b l i s h m e n t a steady s t a t e c o n c e n t r a t i o n  g r a d i e n t , w i l l depend on the e l e c t r o d e  d e n s i t y and on the s l a g c o m p o s i t i o n  trode i n a Ca F  layer.  c u r r e n t of 600 mA.  slag.  Anodic s a t u r a t i o n began a t a  2 on an e l e c t r o d e whose t o t a l exposed area was 2.75 cm .  Approximately 3.2 s e c . was r e q u i r e d  to s a t u r a t e the s u r f a c e and the coulom-  b i c d e n s i t y was t h e r e f o r e 0.70 c o u l . cm.  -2  Fe •> Fe  F o r the r e a c t i o n  2+  + 2e  n = 2, and the number of moles o f i r o n corroded moles, cm.  -2  , which i s e q u i v a l e n t  to 2.6 x 10  s a t u r a t i o n s o l u b i l i t y o f FeO i n low oxide  -4  CaF  2  i s equal  d e n s i t y of pure C a F  s l a g s i s low and f o r t h i s  change i n d e n s i t y saturated  a t 1500°C. i s 2.54 gm. cm.  2  o f 1 wt. % i s used..  _3  ( i n c r e a s e ) due t o s a t u r a t i o n w i t h  s l a g per u n i t area w i l l  to 3.6 x 10 ^  gm. o f FeO per s e c . The  s l a g w i l l be l e s s than 5 wt. %, and an assumed v a l u e The  of t h i s  T h i s c a l c u l a t i o n r e f e r s t o a pure i r o n e l e c -  + 2.5 wt. % A ^ O ^  2  current  (oxide type and c o n t e n t ) .  I t i s of i n t e r e s t to c a l c u l a t e the approximate t h i c k n e s s proposed FeO s a t u r a t e d  of  then be 2.6 x 10  and n e g l e c t i n g the  FeO, the weight o f gm. which has a v o l -  - 2 - 3 ume o f 1.03 x 10 all  cm.  .  This l a t t e r value  contains  t h e assumption t h a t  the FeO produced remains i n the v i c i n i t y of t h e e l e c t r o d e  Therefore  the t h i c k n e s s  o f t h e FeO s a t u r a t e d  surface.  l a y e r at a current  density  which i s h i g h enough to produce s a t u r a t i o n w i l l be a p p r o x i m a t e l y 100 m i crons t h i c k .  106  I t i s a l s o of i n t e r e s t t o c a l c u l a t e the t h i c k n e s s s i o n boundary l a y e r i n the s m a l l s c a l e system.  Levich  of a d i f f u -  (27) d e r i v e d an  f o r the d i f f u s i o n boundary l a y e r t h i c k n e s s , < 5 , f o r the case  expression  of c o n v e c t i v e  diffusion.  The d r i v i n g f o r c e f o r mass t r a n s p o r t i s the  d e n s i t y g r a d i e n t produced by movement o f e l e c t r o - a c t i v e s p e c i e s electrode surface.  The e q u a t i o n  case of d i f f u s i o n o f F a r a d a i c  so d e r i v e d should  a l s o apply  t o the  to the  r e a c t i o n p r o d u c t s away from the e l e c t r o d e  s u r f a c e i n t o the b u l k of the s l a g , and i s :  6  =  x  1/4 —  :  —  (4-3)  where x  -  d i s t a n c e from the upper edge o f the p l a t e  Pr -  P r a n d t l No. of the s l a g  g  -  a c c e l e r a t i o n due t o g r a v i t y  C  -  concentration of d i f f u s i n g species a t the p l a t e s u r f a c e  v  -  Kinematic v i s c o s i t y o f the s l a g .  This equation  a p p l i e s to n a t u r a l  convection  at a planar  elec-  trode s u r f a c e , however i n the case o f c y l i n d r i c a l d i f f u s i o n the d i v e r g e n c e of the f l u x would tend By  to d e c r e a s e the d i f f u s i o n boundary l a y e r  assuming that the e l e c t r o d e s u r f a c e i s p l a n a r ,  i n g the f o l l o w i n g  thickness.  <5 can be c a l c u l a t e d u s -  data: x  =  Pr  =  C  =  1 cm. 0.12 w h i c h i s the P r a n d t l No. of l i q u i d NaCl (28) -2 -3 2.54 x 10 gm. cm. f o r 1 wt. % FeO s a t u r a t i o n i n low oxide CaF„ s l a g s  107  p_ p  v  and  6  _  0.6 p o i s e _3 2.54 gm.cm.  =  (0.7K.12) <5 =  =  -1 0.24 cm. s e c .  C4-4)  1/4,980 • 0.0254.1/4 4 • 0.058 L  ;  0.757 cm.  D i f f u s i o n boundary l a y e r t h i c k n e s s e s  i n natural convection are  found t o be s u b s t a n t i a l l y g r e a t e r than i n f o r c e d c o n v e c t i o n , of the d i f f u s i o n a l f l u x e s a r e found t o be c o r r e s p o n d i n g l y  and the v a l u e s  smaller.  The  c a l c u l a t e d v a l u e o f <5 i s much l a r g e r than would be found i n aqueous systems I t i s q u i t e l i k e l y that the a c t u a l d i f f u s i o n boundary l a y e r t h i c k n e s s i n the s m a l l s c a l e system i s s m a l l e r than t h i s c a l c u l a t e d v a l u e , being  determined by thermal c o n v e c t i o n  i t s thickness  i n the s l a g .  There i s no phase diagram i n f o r m a t i o n a v a i l a b l e on the CaF -A1„0 -FeO system, but i n analogy w i t h CaF^  - CaO - FeO system  the CaF^ - A ^ O ^  - MnO system  (30) , ( t h e p a r t i a l phase diagrams of which a r e  g i v e n i n F i g . 32, and show the FeO s o l u b i l i t y and  to be s t r o n g l y  composition  temperature dependent), the system i s b e l i e v e d to e x h i b i t a l a r g e m i a -  cibility is  (29) and the  gap.  Hence the c o r r o s i o n r a t e a t which s u r f a c e s a t u r a t i o n i n FeO  a t t a i n e d would be expected to depend s t r o n g l y on s l a g c o m p o s i t i o n  temperature.  Once the s l a g of t h e anodic  and  e l e c t r o d e s u r f a c e has become  s a t u r a t e d i n FeO, the p o t e n t i a l o f the s u r f a c e may be r e p r e s e n t e d by Fe(s) + 0  -> FeO (slag  + 2e  T h i s p o t e n t i a l w i l l be a p p r o x i m a t e l y s i t i o n as i s seen' from the approximately 8.  (4-5)  sat'n.)  independent o f s l a g compa-  constant  plateau values  i n Fig.  The o v e r a l l p o t e n t i a l d i f f e r e n c e which i s measured between the r e f e r -  ence and working e l e c t r o d e w i l l ct  then be r e p r e s e n t e d  by the v i r t u a l  reac-  CaO  F i g u r e 32.  P a r t i a l phase diagrams o f the system CaF -Ca0-Fe0, o  108  109  tion: Fe  which i s obtained  t c  + CO S  8  +  g r  (FeO) (slag sat'n)  by adding r e a c t i o n s (2-6)  check (4-6) w i t h an exact  and  (4-6)  (4-5)U;  It is d i f f i c u l t  c a l c u l a t i o n because the carbon monoxide p r e s -  s u r e at the r e f e r e n c e e l e c t r o d e i s unknown. t h a t the e f f e c t i v e p r e s s u r e  of CO  However, i f i t i s assumed  at the r e f e r e n c e  e l e c t r o d e i s one  mosphere, the p o t e n t i a l d i f f e r e n c e e x i s t i n g between the r e f e r e n c e saturated  iron electrode  to  (a„ _ at the i r o n s u r f a c e i s u n i t y ) FeO  at-  and  can be c a l -  J  c u l a t e d u s i n g standard temperature of 1480°C  thermochemical d a t a  2  and  a t the measured s l a g  (1753°K).  C(gr) + - | o ( g ) •+ C0(g)  Fe(6)  (31)  + -|- 0 ( g ) 2  A F° = 63,540 c a l . (1753°K)  (4-7)  + " F e 0 " ( l ) A F° = -36,496 c a l . (1753°K)  (4-8)  AF° = + 27,044 c a l (1753°K) = -nFE°  so  E° = -586  mV.  where n = 2 e q u i v a l e n t , mole and  \  F = 23,060 c a l . • v o l t  T h i s c a l c u l a t e d v a l u e of 586 served v a l u e of a p p r o x i m a t e l y 500  mV.  equivalent.  mV.  i s reasonably  c l o s e to the  measured as the b e g i n n i n g  of  ob-  sections  B i n F i g . 8. At h i g h e r  c u r r e n t d e n s i t i e s i n s e c t i o n s B, i t i s thought  the r a t e of e l e c t r o n t r a n s f e r c o u l d be accommodated by r a t i o of Fe  3 + 2 + to Fe Fe  by  increasing  that the  the r e a c t i o n * -* F e  J  + e  (4-9)  1 10  i n the s a t u r a t e d  s l a g l a y e r , which w i l l have the e f f e c t of d i s p l a c i n g the  measured p o t e n t i a l to a h i g h e r v a l u e . sections B i n Figure  8,  T h i s i s seen as the s l o p e of  and w i l l a g a i n be  As mentioned p r e v i o u s l y , i t was  found to be  s c a l e s t u d i e s i n h i g h oxide s l a g s due t r o d e and due  density  observed i f we (1^)  impossible  to A ^ O ^  against  (10 wt.  T h i s seems to i n d i c a t e that as the A^O^  % A^O^)  .  of the l i m i t i n g  i n the s l a g as shown i n content  s a t u r a t i o n of the anodic  current Fig.  33.  of the s l a g i s i n c r e a s e d ,  cance of t h i s i s that as long as anodic m e t a l - s l a g  FeO  concentrations  None the l e s s , a trend  i p w i l l a s y m t o t i c a l l y approach a " l i m i t i n g " v a l u e .  s i t i e s are m a i n t a i n e d below the i  small  p r e c i p i t a t i o n on the e l e c -  p l o t the e x t r a p o l a t e d v a l u e s the mole % of A ^ O ^  composition.  to c a r r y out  to the r i s i n g l i q u i d u s temperature a t kl^O^  above the e u t e c t i c composition can be  independent of s l a g  the  The  practical  signifi-  i n t e r f a c e c u r r e n t den-  curve d u r i n g D.C.  e l e c t r o s l a g melting,  s u r f a c e w i l l not o c c u r ,  and  o x i d a t i o n of  the  m e t a l w i l l be h e l d to a minimum. I t was to see  considered  i f t h e r e were a c t i v a t i o n mechanisms p r e s e n t which would have v e r y  f a s t r i s e and i t would be  decay time at these temperatures.  anodic  To  check t h i s ,  have a t t r i b u t e d to  an  seen as the i n i t i a l r i s e on the o s c i l l o s c o p e  the "IR"  e l e c t r o d e i n a CaF^  + 2.5  c u r r e n t I , as shown i n F i g . 34. cludes  I f such a decay e x i s t e d ,  i n c l u d e d i n the p o t e n t i a l d i f f e r e n c e we  ohmic p o t e n t i a l g r a d i e n t trace.  n e c e s s a r y to examine the p o l a r i z a t i o n r e s u l t s  the presence of any  p o r t i o n of the r i s e times f o r a wt.  % Al^O^  The  s t r a i g h t l i n e b e h a v i o u r of the p l o t  a c t i v a t i o n process  s l a g was  Ferrovac-E  p l o t t e d against  i n these systems  (16).  the ex-  Ill  113  I t i s thought t h a t towards the end  of s e c t i o n s B i n the  p o l a r i z a t i o n curves of F i g . 8, when the r a t e of e l e c t r o n t r a n s f e r 3+ become too h i g h to be gas  accommodated by  i n c r e a s i n g the Fe  e v o l u t i o n at the i n t e r f a c e i s a c h i e v e d 0 ~ 2  by  anodic has  2+ /Fe  ratio,  the r e a c t i o n s :  + 0* + 2e~  (4-10)  20* + 0 ( g )  (4-11)  2  I t was  shown i n F i g . 14 t h a t anodic  p o l a r i z a t i o n at  relatively  h i g h c u r r e n t d e n s i t i e s produces oxygen i n the b u l k of the e l e c t r o d e . 35 i s the o s c i l l o s c o p e t r a c e of a h i g h c u r r e n t d e n s i t y anodic Ferrovac-E  e l e c t r o d e i n a low oxide  r a t e d b e h a v i o u r of the s t e a d y - s t a t e t i c of gas I t was  content  s l a g (250  ppm.  Fig.  p u l s e on  CaO).  The  a ser-  p o r t i o n of the curve i s c h a r a c t e r i s -  e v o l u t i o n on m e t a l e l e c t r o d e s as observed by Gosh and K i n g  observed that anodic  that  (16).  e l e c t r o d e s showed s i g n i f i c a n t d i s s o l u t i o n when  p o l a r i z e d a t h i g h c u r r e n t d e n s i t y i n s e c t i o n B, but not when p o l a r i z e d i n s e c t i o n A. FeO  T h i s b e h a v i o u r i s expected as a r e s u l t of f o r m a t i o n . o f  on the s o l i d  e l e c t r o d e s u r f a c e , the FeO  (^ 1380°C) than the iron'. the f o l l o w i n g mechanism. s a t u r a t e d w i t h FeO, the s o l i d  enough, FeO  a lower m e l t i n g  point  o x i d a t i o n of the e l e c t r o d e i s e x p l a i n e d  D u r i n g the time when the e l e c t r o d e s u r f a c e i s  t h e r e w i l l be s i g n i f i c a n t d i f f u s i o n of oxygen i n t o  e l e c t r o d e d e s p i t e the f a c t  d i c a l l y corroded.  producing  The  having  liquid  that the i r o n i t s e l f  When the oxygen content  forms as a l i q u i d w h i c h f a l l s  i s being  ano-  of the s u r f a c e l a y e r i s h i g h away from the s o l i d  the observed e l e c t r o d e d i s s o l u t i o n .  electrode  by  l i l t  0 . 5 s e c /div time  Figure  35.  A n o d i c p u l s e on p u r e i r o n i n C a F + 250 ppm CaO. The e l e c t r o d e p o l a r i z a t i o n i s seen t o t r a n s f e r from the s a t u r a t i o n c o n d i t i o n t o an a r c , a p o l a r i z a t i o n c o n t i n u e s i = 500 ma.cm o 2  r  115  4.3.1  Apparent t r a n s i t i o n The  time  type of anodic p o l a r i z a t i o n curves r e s u l t i n g from a s i n g l e  p u l s e t e s t as shown i n F i g . 12 were found the m i d d l e  a t v a l u e s of c u r r e n t l y i n g i n  r e g i o n of s e c t i o n s B i n F i g . 8.  At h i g h e r and  lower  current  d e n s i t i e s , the i n d i v i d u a l t r a c e s e x h i b i t e d the u s u a l e x p o n e n t i a l r i s e decay  behaviour. The  shape of the g a l v a n o s t a t i c p o l a r i z a t i o n curve shown i n F i g .  12 i s q u a l i t a t i v e l y two  and  the same as t h a t expected  slow, d i f f u s i o n c o n t r o l l e d p r o c e s s e s  f o r a t r a n s i t i o n between  as d e s c r i b e d i n e q u a t i o n  T h e r e f o r e a n a l y s i s of these curves a c c o r d i n g to the  (1-28).  relationship  .1/2 1/2 . , RT .. T n, - A + — In d nF l/2 t  should i n d i c a t e whether or not the p o l a r i z a t i o n mechanism b e i n g s t u d i e d was  indeed a d i f f u s i o n c o n t r o l l e d r e a c t i o n i n which some i o n i c  i n the s l a g was  b e i n g d e p l e t e d at the e l e c t r o d e s u r f a c e . A,  curves were a n a l y s e d f o r a p p l i e d c u r r e n t s of 0.5 v a l u e s of the c u r r e n t d e n s i t y ( i _ ) ,  2A.,  species  The  transition  and  3A.  the t r a n s i t i o n time T, and  the  and  the  pro-  1/2 duct i T Q  I(A.)  are g i v e n below. i (A. cm. o  2  )  T (sec.)  i x ^ o 1  (A.  2  0.5  0.18;  8.5  2.92  2.0  0.73  1.3  1.14  3.0  1.09  0.8  0.89  I t i s apparent  sec."^ ) 2  t h a t the t r a n s i t i o n time does indeed d e c r e a s e 1/2  the c u r r e n t d e n s i t y i n c r e a s e s , but  the product  i  Q  x  as  i s not constant f o r  a g i v e n s l a g , a c o n d i t i o n w h l c h m u s t . b e s a t i s f i e d f o r such a mechanism.  116  The  d i f f u s i o n overvoltage  i s p l o t t e d according T  to e q u a t i o n l / 2 _ 1/2  t h r e e a p p l i e d c u r r e n t s i n F i g . 36 [ f ( x ) = I n  ]  (1-28) f o r the  and a l t h o u g h  the p o i n t s f o r each c u r r e n t d e n s i t y l i e on s t r a i g h t l i n e s , the s l o p e s i n c r e a s e w i t h the c u r r e n t d e n s i t y . Values o f n can be c a l c u l a t e d by RT e q u a t i n g the measured s l o p e t o and a r e g i v e n below. I(A.)  n (gm. equiv../gm. mole)  0.5  5.2  2.0  3.5  3.0  3.0  The v a l u e s  of n so o b t a i n e d  a r e not unreasonable i n magnitude,  but v a r y over a range which i s much wider than would be expected f o r a correct analysis. product be  i x o  1/2  concluded  Because the v a l u e s  of n and, a l s o , the v a l u e s  a r e found t o be f u n c t i o n s of the c u r r e n t d e n s i t y , i t must  t h a t the observed t r a n s i t i o n curves  do not a r i s e from  s i o n a l d e p l e t i o n o f the s l a g i n some i o n i c s p e c i e s . with  of the  the p r e v i o u s l y proposed mechanism o f anodic  This finding  diffuagrees  c o r r o s i o n of i r o n which  i s a s i n g l e step  d i f f u s i o n process  and would n o t e x h i b i t a t r a n s i t i o n  time b e h a v i o u r .  The apparent t r a n s i t i o n times must t h e r e f o r e be  explained  i n another way. Such an e x p l a n a t i o n can be found i f we examine F i g u r e s 32 and 37.  F i g . 37 i s the o s c i l l o s c o p e t r a c e o b t a i n e d when r e p e t i t i v e  p u l s e s a r e a p p l i e d i n the " t r a n s i t i o n " r e g i o n , at a frequency t a i n s p a r t of t h e ' s a t u r a t e d  FeO l a y e r produced by the p r e v i o u s  anodic  w h i c h mainpulse.  Here i t i s observed t h a t the i n f l e c t i o n i n the p o l a r i z a t i o n curve i s  117  F i g u r e 36.  A p p l i c a t i o n o f t r a n s i t i o n time law to anodic p o l a r i z a t i o n of pure i r o n i n CaF + 2.5 wt.% A1 0 . 2 2  3  118  7  0.2v/|_ div.  1 A ~\ J  Figure  37.  J  L  I  L  I  J  I  I  I  1  2 sec/div. time  Successive anodic pulses a p p l i e d to pure i r o n i n C a F + 2.5 w t . % l ° 3 > s h o w i n g t h e d i s a p p e a r a n c e o f t h e a p p a r e n t t r a n s i t i o n t i m e whe s u r f a c e s a t u r a t i o n i s r e t a i n e d between p u l s e s . A  2  2  119  g r a d u a l l y removed with, s u c c e s s i v e p u l s e s . i t i o n curves a r e t h e r e s u l t o f c h e m i c a l chemical  ones, and can be e x p l a i n e d  Fe -* Fe  2+  T h i s i n d i c a t e s t h a t the t r a n s -  phenomena r a t h e r than e l e c t r o -  using  the s i n g l e e l e c t r o d e r e a c t i o n .  + 2e  I f we examine t h e p a r t i a l phase diagrams of the CaF^-CaO-FeO system ( F i g . 3 2 ) , we see a two l i q u i d r e g i o n i n a c o m p o s i t i o n  range  through w h i c h the s l a g a t the e l e c t r o d e s u r f a c e must pass i n order t o 2+ achieve curve,  Fe  saturation.  Therefore  the f i r s t p o r t i o n o f the t r a n s i t i o n  shown as l i n e A-A' i n F i g . 32, r e p r e s e n t s  the simple d i s s o l u t i o n  2+ of Fe  , while  the i n f l e c t i o n p o i n t i n d i c a t e s the b e g i n n i n g 2+ s l a g system as Fe s a t u r a t i o n i s approached.  liquid  4.4  The CaF  2  o f the two  Anodic p o l a r i z a t i o n of pure i r o n i n CaO s l a g s  anodic  p o l a r i z a t i o n b e h a v i o u r of pure i r o n e l e c t r o d e s i n  - CaO s l a g s i s shown i n F i g . 10.  Comparisons of F i g u r e s  8 and 10  show t h a t the p l a t e a u s e c t i o n B i s much more pronounced i n Al^O^  contain-  i n g s l a g s than i n CaO c o n t a i n i n g s l a g s where the o v e r p o t e n t i a l a t e q u i v a l e n t c u r r e n t d e n s i t y i s seen t o d e c r e a s e more r a p i d l y w i t h CaO c o n t e n t . of C a F  2  T h i s ,-is p r o b a b l y  - CaO l i q u i d s  compared w i t h C a F  2  t h e combined r e s u l t of t h e lower v i s c o s i t y  C32) and t h e i r h i g h e r  solubility  - A l ^ ^ l i q u i d s at equivalent  a l s o observed t h a t anodic d e n s i t i e s and CaO contents  increasing  oxide  (30) of F e content.  i r o n e l e c t r o d e s m e l t e d at much lower than they d i d i n a n . e q u i v a l e n t  A1„0„  2 +  , as  I t was current content.  120  One can s p e c u l a t e , from  the r e s u l t s i n F i g . 10, t h a t an ESR s l a g  con-  t a i n i n g 25 wt. % CaO, a t a g i v e n c u r r e n t d e n s i t y , would n o t s a t u r a t e 2+ a m e l t i n g i r o n e l e c t r o d e i n Fe kl^O  content would.  lower  oxygen c o n t e n t when melted  of  , whereas an ESR s l a g of the same  T h i s might r e s u l t i n the f i n a l m e t a l h a v i n g a through  the CaO c o n t a i n i n g s l a g because  the lower o x i d a t i o n r a t e at the anodic s u r f a c e .  4.5  In  C a t h o d i c p o l a r i z a t i o n of pure i r o n i n ESR s l a g s  order t o p r o v i d e a F a r a d a i c mechanism a t the s l a g / m e t a l  c a t h o d i c i n t e r f a c e , we may p o s t u l a t e any combination  of the f o l l o w i n g  reactions: Ca  2 +  + 2e~ -> Ca*  Ca* + (Ca°) . slag  (4-13)  Ca* -> Ca°g  (4-14)  3+ Al  * + 3e ^ A l  A l * -> [ A l ]  for  (4-15) (4-16)  p E  A l * + (Al°) . slag  (4-17)  A l * ->  (4-18)  Al°  £  the a p p r o p r i a t e s l a g c a t i o n In  (4-12)  composition.  the case of t h e o v e r a l l r e a c t i o n  t i a l seen by t h e measuring c i r c u i t (CaO)  slag  (4-12) and (4-14), the p o t e n -  i s r e p r e s e n t e d by t h e e q u a t i o n :  + C -y CO + C a , . gr g (g)  (4-19)  S i m i l a r l y , the p o t e n t i a l seen a t an e l e c t r o d e which, i s c a t h o d i c a l l y p o l a r i z i n g due t o r e a c t i o n s (4-15) and (4-16) i s r e p r e s e n t e d by the  121  reaction: (Al.O,) . + 3 C + 3CO + 2[A1]_, 2 3 slag gr g Fe  (4-  J  giving potentials:  and  AE°. ., 4-19  = 800  mV  E°. . 4-20  = 400  mV  at 1812°K, f o r R a o u l t i a n standard  states.  However, as the observed  t i o n must i n v o l v e s o l u t i o n of A l i n both the m e t a l and i n the s l a g , we would not tion  reac-  the s l a g and  expect to see these p o t e n t i a l s as the  Ca  polariza-  values. The  form of the c a t h o d i c r\/ln  i  curves  at the lower c u r r e n t  o d e n s i t i e s i s again products  t h a t to be expected from the d i f f u s i o n of r e a c t i o n  away from the c a t h o d i c i n t e r f a c e .  One  might expect to observe  a l i m i t i n g p o t e n t i a l at i n t e r f a c e s a t u r a t i o n but it  d i d i n the anodic  t h i s w i l l not  occur  as  cases because t h e r e i s no e l e c t r o c h e m i c a l mechanism  by which the i n t e r f a c e can remain s a t u r a t e d as the c u r r e n t d e n s i t y i s i n c r e a s e d above the i n i t i a l  s a t u r a t i o n current density.  r e n t d e n s i t i e s , i t i s b e l i e v e d t h a t the c a l c i u m gas  At h i g h e r  cur-  produced forms a  s o f t a r c which i s v e r y s t a b l e and  the i n c r e a s e i n measured  polarization  r e s u l t s from the a r c r e s i s t a n c e .  T h i s concept i s supported  by F i g . 38  which i s a p l o t of the c a t h o d i c o v e r p o t e n t i a l a g a i n s t the working c u r r e n t of a pure i r o n e l e c t r o d e i n a CaT?^ + 1 wt.  comes l i n e a r a f t e r a c u r r e n t of 5 A. t i c of an arc p r o c e s s .  CI.8 A. cm.  In s l a g s w i t h a h i g h e r  t h a t t h i s v e r y s t a b l e s o f t a r c does not d e n s i t i e s are a t t a i n e d .  % A^O^ -2  T h i s i s one way  slag.  The  curve  be-  ] which i s c h a r a c t e r i s -  oxide content,  i n i t i a t e u n t i l higher i n which the c a t h o d i c  i t appears current polariza-  123  t i o n curves  can be r e a s o n a b l y  explained.  Another p o s s i b l e e x p l a n a t i o n of the l i n e a r b e h a v i o u r shown i n F i g . 38 i s that the c u r r e n t d e n s i t y , i ,  and  Q  c o n v e c t i o n boundary l a y e r  are r e l a t e d In such a f a s h i o n t h a t a l i n e a r o v e r p o t e n t i a l - c u r r e n t d e n s i t y p l o t i s obtained. p o t e n t i a l s do  T h i s i s based on the assumption t h a t the measured  indeed  r e s u l t from a change i n a c t i v i t y  c i e s at the i n t e r f a c e , and of a N e r n s t achieved,  equation.  with  T h i s would apply when a s t e a d y - s t a t e  the d i f f u s i o n o v e r v o l t a g e  curved  given  been s u b s t i t u t e d f o r a c t i v i t y ) .  (concentra-  assume t h a t the  represents  the  initial  the c a t h o d i c  de-  the r e a c t i o n :  + 3e  (Al)  t h a t the l i n e a r p o r t i o n r e p r e s e n t s  sequently  s p e c i e s at  c u r r e n t passage  I f we  p o r t i o n of the p l o t g i v e n i n F i g . 39,  Al  condition i s  by  of the d i f f u s i n g  c i s the c o n c e n t r a t i o n b e f o r e  p o s i t i o n of A l i n the i r o n by  and  spe-  RT . c ( i ) — In — nF c  where c ( i ) i s the imposed c o n c e n t r a t i o n  t i o n has  of a d e p o s i t e d  t h a t t h i s p o t e n t i a l can be d e s c r i b e d i n terms  _ n., = d  i n t e r f a c e , and  over-  Fe  the d e p o s i t i o n of Ca which sub-  d i s s o l v e s i n the s l a g , the c u r r e n t d e n s i t y at which the  curve 3+  becomes l i n e a r should be the l i m i t i n g d i f f u s i o n c u r r e n t d e n s i t y of A l i o n s i n the s l a g .  The  lated using equation  when i =  c =  0.  d i f f u s i o n boundary l a y e r can t h e r e f o r e he (1-11).  1 = D . nF  Cc 6  c)  calcu-  124  For the anodic The  r e a c t i o n g i v e n above, n = 3 and i ^ = 1.8 A.  slag contains  1 wt. % A ^ O ^  cm.  -2  (M.Wt. = 102 gm) , and the den-  -3 -4 s i t y of the s l a g i s a p p r o x i m a t e l y 2.6 gm.cm. , t h e r e f o r e c = 2.55 x 10 moles,  cm. F = 96,500 c o u l . gm. D = 8.5 x 10  to be 0.347 cm.  Therefore  t i o n at a cathodic reasonable  value  -5  cm.  2  equiv. sec.  -1  (33) and 6 can be c a l c u l a t e d  the d i f f u s i o n boundary l a y e r f o r A ^ O ^  deple-  i r o n s u r f a c e i s a p p r o x i m a t e l y 3500 microns t h i c k , a  f o r s u c h a system.  When the e l e c t r o d e c u r r e n t  density  2+ exceeds the l i m i t i n g  c u r r e n t d e n s i t y f o r A l d e p o s i t i o n , Ca  i o n s must  d e p o s i t , and the l i n e a r p o r t i o n of the curve must c o r r e s p o n d , i n the absence of a s t a b l e s o f t a r c , to the r e a c t i o n : Ca  + 2e~ •> [Ca] ,  2 +  slag where the measured o v e r p o t e n t i a l i s determined by the Ca a c t i v i t y a t the electrode i n t e r f a c e .  For Nernst type b e h a v i o u r ,  In c, and from e q u a t i o n  (1-11),  i s proportional t o . c  i i s p r o p o r t i o n a l to -r (c  =0).  There-  f o r e i and 6 must be r e l a t e d i n such a way that the c o n c e n t r a t i o n ,  c, of  Ca i n the s l a g a t the i n t e r f a c e , v a r i e s to produce the r e l a t i o n s h i p : i and  a  In c  therefore n. a In c. d Although i t i s d i f f i c u l t  electrolysis, i t i s felt  to c o n c e i v e o f s u c h b e h a v i o u r  that t h i s e x p l a n a t i o n  i s more r e a l i s t i c  i d e a of e s t a b l i s h i n g a s o f t a r c a t these low c u r r e n t d e n s i t i e s .  during t h a t the  125  In h i g h oxide content be d e p o s i t e d  together  s l a g s c o n t a i n i n g Al^O^,  such t h a t some A l w i l l d i s s o l v e i n t h e I r o n e l e c -  trode w h i l e A l and Ca d i s s o l v e i n the s l a g . at  A l and Ca should  The f a c t  t h a t A l i s produced  a c a t h o d i c s u r f a c e was shown by- t h e e l e c t r o n m i c r o p r o b e s t u d i e s o f an  c a t h o d i c i r o n e l e c t r o d e as d i s c u s s e d i n S e c t i o n  4.6 The  (2-5).  .  P o l a r i z a t i o n of Fe-Cr a l l o y s and pure chromium anodic  and c a t h o d i c p o l a r i z a t i o n b e h a v i o u r o f AISI 430  s t a i n l e s s s t e e l e l e c t r o d e s was shown i n F i g u r e s 15 and 16. i n f o r m a t i o n can be drawn from these  curves  l e n t c u r r e n t d e n s i t y and s l a g composition,  Very  little  except t o say t h a t a t an e q u i v a the degree o f anodic  polariza-  t i o n i s l e s s on a s t a i n l e s s s t e e l e l e c t r o d e than on a pure i r o n e l e c t r o d e . No B r e g i o n s , o r p l a t e a u s were found on t h e anodic p o l a r i z a t i o n curves o f these e l e c t r o d e s .  A t low c u r r e n t d e n s i t i e s , the anodic r e a c t i o n i s  thought to be Cr + C r As  3 +  + 3e~  (4-21)  the c u r r e n t d e n s i t y i s i n c r e a s e d , and the Cr c o n c e t n r a t i o n a t the i n t e r -  f a c e d e c r e a s e s due t o p r e f e r e n t i a l anodic  corrosion, iron w i l l  3+ corrode  to produce a b u i l d up of Cr  s t a r t to  2+ and Fe  i o n s a t the e l e c t r o d e  inter-  face. The ilar  c a t h o d i c p o l a r i z a t i o n curves  t o the c a t h o d i c curves  ( F i g . 16) show a b e h a v i o u r s i m -  f o r pure i r o n shown i n F i g . 9.  T h i s would  i n d i c a t e t h a t the c a t h o d i c r e a c t i o n i s n o t a p p r e c i a b l y i n f l u e n c e d by t h e presence o f Cr as an a l l o y i n g  element i n the I r o n  matrix.  126  The 430  c o n c e n t r a t i o n g r a d i e n t of Cf at the s u r f a c e of an A I S I  s t a i n l e s s s t e e l shown i n F i g . 17 g i v e s ample e v i d e n c e  e n t i a l anodic  c o r r o s i o n of Cr from Fe-Cr a l l o y s .  As  of the p r e f e r -  indicated previous-  l y the removal of Cr took p l a c e at a c u r r e n t e f f i c i e n c y of only T h i s means t h a t even at t h i s r e l a t i v e l y low r e a c t i o n was  c u r r e n t d e n s i t y , the  f o r o n l y 10% of the c u r r e n t passage.  that Cr removal accounts  However, no p l a t e a u was  because the p o t e n t i a l s measured were e s t a b l i s h e d by Cr and Fe such t h a t the e f f e c t of the Cr was anodic p o l a r i z a t i o n curves  f a c e has  f o r the Fe -1 wt.  % Cr  %.  Upon passage of c u r r e n t ,  the Cr content  i r o n takes p l a c e c a u s i n g  As  the cur-  at the s u r f a c e and anodic  the measured p o t e n t i a l d i f f e r -  ence between the r e f e r e n c e and working e l e c t r o d e s to i n c r e a s e to Iron electrode.  the  230 mV.,  at the s u r f a c e .  r e n t passage p r o c e e d s , the Cr i s q u i c k l y d e p l e t e d  p l a t e a u v a l u e of an anodic  alloy  the concept of Cr d e p l e t i o n  to produce a p l a t e a u at approximately  v a l u e of which i s determined by  c o r r o s i o n of  plateau.  P r i o r to c u r r e n t passage, the e l e c t r o d e s u r -  a Cr c o n c e n t r a t i o n of 1 wt.  electrode polarizes  observed  c o r r o s i o n of both,  to mask the Fe  shown i n F i g . 18 can a l s o be e x p l a i n e d u s i n g at the e l e c t r o d e s u r f a c e .  chromium.  p o l a r i z a t i o n p l a t e a u e s t a b l i s h e d by  s a t u r a t i o n i n the s l a g because of the f a c t  The  anodic  accounted f o r i n the main by c o r r o s i o n of i r o n , not  One might expect to observe an anodic FeO  10%.  the  Upon c u r r e n t i n t e r r u p t i o n ,  the measured p o l a r i z a t i o n decay i s t h e r e f o r e t h a t of an i r o n  electrode  as shown by decay p o l a r i z a t i o n curve i n F i g . 18. The  anodic  p o l a r i z a t i o n curve of pure chromium CFig. 19 ) e x h i -  b i t s a p l a t e a u at a p o t e n t i a l between 200  and  300  from s a t u r a t i o n of the s l a g at the i n t e r f a c e i n Cr  mV. 3+  This plateau a r i s e s i o n s and  i s therefore  127  comparable to the p l a t e a u s found f o r anodic Al^O^  slags.  The  i r o n e l e c t r o d e s i n CaF^  p o t e n t i a l d i f f e r e n c e e x i s t i n g between the  -  graphite  3+ r e f e r e n c e e l e c t r o d e and the  the Cr  saturated surface i s represented  by  equation: 3.C,  . + Cr.O. Cgr.) (g) 2  •> 2 C r , , + 3 CO, > ^s) ^  3  (4-22)  From the f r e e e n e r g i e s g i v e n below (T = 1809°K), 3 C r , . + -| 0„ (s) 2 2  2 Cr  . + |  r  -> 3 CO, ( g )  0 (g)  and  rx.  C r 2  °3(e)  A F  °  =  1 5  mV.  T h i s c a l c u l a t e d v a l u e agrees v e r y w e l l w i t h of the  (4-24)  - 9,481 c a l .  = -34,526 c a l .  = -250  AE°  (4-23)  -*•  ^  AF°  . AF° = -194,007 c a l . (g)  the observed  value  plateau.  4.7 The  P o l a r i z a t i o n of pure n i c k e l  s m a l l s c a l e s t u d i e s c a r r i e d out on an anodic n i c k e l e l e c -  trode i n a C a F  2  + Al^O^  s l a g were not s u i t a b l e f o r p r e s e n t i n g as a p o l a r i -  z a t i o n curve because at the lower c u r r e n t d e n s i t i e s , the e l e c t r o d e would not p o l a r i z e to a steady  s t a t e v a l u e i n the time a v a i l a b l e d u r i n g an i n -  d i v i d u a l p u l s e experiment.  Steady s t a t e p o l a r i z a t i o n was  not  achieved.  -2 until  the c u r r e n t d e n s i t y was  i n c r e a s e d to 1.1  A.  scope t r a c e of t h i s t e s t i s shown i n F i g . 20, and t i o n p o t e n t i a l i s approximately reaction:  700 mV,  cm.  The  oscillo-  the observed  which should  correspond  polarizato  the  128  C, . + NiO, . -* N i , . + CO. (gr) (s) (s) (g) The  (4-25)  f r e e energy change f o r t h i s r e a c t i o n i s c a l c u l a t e d as f o l l o w s  at  1726°K: C  (gr)  2 ° 2 ^  +  Ni, .  +  i 0  The the p l a t e a u  rx  AE°  . AF° =  -21,325 c a l .  - -904  mV.  d i s c r e p a n c y between the observed and  lower than the  n i c k e l , then a c a l c u l a t e d  explained  by  I f the  to whether or not  containing  + A^O^  brium c o n d i t i o n s , dissolved  system temperature was  A l and  slags.  i n CaF^ found that  Ca  sig-  Bell  s l a g s and the  examination  were p r e s e n t e d i n F i g . i t i s soluble  Ca w i l l be  (21)  considered  i n n i c k e l , but  the  under  as long  pre-  interaction equili-  i n t e r a c t i o n between the o x i d e s s u c k that  21.  d e p o s i t e d i n the  l i q u i d n i c k e l , and  i n l i q u i d n i c k e l was  of  smaller.  e l e c t r o n m i c r o p r o b e r e s u l t s o b t a i n e d upon the  the q u e s t i o n a r i s e s as  during  assumed temperature of the m e l t i n g p o i n t AE° would a l s o be  for  time needed  p o l a r i z a t i o n i n t h i s sytem, a time  Although c a l c i u m i s i n s o l u b l e i n i r o n (34),  between CaO  calculated values  the r e l a t i v e l y l o n g  of a c a t h o d i c a l l y p o l a r i z e d n i c k e l e l e c t r o d e  sence of A ^ C ^  (4-27)  = -41,671 c a l .  h e i g h t might be  The  (4-26)  (- ^  e n t i r e system i s c o o l i n g .  nificantly  = -62,996 c a l .  s  to e s t a b l i s h the s t e a d y - s t a t e which the  AF°  NiO,  Qg)  AF° and  ->  o  2  2  (S)  CO^  as  and  alumina  i s p r e s e n t i n a s l a g , alumina w i l l r e a c t with, l i q u i d n i c k e l to a much, greater  e x t e n t than CaO.  He  stated  t h a t Ca would not  dissolve in  the  129  m e t a l even when the CaO  a c t i v i t y i n the s l a g was  a c t i v i t y was  l e s s than 0.00018, w h i c h was  CaF  % AlyO^  We  2  + 8 wt.  must t h e r e f o r e  21, were not  unless  c e r t a i n l y not  the  A^O^  the case i n  the  s l a g used f o r e l e c t r o l y s i s of the n i c k e l e l e c t r o d e .  conclude t h a t the Ca c o n t a i n i n g  e l e c t r o c h e m i c a l l y formed, but  n i c k s l i n the as r e c e i v e d  4.8 The  0.56,  regions  shows i n F i g .  t h a t t h i s c a l c i u m was  in  the  condition.  Anodic p o l a r i z a t i o n of  cobalt  anodic p o l a r i z a t i o n b e h a v i o u r of c o b a l t was  to t h a t of n i c k e l i n t h a t s t e a d y - s t a t e u n t i l the c u r r e n t d e n s i t y was  high.  very s i m i l a r  p o l a r i z a t i o n could not be  attained  F i g . 22 shows the o s c i l l o s c o p e t r a c e -2  obtained  at a c u r r e n t  d e n s i t y of 2.0  p o t e n t i a l d i f f e r e n c e of almost 800 C  The  (gr)  +  CO  °(s) *  A.  mV. C  cm.  T h i s shows a measured  which corresponds to the  °(Y)  + C 0  reaction:  (g)  <4-28)  f r e e energy change f o r t h i s r e a c t i o n i s c a l c u l a t e d at 1766°K,  melting  p o i n t of c o b a l t , as  C  (gr)  +  2 °2  ( g )  +4-0  Co  2  AF° and  AE°  +  follows:  C0  AF° = -63,803 c a l .  C g )  -*• CoO 2  (4-30)  L S )  = -37,793 c a l .  = -819  T h i s f i g u r e of 819 I f the r i s e times i n F i g u r e s  mV. mV.  agrees v e r y w e l l w i t h the measured  20 and  22 are compared, i t i s apparent  the c o b a l t r i s e time i s much f a s t e r and t h e r e f o r e kept to a minimum.  The  value. that  the degree of system c o o l i n g i s  faster rise  times so observed must  a r e s u l t of a d i f f e r e n c e i n s a t u r a t i o n s o l u b i l i t y of CoO slags.  (4-29)  AF° = -26,010 c a l .  (g) rx  the  and  NiO  be  i n CaF  2  130  In s i l i c a t e m e l t s , of oxides Ni,  of t r a n s i t i o n metals d e c r e a s e s a c c o r d i n g  Co, and  sently  Anodic p o l a r i z a t i o n of an Fe-C  s t a t e d i n s e c t i o n (2.5.7) the anodic  p o l a r i z a t i o n of  d e s p i t e the f a c t  This  an ob-  that t h e r e i s o f t e n ,a bubble n u c l e a t i o n problem I t i s not  s u r p r i s i n g , however,  t h a t carbon monoxide e v o l u t i o n o c c u r r e d because, a c c o r d i n g  to D i s t i n  (35), t h e r e w i l l be no n u c l e a t i o n b a r r i e r at an i r o n s u r f a c e con-  t a i n i n g 0.8  wt.  % C when i t i s i n c o n t a c t w i t h FeO  From t h i s experiment one D.C.  pre-  carbon l o s s must have taken p l a c e by e v o l u t i o n of carbon-monoxide  a s s o c i a t e d w i t h carbon-monoxide e v o l u t i o n .  al.  Fe,  alloy  e l e c t r o d e s i g n i f i c a n t l y d e c r e a s e d i t s carbon c o n t e n t .  served  Cr,  studied.  As  gas  to the o r d e r  i t i s p o s s i b l e t h a t a s i m i l a r trend e x i s t s i n the s l a g s  4.9  Fe-C  i t i s found that the s a t u r a t i o n s o l u b i l i t y  ESR  saturated  et. 1  slag.  can conclude t h a t carbon l o s s d u r i n g  c o u l d be q u i t e s u b s t a n t i a l .  4.10  High c u r r e n t d e n s i t y p o l a r i z a t i o n  The v i s u a l o b s e r v a t i o n d e n s i t i e s on b o t h anodic  and  of a r c e f f e c t s at v e r y h i g h  cathodic  surfaces  current  i s supported by  behaviour of the p o l a r i z a t i o n curves shown i n F i g . 13.  the  It i s evident  t h a t at s u f f i c i e n t l y h i g h c u r r e n t d e n s i t i e s the e l e c t r o d e s u r f a c e  dis-  p l a y s an e s s e n t i a l l y l i n e a r o v e r p o t e n t i a l / c u r r e n t d e n s i t y b e h a v i o u r which i s unexplainable be  by a d i f f u s i o n c o n t r o l l e d mechanism.  However, i t would  the case i f the observed p o l a r i z a t i o n r e s u l t e d from the r e s i s t a n c e of  131  a gas  film  around the e l e c t r o d e  of two ways. Ca, , (g)  Firstly,  (36).  Such an a r c could a r i s e i n e i t h e r  the e l e c t r o d e r e a c t i o n c o u l d  0„ , , or F „ , at a r a t e s u f f i c i e n t 2(g) 2(g)  N  envelope around the e l e c t r o d e .  evolve  gas,  either  to c r e a t e a s t e a d y - s t a t e  gas  At the h i g h temperature at the t i p ,  s u f f i c i e n t e l e c t r o n t r a n s p o r t would be p o s s i b l e i n t h i s envelope to f e r the e l e c t r o d e r e a c t i o n to i t s o u t e r the e l e c t r o d e r e a c t i o n , and 35 shows the f o r m a t i o n The  (slag/gas)  trans-  s u r f a c e , thus r e t a i n i n g  hence the a r c would be s e l f - s u s t a i n i n g . F i g .  of such a c o n d i t i o n i n a CaF^  + 250  ppm.  CaO  slag.  e l e c t r o d e f i r s t p o l a r i z e s , i n t h i s case a n o d i c a l l y , then e v o l v e s  at a h i g h e r  potential.  The  anodic  r e a c t i o n here would be  mation of a s a t u r a t e d  l a y e r of FeO  (4-2), which would be  i n s t a n t l y f o l l o w e d by  r a t e of these r e a c t i o n s i s i n s u f f i c i e n t imposed by  the c u r r e n t  a gaseous s p e c i e s  source,  the i n i t i a l  at the e l e c t r o d e s u r f a c e , by reaction  (4-9).  for-  reaction  When the  to s u s t a i n the c u r r e n t  r e a c t i o n (4-11) w i l l be  gas  density  initiated,  to form the a r c , which produces a s i g n i f i c a n t  evolving  increase  i n the p o l a r i z a t i o n p o t e n t i a l . T h i s r e g i o n of a r c i n g i s s e c t i o n C i n F i g . 30.  This s i t u a t i o n i s probably  the phenomenon r e f e r r e d t o i n the  litera-  t u r e as a " s o f t " a r c , s i n c e a l t h o u g h the e f f e c t i s v i s i b l e as a sudden i n c r e a s e i n r a d i a t i o n , i t has  no c h a r a c t e r i s t i c a r c s p l u t t e r .  The  initia-  t i o n of s u c h an a r c i s v e r y s i m i l a r to the i n i t i a t i o n of the "anode e f f e c t " i n aluminum e l e c t r o l y s i s  (37), and  current d e n s i t y required f o r anodic on the o x i d e content the FeO o n l y use  of the s l a g .  as w i t h the anode e f f e c t ,  arc formation As  the  i s s t r o n g l y dependent  i n t h e case of the t r a n s i t i o n i n t o  s a t u r a t i o n c o n d i t i o n at lower anodic  c u r r e n t d e n s i t i e s , we  an approximate e x t r a p o l a t i o n to extend our  f i n d i n g s to  can  higher  132  TABLE XI  CRITICAL CURRENT DENSITY ESTIMATION  ELECTRODE POLARITY  ESTIMATED CURRENT DENSITY FOR ARC INITIATION  SLAG CaF„  + wt. % oxide  +  25 A 1 0  3  250  25 A 1 0  3  400  5 A1 0  3  150  5 A1 0  3  200  2  2  +  2  2  +  +  +  A. cm  0  10  0  25  5 CaO  300  5 CaO  400  25 CaO  500  25 CaO  500  -2  133  oxide  slags.  However, s i n c e the c u r r e n t d e n s i t y at the p o i n t of a r c i n -  i t i a t i o n marks the upper l i m i t process  e l e c t r o d e , we  able operating  f e e l t h a t such, an a p p r o x i m a t i o n w i l l p r o v i d e  information.  f i g u r e s are shown i n T a b l e The  The  anodic  and  cathodic  current  valu-  limitation  XI.  e l e c t r o d e c u r r e n t d e n s i t y i n the U.B.C. e l e c t r o s l a g f u r -  nace i s a p p r o x i m a t e l y 100 A. furnace,  f o r the working o f a s t a b l e e l e c t r o s l a g  cm.  -2  , while  i n a commercial e l e c t r o s l a g ,  the c u r r e n t d e n s i t y on an 18 i n . diameter e l e c t r o d e at a  current  _2 of 15 kA.  i s approximately 10 A. cm.  are below the estimated a t i o n i n a 25 wt.  values  % Al^O^  Both of these c u r r e n t d e n s i t i e s  of c r i t i c a l  slag.  On  c u r r e n t of 450 4  ly  A.,  initi-  the other hand, the c u r r e n t d e n s i t i e s  i n e l e c t r o s l a g w e l d i n g are v e r y h i g h . a process  current density f o r arc  For a w i r e d i a m e t e r of 1/8  i n . and  the e l e c t r o d e c u r r e n t d e n s i t y i s approximate-  -2  3 x 10  A.  T h i s c u r r e n t d e n s i t y i s f a r g r e a t e r i.than t h a t r e r  cm.  q u i r e d f o r a r c i n i t i a t i o n and as a submerged a r c  e l e c t r o s l a g w e l d i n g must t h e r e f o r e o p e r a t e  process.  A second p o s s i b l e mechanism f o r such a r c i n i t i a t i o n i s heat generation  by J o u l e h e a t i n g  b o i l i n g i n the s l a g . point  (38)  sity  T h i s has been c o n s i d e r e d  i n analogy w i t h  the p r e s e n t to b o i l  case.  i n the p o l a r i z e d s l a g l a y e r l e a d i n g to  from a t h e o r e t i c a l s t a n d -  the L e i d e n f r o s t phenomenon, and may  operate i n  I t i s p o s s i b l e to c a l c u l a t e the r e q u i r e d c u r r e n t  the s l a g at the e l e c t r o d e t i p u s i n g  Consider  local  den-  the f o l l o w i n g model.  a " c o n i c a l p o l a r i z e d e l e c t r o d e w i t h a s u r f a c e a r e a of  2 11.3  cm.  (an average e l e c t r o d e t i p ) . .  J o s h l (32)  t r a n s f e r c o e f f i c i e n t , h, between s l a g and  c a l c u l a t e d the heat  e l e c t r o d e to  be:  134  h = 1 cal.°C I f the  -1  sec.  -1  cm.  assumption i s made t h a t  -2  a l l of heat generated by  passage through, the p o l a r i z e d s l a g l a y e r i s t r a n s f e r r e d at t h i s v a l u e of h, i t i s p o s s i b l e  to c a l c u l a t e the  to the  current  current  electrode  density  needed  to r a i s e the s l a g temperature at the i n t e r f a c e to i t s b o i l i n g p o i n t .  The  heat t r a n s f e r e q u a t i o n i s  q = h where A i s s u r f a c e  the m e t a l (Fe)  • AT  c a l . sec.  area of t r a n s f e r , and  a c r o s s the I n t e r f a c e . and  • A  By  AT  assuming t h a t  i s at i t s m e l t i n g  AT  = B.P._  (4-31) i s the temperature  difference  the s l a g i s a t i t s b o i l i n g p o i n t  point.  _ - M.P._, CaF„ Fe  = 2509°C - 1538°C  1  = 971°C T h e r e f o r e , f o r these  assumptions,  q = 1 x 11.3 = I f the v o l t a g e be  10 V.  971  11,000 c a l . s e c . "  the power d i s s i p a t i o n i s g i v e n  where I Is the  current  i n amps.  c o n v e r t s P to k e a l . s e c .  1  drop i n the p o l a r i z e d s l a g l a y e r i s assumed  P = 101  follows:  x  1  to  by:  watts M u l t i p l y i n g P i n kW  T h e r e f o r e the  current  by  can be  the f a c t o r calculated  0.24 as  135  11 k c a l . s e c .  -1  11 .24  kW.  = 45.9 kW. Therefore  I =  45.9 - = 4.59 kA. 10 = 4590 A.  The  e l e c t r o d e c u r r e n t d e n s i t y would then be -2 = 400 A. cm. T h i s i s an u n r e a s o n a b l y h i g h  the i n i t i a l by  c u r r e n t d e n s i t y , even i n view o f  assumptions and one can t h e r e f o r e conclude t h a t a r c i n i t i a t i o n  t h i s means i s v e r y u n l i k e l y . The  e v o l u t i o n o f gas on t h e working e l e c t r o d e w i l l o n l y  alter  2+ 3+ the measured p o l a r i z a t i o n through i t s e f f e c t on the Fe /Fe r a t i o of the s l a g a t the s l a g / g a s crease  interface.  When gas bubbles a r e e v o l v e d ,  i n p o l a r i z a t i o n overvoltage  the i n -  observed i s due t o the i n c r e a s e i n  ohmic r e s i s t a n c e i n the r e g i o n of the working e l e c t r o d e s u r f a c e . the c a l c i u m ppm), tem,  f l u o r i d e used  and e x t e n s i v e  Although  c o n t a i n e d o n l y a s m a l l q u a n t i t y of oxide (250  precautions  were taken t o e x c l u d e oxygen from the s y s -  t h e r e were no i n d i c a t i o n s that f l u o r i d e s p e c i e s were i n v o l v e d i n the  p o l a r i z a t i o n r e a c t i o n s i n "pure" c a l c i u m i n , f o r example, t h e anodic  f l u o r i d e , as evidenced by changes  p o l a r i z a t i o n times.  low. l e v e l s o f oxygen p r e s e n t  I t i s probable that the  i n t h e f l u o r i d e were s u f f i c i e n t  s a t u r a t e d l a y e r on the working e l e c t r o d e s i n c e o x i d e calcium  f l u o r i d e i s very The  observations  to provide a  s o l u b i l i t y i n "pure"  small. made on t h e c a t h o d i c p r o c e s s  a r c i n i t i a t i o n c o n d i t i o n must be a s s o c i a t e d w i t h  i n d i c a t e that;the  the p r o d u c t i o n  of Ca, v .  136  In CaF^ + CaO s l a g s , t h i s i s e v i d e n t l y the o n l y p o s s i b l e mechanism, and should  occur  a t e l e c t r o d e p o t e n t i a l s o f approximately  to the g r a p h i t e r e f e r e n c e e l e c t r o d e . should  1 v o l t with, r e s p e c t  In CaF^ + Al^O^ s l a g s , the mechanism  remain the same, b u t t h e observed p o t e n t i a l s a t a r c i n i t i a t i o n  will  be markedly d i f f e r e n t from the CaF^ + CaO case due to the e f f e c t o f A ^ O ^ on the CaO  activity.  4.11  E l e c t r o s l a g process  The v a l u e s of anodic trodes  polarization  and c a t h o d i c p o l a r i z a t i o n on m e l t i n g  i n t h e m e l t i n g u n i t g i v e n i n F i g u r e s . 23-+26 agree w e l l w i t h  trapolated small scale p o l a r i z a t i o n r e s u l t s .  electhe ex-  One can t h e r e f o r e conclude  t h a t n e i t h e r the o b v i o u s l y d i f f e r e n t hydrodynamic regime a t the ESR e l e c trode nor the f a c t t h a t the e l e c t r o d e i s c o n t i n u o u s l y m e l t i n g have a s i g nificant  e f f e c t on the p o l a r i z a t i o n r e a c t i o n s .  the s u b s t a n t i a l (8%) 360 Hz r i p p l e p r e s e n t has  no d e t e c t a b l e e f f e c t on the r e a c t i o n s .  overpotential values  obtained  e l e c t r o d e s a r e approximately  We may a l s o conclude t h a t  i n the D.C. r e c t i f i e r The few anodic  circuit  and c a t h o d i c  on m e l t i n g AISI 1095 and AISI 430 s t e e l the same as those measured on pure i r o n e l e c -  trodes a t e q u i v a l e n t c u r r e n t d e n s i t i e s .  T h i s i n d i c a t e s t h a t the a l l o y i n g  elements had no a p p r e c i a b l e d e p o l a r i z i n g e f f e c t and t h a t our f i n d i n g s on pure i r o n e l e c t r o d e s apply  to a l l iron-based m a t e r i a l s .  CHAPTER 5 DISCUSSION OF MELT PROGRAM RESULTS  5.1  The and  Introduction  purpose of the m e l t program was to i n v e s t i g a t e  the c h e m i c a l  thermal e f f e c t s which o c c u r d u r i n g D.C. e l e c t r o s l a g m e l t i n g , and ex-  p l a i n these e f f e c t s i n terms of the F a r a d a i c r e a c t i o n mechanisms proposed i n Chapter 4. The  more important f i n d i n g s 1.  slag melting.  of the m e l t program a r e shown t o be:  Chemical e f f e c t s e x i s t d u r i n g D.C.  electro-  In the case o f pure m e t a l s , o x i d a t i o n  occurs  at the anodic s u r f a c e to produce i n g o t s w i t h a f i n a l oxygen content which i s dependent p r i n c i p a l l y on the e l e c t r o d e polarity. and  D e p o s i t i o n of A l occurs at the cathode s u r f a c e  the A l - 0 i n t e r a c t i o n i s p a r t i a l l y r e s p o n s i b l e f o r  oxygen removal from the i n g o t 2.  pool.  Thermal e f f e c t s a r e p r e s e n t d u r i n g  electroslag melting.  They a p p a r e n t l y a r i s e from excess  heat g e n e r a t i o n i n the p o l a r i z e d metal/liquid a higher slag electrode,  slag  l a y e r a t the l i q u i d  s l a g i n t e r f a c e s . These e f f e c t s r e s u l t not i n temperature b u t , i n the case o f an anodic  I n a higher s p e c i f i c melt r a t e  case of an anodic i n g o t , i n the ingot  D.C.  i n a larger  pool.  137  and, i n the  l i q u i d m e t a l volume  138  3. alloys,  In the ,case of D.C. e l e c t r o s l a g m e l t i n g o f  the chemical  effects result i n significant  of e a s i l y o x i d i z a b l e a l l o y i n g elements.  losses  Alloy depolariza-  tion i s small. 4.  A l l o f these e f f e c t s can be e x p l a i n e d i n  terms of both  F a r a d a i c r e a c t i o n mechanisms and mass  t r a n s f e r phenomena at t h e two m e t a l / s l a g i n t e r f a c e s .  5.2  E f f e c t of e l e c t r o d e p o l a r i t y on oxygen  content  The oxygen a n a l y s i s r e s u l t s of the i n g o t s made i n the melt program show that i n g o t s made on the D.C. e l e c t r o d e n e g a t i v e mode have a much h i g h e r f i n a l oxygen content  than i n g o t s made on the e l e c t r o d e p o s i t i v e mode, i n d e -  pendent o f atmospheric i n t e r a c t i o n s .  T h i s f a c t i s best  amining the melt r e c o r d r e s u l t s o f F e r r o v a c - E the average oxygen content  ingots  of e l e c t r o d e n e g a t i v e  CaF^ + 25 wt. % Al^O^ s l a g i s 480 ppm. 0, w h i l e  i l l u s t r a t e d by ex-  (Table I I I ) i n which  i n g o t s melted  i n g o t s made on the e l e c t r o d e  p o s i t i v e mode i n t h e same s l a g have an average oxygen content 0.  As shown i n the m e l t r e c o r d s ,  p o s i t i v e melting  possible.  through  of 185 ppm.  t h e r e a r e a c t u a l l y t h r e e types of e l e c t r o d e  The f i r s t  type i s the standard  electrode p o s i t i v e  mode i n which t h e maximum r a t e of power i n p u t i s c o n t r o l l e d by the v o l t a g e at which a r c i n g to the " i n s u l a t e d " mold; occurs higher v o l t a g e ,  (~19 V . ) .  To operate a t a  i t was n e c e s s a r y t o p a i n t t h e i n n e r s u r f a c e o f the copper  mold w i t h b o r o n - n i t r i d e p a i n t as d e s c r i b e d electrode p o s i t i v e melting  [+(im)J  -voltages without a r c i n g t o the mold  i n Chapter 3.  In t h i s type o f  i t was p o s s i b l e t o melt at h i g h e r (see i n g o t s 37, 38, and 41).  applied  These i n -  139  g o t s had an average oxygen content  o f 140 ppm. 0 which i s o n l y  lower than that of the normal e l e c t r o d e p o s i t i v e (+) i n g o t s . were made i n the t h i r d  Two i n g o t s  type of e l e c t r o d e p o s i t i v e mode w h i c h i s e l e c t r o d e  p o s i t i v e w i t h a l i v e mold, I + ( l m ) ] . made t o c a r r y c u r r e n t being own leads and c u r r e n t shunt. 90% of the process  slightly  I n t h i s mode the mold was  purposely  connected t o the power r e t u r n c i r c u i t w i t h i t s I t was found t h a t i n t h i s mode approximately  c u r r e n t passed through the l i v e mold c i r c u i t , and i n g o t s  35 and 83 made t h i s way i n a CaF^ + 25 wt. % k l ^ ) ^ s l a g had the h i g h e r oxygen content and  (760 ppm. 0 ) . Three i n g o t s made u s i n g A.C. power (34, 36  40) had an average oxygen content  III,  of 215 ppm. 0.  i n g o t 36 was made u s i n g a l i v e mold c o n n e c t i o n ,  As shown i n T a b l e while  i n g o t 40 was  made i n an i n s u l a t e d mold, but these c o n d i t i o n s appeared n o t t o a f f e c t .the f i n a l oxygen content  to any e x t e n t .  T h i s observed dependence of the oxygen content polarity  can be e x p l a i n e d  Consider  first  mode.  using  the f i n d i n g s of the s m a l l s c a l e s t u d i e s .  the case of the i n g o t being made i n the e l e c t r o d e  The e l e c t r o d e i s c a t h o d i c  w i l l be d e p o s i t e d  a t the m e l t i n g  and the i n g o t anodic electrode t i p .  the s l a g and A l w i l l d i s s o l v e a t a s t e a d y - s t a t e film  on the e l e c t r o d e  on the e l e c t r o d e t i p .  Therefore,  negative  such t h a t A l and Ca  The Ca w i l l d i s s o l v e i n rate i n t o the l i q u i d  iron  the m e t a l d r o p l e t s which detach from  the t i p w i l l c o n t a i n d i s s o l v e d aluminum which w i l l be c a r r i e d to the l i q u i d metal pool ofthe s o l i d i f y i n g  ingot.  c u r r e n t d e n s i t y a t an anodic  i n g o t t i p i s h i g h enough t o s a t u r a t e the s l a g  at  In the U.B.C. e l e c t r o s l a g r i g , the  the i n t e r f a c e i n FeO, and i r o n oxide w i l l be d i s s o l v i n g  i n t o the l i q u i d m e t a l p o o l .  continuously  Depending on the r e l a t i v e c o n c e n t r a t i o n s o f  140  A l and  0 i n the l i q u i d , o x i d e i n c l u s i o n s w i l l n u c l e a t e  f i c a t i o n p r o c e e d s . The i n t e r f a c e represent a very  i n c l u s i o n s t h a t are trapped  the f i n a l oxygen content  been g i v e n  grow as  the  solidifying  of the i n g o t m e t a l .  s i m p l i s t i c view of i n c l u s i o n f o r m a t i o n  c o n s i d e r a t i o n has  by  and  and  solidi-  This i s  entrapment i n which  no  to the problems of i n c l u s i o n n u c l e a t i o n or A l  l o s s from the drops as they pass through the i r o n - o x i d e  saturated  anodic  interface. Ingots made i n the e l e c t r o d e p o s i t i v e [+] or e l e c t r o d e p o s i t i v e with  an i n s u l a t e d mold {+(im)] mode w i l l p i c k up A l and  with  the e x c e p t i o n  Why,  then, do e l e c t r o d e n e g a t i v e  that the e l e c t r o d e i s anodic  T h i s can be e x p l a i n e d s i t e s i n the two  The  the same means  the i n g o t  i n g o t s have a h i g h e r  by c o n s i d e r i n g  cases.  and  0 by  cathodic.  f i n a l oxygen Content?  the l o c a t i o n of the a n o d i c  reaction  i n g o t c u r r e n t d e n s i t y i s approximately  h a l f t h a t of the e l e c t r o d e t i p c u r r e n t d e n s i t y , but  the o p e r a t i n g  i n the U.B.C. e l e c t r o s l a g r i g i s s u f f i c i e n t l y h i g h to s a t u r a t e an i n g o t s u r f a c e i n FeO. equivalent  In e l e c t r o c h e m i c a l  to an anodic  i n g o t oxygen content  e l e c t r o d e t i p , and  terms an anodic  sents  current anodic  ingot surface i s  the observed dependence of  on e l e c t r o d e p o l a r i t y must t h e r e f o r e a r i s e from  ces i n the r a t e of m e t a l o x i d a t i o n at the two Anodic l i q u i d  one-  i r o n surfaces  an u n l i m i t e d supply  are exposed to FeO  of o x i d a n t ,  and  p o s s i b l e anodic saturated  s l a g which  repre-  the r a t e of oxygen d i s s o l u t i o n  c o n t r o l l e d by the l i q u i d m e t a l f l o w c h a r a c t e r i s t i c s at these  faces.  A melting  i m a t e l y 20.0u-thick ( 4 ) .  differen-  surfaces.  must be  e l e c t r o d e has  final  inter-  a l i q u i d f i l m on the s u r f a c e which i s approx-  As w i l l be  shown i n S e c t i o n  oxygen w h i c h d i s s o l v e s i n the m e l t i n g  (5.6),  the amount of  e l e c t r o d e m e t a l i s equal  to 1600  ppm.  141  0 f o r pure i r o n e l e c t r o d e s . lent value  I t was not p o s s i b l e t o c a l c u l a t e an e q u i v a -  f o r the amount of oxygen d i s s o l u t i o n a t an anodic  s u r f a c e , because too l i t t l e the i n g o t p o o l . can s p e c u l a t e  i s known about the f l u i d f l o w  liquid  ingot  conditions i n  However, due to the l a r g e r s u r f a c e area a t the p o o l , one  t h a t more oxygen w i l l be d i s s o l v e d a t an anodic  ingot.  One  must now c o n s i d e r how t h i s oxygen i s removed from the l i q u i d m e t a l , because the observed f i n a l i n g o t oxygen c o n t e n t s oxygen c o n t e n t . could c o n c e i v a b l y drops f a l l  a r e much lower than the c a l c u l a t e d  I n the case o f the anodic  e l e c t r o d e , the metal droplets  l o s e some of t h e i r d i s s o l v e d oxygen d u r i n g  through the s l a g , e s p e c i a l l y when one c o n s i d e r s  the time the  the f a c t  that  oxygen i s s u r f a c e a c t i v e i n i r o n and the drop s u r f a c e t h e r e f o r e p r e s e n t s  a  l a r g e amount of the d i s s o l v e d oxygen t o the r e f i n i n g a c t i o n o f the s l a g . A l s o when the drops reach  the c a t h o d i c i n g o t s u r f a c e , they must pass through  a l a y e r of s t r o n g l y d e o x i d i z i n g s l a g c o n t a i n i n g m e t a l produced by the c a thode r e a c t i o n .  I t i s therefore reasonable  oxygen d i s s o l v e d i n a m e l t i n g  anodic  t o expect t h a t the b u l k o f the  e l e c t r o d e w i l l be removed from the  metal b e f o r e  the m e t a l becomes p a r t of the i n g o t p o o l p r o p e r .  of an anodic  i n g o t p o o l which i s i n c o n t a c t w i t h  o n l y supply metal drops.  an FeO s a t u r a t e d  of deoxidant i s t h a t which i s t r a n s p o r t e d  s l a g , the  to t h e p o o l by the  Then drops w i l l c o n t a i n e l e c t r o c h e m i c a l l y produced aluminum  which w i l l r e a c t w i t h  the d i s s o l v e d oxygen i n the i n g o t m e t a l t o form i n -  c l u s i o n s , some of w h i c h w i l l final  I n the case  i n g o t oxygen content  d i s s o l v e i n the s l a g .  on e l e c t r o d e p o l a r i t y  The dependence of the  can t h e r e f o r e be e x p l a i n e d  i n terms of d i f f e r e n c e s i n oxygen d i s s o l u t i o n r a t e s a t the i n g o t and e l e c t r o d e and t h e chemical products.  i n t e r a c t i o n between the e l e c t r o c h e m i c a l l y produced  142  The  f a c t that l i v e mold e l e c t r o d e p o s i t i v e i n g o t s have the  e s t oxygen content  of any  mode can be e x p l a i n e d  electrochemical reaction s i t e s .  The  anodic  by d i s s o l v i n g oxygen i n the m e l t i n g m e t a l .  r a d i a l one,  to the mold and  orthogonal  to the Cu mold.  s u r f a c e i n any  o t h e r mode  the path of lowest r e s i s t a n c e should  the  be  a  I t seems u n r e a s o n a b l e to  c u r r e n t passes through the s o l i d i f i e d s l a g s k i n  I t would be more p l a u s i b l e to have the c u r r e n t t r a n s f e r r e d  through a " s o f t a r c , " ( s e c t i o n 4.10) and  the  In t h i s mode, the b u l k of  to the e l e c t r o d e t i p .  propose t h a t the p r o c e s s  again consider  s u r f a c e i s the e l e c t r o d e t i p  which behaves i n the same manner as the anodic  c u r r e n t flows  i f we  high-  the copper mold.  As d i s c u s s e d  duct both e l e c t r o n i c a l l y and the a r c / s l a g i n t e r f a c e .  We  i n Section  i o n i c a l l y , with  slag  (1.3.3) such an a r c might conthe p r i n c i p a l F a r a d a i c  Hence the i n g o t / s l a g i n t e r f a c e has  d e o x i d i z i n g c a p a b i l i t y and m e t a l composition.  e s t a b l i s h e d between the l i q u i d  little  thus w i l l not s u b s t a n t i a l l y a l t e r the  site  as  cathodic  electrode  would expect then t h a t the i n g o t p o o l c o n t a i n  large  amounts of d i s s o l v e d oxygen.  5.3  I n c l u s i o n types  As p r e v i o u s l y i n d i c a t e d i n Chapter 1, e l e c t r o s l a g p r o c e s s i n g sures  that no  l a r g e i n c l u s i o n s are p r e s e n t  i n the i n g o t m e t a l .  Electron  microprobe examination of i n g o t specimens produced i n the p r e s e n t program show t h i s  to be  the case.  microprobe p i c t u r e s o b t a i n e d while  F i g . 39 shows the a p p r o p r i a t e  upon examination of i n g o t no.  the p i c t u r e s i n F i g . 40 were o b t a i n e d  s t e e l ingot  (No.  61).  Both i n g o t s c o n t a i n e d  27  en-  melt  electron  (FVE.  el.-ve)  from the AISI 430 s t a i n l e s s s m a l l round i n c l u s i o n s of  high  A.E.I. X  Figure  39.  1200  S e c t i o n from i n t e r i o r of electrode negative ingot containing inclusions.  Ferrovac-E showing A l  A.E.I. X 2100  Al X-ray X 2 I 0 0  F i g u r e 40.  S e c t i o n from i n t e r i o r of AISI 430 s t a i n l e s s s t e e l i n g o t showing A l containing i n c l u s i o n s .  145  aluminum c o n t e n t .  The  same type of i n c l u s i o n s were found i n most of the  i n g o t s examined whether t h e y were made from A I S I 1095 pure n i c k e l .  I n c l u s i o n f o r m a t i o n and  s t e e l or e s s e n t i a l l y  composition during e l e c t r o s l a g pro-  c e s s i n g w i l l not be d i s c u s s e d here, h a v i n g been a d e q u a t e l y covered by (21).  E v i d e n c e of i n c l u s i o n removal by f l o t a t i o n was  found upon examina-  t i o n of the top s u r f a c e of i n g o t 30 as shown i n F i g . 41. number o f alumina i n c l u s i o n s which were about  T h i s shows a l a r g e  to e n t e r the s l a g .  c l u s i o n s were l a r g e r than those found i n the b u l k of the i n g o t and appear  Bell  t o have formed-by a g g l o m e r a t i o n of s m a l l e r i n c l u s i o n s .  These i n they ;  T h i s produced  i n c l u s i o n s w i t h a l a r g e r e f f e c t i v e r a d i u s which were a b l e to f l o a t out of the i n g o t p o o l .  T h i s o b s e r v a t i o n i n d i c a t e s t h a t the measured oxygen content  of the i n g o t s i s lower than the e l e c t r o c h e m i c a l l y produced  oxygen c o n t e n t .  I t must be noted t h a t the f i n a l i n g o t oxygen and aluminum c o n t e n t s i n pure i r o n were f a r h i g h e r than would be produced w i t h alumina.  by e q u i l i b r a t i o n of  T h i s i s shown to be t r u e i n F i g . 42 which i s a p l o t of  iron  exper-  i m e n t a l l y determined A l and 0 c o n t e n t s of pure i r o n i n e q u i l i b r i u m w i t h ; alumina o f u n i t a c t i v i t y at 1600°C (21). of i n g o t s 25, 41, and tents.  The r e s i d u a l A l and 0 c o n t e n t s  83 are shown to l i e w e l l above the e q u i l i b r i u m  T h i s s u p p o r t s the argument t h a t the e l e c t r o c h e m i c a l r e a c t i o n p r o -  ducts are r e s p o n s i b l e f o r the observed With r e s p e c t to removal e l e c t r o d e m e t a l , i t has been noted  ingot  inclusions.  of i n c l u s i o n s p r e s e n t i n the s t a r t i n g (39) t h a t the e l e c t r o d e t i p i s the  at w h i c h these i n c l u s i o n s a r e most e f f i c i e n t l y removed. v i o u s l y , t h i s Is not the case w i t h r e s p e c t to removal formed  con-  inclusions.  site  As d i s c u s s e d p r e -  of e l e c t r o c h e m i c a l l y  0pticalX950  F i g u r e 41.  A l u m i n a i n c l u s i o n s on t h e t o p o f a Ferrovac-E electrode negative ingot.  o F i g u r e 42.  A l and 0 i n l i q u i d i r o n i n e q u i l i b r i u m w i t h u n i t a c t i v i t y A1„0 at 1600°C. E x p e r i m e n t a l data f a l l s w i t h i n the hatched a r e a .  148  5.4  E f f e c t of s p e c i f i c coulombic As p r e s e n t e d i n Chapter  d e n s i t y (Z) on the f i n a l oxygen content  3, i t was  p o s t u l a t e d t h a t the f i n a l i n -  got oxygen content c o u l d be c o r r e l a t e d to a parameter, Z, which took account  the o p e r a t i n g c u r r e n t density- and  the m e l t r a t e .  There may  into indeed  e x i s t such a r e l a t i o n s h i p , but the range of s t a b l e o p e r a t i n g c u r r e n t d e n s i t i e s a v a i l a b l e on the U.B.C. e l e c t r o s l a g r i g i s l i m i t e d .  Hence, we were un-  a b l e t o v a r y Z over a s u f f i c i e n t l y wide range f o r the p o s t u l a t e t o be exami n e d w i t h r e s p e c t to the f i n a l  i n g o t oxygen c o n t e n t .  amount of d a t a , a t r e n d does appear t o e x i s t . Z and seen  t h a t the lowest oxygen content corresponds  Examination  approach appears  the h i g h e s t melt r a t e .  to adequately  5.5 Two  the lowest oxygen content as w e l l However, a l t h o u g h  e n t i r e l y r e a s o n a b l e , the range o f a c c e s s i b l e  data i s i n s u f f i c i e n t  1%  Z,  of the d a t a f o r the e l e c t r o d e n e g a t i v e i n g o t s i n T a b l e I I I  as the lowest v a l u e of Z and  The  t o the lowest v a l u e of  s i m p l y be a r e f l e c t i o n of the h i g h e r m e l t r a t e of i n g o t 31.  shows the same trend i n t h a t i n g o t 77 has  to  examines the v a l u e s of  oxygen content f o r i n g o t s 31->33 i n T a b l e I I I ( e l . +ve.) , i t can be •  but t h i s may  gram.  I f one  D e s p i t e the l i m i t e d ,  argon fume hood produced whereas the argon gas  the p u r i t y of the argon  i n use.  experimental  t e s t our i n i t i a l p o s t u l a t e .  E f f e c t of atmosphere  types of atmospheric  the  control  c o n t r o l were used d u r i n g the melt  pro-  a r e l a t i v e l y o x i d i z i n g atmosphere  (up  cap atmosphere p u r i t y depended, s o l e l y E t i e n n e (4) s t a t e s t h a t the  on  atmospheric  o x i d a t i o n r a t e d u r i n g m e l t i n g i s dependent upon exchange r e a c t i o n s between s l a g and atmosphere w h i c h i n t u r n a r e c o n t r o l l e d by d i f f u s i o n of m u l t i p l e 2+ v a l e n c e i o n s (Fe  3+ /Fe  ) i n the s l a g .  He a l s o concluded  that electrode  149  o x i d a t i o n above the s l a g was unimportant as a means of atmosphere oxygen t r a n s p o r t to the m e l t i n g m e t a l . pressure  H i s f i n a l c o n c l u s i o n i s t h a t the p a r t i a l  of oxygen i n the atmosphere bears l i t t l e  r e l a t i o n to the amount :  of atmospheric o x i d a t i o n except when the a c t u a l r a t e of supply  of atmos-  p h e r i c oxygen i s l e s s than the l i m i t i n g r a t e of oxygen t r a n s f e r a t the slag/atmosphere i n t e r f a c e .  I f one compares the oxygen contents  vac-E i n g o t s made In the argon fume hood of the F e r r o v a c - E is  (23, 72, 24, and 74)with those  i n g o t s made i n the argon gas cap \. (25-K30, and 31+33) i t  obvious t h a t the d i f f e r e n t atmospheres have had v e r y  the oxygen content p o s i t i v e mode.  of Ferro-  little  effect  on  i n e i t h e r the e l e c t r o d e n e g a t i v e mode or the e l e c t r o d e  One must  t h e r e f o r e conclude t h a t the i n g o t oxygen  contents  are a r e s u l t o f m e t a l o x i d a t i o n by the F e O - s a t u r a t e d s l a g l a y e r produced by e l e c t r o c h e m i c a l r e a c t i o n s at the anode. anodic  s l a g l a y e r i s much g r e a t e r  The o x i d i z i n g power of t h i s  than any atmospheric o x i d a t i o n  cesses which might have e x i s t e d d u r i n g m e l t i n g  pro-  i n the U.B.C. e l e c t r o s l a g  rig.  5.6  D i f f u s i o n of oxygen i n t o an anodic  As p r e v i o u s l y d i s c u s s e d is  exposed t o s l a g s a t u r a t e d  (5.2)  i n FeO.  a melting  Etienne  anodic  iron  electrode  Oxygen d i s s o l v e s i n the l i q u i d  e l e c t r o d e f i l m to e s t a b l i s h a s t e a d y - s t a t e the m e l t i n g m e t a l .  electrode  r a t e of oxygen t r a n s f e r to  (4) developed a mathematical model by  consid-  150  irig an i d e a l i z e d s i t u a t i o n i n which, the l i q u i d m e t a l f i l m f l o w s down a conical electrode  t i p under t h e I n f l u e n c e of g r a v i t y , n e g l e c t i n g t h e  e f f e c t s of surface  tension  and momentum t r a n s f e r from the s l a g to the  The t o t a l weight of oxygen Q  l i q u i d metal f i l m .  q  transferred  per u n i t  time a c r o s s the s l a g / m e t a l i n t e r f a c e i s the same as the flow r a t e of oxygen a t the system  exit  (electrode  t i p ) which may be c a l c u l a t e d by  i n t e g r a t i o n of the d i f f u s i o n f l o w e q u a t i o n over the e n t i r e a r e a of the electrode  cone, A.  The f l u x of oxygen can then be w i r t t e n a s :  Q o  using A =  =  2 A[0],  I  / V ift  gm. s e c . "  (5-1)  1  e  cos 6 [0]_£  - c o n c e n t r a t i o n o f 0 i n Fe a t s l a g m e t a l = 0.2 wt. % ( s a t u r a t i o n  interface,  of Fe a t 1550°C).  -3 = 0.014 gm. cm. D  d i f f u s i o n c o e f f i c i e n t of 0 i n Fe o ,m = 3 • 10"  4  cm.  2  sec.  I t has been assumed that compared to the f i l m  1  (40)  the d i f f u s i o n boundary l a y e r i s s m a l l  t h i c k n e s s and the s i t u a t i o n t h e r e f o r e  d i f f u s i o n i n a s e m i - i n f i n i t e medium.  reduces t o  The exposure time, t ^ , of a  surface  element t r a v e l l i n g between the base o f a cone and the t i p i s g i v e n by:  27rcos9 ,2/3/ . m \ 1/3 / R \ 5/3 3W ' \p g s i n e / V cos 6 / m ° y  e  v  1  where  T(5/6) T(l/3) . r(7/6)  ^  l  )  151  W  W  m  -  -1 melt r a t e i n gm. s e c .  =  1.94 gm. s e c . ^ f o r e l e c t r o d e p o s i t i v e  ' -  m  t o t a l v o l u m e t r i c m e l t r a t e i n cm.  W p  y  m  R  6  m  =  sec  -1  _ 3 —1 0.277 cm. s e c .  =  —  =  p  =  -3 7 . 0 — 2.6 = 4.4 gm. cm.  -  v i s c o s i t y of molten  =  0.05 p o i s e .  -  electrode  =  1.59 cm.  -  base angle o f cone  P  3  - p  - e f f e c t i v e d e n s i t y of l i q u i d due t o s l a g buoyancy  iron  iron  radius  = 45° (average o f e l e c t r o d e t i p s ) g  -  a c c e l e r a t i o n due t o g r a v i t y  r (5/6) r q / 3 ) ( 1  j—  , . „. •. . . , - E u l e r i a n f u n c t i o n s which e v o l v e from i n t e g r a t i o n of t w  = 3.259 and  t = 0.976 s e c . f o r the c o n d i t i o n s s t a t e d above, e 2 A = 11.234 cm. and  Q can now be c a l c u l a t e d t o be: o -3 -1 Q = 3.10 x 10 gm. s e c . q  The weigh-t % o f oxygen t r a n s p o r t e d m e t a l i s then found b y d i v i d i n g Q 1600  ppm 0.  q  Into  the m e l t i n g  by W , and I s equal  T h i s f i g u r e of 1600 ppm 0 r e p r e s e n t s  electrode  t o 0.16 wt. % 0 or  the t h e o r e t i c a l amount  152  of oxygen d i s s o l v e d i n the m e t a l d u r i n g e l e c t r o d e p o s i t i v e mode. gen  contents  the m e l t i n g  of pure i r o n i n the  Comparing t h i s w i t h the observed f i n a l  of pure i r o n e l e c t r o d e p o s i t i v e i n g o t s  oxy-  (185 ppm 0 ) , i t  appears that the b u l k o f t h e d i s s o l v e d oxygen i s l o s t between the drop detaching  from the e l e c t r o d e t i p and m e t a l s o l i d i f i c a t i o n i n the i n g o t .  In s e c t i o n s  (5.2) and (5.3) i t was shown that oxygen might be l o s t by  d i s s o l u t i o n o f oxide  i n the s l a g d u r i n g  the time the drops f a l l  the s l a g and by i n c l u s i o n f l o a t a t i o n from the i n g o t p o o l . III)  was made i n the e l e c t r o d e p o s i t i v e l i v e mold  mode  through  Ingot 83 (Table  [+(lm)] and ex-  a m i n a t i o n o f t h e s l a g cap showed t h a t d r o p l e t s of i r o n were r e t a i n e d i n the s l a g cap. The l a r g e r d r o p l e t s were analyzed content  and were found t o c o n t a i n 1750 ppm 0.  t h e o r e t i c a l l y c a l c u l a t e d oxygen content  f o r t h e i r t o t a l oxygen  This f i g u r e i s close  of 1600 ppm 0.  These drops had  been exposed to the s l a g f o r a f i n i t e l e n g t h o f time but s t i l l a h i g h oxygen c o n t e n t . fall  I t t h e r e f o r e appears t h a t oxide  to,the  retained  l o s s during  drop  through the s l a g i s n e g l i g i b l e and t h a t i n c l u s i o n f l o t a t i o n and r e -  moval from the i n g o t p o o l must account f o r the removal of the e l e c t r o c h e m i c a l l y produced oxygen. In order f o r aluminum c o n t a i n i n g  i n c l u s i o n s to nucleate  geneously i n the i n g o t p o o l , t h e r e must be h i g h s u p e r s a t u r a t i o n aluminum and oxygen i n t h e m e t a l . ings  (21).  homo-  of b o t h  This Is In contrast with B e l l ' s  find-  He found that i n g o t s made w i t h A.C. power had low s u p e r s a t u r -  a t i o n s o f aluminum and oxygen and i n c l u s i o n s were n u c l e a t e d f r e e z i n g ingot i n t e r f a c e . range" and occurs  only at the  I n c l u s i o n f l o t a t i o n i n D.C. i n g o t s i s " s h o r t  i n t h e i n g o t p o o l near the s l a g / m e t a l  i n t e r f a c e because  153  t h i s i n t e r f a c e i s the e l e c t r o c h e m i c a l r e a c t i o n s u r f a c e at w h i c h the supersaturations probably  a r e produced.  For t h i s reason  high  inclusion flotation is  independent of c o n v e c t i v e motion i n the p o o l .  5.7  The  Drop s i z e and  drop s i z e and  surface  tension  s u r f a c e t e n s i o n d a t a presented  i n Table  shows a marked dependence of drop s i z e on the e l e c t r o d e p o l a r i t y melting.  VI,  during  These d i f f e r e n c e s must a r i s e from the e f f e c t of oxygen on  i n t e r f a c i a l t e n s i o n at the s l a g / m e t a l  interface.  the  Drops produced at e l e c -  trode p o s i t i v e e l e c t r o d e s are the s m a l l e s t because they have a h i g h oxygen content t i p i n FeO.  a r i s i n g by the mechanism of a n o d i c . s a t u r a t i o n of the  Drops from c a t h o d i c e l e c t r o d e are l a r g e r because they have no  a p p r e c i a b l e oxygen content s o l v e d aluminum produced by  and  i n f a c t should be d e o x i d i z e d by  the c a t h o d i c processes  drops, d e s p i t e the f a c t  that A.C.  (Table X) which should  drop/slag  interface.  s l a g s have a h i g h e r b u l k  d e c r e a s e the i n t e r f a c i a l  T h i s d i s p a r i t y may  simply  o c c u r r i n g a t anodic  A.C.  concentration  t e n s i o n at the  a r i s e from the  :  cathodic In  general  our i n t e r p r e t a t i o n of the e l e c t r o c h e m i c a l phenomena  and  f i n d i n g s of W h i t t a k e r  diss-  negative  e l e c t r o d e c o n t a i n i n g aluminum produced by F a r a d a i c p r o c e s s e s . these f i n d i n g s support  the  taking place.  e l e c t r o d e drops a r e the l a r g e s t , even l a r g e r than e l e c t r o d e  of FeO  electrode  c a t h o d i c e l e c t r o d e t i p s , and  (5).  Our  estimated  values  they agree w i t h  of i n t e r f a c i a l  a r e not u n r e a s o n a b l y d i f f e r e n t from the v a l u e of approximately  the  tension 80Q  dynes.  -2 cm.  found by Y o k a b a s h v i l i e t . a l (41)  tween C a F  9  + 25 % A l - O .  s l a g and m i l d  f o r the i n t e r f a c i a l t e n s i o n  steel.  be-  154  5.8  The  S i g n i f i c a n c e of i r o n i n the s l a g caps  r e s u l t s presented  electroslag processing i s no d e t e c t a b l e  s l a g cap  p i c k up  i r o n i n the form of i r o n - o x i d e .  i r o n i n these s l a g s b e f o r e  observed i r o n contents remelting.  i n T a b l e X,show t h a t s l a g s used  a r i s e by chemical  only a very  c o n t a i n i n g s l a g a t the i n g o t p o o l / s l a g i n t e r f a c e .  FeO  37)  t i o n of FeO system.  thin  the  l a y e r of  These v i s u a l  ob-  the a n a l y s i s shown i n Table  the p o s s i b i l i t y of simple  chemical  X. forma-  i n the s l a g , one must study the thermodynamics of the F e - A ^ O ^  Three p o s s i b l e r e a c t i o n s which may  3  F  e  (l)  + A 1  2  F  e  (l)  +  F e  (l)  2°3(s)  A 1  +  2°3(s)  A 1  2°3(s)  * *  *  3  2  F e  F e  °(l)  °(l)  F e  +  2 [ A 1 }  +  °(l)  produce FeO  A 1  +  i n the s l a g  are:  Fe  ( 5  2°(g)  2  A 1 0  ( 5  (g)  would produce no FeO  during  electroslag melting.  only one-half  produced c a l c i u m  and  3 )  4 )  Therefore The  any FeO  FeO  occur-  produced reac-  amount of e l e c t r o c h e m i c a l l y  aluminum d i s s o l v e d i n the s l a g d u r i n g  5)  hence  of the o v e r a l l e l e c t r o c h e m i c a l  t h e r e must a l s o be an e q u i v a l e n t  "  (5  r i n g i n the s l a g must have an e l e c t r o c h e m i c a l o r i g i n . at the anode r e p r e s e n t s  "  < "  These r e a c t i o n s , however, a r e a l l h i g h l y endothermic and  t i o n , and  FeO  have a s u b s t a n t i a l l y t h i c k e r l a y e r o f  s e r v a t i o n s of the s l a g caps agree w i t h to c o n s i d e r  the b u l k of  Both e l e c t r o d e p o s i -  c o n t a i n i n g s l a g a t the i n g o t p o o l / s l a g i n t e r f a c e .  In order  the  e l e c t r o c h e m i c a l means d u r i n g  In the e l e c t r o d e n e g a t i v e mode ( i n g o t 39),  t i v e s l a g caps ( i n g o t s 31 and  There  they are u s e d ^ t h e r e f o r e  and  i s v e r y c l e a n i n appearance, w i t h  during  melting.  Various electrode on an a n o d i c i r o n  t i p s showing o x i d e electrode t i p .  present  156  One and  cathodic  can envisage a s t e a d y - s t a t e  r a t e of p r o d u c t i o n  o f b o t h anodes  components, t h e b u l k o f w h i c h d i s s o l v e i n the s l a g and con-  tinuously back-react  i n the s l a g as shown below:  (Ca and A l ) . + (FeO) , -> (CaO and A l 0 „ ) + Fe, slag slag z. 3 1 o  Thus s t e a d y - s t a t e present  (5-6)  amounts of t h e e l e c t r o c h e m i c a l p r o d u c t s a r e  i n the t o t a l s l a g cap. When the c u r r e n t i s i n t e r r u p t e d and e l e c -  trochemical  r e a c t i o n s cease, the b a c k - r e a c t i o n w i l l  u n t i l the s l a g caps s o l i d i f i e s the s l a g .  An e q u i v a l e n t  continue  and a r e s i d u a l FeO content  presumably  i s retained i n  amount o f r e s i d u a l Ca and A l must a l s o be r e +  t a i n e d and, i n the case of Ca, w i l l be p r e s e n t the s o l i d i f i e d quate to d e t e c t One  slag.  as Ca  2+ or C a  2  ions i n  P r e s e n t l y a v a i l a b l e a n a l y t i c a l methods a r e i n a d e -  t h i s amount w i t h o u t  ambiguities.  must a l s o c o n s i d e r why the FeO c o n t a i n i n g s l a g i n an e l e c -  trode p o s i t i o n s l a g cap i s found near the i n g o t / s l a g i n t e r f a c e .  This i s  not  immediately c l e a r , but i t must a r i s e from d e n s i t y e f f e c t s i n the s l a g  cap  s i n c e the FeO was produced i n the e l e c t r o d e r e g i o n .  p i c t u r e s o f c l e a n e l e c t r o d e t i p s produced by m e l t i n g negative melting  F i g . 43 shows  i n the e l e c t r o d e  o r A.C. mode and a h e a v i l y o x i d i z e d e l e c t r o d e t i p produced by i n the e l e c t r o d e p o s i t i v e mode.  T h i s lends  posed model r e q u i r i n g FeO s a t u r a t i o n a t an anodic 5.9' An  Melt  r a t e and heat  a t the e l e c t r o d e t i p .  t i o n when m e l t i n g  Ferrovac-E  electrodes  t o the p r o -  surface.  generation  important e f f e c t a s s o c i a t e d w i t h  heat g e n e r a t i o n  support  electrode p o l a r i z a t i o n i s  The average s p e c i f i c power consumpi n CaF  0  + 25 wt. % A l 0, s l a g s i s  157  12.6 in  kWs. • gm.  i n the e l e c t r o d e n e g a t i v e mode and 10.6 kWs. • gm.  the e l e c t r o d e p o s i t i v e mode.  We know from our s m a l l s c a l e s t u d i e s  t h a t a t the o p e r a t i n g e l e c t r o d e and i n g o t c u r r e n t d e n s i t i e s , an anodic s u r f a c e i s p o l a r i z e d t o a g r e a t e r degree than a c a t h o d i c s u r f a c e . heat  g e n e r a t i o n due t o c u r r e n t passage through these anodic and c a t h o d i c  p o l a r i z a t i o n r e s i s t a n c e s i s t h e r e f o r e g r e a t e r a t an anodic it  The  i s e v i d e n t t h a t the heat  e l e c t r o d e , and  t r a n s f e r p a t t e r n i n the e l e c t r o d e s l a g / i n t e r -  f a c e r e g i o n i s such that the heat  generated  i n the p o l a r i z e d slag layers  is  t r a n s f e r r e d e f f i c i e n t l y i n t o the e l e c t r o d e m e l t i n g p r o c e s s .  is  thus seen i n s p e c i f i c m e l t i n g e f f i c i e n c y and n o t d i r e c t l y  temperature d i s t r i b u t i o n . ingot surface manifests than i s observed  S i m i l a r l y , the heat  i t s e l f by p r o d u c i n g  The e f f e c t  i n a slag  g e n e r a t i o n a t an anodic  a g r e a t e r l i q u i d m e t a l volume  i n a cathodic ingot.  I t was a l s o noted  t h a t A.C. power produced t h e most  efficient  -1 m e l t i n g w i t h a s p e c i f i c power consumption o f 7.7 kWs. • gm.  If polari-  z a t i o n e f f e c t s d i d n o t e x i s t , then t h e r e i s no reason why t h e r e should be any d i f f e r e n c e s i n the m e l t i n g e f f i c i e n c y between A.C. and D.C. m e l t i n g . T h i s d i s p a r i t y can be e x p l a i n e d by u s i n g an e l e c t r o c h e m i c a l argument. The  c a t h o d i c products  d u r i n g D.C. m e l t i n g a r e c a l c i u m and aluminum, both  of which a r e s o l u b l e i n CaF2 s l a g s , and s u b s t a n t i a l l y i n c r e a s e the s l a g , conductivity.  J o s h i (32) found  a 35% i n c r e a s e i n s l a g c o n d u c t i v i t y when  m e l t i n g w i t h D.C. power as compared to A.C. power. t i v i t y s l a g , heat creases. this  g e n e r a t i o n i s decreased  With a h i g h e r  and t h e m e l t i n g  conduc-  e f f i c i e n c y de-  T h i s i s i n agreement w i t h t h e m e l t i n g e f f i c i e n c i e s observed i n  study.  1 58  5.10  The  Calcium o x i d e  s l a g s and i n g o t p o r o s i t y  i n s o l u b i l i t y of calcium  i n i r o n was q u i t e apparent when  Armco i r o n i n g o t s were made i n CaF^ + CaO s l a g s . chemical  The c a t h o d i c e l e c t r o -  p r o d u c t i s c a l c i u m w h i c h d i s s o l v e s i n the s l a g w h i l e ,  e l e c t r o d e n e g a t i v e mode, the i n g o t i s e f f i c i e n t l y o x i d i z e d . f y i n g Ingot m e t a l t h e r e f o r e had a h i g h oxygen content  i n the  The s o l i d i -  but no d i s s o l v e d  aluminum and the r e s u l t was carbon-monoxide blow h o l e f o r m a t i o n  by the.  carbon-oxygen r e a c t i o n :  [ C ]  The cient  Fe  +  [ 0 ]  Fe  carbon content  "  C  O  g  (  5  "  7  )  of Armco i r o n i s .012 wt. % and was s u f f i -  to produce i n g o t s o f h i g h p o r o s i t y .  In the case o f i n g o t 52  melted i n the e l e c t r o d e n e g a t i v e mode i n a 25 wt. % CaO s l a g , bubble f o r m a t i o n was so d r a s t i c that i r o n bubbles would form on the top o f the l i q u i d p o o l and r i s e up t o c o n t a c t  the m e l t i n g  electrode.  This i s ad-  m i t t e d l y an extreme case, b u t such c o n d i t i o n s cannot be t o l e r a t e d d u r ing  commercial e l e c t r o s l a g p r o c e s s i n g .  5.11  E f f e c t of i n c r e a s i n g i n g o t / e l e c t r o d e diameter  ratio  In an attempt to produce a wider v a r i a t i o n i n Z, i n g o t s were prepared u s i n g  t h e e l e c t r o d e / i n g o t diameter r a t i o as a v a r i a b l e .  75 and 76 were made u s i n g 3.2 cm. diameter F e r r o v a c - E  Ingots  electrodes but  they were melted i n t o a 7.6 cm. d i a . copper mold i n s t e a d of the u s u a l 5.5  cm. d i a . copper mold.  Ingot 76 was made i n the e l e c t r o d e  negative  159  mode and had a f i n a l average oxygen c o n t e n t what g r e a t e r  than t h e 480 ppm. 0 measured as t h e average oxygen  of 5.5 cm. i n g o t s .  the range o f oxygen contents The h i g h e r  of 153 ppm. 0 w h i c h l i e s i n s i d e  measured f o r 5.5 cm. e l e c t r o d e p o s i t i v e i n -  f i n a l oxygen content  o f i n g o t 75 must have a r i s e n i n  p a r t , because o f the lower c u r r e n t d e n s i t y a t the anodic a lower c u r r e n t d e n s i t y which s t i l l produced more e f f i c i e n t higher  content  Ingot 76 was made i n t h e e l e c t r o d e p o s i t i v e mode  [+(lm)] and had an average oxygen content  gots.  o f 548 ppm. 0 which i s some-  saturated  ingot  surface,  the i n t e r f a c e i n FeO and  n e t e l e c t r o c h e m i c a l o x i d a t i o n of t h e i r o n .  The  c u r r e n t d e n s i t y a t the c a t h o d i c e l e c t r o d e i n t e r f a c e may have r e -  sulted i n less e f f i c i e n t  aluminum t r a n s f e r to the m e l t i n g  t h e r e f o r e l e s s e f f i c i e n t d e o x i d a t i on of t h e i n g o t p o o l .  e l e c t r o d e and These r e s u l t s  would i n d i c a t e t h a t t h e i n g o t p o o l / s l a g i n t e r f a c e i s the most  important  e l e c t r o c h e m i c a l r e a c t i o n s i t e d u r i n g D.C. e l e c t r o s l a g m e l t i n g .  5.12  E f f e c t o f aluminum a d d i t i o n a t the e l e c t r o d e d u r i n g  melting  Experiments were c a r r i e d out i n which aluminum w i r e was attached  to Ferrovac-E  electrodes  t o make i n g o t s 80 and 81.  In these  i n g o t s , i f a l l the aluminum from t h e w i r e was d i s s o l v e d i n the i r o n during melting,  t h e aluminum content  would be 2000 ppm.  The aluminum  a n a l y s i s r e s u l t s g i v e n i n T a b l e VLT show t h a t t h e r e was e f f i c i e n t f e r of aluminum i n t o t h e i n g o t m e t a l .  These aluminum a n a l y s i s f i g u r e s  a r e t o t a l aluminum f i g u r e s because b o t h m a t r i x  aluminum and aluminum  i n i n c l u s i o n s a r e counted b y the n e u t r o n a c t i v a t i o n technique. than normal t o t a l oxygen content  trans—  The lower  o f i n g o t 80 (259 ppm. 0 as compared t o  an average of 480 ppm. 0) i n d i c a t e s t h a t the added aluminum was r e s p o n -  160  s i b l e f o r a higher  than normal r a t e of i n c l u s i o n f o r m a t i o n and  The p r e s e n c e of the excess  aluminum at the m e l t i n g c a t h o d i c e l e c t r o d e  i n c r e a s e d the drop s i z e to 3.46 comparable to the A.C. oxygen content expected content  as shown i n T a b l e V I , a drop The  e f f e c t of aluminum w i r e on  i n view of our other o b s e r v a t i o n s . (576 ppm.  trode m e t a l , but  0) was  of 185  The  final  i n g o t oxygen  ppm.  0.  One  would have expected  e x c e s s i v e oxygen d i s s o l u t i o n i n the m e l t i n g  to the i n g o t which had  elec-  a final Al  content  T h i s agrees w i t h the r e s u l t s g i v e n i n T a b l e VI which show not a p p r e c i a b l y g r e a t e r than the drop  of a normal e l e c t r o d e p o s i t i v e i n g o t . on the b a s i s that b o t h oxygen and i n t o the i n g o t p o o l . would be h i g h e r  The  oxide  These r e s u l t s c o u l d be  i n c l u s i o n s subsequently  i n aluminum content than  e f f e c t o f adding  A I S I 1095  explained  precipitating  normal e l e c t r o d e p o s i t i v e i n -  aluminum i s to r e t a i n oxygen as i n g o t o x i d e  5.13  A l l o y losses during  Hence the  net  particles.  remelting  steel  i n g o t a n a l y s i s r e s u l t s g i v e n i n T a b l e V I I I show t h a t  were s i g n i f i c a n t l o s s e s i n carbon and manganese l o s s .  size  aluminum are t r a n s f e r r e d e f f i c i e n t l y  g o t s , and might be l e s s e f f i c i e n t l y removed i n t o the s l a g .  The  the  t h i s a p p a r e n t l y does not o c c u r even though the aluminum  the drop s i z e of i n g o t 81 was  5.13.1  the  much g r e a t e r than the normal e l e c t r o d e p o s i -  transferred e f f i c i e n t l y ppm.  size  of i n g o t 81 made i n the e l e c t r o d e p o s i t i v e mode i s un-  aluminum to prevent  of 2621  gm.  drop s i z e .  t i v e i n g o t oxygen content  was  removal.  Ingot  there  s i l i c o n d u r i n g r e m e l t i n g , hut  67 m e l t e d i n the e l e c t r o d e p o s i t i v e mode l o s t  no 6%  161  of  its initial  carbon but no s i l i c o n , w h i l e i n g o t 68 m e l t e d  i n the e l e c -  t r o d e n e g a t i v e mode l o s t 7% of i t s carbon and 27% o f i t s i n i t i a l content. produced  silicon  These a l l o y l o s s e s must be a s s o c i a t e d w i t h the a n o d i c a l l y l a y e r of FeO c o n t a i n i n g s l a g a t t h e anodic i n t e r f a c e whether  t h i s i n t e r f a c e be a t the e l e c t r o d e o r t h e i n g o t p o o l .  I n t h e case of  i n g o t 67 t h e carbon l o s s s h o u l d occur a t t h e e l e c t r o d e .  The l a c k of  s i l i c o n l o s s i n t h i s mode of m e l t i n g can be e x p l a i n e d i f one c o n s i d e r s that a continuous s u p p l y of h i g h carbon l i q u i d metal i s exposed to t h e o x i d i z i n g i n t e r f a c e a t such a r a t e t h a t the carbon l e v e l w i l l never be d e p l e t e d s u f f i c i e n t l y t o p e r m i t s i l i c o n o x i d a t i o n . Why then i s s i l i c o n l o s s e x p e r i e n c e d i n the e l e c t r o d e n e g a t i v e mode?  The a n o d i c i n g o t i s  b e i n g o x i d i z e d d u r i n g m e l t i n g t o such an e x t e n t that t h e carbon a t the i n t e r f a c e i s s u f f i c i e n t l y d e p l e t e d t o a l l o w s i l i c o n t o o x i d i z e . assuming an i n g o t p o o l / s l a g i n t e r f a c e temperature  By  of 1600°C, and n e g l e c -  t i n g the i n t e r a c t i o n e f f e c t s between carbon and s i l i c o n ,  i t can be shown  that the e q u i l i b r i u m oxygen content o f i r o n w i t h 0.975 wt. % carbon i s approximately  20 ppm. 0, an oxygen l e v e l too low to o x i d i z e  S i l i c o n w i l l not o x i d i z e at t h i s  temperature  until  silicon.  the oxygen l e v e l i s  above a p p r o x i m a t e l y 80 ppm. 0 which c o u l d not be reached under e q u i l i b r i u m c o n d i t i o n s u n l e s s t h e carbon l e v e l i s d e p l e t e d t o a p p r o x i m a t e l y 0.2 wt. % at  an i n t e r f a c e .  Therefore s i l i c o n o x i d a t i o n only occurred i n the e l e c -  t r o d e n e g a t i v e mode (anodic i n g o t ) because t h e degree  of mixing  at the  i n g o t p o o l / s l a g i n t e r f a c e was low. Hence t h e r a t e o f carbon t r a n s p o r t to  t h i s i n t e r f a c e was i n s u f f i c i e n t  t o accommodate t h e r a t e o f o x i d a t i o n .  In  the e l e c t r o d e p o s i t i v e case, the r a t e , o f carbon s u p p l y t o the a n o d i c  162  i n t e r f a c e was s u f f i c i e n t l y h i g h and no s i l i c o n l o s s e s were observed. The f a c t that no s i l i c o n c o n t a i n i n g i n c l u s i o n s were found i n these i n gots tends to s u b s t a n t i a t e the i d e a t h a t s i l i c o n l o s s e s took p l a c e at the i n g o t p o o l / s l a g i n t e r f a c e . Table V are higher  The oxygen contents  reported i n  than b o t h of the C-0 o r S i - 0 e q u i l i b r i u m v a l u e s .  T h i s s u p p o r t s the p r o p o s a l  that i n c l u s i o n p r e c i p i t a t i o n i s taking  place  i n the i n g o t p o o l s u r f a c e r e g i o n and n o t i n the b u l k of the p o o l .  5.13.2.  A I S I 430 s t a i n l e s s s t e e l The i n g o t a n a l y s e s  information given i n Table  chromium, s i l i c o n , - and s u l p h u r were l o s t d u r i n g stainless steel,  IX shows  the m e l t i n g  that  o f A I S I 430  and t h a t the t o t a l a l l o y l o s s e s were g r e a t e s t i n the  electrode negative  mode.  These o b s e r v a t i o n s  a g a i n show t h a t the anodic  i n g o t s u r f a c e i s c a p a b l e o f more e f f i c i e n t o x i d a t i o n of a l l o y i n g e l e ments than i s the anodic  electrode t i p .  The lower r a t e of m e t a l  p o r t through the i n g o t s u r f a c e produces a l o n g e r o x i d i z i n g anodic  trans-  exposure time t o the  s l a g i n t e r f a c e than i s encountered at an anodic  A c a l c u l a t i o n f o l l o w i n g the l i n e s i n d i c a t e d i n S e c t i o n  electrode.  (5.5) was done f o r  the known melt r a t e and e l e c t r o d e diameter of t h i s m a t e r i a l , and Q , the o -3 f l u x of oxygen i n t o an anodic e l e c t r o d e t i p was found to be 2.0 x 10 gm. sec. ^  I f i t i s assumed t h a t an e q u i v a l e n t  amount of chromium i s o x i d i z e d  3+ to the Cr ium  s t a t e and d i s s o l v e s i n t h e s l a g , t h i s corresponds to a chrom-  l o s s of 0.22 wt. % from the a l l o y , which i s c l o s e to the observed  chromium l o s s o f 0.19 wt. % i n t h e e l e c t r o d e p o s i t i v e mode. i t y o f oxygen i n pure Fe-Cr a l l o y s i s r e l a t i v e l y h i g h ,  The s o l u b i l -  (~400 ppm a t 1600°C  i n Fe-20 wt. % Cr) but the p r e s e n c e of 0.26 wt. % S i i n t h e a l l o y  decreases  1 63  the oxygen s o l u b i l i t y to 100  ppm.  0.  In the absence of d i s s o l v e d A l  would expect to f i n d S i c o n t a i n i n g i n c l u s i o n s i n t h i s m a t e r i a l . t h i s was  not observed, and  one  However  the i n c l u s i o n s found c o u l d o n l y have been  formed i n a s i l i c o n - d e p l e t e d r e g i o n .  5.14  Remelting of n i c k e l e l e c t r o d e s  The m e l t r e c o r d r e s u l t s of the two T a b l e V. gen  n i c k e l i n g o t s are g i v e n i n  Ingot 69 melted i n the e l e c t r o d e n e g a t i v e  l e v e l of 7 ppm.,  while  s u l a t e d mold mode has  mode has  a low  i n g o t 71 m e l t e d i n the e l e c t r o d e p o s i t i v e i n -  a r e l a t i v e l y h i g h oxygen content  of 326  ppm.  T h i s i s c o n t r a r y to the f i n d i n g s f o r pure i r o n i n g o t s i n which negative  i n g o t s had  71 were found to be very  the h i g h e r  oxygen c o n t e n t .  and  e l e c t r o c h e m i c a l means.  inclusions i n ingot  The  a ,  dependence of  final  I t i s p o s s i b l e that  e l e c t r o d e of i n g o t 69  the  was  the carbon-oxygen r e a c t i o n i n the i n g o t p o o l removed most of  the a n o d i c a l l y produced oxygen. aluminum and  However, i n the e l e c t r o d e p o s i t i v e case,  p o s s i b l y c a l c i u m were d e p o s i t e d  e f f i c i e n t l y to produce a s u f f i c i e n t w h i c h consumed the anodic trapped  electrode  aluminum c o n t a i n i n g , whereas i n g o t 69 c o n t a i n e d  r a t e of aluminum d e p o s i t i o n i n the c a t h o d i c and  0.  of these n i c k e l i n g o t s appears to have a r i s e n from a com-  b i n a t i o n of chemical  low,  The  l a r g e number of carbon-monoxide blow h o l e s .  oxygen content  oxy-  during  concentration  of d e o x i d i z i n g elements  oxygen to form i n c l u s i o n s . H o s t of t h e s e were  ingot s o l i d i f i c a t i o n .  i n g o t s were made from n i c k e l w i t h electrode negative  i n the i n g o t p o o l more  The  c o r o l l a r y of t h i s i s t h a t i f  a n e g l i g i b l e carbon c o n t e n t ,  i n g o t would have the h i g h e r  oxygen  content.  then  the  164  5.15  The  A.C.  melting  of pure  iron  e l e c t r o c h e m i c a l phenomena o c c u r r i n g at the two  i n t e r f a c e s d u r i n g A.C.  melting  asymmetry of the anodic  and  are extremely d i f f i c u l t  to study.  The  c a t h o d i c p o l a r i z a t i o n curves on pure  iron  suggests a mechanism by which a degree of e l e c t r o c h e m i c a l could occur. Bell  I f t h i s was  slag/metal  so, then t h e r e would be a net  (21) measured oxygen contents  i n pure i r o n A.C.  rectification  chemical  effect.  i n g o t s which were  i n excess of the t h e o r e t i c a l l y p r e d i c t e d oxygen l e v e l t h a t would be duced by  e q u i l i b r a t i o n w i t h Al^O^  s u b s t a n t i a l D.C. from the A.C.  current  (5-^-10 A.)  power source.  I t can  m i c a l r e c t i f i c a t i o n does occur i r o n , and  t h a t t h i s D.C.  containing slags. i n the m e l t i n g t h e r e f o r e be  d u r i n g A.C.  He  pro-  a l s o measured a  u n i t which d i d not concluded t h a t  arise  electroche-  e l e c t r o s l a g processing  of pure  c u r r e n t d e p o s i t s oxygen i n the m e t a l which g i v e s  r i s e to n o n - m e t a l l i c i n c l u s i o n s .  5.16  E l e c t r o c h e m i c a l phenomena i n commercial  I t i s n e c e s s a r y to c o n s i d e r to commercial D.C.  ESR  ESR  the f i n d i n g s of t h i s study i n r e l a t i o n  practices.  I t i s apparent t h a t the c u r r e n t d e n s i t y at the anodic whether i t be  the e l e c t r o d e or the i n g o t , should  be m a i n t a i n e d below  c u r r e n t d e n s i t y a t which i n t e r f a c e s a t u r a t i o n i n FeO  occurs.  m e t a l o x i d a t i o n d u r i n g m e l t i n g w i l l be kept to a minimum and l o s s e s w i l l be  l e s s extreme.  t i e s i n commercial ESR  In view of the f a c t  furnaces  nace., t h i s c o n d i t i o n should  surface, the  In t h i s  way,  alloy  t h a t the c u r r e n t  densi-  are lower than those used i n U.B.C. f u r -  be s a t i s f i e d when u s i n g  slags with  a high  165  oxide content. The mechanism o f drop f o r m a t i o n on a commercial e l e c t r o d e i s one of m u l t i p l e drop f o r m a t i o n and i s t h e r e f o r e q u i t e d i f f e r e n t from a s m a l l e r f u r n a c e i n which drops  form one a t a time.  I t i s n o t known what  e f f e c t t h i s w i l l have on the r a t e a t which the F a r a d a i c r e a c t i o n e n t e r the m e l t i n g m e t a l .  products  I t i s also d i f f i c u l t to d e f i n e the e f f e c t i v e  e l e c t r o a c t i v e s u r f a c e area and any model developed t i p w i l l be v e r y  that i n  f o r such an e l e c t r o d e  complicated.  C o n v e c t i v e movement i n the l i q u i d  i n g o t p o o l w i l l be important  w i t h r e s p e c t t o i n c l u s i o n n u c l e a t i o n i n the l i q u i d m e t a l and i n c l u s i o n removal by f l o t a t i o n . liquid  L i t t l e i s known about the f l o w p a t t e r n s i n the  p o o l and i t i s t h e r e f o r e unreasonable  nomena .  t o s p e c u l a t e about such phe-  CHAPTER 6  CONCLUSIONS  The  observations  on t h e b a s i s o f the anodic  made i n the m e l t program can be i n t e r p r e t e d and c a t h o d i c F a r a d a i c r e a c t i o n mechanisms  proposed t o e x p l a i n t h e r e s u l t s of the s m a l l s c a l e s t u d i e s . dominant anodic CaF  2  r e a c t i o n a t pure i r o n s u r f a c e s  + CaO s l a g s i s the anodic  Fe  -y  The p r e -  i n CaF^ + Al^O^ s l a g s and  c o r r o s i o n of i r o n a c c o r d i n g t o :  2+ Fe + 2e  At s u f f i c i e n t l y h i g h c u r r e n t d e n s i t i e s the s l a g adjacent  t o the anodic  2+ i n t e r f a c e becomes s a t u r a t e d i n Fe to i r o n oxide  saturation.  of the e l e c t r o d e m e t a l . posed t o be t h e  i o n s , which i s f o r m a l l y  This s a t u r a t i o n c o n d i t i o n leads  equivalent to o x i d a t i o n  The c a t h o d i c r e a c t i o n a t i r o n s u r f a c e s  i s pro-  d e p o s i t i o n of aluminum a t low c u r r e n t d e n s i t i e s and the  c o d e p o s i t i o n of both aluminum and c a l c i u m Aluminum d i s s o l v e s i n i r o n and c h e m i c a l  at higher  current d e n s i t i e s .  r e a c t i o n i n the ingot pool be-  tween the e l e c t r o c h e m i c a l l y produced oxygen and aluminum produces alumina or aluminum c o n t a i n i n g i n c l u s i o n s i n the f i n a l i n g o t m e t a l . elusions are t y p i c a l l y small anodic  (<20 microns) i n diameter.  The proposed  and c a t h o d i c r e a c t i o n s have a h i g h net r e v e r s i b i l i t y  current d e n s i t i e s , t h e r e f o r e the s l a g composition changed d u r i n g  These i n - '  even a t h i g h  remains e s s e n t i a l l y u n -  remelting.  At e q u i v a l e n t e l e c t r o d e anodic i n the U.B.C. e l e c t r o s l a g f u r n a c e  and c a t h o d i c  a higher 166  current densities  degree o f anodic p o l a r i z a t i o n  167  i s observed.  The  manifests i t s e l f p o s i t i v e mode. lished  heat g e n e r a t i o n  due  At e x c e s s i v e l y t i p and  high current d e n s i t i e s , the m e l t i n g  to a r c heat g e n e r a t i o n  As has  to t h i s p o l a r i z a t i o n  electrode  an a r c i s  at the e l e c t r o d e t i p .  been observed by other workers, a l l o y l o s s e s of  s t u d i e s on  melting.  arization.  Thus the p o l a r i z e d anode behaves e s s e n t i a l l y  a pure i r o n  surface.  Results  alloy  from  f a c e i s the most important r e a c t i o n s i t e w i t h r e s p e c t deoxidation  reactions.  e s s e n t i a l l y independent of the  to  depol-  as i f i t were  I t appears, from t h i s work, t h a t the i n g o t p o o l / s l a g  and  easily  the g a l v a n o s t a t i c p u l s i n g of i r o n a l l o y anodes  demonstrate t h a t such l o s s e s a r e a s s o c i a t e d w i t h n e g l i g i b l e  oxidation  estab-  c o n d i t i o n s become extremely  o x i d i z e d elements are v e r y s u b s t a n t i a l d u r i n g D.C. the p r e s e n t  resistance  as a lower s p e c i f i c power consumption i n the  at the e l e c t r o d e  unstable  due  inter-  electrochemical  Such r e a c t i o n s occur i n a manner  atmosphere.  REFERENCES  1.  M. E t i e n n e and A. M i t c h e l l : P r o c . Sec. Symp. on ESR t e c h n o l o g y , P a r t I I , M e l l o n I n s t i t u t e , Sept. 1969.  2.  G.K. Bhat, J.B, T o b i a s , and R.L. 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