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Stress corrosion cracking of 316 stainless steel in caustic solutions Crowe, David Charles 1982

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STRESS CORROSION CRACKING OF 316 S T A I N L E S S STEEL IN CAUSTIC SOLUTIONS  by  DAVID B.Sc,  CHARLES  (Mechanical  The U n i v e r s i t y  of  CROWE Engineering),  Manitoba,  1977  A T H E S I S SUBMITTED IN P A R T I A L FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF A P P L I E D SCIENCE  :"  in  THE FACULTY OF GRADUATE STUDIES Department  of Metallurgical  We a c c e p t t h i s to  the  thesis  required  as  Engineering  conforming  standard  THE UNIVERSITY OF B R I T I S H COLUMBIA May 1982  0  David Charles Crowe,  1982.  In  presenting  requirements of  British  it  freely  agree for  this f o r an  available  that  I agree  understood  that  that  may  be  fulfilment at the  University shall  and  I  study.  copying  granted  by  f i n a n c i a l gain  or publication  shall  n o t be  allowed  Department  of  M&M/fu'ry/cq-l  The University of British 2075 W e s b r o o k P l a c e Vancouver, Canada V6T 1W5  Date  thesis  of  my  It i s  of this without  E^/i/ie<?r/'/?Q  Columbia  further  t h e head  permission.  make  of this  h i s or her representatives. copying  of the  the Library  f o r extensive  purposes  o r by  degree  f o r reference  permission  scholarly  in partial  advanced  Columbia,  department  for  thesis  thesis my  written  ABSTRACT  Stress steel and  was  corrosion  studied  2.5 m o l / k g  statically (SSRT).  cracking  i n hot (92°C)  NaOH  + 0.423  controlled  Anodic  slow  steel,  red  in the transpassive  unless tion,  together  kinetics  Cracking  was  investigated and  Region  The  and  that  rupture  techniques crack  electron  NaOH  also  determined SCC  were  of stress  crack  that  mechanism  ii  behavior  of cracking  process,  i n 3.35, 8 and  with  Na S. 2  intensity  (Kj),  fractography  Region  I  was films  (Kj-dependent)  were  observed.  associated  occurred  solu-  to study the  and c o r r o s i o n  SCC was  no  region,  + Na^S  + 0 . 4 2 3 mol/kg  diffraction.  occur-  region.  used  microscopy  for ;  NaOH, w i t h  I n t h e NaOH  ( E ) . Crack  instabilities  and d i s s o l u t i o n  techniques  propagation  as a f u n c t i o n  indicated  the basic  were  condition.  II (K-j-independent)  at.which  testing  NaOH  potentio-  i n the active-passive  corrosion  by e l e c t r o n  results  potentials  rate  i n 3.35 m o l / k g  ( T ) and p o t e n t i a l  by s c a n n i n g  of  2  i n the active-passive  mechanics  studied  o f 3.35 m o l / k g  f o r N i , C r and Fe.  NaOH, a n d 12 m o l / k g  temperature studied  those  316 s t a i n l e s s  N a S b y means  strain  to crack  detected  of stress  mol/kg  mol/kg  region  i n the s e n s i t i z e d  Fracture  12  with  tendency  SCC was  solutions  p o l a r i z a t i o n curves  the  detectable  (SCC) o f type  with  i n passive involved  dissolution  a  films film  processes  exerting of  the  predominant  sulfide  processes  could  rate  containing not  be  control  in  solution,  eliminated  iii  as  Region hydrogen a  II.  In  the  case  embrittlement  contributing  factor.  TABLE OF CONTENTS  Page Abstract  •  •  n  Table o f Contents  i  L i s t o f Tables  ,  v  L i s t of Figures  v  L i s t o f Symbols and A b b r e v i a t i o n s  2.  i  i  l  xi  Acknowledgement  1.  i  i  v  x  i  n  INTRODUCTION  1  1.1  Temperature  2  1.2  Electrochemical  1.3  Composition and C o n c e n t r a t i o n  1.4  A l l o y Composition  1.5  Thermomechanical  1.6  Stress  1.7  Mechanisms o f SCC  1.8  Present  Potential  12 o f the Environment  —  5  Effects  7  Intensity  8 8  Objectives  11  EXPERIMENTAL 2.1  2.2  4  12  P o l a r i z a t i o n Curves  ...  12  2.1.0  Introduction  12  2.1.1  M a t e r i a l s and P r e p a r a t i o n  12  2.1.2  Procedure  16  Slow S t r a i n Rate T e s t  17  2.2.0  Introduction  17  2.2.1  M a t e r i a l s and P r e p a r a t i o n  18  2.2.2  Procedure  . iv.  20  Page  2.3  2.4  3.  F r a c t u r e Mechanics T e s t i n g  23  2.3.0  Introduction  23  2.3.1  M a t e r i a l s and P r e p a r a t i o n  27  2.3.2  Procedure  30  Electron D i f f r a c t i o n Analysis of.Surface.Films  32  2.4.0  Introduction  32  2.4.1  Sampling f o r C o r r o s i o n  2.4.2  Procedure  Film Analysis  33  RESULTS 3.1  3.2  3.3  32  •  35  Anodic P o l a r i z a t i o n Curves  35  3.1.0  NaOH  35  3.1.1  NaOH + Na S  39  2  Slow S t r a i n Rate T e s t s  45  3.2.0  NaOH  45  3.2.1  NaOH + NagS  52  F r a c t u r e Mechanics T e s t i n g 3.3.0  NaOH  ........  52  ...  52  3.3.0.0  E f f e c t of Stress  3.3.0.1  Temperature  3.3.0.2  E f f e c t o f NaOH C o n c e n t r a t i o n  60  3.3.0.3  E f f e c t o f Applied Potential  60  3.3.0.4  E f f e c t o f Cold Work  63  3.3.0.5  Fractography  63  3.3.0.6  pH Measurement  71  v  Intensity  Effect  ..  52 56  Page  3.3.1  NaOH + Na S  77  2  3.3.1.0  2.5 mol/kg NaOH + 0.423 mol/kg Na S 2  ( S i m u l a t e d White L i q u o r )  3.4 4.  3.3.1.1  12 mol/kg NaOH + 0.423 mol/kg Na S .....  77  3.3.1.2  Fractography  79  3.3.1.3  S o l u t i o n Appearance  79  2  E l e c t r o n D i f f r a c t i o n A n a l y s i s o f S u r f a c e Films i n NaOH..  82  DISCUSSION  86  4.1  I n t e r p r e t a t i o n o f Anodic P o l a r i z a t i o n Curves  86  4.2  SCC S u s c e p t i b i l i t y  92  4.3  Crack Growth Rates and t h e Mechanism o f C r a c k i n g  97  4.3.1  Potential  97  4.3.2  D i s s o l u t i o n Rate and Crack Growth Rate  4.3.3  K i n e t i c s o f Crack Growth Rates  4.4  4.5 5.  77  .  Dependence o f Crack Growth Rate  Fractography and t h e D i s s o l u t i o n Mechanism 4.4.0  Corrosion Deposits  4.4.1  F r a c t u r e Mode ..  —  98 101 .........  .  103 105  E l e c t r o n D i f f r a c t i o n A n a l y s i s o f S u r f a c e Films i n NaOH..  SUMMARY  103  -..  BIBLIOGRAPHY  109 HI 113  vi  LIST  OF  TABLES  Table  Page  I  Chemical  Composition o f S t e e l s  14  II  Mechanical P r o p e r t i e s o f SSRT Specimens  19  III  Summary o f F r a c t u r e Mechanics  57  IV  Electron Diffraction  T e s t Data  P a t t e r n Data  vii  85  LIST OF FIGURES Figure  1  Page  T e s t c e l l f o r p o l a r i z a t i o n s t u d i e s a) t e s t e l e c t r o d e b) Luggin c a p i l l a r y c) c o u n t e r e l e c t r o d e d) temperature probe 3) n i t r o g e n purge f ) l i d g) beaker  15  Slow s t r a i n r a t e t e s t c e l l a) specimen b) Luggin c a p i l l a r y c) c e l l d) l i d e) r e f l u x condenser f.) temperature probe g) n i t r o g e n purge h) T e f l o n c e l l bottom i ) counter electrode  21  3  T-notch double c a n t i l e v e r  25  4  F r a c t u r e mechanics t e s t i n g c e l l c) p i n s d) c e l l l i d e) beaker  2  5.  6  7  8  9,  beam specimen a) specimen  b) g r i p s .  29  Anodic p o l a r i z a t i o n c u r v e , 316 s t a i n l e s s s t e e l r o d , 3.35 mol/kg NaOH, 92 °C  36  Anodic p o l a r i z a t i o n c u r v e , 316 s t a i n l e s s s t e e l 3.35 mol/kg NaOH, 92 °C  37  Anodic p o l a r i z a t i o n curves a t s e l e c t e d 316 s t a i n l e s s s t e e l r o d , 92 °C  plate,  NaOH c o n c e n t r a t i o n s , 38  Anodic p o l a r i z a t i o n , curves a t s e l e c t e d temperatures, 316 s t a i n l e s s s t e e l r o d , 3.35. mol/kg NaOH Anodic p o l a r i z a t i o n  40  c u r v e s , chromium, 3.35 and 8 mol/kg  NaOH, 92 °C  41  10  Anodic p o l a r i z a t i o n c u r v e , n i c k e l , 3.35 mol/kg NaOH, 92 ° C .  42  11  Anodic p o l a r i z a t i o n  43  12  Anodic p o l a r i z a t i o n c u r v e , 316 s t a i n l e s s s t e e l mol/kg NaOH + 0.423 mol/kg Na S, 92 °C  c u r v e , i r o n , 3.35 mol/kg NaOH, 92 °C ... r o d , 2.5  2  13  14  46  Anodic p o l a r i z a t i o n curves a t s e l e c t e d temperatures, 316 s t a i n l e s s s t e e l p l a t e , 2.5 mol/kg NaOH + 0.423 mol/kg Na S  47  E f f e c t o f p o t e n t i a l on p e r c e n t r e d u c t i o n i n area d u r i n g slow s t r a i n r a t e t e s t s , 3.35 mol/kg NaOH, 92 °C. Anodic p o l a r i z a t i o n curve f o r 316 s t a i n l e s s s t e e l r o d  48  2  15  44  Anodic p o l a r i z a t i o n curves i n s o l u t i o n s o f s e l e c t e d NaOH c o n c e n t r a t i o n w i t h Na^S, 316 s t a i n l e s s s t e e l r o d , 92 °C  viii  Page  Figure  16  SSRT specimen a f t e r t e s t i n g  17  SSRT specimen a f t e r t e s t i n g a t -0.95'. V  18  S u r f a c e f i l m on SSRT specimens t e s t e d a) 0.25 V , b) -1.00 V S C E  19  a t -0.10 V  S C E  i n NaOH s o l u t i o n . . i n NaOH s o l u t i o n . .  S C E  i n NaOH  50 50  solution 51  $ C E  E f f e c t o f p o t e n t i a l on p e r c e n t r e d u c t i o n i n area d u r i n g slow s t r a i n r a t e t e s t s 2.5 mol/kg NaOH + 0.423 mol/kg Na S, 92 °C. Anodic p o l a r i z a t i o n curve f o r 316 s t a i n l e s s s t e e l rod ........  53  SSRT specimen a f t e r t e s t i n g Na S s o l u t i o n  54  2  20  a t -1.15 V _  r F  i n NaOH +  2  21  22  Crack growth r a t e versus s t r e s s i n t e n s i t y a t s e l e c t e d temperatures i n 3.35 mol/kg NaOH, -0.10  55  Crack growth r a t e v e r s u s s t r e s s i n t e n s i t y a t s e l e c t e d temperatures i n 12 mol/kg NaOH,,-0.10 V -..  58  A r r h e n i u s p l o t o f t h e Region I I c r a c k growth r a t e s i n 3.35 and 12 mol/kg NaOH, -0.10 V  59  Crack growth r a t e versus s t r e s s i n t e n s i t y a t s e l e c t e d c o n c e n t r a t i o n s , 92 °C, -0.10 V  61  S C E  23  $ C E  24  NaOH  $ C E  25  26  Crack growth r a t e versus s t r e s s i n t e n s i t y a t s e l e c t e d p o t e n t i a l s i n 3.35 mol/kg NaOH 92 °C  62  F r a c t u r e s u r f a c e a f t e r t e s t i n g i n 3.35 mol/kg NaOH, 92 °C, -0.10 V , 45 - 47 M P a * ¥  64  S C E  27  28  29  F r a c t u r e s u r f a c e s a f t e r t e s t i n g i n 3.35 mol/kg NaOH, 92 °C, -O.lO.V-pp-at a) 38-40 MPav^m, b) 46-48 MPavfii", b c ) 55-60 MPa^m r.... Crack b r a n c h i n g . S t r e s s i n t e n s i t y r i s e s from 75 t o 105 MPavrn i n t h i s view  68  D e p o s i t s on f r a c t u r e s u r f a c e a f t e r t e s t i n g NaOH, 92 °C, -0.10 V  69  i n 3.35 mol/kg  $ C E  30  65,66  F r a c t u r e s u r f a c e s a f t e r t e s t i n g i n 3.35 mol/kg NaOH, -0.10 V , a) 82 °C, 33-34 MPavrrT, b) 72 °C, 38-40 MPa^.. S C E  ix  70  Figure  31  Page  i n 12 mol/kg NaOH, 92 °C,  Fracture surface a f t e r t e s t i n g -0.10 V , 28-29 MPavfiT  72  $ C E  32  33  Intergranular facet displaying intersecting transgranular cracking. This i s a m a g n i f i c a t i o n o f the center o f F i g u r e 31  73  C o r r o s i o n d e p o s i t s a f t e r t e s t i n g i n 3.35 mol/kg NaOH, 92 °C, a) -0.175 V . , 36 MPaVm, b) -0.10 42-44 MPav¥, c) 0.00 V , 41-43 MPav^m .... 74,75 c  r F  $ e E  34  F r a c t u r e s u r f a c e s a f t e r t e s t i n g i n 3.35 mol/kg NaOH, 92 °C, a) -0.175 V , 33-35 MPa^/m, b) 0.00 V , 43-44 M P a v € ...  76  Crack growth r a t e versus s t r e s s i n t e n s i t y + 0.423 mol/kg Na S, 92 °C, -1.175 V  78  S C E  35  s  Fracture surfaces a f t e r testing mol/kg Na S, 92 °C, -1.175 V b) 50-54^MPa^rT 9  37  Q  r  F  Corrosion deposits a f t e r t e s t i n g mol/kg Na S, 92 °C, -1.T75 V , 2  E  i n 12 mol/kg NaOH  $ C E  2  36  e  $ C E  i n 12 mol/kg NaOH + 0.423 a) 37-39 MPav¥, ... i n 12 mol/kg NaOH + 0.423 50-54 MPa^rrT ..  81  38  Electron  39  Diameter o f d i f f r a c t i o n r i n g s  40  Anodic p o l a r i z a t i o n c u r v e s , 316 s t a i n l e s s s t e e l , 3.35 mol/kg NaOH, 92 °C. I d e n t i f i c a t i o n o f r e a c t i o n s  88  Anodic p o l a r i z a t i o n c u r v e s , 316 s t a i n l e s s s t e e l , NaOH + Na S, 92 °C. I d e n t i f i c a t i o n o f r e a c t i o n s  91  41  d i f f r a c t i o n p a t t e r n from c o r r o s i o n , f i l m  80  Q  x  versus / h ^ + k  c  +• 1  83 L  84  LIST  OF SYMBOLS AND A B B R E V I A T I O N S  Symbol  crack  a a (  length  lattice  parameter  crystal  d-spacing  d diameter  of  diffraction  pattern  ring  D electrochemical  potential  E E  ( •CORR  corrosion  potential  F  Faraday  h,k,l  Miller  indices  i_  anodic  current  camera  constant  stress  intensity  ISCC  (9.85 x 1 0  threshold  P  1 oad  Q  apparent  p  density  R  gas  r  plastic  temperature  v  crack SHE  volts  electron  for  mode  I  diffraction opening  intensity  (8v314  energy  kJ/mol  deg)  size  stress  T  V  for  activation  zone  A-s)  density  stress  constant  yield  4  °K  velocity with  respect  electrode  xi  to  the  standard  hydrogen  V  $  C  E  volts  with  respect  to the standard  electrode W  equivalent  weight  Abbreviation  SCC  stress  SSRT  slow  TN-DCB  T-notch  corrosion  strain  rate  double  cracking test  cantilever  xii  beam  calomel  ACKNOWLEDGEMENT  I for also  am  his unfailing been  MacLeod, Kent  very  parents  support  which  provided Council.  generous  have  Desmond  and p a t i e n c e . . me.  The s t a f f  L. F r e d e r i c k ,  H. Tump,  Tromans,  E. A r m s t r o n g  has  R. a n d K.  thanks.  given  support  I received  supervisor,  in helping  M. M a g e r ,  throughout  this  me  a great  amount  of  understanding  work.  has been  provided  f o r two y e a r s .  by t h e N a t i o n a l Their  research  encouragement  special  Financial ship  t o my  E. K l a s s e n ,  deserve  My and  indebted  Science  contributions  xiii  by A l c a n  Additional  and E n g i n e e r i n g  have  been  as a f e l l o w support  was  Research  gratefully  received.  1  1.  INTRODUCTION Stress  corrosion  cracking  (SCC)  f a i l u r e process caused by the stress  and  corrosion.  caustic  Such s t e e l s include  s t e e l , which i s the  subject  are  exposed to  type 316  alumina  in c r e v i c e s  vestigated  The  e f f e c t s of c a u s t i c  extensively  and  a better  oped hand i n hand with b e t t e r  Several  Unwanted  control  is  have been i n -  understanding has  techniques of  zones  devel-  investigation.  techniques have been employed f o r study of  Early studies caustic  solutions  of  production,  or splash  in b o i l e r s , tanks or other equipment where pH practiced.  austenitic  used in a v a r i e t y  process streams.  d e p o s i t s also b u i l d up  hot,  of t h i s t h e s i s .  processes such as wood p u l p i n g ,  or a l k a l i n i z a t i o n of chemical caustic  of a t e n s i l e  i s of p a r t i c u l a r concern in  Sodium hydroxide s o l u t i o n s chemical  action  s i t u a t i o n s wherever s t e e l s are  solutions.  stainless  conjoint  It occurs in many d i f f e r e n t a l l o y -  environment combinations and industrial  i s a time dependent  of s t r e s s c o r r o s i o n  solutions  SCC.  of s t a i n l e s s s t e e l i n  were done using loaded t e n s i l e specimens^ 1-3  or p r e s s u r i z e d  tube specimens.  loaded l o n g i t u d i n a l  sections  In most of these t e s t s the  U-bend,. specimens  and  of pipe a l s o have been used.  electrochemical  potential  was  4-7  not g  measured or c o n t r o l l e d .  More r e c e n t l y ,  straining  electrode  2 or  slow  slow  strain  strain  rapid  test  ibility  to  mechanics  rate  rate  testing  test  has  been  for  determining  SCC.  Another  specimens  to  has  been  employed.^'^  used  primarily  potential  recent  allow  regions  method  study  of  as  growth  a  for  employs  The relatively  suscept-  fracture  of  a  crack  of  12-15 well  characterized  related required  to  the  for  Studies have  shown  geometry.  stress  crack  of  of  temperature,  ment,  electrochemical  sent in  1 .1  , several  of  and  . reviews.  stress  in  crack  at  the  growth crack  rate  is  tip.  Time  minimized.  steels are  and  potential,  SCC  Kj,  variables  composition  effects,  knowledge  is  stainless  several  clude  mechanical  intensity,  initiation  SCC  that  The  in  solutions  important.  These  concentration  alloy  of  composition,  intensity.  Fe-Cr-Ni  caustic  alloys  Much has  of  been  in-  the  environ-  thermothe  pre-  summarized  16-21  Temperature  The  effect  of  temperature  was  investigated  systematically  3 by  Snowden  creased and  1.2  who  showed  dramatically  Staehle  also  the  observed  Electrochemical  The  as  that  importance  time-to-failure temperature 22 this.  was  of  specimens  decreased.  inAgrawal  Potential  of  electrochemical  potential  to  SCC  was  3 not  studied  600,  Incoloy  steel open  until  800 and t y p e  cracked circuit  potential  recently.  Theus,^  304, o b s e r v e d  i f the potential corrosion  dependence  i n a study  was m o r e  potential.  Inconel  that  t h e 304 s t a i n l e s s  than  30 mV  Morris  o f SCC o f A l l o y  of  above t h e  has i n v e s t i g a t e d t h e  600.  2 3  8 Park  et a l .  electrodes near  in boiling  the corrosion  failure very  i n a study  times  short  20 N NaOH,  found  i n the primary  to failure  steel  a short  p o t e n t i a l , no f a i l u r e  decreasing  time  o f 304 s t a i n l e s s  straining  time-to-failure  i n the passive transpassive  i n the secondary  passive  region,  region  and  region.  ?4 Dahl  et a l .  stainless anodic  steel  current  found was  peak  that  i n 2 0 % NaOH  susceptible  to cracking  and a t c a t h o d i c  ization a  studies  better  the  et a l .  Cr-9Ni  at the active  26 and Agrawal  of Fe-Cr-Ni  understanding  alloy  18  currents.  25 Long  a t 225°C,  i s affected  et a l .  alloys  o f how by t h a t  have  i n NaOH  conducted  solutions  polar-  to  gain  the p o l a r i z a t i o n behavior of of i t s  constituents.  27 Okada and  et a l .  Staehle  cracking potential  which  mode.  quoted  unpublished  indicated  I t was  that  by  the potential  intergranular  and t r a n s g r a n u l a r  work  at the  Subramanyam affected the  transpassive  at the active-passive  potentials  for  304 s t a i n l e s s  that  cracking  1.3  Composition  Higher  steel  i n 7 0 % NaOH.  mode was  a function  3  observed  potential.  o f the Enviornment  o f NaOH  have  been  The c o n c e n t r a t i o n  observed  to affect  granular  t o mixed  shown  to reduce  the fractography,  of caustic  changing  was  i t from  inter-  intergranular-transgranular  a s t h e NaOH  The  et a l .  7 22  time-to-failure.  steel  of  and C o n c e n t r a t i o n  concentrations  Park  concentration  addition  o f other  i n 304 2 28 increased. '  was  species  stainless  t o t h e s o l u t i o n may  shift  9 29 30 the  free  tives  corrosion  may  potential  SCC.  into  Park  tested steel of  inhibit  shift  a region 9 29  the free  i n a noble  Addition solution)  potential  corrosion  e t a-l. '  inhibitors  Na^S  potential.  '  In t h i s  by s h i f t i n g  where  there  corrosion  potential  of sulfur has been  this  31  Early  white  to shift More  f o r 304  effect  to simulated  steel.  corrosion  the i n h i b i t o r s  i n 20 N NaOH.  shown  addi-  i s no s u s c e p t i b i l i t y  that  d i d not consider  way many  the free  concluded  direction  f o rmild  '  on  they stainless  observations 3 potential.  liquor  the free  sulfur  to  (NaOH  +  corrosion  i s needed  as t h e  32 Na^S^O^  concentration  passivating  inhibitor  i s increased. f o r mild  P o l y s u l f i d e acts  and s t a i n l e s s  steels.^  as a  Theus affect  and S t a e h l e '  have  1  the fracture  mode.  observed  They  quoted  that  sulfide  work  on 304  may stainless  34 steel  O  which 50%  i n 5 0 % NaOH w i t h  H  0.03 m o l / l i t e r  intergranular cracking  NaOH  at higher  was  temperature,  Na S 2  observed.  a t 180°C i n  In o t h e r  work  transgranular cracking  with  was  observed. The was  fractography  used  instead  NaCl for  1 .4  321  stainless  The  effect  improved  1  to increase  i n 3%  NaOH.  of composition  Increasing  a higher  amounts  nickel  but the s t i l l reduced  Mcllree at  t o change  when  K0H  2  observed steel  observed  the t i m e - t o - f a i l u r e  3 5  of the alloy  aerated  5 0 % NaOH  content  were  also  nickel  been, i n -  i n the alloy  a t 300 ° C . was  content  found used  At  10-15%  t o be with  bene-  20-25%  resistance to cracking.  and M i c h e l s  300 ° C , n i c k e l  content  higher  has  o f chromium  r e s i s t a n c e t o SCC i n 5 0 % NaOH  chromium,  chromium  also  Composition  vestigated.  ficial  o f NaOH.  has been  Alloy  was  6  found  increased solution  necessary  that  i n deaerated  5 0 % NaOH  t h e r e s i s t a n c e t o SCC b u t i n an  both  high  to increase  chromium  and high  SCC r e s i s t a n c e .  nickel  Potential  6 was  not  measured  The  or  influence  controlled.  of  nickel  36 in  MgCl^  nickel  solutions.  content  to  has  been  37  investigated  thoroughly  36  '  C o p s on  produced  time-to-failure  a  i n Mg C l ^ •  curve A  relating  minimum  time-  13 to-failure for  occurred  stainless  curve hold  f o r about  8%  steel  in  NaCl  i s reproduced  by  plotting  stress  stainless  intensity  steel  of  solution  Speidel  at  the  105  °C  stress  versus  ( K J ^ Q Q )  18%  nickel.  chromium.  j  i  that  the  corrosion  nickel K  observed  thresh-  content  s  the  Copson  in  a  stress  in4  tensity found  below  that  stainless was  which  i s not  increased with  K J ^ Q Q  steel  cracking  in  i n c r e a s e d the  NaOH crack  detected.  increasing  solutions. path  As  changed  the  from  Sednks  et  al .  nickel  content  for  nickel  content  t r a n s g r a n u l a r to  intergranular. The  amount  identified  as  of  nickel  important  and  in  chromium  the  i n the  stainless  alloy  overlay  has  used  been  in  38 kraft  pulp  corrosion persed average was  digesters. to  throughout level  observed  mately  attack  13%  Other  of in  of  the  Crooks  and  small  areas  overlay  alloying places  chromium  and  studies  have  8%  of  lining  elements  where  Linnert  alloy  nickel.  considered  attributed  low and  in  alloy  content Rapid  the  content  frequently a  the  lining. was  of  dis-  low  Martensite  below  attack  effect  overlay  approxi-  occurred  carbon,  39  there  '  40  7 1 6 39 41 42 molybdenum.,' silicon,  1.5  Thermomechanical  Sensitization the  alloy  formed  i s heated  effect  denuded  43  and  phosphorus.  40  Effects  ( i . e . carbide  p r e c i p i t a t i o n ) occurs  i n t h e range  at the grain  chromium  aluminum,  boundaries,  path  500 t o 8 0 0 ° C . and these  f o r intergranular  Carbides are  produce  corrosion.  o f s e n s i t i z a t i o n on SCC o f s t a i n l e s s  when  steel  a  continuous  44  The  i n NaOH 5  solutions cluded  has been  that  steel  investigated.  Wilson  and Aspden  s e n s i t i z a t i o n i s n o t damaging  i n 1 0 % NaOH  a t 316 °C a n d 332 °C  con-  t o 304 s t a i n l e s s  o r i n 5 0 % NaOH  a t 316 ° C  39 Wilson  et a l .  found  that  f o r 304 s t a i n l e s s s t e e l  NaOH  a t 371 °C s e n s i t i z a t i o n h a d no e f f e c t  also  h a d no e f f e c t on t h e l i f e  in  1 0 % NaOH  granular. did  a t 316 ° C . They  not induce  Also,  they  steel  was The  that  susceptibility  found  that  cases grain  i n 1 0 % NaOH  of cold  work  cracking boundary  i n 10-50%  NaOH  a t 316 °C  r e s i s t a n t t o SCC b u t t y p e effect  on c r a c k i n g .  It  o f 3 1 6 L o r 316 s t a i n l e s s  In b o t h  concluded  i n 50%  was  steel  trans-  carbides  per se  a t 149-371 ° C . t h e 304 s t a i n l e s s  316 was n o t .  has a l s o  been  investigated.  6 45 ' 46  Cold Asaro  work  may  induce  partial  et a l . ^ identified  granular Mcllree  martensitic  martensite  SCC o f 304 s t a i n l e s s 6 and M i c h e l s obtained  steel  transformation.  formation  with  i n hot c a u s t i c  evidence  that  stress  transsolutions. relief  of  304  stainless  steel  f o r four  hours  a t 593 °C  d i d not improve  45 resistance  i n 5 0 % NaOH  found  cold  and  that  304L  1.6  size  Stress  The  steel  though  found  on t h e c r a c k i n g  4  Mechanisms  i n hydroxide  4 8  -  no e f f e c t  i n t e n s i t y on c r a c k i n g solutions  shown  t o be i m p o r t a n t  described  In p a r t i c u l a r , t h e p o t e n t i a l  Within  have  Most have  only  these  proposed to account  quantitatively  ments  process  or controlled;  mechanisms  of  examined  i n other  i n the l i t e r a t u r e ,  or  been  39  o f SCC  corrosion  recognized.  time.  rate  has n o t been  in.the s t r e s s  measured  on f a i l u r e  4 9  the research  defined.  o f 304  1 0 % a n d 5 0 % NaOH.  t o have  of stress  i t has been  , 2  In  was  i n both  and Aspden  Intensity  systems. -' '  1.7  h a d no e f f e c t  steel  influence  stainless even  rolling  stainless  Grain  a t 294 a n d 332 ° C . W i l s o n  been  have  n o t been  recently  the variables  fully  may  controlled  n o t have  been  has i t s i m p o r t a n c e been  l i m i t a t i o n s , several f o r the cracking  mechanisms  process.  e s s e n t i a l l y q u a l i t a t i v e rather  have  These than  predictive.  mechanistic centered  descriptions  on t h e f i l m  o f SCC i n c a u s t i c  rupture  environ-  and d i s s o l u t i o n  9  mechanism. to  this  This  model,  has  been d e s c r i b e d  a protective  steps.  Localized  surface  until  repassivation  dissolution  occurring  geometry  the a t t a c k e d  of  dissolution In  this  tion  cycles  model,  rate  solution  the  in  morphology  of  tunneling  at  the  repassivation  area  crack  is  potential the  principal  factors the  propagation  believed  crack  is  exposed  t h e amount  film  of  the  rupture  to  the  dissolu-  stage.  in  and  cracking.  Film  and t r a n s i e n t  some i n s t a n c e s  slip  influencing  related  ranges  by  constitute  the d i s s o l u t i o n  in  newly  Subsequent  localized  According  ruptured  complete, with  spent  In  the  at  rate  are  susceptibility.  is  cracking  various  currents  is  area.  the  film  occurs  before  at  and t i m e  instability  surface  dissolution  21  by S t a e h l e .  dis-  determining  detailed to  dissolution arise  via  tip. ^'^ 2  51 Vermilyea model  i n which  sivation  film  presented crack  is  ing  on t h e  SCC  of  strain  Scully cracking,  ruptured that  strain  steel in 53  a detailed  advances  rate  again. the  would  Diegle  ratio  film  by d i s s o l u t i o n  Further dissolution  evidence tip  formulated  the crack  occurs.  surface  to  has  of  ahead  of  before  repas-  o c c u r when t h e 52  and V e r m i l y e a  crack  tip  the c r a c k  have  corrosion  must e x c e e d a c r i t i c a l  gradient  rupture  value,  tip,  to  rate  dependpromote  NaOH.  has  described  requiring  a model  a critical  delay  for in  dissolution repassivation  controlled time  10 during  which  dissolution  occurs.  In  a  later  paper,  54  Scully  55 has  quoted  during  the  ing  in  a  has  noted  systems  by  Newman  showing  repassivation  Cr-Mo  steel  that  like  solution and  work  not  commented  composition  exposed  Vermilyea's  steel  may  event  in  be  that  is  fitted  to  model  may  than  about  surprisingly  be  the  charge  analysis 100  at  more  solution  different  evidence  the  the  NaOH  8M  hydroxide  much  that  passed of  °C.  crackScully  applicable  where the  the  crack  bulk  crack  tip  to tip  solution solution  lacking.  8 Park where  the  i b i l i t y of  et  using  surface  of  current  trode.  a l .  was  a  high  strain  ruptured,  have  stainless  steel  in  on  metal  surface  a  Their  bare  work  supported  rate  related  hydroxide  the  loading  to  film  the  SCC  solution  that  on  rupture  a  technique  to  susceptthe  filmed  arid  ratio elec-,  dissolution  model . 56 Bignold potential the  dependence  existence  ceases.  butions  developed  His  in  a  of  an  model  of  Doig  distribution  of  and  how  affect  Very  may  l i t t l e  to  cracking  explain  rate  potential  based  the  this  model  anodic is  crack.  a  on  and  potential crack  quantitative  and  Flewitt in  a  observed  aspect  above  potential 57  the  which  and  have  stress  ratio,  and  cracking  current  also  corrosion  d i s t r i -  considered crack,  propagation. information  is  available  to  11  support  or disprove  The  role  stainless loss  t h e mechanisms  o f hydrogen  steels  embrittlement  has been  of ductility  after  suggested.  investigated  cathodic  in caustic  SCC o f  by H o l z w o r t h .  charging  was  58  The  related  to the 59  amount  of martensitic  have  studied  less  steel  slopes  conditions which  steel  within  could  a crack  generate  They  hydrogen  at a  have  densities  f o r a range  stain-  tabulated and  o f NaOH  that  under  and S e t o  evolution  current  no o n e h a s e s t a b l i s h e d  exist  O'Brien  solution.  exchange  stainless  However  present.  o f hydrogen  i n NaOH  potentials,  on 3 0 4 L  tions.  tions  t h e mechanism  electrode  reversible  phases  Tafel  concentra-  thermodynamic  freely  corroding  and p r o m o t e  condi-  hydrogen  embrittlement.  1.8  Present  The ques,  present  investigation  particularly  quantitative solutions sulfide the  Objectives  and f r a c t u r e  information  o f NaOH w i t h  simulates  Kraft  SSRT  white  has employed  improved  mechanics,  on SCC i n s o l u t i o n s  Na,,S a d d e d . liquor  to obtain  new  o f NaOH a n d  The s o l u t i o n  wood-pulping  techni-  containing  solution  used i n  process.  During  the experimental  electrochemical  potential,  investigation  environment  the temperature,  composition  and  12 concentration, and  stress  composition,  i n t e n s i t y have  quantitative mechanisms  2.  alloy  data  and  useful  for  been for  thermomechanical  controlled  in  understanding  designing  properties,  order  rate  to  provide  controlling  equipment.  EXPERIMENTAL  2.1  Polarization  2.1.0  Curves  I n t r o d u c t i on Electrochemical  when  submersed  in  reactions  aqueous  occur  solution.  on  The  metal rate  electrodes  and  direction 6 0  of  each  reaction  To  apply  a  corroding an  potential conditions  external  (with  respect  record  of  to  the  The  reactions  which  take  and  Polarization Two  requires In  a  a  may  be  place  potential from  that  that  of  current  reference and  this  versus used  be  on  the  obtain  is  freely  supplied  electrode) current  electrode.  under  test,  potential to  the  found  potentiodynamic  range  current  curve  Materials  the  standard  through  curve.^  2.1.1  a  on  different  circuit.  continuously The  depends  is  the  potential  is  varied  recorded.  a polarization  information  s u r f ace. ^  via  on  the  ^  5  Preparation  curves  AT SI  Type  316  chromium  were  examined.  were  determined  stainless steels,  pure  for  iron,  five  materials.  nickel  and  13 Type rod  and  316  as  plate.  stainless  10.6  The  analysis,  mm  thick  determined  separately  results  the  Armco vacuum  iron  arc  Purified  was  nickel  carbon  irregular  chunks  the are  annealed  and  determined  in Table  as  0.46  15  mm  1 cm  mm  diameter  pickled  content  LECO  was  method.  The  I.  sheet.  diameter metal  mm  spectroscopic  Carbon  given  chromium  than  by  accurate  were  fused  less  9.5  more  received  free  as  Machinery.  by  discs  received  rolled,  were  CAE  analyses  was  hot  compositions  courtesy of  of  steel  High  and  was  purity  6.5  mm  i n the  thick.  form  of  long.  2 v l cm  Electrodes pure  nickel  wire  was  and  tube  acrylic polished clean test  were  was  through  set  in a  plastic. to  spot  600  The grit,  with  were  welded  3 mm disc  to  PTFE of  cut  from  each  each  electrode  tubing.  The  "Quiekmount"  electrode  face,  in  stock. and  the  electrode  selfsetting  i t s mounting,  with  final  polishing  distilled  water  just  A  being  before  done  placing  was on  a  into  the  cel1.  a  600  a  Teflon a  passed  paper  The  by  wire  i n area  ml  test  cell,  as  shown  schematically  \(polytetraf1uorethylene) lid.  The  temperature  of  Teflon the  in  Figure  beaker  solution  T e f l o n - c o a t e d t h e r m i s t o r temperature  probe  1,  fitted  was  was with  measured  connected  to  Table  Rod  I  Chemical  Composition  of  Type  316  Steels  Material C  Mn  Si  Ni  Cr  Mo  P  S  Fe  Batch  1  0.09  1 .67  0.4  11 .33  17.33  2.57  0.012  0.021  bal  .wt%  Batch  2  0.09  1 .69  0.38  11.52  16.6  2  0.014  0.014  tial  . wt%  Plate  Material: Mn  Si  Ni  Cr  Mo  P  S  Fe  1 .75  0.38  •10.29  15.7  2.0  0.015  0.015  bal  C -0.09  wt%  15  U —  rtltt  it—t-r—  T~l  Ti-  ll J  M-L'  J  • i  'dL  L  Hi  11i  LJ  e  1  11  c ! L.  J  Figure  1:  Test  cell  for polarization  a)  test  electrode  c)  counter  e)  nitrogen  g)  beaker.  electrode purge  studies  (b)  Luggin  capillary  (d)  temperature  (f)  1 id  probe  16 a  temperature  rent  to a heating  Temperature counter  was  reference  were  openings  denser,  regulated  coated  Luggin  with  thread  minimized  was  first  the  chemicals.  The  tial  2.1.2  were  hydrate.  Applied  with  with  was  There  reflux  purge thread The  con-  line. a n d was  cotton  The L u g g i n  reagent water USP  of current  was  capilexternal  NaOH  (Model  recorded  adding  the t e s t s .  by u s i n g  a  173) e q u i p p e d  178), logarithmic  were  which  before  throughout  (Model  pellets  employed  the t e s t  potentiostat (Model  grade  nitrogen  during  376) and programmer  and l o g a r i t h m  probe,  a cotton  continued  scanned  probe  capil-  the l i d .  K C l s a l t b r i d g e t o an  Distilled  Research  an e l e c t r o m e t e r (Model  made  The p u r g e  graphite  and t h e L u g g i n  through  formation.  enclosed.  a t 24°°C.  and purged  p o t e n t i a l was  converter  Two  ° C ) KCl s o l u t i o n .  bubble  the cur-  was  and n i t r o g e n  contained  ( a t 24  electrode  boiled  Princeton  cell  °C.  inserted  v i a a saturated  solutions  Na£S'9H20  with  capillary  vapor  calomel  and  were  +1  electrode  thermometer,  saturated  connected  The  the test  to within  the working  regulated  i n the l i df o r the temperature  filled  standard  The c o n t r o l l e r i n which  electrode  Teflon  Teflon  lary  mantle  electrodes,  lary  The  controller.  175).  current The  o n a n X-Y  potenrecorder.  Procedure The  test  s o l u t i o n was  placed  i n the c e l l ,  then  brought  17 to  the  test  cathodic to  temperature.  potential  remove  scanned  any  in  the were  2.2  Strain  2.2.0  respect  correlate  at  to  slow  tensile a  very  during  1  for  the  and  30  a  minutes  potential  mV/see.  was  Current  and  automatically.  is  that  the  test  is  before  rate  test  is  ductile,  final  area  test  i t has is  rate  test to  until  i t  curves  known in  potential  order  to  behavior.  (SSRT) SCC.  surrounded  at  provides '  by  A  test  fails.  a  quick  waisted,  solution,  Potential  is  test  cylindri-  is  pulled  controlled  test.  The  fracture  conducted  polarization  susceptibility  conducted  occurs.  were  electrochemical  strain  slow  the  failure  tests  anodic  specimen,  the  If  the  with  determine  cal  applied  s  at  inserted  Tests  corrosion  SCC  The to  Rate  a  Afterwards,  direction  recorded  w  was  Introduction Stress  with  specimen  V$QIT)  films.  anodic  potential  Slow  1.25  (<  oxide  The  conducted and of  under is  necked  larger.  the the  fractured  conditions  partly  inert  specimen  shorter  By  under  of  fully  measuring  necks  before  surface  because or  conditions,  the  is  the  fracture  small.  susceptibility,  If  specimen  fractures  and  the  area  areas  of  the  the  cracking  the  final  so  of  the  fractures  18 as  a  fraction  of  the  original  specimen,susceptibi1ity the in  percent inert  ted  reduction  The  potential  be  in area  conditions.  versus  can  cross-sectional  to  determined. will  be  percent  of  the  If s u s c e p t i b l e ,  less  than  reduction  determine  area  in  i t would area  is  be  plot-  regimes  of  SCC  suscept-  received  as  9.5  mm  diameter  The  material  i bi1ity.  2.2.1  M a t e r i a l s and Type  rod,  was  316  used  composition 25.4  cm  drical  stainless  to  has  long, gage  Preparation steel,  fabricate been  listed  threaded  section  the  4  at mm  SSRT  in  each in  specimens.  Table  I.  end.  A  diameter  The  specimens  central  was  were  25.4.mm  machined  in  cylin-  each  specimen.  Most after in  specimens  being  water.  hour,  annealed Three  stainless  properties  The paper ped  and  with  for  specimen  gage  2  Pak)  for  was  the  histories  leaving  only  was  and  then to  The are  the  gage  quenched for  T  quenched.in  that  used  were  by  enclosed  mechanical listed  p o l i s h e d with Each  tested  then  • annealed  specimens  envelopes.  s e c t i o n s were  One  hour,  similar  wi t h c h l o r e t h a n e .  tape,  1  being  hours*  treating  3 material  degreased Teflon  heat  (Sen  the  for  received.  °C  after  treatment  During steel  °C  as  1050  tested  650  Sensitization  others.^>64 in  at  tested  at  were  sensitized  water.  were  specimen section  in  Table  3/0 was  II.  emery wrap-  exposed.  Table  II  Mechanical  -Material Hi s t o r y  Properties  Yield Strength a t 92 °C MPa  of  SSRT  Ultimate T e n s i Te Strength a t 92 °C MPa'  Specimens  Hardness HRB  A s - r e c e i ved  283  530  87  Annealed  170  -  72  170  -  70  Annealed &  Sensitized  20 The  in  specimens  The  cell  Figure  2.  used  in  the  into  the  threaded into  a  and  stored  used  for  Its  construction  to  ends  of  specimen  the  model the  cell,  was  in  the  record made  during  was  leakage  reference test  during  as  described  elongation in  Section  roll  a  the  cell  tightly  solution. which  The  pinned tape,  thermistor  Section  same  the  fitted  to  probe  2.1.1.  a  (SCE)  type  of  Heating  respect  the  that  grips  in  electrode  with  schematically  the  with  described  load  versus  of  into  regulated  with  needed.  specimen  t e s t i n g machine.  measured  the  The  A  room  and  tempera-  controlled  potentiostat chart  was  test.  used  used  to  Solutions  were  2.1.1.  Procedure The  placed  in  specimen  Instron.  heated purge  prevent  Heating  SSRT the  and  Nitrogen to  s i m i l a r to  polarization studies.  2.2.2  mixed  V  until  illustrated  screwed  c o n t r o l l e r as  potential  0.005  is was  prevent  Instron  ture.saturated calomel +  desiccator  SSRT  bottom  temperature  to  the  a  cell  around  The  in  polarization studies.  floor  wrapped  were  to  was  The  inserted solution  temperature begun  oxidation  t a p e was  was  of  wrapped  was  around  in the  the  test  for  the  test,  poured  immediately sulfide  in  to  those cell  into  and  and  freshly  the  provide tests  cell  cell.  stirring using  heat  and  sulfide.  applied.  Slow s t r a i n a) specimen  rate t e s t  b) Luggin c a p i l l a r y  d) l i d e) r e f l u x probe bottom  cell  g) nitrogen  condenser purge  c) c e l l  f ) temperature  h) T e f l o n  i ) counter e l e c t r o d e  cell  22 A  cathodic  specimen face  for  film.  After  potential  30  minutes  The  another  of  -1.25  prior  to  was  the  test  potential  was  then  minutes,  the  crosshead  30  set  applied  to  to  reduce  the  of  the  in  load  motion  to  give  a  strain  rate  of  3.3  x  the  any  test  Instron  10"  sur-  potential.  - 6 set  to  was  -1 S~.•; a n d  the  recorded.  Each  test  SCC  occurred,  was  removed  faces cent  was  lasted; the  from  and  in  of  the  tests,  in  'Quickmount' mounted  HN0 , 3  10  ml  Examination phot  with  a  the  in  a  epoxy  of  of  the  and  18  from  the  the  fracture.  When  the  of  the  If  specimen  fracture  microscope  specimens  electron failed  polished  and  chloride  diameter  scanning  CH C00H, and  shorter.  with  and  sur-  the  per-;  calculated.  then  3  ending  travelling  surfaces  one-half  photography  to  etched  ml  HCl  were  were  examined  microscope.  gage 1  section  ym w i t h  with  with  was  drops  conducted  of  some  mounted  diamond  a mixture  4  In  of  paste. 10  ml  glycerol.  with  a  Zeiss  test  Ultra-  microscope.  aliquot  of  solution  concentration  Luggin  TTT80)  was  was  mine  meter  time  specimen  optical  An  hours,  cell ,  area  fracture  photographed  The  the  measured  reduction  The  test  -\. 4 8  and  capillary. an  was  taken  from  one  resulting  from  leakage  An  automatic  automatic burette  to of  deterKC1  titrator  (Radio-  (Radiometer  ABU-80)  23 were  employed  in  2. 3  Fracture  Mechanics  2.3.0  the  provide a  given  this  and  studies  information  promoting to  Testing'  Introduction Polarization  in  analysis.  on  the  environment,  and  providing  by  useful  slow  strain  potential  susceptibility.  understanding  and  on  the  which  SCC  mechanics  information  information  tests  for  occur  reactions  testing  on  together  may  electrochemical  Fracture giving  at  rate  can  kinetic  equipment  add  factors  designers  and  operators.  Specimens notched lems  and  and  designed  may  be  so  related  intensity,  in  fatigue  provide  are  crack  used  Kj,  length  pre-cracked  data that  by  a  can  to  f r a c t u r e mechanics  on  crack  load, known  then  give  a  to  overcome  growth  crack  length,  calculated  measure  of  the  are  and  The  stress  a  given  intensity  prob-  specimens intensity  relation.  for  pre-  initiation  kinetics.  Kj-calibration  be  testing  The load  of  stress and  stress  at  4.ftfiR the  crack  tip.  stress  intensity  ments,  there  cracking  can  raised  above  region  of  of  '  of  throughout  is  often  be  detected;  this  a  crack  crack the  stress  value,  Kj-dependent  K -independent T  Rate  this crack  growth  test.  may  In  be  corrosive  intensity  below  is called  K^^Q.  growth  velocity  is  growth  rate  rate  called  related  Region  extends  environ-  which 65  rises  to  no  AS  KJ. I S  too.  This  I.  through  A  region  24 intermediate growth K .; a  rate  this  I(  number  t i c s .  1  3  becomes  '  4  '  8  l  again  This  difficult  would  until  behavior  The  Kj'=  Finally  failure  has been of alloys  the  occurs  at  observed  by  and  and  region  cracking toward  above  K  for this  to designers  the causes  in that  just  cracking  a large  important  studying  place  during  spends  because  Kj i s d e c r e a s e d  I i s of interest  kinetics  by  as  to take  increases  II i s a l s o  The  t° determine  slow  have  i s cracking  independent  II.  solu-  9  infinitely  researchers  nated  4  s  Region  growth  III.  i s Region  of investigators f o r a variety  Kj v a l u e  Region  this  to increase  i s Region  Initiation  which  o f K^;  begins  ^ISCC  the  values  portion  J  ^ j 5 Q Q *  C  . Q  because  C  specimen  because  because  design  equipment  of i t s l i f e  i t i s of special o f SCC  rate  there.  interest  the  to  crack  are c o n t r o l l e d solely  by  stress  processes.  T-notch Russel  double  c a n t i l e v e r beam  and T r o m a n s .  Kj c a l i b r a t i o n  (T.172. x 1 0 ) 5  14  ( T N - D C B ) was  It is illustrated  f o r t h e TN-DCB  specimen  origi-  in Figure  i s given  3.  by:  P ( a ) ' ' [2 . 4 3 - 3 . 62 ( a / 0 . 032 ) 0  +  14.5(a/0.032)  +  26.5(a/0.032) ]  2  4  5  - 24. 6 ( a / 0  .03'2)  3  . . . (1 )  25  Figure  3  T-notch  double  cantilever  beam  specimen.  26 Errors  due  to  the  downward  growth  of  cracks  have  been  calcu-  48 lated in  by  this  1.  Russel ,  but  present  TN-DCB  It  is thin, not  been  considered  to  be  negligible  study.  The  will  have  has  been  so  the  differ  used  here  crack  because:  length  g r e a t l y from  measured  the  crack  in  the  front  surface  at  the  cen-  ter.  2.  Research  has  been  done  on  14 this  specimen  comparison  1.  of  has  The  very  loads  crack  are  below  2.  that  stress  specimen  not  be  on  yield  To  fulfil  similarity different  apply  to  to  produce  used,  using  will  aid  environments.  pre-crack  fatigue and  preclude  the  prestress  SCC  testing  intensity.  at  so  high  plane stress  strain  conditions  intensities,  would  depending  strength.  the  thickness  plane  a  accurately  criterion  for  plane-strain testing 2  specimen  in MgC^  disadvantages:  to  is thin,  satisfied  these  required  required  The  in  some  difficult  intensities  This  behavior  specimen  material  48  design.  The  low  this  strain  must  be  >'2.5  conditions  are  (K/a ) y  the  66 .  satisfied  For only  the to  material a  stress  27 intensity  o f . 17 MPa/nT ( a s s u m i n g -a  value  spite  of this,  which  point  straight  t e s t i n g was  gross  plastic  2.3.1  Materials Type  Its  The  plate  foil  Further  and  was  mm  envelope  cold  a single  i n Table  rolling  Hardness  measured  condition  One final  were  was  free  Specimens cracks  would  were  was  then  were  25.  plane-  made  work.  machined  .at  from  with  a thickness plate  strength  had a  of yield  o f 830  MPa.  and worked  by R u s s e l .  which  i n water.  was  14  '  rolled  48  to  1 0 5 0 °C f o r 1 h o u r . measured  the r o l l e d  i n the r o l l i n g  t o 600 g r i t  quenched  employed  Hardness  in a stainless  material  of material  used.  thickness,  then  tensile  to those  was  I.  worked  The p l a t e  annealed  propagate  polished  The c o l d  and u l t i m a t e  HRC  bf cold  I t was  the material  work.  identical  specimen  thickness  left  plate  t o an i n t e r m e d i a t e  (Sen P a k ) .  and 25% c o l d  through  suggested.^  from  listed  cold-rolled  o f 475 MPa  mens  propagation  were  Preparation  has been  strength  it  as has been  The c r a c k s  ?. .a itn.e.a l e d:. .'at. 1 0 5 0 °C f o r 30 m i n u t e s  steel  3.2  zones  to avoid  In  up t o 1 0 0 •-.MRa/m" a t  began.  316 s t a i n l e s s s t e e l  composition  then  deformation  and d i d n o t seem  stress  conducted  = 475 M P a ) .  HRB  on o n e s i d e  Thus,  76.  material  direction.  i t s  The  and t h e  so  that  speci-  surface  28 was  scribed  measure  with  crack  Starter lers  saw.  stress  leaving  specimens  were  fatigue  pre-cracked  The  fatiguing  value  of exposed of  solutions  continued  trolled  to +  Each  were to  were  used  °C, and  t o an  load  loaded  area  used  that  wrapped  with  a  jewel-  to  reduce  o u t a t a maximum  t o be  used  i n the  test.  for pre-cracking.  with  Teflon  i n the v i c i n i t y  was  i n the t e s t s  tape  of the  crack  of a  similar  were  mixed  potential  drop by  was  standard  mounted  controlled calomel  resulting  a calibrated  from  and  loaded  as  Monsanto) the  cracking.  homemade  con-  electrode.  cell  with  was  t o + 0.005 V  ( H o u n s f i e l d and in series  (Section  nitrogen  temperature  in i t s test  a spring  studies  i n t h e same way,  the t e s t s ,  Tensometers with  were  i n the p o l a r i z a t i o n  external  4.  mostly,  carried  utilized  were  throughout  specimen  minimize  mens  1  in Figure used  to  propagation.  2.1.1). purging  was  than  m a c h i n e was  to that  shown  lower  corrosion cells  respect  used  specimen  construction  with  t o be  i n each  the specimens  i t s region  The  apart  sawn  fatigue  a band  The  1 mm  were  intensity  Finally  and  cracks  time.  A Sonntag  lines  velocity.  The  initiation  fiducial  device  specimen  Some  speci-  which  used  a  Fracture a)  mechanics  specimen  c)pins e)  beaker.  b)  testing cel grips  d ) c e l l l i d  30 weighted  lever  The slow  potential  strain  including  2.3.2  rate  dic  potential  test.  Load  potential about  i n t h e same  A variety  manner  as f o r t h e  of potentiostats  549 a n d W e n k i n g  was  added,  was  was  and c e l l  models  applied  30 m i n u t e s  because  i t s t i p and a b u b b l e  level  was  OPA  was  69 a n d  used,  68TS10.  The length. scribed  cell  was  opened  This  was  done  on  calculated  cases  where  only,  the other  length  stress  cracking  to maintain  every  was  intensity  was  proper  sawed  were  on o n e  The  by  water.  the a i d of load,  the crack  the.scale  crack  recorded. side  out  solution.  daily  t o measure  In  length those  o f the specimen  periodically  loading  replaced  dissolved  fresh  to the  the  the c i r c u i t . . with  catho-  prior  h a d t o be  distilled  Time,  No  establishing  maintained  few days  surface.  films  thread  periodically  occurred  side  after  breaking  of boiled  applied.  surface  v i s u a l l y with  the specimen  was  the cotton  i n the c e l l  a few m i l l i l i t e r s  i n the tensometer,  capillary  formed,  replaced  of solution  placed  to reduce  The L u g g i n  p e r week  solution  were  and p o t e n t i a l  applied  control.  once  adding  and  applied  tests.  specimen  solution  test  load.  Procedure  the  The  the  was  ECO m o d e l  The  of  to apply  geometry.  to a  matching  31 At cell the pH  the completion  was  lowered  specimen indicator  pressed  away  was  absorbed paper  showed  room  temperature.  with  that  Teflon  distilled  draining this  corrosion  sonically  with  an  HCl,  2-Butyne-l,4  The  water  versus  crack  tion  This  was  intervals  so t h a t The  h a s a pH  before  being  deposits  were  solution  ( 3 5 % aqueous no  the  paper  o f <\* 14 a t  was  rinsed  sectioned. cleaned  ultra-  o f 3 ml  s o l u t i o n ) plus  i n an  was  indicator  composed  artifacts.  examined  tissue.  o f 0.5,  and t h e s p e c i m e n  acid  produced  lengths  measured  was  i f three  Otherwise was  diol  paper  of  50 ml  68  ETEC  scanning  electron  keV.  A line  method  available. ted.  a t 20  time.  squares  which  fractography  microscope  The  inhibited  pH  solution  and e t h a n o l  heavy  absorbent  the crack.  removed  the c o r r o s i o n  and t h e s u r f a c e  at the crack,  water  with  distilled  with  from  crack  was  Specimens  4 ml  q u i c k l y with  (pHydrion)  tape  of the t e s t s ,  the specimen  t h e specimen  solution  The  from  dried  paper  against  o f some  of the equation  fitted  the test  to t h i s  approximately  the line  the case  during  f o r the l i n e  using  colinear  through  f o r most  data  Region  two  plotted  the  points  points  I data.  yielded  were  was  least were calcula-  Differentia-  the v e l o c i t y .  The  velocity  has  been  plotted  as  a  constant  value  (stress  i n t e n s i t y independent  cracking).  ted  95%  using  for  confidence  limits  for  Error  constants  Region  was  II  calcula-  from  Student's  69 t-distribution activation  confidence  2.4  Electron  Stanton.  calculated  in  Values  the  same  for  manner  apparent using  Diffraction;  Analysis  of  Surface  Films  Introduction  fracture  film  of and  dissolved the  tion  the  provide  films  the  may  matrix  show  cracking. of  The  various  crystal  remaining  important  Differences  during  the  may  corrosion  cracking.  products  of  of  surfaces^  mechanism  of  were  by  limits.  Analysis  the  described  energy  90%  2.4.0  as  in  which film  clues  on  the  about  composition elements  structure  between  have may  may  provide  Corrosion  Film  been  be  dissolution reactions.  structure  the  formed  Examina-  insight  into  these  reactions.  2.4.1  3.35 saw in  Sampling  for  Fracture  mechanics  mol/kg to  expose  shallow  percent After  NaOH  at  the  dishes  bromine  2 'days,  in  the  92  °C  (TN-DCB) were  fracture and  sections  specimens  sectioned  surfaces.  submerged  methanol  Analysis  in  a  from  with  a  Sections solution  as  described  were  tapped  by  tests  in  jewellers were  of  1  placed volume  Nikiforuk.^  lightly  to  dislodge  33 the  film  and t h e n  were  left  were  picked  grid  and p l a c e d  floating  sequently, covered  removed.  a fine  the films  were  a support  stored  in a desiccator.  to dry.  picked  o f carbon  specimen  o f bromine.  a fine  copper  and p l a c e d  the grid  These  microscope  traces  up w i t h  Afterwards  mixture.  film  with  on  Sub-  grid  absorbent  films  was  Procedure The  holder  grating  and p l a c e d  Hitachi  model  with  HU-11A,  aperture  electron  diffraction  After diffraction evaporated  i t , was  was  used ring  to select pattern  plate  film  standard  was  f o r the gold  the area  was  be  A  from  field  which  an  obtained.  exposed ring  sample  microscope,  a t 100 k e V .  to the  pattern  photographed.  Using  crystal  standard.  diffraction  pattern.  in a  electron  o f an This  served  standard.  d i s t a n c e s , d, between  corresponding  inserted  electron  would  pattern, the d i f f r a c t i o n gold  known  fraction  on  f o r examination  the photographic  a calibration  The  film  i n the transmission  limiting  were  of the surface  electron  t o remove  film  paper  as  copper  i n methanol  filter  2.4.2  pieces  i n the methanol-bromine  up w i t h  with  Small  rings  i n the  The d i a m e t e r ,  were  equation  planes  2,  measured i t was  on  D,  lattice  of the  the  dif-  possible to  34 determine  the camera  c o n s t a n t , . <:  Dd  The tion  pattern  camera and  diameters  ...(2)  of the rings  were  of the u n i d e n t i f i e d  constant,  the r e l a t i v e  the ,d-spacing intensities  in  t h e powder  diffraction  in  the f i l m .  These  cal  =  estimates  were  surface was  were  on  film,  calculated.  confirmed  to  1  by  diffrac-  using  The  the  d-spacing  those  listed  the  compound  identify comparison  intensities  the  and  matched with  catalog^  of relative  measured  with  for electron  theoreti-  diffrac-  72 tion  i n FegO^  found  t o be  cubic  symmetry d  where  a  Miller  to  calculated  hkl  indices  calculated  of spinel  a /(h +k +l ) -  =  2  2  2  0  The  patterns  structures  were  exhibiting  ,  5  o  parameter  of the plane  +k  +1  From  using  this,  equation  of  ; was  of the d i f f r a c t i o n  the data.  Birley.  f o r which:  factor /h  rings  by  characteristic  i s the l a t t i c e  The the  as  and  h,  1  k and  are the  interest.  plotted  pattern  the l a t t i c e 3.  ...(3)  versus  and  the diameter  a line  was  fitted  parameter,  a ,  could  Q  of  be  3.  RESULTS  3.1  Anodic  3 . 1 . 0  P o l a r i z a t i o n Curves  NaOH The  anodic  polarization  respect  t o two d i f f e r e n t  calomel  electrode,  ^SHE"  These  V  $CE'  SHE  effects  5 shows  mol/kg  NaOH  located  at  primary  transpassive  0 . 2 5  V  Figure material. passive  The  S  t  h  e  s  t  a  n  been  scales: d  a  r  c  drawn  the  with  standard  hydrogen  l  C  6 shows  I t was  "nose"  concentration  1  6  V  electrode,  SCE  ••'  V  S  C  region was  for 3 1 6 stainless  The a c t i v e - p a s s i v e E  was  flat  o f NaOH  increased  f o r thermal  was  a secondary  a single  i s shown  rodi n  transition  and d o u b l e  observed  peaked.  The A  at - 0 . 1 0 V ^ r ^ .  passive  curve  N  C  '  A  From  region.  f o r the plate  5 except  to Figure  steel  - 0 . 7 5 to - 0 . 2 5 V ^ F .  from  the polarization  similar  to account  ( 4 )  junctions.  extended  exhibited  effect  4  t h e curve  range  there  E  2  to the potential  - 1 . 0 0  passive  to  d  a t 9 2 °C.  primary  0  n  and l i q u i d  Figure  "nose"  a  - ° '  =  No c o r r e c t i o n was made  3 . 3 5  potential  have  a r e r e l a t e d by:  V  gradient  curves  that  the a c t i v e -  peak.  in Figure  the current  7 .  density,  Increasing the  and d i s p l a c e d  36  0.50  CURRENT DENSITY  Figure  5  Anodic 316 3.35  polarization  stainless mol/kg  A/m  curve,  steel rod,  NaOH,  2  92 ° C .  ure  6  Anodic 316 3.35  polarization  stainless mol/kg  steel  NaOH, 92  curve, plate, °C.  38  0.50  CURRENT DENSITY  A/m  •  2  i  Figure  7  Anodic at 316  polarization  selected  NaOH  stainless  curves  concentrations,  steel  rod,  92  °C.  39 the  primary  (more  transpassive  active)  Raising 3.35  mol/kg  peak  as  in was  two  The  3.35  the  9  at  Iron shown  3.1.1  potenti al  to  lower  in  Figure  NaOH  for  the  The  the  of  potential  The in  potential, the  was  the  density  in  transpassive  curves  transpassive  the  more  E ^ O R R ' active  active-passive  solution  of  polarization  NaOH.  in  current  8.  potential  and  the  for  current  concentrated  was  in  the in  8  transition  at  -1.10  after  density solu-  passive  region  yellow  chromium  region  mol/kg  completion  NaOH,  V^rf of  n  the  scan.  was  region  the  lowered  lower  mol/kg  The There  and  illustrates  to  latter.  anodic  increased  corrosion  accounting  corrosion  temperature  concentrations  tion. in  the  illustrated  shifted  and  values.  NaOH  Figure  peak  in  polarization an  active-passive  more  had  an  +  Figure stainless  noble  Figure  NaOH  behavior  of  peak  nickel at  is  -0.85  shown  V ^ ^ ' a n d  in a  Figure passive  potentials.  active-passive  current  peak  at  -1.05  V<-  C E  11.  Na S 2  12  steel  shows rod  10.  in  the  polarization  2.5  mol/kg  NaOH  curve +  0.423  for  the  mol/kg  316 Na S 2  as  40  Figure  8  Anodic at  polarization  selected  316 3.35  temperatures,  stainless mol/kg  curves  steel  NaOH.  rod,  41  Figure  9  Anodic  polarization  chromium, NaOH,  92  3.35 °C.  and  curves, 8  mol/kg  Figure  10  Anodic 3.35  polarization  mol/kg  NaOH,  92  curve, °C.  nickel,  Figure  11  Anodic 3.35  polarization  mol/kg  NaOH,  92  curve, °C.  iron,  44  Figure  12  Anodic  polarization  316  stainless  2.5  mol/kg  Na S, 2  92  steel  NaOH  °C.  curve,  :! i ;  rod,  + 0.423  mol/kg  45 at  92  was  °C.  The c u r r e n t  o f t h e same  passive  current  Figure tions  peak  region  current  increased,  between  o f magnitude  -1.15 and -0.75 V^^.  as t h a t  o f the a c t i v e -  i n the s o l u t i o n without  1 3 i 11 u s t r a t e s  o f NaOH.  active The  order  density  the effect  sulfide.  of increasing  The' c o r r o s i o n , p o t e n t i a l , E Q O R R ' a n d an a c t i v e - p a s s i v e  peak  transition  became  larger  a s t h e NaOH  a n d was much  larger  than  that  was was  c o n c e n t r a - i <> i nthe evident.  concentration  was  i n the sulfide  free  s o l u t i ons.  The mol/kg The  effect  Na S 2  current  temperature observed seen  3.2  increased.  and E Q Q  Two  solutions. to those  Strain  Rate  percent  specimens minimum  c  increased  14 f o r p l a t e R  small  decreased  R  current  transition,  The c u r r e n t  shown  NaOH  i n Figure  +  material. as t h e  peaks  similar  peaks  0.423  were  were to those insigni-  13.  Tests  NaOH  SSRT  r  was  compared  The  c  density  i n 2.5 m o l / k g  i n Figure  at the a c t i v e - p a s s i v e  Slow  3.2.0  V  is illustrated  i n t h e NaOH  ficant  The  o f temperature  .  This  reductions  are plotted  i n c r o s s - s e c t i o n a l area  versus  reduction  i n area  indicated  that  potential  occurred  the greatest  i n Figure  of the 15.  a t -0.50 t o -1.50 susceptibility  t o SCC  ii  46  Figure  13  Anodic  polarization  solutions  of  selected  centration  with  steel  92  rod,  curves  Na^S,  °C.  NaOH 316  in constainless  Figure  14  Anodic  polarization  selected stainless NaOH  curves  at  t e r m p e r a t u r e s , 316 steel  + 0.423  plate.  mol/kg  2.5  Na S. 9  mol/kg  48  Figure  15  Effect  of  reduction strain  in  rate  NaOH, 92 curve  potential  for  on  percent  area  during  slow  tests,  3.35  mol/kg  °C. 316  Anodic  polarization  stainless  steel  rod.  49 was  near  the primary  Figure the  16  specimen  attacked  occurred  V"sCE  near  more  surface  potential.  seen be  very  in  Figure  -0.50  than  specimen One  severely  surface  cracks.  and  indicated region was  was  The V  -0.15  s c £  surfaces  ,were  that  of  of  cracked  cracking  but not i n the primary  absent as  film  18a.  at lower  poten-  shown  -0.95  dark  thinner  V  S  C  E  at  showed  the a s - r e c e i v e d  slightly  material.  V , , ^  a t -1.15  a t -0.85  was  V ^ i r showed  slightly no  dif-  material.  observed  At  a t -0.10  tested  tested  i t appeared  much 18b.  The  transition,  tested  t o SCC  I t was  in Figure  to  -  cracking  the as-received  V < . £ £ ,  E  passive  specimen  susceptible.  The  Q  region  17.  sensitized  0.25  0,  the active-passive  from  S  sections  Surface  annealed  ference  V  of the f r a c t u r e  intergranular  fracture  susceptibility  A  a t -0.10  i n the secondary  in Figure  less  At  The  peak.  the appearance  a t 0.10,  region.  An  the  tested  tested  similarly.  tials  shows  and e x h i b i t e d  specimens  passive  transpassive  on  the specimens  red-brown  t o be  thick  above and  varied  about  fairly  -1.0  V j - ^ , the surface  with  a distorted  with V ^ ^ ^  -0.50 brittle,  film  appearance  as  appeared as  shown  50  Figure  16  SSRT  specimen  after  testing  a t -0.10 V -  r F  O L C  in  Figure  17  NaOH  SSRT in  solution.  specimen  NaOH  after  solution.  testing  a t -0.95 V  b Figure  18  Surface solution  film  on  a ) 0.25  SSRT  specimens  V,, -, ri  b)  tested  -1.00  V-  r F  i n NaOH .  52 The  solution  [Cl"].  from  one t e s t  The c o n c e n t r a t i o n  (-0.25  Vc^rr)  was ^ 0.03 m o l / k g  was  analyzed f o r  after  a 2 day  test.  3.2.1  NaOH The  SSRT  + Na^S; percent  specimens  reductions  in cross-sectional  are i l l u s t r a t e d  i n Figure  1 9 . Some  bility  t o SCC i n t h e a c t i v e  region  crease  in reduction  a t - 1 . 1 5 Vc-^^.  conducted ions  as e v i d e n c e d  The ^SCE the  at higher  ^  s  "* u s t r a t e d 11  film  No e f f e c t tested  3.3.  o f the specimen  i n Figure  between  20.  There  -0.85 and -0.95  of sensitization  was  by t h e d e were n o t  of  sulfide  above  tested were  -0.75  V  S  C  E  .  a t -1.15  no c r a c k s i n  V ^ Q ^ .  observed  i n specimens  Mechanics  Testing  NaOH  3.3.0.0  3.35  region  suscepti-  Tests  in current  of the  a t -1.00 and -1.15 V$CE"  Fracture  3.3.0  indicated  p o t e n t i a l s due t o o x i d a t i o n  by t h e i n c r e a s e  fractured  surface  i n area  was  area  Effect  of Stress  Figure  21  mol/kg  Intensity  i s a plot  of crack  NaOH, 92 ° C , - 0 . 1 0 V  $  C  E  .  growth  rate  The d a t a  versus  show  Kj i n  Region  I  53  % REDUCTION IN AREA 50 -i  CURRENT  Figure  19  Effect area  DENSITY  A/m  slow  NaOH  Anodic  polarization rod.  on  strain  mol/kg  steel  70  80  1  1  1  90 1  2  of potential  during  60  +0.423  percent rate  mol/kg curve  reduction  tests,  Na S, 2  f o r 316  in  2.5  92 ° C . stainless  Figure 20  SSRT specimen a f t e r t e s t i n g at -1.15  V  $ C E  i n NaOH + Na S 2  solution  10  >—• t>  •  r  e—e— T°C  e- 92 A-82 B-72  10  20  30  40  STRESS  Crack at  growth  selected  NaOH,  -0.10  INTENSITY  rate  versus  temperatures V  c  r  c  .  50  60  70  80  MPavTTT  stress in  3.35  intensity mol/kg  56 (Kj. d e p e n d e n t ) Region so  I data  slow.  data  and Region  K  used  3.3.0.1  were  iscC  72  was  a  less  TO MPa/m .  r  e  c  obtained i n Table  !  t  0  D  e  Figure  21  than  rate  data  i n Table  a r e summarized  i n 3.35 m o l / k g  NaOH  I I I a n d a r e shown  effect  was  evaluated  decreasing  III.  i n Figure  assuming  V  = V  V  = crack  22.  tests.  a t 9 2 , 82  in Figure  on R e g i o n  a simple  °C a r e a l s o The  potential  Crack  II crack!  Arrhenius:  growth  growth  rate law:  e x p (-Q/RT)  ...(5)  where  V = Q  growth  experimental  Q = apparent R  = gas  The 1/T  logarithm  a s shown  rate constant  activation  energy  constant  T = temperature  o f crack  i n Figure  23.  21.  temperature.  o f the temperature  Q  The  NaOH  a t 92 a n d 82  a t -0.10 V^^^ i n a l l t h e s e  The  was  i n Table  I I I and i l l u s t r a t e d  i n 12 m o l / k g  with  cracking  Effect  °C a r e l i s t e d  decreased  rate  because  growth  maintained  rate  e  to obtain  Temperature  Results listed  PP  behavior.  difficult  to construct  Crack and  a  I I (Kj. i n d e p e n d e n t )  (°K)  growth  r a t e was  A line  was  plotted  fitted  versus  t o t h e 3.35  57  TABLE  III  Summary o f F r a c t u r e M e c h a n i c s T e s t  NaOH Concentration mol/kg  T  3.35  92  Kj Range SCE  -0.10  MPa^n 10  - 10.5  18  - 22.6  22.6 - 27  3.35  82  72  92  12  12  3.35  3.35  92  82  92  92  -0.10  -0.10  -0.10  -0.10  -0.10  0.0  -0.175  •10 4.29 + 2.15)xlC' -9 4.04 + 0.81)x!0 6.17 + 1.23)xlO"  - 30.7  7.26 + 1.82)xl0"  33  - 52.7  50  - 39.3  1.02 + 0.08)xl0" -8 0.94 + 0.27)xl0 -8 1.02 + 0.34)xl0 -8 1.04 + 0.17)xl0 -8 0.90 + 0.18)xl0  -  98.3  40  - 110.3  30  - 45.7  26  35.5  2.5  35.6  72.1  5.39 + 0.90)xl0"  20  - 23.5  Number o f Observations  Crack Growth Rate m/s  27  40  3.35  Data  +0.19)xl0"  1.05 + 0.55)xl0  23.5 - 34  2.09 + 0.22)xl0  34  - 75.4  3.09 + 0.25)x1O  15  27.6  -8 0.82 + 0.16)xl0  27.6  73.1  1.50 + 0.55)xl0  -9  36.3 - 63.1  -8 1.36 + 0.16)xl0 -8 0.99 + 0.09)xl0 -8 1.35 + 0.11 )xl0  25.8 - 30.9  2.29 + 0.02)xl0  30.9 - 94.4  0.94 + 0.13)xl0  30  - 35.2  2.23 + 0.29)xl0"  6  35.2 - 64.8  3.01 + 0.17)xlC"  17  16  - 52  15  - 28.8  25  46  i  -9  (0.31 + 0.04)xl0"  58  10  20  30  AO  50  STRESS INTENSITY (K,)  Figure  22  Crack  growth  intensity in  at  12 m o l / k g  rate  versus  selected NaOH,  60  70  MPa/m"  stress  temperatures  -0.10  V  c  r  r  .  [ NaOH ] mol/kg 0-3.35 A-12.0  _l  I  I  I  2.8  27  1 / T x 10  Arrhenius crack  plot  growth  12 m o l / k g  of  rates  L_ 2.9  3  the Region in  3.35.  NaOH, -0.10 V,,-,-.  II and  60 mol/kg  NaOH  parent  a c t i v a t i o n energy,  the  line,  tion  data  was  energy  3.3.0.2  tion at  Effect  appeared  greater  3.3.0.3  effect  Effect Test  3.35  mol/kg  The  crack  with  10  days.  cracking  NaOH,  <  rate  indetectable  The d a t a  of  activa-  t o be ^ 37  kJ/mol.  is illustrated  the errors  of Figure was  of  in  concentra-  o f measurement  23 s u g g e s t s  small  that,  a t 92 ° C , i t  had  Potential  obtained  was  a t 0, -0.110 a n d - 0 . 1 7 5  fastest  $  C  test,  15 d a y s . m/s  i n Table  susceptibility  E  A test  were  conducted  Crack  $  C  period.  -  1  i n agreement  conducted at  i n 3.35 m o l / k g no c r a c k i n g i n  a n d 30 MPa/IT, s h o w e d  E  growth  a n d < 1.92 x 1 0  i n the test  tests  25.  i n t h e SSRT.  a n d 30 MPa/jrT, s h o w e d  a t -0.85 V  in  I I I and F i g u r e  a t -0.10 V ^ Q ^ ,  f r a c t u r e mechanics  potentials.  Another  1 0  The  The e f f e c t  and w i t h i n  o f maximum  ° C , -1.15 V  2.89 x 1 0 "  III.  92 °C a r e shown  addition,  after  concentration  i n Table  of Applied  growth  a t 92  estimated  of concentration  results  active-passive  the gradient  a t 82 ° C .  the potential  In  NaOH  o f NaOH  NaOH.  the effect  from  The ap-  Concentration  t o be s m a l l  8 a n d 12 m o l / k g  method.  (90% confidence).  NaOH was  o f NaOH  effect  squares  calculated  8 kJ/mol  24 a n d t a b u l a t e d  although a  60 ±  the least  i n 12 m o l / k g  The Figure  using  rates 0  m/s  would  have  been  r e s p e c t i v e l y t o be  no  61  10  o x  WW  PT  1.0 LU  t NaOH ] m o l / k g  5 o or  0-  3.35  ID 0.1  A -  8.0  i£  B-  12.0  o < or o  10  20  30  40  50  60  70  80  S T R E S S INTENSITY <Kj) MPavAm"  Figure  24  Crack at - ° -  growth  selected  1  0  V  S C E :  rate  versus  stress  NaOH c o n c e n t r a t i o n s ,  intensity 92  °C,  62  10  £  < or o or ID it o < or o  fcfr j^a oft (a • • • 0.1  V  -i  10  1  i  20  30  STRESS  Figure  25  S C E  A -  Crack at  growth  selected  NaOH,  92  °C.  i_  -0.175  G -  -0.10  B-  0.0  G -  -1.15  V -  -0.85  _l  40  i_  50  60  70  80  INTENSITY (K,) MPaVm"  rate  versus  potentials  stress  i n 3.35  intensity mol/kg  63 3.3.0.4  obtained cold  of  All  the  of  with  not  creased  be  growth  28  test  crack  the  face,  other  In  -0.10  V  stress  a  $  C  in  been  Table  Annealed and  E  92  intensity  no <  1  10  were  material  °C  but not  ^  was m/s  (nb  cracking be  deformation  cracking x  III  could  considerable  month,  have  in-  of  observed. to  be  un-  period.  f r o n t was  on in  one an  the  bottom  in  of  Figure  a  cracked  specimen  illustrates  the  cracking Figure  occurred 27a  the  the  26  and  increasing  higher were  random  the  tests.  sometimes  on  manner.  direction  is  from  Small K  predominantly  uncleaned  V ^ Q ^  (cleaned).  there  specimen,  NaOH was  an  -0.10 of  during  the  photograph.  shows  at  at  the  cracking  mol/kg  effect  fractography  of  straight  apparently  3.35  granular.  not  face  a l l fractographs  Cracking  In  at  after  data  material.  The  would  the  i t led  the  tested  Even  Sometimes  to  worked  rate  Fractography The  top  growth  MPa/nT w i t h o u t  rate  in  3.3.0.5  cold  initiated.  above  detectable  Work  crack  also  material.  Crack  Cold  25%  w o r k ) was  could  the  Effect  of  °C.  stress  areas  -levels  remnants  92  fracture  as  of  inter-  surface  Figure intensity  of  27 on  transgranular  shown  corrosion  in  Figure  film  27c.  adhering  64  Figure  26  Fracture 3.35 45-47  surface  mol/kg  NaOH,  MPa/m".  after 92  testing  in  ° C , -0.10  V  $  C  E  I  66  c  Figure  27  Fracture in V  $  c)  surfaces  3.35 m o l / k g C  E  NaOH,  a t a ) 38-40  55-60  MPa/m.  after  testing  92 ° C ,  MPa/m,  -0.10  b) 4 6 - 4 8  MPa/m,  67 to  the g r a i n s ;  described  At  on  ahead  tion  surface  film  surface, front. dark  becoming I t was  mounds  fracture like  form  shown  were  in Figure  shows  graphy the  that  between  were  the surface  branching  to  initiate  formed and  of the fractography  where  emerged  was  o f a straw  red i n color.  was  found  Further  as i n F i g u r e black.  29a.  Finally,  i n the oldest  the forma-  A t -0.10 V ^ r r *  films.  mat  deposits  portion  from  along  In t h a t  a  fine  covered  progressively thicker further  appeared  Comparison 27b  aspect  the appearance  rust  surface  crack:,  appeared  but a c t u a l l y  beneath  and s u r f a c e  with  the c l e a n i n g (as  crack.  interesting  of deposits  60 MPa/m  28. C r a c k s  cases  tunnelled  o f t h e main  An  about  in Figure  i n some  cracks  o f f during  2.3.2).  intensity,of  as s e e n  the surface  internal  d i d n o t come  in Section  stress  occurred,  these  the area  in a  the  the  crack  crack, the needle-  of the crack,  as  29b.  of Figure there 72  was  a n d 92  30 no  (cleaned  surfaces)  significant  °C when  other  with  Figure  difference in fractoconditions  were  held  same.  The  fractography  was  affected  by t h e c a u s t i c  concentration.  68  Figure  28  Crack  branching.  Stress  rises  from  105  view.  75  to  intensity  MPa/m  in  this  Figure  29  Deposits in  3.35  on  fracture  mol/kg  surface  NaOH, 92  °C,  after -0.10  testing V  c  r  r  .  Figure  30  Fracture mol/kg MPa/m,  surfaces  NaOH,  after testing  -0.10 V  $ C E  ,  b) 72 ° C , 3 8 - 4 0  i n 3.35  a ) 82 ° C 3 3 - 3 4 MPa/m.  71 Figure  31 s h o w s  tested  i n 12 m o l / k g  the  center  NaOH.  near  facets black  further  Figure deposits. deposits  33 s h o w s  of  failure  by  comparing  3.3.0.6  pH  was  Figures  that  been  a  cracks.  were  were  a  differ-  bronze  crystalline became  brown  then  tip.  of potential  on  corrosion  r a i s e d , the density  the surfaces  t o be e s s e n t i a l l y  were  cleaned,  t h e same,  of the t h e mode  a s c a n be  seen  3 4 a , 34b a n d 2 7 a .  Measurement  Measurements the  They  specimen  view o f  have  surface  The d e p o s i t s  the effect  When  shown  may  t i p and d i d n o t e x h i b i t  As t h e p o t e n t i a l was increased.  what  solution.  the crack  on a  32, a m a g n i f i e d  transgranular  magnification. from  fracture  on t h e f r a c t u r e  caustic  the crack  a t high  several  deposits  i n the stronger  Figure  31, i l l u s t r a t e s  through  Corrosion  color  of transgranular  of Figure  cross-section  ent  area  s o l u t i o n which o f the bulk  o f pH, v i a i n d i c a t o r p a p e r ,  drained  solution  out o f the crack  \(ite.  was  ^ 14 a t 25 ° C ) .  showed  that  t h e same a s  72  Figure  31  Fracture mol/kg MPa/m.  surface  NaOH,  92  after  testing  ° C , -0.10  V  $  C  E  i n 12 ,  28-29  73  Figure  32  Intergranular secting is  facet  transgranular  a magnification  Figure  displaying  31.  of  inter-  cracking.  This  the  of  center  74  b  75  Figure  33  Corrosion in  deposits  3.35 m o l / k g  a)  -0.175  V  b)  -0.10 V  c)  0.00 V  c  $  r  S  C  c  C  E  ,  after  NaOH, E  ,  testing  92 °C  36 MPa/m 42-44  MPa/m.  4 1 - 4 3 MPa/m.  77 3.3.1  NaOH  3.3.1.0  +  Na S 2  2.5 m o l / k g White  One  + 0.423 test  within  specimen,  at  - 1 . 1 0 V<*££  11  days  to  30 MPa/m  3:3:1:1  92  2  x 10  f o r 13 d a y s ,  was  observed  NaOH  °C, a t a c t i v e - p a s s i v e  -  1  ^ m/s).  with  2  cracking  no e f f e c t  raised  (v^-3.6  + 0.423  growth  Another test  The s t r e s s  potentials.  was  crack  started  d i d not occur  within  i n t e n s i t y was  raised  (v<<2.2  x 10  x 10"  1 0  m/s).  ^ m/s).  - 1  Na S 2  the relation  rate  mol/kg  t o 100 °C f o r 8 d a y s b u t  mol/kg  35 i l l u s t r a t e s  and c r a c k  Na S  (Simulated  2  w e r e c o n d u c t e d i n 2.5  a n d 20 MPa/m, b u t  12 m o l / k g  intensity  Na S  a t - 1 . 0 0 V ^ ^ ^ a n d 30 MPa/m, d i d n o t  (v<4.1  Figure  mol/kg  Na S,  t h e t e m p e r a t u r e was  cracking  mol/kg  tests  ( v < 2.6 x 1 0 " ^ m / s ) .  Finally, no  mechanics  mol/kg  7 days  + 0.423  Liquor)  Fracture NaOH  NaOH  between  i n 12 m o l / k g  a t 92 ° C a n d - 1 . 1 7 5 V ^ ^ ^ .  NaOH  The c r a c k  stress  +  0.423  growth  rate  -9 was  found  between 8 data the  points  between  potential  was  ceased  potential  suggested 0.423  ( 1 . 4 8 + 0 . 4 8 ) x 10  m/s  f o r 6 data  30 a n d 37.1 MPa/m, a n d ( 2 . 5 7 + 0 . 5 9 ) x 1 0 ~  cracking the  t o be  mol/kg  raised  during  was  that  37.1 a n d 5 7 . 9 MPav^m.  reset  cracking  Na S 2  t o -1.15  that  During  period.  Cracking  may  occur  i n t h e 2.5 m o l / k g  V  even  C  E  ,  m/s f o r the test,  .  This  resumed  V  $  9  f o r a few d a y s and  t o -1.175  a t -1.175  points  though  when  observation NaOH +  i t d i d not at  78  10  0)  £  > i-ob< or  o  cr „ ID 0.1 < cr o  -J 10  figure  35  Crack  1 1 1 i i 20 30 40 50 60 STRESS INTENSITY (K,) MPa^rfT  growth  in  12 m o l / k g  92  ° C , -1.175  rate  versus  NaOH  + 0.423  V  c  r  c  .  stress mol/kg  i 70  I 80  intensity Na S, 2  79  -1.15  V  S C E  3.3.1.2  .  Fractography In  tion  i s from  The Na S 2  is  al1.fraetographs  the top t o t h e bottom  fractography  illustrated  i n cracking  intensity than  tested.  those  Corrosion color,  with  During  removal  hydrogen were  3.3.1.3  The s u r f a c e s  were  shown  o f green  of deposits smell  the range  dif-  of stress  considerably  i n Figure when  37 w e r e  rinsed  with  i n the inhibited  was  Solution  rougher  produced  black i n  distilled  acid  indicating  water.  solution, a  the deposits  Appearance  solution  taken  was  brown  in color.  led  water  ( i . e . reduction  this  no s i g n i f i c a n t  It  sulfides.  The  Dilution  i n t h e NaOH +  i n NaOH.  deposits  sulfide  propaga-  and i n t e r g r a n u l a r .  was  within  a tinge  metal  36. T h e r e  morphology  obtained  cracked  transgranular  i n Figure  of crack  of the photograph.  o f t h e specimen  s o l u t i o n was m i x e d  ference  the d i r e c t i o n  with  effect.  fresh  When  strong  from  the c e l l  diluted  with  during  the addition  o f pH), the s o l u t i o n caustic  the test of  turned  distilgreen.  s o l u t i o n d i d n o t have  b Figure  36  Fracture NaOH a)  surfaces  + 0.423  after  mol/kg  3 7 - 3 9 MPa/m  testing  Na S, 2  b) 50-54  i n 12  mol/kg  92 ° C , - 1 . 1 7 5 MPa/m.  V  S  C  E  81  Figure  37  Corrosion  deposits  12 m o l / k g  NaOH + 0.423  92  ° C , -1.175  after  testing  mol/kg  V Q p p j 50-54  in  Na S,  MPa/m.  2  82 3.4  Electron  An shown this  example  i n Figure sample.  diffraction the  Diffraction  data  Analysis  of a typical 38.  Figure pattern  i n Table  Table 39  IV.  electron  Films  diffraction  i n NaOH  pattern i s  IV i s an a n a l y s i s , o f t h e d a t a f o r  i s a plot  rings,  of Surface  o f . D, t h e d i a m e t e r  versusvh  The l a t t i c e  +k  +1  of the  obtained  parameter,  from  a , for Q  the  o  cubic  spinel  intensities in  the x-ray  ties  0.01  A according  of the d i f f r a c t i o n powder  calculated  values 8.8  was 8 . 5 0 +  obtained  + 0 . 2 , 9.3  diffraction  f o r electron  a n d 9.3  f i l e  were 7  1  parameter  + 0.2  A.  3.  The  c o n s i s t e n t with  and w i t h  diffraction  f o r the l a t t i c e + 0.3  rings  to equation  the i n t e n s i 72  i n Fe^O^. were  Fe^O  8.5  Other + 0.1,  Figure  38  Electron corrosion  diffraction film.  pattern  from  84  /h +k +l 2  Figure  39  Diameter  A W .  of  2  diffraction  2  rings  versus  Table  IV  Ring Number  Electron-  Di a m e t e r Inches  Di f f r a c t i o n d  6  A  P a t t e r n ' Data  1  Intensity  h,  k,  1  0  /h  2 +  k  2 +  l  1  1.1.2  2.97  weak  2  2  2  1 .22  2.73  spots  c  -  3  1 .31  2.54  weak  3  1  1  3.32  4  1 .56  2.13  wea k  4  0  0  4.  5  1.72  1 .94  spot  3  3  1  4.36  6  1 .95  1 .71  very  weak  4  2  2  4.90  7  2.10  1 .60  v.v.  weak  2.25  1 .48  m e d i um  3= 1 4  3 1 0  5.20  8  3 5 4  9  2.44  1 .36  spot  6  2  0  6.32  10  2.62  1 .27  v.v.  5  3  3  6.56  11  2.72  1 .22  m e d i um  4  4  4  6.93  12  3.00  1.11  v.v.  weak  3.16  1 .05  v.v.  weak  3 4 0  1 2 0  7.68  13  7 6 8  weak  2.83  -  :  5.66  8.00  2  86 4.  DISCUSSION  4.1  Interpretation  of Anodic  Electrochemical be  identified peaks  a  species  metal  that be  result  reactions  The  reactions more  from  the current  It i s believed i n terms  take  stability and t h u s  peaks  may  may  Anodic  place  t o change  stable at  of a species  may  the possible be  identified.  f o r t h e 316 s t a i n l e s s s t e e l r o d  i n 3.35 m o l / k g . N a O H  interpreted  which  an E-pH d i a g r a m ,  cause  on t h e s u r f a c e  thermodynamically  The thermodynamic  which  5.  occurring  Curves  the p o l a r i z a t i o n curves.  p o l a r i z a t i o n curve  material Figure  from  t o a form  potential.  determined  reactions  by e x a m i n i n g  current  'Polarization  a t 92'°C  has been  t o be a c o m p o s i t e  of the behavior  presented  in  curve  which  may be  o f the major  alloy  con-  stituents .  The with Lee  p o l a r i z a t i o n behavior  reference  t o t h e E-pH e q u i l i b r i u m  f o r the Cr-H 0  7 3  2  3.35  mol/kg  Figure  o f chromium  NaOH  system 31 °C.  a t 92  9 corresponded  a t 100 ° C .  was  diagram T h e pH  The t r a n s p a s s i v e  considered constructed  by  i s -v 1 2 . 5 - 1 3  in  region i n  t o t h e d i s s o l u t i o n o f chromium  oxide  to  2- 73 Cr0  4  .  dynamic ions. passive  The y e l l o w  scanning  i n NaOH  By i n s p e c t i o n , region  color  of the solution  confirmed  i t was  observed  above  after  the presence  concluded - 0 . 2 5 V^r  that  potentio2-  o f CrO^  the primary  i n Figure  40  trans-  resulted  87 from  formation  The  o f CrO^  2-  p o l a r i z a t i o n behavior  of nickel  was  analyzed  using  74 an  E-pH  Ni-H 0  system  2  above  diagram  constructed  a t 100  the corrosion  °C.  by Cowan  and S t a e h l e  f o r the  The i n c r e a s i n g c u r r e n t  potential  i n Figure  10 may  observed have  been  due  74 to  formation  o f HNi0^-  HNi0 -  +  2  E  HNiO-/Ni  The to  =  +  +  2e"  7.  Ni  +  2H 0  of the current  formation  active-passive  peak  t o t h e peak  film. V^^  a t -0.85 a t -0.9  E  V  ...(7a)  $ H E  -0.85 V < , ^  peak,  of a passivating  ...(7)  2  -°-837-0.074 pH + 0.037 l o g [ H N i 0 ~ ] ,  potential  responded  3H  v i a equation  I t was  noted  for nickel  V<j r£ C  was  E  attributed that  (Figure  f o r the alloy  the 10)  cor-  ( F i g u r e s 5 and  40).  75 Dissolution been  the major  current  peak  of iron  reaction  seen  E  HFe0 - / F e  The by  =  potential  +  v i a equation  contributing  in Figure  HFe0 - + 3 H 2  t o HFeO^  + 2e"  =^  to the  have  active-passive  Fe + 2H 0  ...(8)  2  of the active-passive o f an o x i d e  may  11.  0.503-0.111 pH + 0.037 l o g [HFeO"],  the formation  8  film.  V  ...(8a)  $ H E  transition The c u r r e n t  would peak  be  defined  observed  Figure  40  Anodic  polarization  steel,  3.35  mol/kg  Identification  of  curves, NaOH,  92  reactions.  316 °C.  stainless  89 at  -1.05 in  V  Figure  Thus, a  of  o f one  Figure iron  Figure  40  was  the of  may  tent  plate  increase  According  tion  species  to  solved  species,  higher  pH  before  enough  of  at  higher  the  current  been  at  -1.05  identified  active-passive plate by  peak  material.  the  lower  densities 7)  may  to  the  diagrams,  pH  13.  However, above then  increase  higher  with  due  Iron  chromium  the  the  d i s s o l u t i o n has species  temperature 8)  do  significantly  d i d not  because over  nickel  con-  to  10~  this  using  and  mol/kg  passivity  may  of  dis-  occur.  in  the  to  provide  cause  a  reached concentra-  passivity.  the  current  thermodynamic range  be  of  concentra-  region  Thus,  E-pH  chromium  concentration  increase  the  con-  concentration  d e n s i t i e s must  occurred  adequate  caustic  explained  i f the  the  current  be  as  iron,  d i s s o l u t i o n must  (Figure  change  constituents.  (Figure  To  more  dissolved  alloy  enhanced  significantly not  peak  has  f o r the  diagrams  pH.  solutions  Increased  the  peaks  that  is increased  f o r the  extend  tion  been  increased  passivate  assumed  major  i n peak  diagrams.  dissolved  to  material.  was  not  the  have  centration  may  current  pronounced  dissolution  The  corresponded  illustrates  more  i n the  11  40.  each  reaction  to  on  SCE  of  density  stabilities  temperature  90 as  shown  by t h e E-pH  The Na S in  polarization  solution  2  sulfide  dynamics  observed  solution steel  may  S 0 2  E  2  2 3  "  illustrates  oxidize  " + 6H  2"  how  solution  =  °-  reversible  0 3 4  o f data  the behavior  on  was h i d d e n  The p o l a r i z a t i o n  current  thermo-  to thiosulfate  -°-  2S ~ 2  P  0 5 6  potential  H  +  current  i n the sulfide  of polarization  7 6  of the  of sulfide.  v i a equation  ...(9)  2  2  o f equation  The  9.  3H 0  0.0093 l o g ^ [ S 0  +  current  i n the sulfide  f o rthe oxidation  + 8e"  +  with  the active-passive  t h e sum o f t h e c u r r e n t plus  i n t h e NaOH +  solutions.  solution.  was  by a n a l o g y  due t o l a c k  i n t h e NaOH  i n NaOH  sulfide  The  41  7 3 - 7 5  of the steel  analyzed  solutions  in sulfide  containing  behavior  has been  free  Figure peak  diagrams.  2 3  ]/[S  9, - 0 . 9 5 9  |»V  2  V  S  C  E  ..(9a)  $ H E  ,  was  fi i  calculated  after  Tromans  using  the average  pH v a l u e  given  49 by was  Singbeil. hidden  Hence,  the p o l a r i z a t i o n  a t > - 0 . 7 5 V<.£ 75  Biernat for  and Robins  the Fe-S-H 0  their  2  data,  iron  by o x i d a t i o n  E  and S-H 0 sulfides  of the steel  of sulfide.  76 '  2  behavior  have systems  constructed  E-pH  a t 100 ° C .  are stable  a t low  diagrams  According  potential.  to  91  Figure  41  Anodic  polarization  curves,  steel,  NaOH  92  +  Identification  Na S, 2  of  °C.  reactions.  316  stainless  Similarly, diagram  MacDonald  f o r t h e Ni-S-H^O  formation  of nickel  g r a m was a v a i 1 a b l e  For current have  and S y r e t t  higher  the  major  FeS  when  s u l f i d e s a t low p o t e n t i a l .  concentrations were  observed current  constructed  f o rmild peak  has been  larger  (Figure  steel.  49  13).  61 62 ' '  4.2  SCC  behaviour  solution  was  o f the slow indicated  associated  with  p o l a r i z a t i o n study  and  d i s s o l u t i o n t o chromate region  The potential may  have  allowing have  peak  anodic  Similar  peaks  attributed to deposition i s exceeded.  where  surface range formed fast  occurred  strain  that  The  sive  E-pH d i a -  of  l  Susceptibility  Comparison tion  product  No  The p o s i t i o n o f  fi  its solubility  possible  system.  o f NaOH,  observed  a n E-pH  a t 25 ° C , i n d i c a t i n g  f o r t h e Cr-S-H^O  densities  been  system  have  lution  model,  1 9  that  o f a specimen deeply  where  the surface  i n NaOH film.  of passive  greatest  film  transpas^  (Figure 15).  i n the susceptible  film  dissolution.  the released  was  (Figure  release  polariza-  i n the primary  tested  cracked  According  with  of the passive  breakdown  occurred  by t h e s u d d e n  the film.  instability  SCC s u s c e p t i b i l i t y  was  tests  SCC s u s c e p t i b i l i t y  showed  localized  behind  rate  was  16).  The  broken,  Rupture  thus  of the film  of dislocations  to the film  cracks  rupture  d i s l o c a t i o n s would  piled  may up  and d i s s o form  a  slip  93 step of  of  bare  chromate  ing  in  ions  A  SCC  ruled  which  would  formation  iron-nickel  be  metal  of  film  at  have  deep  could  mechanism  out  would  then  be  caused  cracks  dissolved.  rapid  before  Formation  dissolution  result-  repassivation  with  an  occur.  involving  primary  hydrogen  transpassive  embrittlement  potentials.  In  would pH  12.5  75 solutions,  hydrogen  only  -1.17  below  = H  A  /H  pH  +•  +  0.057-0.074  2e"  pH  -  H  a  measurably Thus  10  would  occur  0.037  _  2  log  a  of  solution  draining  out  of  a  mechanics  specimen  showed  lower  the  solution  than  local  crack  hydrogen  evolution  1  0  )  2  fracture in  (  ...(10a) H  in  tion.  equation  2  measurement  crack  via  Vc,^. 2H  E  evolution  was  stress that  in  pH  the  unlikely  corrosion was  bulk even  not solu-  inside  cracks.  49 In the  316  contrast stainless  active-passive these No  chromium  behavior  steel  may  film film  of  showed  potential.  potentials  significant  stable  to  have  The  no  steel  may  minimize  in  NaOH  solutions,  s u s c e p t i b i l i t y to  absence  resulted  instability may  mild  from  of a  occur  SCC  at  susceptibility number in  dissolution  this of  of  the  at  factors.  range. iron  and  A  94 nickel  and  causing  the  strain  rate  occurred  cycles  of  rate  was  solution  cracking  the  of  taken  rapid  below  in  at  the  the  at  this  that  this  period  have  rate  tests  The  were  p o t e n t i a l , then which  low  indicated  Very  rapid  hydrogen  little  have  in  dis-  was been  de-  reducing  dis-  Additionthe  cracking  repassivation is  the  dissolution  cracking.  important  then  current  that  would  by  cycles  repassivation  sustain  embrittlement  during  occurred  k i n e t i c s , perhaps  to  the  have  repassivation  d i s s o l u t i o n stage  required  would  repassivation  before  that  progress.  potential.  place  factor  been  failure  to  rate.  peak  repassivation  i f hydrogen  mechanism  strain  cracking  might  have  time  d i s s o l u t i o n and the  alternate  case,  d i s s o l u t i o n and  low  by  had  slow  relatively  creased  reduce  the  that  active-passive  Time  ally,  in  In had  rupture,  complete.  solution  may  high.  too  affected  at  susceptibility  was  duration  density  An  of  film  have  instability.  absence  cracking  and  would  prevent  before  If  rate  thus  absorbed  might  into  the  lattice.  o  Park 20N  NaOH  metal lowest  et in  al. terms  surface in  the  to  of  cracking  the  of  explained  the  that  on  primary  observed rate  have  a  of  filmed  transpassive  near  cracking  ratio  SCC  was  the  of  for  stainless  current  density  metal.  This  region.  corrosion  lower  304  ratio  Although  potential  alloys  on  with  (/v-0.9 higher  steel a  in  bare was  they V  S  H  E  ) ,  chromium  95 content. Park  et  tions  The al.  at  in that  the  b e e n due t o rate,  present  and  °C.  from  their  regime  ally for  form  study, tion  iron  In white  same  perhaps  Santarini  allow and  the  film  a  17  Cr-13Ni  the  open  i n 50%  circuit  -1.00  V$QI£-  alloy  would  and  steel  This be  nickel  was  strain  dissolving,  would  this  be  as  present  study  was  he  as  NaOH +  0.423 m o l / k g  to  V^^^.  potential  chromium  dangerous  i n the  was  model  susceptibility  rose  present  and  concentra-  potentiostatic  i n the  solution,  at  thermodynamic-  region  observed  in potential,  NaOH  i n the  temperature  -1.175  soluhave  potential  lower  mol/kg  of  composition.  the  shift  may  faster  not  included  2.5  -1.15  the  difference  was  of  that  i n NaOH  solutions,  identified  result  because  Also,  liquor) at  i n the  Santarini  The  not  then  a passive  here.  would  test,  -1.13  stable. SCC.  SSRT  to  a  from  observed  The  concentrated  alloy  the  not  potential.  tested  7 8  During  where  would  more  different  -1.17  c r a c k i n g was  corrosion  Santarini 130  investigation,differed  observed  and  so  by  proposed.  to  According  Na S  (simulated  2  SCC to  was  detected  equation  10,  by  H^  2may  be  may  retard  of  adsorbed  fuse  evolved  into  effect  of  the  at  this  hydrogen  hydrogen.  the  metal  sulfide  to (H S 9  potential. evolution This cause  The and  dissolved hydrogen  solution)  presence so  of  promote  hydrogen  S  absorption  may  then  embrittlement.  i n p o i s o n i n g the  ions  dif-  The  recombination  96 reaction  10 h a s b e e n  The formed  sulfide  at this  allowing  shown  also  may  experimentally. have  weakened  p o t e n t i a l , making  d i s s o l u t i o n to occur.  the passive  i t less  Another  79  surface  protective  possibility  film  and t h u s may  be  that  2the  sulfide  bed  onto  affected  the repassivation  the surface  formation  instad  of passive  repassivation  rate  film  may  as  shown  NaOH The  by B e r k o w i t z  leave  a t -1.15  i n the slow  perly  adjacent have  resulted  dissolution slower  2  solution.  already An  slowed  area  specimen  solution  Critical  s t r a i n s would  softer  i n the test tested  t o be  than  when  slower  longer  at non-susceptible  n o t have  chromium thus  in  formed  i n pro-  content  allowing  may  more  r  Alternately,  longer  time  fora  !  i n t h e NaOH +  because  reduction  specimen.  developed  The  had  material.  i n the material Annealed  might  in  annealed  as-received  was g r e a t e r .  potentials  sulfide  transpassive  percent  the work-hardened  elongation  o f t h e chromium  at the  had a l a r g e r  n o t have  (Figure 15).  susceptibility  tested  mol/kg  operative.  d i d an a s - r e c e i v e d  was  mens  mechanism  allow  kinetics.  than  The  i n 3.35  complete.  the repassivation  material  later  was  no e f f e c t  i n NaOH  at failure  Lower  i t showed  SSRT  fora  tests  may  repassivation  d i d not a f f e c t  Perhaps  annealed  potential  up some  k i n e t i c s might  embrittlement  rate  the film  repassivation  Sensitization Na S  strain  boundaries.  i n slower  before  bare  adsor-  the rate of  repassivation.  susceptibility  tied  and hence  repassivation  hydrogen  some  have  to grain  have  may c a t a l y z e e v o l u t i o n o f t o g r e a t e r hydrogen absorption 79 Horowitz.  caused  carbides  may  reducing  the surface  Sensitization  tergranular  S  surface leading  and  s e n s i t i z a t i o n would  also  o f 0H~, t h u s  and slowing  p e r i o d o f time. This bare adsorbed hydrogen thereby  rate.  until specie  have. v . .  97 showed  higher  result may  with  percent  reductions  the annealed  reduce  specimen  susceptibility  in  area,  indicated  but does  too.  that  The  stress  not n e c e s s a r i l y  relief  prevent  SCC.  4.3  Crack  SCC  the  by t h e s l o w  correlated  means  described variety  4.3.1  of conditions  curves.  Dependence mechanics  In  NaOH,  0.0  t h e 3.35 m o l / k g and -0.175 was  SSRT.  confirmed  V  expected  $  C  E  than  because  of the region  Fracture  the absence  t h e 3.35 m o l / k g  by  attributed  and d i s s o l u t i o n  mechanism.  growth  conclusive  rates  evidence  under  to As  a  regarding  mechanism.  Growth  Region  t h e SSRT  II crack $  C  E  and -0.175  tests  result  dependent.  a t -0.10 V 0.0  Rate  confirmed  potential  of cracking  NaOH.  regions  instability  growth  (Figure V  S  C  E  o f SCC s u s c e p t i b i l i t y  mechanics  regions  was  testing  was  and t h e s e  Cracking  o f Crack  SCC s u s c e p t i b i l i t y  Cracking  for film  of the crack  provided  of  i n the potential  tests,  ranges  of the dissolution  extremities  in  rate  rupture  a study  Potential  result  by  greatest  strain  of a film  Fracture  at  was  to potential  below,  nature  that  and t h e Mechanism  of the p o l a r i z a t i o n  operation  the  Rates  susceptibility  indicated were  Growth  was  25).  were  slower This  at the  determined  a t -0.85 and -1.15 V at active-passive potentials  98 The  cracking  dependent, 0.423  was  NaOH  +  The V  b  u  n  specimens was of  due  s  w  a  to  n  Thus,  o  fastest  Fracture Regions  cracking dependent probable.  kinetics  crack SCC  i n the  -1.00  f o r the  that  Rate  of  the  were  fracture  the  and  rod  the  the  v  i n Region only,  -  mol/kg  -1.10  V  plate,  Growth  provided Kj. p l o t s .  was  II were  of which  the  .  may  per-  have  insufficient  of  tests  that  of  mainly  specimens.  rate  crack  and  crack  greatest  growth suscept-  Rate crack Kj  E  1.15  high  dependence  ranges  C  difference  potentials  and  $  SSRT.  Crack  tests  poten-  i n 2.5  relatively  mechanics  consistent  +  mechanics the  strain  potential  and  the  or  Perhaps  potential slow  by  NaOH  s u s c e p t i b i l i t y at ^ -  fracture  predicted  II o f  at  potential  mol/kg  tested  s u s c e p t i b i l i t y , which  tests  processes  Specimens  >  active-passive  mechanics  I and  i n 12  was  s t o p p e d when  Alternately, at  SCC  Dissolution  was  E  material.  in those  as  C  some  investigation  rate  in  E  $  compositions of  i n area  mechanics  4.3.2  plate  indicated  SCC  C  observed  t  nickel.  fracture  to  S  V  solutions  observed  d i d not  2  cracking  the  rate  ibility  Na S  from  and  V  indicated  marginal  initiate  was  -1.175  different  reduction  growth  s  containing  cracking  -1.15  mol/kg  made  indicated to  i  chromium  cent  to  results t  at  2  0.423  t  The  Na S  raised  SSRT  SCE  also.  mol/kg  tial  in sulfide  growth  rates  independent  result  dissolution  of  stress  i s the  in-  most  99 An the  attempt  was  dissolution  made  rate  to  via  relate  the  Faraday's  i  crack  law.  growth  rate  80  W  a  to  ...(11)  where v  =  crack  =  anodic  W  =  equivalent  F  =  Faraday  (9.65  p  =  density  of  i  The  a  anodic  polarization  of  nickel  14.3  crack  0.49  x  m/s.  to  was  10"  growth  expected  rate  because  what  their  None enough  to  of  the  m/s,and  1 0  Crack  Cr(VI)  Fe(III)  with  or  -0.10  velocity calculated  iron  of  V<-  CE  the  was:  1.79  to  calculated  than  Ni:  0.3  anodic x  10~  crack  crack  observed  of  was from  species  or  The  2  i t was  the  for  iron  A/m ).  growth  solution  what  that  density  m/S.  1 0  from  density  current  Ni(II)  would  obtained  current  calculated  about  rates  , were  the  2  sulfide rate  a  larger  from  in  the  i  A/m ,  growth  for  metal  A«S)  2.0  nickel  dissolution  account  the  4  much  uncertainty  the  TO  calculated  for  rate  crack  x  of  metal  Fe:  2  for  J  weight  At  A/m ,  d i s s o l u t i o n to  density  densities,  chromium  for  rate  current  current  curves.  dissolution (Cr:  growth  Similarly rate  was  1.02 not  x  10"  1 1  compared  dissolution would  be  formed;  be.  growth rate  rates in  NaOH  was (^1  large xlO  m/s).  1 00 However, ring  during  cracking,  uniformly  ted  over  at defects  density  sites  et a l .  stainless  steel  8  have  would  shown  electrode  electrode.  This  but would  film.  n o t have have  Therefore,  have  by t h e p o l a r i z a t i o n  Park  filmed  the surface,  i n the oxide  at defect  indicated  d i s s o l u t i o n would  been  much  been  occur-  been  concentra-  the true  current  higher  than  that  diagrams.  that  current  i n NaOH  density  i s about  i s corroborated  on a  100 t i m e s  by H o a r  bare  that  and J o n e s  on a 91 82  for  mild  steel  for  iron  i n NaOH  densities static  It  anodic  a drop  weight  on s t r a i n i n g e l e c t r o d e s then  cracking  f o r the observed  has been  cannot  density  crack  using  electrodes,  account  rate  s t r a i n i n g i n NaOH, a n d by D i e g l e  be c a l c u l a t e d  that from  apparatus.  rates  growth  density  rate.  still  the magnitude  i n crack  (Figure  21) c o r r e l a t e d  (Figure  8).  A t 92 ° C , c r a c k  related  with  increase  potentials  growth  (Figure 7).  rate with  over  on  would  NaOH  increases  growth  density  S  C  E  (Figure over  density 24)  in in  , i n -  o f temperature  i n current  rate  increases  a t -0.10 V  a range  growth  o f the current  However,  increases  in current  than  This  o f crack  c o r r e l a t e with  I n 3.35 m o l / k g  creases  be a l s o .  the magnitude  may  higher  rates.  on t h e p o l a r i z a t i o n d i a g r a m . current  may  Vermilyea  If current  a r e 100 t i m e s  cracking  concluded  and  cor-  a range o f  ;i.  101 It  already  qualitatively fortunately, curves  sistent  confirmed  4.3.3  in  trolled  Growth  were  by c h a r g e  are  Un-  polarization  densities  were  but  con-  neither  of the dissolution  process.  Rates energy  a t -0.10 V  transfer,  rates  for quantitatively  f o r crack  i n 3.35 m o l / k g  consistent  on  mechanism  the nature  activation  (both  Current  growth  mechanism.  densities  dissolution  t o be ^ 60 k J / m o l  energies  a dissolution  rates.  o f Crack  NaOH  the crack  t o be i n a d e q u a t e  growth  apparent  that  of current  i t nor revealed  12 m o l / k g  tion  with  a localized  Kinetics  found  shown  crack  The was  been  with  shown  consistent comparison  have  predicting  has been  S  with  C  ) .  E  NaOH These  growth  a n d ^ 37 k J / m o l apparent  a dissolution  f o r which  values  rate  activa-  process  may  range  con-  from  21-  o o  105 to  kJ/mol. result  1iquid  solely  energy  attempt  energies  by d i f f u s i o n  through  to correlate  with  that  on t h e p o l a r i z a t i o n  energy  varied  from  66.8 + 1 9 . 9  t o 3 3 . 9 + 3.8  were  too  (transport)  kJ/mol  fact  that  the  best  correlation  growth  of the dissolution diagram  (Figure  at the primary  large  i nthe  with  supports  energy  the film  current  8).  The  activa-  transpassive passive  of the current  the apparent  activation  a t -0.10 V  at the secondary  the a c t i v a t i o n  rate  the apparent  9 3 . 8 + 33.1 k J / m o l  kJ/mol  The  crack  control  was made  f o r cracking  measured  peak  from  activation  phase.  An  tion  The a p p a r e n t  activation  rupture  and  current  minimum. peak  gave  energy f o r dissolution  102 model rent  in  which  peak  may  The solution 12  the  greatly  apparent was  mol/kg  magnitude  of  affect  solution  was  of  growth  energy  charge  more  transient  crack  activation  indicative  the  in  dissolution  cur-  rate.  the  transfer  representative  3.35  mol/kg  while of  that  the  NaOH in  lower  the values  49 obtained the  12  the  mixed  in  mixed  mol/kg  charge  solution,  charge  energy  The with  for  values  results  data  of  Russel  67  25%  same 304  plate  used  stainless  in  found  cold  in  present  in  22%  activation  energy  of  kJ/mol  nickel  straining  The  for  lower  activation  attributed  to  that  might  metal of  there  in  be  a  due  rapidly  of  to  an  emerging  4  '  4  consistent  other  steel  energy from  Speidel  8  found  an  in-  ,  of the  testing  apparent  3  static  after in  the  increase slip  in  obtained  electrodes  straining  disarray' reduction  1  Staehle  and  in  activa-  were by  stainless  kJ/mol.  energy  'lattice  dissolution  atoms  72  an  activation  solution 1  part  dissolution. of  obtained  study.  NaCl  greater of  environments  316  In  solutions.  apparent  worked  the  steel  an  a  calculation  energy  other  play  control.  control  insulfide  activation  obtained  for  may  precluded  cracking  vestigators. kJ/mol  diffusion  transfer-diffusion  Insufficient tion  transfer-diffusion  42-75.3  in  electrodes 85  Hoar  who  activation the  "pockets."  1NH S0 . 2  4  was  predicted energy  internal  for  energy  103 8 6  Petit solutions the  studied  of  sulfuric  activation  currents  at  region).  transpassive  acid  energies  the  Impedance  50.5-70.0  of  sodium  the  reaction  peak  measurements stages.  kJ/mol  at  The  a  behavior  and  transpassive  occurred in several were  the  or  of  sulfate. rates  in  the  showed  polarization  He  (equivalent  the  of  to  passive  reactions  energies  rate  in  determined  secondary  that  activation  nickel  quoted  33  mV/min.  1 c  Staehle ated is  with  and Thus the  to  that  that  the  nickel  rent  4.4  4.4.0  film-free  rate  showed  that  of  crack  cracking  rate  by  transient  following  film  He  rate  with  by  associrupture  presented  for.'partial  increased  following  cracking  metal  energies  energy  reduce  current  propagation.  activation  activation  the  currents',  nickel  reducing  content.  the  rupture,  and  chromium  increasing  the  size  of  size  of  would the  cur-  transient.  Fractography  Corrosion  precipitated  was  and  Dissolution  deposits  from  deposits  saturated the  the  Mechanism  Deposits  corrosion  crystalline crack  of  transient  the  The have  the  would  current  increase  suggested  corrosion  related  results  has  solution.  indicated with  composition  of  crystalline  deposits  metal  deposits in  shown  The  that  Figure presence  the  species.  could  Figure  in  29b  not  be  29 of  solution  appear these within  Unfortunately determined.  resembled  to  hematite  the the  The  _  104 platelets  At  higher  deposits the  observed  were  higher  tion,  during where  creased  subsequently  may  They  a longer  film.  soluble  from  species  V m a y  distances  control  were  V  also  a t these  exposed  formed  thus  occur.  significantly  by  a s t h e p o t e n t i a l was i n 33).  passive  equilibrium  This  may  have  to secondary differing  deposition  concentrations f o r  The heavy  diffusion  was  found  potential  was  i n the secondary  were  potential  i n t h e 3.35 m o l / k g been  solu-  mouth  longer,  to  deposition  have  t o have  the crack  reflected  with  into  deposits  processes  at  over  a grain.  deposits  may  near  potentials.  interfered than  from  ions  p r e c i p i t a t e d t o form the  (Figure  have  Corrosion  fusion  S C E  faster, the  dissolution at  more  affected  greater  the d i f f e r e n t  have  have  had been  of transition  smaller  Heavy  was  t o 0.0  I t may  carried  thicker  deposits  -0.175  was  The f a s t e r  forprecipitation  Deposition  from  resulting  were  time  cracking  p r e c i p i t a t e s appeared  surface  corrosion  the result  passive  The  have  the fracture  potential.  0.0  may  These  solutions.  where  thicker.  temperatures  cracking.  The  been  slightly  deposit.  allowing  i n NaOH  temperatures,  and t h e s e  thicker  on i r o n  thicker  affected  i n t h e 12 m o l / k g  than  those  NaOH, t h u s  NaOH s o l u t i o n .  passive found  a t t h e same  indicating  i n the stronger  region.  that  dif-  NaOH s o l u t i o n .  105 Indeed, a  the apparent  low v a l u e  diffusion  which  consistent  deposits  solutions.  fracture  surfaces  of  iron  As  previously  test  was  energy with  confirmed mixed  this,  being  activation-  control.  Corrosion taining  activation  sulfide  turned  were  found  The o b s e r v a t i o n was  i n agreement  formation  stated,  green  also  on  iron  solution  when  i n the s u l f i d e  o f metal with  previous  i n NaOH  taken  neutralized  sulfides  from  conon t h e  observations  + Na^S  solution.  the c e l l  during  with  the addition  have  studied  of  5 1  the distil-  88 led  water.  sulfide green This  solutions  color was  the  and S h o e s m i t h o f pH  i s derived  evidence  mechanics ever  Taylor  that  tests.  not rule deposits  12-13 and have  that  the  colloidal  NaFeS^  in solution.  iron  dissolved  during  the  The f o r m a t i o n  have  determined  alkaline  from  out hydrogen may  green  of these  deposits  embrittlement  formed  fracture  at this  by d i s s o l u t i o n  from  would  how-  potential  as  the crack  wal1s.  4.4.1  Fracture The  Mode  cracking  Intergranular  progressed  segregation  may  mostly have  i n an  been  intergranular  a factor  mode.  i n the faster  89 dissolution  at grain  intergranular ted have  there.  boundaries.  cracking  Dislocation  resulted  may  have  Another been  pile-ups  in a greater  strain  that  factor  strain  at the grain rate  causing  was  the  concentra-  boundaries  i n the adjacent  may  area  106 when in  released.  Figures  took ing  place from  offering  there.  Larger  dissolution  inherent  an  at the may  the  could  faster  evidence  take  have  provided  sites  boundaries. the  This  The  grain  adsorption  may  greater  boundaries due  to  their  have i n -  species  imperfections  result-  for  disarray.may  o f damaging  straining  steps  dissolution rate The  surfaces  substantial  would  surface  place.  grain  slip  disarray.  of  that  the  numerous  of adsorption  number  on  more  increased  atomic  observed  by  offering  at which  adsorp4r"  have  stimulated  dissolution.  At have  were  and  grain  have  rate  increased  tton  straining  local  creased  bands  26-33,  this  themselves  Slip  the  been  primary  transpassive  e s s e n t i a l to the  potentials, adsorption  d i s s o l u t i o n process.  may  Knoedler  and  90 Heusler  have  suggested  that  the  oxidation  of  chromium  to  2CrO^ The Most the  proceeds  intermediates of  the  rate  equation  The with  via six consecutive  surface  mechanism  the  step.  12. Cr(0H)  +  x + 1  that  adsorption, suggested  surface  i s covered  determining  fractography 0H~  cover  with  with  The  and  thus  +  x  and  adsorbed  reactions. monolayer.  i t s oxidation  are  summarized  OH"  in this  supported  Knoedler  an  reactions  observed  transfer  Cr(IV),  e" = F = * C r ( 0 H )  was  by  charge  being by  ...(12)  study  was  consistent  the d i s s o l u t i o n  Heusler.  107 Figure  27 i l l u s t r a t e s  fractography. high of  stress  of  Transgranular  oxide  at this  sites film  high  within  or high  of stress  cracking  i n t e n s i t y i n 3.35 m o l / k g  deformation  competing  the effect  stress  the grain strain  was  i n t e n s i t y on  observed  NaOH.  energy  have  by s e v e r e  sites  at  The g r e a t e r  i n t e n s i t y may  either  only  amount provided  disruption  f o r adsorption. 48  This for  observed  effect  was  316 s t a i n l e s s s t e e l  transgranular  opposite  to the r e s u l t s  i n MgCL,,.  SCC a n d i n c r e a s i n g  observed  a s K j was  increased  observed  by R u s s e l  may  have  A decreasing intergranular  i n MgC^. resulted  differences specimens  could  were  be r u l e d  t h e same  boundary  segregation  granular  failure  effect  cracking  this  behavior  tempera-  Metallurgical  and p r e p a r a t i o n studies.  of  If grain  important  to the inter-  obviously  had a  different  i n MgCl,,.  In  peak,  they  of  SCC w e r e  of C l " .  as f o r t h e p r e s e n t  in caustic,  fraction  different  o u t as t h e p l a t e  e f f e c t s were  Russel  The d i f f e r e n t  from  t u r e , p o t e n t i a l , p H , o r by t h e p r e s e n c e  of  t h e NaOH over  solutions,  the range  and s e c o n d a r y range,  cracking  there  was  of primary  passive  i n t h e mode o f  transpassive,  potentials  mode may  no c h a n g e  change.  transpassive  investigated. Some  evidence  Beyond i nthe  27 literature  supports  this  possibility.  quoted a study  by Subramanyam  cracking  depended  mode  potentials,  and S t a e h l e  on e l e c t r o d e  intergranular  Okada  cracking  et a l .  which  potential. was  favored  have  showed At  that  transpassive  i n 304 s t a i n l e s s  108 steel  i n 7 0 % NaOH  transgranular  The  cracking  cracking  temperature  of crack  cated  that  change  Figure  crack  31  able.  illustrates  A clue  energy.  That  Surface  that  NaOH  might value  diffusion  dissolution.  rate  Kj v a l u e s  a t those  This  playing may  indi^ a t 92 °C  higher  process  already  solution  a larger  have  have  role  less  i n t h e 12 m o l / k g  exhibited  mixed  i n mixed the  which  favorwith  solu-  control  may  have  in a  influences.  been con-  cracking Thus  as i n t e r g r a n u l a r .  cracking NaOH  of  fractography.  foractivation  be a s f a v o r a b l e  to the intergranular  surfaces  occur-  of the activation  important  by d i f f u s i o n  could  cracking  in fractography  affected  boundaries  been  cracking.  i n the concentrated  dissolution rate  down  cracking  contrast  fracture  slowed  t h e change  that  at grain  d i s s o l u t i o n may  of  t h e o r i e n t a t i o n was  i n the value  i n turn  trolled  In  when  suggested  was  t h e mode  some t r a n s g r a n u l a r  be f o u n d  i n increasing  transgranular  affected  perhaps  imperfections  important  Na^S  o f an e f f e c t  a t high  growth  to understanding  concentrations  the  mode  with  of the substantial  The a b s e n c e  i n cracking  concentration  i n 12 m o l / k g  tion,  d i d n o t change  intensities.  Caustic  red  rate.  n o t due t o h i g h e r  stress  solutions  26 a n d 3 0 ) i n s p i t e  growth  potential  predominated.  mode, i n NaOH  (Figures  range  was  but a t the active-passive  i n t h e NaOH,  + 0.423  mol/kg  intergranular-transgranular  109 cracking. the  The l a r g e r  sulfide  solution ions.  tion  containing  may  have  of transgranular  solution  resulted  The s u l f i d e  fections  proportion  may  at the grain  than  from  have  o f 0H~ a n d l e s s e n i n g  i n the straight  the presence  adsorbed  boundaries  cracking NaOH  of the s u l f i d e  on t h e l a t t i c e  thereby  imper-  decreasing  the dissolution  in  rate  the adsorp-  at the grain  2boundaries. then,  If adsorption  similarly,  growth  rate  i t may  reduces  be r e s p o n s i b l e  i n t h e NaOH  Alternately,  of S  + Na S  from  testing  Hydrogen  embrittlement  i n a much was  f o r the slower  rate crack  solution.  2  the difference  resulted  dissolution  i n fractography different  possible  may  have  potential  at this  range.  potential  and  2_ may  have  caused  important gen  the transgranular  i n causing  recombination.  ditions  hydrogen Lack  has p r e c l u d e d  In  contrast  cracking.  embrittlement  of experimental  evaluation  of this  to the present  304  stainless  steel  showed  50%  NaOH w i t h  0.03 m o l / l i t e r  result,  mostly Na S 2  S  may  have  by p o i s o n i n g  results  been hydro-  at these  con-  possibility.  Asaro  3 4  intergranular a t 180 ° C .  found  that  cracking  The  in  difference  2may  have  ferent  4.5  resulted  potential  Electron  The  from  either  S  concentration  or  dif-  (unknown).  Diffraction  electron  lower  Analysis  d i f f r a c t i o n ring  of Surface  patterns  Films  were  i n NaOH  consistent  no with  a spinel  meters  were  published  type  of structure  generally  data  a little  f o r Fe-b.ased  71  '  72  but the l a t t i c e  higher  spinels  than  91  that  para-  expected ° A  e . g . 8.50  from  versus  o  ^  8.38  A  ferent  f o r Fe^O^.  valency  Nikiforuk  state  o r an  obtained  7 0  parameters  The d i f f e r e n c e  formed  irregular  reasonable  on  may  have  lattice  values  stainless steels  been  due t o a  arrangement  f o r spinel i n MgCl^  dif-  here.  lattice  using  t h e same  method.  There  were  analysis. of  The f i l m  the film  deposit  was  patterns surface  some  fundamental analyzed  was  on t h e f r a c t u r e thick  may  but from  been  was  Much  opaque  f o r material  the sides  with  this  method  not a r e p r e s e n t a t i v e  surface.  and as such  n o t have  problems  of the  sample  surface  to electrons. from  of the specimen.  of  The  the fracture The  method  72 described  by B i r l e y  patterns these  from  oxides  The E-pH  the unstripped  problems.  through  f o r obtaining  In t h a t on s u r f a c e  surface  diagrams  film  was  indicate  electron  fracture  method,  surface  the electron  that  FeOOH  might  removal  the s o l u t i o n  or i n subsequent  8  (  Y  - F e  2  0  3  FeOOH  may  be f o r m e d  The  5  beam  tentatively identified  potentials.  HFe 0  would  avoid passes  asperities.  transpassive from  diffraction  have  as at  Fe^P^. primary  dehydrated  handling  to  on form  ) ^via:  5  FeOOH  HFe 0 5  8  + 2H 0 2  ...(13)  Ill Birley  7  2  has n o t e d y-Fe^O^  between  technique  that  i t i s almost  (HFe 0 ) 5  because  and F e 0  g  3  4  impossible  to  by t h e e l e c t r o n  of the limitations  distinguish diffraction  i n the accuracy  of the  t e c h n i que.  5.  SUMMARY  Polarization mechanics  tests,  techniques to  reveal  caustic  have  studies, electron  been  slow  rate  d i f f r a c t i o n , and  employed  t h e mechanism  strain  of  to obtain  cr.ackihgj  tests,  fracture  fractographic  engineering  d a t a and  o f 316 s t a i n l e s s  steel in  solutions.  In  NaOH  transpassive  solution,  cracking  potential  range  occurred  where  i n the primary  t h e chromium  passive  film  2was  unstable  and d i s s o l u t i o n  ring.  At active-passive  showed  some  film  potentials,  s u s c e p t i b i l i t y , and t h a t  instability  and i r o n  at primary  appeared  t o be d i s s o l u t i o n ,  also  There affected  passive  In active  was  transpassive  evidence  cracking  may  kinetics  may  was  sensitized have  occur-  material  resulted  from  The mechanism o f  potentials  which  that  to CrO^  only  dissolution.  cracking  step.  o f chromium  i n NaOH  involve  diffusion  an  solution  adsorption  i n the l i q u i d  at potentials  phase  i n the secondary  region.  t h e NaOH  + Na S  potentials.  2  Film  solutions,  cracking  instability  occurred at  at these - p o t e n t i a l s  may have  resulted  from the  embrittlement could t i a1 s .  not  presence  be r u l e d o u t  of s u l f i d e .  Hydrogen  at a c t i v e - p a s s i v e  potent  11 3 BIBLIOGRAPHY  1.  J . E. Truman (1966).  2.  P.  P.  Snowden,  J . I . S . I . . 197  p p . 1 36-141  (1 961 ) .  3.  P.  P.  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