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A study of the stress corrosion cracking of mild steel in alkaline and alkaline sulphide solutions Singbeil, Douglas Lloyd 1981

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A STUDY OF THE STRESS CORROSION CRACKING OF MILD STEEL IN ALKALINE AND ALKALINE SULPHIDE SOLUTIONS by DOUGLAS LLOYD SINGBEIL B . S c , The Un ivers i ty of B r i t i s h Columbia, 1977 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES The Department of Meta l lu rg i ca l Engineering We accept th i s thes i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA J u l y , 1981 © Douglas Lloyd S i n g b e i l , 1981 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and study. I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e copying o f t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the head o f my department or by h i s o r her r e p r e s e n t a t i v e s . I t i s understood t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be allowed without my w r i t t e n p e r m i s s i o n . Department of Metallurgical Engineering The U n i v e r s i t y o f B r i t i s h Columbia 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 Date J u l Y 2 8> 1 9 8 1 ABSTRACT The s t r e s s c o r r o s i o n c r a c k i n g (SCC) o f an At ST C -1018 m i l d s t e e l was i n v e s t i g a t e d i n t h r e e s o l u t i o n s , composed o f 12 .5 m o l / k g NaOH, 3.35 m o l / k g NaOH and 2.5 m o l / k g NaOH + 0 .42 m o l / k g Na,,S, r e s p e c t i v e l y . The p o t e n -t i a l o f maximum s u s c e p t i b i l i t y to SCC o f s t e e l i n t he l a t t e r two s o l u t i o n s was a s s e s s e d by a s l o w s t r a i n r a t e t e c h n i q u e . I t was f o u n d t o be s l i g h t l y h i g h e r t h a n t h e a c t i v e - p a s s i v e t r a n s i t i o n i n each s o l u t i o n ( - 1 . 0 0 v " s c e i n 3.35 m o l / k g NaOH and - 0 . 8 8 V i n 2.5 m o l / k g NaOH + 0.42 m o l / k g N a 2 S ) . A f r a c t u r e m e c h a n i c s t e c h n i q u e , u t i l i z i n g f a t i g u e p r e c r a c k e d d o u b l e c a n t i l e v e r beam s p e c i m e n s , was t h e n used t o s t u d y t h e e f f e c t s o f s t r e s s i n t e n s i t y , t e m p e r a t u r e and e l e c t r o c h e m i c a l p o t e n t i a l on c r a c k v e l o c i t y i n a l l t h r e e s o l u t i o n s . Bo th s t r e s s i n t e n s i t y d e p e n d e n t ( r e g i o n I) and s t r e s s i n t e n s i t y i n d e p e n d e n t ( r e g i o n I I ) c r a c k v e l o c i t y b e h a v i o r was f o u n d . A p p a r e n t a c t i v a t i o n e n e r g i e s f o r r e g i o n II o f ~ 24 k J / m o l were d e t e r m i n e d a t b o t h E and - 1 . 0 0 V i n 12 .5 m o l / k g NaOH. C r a c k v e l o c i t i e s c o r r s e e 3 -9 o f t h e o r d e r o f 10 m/s were m e a s u r e d a t E „ i n c o r r 12 .5 m o l / k g NaOH and a t - 1 . 0 0 V and - 0 . 8 8 V n a i n 3 see see 3.35 m o l / k g NaOH and 2.5 m o l / k g NaOH + 0 .42 m o l / k g N a 2 S , 11 r e s p e c t i v e l y . The c r a c k v e l o c i t i e s measured a t - 1 .00 V s c g - 8 i n 12.5 mol/kg NaOH were of the o r d e r of 10 m/s. The f r a c t o g r a p h y o f the c r a c k s was t r a n s g r a n u l a r i n 12.5 mol/kg NaOH at E c o r r - A mixed i n t e r g r a n u l a r - t r a n s g r a n u l a r f r a c t o g r a p h y was ob se rved a t the a c t i v e - p a s s i v e t r a n s i t i o n i n a l l t h r e e s o l u t i o n s . The r e s u l t s of the two t e c h n i q u e s were compared and d i s c u s s e d , as was the r o l e of s t r e s s i n t e n s i t y and p a s s i v a -t i o n r a t e i n f r a c t u r e mechan ics e x p e r i m e n t s . Anod i c d i s -s o l u t i o n , hydrogen e m b r i t t l e m e n t and a d s o r p t i o n mechanisms were c o n s i d e r e d . I t was d e c i d e d t h a t the r e s u l t s a t E „ „ c o r r i n 12.5 mol/kg NaOH c o u l d bes t be accoun ted f o r by a hydrogen e m b r i t t l e m e n t mechanism, perhaps a s s i s t e d by anod i c d i s s o l u t i o n . Hydrogen e m b r i t t l e m e n t was e l i m i n a t e d as a p o s s i b l e mechanism a t the a c t i v e - p a s s i v e t r a n s i t i o n i n a l l t he s o l u t i o n s . The most l i k e l y mechanism was thought to be one i n v o l v i n g mixed a c t i v a t i o n - d i f f u s i o n c o n t r o l l e d d i s s o l u t i o n . A p p l i c a t i o n s o f the r e s u l t s t o the pu lp and paper i n d u s t r y were c o n s i d e r e d . i i i TABLE OF CONTENTS Abstract i i ' Table of Contents , iv L i s t of Tables v i i i L i s t of Figures x L i s t of Symbols and,Abbreviations xvi Acknowledgement x v i i i Chapter 1 INTRODUCTION 1 1.1 Stress Corrosion Cracking . . 1 1.2 SCC Techniques 3 •1.3 SCC of Mi ld Steel in A l ka l i ne Solut ions 12 1.4 Or ig ins of the Present Work 1 8 2 EXPERIMENTAL • 2 2 2.1 Scope of the Present Work 2 2 2.2 Mater ia l s 2 3 2.2.1 Steels 2 3 2.2.2 Solut ions 2 7 2.3 Equipment and Apparatus 2 7 2.4 Experimental Procedures 3 2 2.4.1 SSRT Experiments 3 2 2.4.2 Fracture Mechanics Experiments 3 4 2.4.3 Anodic P o l a r i z a t i o n Curves and L inear 3 9 O Q Po l a r i z a t i on Experiments 2.4.4 Sulphide and Chlor ide Analys i s 4 1 i v Chapter Page 3 RESULTS . . . . 42 3.1 Anodic P o l a r i z a t i o n Curves and Linear P o l a r i z a t i o n Results 4 2 3.2 SSRT Results 4 7 3.2.1 3.35 mol/kg NaOH 4 7 3.2.2 2.5 mol/kg NaOH + 0.42 mol/kg Na 2S 50 3.3 Fracture Mechanics Results 5 3 3.3.1 General 5 3 3.3.2 12.5 mol/kg NaOH 5 7 3.3.2.1 The E f f ec t of Stress Intens i ty . . . 5 7 3.3.2.2 The E f f e c t of Temperature and Potent ia l 5 7 3.3.3 3.35 mol/kg NaOH 6 0 3.3.3.1 The E f f ec t of Stress Intens i ty . . . 6 0 3.3.3.2 The E f f ec t of Potent ia l 6 3 3.3.4 2.5 mol/kg NaOH + 0.42 mol/kg Na 2S 6 3 3.3.4.1 The E f fec t o f Stress Intens i ty . . . 6 3 3.4 Fractography 6 6 3.4.1 General 6 6 3.4.2 12.5 mol/kg NaOH 7 2 3.4.2.1 The E f f ec t of Stress Intens i ty . . . 72 3.4.2.2 The E f fec t of Temperature and Potent ia l 75 v Chapter Page 3 RESULTS (continued) 3.4.3 3.35 mol/kg NaOH 78 3.4.3.1 The E f f ec t of Stress Intens i ty . . . 78 3.4.3.2 The E f f ec t of Potent ia l 80 3.4.4 2.5 mol/kg NaOH + 0.42 mol/kg Na 2S 80 3.4.4.1 The E f f ec t o f Stress Intens i ty . . . 80 4 DISCUSSION 83 4.1 General 83 4.1.1 SSRT 83 4.1.2 Fracture Mechanics Technique 85 4.1.3 K inet ic s 87 4.1.4 Fractography 89 4.2 Mechanisms 9 4 4.2.1 . The Role o f Stress Intens i ty and Pass ivat ion 95 4.2.2 Anodic D i s so lu t ion 9 8 4.2.3 Hydrogen Embrittlement 1 ° 7 4.2.4 Adsorption of Damaging Anions 112 4.2.5 Assessment of Mechanisms ^ 2 4.3 Industr ia l Implications 116 5 CONCLUSION 121 5.1 Conclusions 121 5.2 Suggestions f o r Future Work 123 v i BIBLIOGRAPHY APPENDIX A APPENDIX B .. LIST OF TABLES Table Page I Y i e l d Strengths and Chemical Composition of Stee ls 24 II Co r re l a t i on Between S o l u t i o n , P o t e n t i a l , Fatigue Level and Steel Batch in Fracture Mechanics Experiments 35 III Results of L inear Po l a r i z a t i on Experiments 46 IV Comparison of Crack Ve loc i t y and Steel Batch 56 V E f f e c t of Stress Intens i ty on Crack Growth in 12.5 mol/kg NaOH at E c Q r r and 92° C 58 VI E f f e c t of Temperature and Potent ia l on Crack Growth in 12.5 mol/kg NaOH at Kj = 31-43 MPa^T 61 VII E f f e c t of Stress Intens i ty on Crack Growth in 3.35 mol/kg NaOH at -1.00 V- and 92° C 64 3 see VIII E f f e c t of Stress Intens i ty on Crack Growth in 2.5 mol/kg NaOH + 0.42 mol/kg Na2S at -0.88 V s c e and 92° C 6 7 IX Value's, o f i Predicted at the Test Potent ia l a from i Values 101 co r r X Ca lcu lated Overpotentia ls Based on Estimated Values of i „ on Bare Metal 1 0 8 cor r v i i i Table Page XI Ca lcu lated pH and E H+^ H f o r a l l Solut ions at 100° C 1 1 1 BI A c t i v i t y Coe f f i c i en t s ( Y O H - ) and A c t i v i t i e s ( a Q H - ) f o r OH" and Calcu lated pH fo r a l l Three Solut ions 135 i x L I ST OF FIGURES Figure Pa__e 1. S t r e s s - s t r a i n curve f o r specimens tested in SCC suscept ib le and i ne r t environments using SSRT, where A and A are the areas under the o sec respect ive curves 6 2 Schematic of a t yp i ca l log v - K- curve, showing regions I, II and III 9 3 Schematic o f the Kraft cyc le 20 4 DCB specimen geometry 21 5 SSRT te s t c e l l geometry 29 6 Fracture mechanics te s t c e l l geometry 31 7 Anodic p o l a r i z a t i o n curves of mild s tee l in 3.35 mol/kg NaOH and 2.5 mol/kg NaOH + 0.42 mol/kg Na 2S at 92° C 43 8 Anodic p o l a r i z a t i o n curves of mi ld s tee l in 12.5 mol/kg NaOH at se lected temperatures, 45 9. E f f e c t of potent ia l upon reduct ion in area f o r 3.35 mol/kg NaOH superimposed upon the anodic p o l a r i z a -t i on curve obtained in the same so lu t ion 48 10 Appearance of f i n a l f r ac tu re region a f t e r SSRT te s t in 3.35 mol/kg NaOH a) E = - 1.00 V b) E = - 0.62 V see see 49 x Figure Page 11 E f f e c t of potent ia l upon reduct ion in area f o r 2.5 mol/kg NaOH + 0.42 mol/kg Na,,S superimposed upon the anodic p o l a r i z a t i o n curve obtained in the same so lu t ion 51 12 Appearance of f i n a l f r ac tu re region a f t e r SSRT te s t in 2.5 mol/kg NaOH +0.42 mol/kg Na 2S a) E = -0.90 V see b) E = -0.50 V 52 see 13 Macroscopic view of s t ress corros ion crack surface of DCB specimen tested in 12.5 mol/kg NaOH at 92° C, E c o r r a n d KI = 2 3 ' 8 " 2 5 A M P a v ^ " * • 5 4 14 E f f ec t of s t res s i n t e n s i t y on crack v e l o c i t y in 12.5 mol/kg NaOH at 92° C and E c Q r r 59 15 Arrhenius p lo t of the region II crack v e l o c i t i e s in 12.5 mol/kg NaOH at E c Q r r and - 1.00 V s c e , Kj = 31 - 43 MPa^m 62 16 E f f ec t of s t ress i n t e n s i t y on crack v e l o c i t y in 3.35 mol/kg NaOH at 92° C and - 1.00 V s c e 65 17 E f fec t of s t ress i n t e n s i t y on crack v e l o c i t y in 2.5 mol/kg NaOH + 0.42 mol/kg Na 2S at 92° C and - 0.88 V „ Q 68 see xi gure 18 Comparison of the corros ion product on the s tress corros ion crack surface in d i f f e r e n t environments. a) 12.5 mol/kg NaOH, E •. K- = 31.0-33.7 MPa/m b) 12.5 mol/kg NaOH, -1.00 V s c e , K.= 31.3-43.1 HPaJm c) 3.35 mol/kg NaOH, -1.00 V s c e , K-= 20.7-24.3 MPa.vfiT. d) 2.5 mol/kg NaOH + 0.42 mol/kg Na 2S, -0.88 V s c e , K- = 25.9-26.7 MPavfiT 19 B r i t t l e overload f a i l u r e near the crack t i p . a) before c leaning with i n h i b i t e d ac id b) a f t e r c leaning with i n h i b i t e d ac id 20 Comparison of fractography between specimens machined from d i f f e r e n t batches of s tee l at -1.00 V and 92° C. see a) Batch A, 12.5 mol/kg NaOH, K-= 31.0-33.8 MPa/m ,. b) Batch B, 12.5 mol/kg NaOH, K.:= 31.8-37.4 MPa/m c) Batch A, 3.35 mol/kg NaOH, K.= 30.3-32.7 MPa/m d) Batch B, 3.35 mol/kg NaOH, K-= 29.7-33.3 MPa/m . . x i i Figure 21 Var i a t i on of fractography with s t ress i n ten s i t y 12.5 mol/kg NaOH at 92° C and E c o r r . a) Kj = 17.9 - 18.1 MPavffi" b) Kj = 23.8 - 25.4 MPavmT c) Kj = 30.1 - 32.7 MPavfiT d) KT = 35.0 - 37.8 MPavfiT 22 Var i a t i on of fractography with temperature in 12.5 mol/kg NaOH at E c o r r -a) 55° C, Kj = 34.5 - 37.7 MPavffi" b) 70° C, Kj = 34.0 - 35.0 MPaVfiT c) 105° C, Kj = 35.4 - 41.9 MPavfif d) 115° C, Kj = 35.2 - 41.1 MPavffi" 23 Var i a t i on of fractography with temperature in 12.5 mol/kg NaOH at - 1.00 V s c e . a) 70° C, Kj = 31.3 - 34.8 MPavfif b) 92° C, Kj = 31.8 - 37.4 MPa*€ c) 105° C, KT = 31.3 - 43.1 MPavfii" x i i i Figure Page 24 Var i a t i on of fractography with s t ress i n ten s i t y in 3.35 mol/kg NaOH at 92° C and - 1.00 V s c e -a) Kj = 15.5 - 16.1 MPa^ m" b) Kj = 20.7 - 24.3 MPavm" c) Kj = 29.7 - 33.3 MPav'm d) Kj = 35.7 - 43.0 MPavfiT 79 25 Var i a t i on of fractography with s t ress i n t e n s i t y in 2.5 mol/kg NaOH + 0.42 mol/kg Na2S at 92° C and - 0.88 V . see a) Kj = 15.5 - 15.6 MPaVm b) Kj = 21.2 - 22.2 MPav'm c) Kj = 31.4 - 32.5 MPavfiT d) Kj = 36.0 - 38.9 MPav€ 81 26 Comparison of uncorroded fat igue pre-crack surfaces of Batch A and Batch B s tee l a) Batch A b) Batch B 92 27 Anodic p o l a r i z a t i o n curve of bare metal surface under a c t i v a t i o n contro l and mixed a c t i v a t i o n - d i f f u s i o n contro l . 103 xi v Figure Page 28 E f f ec t of sulphur add i t ion upon E c Q r r in 2.5 mol/kg NaOH + 0.42 mol/kg Na,,S superimposed on the anodic p o l a r i z a t i o n curve f o r the sulphur f ree so lu t i on Al Stress corros ion crack surface of specimen machined from a 32 mm x 32 mm bar. Tested in 3.35 mol/kg NaOH at -1.00 V and 92° C. 3 see KT = 48-53 MPav'm xv LIST OF SYMBOLS AND ABBREVIATIONS Symbols a OH" B b_ 'OHP d D E E E F H i . co r r "rev corr 'IC VISCC crack length chemical a c t i v i t y o f OH~ thickness of f r ac tu re mechanics specimen anodic Tafel slope cathodic Tafel slope bulk concentrat ion concentrat ion at the outer Helmholtz plane 3 3 dens i ty of i ron (7.86 x 10 kg/m ) d i f f u s i o n c o e f f i c i e n t potent ia l f ree corros ion potent ia l thermodynamically r eve r s i b l e potent ia l Faraday (9.85 x 10 4 A.s) DCB specimen beam height anodic current dens i ty corros ion current density d i f f u s i o n current density exchange current dens i ty s t ress i n ten s i t y f ac to r f o r mode I opening c r i t i c a l s t ress i n t e n s i t y f a c to r threshold SCC s tress i n t e n s i t y f a c to r x v i Symbol s Q R R"1 P r y T v V see w W Y0H" 6 A n D n T n a ys Abbreviations DCB LEFM OHP SEM SCC see d i s soc iat ion constant for water load apparent act ivat ion energy gas constant (8.314 kJ/mol.deg) reciprocal po lar izat ion resistance crack t i p p l a s t i c zone radius absolute temperature crack ve loc i ty volts with respect to the saturated calomel reference electrode equivalent weight of iron DCB specimen length a c t i v i t y coe f f i c i en t of OH d i f fu s ion layer thickness act ivat ion overpotential d i f fu s ion overpotential to ta l overpotential y i e l d stress double cant i lever beam l i nea r e l a s t i c fracture mechanics outer Helmholtz plane scanning electron microscope stress corrosion cracking saturated calomel reference electrode x v i i ACKNOWLEDGEMENT S p e c i a l t h a n k s go t o my s u p e r v i s o r , D r . D. T r o m a n s , f o r h i s g u i d a n c e and c o u n s e l t h r o u g h o u t t h e p r o j e c t . D. Crowe and L. F r e d e r i c k d e s e r v e c r e d i t f o r t h e h e l p t h e y g a v e , and f o r s h a r i n g an o f f i c e w i t h me. I a p p r e c i a t e t h e work done f o r me by R. M c L e o d , H. Tump, E. K l a s s e n , P. M u s i l and many o t h e r s i n t h e d e p a r t m e n t . I wou ld l i k e t o t h a n k a l l t h e p e o p l e i n t h e M e t a l l u r g y d e p a r t m e n t f o r s u p p o r t i n g me when I most needed i t , and f o r mak ing my s t a y h e r e bo th e n j o y a b l e and w o r t h w h i l e . F i n a n c i a l a s s i s t a n c e f r om t h e N a t i o n a l S c i e n c e and E n g i n e e r i n g R e s e a r c h C o u n c i l was g r a t e f u l l y a p p r e c i a t e d . I am g r a t e f u l t o H. G i l l i s f o r t h e h e l p , u n d e r -s t a n d i n g and e m o t i o n a l s u p p o r t she p r o v i d e d . x v i i i I t wou ld no t seem, t h e r e f o r e , t h a t we may a n t i c i p a t e f a i l u r e s o f t h i s t y p e o c c u r r i n g i n u n s u s p e c t e d p l a c e s , so t h e i n d u s t r i a l i m p l i c a t i o n s o f t h i s phenomenon do no t loom l a r g e . F a i l u r e s a r e r a r e and w i l l become more so as u n d e r s t a n d i n g s p r e a d s . . . . " * * D i s c u s s i o n o f p a p e r by Hodge , J . C . and M i l l e r , J . L . , T r a n s o f ASM, 2 8 , 2 5 , ( 1 9 4 0 ) . x i x C h a p t e r 1 INTRODUCTION 1 .1 S t r e s s C o r r o s i o n C r a c k i n g S t r e s s c o r r o s i o n c r a c k i n g (SCC) can be d e f i n e d as t h e p e n e t r a t i o n and c r a c k i n g o f a m e t a l unde r t h e c o n j o i n t a c t i o n o f a t e n s i l e s t r e s s and a c o r r o s i v e e n v i r o n m e n t a t a r a t e i n e x c e s s o f t h a t p r o d u c e d by e i t h e r f a c t o r a c t i n g s i n g l y . I n d u s t r i a l s u r v e y s ^ show t h a t SCC c o n s t i t u t e s a p p r o x i m a t e l y 33% o f a l l c o r r o s i o n r e l a t e d f a i l u r e s i n t h e c h e m i c a l i n d u s t r y . SCC i s p a r t i c u l a r l y d a n g e r o u s b e c a u s e i t i s n o t r e a d i l y d e t e c t a b l e and t h e f i r s t w a r n i n g o f a s t r e s s c o r r o s i o n p r o b l e m i s o f t e n t h e u n t i m e l y and c a t a s -t r o p h i c f a i l u r e o f a v i t a l p i e c e o f e q u i p m e n t . A s i d e f r om the p o s s i b i l i t y o f s e r i o u s i n j u r y to p e r s o n n e l i n t h e v i c i n i t y o f t h e f a i l u r e , t h e a t t e n d a n t l o s s o f p r o d u c t i o n and t h e c o s t o f r e p a i r s can be p r o h i b i t i v e l y e x p e n s i v e . S i n c e 1 9 4 0 , when t h e w r i t t e n d i s c u s s i o n o f a p a p e r on t h e SCC o f a u s t e n i t i c s t a i n l e s s s t e e l s i n c l u d e d t h e comment on 2 t h e f r o n t i s p i e c e o f t h i s t h e s i s , t h e s t u d y o f SCC has become a m a j o r c o n c e r n o f b o t h i n d u s t r i a l and a c a d e m i c r e l a t e d c o r r o s i o n r e s e a r c h . 1 2 T r a d i t i o n a l l y , SCC was a s s o c i a t e d w i t h a few s p e c i f i c m e t a l - e n v i r o n m e n t c o m b i n a t i o n s , such as a u s t e n i t i c s t a i n l e s s s t e e l s e x p o s e d t o c h l o r i d e s o l u t i o n s , b r a s s i n ammon i aca l s o l u t i o n s and m i l d s t e e l i n a l k a l i n e s o l u t i o n s . H o w e v e r , i t i s now r e c o g n i z e d t h a t SCC may o c c u r i n a w ide v a r i e t y o f m e t a l - e n v i r o n m e n t - c o m b i n a t i o n s , i n c l u d i n g e x p o s u r e t o e n v i r o n -3 ments as i n n o c u o u s as d i s t i l l e d w a t e r , and t h a t no s i n g l e , 4 u n i f y i n g t h e o r y can a c c o u n t f o r a l l o c c u r r e n c e s o f SCC. A l a r g e number o f v a r i a b l e s a f f e c t t h e SCC p r o c e s s i n a d d i t i o n t o t h e m e t a l a n d . s p e c i f i c e n v i r o n m e n t . T h e s e i n -c l u d e t h e c h e m i c a l c o m p o s i t i o n and m e t a l l u r g i c a l h i s t o r y o f t h e m e t a l , t h e c o n c e n t r a t i o n o f damag ing i o n ( s ) i n s o l u t i o n , t h e e l e c t r o c h e m i c a l p o t e n t i a l o f t h e m e t a l i n s o l u t i o n , t h e p r e s e n c e and n a t u r e o f p a s s i v e f i l m s on t h e m e t a l s u r f a c e , t e m p e r a t u r e , t h e n a t u r e and k i n e t i c s o f c o m p e t i n g a n o d i c and c a t h o d i c r e a c t i o n s on t h e m e t a l s u r f a c e and t h e e f f e c t s o f r e s i d u a l o r a p p l i e d s t r e s s e s on t he m e t a l . A f u r t h e r c o m p l i -c a t i o n i s t h a t t h e e n v i r o n m e n t a t t h e t i p o f a s t r e s s c o r -r o s i o n c r a c k may d i f f e r g r e a t l y f r o m t h e b u l k c o n d i t i o n s commonly u sed t o c h a r a c t e r i z e SCC. F u r t h e r m o r e , some o r a l l o f t h e s e v a r i a b l e s may i n t e r a c t s y n e r g i s t i c a l l y , as a l r e a d y i n d i c a t e d f o r t h e c o m b i n a t i o n o f t e n s i l e s t r e s s and c o r -r o s i v e e n v i r o n m e n t . 1. 2 SCC Techn iques In a cco rdance w i t h the m u l t i p l i c i t y of v a r i a b l e s wh ich a f f e c t SCC, many d i f f e r e n t t e c h n i q u e s have been used to s tudy SCC phenomena. These range from t r a d i t i o n a l e x p e r i -ments where the l i f e t i m e of s t r e s s e d samples p l a c e d i n c o r -r o s i v e env i r onment s i s measured, t o t r i b o e l 1 i p s o m e t r i c e x p e r i m e n t s , where p a s s i v e f i l m growth k i n e t i c s and c u r r e n t 5 d e n s i t y a re measured as a f u n c t i o n o f t i m e . Reviews of 6 7 some t e c h n i q u e s have been w r i t t e n by P o u r b a i x and P a r k i n s . S i n c e SCC can be a r e l a t i v e l y s l o w , t ime dependent phenomenon by l a b o r a t o r y s t a n d a r d s , most s t r e s s c o r r o s i o n t e s t i n g i s a c c o m p l i s h e d by i n c r e a s i n g the r e l a t i v e a g g r e s -s i v e n e s s of the t e s t env i ronment towards SCC w i t h r e s p e c t t o i n d u s t r i a l s i t u a t i o n s . Th i s can have two major e f f e c t s on the b e h a v i o r of the m e t a l . I t can dec rea se the i n i t i a t i o n t ime o f the s t r e s s c o r r o s i o n c r a c k s (which can be a s i g n i -f i c a n t f r a c t i o n o f the t o t a l t e s t t i m e ) . a n d / o r i t can i n -c r ea se the p r o p a g a t i o n r a t e o f the s t r e s s c o r r o s i o n c r a c k s once i n i t i a t i o n has o c c u r r e d . Some common methods used to i n c r e a s e the s e v e r i t y o f SCC t e s t s i n c l u d e i n c r e a s i n g the t e m p e r a t u r e , chang ing the s o l u t i o n c o n c e n t r a t i o n , c o n -t r o l l i n g the specimen p o t e n t i a l , and i n t r o d u c i n g a notch or p r e c r a c k i n t o the s pec imen . Two r e l a t i v e l y new t e c h n i q u e s of p a r t i c u l a r i n t e r e s t 4 t o t h i s s t u d y a r e t h e s l ow s t r a i n r a t e t e c h n i q u e (SSRT) and t h e f r a c t u r e m e c h a n i c s a p p r o a c h . Both have t h e a d v a n t a g e o f a c c e l e r a t i n g t h e s t r e s s c o r r o s i o n p r o c e s s w i t h o u t r e q u i r i n g t h a t a change be made i n t he t e s t e n v i r o n m e n t and t h e y compare w e l l w i t h t r a d i t i o n a l m e t h o d s . In a d d i t i o n , t h e y c o n t r i b u t e u n i q u e i n f o r m a t i o n t o t h e u n d e r s t a n d i n g o f SCC. The SSRT was f i r s t e x t e n s i v e l y u sed by P a r k i n s and o h i s c o w o r k e r s , and has s i n c e been f o u n d t o be a u s e f u l method o f a s s e s s i n g t h e SCC b e h a v i o r o f m e t a l s i n a number o f e n v i r o n m e n t s . T h e r e a r e two m a j o r a d v a n t a g e s t o t h e SSRT. The f i r s t i s t h a t a s l ow s t r a i n r a t e t e s t r a r e l y l a s t s l o n g e r t h a n 48 h o u r s , compared t o 60+ days f o r t h e l o n g e s t c o n s t a n t l o a d o r c o n s t a n t s t r a i n t e s t s . The s e c o n d i s t h a t a s l ow s t r a i n r a t e t e s t a l w a y s ends w i t h a p o s i t i v e r e s u l t . The s p e c i m e n f a i l s e i t h e r p r e m a t u r e l y i n a b r i t t l e f a s h i o n , i n d i c a t i n g t he o c c u r r e n c e o f SCC , o r by norma l d u c t i l e o v e r l o a d . The SSRT i s b a s e d on t h e p r e m i s e t h a t SCC i s d e p e n d e n t upon t h e l o c a l s t r a i n r a t e p r o d u c e d a t t h e t i p o f a c r a c k by t h e t e n s i l e s t r e s s , r a t h e r t h a n upon the t e n s i l e s t r e s s p e r s e . T h i s was d e m o n s t r a t e d by P a r k i n s , w i t h p r e c r a c k e d c a n t i l e v e r beam s p e c i m e n s . ^ He showed t h a t a t c o n s t a n t s t r e s s i n t e n s i t y , c r a c k s wou ld p r o p a g a t e o n l y i f t h e c o n d i t i o n s w h i c h c a u s e d c r a c k i n g were e s t a b l i s h e d b e f o r e c r e e p i n t h e p l a s t i c zone ahead o f t h e c r a c k t i p was 5 e x h a u s t e d . A t y p i c a l s l ow s t r a i n r a t e t e s t c o n s i s t s o f s u b -j e c t i n g a s t a n d a r d , w a i s t e d , t e n s i l e t e s t s p e c i m e n t o a v e r y s l o w , c o n s t a n t s t r a i n r a t e u n t i l f a i l u r e o c c u r s , w h i l e e x -p o s i n g i t to - an a p p r o p r i a t e e n v i r o n m e n t . S i n c e t h e o c c u r -r e n c e o f SCC w i l l a f f e c t t h e m e c h a n i c a l b e h a v i o r o f t h e s p e c i m e n , SCC s u s c e p t i b i l i t y i s b e s t c h a r a c t e r i z e d by p a r a -m e t e r s w h i c h a s s e s s t h e r e l a t i v e d u c t i l i t y o f t h e t e s t s p e c i m e n . T h i s i s i l l u s t r a t e d s c h e m a t i c a l l y by F i g u r e 1, w h i c h compares c o n s t a n t t e m p e r a t u r e s t r e s s - s t r a i n c u r v e s f o r a s p e c i m e n t e s t e d i n i n e r t and SCC s u s c e p t i b l e e n v i r o n -m e n t s . Some commonly u sed p a r a m e t e r s i n c l u d e % r e d u c t i o n i n a r e a , t i m e t o f a i l u r e , and % e l o n g a t i o n . V i s u a l c o n f i r m a -t i o n o f SCC , by o b s e r v a t i o n o f s e c o n d a r y c r a c k s on t h e s p e c i -men s u r f a c e , i s a l s o r e c o m m e n d e d . ^ S t u d i e s have shown t h a t t h e r e i s b o t h an u p p e r and 12 13 l o w e r l i m i t on t h e s t r a i n r a t e w h i c h w i l l p r o d u c e SCC. ' The u p p e r l i m i t o c c u r s as a r e s u l t o f i n s u f f i c i e n t c r a c k a d v a n c e w i t h r e s p e c t t o t h e amount o f s t r a i n o c c u r r i n g , w h i l e t h e l o w e r l i m i t i s t h o u g h t t o be due to a protect ive f i l m ; which e s t a b l i shes !; i t s e l f , o v e r t h e c r a c k t i p f a s t e r t h a n i t can be r u p t u r e d by t he s t r a i n . The opt imum s t r a i n r a t e f o r p r o d u c i n g SCC v a r i e s w i t h t he m e t a l , e n v i r o n m e n t , e l e c t r o -c h e m i c a l p o t e n t i a l , and t e m p e r a t u r e , and must be S C C 0 STRA IN ( 6 ) Figure 1 S t r e s s - s t r a i n curve f o r specimens t e s t e d i n SCC s u s c e p t i b l e and i n e r t environments using SSRT, where A and A „ are the o s c c areas under the r e s p e c t i v e curves. e x p e r i m e n t a l l y d e t e r m i n e d f o r each s y s t e m s t u d i e d . 7 The SSRT has p r o v e n u s e f u l f o r q u i c k l y a s s e s s i n g b o t h t h e s e v e r i t y o f c r a c k i n g and s u s c e p t i b i l i t y o f m e t a l s to SCC u n d e r a v a r i e t y o f c o n d i t i o n s . I t has been f o u n d t o be a more s e v e r e t e s t o f SCC t h a n t r a d i t i o n a l m e t h o d s , and has p r o d u c e d SCC i n m e t a l - e n v i r o n m e n t c o m b i n a t i o n s p r e -v i o u s l y t h o u g h t t o be immune t o SCC , o r where SCC was d i f -f i c u l t t o i n i t i a t e . 1 0 ' ^ 4 F o r t h i s r e a s o n , s l ow s t r a i n r a t e t e s t s may be c o n d u c t e d i n t h e same e n v i r o n m e n t s e n c o u n t e r e d i n d u s t r i a l l y r a t h e r t h a n i n t h e more a g g r e s s i v e e n v i r o n -ments n o r m a l l y u sed i n l a b o r a t o r y t e s t s . Q u a n t i t a t i v e d a t a , i n t h e f o r m o f s t r e s s c o r r o s i o n c r a c k v e l o c i t i e s have a l so 1 5 been e s t i m a t e d f^om s l ow s t r a i n r a t e t e s t s . The f r a c t u r e m e c h a n i c s a p p r o a c h t o SCC i s an e x t e n s i o n o f t h e l i n e a r e l a s t i c f r a c t u r e m e c h a n i c s (LEFM) a p p r o a c h u sed to e x p l a i n and q u a n t i f y b r i t t l e f a i l u r e o f h i g h s t r e n g t h l fi m a t e r i a l s . The use o f a f r a c t u r e m e c h a n i c s t e c h n i q u e t o s t u d y SCC makes i t p o s s i b l e t o f o l l o w t h e g rowth c h a r a c t e r -i s t i c s o f a s i n g l e , p l a n a r c r a c k as a f u n c t i o n o f t h e s t r e s s i n t e n s i t y a t t he t i p o f t h e c r a c k . F o r a t y p i c a l s t r e s s c o r r o s i o n c r a c k , t h e s t r e s s i n t e n s i t y a t t h e t i p can be c h a r a c t e r i z e d by K-, t h e s t r e s s i n t e n s i t y f a c t o r f o r mode I o p e n i n g . The m a g n i t u d e o f Kj i n c r e a s e s w i t h i n c r e a s i n g a p p l i e d s t r e s s and c r a c k l e n g t h . At s u f f i c i e n t l y h i g h 8 v a l u e s , i t r eaches K j ^ , the c r i t i c a l s t r e s s i n t e n s i t y a t which spon taneou s , u n s t a b l e f a s t f r a c t u r e o c c u r s . In most SCC f r a c t u r e mechan ics e x p e r i m e n t s , the c r a c k v e l o c i t y , v i i s measured as a f u n c t i o n o f K j . S i n c e the c r a c k v e l o c i t y commonly spans s e v e r a l decade s , da ta are p l o t t e d as l o g v v s . K j . The shape o f a t y p i c a l l o g v -Kj cu r ve i s shown i n F i g u r e 2. Three d i s t i n c t r e l a t i o n -s h i p s between l o g v and Kj a re a p p a r e n t . In r e g i o n I, a t low Kj v a l u e s , the c r a c k v e l o c i t y i s s t r e s s dependent . In r e g i o n I I , the v e l o c i t y reaches a p l a t e a u v a l ue which i s i ndependent o f changes to K j . F i n a l l y , at h igh Kj v a l u e s , r e g i o n I I I c r a c k growth o c c u r s , which i s once aga in s t r e s s dependent . The upper l i m i t o f the cu rve o c c u r s when Kj -•• K j ^ and the l ower l i m i t i s r eached when Kj = ^ ISCC ' t h r e s h o l d s t r e s s i n t e n s i t y below wh ich s u b - c r i t i c a l c r a c k p r o p a g a t i o n i s not d e t e c t a b l e . In p r a c t i c e , Kj^^.^ i s d i f f i c u l t to q u a n t i f y e x p e r i m e n t a l l y and i s u s u a l l y g i v e n some a r b i t r a r y d e s i g n a t i o n , such as the s t r e s s i n t e n s i t y below wh ich SCC i s not ob se r ved a f t e r a s e t p e r i o d . o f t ime or the s t r e s s i n t e n s i t y below which the c r a c k v e l o c i t y i s equa l to or s l owe r than the r a t e of d i s s o l u t i o n of the meta l by g e n e r a l c o r r o s i o n . The e x i s t e n c e of a Kj i ndependent r e g i o n i n a l o g v - Kj 9 i I Figure 2 Schematic of a t y p i c a l log v - Kj curve, showing regions I, II and I I I . p l o t makes i t p o s s i b l e t o s e p a r a t e s t r e s s dependent from s t r e s s i ndependent mechanisms. Th i s i s p a r t i c u l a r l y u se -f u l i n d e t e r m i n i n g the e f f e c t s of t e m p e r a t u r e , v i s c o s i t y , and s o l u t i o n c o m p o s i t i o n on the s t r e s s c o r r o s i o n c r a c k 21 22 v e l o c i t y . ' The n a t u r e o f the dependence o f SCC on the se v a r i a b l e s can l e a d to the e l u c i d a t i o n of a mechanism f o r SCC i n the m e t a l - e n v i r o n m e n t c o m b i n a t i o n under s t u d y . G e n e r a l l y , the complex i n t e r a c t i o n which o c cu r s between s t r e s s i n t e n s i t y and the env i ronment i n r e g i o n s I and I I I o f the l o g v - K- cu r ve make i t d i f f i c u l t to t r e a t the da ta m e c h a n i s t i c a l l y . Ano the r advantage of the f r a c t u r e mechan ics approach i s t h a t i t a l l o w s , a t l e a s t t h e o r e t i c a l l y , the s e p a r a t i o n o f the c r a c k i n i t i a t i o n s t age from t h a t o f c r a c k p r opaga -t i o n . T h i s i s e s p e c i a l l y u s e f u l i n k i n e t i c s t u d i e s . S e v e r a l d i f f e r e n t methods may be employed t o measure the r a t e o f c r a c k advance i n a f r a c t u r e mechan ics t e s t . ^ These i n c l u d e d i r e c t o p t i c a l o b s e r v a t i o n , c r a c k open ing d i s p l a c e m e n t measurements , e l e c t r i c a l r e s i s t a n c e measure-ments , u l t r a s o n i c m o n i t o r i n g and a c o u s t i c e m i s s i o n . Each method has i t s own p a r t i c u l a r advantages and d i s a d v a n t a g e s and the m e r i t s of one t e c h n i q u e over ano the r depend on the r e q u i r e m e n t s of s p e c i f i c e x p e r i m e n t s . The c h o i c e of specimen geometry i s i m p o r t a n t i n 11 f r a c t u r e mechan ics e x p e r i m e n t s . For a s p e c i f i c c r a c k geomet ry , Kj i s g i v e n by e q u a t i o n 1: Kj = |- Y (a) . . . . 1 Where P i s the a p p l i e d l o a d , B i s the spec imen t h i c k n e s s and Y (a) i s a f u n c t i o n of the c r a c k l e n g t h , a , and the l o a d i n g geometry . Perhaps the most c r i t i c a l d imens i on i s the spec imen t h i c k n e s s . I t has been shown t h a t f o r c o n s t a n t P/B r a t i o , Kj may o n l y be c o n s i d e r e d u s e f u l i f a c e r t a i n 1 o t h i c k n e s s i s exceeded . In ve ry t h i n s pec imens , a p l ane s t r e s s s t a t e e x i s t s a t the c r a c k t i p and a l a r g e p l a s t i c zone forms ahead o f the t i p . The s i z e of the p l a s t i c zone i s s t r o n g l y a f f e c t e d by specimen t h i c k n e s s ; as the spec imen t h i c k n e s s i s i n c r e a s e d , the s i z e of the p l a s t i c zone dec rea se s w i t h d i s t a n c e from the f r e e s u r f a c e . E v e n t u a l l y , a l i m i t i n g t h i c k n e s s i s reached where p l ane s t r a i n c o n d i t i o n s p redomina te a l ong the c r a c k f r o n t and p l ane s t r e s s e x i s t s o n l y a t the extreme edges of the spec imen . Under p l ane s t r a i n c o n d i t i o n s , the s i z e of the p l a s t i c zone i s c o n s t a n t and i ndependent of p o s i t i o n a l ong the c r a c k f r o n t . LEFM t e c h n i q u e s can s u p p l y v a l i d e n g i n e e r i n g da ta o n l y when p l ane s t r a i n can be app rox imated a c r o s s the c r a c k f r o n t . T h e r e f o r e , a minimum specimen t h i c k n e s s , 1 2 r e l a t e d t o K. and the y i e l d s t r e n g t h o f t h e m a t e r i a l , ~.ayS'.*o i s r e q u i r e d . E q u a t i o n 2 shows t h e t h i c k n e s s c r i t e r i o n u s u a l l y a d o p t e d , w h i c h i s t h a t o f Brown and S r a w l e y 1 3 and ASTM E 3 9 9 - 7 8 a . .2 The c o n s i d e r a t i o n o f p l a n e s t r a i n v s . p l a n e s t r e s s i n SCC f r a c t u r e m e c h a n i c s e x p e r i m e n t s i s i m p o r t a n t b e c a u s e i t has been f o u n d , t h a t u n d e r some c o n d i t i o n s , s t r e s s c o r r o s i o n c r a c k s w i l l p r o p a g a t e v e r y s l o w l y , i f a t a l l , t h r o u g h p l a n e s t r e s s p l a s t i c z o n e s . ^ F r a c t u r e m e c h a n i c s t e c h n i q u e s have been s u c c e s s f u l l y a p p l i e d t o t h e s t u d y o f SCC b e h a v i o r o f a number o f m a t e r i a l s . 21 22 T h e s e i n c l u d e a luminum a l l o y s , t i t a n i u m , h i g h s t r e n g t h 23 24 25 s t e e l s , b r a s s e s , and s t a i n l e s s s t e e l s . 1 .3 SCC o f M i l d S t e e l i n A l k a l i n e S o l u t i o n s The f i r s t r e p o r t e d f a i l u r e s by t h e SCC o f m i l d s t e e l i n h o t , a l k a l i n e e n v i r o n m e n t s were i n r i v e t e d b o i l e r s on 26 s team l o c o m o t i v e s n e a r t h e t u r n o f t h e c e n t u r y . F a i l u r e s o c c u r r e d i n t he v i c i n i t y o f t h e r i v e t s and were a s s o c i a t e d w i t h a w h i t i s h , " c a u s t i c " p r e c i p i t a t e d e p o s i t e d on t h e s u r f a c e o f t h e s t r e s s c o r r o s i o n c r a c k s . A l t h o u g h r e p l a c e -ment o f r i v e t e d b o i l e r s by w e l d e d ones h e l p e d t o e l i m i n a t e f a i l u r e , c a u s t i c c r a c k i n g o f m i l d s t e e l s c o n t i n u e d t o be a p r o b l e m i n many i n d u s t r i e s . A t l e a s t two s u r v e y s o f i n d u s t r i a l o c c u r r e n c e s o f SCC i n a l k a l i n e e n v i r o n m e n t s a p p e a r e d d u r i n g t h e 1 9 5 0 ' s . One was a s u r v e y o f s e v e r a l i n d u s t r i e s by t h e N a t i o n a l A s s o c i a t i o n o f C o r r o s i o n 2 7 E n g i n e e r s ( N A C E ) , and t h e o t h e r d e t a i l e d e x p e r i e n c e i n 2 8 t h e a l u m i n a p r o c e s s i n d u s t r y . A l a b o r a t o r y s t u d y o f 29 c a u s t i c c r a c k i n g was a l s o p u b l i s h e d a t t h i s t i m e . D e s p i t e a g r o w i n g a w a r e n e s s and u n d e r s t a n d i n g o f c a u s t i c c r a c k i n g , i t r e m a i n s a p r o b l e m and f a i l u r e s c o n t i n u e t o o c c u r . Some r e c e n t e x a m p l e s i n c l u d e f a i l u r e s i n p i p e s o f 30 an u n d e r g r o u n d ho t w a t e r h e a t i n g s y s t e m i n Denmark, a c a t h o d i c a l l y p r o t e c t e d n a t u r a l gas p i p e l i n e i n t h e U n i t e d 31 32 S t a t e s , a c l a r i f i e r r a k e i n a C a n a d i a n - pu lpm i11 and a 33 d i g e s t o r f a i l u r e i n an A labama p u l p m i l l . A l t h o u g h t h e r e s u l t s o f e a r l y work on t h e SCC o f m i l d s t e e l i n a l k a l i n e s o l u t i o n s were o f t e n c o n f u s i n g and c o n -t r a d i c t o r y , p r i m a r i l y due t o p o o r c o n t r o l and u n d e r s t a n d i n g o f e x p e r i m e n t a l v a r i a b l e s , a c l e a r p i c t u r e has begun t o 34 35 26 e m e r g e . P a r k i n s , ' R e i n o e h l and B e r r y , and C a r t e r and 36 H y a t t have a l l r e v i e w e d t h e s u b j e c t i n r e c e n t y e a r s . R e c e n t work shows t h a t bo th s o l u t i o n c o m p o s i t i o n and t e m -p e r a t u r e s t r o n g l y i n f l u e n c e t h e SCC o f m i l d s t e e l i n a l k a l i n e s o l u t i o n s . SCC has been r e p o r t e d i n s o l u t i o n s v a r y i n g i n c o n c e n t r a t i o n f r o m 4% t o 75% N a O H , 8 ' 2 7 ' 2 9 w i t h t h e most 2 6 s e v e r e c r a c k i n g o c c u r r i n g a t ~ 33% NaOH. No SCC has been r e p o r t e d o u t s i d e t h i s r a n g e . Most f a i l u r e s o c c u r a t e l e -v a t e d t e m p e r a t u r e s (> 7 0 ° C ) , w i t h t he minimum t e m p e r a t u r e r e q u i r e d t o c a u s e SCC d e c r e a s i n g w i t h i n c r e a s e d NaOH c o n -27 37 38 c e n t r a t i o n . ' ' The c a r b o n c o n t e n t o f t h e s t e e l a l s o p l a y s a m a j o r r o l e i n d e t e r m i n i n g SCC s u s c e p t i b i l i t y . Maximum s u s c e p t i b i l i t y t o SCC o c c u r s a t a c a r b o n c o n t e n t between 0 and 0 . 0 2 % , and d e c r e a s e s w i t h i n c r e a s e d c a r b o n 34 38 c o n c e n t r a t i o n i n t h e s t e e l . ' A c a r b o n c o n t e n t o f g r e a t e r t h a n 0 .20% a p p e a r s t o p r o v i d e a measure o f p r o -3 6 t e c t i o n f r o m SCC. L i t t l e i s known a b o u t t h e e f f e c t s o f o t h e r component s i n t h e s t e e l on t h e SCC b e h a v i o r o f m i l d 35 36 s t e e l i n a l k a l i n e s o l u t i o n s . ' I t has o f t e n p r o v e d d i f f i c u l t t o i n i t i a t e SCC i n a l k a l i n e s o l u t i o n s i n t he l a b o r a t o r y , u s i n g t r a d i t i o n a l m e t h o d s . ^ S low s t r a i n r a t e t e s t s have been u sed t o show t h a t SCC does no t i n i t i a t e a t s t r e s s e s be low t h e y i e l d s t r e n g t h .of t h e m a t e r i a l and t h a t a minimum s t r a i n r a t e i s 8 39 r e q u i r e d . ' E x a m i n a t i o n o f s t r e s s c o r r o s i o n c r a c k s p r o d u c e d by i n d u s t r i a l f a i l u r e s , SSRT e x p e r i m e n t s , and t r a d i t i o n a l methods has shown t h a t SCC o f m i l d s t e e l f o l l o w s a p r e d o m i n a n t l y i n t e r g r a n u l a r p a t h i n a l k a l i n e 8 27 37 s o l u t i o n s , ' ' a l t h o u g h c a s e s o f t r a n s g r a n u l a r c r a c k i n g have been o b s e r v e d . ' 1 5 S e v e r a l s t u d i e s have l i n k e d t h e o c c u r r e n c e o f SCC t o p o t e n t i a l s n e a r t h e a c t i v e - p a s s i v e t r a n s i t i o n o f m i l d s t e e l i n a l k a l i n e s o l u t i o n s , as d e f i n e d by a n o d i c p o l a r i z a t i o n 8 3 7 40 c u r v e s . ' ' T h u s , SCC can be c o r r e l a t e d w i t h t h e change i n s u r f a c e c o v e r a g e o f t h e m e t a l f r o m a f r e e l y c o r r o d i n g s u r f a c e ( a c t i v e s t a t e ) t o c o m p l e t e c o v e r a g e by a p r o t e c t i v e f i l m ( p a s s i v e s t a t e ) . R e i n o e h l and B e r r y have shown t h a t t h e f r e e l y c o r r o d i n g p o t e n t i a l ( F - c o r r ) o f m i l d s t e e l a t 93° C may r e s t a t e i t h e r a c t i v e o r p a s s i v e p o t e n t i a l s , d e p e n d i n g upon t h e NaOH c o n c e n t r a t i o n i n s o l u t i o n and t h e 2 6 p r e v i o u s e l e c t r o c h e m i c a l h i s t o r y o f t h e s p e c i m e n . The a b i l i t y o f s o l u t i o n a d d i t i o n s o r i m p u r i t i e s t o a f f e c t t h e SCC b e h a v i o r o f m i l d s t e e l i n a l k a l i n e s o l u t i o n s a p p e a r s t o depend p r i m a r i l y upon t h e i r e f f e c t on E c o r r a n d / o r a n o d i c p o l a r i z a t i o n c u r v e o f t h e m i l d s t e e l . The a d d i t i o n o f s m a l l amounts o f o x i d i z i n g a g e n t s such as 0 2, NaN03, and PbO t o NaOH s o l u t i o n s has t h e e f f e c t o f s h i f t i n g E c o r r o f t h e s t e e l i n t o t h e SCC s u s c e p t i b l e r e g i o n , o p c o A. w h i c h c a u s e s SCC t o o c c u r . ' ' The a d d i t i o n o f l a r g e r amounts o f t h e same s u b s t a n c e s , h o w e v e r , s h i f t s E c o r r i n t o t h e p a s s i v e r e g i o n and c o n f e r s p r o t e c t i o n f r om SCC. O t h e r s u b s t a n c e s have been shown to s h i f t t h e p o t e n t i a l o f t h e a c t i v e - p a s s i v e t r a n s i t i o n and t h u s change t h e p o t e n t i a l 2 fi 37 41 r e g i o n o f SCC s u s c e p t i b i l i t y . ' O n l y a few s u b s t a n c e s , such as q u e b r a c h o and v a l o n e a t a n n i n s , w h i c h a c t as SCC 16 i n h i b i t o r s , a p p e a r t o a f f e c t t h e SCC b e h a v i o r o f m i l d s t e e l i n a l k a l i n e s o l u t i o n s w i t h o u t c h a n g i n g t h e e l e c t r o -8 38 c h e m i c a l b e h a v i o r o f t h e m e t a l . ' The c o r r e l a t i o n o f SCC w i t h p a s s i v e f i l m b e h a v i o r has l e d t o s e v e r a l i n v e s t i g a t i o n s o f t h e p r o p e r t i e s o f t h e s e f i l m s . A l t h o u g h p a s s i v e f i l m s i n g e n e r a l a r e d i f -42 43 44 f i c u l t t o c h a r a c t e r i z e , c y c l i c v o l t a m m e t r y e x p e r i m e n t s ' have shown t h a t i n i t i a l f i l m f o r m a t i o n o c c u r s i n t h e a c t i v e r e g i o n as an a d s o r b e d l a y e r o f F e ( 0 H ) 2 - A t h i g h e r p o t e n t i a l s , f u r t h e r o x i d a t i o n o f t h e i r o n o c c u r s and t h e p a s s i v e f i l m becomes e i t h e r FeOOH o r FegO^. T h i s o c c u r s a t t h e a c t i v e -p a s s i v e t r a n s i t i o n . At s t i l l h i g h e r p o t e n t i a l s , t h e FeOOH 3Q o r Fe^O^ o x i d i z e s t o f o r m y-fe^O^. I t has been shown t h a t t h e f i l m s f o r m e d a t SCC s u s c e p t i b l e p o t e n t i a l s i n a l k a l i n e s o l u t i o n s a r e more b r i t t l e t h a n t h e y~^e2°3 f l l m s f o rmed a t h i g h e r p o t e n t i a l s a n d , t h e r e f o r e , a r e more l i k e l y to r u p t u r e 45 unde r t h e a p p l i c a t i o n o f a t e n s i l e s t r e s s . O t h e r 39 46 47 s t u d i e s ' ' have shown t h a t f i l m r u p t u r e a t SCC s u s c e p -t i b l e p o t e n t i a l s p r o d u c e s l a r g e c u r r e n t t r a n s i e n t s , a s s o c i a t e d w i t h t h e d i s s o l u t i o n o f i r o n f r o m t h e e x p o s e d s u r f a c e b e f o r e r e p a s s i v a t i o n can o c c u r . At o t h e r p o t e n t i a l s r u p t u r e o f t h e p a s s i v e f i l m c a u s e s l i t t l e o r no i n c r e a s e i n t h e a n o d i c c u r r e n t d e n s i t y . T h i s b e h a v i o r has been r e -l a t e d t o t h e t h e r m o d y n a m i c s t a b i l i t y o f a s o l u b l e i r o n s p e c i e s , HFeO^, w i t h i n t h e p o t e n t i a l r e g i o n where t h e s e id ai 4 6 , 4 7 39 c u r r e n t t r a n s i e n t s a r e p r o d u c e d and t h e r a t e o f r e p a s -s i v a t i o n o f t h e e x p o s e d m e t a l Most o f t h e mechan i sms w h i c h have been p r o p o s e d t o e x p l a i n c a u s t i c c r a c k i n g o f m i l d s t e e l a r e b a s e d upon e l e c t r o c h e m i c a l d i s s o l u t i o n o f t h e m e t a l a t t h e c r a c k t i p , w h i l e t h e s i d e s o f t h e c r a c k r e m a i n p r o t e c t e d by p a s s i v e f i l m s . T h i s i s c o n s i s t e n t w i t h t h e t h e r m o d y n a m i c s t a b i l i t y o f HFeO^ w i t h i n t h e r a n g e o f p o t e n t i a l s i n w h i c h SCC o c c u r s , and a l s o w i t h t h e c o r r e l a t i o n between SCC and t h e r e g i o n o f t h e a c t i v e - p a s s i v e t r a n s i t i o n . O p i n i o n has been d i v i d e d , h o w e v e r , as t o t h e method by w h i c h s t r e s s c o r r o s i o n c r a c k 46 p r o p a g a t i o n o c c u r s . Hoar and J o n e s f e l t t h a t SCC o c c u r s v i a s t r a i n - a s s i s t e d d i s s o l u t i o n , w i t h t h e r o l e o f s t r a i n b e i n g t o c r e a t e f r e s h s u r f a c e a t t h e c r a c k t i p and t o p r e -35 v e n t r e p a s s i v a t i o n o f t he e x p o s e d m e t a l . P a r k i n s a l s o s u p p o r t e d a s t r a i n a s s i s t e d , c o n t i n u o u s d i s s o l u t i o n 48 49 50 m e c h a n i s m . S c u l l y ' and S t a e h l e have b o t h s u p p o r t e d s l i p - d i s s o l u t i o n m o d e l s whereby f i l m r u p t u r e i s c a u s e d by t h e emergence o f s l i p s t e p s a t t h e t i p o f t h e c r a c k and t h e s u b s e q u e n t d i s s o l u t i o n o f e x p o s e d m e t a l i s c o n t r o l l e d by 51 r e p a s s i v a t i o n k i n e t i c s . V e r m i l y e a and D i e g l e p r o p o s e d t h a t f i l m r u p t u r e o c c u r s ^ w h e n a c r i t i c a l s t r a i n has b u i l t up i n t h e p a s s i v e f i l m a t t h e c r a c k t i p . D i s s o l u t i o n o c c u r s u n t i l r e p a s s i v a t i o n o f t h e s u r f a c e i s c o m p l e t e , a t 18 52 w h i c h p o i n t t h e c y c l e r e p e a t s i t s e l f . P e r d i u s e t a l* a r g u e d t h a t a n o d i c d i s s o l u t i o n a t t h e c r a c k t i p c a u s e s a d r o p i n pH o f t h e c r a c k t i p s o l u t i o n u n t i l t h e e v o l u t i o n o f h y d r o g e n i s t h e r m o d y n a m i c a l l y p o s s i b l e . T h i s h y d r o g e n t h e n e n t e r s t h e s t e e l and s u b s e q u e n t l y c a u s e s e m b r i t t l e -ment o f t h e m e t a l ahead o f t he c r a c k t i p . M a z i l l e and 3 8 U h l i g s u p p o r t e d a mechan i sm whereby damag ing a n i o n s a d s o r b o n t o t h e m e t a l a t t h e c r a c k t i p and weaken t h e s t r a i n e d m e t a l bonds enough t o a l l o w m e c h a n i c a l f a i l u r e t o o c c u r . O t h e r a u t h o r s have a t t e m p t e d t o model s p e c i f i c a s p e c t s o f t h e SCC o f m i l d s t e e l i n a l k a l i n e s o l u t i o n s , 53 b a s e d on a n o d i c d i s s o l u t i o n t h e o r i e s . T h u s , B i g n o l d , D o i g 54 55 and F l e w i t t and M e l v i l l e have a l l exam ined t h e e f f e c t s o f a change i n p o t e n t i a l down t h e c r a c k l e n g t h on S C C , and 56 Mogensen e t . a l . have i n v e s t i g a t e d t h e d i s s o l u t i o n and r e p a s s i v a t i o n k i n e t i c s o f o x i d e f o r m a t i o n a t v a r i o u s p o t e n t i a l s . None o f t h e p r o p o s e d mechan i sms s p e c i f y an i n t e r g r a n u l a r o r t r a n s g r a n u l a r c r a c k p a t h and e x p l a n a t i o n s have been d e v e l o p e d t o a c c o u n t f o r t h e o c c u r r e n c e o f b o t h modes o f p r o p a g a t i o n . 1.4 O r i g i n s o f t h e P r e s e n t Work S e v e r a l i n d u s t r i e s d e a l w i t h h i g h l y a l k a l i n e s o l u t i o n s on a r e g u l a r b a s i s . One o f t h e s e i s t h e p u l p and p a p e r 1 9 i n d u s t r y . In t h e K r a f t p r o c e s s , a s o l u t i o n known as w h i t e l i q u o r , p r i m a r i l y c o n t a i n i n g 2-3 M NaOH + 0.2 - 0 .6 M'-NagS, i s u sed t o d i s s o l v e t h e l i g n i n i n wood c h i p s , l e a v i n g c e l l u l o s e f i b r e s o r p u l p . The c h i p s and w h i t e l i q u o r come i n t o c o n t a c t w i t h each o t h e r i n l a r g e p r e s s u r e v e s s e l s c a l l e d d i g e s t o r s , a t 1 5 0 - 1 7 0 ° C and 6 8 7 - 7 5 0 k P a . When t h e d i g e s t i o n i s c o m p l e t e , t h e s p e n t l i q u o r , c o n t a i n -i n g l i g n i n and o t h e r s o l u b l e r e s i d u e f r om t h e wood c h i p s , as w e l l as o x i d i z e d s u l p h u r s p e c i e s , i s s e p a r a t e d f r o m t h e p u l p . The s p e n t , o r b l a c k l i q u o r , as i t i s now c a l l e d , i s t h e n s u b j e c t e d t o a s e r i e s o f o p e r a t i o n s d e s i g n e d t o r e -s t o r e t h e NaOH and Na^S c o n c e n t r a t i o n s t o t h e i r o r i g i n a l l e v e l and t o remove i m p u r i t i e s f r o m t h e s o l u t i o n . The f r e s h w h i t e l i q u o r , a t 9 2 ° C and a t m o s p h e r i c p r e s s u r e , i s c l a r i f i e d i n l a r g e s e t t l i n g t a n k s and s t o r e d u n t i l r e -t u r n e d t o t he d i g e s t o r s f o r f u r t h e r u s e . F i g u r e 3 i s a s c h e m a t i c d i a g r a m o f t h e K r a f t p r o c e s s and e m p h a s i z e s i t s 57 c y c l i c n a t u r e . B e s i d e s NaOH and Na^S, w h i t e l i q u o r a l s o c o n t a i n s a number o f i m p u r i t i e s , such as Na2$20.-, Na^SO^, Na^CO^ and NaCl , w h i c h c a n n o t be removed by t h e p r o c e s s . W h i t e l i q u o r c o n t a c t s m i l d s t e e l a t t h r e e p o i n t s i n t h e K r a f t p r o c e s s ; i n t h e d i g e s t o r s , and i n t h e w h i t e l i q u o r c l a r i f i e r s and s t o r a g e t a n k s . A l t h o u g h c a u s t i c c r a c k i n g o f m i l d s t e e l i s a m a j o r p r o b l e m i n t h e p u l p and 20 WATER CHIPS \ digestor WATER \ / pulp washer PULP / weak black liquor storage evaporator \ white liquor storage 1 strong black liquor storage MAKEUP CHEMICALS mud wasner white liquor c l a r i f i e r mud thickener causticizer \ green liquor c l a r i f i e r / dissolving tank molten chemical LIME STONE / lime k i l n dregs washer weak liquor storage Figure 3 Schematic of the K r a f t c y c l e . p a p e r i n d u s t r y , " " " v e r y l i t t l e r e s e a r c h has been done on s o l u t i o n s i n t h e t e m p e r a t u r e and c o n c e n t r a t i o n r ange o f i n t e r e s t t o t h e i n d u s t r y , o r on t h e e f f e c t o f s u l p h i d e i o n s on t h e c a u s t i c c r a c k i n g o f m i l d s t e e l . The p r e s e n t work was u n d e r t a k e n w i t h t h e a n t i c i p a t i o n o f b e g i n n i n g t o r e c t i f y t h i s s i t u a t i o n . The e n v i r o n m e n t s c h o s e n f o r s t u d y were s i m i l a r t o t h a t e n c o u n t e r e d i n w h i t e l i q u o r c l a r i f i e r s and s t o r a g e t a n k s . 41 P r e l i m i n a r y work by Tromans,, and c o n f i r m e d i n d e -p e n d e n t l y by Wens l ey and C h a r l t o n , 5 8 showed t h a t t h e a d -d i t i o n o f s u l p h i d e i o n s , a t t h e l e v e l f o u n d i n t h e K r a f t p r o c e s s , t o NaOH s o l u t i o n s r a i s e d t h e a c t i v e - p a s s i v e t r a n s i -t i o n p o t e n t i a l o f m i l d s t e e l by ~ 1 0 0 m V . I t was p o s t u l a t e d by Tromans t h a t t h e p o t e n t i a l a t w h i c h m i l d s t e e l was most s u s c e p t i b l e t o SCC wou ld s h i f t a c c o r d i n g l y . The g o a l s o f t h e p r e s e n t w o r k , t h e r e f o r e , were t o d i s c o v e r w h e t h e r t h i s s h i f t i n SCC s u s c e p t i b l e p o t e n t i a l s d i d o c c u r , and t o i n -v e s t i g a t e t h e r e l a t i v e s e v e r i t y o f SCC i n m i l d s t e e l e x -posed t o a s i m u l a t e d w h i t e l i q u o r c o n t a i n i n g NaOH and Na^S compared t o a s i m i l a r s t r e n g t h p l a i n NaOH s o l u t i o n . I t was hoped t h a t t h e r e s u l t s o f t h i s i n v e s t i g a t i o n wou ld p r o v i d e i ) i n s i g h t i n t o t h e mechan i sm o f c a u s t i c c r a c k i n g o f m i l d s t e e l and i i ) d a t a o f use t o t h e p u l p and p a p e r i n d u s t r y i n a v o i d i n g SCC p r o b l e m s a s s o c i a t e d w i t h t h e e x p o s u r e o f m i l d s t e e l t o w h i t e l i q u o r . C h a p t e r 2 EXPERIMENTAL 2.1 S cope o f t h e P r e s e n t Work D u r i n g t h e c o u r s e o f t h i s w o r k , t h r e e d i f f e r e n t s o l u t i o n c o m p o s i t i o n s and two t e s t methods were e m p l o y e d . A SSRT was a d o p t e d t o i n v e s t i g a t e t he e f f e c t o f p o t e n t i a l on t he SCC o f m i l d s t e e l i n s o l u t i o n s composed o f 3.35 m o l / k g NaOH and 2.5 m o l / k g NaOH + 0 .42 m o l / k g N a 2 S . T h i s t e c h n i q u e had a l r e a d y been used s u c c e s s f u l l y t o i n v e s t i -g a t e t h e SCC o f m i l d s t e e l i n more c o n c e n t r a t e d NaOH 8 1 1 3 9 s o l u t i o n s by o t h e r w o r k e r s . ' ' The p u r p o s e o f t h e s l ow s t r a i n r a t e e x p e r i m e n t s was t o e s t a b l i s h t h e c o n d i t i o n s o f maximum s u s c e p t i b i l i t y t o SCC t o be u sed f o r t h e s u b s e q u e n t f r a c t u r e m e c h a n i c s e x p e r i m e n t s . The m a t e r i a l u sed i n t h e s l ow s t r a i n r a t e e x p e r i m e n t s was a n n e a l e d b e f o r e use i n o r d e r t o e s t a b l i s h a known m e t a l l u r g i c a l h i s t o r y . Ba sed on t h e r e s u l t s o b t a i n e d f r om the s l ow s t r a i n r a t e e x p e r i m e n t s , a f r a c t u r e m e c h a n i c s s t u d y was i n i t i a t e d w h i c h i n v e s t i g a t e d t h e e f f e c t s o f s t r e s s i n t e n s i t y and p o t e n t i a l on t he s t r e s s c o r r o s i o n c r a c k v e l o c i t y o f m i l d s t e e l i n t h e two s o l u t i o n s . To t h e b e s t o f t h e a u t h o r ' s 22 k n o w l e d g e , no p r e v i o u s f r a c t u r e m e c h a n i c s s t u d i e s had been made o f t h e SCC o f m i l d s t e e l i n any c o n c e n t r a t i o n o f NaOH s o l u t i o n . Fo r t h i s r e a s o n , p a r a l l e l f r a c t u r e m e c h a n i c s e x p e r i m e n t s were c o n d u c t e d i n a 12 .5 m o l / k g (33%) NaOH s o l u t i o n i n o r d e r t o compare t h e r e s u l t s o f t h e f r a c t u r e 8 26 37 38 m e c h a n i c s t e c h n i q u e w i t h t h o s e o f o t h e r t e c h n i q u e s . ' ' ' S i n c e p r e v i o u s w o r k ^ had i n d i c a t e d t h a t c a u s t i c c r a c k i n g o f m i l d s t e e l was d i f f i c u l t to o b t a i n i n t h e l a b o r a t o r y , t he m a t e r i a l u sed i n t h e s e e x p e r i m e n t s was t e s t e d i n t h e h i g h y i e l d s t r e n g t h , a s - r e c e i v e d , c o l d worked c o n d i t i o n t o e n s u r e t h a t p l a n e s t r a i n c o n d i t i o n s were met i f t h e m a t e r i a l p r o v e d t o u n d e r g o SCC o n l y a t h i g h s t r e s s i n t e n s i t i e s . De-t a i l e d f r a c t o g r a p h y was c o n d u c t e d on the c r a c k s u r f a c e s o f a l l s p e c i m e n s . 2 . 2 . M a t e r i a l s 2.2.1 S t e e l The s t e e l u sed i n t h i s work was n o m i n a l l y an AISI C -1018 m i l d s t e e l . T h r e e b a t c h e s o f s t e e l were u s e d . One b a t c h was r e c e i v e d as 9.5 mm d i a m e t e r c o l d drawn r o d , w h i l e t h e o t h e r b a t c h e s were r e c e i v e d as c o l d drawn b a r s t o c k . The b a r s were r e c e i v e d w i t h a s q u a r e c r o s s - s e c t i o n o f 2 5 . 4 mm x 25 .4 mm. The c h e m i c a l c o m p o s i t i o n and y i e l d s t r e s s o f e a c h o f t he b a t c h e s i s g i v e n i n T a b l e I. The y i e l d s t r e s s a t 9 2 ° C d i d not change s i g n i f i c a n t l y f r om t h a t g i v e n a t 2 0 ° C f o r t h e ba r s t o c k . T a b l e I Y i e l d S t r e n g t h and C h e m i c a l C o m p o s i t i o n o f S t e e l s B a t c h S i ze (mm) Y i e l d S t r e n g t h ' (MPa) E 1 e m e n t * * C Mn P S S i Ni Cr Mo Cu r o d 9.5 d i a 297 0 .17 0.65 0 .012 0.01 ND ND ND.: ND •0.01 b a r A 25.4 x 25 .4 631 0.16 0.72 0 . 0 0 5 / 0.02; 0 .23 0 .05 0 .15 ND 0 .37 b a r B 25.4 x 25 ,4 656 0 .20 0 .58 0 .014 0.02 0 .25 0 .04 0 .16 0 .04 0.12 * r o d a t 92° C, b a r s a t 2 0 ° C * * w e i g h t p e r c e n t a g e ND no t d e t e c t e d ro S low s t r a i n r a t e t e n s i l e t e s t s p e c i m e n s were m a c h i n e d f r o m t h e 9.5 mm d i a m e t e r r o d w i t h a r e d u c e d gage s e c t i o n o f 5 mm d i a m e t e r and '25.4mm l e n g t h . Each s p e c i m e n was 254 mm l o n g and had t h r e a d e d ends t o r e c e i v e g r i p s f o r t h e t e n s i l e t e s t i n g m a c h i n e . The gage s e c t i o n s were p o l i s h e d w i t h 320 g r i t p a p e r , d e g r e a s e d i n c h l o r o e t h a n e and c l e a n e d i n a 10 v o l % HC1 s o l u t i o n . A f t e r r i n s i n g w i t h w a t e r f o l -l owed by e t h a n o l , each s p e c i m e n was d r i e d and s e a l e d unde r vacuum i n a V y c o r c a p s u l e . The s p e c i m e n s were s u b s e q u e n t l y a n n e a l e d a t 9 2 0 ° C f o r 45 m i n u t e s , a i r c o o l e d and s t o r e d i n t h e i r c a p s u l e s u n t i l u s e d . D o u b l e c a n t i l e v e r beam (DCB) f r a c t u r e m e c h a n i c s s p e c i m e n s were m a c h i n e d f r o m t h e s q u a r e ba r a c c o r d i n g t o t he g e o m e t r y shown i n F i g u r e 4. Beam h e i g h t (H) was 11 .6 mm, s p e c i m e n l e n g t h f r o m t h e l o a d i n g l i n e (W) was 6 6 . 7 mm and s p e c i m e n t h i c k n e s s (B) was 25 .4 mm. The l e n g t h o f t h e i n i t i a l m a c h i n e d c r a c k f r o m t h e l o a d i n g l i n e was 2 5 . 4 mm and t h e beams were t h r e a d e d t o r e c e i v e g r i p s f o r t h e a p p l i c a t i o n o f l o a d ( P ) . The f a c e s p e r p e n d i c u l a r t o the c r a c k p l a n e were p o l i s h e d t o 600 g r i t , washed i n w a t e r , t h e n e t h a n o l and d r i e d . The s p e c i m e n s were s t o r e d i n a d e s i c c a t o r u n t i l u s e d . 1 g Brown and S r a w l e y have shown t h a t t h e Kj c a l i -b r a t i o n f o r t h e DCB g e o m e t r y i s g i v e n by e q u a t i o n 3. Figure 4 DCB specimen geometry. 27 Kj = P , a ( 3.45 + 2.415 H The c a l i b r a t i o n i s v a l i d f o r t h i s type o f specimen f o r a/W < 0.6 i f W/H > 5. 2.2.2 S o l u t i o n s A l l s o l u t i o n s were f r e s h l y p repa red p r i o r t o each t e s t . Reagent grade sodium h y d r o x i d e p e l l e t s s u p p l i e d by Amachem, and r eagen t grade h y d r a t e d sodium s u l p h i d e c r y s t a l s ( N a 2 S , 9 H 2 0 ) s u p p l i e d by Matheson, Coleman and B e l l , were used to p repa re the s o l u t i o n s . S i n g l y d i s t i l l e d water was used at a l l t imes and a l l s o l u t i o n s were p repa red on a mo la l (g s o l u t e / k g s o l v e n t ) b a s i s . The NaOH s o l u t i o n s were p repa red by add ing the r e q u i s i t e amount of NaOH to the d i s t i l l e d w a t e r . The s i m u l a t e d w h i t e l i q u o r s o l u t i o n s were p repa red by f i r s t b o i l i n g the d i s t i l l e d water and then pu r g i n g i t w i t h N 2 g a s , w h i l e c o o l i n g to room t e m p e r a t u r e . NaOH p e l l e t s were added under a N 2 atmosphere and were f o l l o w e d by the N a 2 S - 9 H 2 0 c r y s t a l s . The nominal s o l u t i o n c o m p o s i t i o n s were 12.5 mol/kg NaOH, 3.35 mol/kg NaOH and 2.5 mol/kg NaOH + 0.42 mol/kg Na 2 S . The t o t a l N a + m o l a l i t y of the s i m u l a t e d w h i t e l i q u o r and the c o r r e s p o n d i n g NaOH s o l u t i o n was 3.35 mol/kg N a + . 2.3 Equipment and Appa ra tu s Slow s t r a i n r a t e t e s t s were c a r r i e d out on a f l o o r 28 mounted I n s t r o n t e n s i l e t e s t i n g m a c h i n e c a p a b l e o f a c h i e v --4 i n g a minimum c r o s s - h e a d s p e e d o f 8 .47 x 10 mm/s. A s c h e m a t i c d i a g r a m o f t h e t e s t c e l l u sed i s shown i n F i g u r e 5. The c e l l was c o n s t r u c t e d e n t i r e l y o f FEP o r PTFE f l o o r o -c a r b o n p o l y m e r s and was f i t t e d w i t h a N 2 pu r ge l i n e , r e f l u x c o n d e n s e r , and L u g g i n c a p i l l a r y , a l l c o n s t r u c t e d o f t h e same m a t e r i a l s . A FEP c o a t e d t h e r m i s t o r p r o b e (YSI model 400) was i n s e r t e d i n t o t h e c e l l , and two s p e c t r o g r a p h ! ' c g r a d e g r a p h i t e c o u n t e r e l e c t r o d e s were p o s i t i o n e d a t o p -p o s i t e s i d e s o f t h e c e l l . The c e l l was h e a t e d by an e x t e r n a l l y wrapped e l e c t r i c a l r e s i s t a n c e h e a t i n g t a p e . T e m p e r a t u r e c o n t r o l o f + 1° C was m a i n t a i n e d by c o n n e c t i n g t h e h e a t i n g t a p e t h r o u g h a V a r i a c power t r a n s f o r m e r t o a YSI model 71 o r 71A t e m p e r a t u r e c o n t r o l l e r . An e x t e r n a l s a t u r a t e d c a l o m e l r e f e r e n c e e l e c t r o d e ( s e e ) , m a i n t a i n e d a t room t e m p e r a t u r e , was c o n n e c t e d to t h e c e l l v i a t h e s a t u r a t e d KC1 f i l l e d L u g g i n c a p i l l a r y . The L u g g i n c a p i l l a r y was p o s i t i o n e d ~ 1 mm f rom t h e gage s e c t i o n o f t h e s p e c i m e n . P o t e n t i a l c o n t r o l o f t h e t e s t s p e c i m e n was m a i n t a i n e d by a P r i n c e t o n A p p l i e d R e s e a r c h (PAR) model 173 p o t e n t i o s t a t . DCB s p e c i m e n s were f a t i g u e p r e c r a c k e d by a S o n n t a g model S F - l - U f a t i g u e t e s t i i n g m a c h i n e . F a t i g u e g r i p s were c o n s t r u c t e d o f h i g h s t r e n g t h t o o l s t e e l and were h e a t t r e a t e d t o p r o v i d e t o u g h n e s s . A u n i v e r s a l j o i n t i n one g r i p was p r o v i d e d i n l a t e r t e s t s t o e n s u r e even l o a d i n g 29 LUGGIN CAPILLARY N 2 PURGE PTFE CAP -H FEP BEAKER SPECIMEN COUNTER ELECTRODE J SOLUTION ^COUNTER ELECTRODE PTFE SEAL Figure 5 SSRT test c e l l geometry. 30 a c r o s s t h e c r a c k p l a n e . F r a c t u r e m e c h a n i c s t e s t s were c a r r i e d ou t i n a c e l l , c o n s t r u c t e d e n t i r e l y o f T e f l o n , s i m i l a r t o t h e one e m p l o y e d i n t he s l ow s t r a i n r a t e t e s t s . A s c h e m a t i c o f t h e c e l l i s shown i n F i g u r e 6. S t i r r i n g was p r o v i d e d by a T e f l o n c o a t e d m a g n e t i c s t i r r i n g b a r on t h e bo t tom o f t h e c e l l . The c e l l was h e a t e d by a 600 ml b e a k e r - t y p e h e a t i n g m a n t l e and t e m p e r a t u r e c o n t r o l was m a i n t a i n e d i n t h e same manner as f o r t h e s l ow s t r a i n r a t e c e l l . T e m p e r a t u r e was m o n i t o r e d by a T e f l o n c o a t e d t h e r m o m e t e r . Two s p e c t r o g r a p h i c g r a d e g r a p h i t e c o u n t e r e l e c t r o d e s were mounted i n t h e c r a c k p l a n e on o p p o s i t e s i d e s o f t h e s p e c i m e n . A L u g g i n c a p i l l a r y , f i l l e d w i t h an a g a r / s a t u r a t e d KC1 g e l and c o n n e c t e d t o an e x t e r n a l SCE a t room t e m p e r a t u r e , was s i t u a t e d ~ 1 mm f r o m t h e s p e c i m e n s u r f a c e and j u s t be low t h e l e a d i n g edge o f t h e f a t i g u e p r e c r a c k . A c o t t o n t h r e a d r a n t he l e n g t h o f t h e L u g g i n c a p i l l a r y i n o r d e r t o m a i n t a i n s t a b l e p o t e n -t i a l c o n t r o l o v e r l o n g t i m e p e r i o d s . The p o t e n t i a l was c o n t r o l l e d by Wenk ing mode l s 68FR0 .5 and 68TS10 o r ECO model 541 p o t e n t i o s t a t s . The c e l l a s s e m b l y was mounted i n a h o r i z o n t a l H o u n s f i e l d o r Monsanto t e n s i l e t e s t i n g m a c h i n e , w h i c h was u s e d t o a p p l y a c o n s t a n t l o a d to t h e s p e c i m e n . I n d e p e n d e n t a n o d i c p o l a r i z a t i o n c u r v e s were g e n e r a t e d by t h e PAR model 173 p o t e n t i o s t a t , e q u i p p e d w i t h a PAR model LUGGIN C A P I L L A R Y ' P T F E C A P P — 3 ' - ' y * f f f S < F E P B E A K E R N 2 P U R G E R E F L U X > ~ — ' > 7 7 .r — P o SPECIMEN SOLUTION ure 6 Fracture mechanics test c e l l geometry. 376 l o g a r i t h m i c c u r r e n t c o n v e r t e r and model 178 e l e c t r o -m e t e r p r o b e and c o n n e c t e d t o a PAR model 175 u n i v e r s a l p rogrammer and a H o u s t o n O m n i g r a p h i c model 2000 X-Y r e -c o r d e r . F r a c t o g r a p h y was e x a m i n e d on an ETEC A u t o s c a n s c a n n i n g e l e c t r o n m i c r o s c o p e (SEM) u s i n g s e c o n d a r y e l e c t r o n i m a g i n g mode and 20 KeV e x c i t a t i o n . C h l o r i d e c o n t a m i n a t i o n f r o m the L u g g i n c a p i l l a r y , and s u l p h i d e i o n c o n c e n t r a t i o n were m e a s u r e d w i t h a R a d i o m e t e r model RTS 822 a u t o m a t i c + 2 -t i t r a t i o n s y s t e m e q u i p p e d w i t h an O r i o n Ag /S s o l i d membrane s p e c i f i c i o n e l e c t r o d e and d o u b l e j u n c t i o n r e f e r -ence e l e c t r o d e . 2 . 4 E x p e r i m e n t a l P r o c e d u r e s 2.4-1 SSRT E x p e r i m e n t s A t e n s i l e t e s t s p e c i m e n was removed f r o m i t s c a p s u l e b e f o r e each t e s t , and e l e c t r o c h e m i c a l l y p o l i s h e d f o r 60 s a t 30 V i n a s o l u t i o n c o n t a i n i n g 1 2.5 g c h r o m i c t r i o x i d e , 135 ml a c e t i c a c i d and 7 ml w a t e r . A f t e r r i n s i n g w i t h w a t e r , t h e n e t h a n o l , and d r y i n g , t he gage d i a m e t e r was m e a s u r e d w i t h an o p t i c a l t r a v e l l i n g m i c r o s c o p e . A l l a r e a s o f t h e s p e c i m e n e x p o s e d t o t h e s o l u t i o n , e x c e p t f o r t h e gage s e c t i o n , were wrapped w i t h T e f l o n t a p e t o r e -duce c u r r e n t r e q u i r e m e n t s and m i n i m i z e c h a n g e s i n t h e s o l u t i o n c o m p o s i t i o n d u r i n g a t e s t . The s p e c i m e n and t e s t c e l l were t h e n a s s e m b l e d and a t t a c h e d t o t h e I n s t r o n . The c e l l was f l u s h e d w i t h N 9 gas b e f o r e a d d i n g ~ 500 mls o f t e s t s o l u t i o n to the c e l l under a N 2 a tmosphere . A sma l l t e n s i l e l o a d was a p p l i e d to the specimen w h i l e the c e l l s o l u t i o n was brought to the t e s t t empe ra tu re of 92° C. The specimen p o t e n t i a l was m o n i t o r e d , but not c o n t r o l l e d , d u r i n g t h i s t i m e . When the t e s t t empe ra tu re was r e a c h e d , and the rma l e q u i l i b r i u m o f the appa ra tu s a c h i e v e d , a p o t e n t i a l o f -1 .25 V s c e w a s a P P l 1 e d t 0 the spec imen . A f t e r 120 s . , an anod i c p o l a r i z a t i o n scan a t 1 mV/s was i n i t i a t e d u n t i l the d e s i r e d — 6 t e s t p o t e n t i a l was r e a c h e d . A s t r a i n r a t e o f 3.3 x 10 /s was then a p p l i e d to the spec imen u n t i l f a i l u r e o c c u r r e d . T y p i c a l l y , t h i s was 18-20 hours a f t e r the s t a r t of the t e s t . Temperature and p o t e n t i a l were m a i n t a i n e d c o n s t a n t t h r o u g h -out the t e s t , and N 2 gas was bubb led c o n t i n u o u s l y th rough the c e l l to p r o v i d e a s t i r r i n g a c t i o n and p r e v e n t o x i d a t i o n of s u l p h i d e i o n s . At the c o n c l u s i o n o f a t e s t , the two h a l v e s o f the t e s t specimen were removed from the c e l l , c a r e f u l l y washed w i t h w a t e r , r i n s e d i n e t h a n o l , and d r i e d . The r e d u c t i o n i n c r o s s - s e c t i o n a l a rea was measured, and the f a i l e d ends were examined w i t h the SEM f o r s i g n s of secondary c r a c k i n g on the s u r f a c e near the f r a c t u r e t i p . The gage s e c t i o n s of a few specimen h a l v e s were s e c t i o n e d l o n g i t u d i n a l l y , and the f a c e s p o l i s h e d to a 1 ym diamond f i n i s h . The p o l i s h e d f a c e s were ' e t c h e d w i t h a 2 v o l % HNO^ i n e t h a n o l s o l u t i o n to r e v e a l f e r r i t e and p e a r l i t e g r a i n b o u n d a r i e s , 34 r and t h e c r a c k p a t h s o f s e c o n d a r y c r a c k s were e x a m i n e d by SEM. 2 . 4 . 2 F r a c t u r e M e c h a n i c s E x p e r i m e n t s T h r o u g h o u t t h e w o r k , t h r e e d i f f e r e n t p r o c e d u r e s were u sed to f a t i g u e p r e c r a c k t h e s p e c i m e n s . A l l o f t h e s p e c i m e n s m a c h i n e d f r om b a r A were f a t i g u e c r a c k e d a t a Kj l e v e l o f 12 + 11 MPa*^. A l l o f t h e s p e c i m e n s m a c h i n e d f r o m b a r B and t e s t e d i n 3.35 m o l / k g NaOH were f a t i g u e c r a c k e d a t a K- l e v e l o f 7 +_ 5 MPa/m. In t he above c a s e s , t h e f a t i g u e c r a c k was e x t e n d e d 2-4 mm f rom t h e i n i t i a l m a c h i n e d c r a c k . The r e m a i n d e r o f t h e s p e c i m e n s m a c h i n e d f r om b a r B, i n c l u d i n g t h o s e t e s t e d i n 2.5 m o l / k g NaOH + 0 . 4 2 m o l / k g Na^S and 12 .5 m o l / k g NaOH a t - 1 . 0 0 V s c e , were f a t i g u e c r a c k e d a t two l e v e l s . A f a t i g u e c r a c k was i n i t i a t e d and e x t e n d e d ~ 2 mm a t a K- l e v e l o f 12 + 11 MPa/m b e f o r e t h e f a t i g u e l o a d was d r o p p e d t o 7 + 5 MPa/m and t h e f a t i g u e c r a c k e x t e n d e d a f u r t h e r ~ 1 mm. In most c a s e s , t he maximum f a t i g u e s t r e s s i n t e n s i t y was l o w e r t h a n the t e s t s t r e s s i n t e n s i t y . The c o r r e l a t i o n between s t e e l b a t c h , f a t i g u e p r e c r a c k s t r e s s i n t e n s i t y and s o l u t i o n c o m p o s i t i o n i s shown i n T a b l e I I . P r i o r to a s s e m b l y i n t h e c e l l , t he s p e c i m e n s were wrapped i n T e f l o n t a p e so t h a t o n l y a s m a l l a r e a a r o u n d t h e c r a c k t i p was l e f t e x p o s e d t o the s o l u t i o n . As f o r T a b l e II C o r r e l a t i o n Between E n v i r o n m e n t , S t e e l B a t c h and F a t i g u e S t r e s s I n t e n s i t y . Env i ronment B a t c h F a t i g u e S t r e s s I n t e n s i t y (MPa/m) 12 .5 m o l / k g NaOH, E c Q r r A 12 + 11 " • - 1 ' 0 0 V s c e B 12 + 11 7 + 5 3.35 m o l / k g NaOH, - 1 . 0 0 V s c e B 7 + 5 2.5 m o l / k g NaOH + 0.42 m o l / k g N a 2 S , - 0 . 8 8 V see B 12 + 11 7 + 5 co cn 36 t h e s l ow s t r a i n r a t e t e s t s , t h i s was done t o m i n i m i z e t h e t o t a l c u r r e n t r e q u i r e m e n t s and c h a n g e s i n s o l u t i o n com-p o s i t i o n d u r i n g p o t e n t i o s t a t i c c o n t r o l . The s p e c i m e n and t e s t c e l l were a s s e m b l e d and mounted i n t h e t e n s o m e t e r f r a m e . F i r s t , t h e c e l l was f l u s h e d w i t h N 2 g a s , i f t h e s o l u t i o n t o be added c o n t a i n e d s u l p h i d e i o n s . O t h e r w i s e ~ 550 mis o f s o l u t i o n were p o u r e d i n t o t h e c e l l and b r o u g h t t o t e m p e r a t u r e . The s p e c i m e n p o t e n t i a l was m o n i t o r e d , but n o t c o n t r o l l e d , d u r i n g t h i s t i m e . When t h e t e s t t e m p e r a t u r e was r e a c h e d ( 9 2 ° C u n l e s s o t h e r w i s e n o t e d ) , t h e p o t e n t i a l o f most o f t h e s p e c i m e n s t e s t e d u n d e r f r e e l y c o r r o d i n g c o n -d i t i o n s was m e a s u r e d . O c c a s i o n a l l y , E c o r r w a s f o u n d t o r e s i d e i n t h e p a s s i v e r e g i o n . In t h e s e c a s e s , a p o t e n t i a l o f - 1 . 2 5 V $ c e was i m p r e s s e d on t h e s p e c i m e n f o r a few m i n u t e s , w h i c h s e r v e d t o r e t u r n E t o t he a c t i v e r e g i o n . A f t e r c o r r a l l o w i n g E . c o r r t o s t a b i l i z e , t h e L u g g i n c a p i l l a r y was r e -moved f r om the e e l 1 . S p e c i m e n s t e s t e d u n d e r p o t e n t i o s t a t i c c o n t r o l were p o l a r i z e d a t - 1 . 2 5 V f o r a few m i n u t e s , r see t h e n an a n o d i c p o t e n t i a l s c a n was i n i t i a t e d , e i t h e r m a n u a l l y o r a u t o m a t i c a l l y , a t 1 mV/s u n t i l t h e d e s i r e d t e s t p o t e n t i a l was r e a c h e d . A l o a d , p r e c a l c u l a t e d to g i v e t h e d e s i r e d s t r e s s i n t e n s i t y a t t h e t i p o f t h e c r a c k , was t h e n a p p l i e d t o t h e s p e c i m e n . D u r i n g t e s t s w i t h o u t p o t e n t i a l c o n t r o l , t h e L u g g i n c a p i l l a r y was o c c a s i o n a l l y r e i n s e r t e d i n t o t h e c e l l and E o f t h e s p e c i m e n m e a s u r e d . The c u r r e n t p a s s e d by t h e 37 p o t e n t i o s t a t t o c o n t r o l t h e s p e c i m e n p o t e n t i a l was r e c o r d e d p e r i o d i c a l l y f o r t h o s e s p e c i m e n s u n d e r p o t e n t i o s t a t i c c o n t r o l . The a p p l i e d l o a d and s o l u t i o n t e m p e r a t u r e were m o n i t o r e d d a i l y t h r o u g h o u t t h e c o u r s e o f a t e s t . N 2 gas was b u b b l e d . t h r o u g h s o l u t i o n s c o n t a i n i n g t h e s u l p h i d e i o n s t o p r e v e n t o x i d a t i o n o f s u l p h i d e , and a l i q u o t s were r e -moved e v e r y 48 h r s t o c h e c k t h e s u l p h i d e c o n c e n t r a t i o n . On o c c a s i o n , N a 2 S - 9 H 2 0 c r y s t a l s were added t o t h e s o l u t i o n i n t h e c e l l t o m a i n t a i n t h e c o r r e c t s u l p h i d e c o n c e n t r a t i o n . The c h l o r i d e c o n c e n t r a t i o n o f a number o f p l a i n NaOH t e s t s o l u t i o n s was m o n i t o r e d p e r i o d i c a l l y t o c h e c k f o r c o n t a m i n a t i o n f r om t h e L u g g i n c a p i l l a r i e s . A few mis o f d i s t i l l e d w a t e r were added t o t h e c e l l e v e r y few days t o make up f o r e v a p o r a t i o n l o s s e s . As i t p r o v e d i m p o s s i b l e to m o n i t o r t h e c r a c k g rowth d u r i n g t h e c o u r s e o f a t e s t , a l l e x p e r i m e n t s were c o n t i n u e d u n i n t e r r u p t e d f o r 200 - 400 h r s . They were t e r m i n a t e d when i t was e s t i m a t e d t h a t t h e c r a c k had e x t e n d e d 3-4 mm. At t h e c o n c l u s i o n o f an e x p e r i m e n t , t h e t e s t s o l u t i o n was l o w e r e d f r o m t h e s p e c i m e n and r e p l a c e d w i t h l i q u i d N,,. An a l i q u o t o f t e s t s o l u t i o n was removed t o measu re t h e f i n a l c h l o r i d e o r s u l p h i d e c o n c e n t r a t i o n . When the s p e c i m e n had c o o l e d s u f f i c i e n t l y i n t h e l i q u i d N 2 , i t was o v e r l o a d e d t o f a i l u r e and removed f r om the t e n s o m e t e r . The pH o f t h e f r o z e n l i q u i d n e a r t h e c r a c k t i p was m e a s u r e d as i t m e l t e d , u s i n g H y d r i o n n a r r o w r a n g e pH p a p e r . Both h a l v e s o f t h e s p e c i m e n were t h e n washed i n w a t e r , r i n s e d w i t h e t h a n o l and d r i e d . The i n i t i a l c r a c k l e n g t h f r om t h e l o a d i n g l i n e o f t h e s p e c i m e n t o t h e v i s i b l e l i m i t o f f a t i g u e c r a c k g r o w t h , and t h e i n c r e m e n t o f s t r e s s c o r r o s i o n c r a c k g rowth f r o m t h e v i s i b l e l i m i t o f f a t i g u e g rowth t o t he f a r t h e s t v i s i b l e e x t e n s i o n o f t h e s t r e s s c o r r o s i o n c r a c k were a c c u r a t e l y m e a s u r e d f o r e a c h s p e c i m e n h a l f w i t h an o p t i c a l t r a v e l l i n g m i c r o s c o p e . S p e c i m e n s were s t o r e d i n a d e s i c c a t o r u n t i l r e q u i r e d f o r f u r t h e r a n a l y s i s . The f r a c t o g r a p h y o f t h e c r a c k s u r f a c e was f i r s t e x a m i n e d , u s i n g t h e SEM, w i t h t h e c o r r o s i o n f i l m i n p l a c e . The c o r r o s i o n f i l m was t h e n removed by i m m e r s i n g t h e f r a c t u r e d s p e c i m e n i n t o an i n h i b i t e d c l e a n i n g s o l u t i o n com-po sed o f 4 mis 35% 2 b u t y n e - 1 , 4 - d i o l '+ 3 mis HC! + 5 0 : m l s : d i s t i l l e d w a t e r , and s u s p e n d i n g i t i n an u l t r a s o n i c c l e a n e r f o r 2 0 - 3 0 m i n s . The s p e c i m e n was t h e n washed w i t h w a t e r , r i n s e d i n e t h a n o l , d r i e d , and t h e f r a c t o g r a p h y o f t h e c l e a n e d c r a c k s u r f a c e e x a m i n e d w i t h t h e SEM. The c r a c k p a t h s o f s e c o n d a r y c r a c k s were e x a m i n e d i n a few s p e c i m e n h a l v e s by s e c t i o n i n g them l o n g i t u d i n a l l y a t t h e p o i n t o f f a r t h e s t e x t e n s i o n o f t h e s t r e s s c o r r o s i o n c r a c k and p o l i s h i n g t h e s i d e o f t h e s e c t i o n e d p i e c e t o a 1 ym d iamond f i n i s h . The f e r r i t e and p e a r l i t e g r a i n b o u n d a r i e s were r e v e a l e d by e t c h i n g t h e p o l i s h e d f a c e w i t h a 2 v o l % HNO^ i n e t h a n o l s o l u t i o n . The e t c h e d s u r f a c e s were exam ined w i t h t h e SEM. A few s p e c i m e n s were removed f r om t h e t e n s o m e t e r b e f o r e b e i n g b r o k e n open i n l i q u i d N,,, r i n s e d w e l l w i t h w a t e r , and s e c t i o n e d l o n g i t u d i n a l l y t o r e v e a l t h e c r a c k . The f a c e o f t h e s e c t i o n e d p i e c e was p o l i s h e d t o a 1 ym d iamond f i n i s h and e t c h e d as a l r e a d y d e s c r i b e d . The c r a c k p a t h was t h e n e x a m i n e d w i t h t h e SEM. The s p e c i m e n h a l v e s were n e x t s e p a r a t e d by o v e r l o a d i n g t h e s p e c i m e n i n l i q u i d N 2 , and t h e f r a c t o g r a p h y o f t h e c r a c k s u r f a c e was e x a m i n e d as d e s c r i b e d b e f o r e . 2 . 4 . 3 A n o d i c P o l a r i z a t i o n C u r v e s and L i n e a r  P o l a r i z a t i o n E x p e r i m e n t s A n o d i c p o l a r i z a t i o n c u r v e s o f m i l d s t e e l i n a l l t h r e e s o l u t i o n s were run p o t e n t i o d y n a m i c a l l y u s i n g t h e PAR p o t e n t i o s t a t and a s s o c i a t e d e q u i p m e n t . S p e c i m e n s were m a c h i n e d f r o m t h e s q u a r e b a r s t o c k t o d i m e n s i o n s o f ~ 10 mm x 10 mm x 10 mm and t a p p e d t o a c c e p t a s t a n d a r d PAR s p e c i m e n h o l d e r . The e x p o s e d f a c e s were p o l i s h e d t o 600 g r i t , washed w i t h w a t e r , r i n s e d i n e t h a n o l and d r i e d b e f o r e e a c h p o l a r i z a t i o n s c a n . A T e f l o n f r a c t u r e m e c h a n i c s c e l l ( F i g u r e 6) was m o d i f i e d t o a c c e p t t h e PAR s p e c i m e n h o l d e r and s p e c i m e n . The n i t r o g e n p u r g e d t e s t s o l u t i o n was added t o t h e c e l l and b r o u g h t t o t e m p e r a t u r e b e f o r e t h e s p e c i m e n was a d m i t t e d t o t h e c e l l . The s p e c i m e n was p o l a r i z e d a t - 1.25 V f o r 120 s b e f o r e an a n o d i c s c a n a t 1 mV/s see was i n i t i a t e d . N,, gas was c o n t i n u o u s l y b u b b l e d t h r o u g h t h e c e l l , and the s o l u t i o n s t i r r e d by a T e f l o n c o a t e d m a g n e t i c s t i r r e r on t h e b o t t o m o f t h e c e l l . In t h e 12 .5 m o l / k g NaOH s o l u t i o n , s u c c e s s i v e p o l a r i z a t i o n s c a n s were run a t d i f f e r e n t t e m p e r a t u r e s i n t h e same s o l u t i o n . The s p e c i m e n was removed f r o m t h e c e l l and r e p o l i s h e d between t e s t s . C o m p a r a t i v e t e s t s were made on t h e SSRT t e n s i l e s p e c i m e n s i n 3.35 m o l / k g NaOH and 2.5 m o l / k g NaOH + 0 .42 m o l / k g N a 2 S . The c o r r o s i o n c e l l and p r o c e d u r e p r e v i o u s l y d e s c r i b e d f o r t h e s l ow s t r a i n r a t e e x p e r i m e n t s was u s e d , and t h e e x p o s e d a r e a o f t h e gage s e c t i o n was t a k e n as t h e s p e c i m e n s u r f a c e a r e a . No a t t e m p t was made t o c o r r e c t f o r the s m a l l p o t e n t i a l d i f f e r e n c e s a r i s i n g f r om c o n c e n t r a -t i o n and t h e r m a l g r a d i e n t s i n t h e s a l t b r i d g e . L i n e a r p o l a r i z a t i o n e x p e r i m e n t s were c o n d u c t e d u s i n g t he s p e c i m e n s and a p p a r a t u s d e s c r i b e d above f o r t he d e t e r -m i n a t i o n o f t h e a n o d i c p o l a r i z a t i o n c u r v e s . The s p e c i m e n s were c a t h o d i c a l l y p o l a r i z e d a t - 1 . 4 0 V s c e f o r ~ 15 mins i n o r d e r t o remove any f i l m s f r o m t h e s u r f a c e . They were t h e n a l l o w e d t o r e t u r n t o E . A f t e r E had s t a b i l i z e d , c o r r c o r r t h e s p e c i m e n s were p o l a r i z e d a t 3 mV and 6 mV, r e s p e c t i v e l y , i n b o t h t h e c a t h o d i c and a n o d i c d i r e c t i o n s , and t h e c u r r e n t a t each p o t e n t i a l was m e a s u r e d . 2 . 4 . 4 S u l p h i d e and C h l o r i d e A n a l y s i s S u l p h i d e a n a l y s i s was c a r r i e d o u t a c c o r d i n g t o 59 a p r o c e d u r e s i m i l a r to t h a t d e v e l o p e d by Papp and u sed by R a u d s e p p , 6 0 w i t h t h e e x c e p t i o n t h a t A g N 0 3 was u sed as a t i t r a n t i n s t e a d o f H g ( N 0 3 ) 2 . A l i q u o t s o f 5 mis were w i t h -drawn f rom t h e s u l p h i d e c o n t a i n i n g t e s t s o l u t i o n and i m -m e d i a t e l y d i l u t e d t o 100 mis v o l u m e t r i c a l l y . A 10 ml a l i q u o t o f t h i s s o l u t i o n was p i p e t t e d i n t o ~ 40 mis o f d e i o n i z e d w a t e r c o n t a i n i n g ~ 2 mis o f 10 N NaOH. The p r e -p a r e d s a m p l e s were t h e n t i t r a t e d i n t r i p l i c a t e w i t h 0 .100 N AgNOg t o t h e s u l p h i d e e n d p o i n t by t h e a u t o m a t i c t i t r a t o r . The p r o c e d u r e f o r t h e d e t e r m i n a t i o n o f c h l o r i d e i o n s i n s o l u t i o n was s i m i l a r t o t h a t g i v e n a b o v e . A l i q u o t s o f 5 mis were w i t h d r a w n f r o m t h e t e s t s o l u t i o n and d i l u t e d to 100 mis v o l u m e t r i c a l l y . A 10 ml a l i q u o t o f t h i s s o l u t i o n was t h e n p i p e t t e d i n t o ~ 40 mis o f d e i o n i z e d w a t e r . T h i s s o l u t i o n was a c i d i f i e d to pH 3-4 w i t h d i l u t e HNO^. t h e n t i t r a t e d w i t h 0 .100 N A g N 0 3 . No c h l o r i d e i o n d e t e r m i n a -t i o n s were made i n t h e p r e s e n c e o f s u l p h i d e i o n s . C h a p t e r 3 RESULTS 3.1 A n o d i c P o l a r i z a t i o n C u r v e s and L i n e a r P o l a r i z a t i o n R e s u l t s The a n o d i c p o l a r i z a t i o n c u r v e s d e t e r m i n e d f o r t h i s 41 58 work a r e s i m i l a r t o t h o s e p r e s e n t e d e l s e w h e r e . ' F i g u r e 7 shows r e p r e s e n t a t i v e a n o d i c p o l a r i z a t i o n c u r v e s o f t h e m i l d s t e e l i n t h e 3.35 m o l / k g NaOH and 2.5 m o l / k g NaOH + 0 .42 m o l / k g Na,,S s o l u t i o n s . Bo th c u r v e s have s i m i l a r f e a t u r e s , w i t h E j u s t l o w e r t h a n - 1.1 V and a w e l l c o r r see d e f i n e d a c t i v e peak f o l l o w e d by a t r a n s i t i o n t o p a s s i v e b e h a v i o r . The i m p o r t a n t d i f f e r e n c e s between t h e two c u r v e s a r e i ) t h e much h i g h e r a n o d i c c u r r e n t d e n s i t y a t t a i n e d by t h e s t e e l i n s i m u l a t e d w h i t e l i q u o r , a t b o t h t h e a c t i v e peak and i n t h e p a s s i v e r e g i o n , i i ) t h e s h i f t i n t h e o n s e t o f p a s s i v e b e h a v i o r f r o m - 1.03 V i n t h e NaOH s o l u t i o n t o r see - 0 .89 V s c e i n t h e s u l p h i d e c o n t a i n i n g s o l u t i o n , and i i i ) t h e r a p i d i n c r e a s e i n c u r r e n t d e n s i t y i n t h e s u l p h i d e c o n -t a i n i n g s o l u t i o n a t p o t e n t i a l s h i g h e r t h a n - 0 .75 V _ c e > due to t h e o x i d a t i o n o f s u l p h i d e t o h i g h e r o x i d a t i o n s t a t e s . The a n o d i c p o l a r i z a t i o n c u r v e s o f m i l d s t e e l i n 42 43 Figure 7 Anodic p o l a r i z a t i o n curves of mi ld s tee l i n 3.35 mol/kg NaOH and 2.5 mol/kg NaOH + 0.42 mol/kg Na 2S at 92° C. 44 12 .5 m o l / k g NaOH a t d i f f e r e n t t e m p e r a t u r e s a r e shown i n F i g u r e 8. T h e s e c u r v e s a l s o e x h i b i t an a c t i v e - p a s s i v e t r a n s i t i o n . R a i s i n g t h e t e m p e r a t u r e s e r v e d t o i n c r e a s e t h e a n o d i c c u r r e n t d e n s i t y , a t b o t h t h e a c t i v e peak and i n t h e p a s s i v e r e g i o n , and t o d e c r e a s e E c Q r r - I n c r e a s i n g t h e NaOH c o n c e n t r a t i o n f r o m 3.35 m o l / k g NaOH ( F i g u r e 7) t o 12 .5 m o l / k g NaOH ( F i g u r e 8) a t 9 2 ° C c a u s e d t h e peak a n o d i c c u r r e n t d e n s i t y t o i n c r e a s e and E t o d e c r e a s e . No J c o r r s i g n i f i c a n t d i f f e r e n c e s were n o t e d be tween t h e a n o d i c p o l a r i z a t i o n c u r v e s o f t h e d i f f e r e n t b a t c h e s o f s t e e l . The r e s u l t s o f t h e l i n e a r p o l a r i z a t i o n e x p e r i m e n t s i n each s o l u t i o n were p l o t t e d as c u r r e n t d e n s i t y v s . p o t e n t i a l and t h e b e s t s l o p e o f e a c h l i n e was c a l c u l a t e d by a l e a s t s q u a r e s m e t h o d . The s l o p e o f t h e l i n e , ( A i / A E ) F , i s t h e r e c i p r o c a l p o l a r i z a t i o n r e s i s t a n c e , _ , . c o r r R~ , and can be r e l a t e d t o t h e c o r r o s i o n c u r r e n t d e n s i t y , 69 i c o r r > by e q u a t i o n 4: R" 1 - 2 .303 / b a + ;lbc1 \ i . c o r r . . . . 4 ; A b a ' b c ' / ' where b and b a r e t h e a n o d i c and c a t h o d i c T a f e l c o n s t a n t s , a c x r e s p e c t i v e l y . D o i g ^ a n d F l e w i t t have g i v e n t h e v a l u e o f b c as ~ 0 .06 V / d e c a d e ( f o r h y d r o g e n r e d u c t i o n ) and t h a t o f b 3 as : ~ 0.72 V / d e c a d e a t . 9 2 ° C i n 8 M NaOH s o l u t i o n s . 5 4 ' 6 8 a 45 to" 2 IO"1 I 10 io2 10* CURRENT DENSITY ( i ) , A/m* F i g u r e 8 Anodic p o l a r i z a t i o n curves of m i l d s t e e l i n 12.5 mol/kg NaOH at s e l e c t e d temperatures. T a b l e I I I R e s u l t s o f L i n e a r P o l a r i z a t i o n E x p e r i m e n t s 12.5 m o l / k g NaOH 3.35 m o l / k g NaOH 2.5 m o l / k g NaOH + 0 .42 m o l / k g N a 2 S Bar A Bar B Bar A Bar B Bar A Bar ;B ^ c o r r ^ s c e ^ - 1 . 2 4 -1 .24 - 1 . 1 5 - 1 . 1 5 - 1 . 1 6 - 1 . 1 6 Rp 1 ( n " V 2 ) 140 + 40 150 +20 55 + 25 68 + 10 36 + 25 52 + 17 i ( A / m 2 ) c o r r v ' ' 2.0 + 0.6 2.1 + 0.3 0.78 + 0.35 0.97 + 0.14 0.52 + 0.37 0.74 + 0.23 47 U s i n g t h e s e v a l u e s f o r t h e T a f e l c o n s t a n t s , E , R - 1 and c o r r p "I 'corr f o r each s o l u t i o n and b a t c h o f s t e e l a r e t a b u l a t e d i n T a b l e I I I . A t a r n i s h f i l m was o b s e r v e d to fo rm on t h e s u r f a c e o f t h e s p e c i m e n s a t E „ i n a l l s o l u t i o n s . The r c o r r f i l m f o r m e d i n t h e s i m u l a t e d w h i t e l i q u o r was b l a c k e r and t h i c k e r t h a n t h a t f o r m e d i n t h e p l a i n NaOH s o l u t i o n s . The v a l u e s o f R - 1 and i i n T a b l e I I I r e p r e s e n t p c o r r r t he a v e r a g e o f two o r t h r e e e x p e r i m e n t s f o r each b a t c h o f s t e e l . F r e s h s o l u t i o n was u sed f o r each e x p e r i m e n t . W i t h i n t h e a c c u r a c y o f t h e e x p e r i m e n t s , no s i g n i f i c a n t d i f f e r e n c e i n w a s o b s e r v e d between t h e two b a t c h e s o f s t e e l c o r r i n any o f t h e s o l u t i o n s . The v a l u e o f i i n 12 .5 J c o r r m o l / k g NaOH was a b o u t d o u b l e t h a t o f t he o t h e r two s o l u t i o n s , T h e r e was no s i g n i f i c a n t d i f f e r e n c e o b s e r v e d between t h e v a l u e s o f i i n 3.35 m o l / k g NaOH and the s i m u l a t e d w h i t e c o r r 1 i q u o r . 3.2 SSRT R e s u l t s 2.3.1. 3.35 m o l / k g NaOH F i g u r e 9 shows t h e r e s u l t s o f t h e s l ow s t r a i n r a t e e x p e r i m e n t s c o n d u c t e d i n 3.35 m o l / k g NaOH s u p e r i m p o s e d upon t h e a n o d i c p o l a r i z a t i o n c u r v e o f t h e s t e e l i n t h e same s o l u t i o n . The r e s u l t s a r e p l o t t e d as % r e d u c t i o n i n a r e a vs, p o t e n t i a l . The minimum % r e d u c t i o n i n a r e a , w h i c h o c c u r s a t - 1.00 V s c e , c o r r e s p o n d s t o t h e p o t e n t i a l a t w h i c h t he F i g u r e 9 E f f e c t of p o t e n t i a l upon r e d u c t i o n i n area f o r 3.35 mol/kg NaOK superimposed upon the anodic p o l a r i z a t i o n curve o b t a i n e d i n the same s o l u t i o n . 49 Figure 10 Appearance of f i n a l f r a c t u r e reg ion a f t e r SSRT t e s t in 3 . 3 5 mol/kg NaOH. m i l d s t e e l was most s u s c e p t i b l e t o SCC. T h i s p o t e n t i a l c o i n c i d e s w i t h t h e t r a n s i t i o n o f t h e s t e e l f r o m a c t i v e t o p a s s i v e b e h a v i o r . At t h e s t r a i n r a t e e m p l o y e d , SCC a l s o o c c u r r e d w i t h i n a r e g i o n of + 50 mV f r o m t h i s p o t e n t i a l . A d e c r e a s e i n t h e % r e d u c t i o n i n a r e a o f a s p e c i m e n f r o m the mean l e v e l was a l w a y s a c c o m p a n i e d by t h e p r e s e n c e o f numerous s e c o n d a r y c r a c k s p e n e t r a t i n g t h e s u r f a c e o f t h e s p e c i m e n n e a r where t h e f a i l u r e o c c u r r e d . T h i s c o n -f i r m e d SCC as t h e c a u s e o f p r e m a t u r e f a i l u r e . F i g u r e 10 shows r e p r e s e n t a t i v e p h o t o g r a p h s o f t he a p p e a r a n c e o f s p e c i m e n s t e s t e d t o f a i l u r e a t SCC s u s c e p t i b l e (- 1.00 V ) j U C and n o n s u s c e p t i b l e (- 0 .62 \ ' S Q Q ) p o t e n t i a l s r e s p e c t i v e l y . E x a m i n a t i o n o f s p e c i m e n h a l v e s w h i c h had been s e c t i o n e d l o n g i t u d i n a l l y , p o l i s h e d and e t c h e d showed t h a t s e c o n d a r y c r a c k i n g was p r i m a r i l y i n t e r g r a n u l a r , be tween f e r r i t e -f e r r i t e g r a i n s . a n d f e r r i t e - p e a r l i t e g r a i n s . T r a n s g r a n u l a r c r a c k s were o b s e r v e d t h r o u g h b o t h p e a r l i t e g r a i n s and f e r r i t e g r a i n s . 3 . 2 . 2 2.5 m o l / k g NaOH + 0 .42 m o l / k g Na 2 S The r e s u l t s o f t h e s l ow s t r a i n r a t e e x p e r i m e n t s i n t h e s i m u l a t e d w h i t e l i q u o r were s i m i l a r t o t h o s e o b t a i n e d i n t h e p l a i n NaOH s o l u t i o n , as shown by F i g u r e 11 . The minimum % r e d u c t i o n i n a r e a o c c u r r e d a t - 0 .88 V _ „ „ , a Figure 11 E f fec t of potent ia l upon reduction in area fo r 2.5 mol/kg NaOH + 0.42 mol/kg Na2S superimposed upon the anodic p o l a r i z a t i o n curve obtained in the same so l u t i on . a) E = - 0.90 V see p o t e n t i a l j u s t s l i g h t l y h i g h e r t h a n t h e a c t i v e - p a s s i v e t r a n s i t i o n p o t e n t i a l . S p e c i m e n s w h i c h showed a r e d u c e d % r e d u c t i o n i n a r e a a l s o e x h i b i t e d numerous s e c o n d a r y s u r -f a c e c r a c k s , w h i l e no c r a c k s were v i s i b l e on t h e o t h e r s p e c i m e n s . . F i g u r e 12 shows s p e c i m e n s t e s t e d a t SCC s u s c e p t i b l e (- 0 .90 V ) and n o n s u s c e p t i b l e (- 0 .50 V ) see see p o t e n t i a l s , r e s p e c t i v e l y . S e c o n d a r y c r a c k i n g was f o u n d t o be a l m o s t e n t i r e l y i n t e r g r a n u l a r , w i t h f e w e r t r a n s -g r a n u l a r c r a c k s t h a n i n t h e 3.35 m o l / k g NaOH s o l u t i o n . 3.3 F r a c t u r e M e c h a n i c s R e s u l t s 3.3.1 G e n e r a l A f t e r t he DCB s p e c i m e n s were s p l i t open i n l i q u i d n i t r o g e n , i t was u s u a l l y q u i t e e a s y t o d i s t i n g u i s h v i s u a l l y t h e b o u n d a r i e s between t h e f a t i g u e p r e c r a c k , s t r e s s c o r r o s i o n c r a c k and b r i t t l e o v e r l o a d r e g i o n s , as shown i n F i g u r e 13 . F e a t u r e s o b s e r v e d i n F i g u r e 13 a r e t y p i c a l o f most o f t h e s p e c i m e n s t e s t e d . The l e a d i n g edge o f t h e s t r e s s c o r r o s i o n c r a c k s commonly d e s c r i b e d a s h a l l o w "W". The c r a c k was n e v e r f o u n d to p r o p a g a t e on t h e s u r f a c e o f t h e s p e c i m e n s , and t h e e x t e n t o f c r a c k f r o n t c u r v a t u r e v a r i e d f r o m s p e c i m e n t o s p e c i m e n . G e n e r -a l l y , t h e c r a c k f r o n t a p p e a r e d t o be s t r a i g h t e r f o r l o n g e r c r a c k s and had a more p r o n o u n c e d c u r v a t u r e f o r s h o r t e r c r a c k s . The shape o f t h e c r a c k f r o n t i s d i s c u s s e d f u r t h e r i n A p p e n d i x A. The most o b v i o u s m a c r o s c o p i c f e a t u r e s o f 54 fat igue SCC overload Figure 13 Macroscopic view of stress corrosion crack surface of DCB specimen tested in 12.5 mol/kg NaOH at 92° C, E„ r t v ^ and KT = 23.8 - 25.4 MPav'm. corr I t h e c r a c k s u r f a c e were t h e s e r i e s o f r i d g e s r u n n i n g p a r a l -l e l t o t h e d i r e c t i o n o f c r a c k p r o p a g a t i o n . They o c c u r r e d a c r o s s t h e e n t i r e f a t i g u e and s t r e s s c o r r o s i o n c r a c k s u r -f a c e s , as o b s e r v e d i n F i g u r e 13 . The pH o f t h e l i q u i d f r o z e n a t t h e c r a c k t i p o f t h e s p e c i m e n s , a f t e r t h e t e r m i n a t i o n o f t h e t e s t , was 14 i n a l l t h r e e s o l u t i o n s , as was t h e room t e m p e r a t u r e b u l k pH o f a l l t h r e e s o l u t i o n s . The c h l o r i d e i o n c o n c e n t r a t i o n i n t h e t e s t s o l u t i o n as a r e s u l t o f l e a k a g e f r om t h e L u g g i n c a p i l -l a r y was u s u a l l y between 0 .002 M .and 0.2 M a t t h e end o f a t e s t . Too few t e s t s were c o n d u c t e d t o o b t a i n a good c o r -r e l a t i o n between the r e s u l t s o f t h e two b a t c h e s of. s t e e l . In 3.35 m o l / k g NaOH a t - 1.00 V , t h e r a t i o o f c r a c k 3 see v e l o c i t i e s between a s p e c i m e n m a c h i n e d f r om each ba r and t e s t e d a t t h e same s t r e s s i n t e n s i t y was 1 .2 , w i t h t h e ba r A s p e c i m e n b e i n g t h e f a s t e s t . The r a t i o was 3 i n 12 .5 m o l / k g NaOH a t - 1.00 V s c e > w i t h t h e ba r B s p e c i m e n b e i n g f a s t e r . The r e s u l t s a r e shown i n T a b l e IV. An a v e r a g e c r a c k v e l o c i t y , v , f o r e a c h s p e c i m e n was d e t e r m i n e d by m e a s u r i n g t h e i n c r e m e n t o f s t r e s s c o r r o s i o n c r a c k g rowth w h i c h o c c u r r e d d u r i n g t he t e s t . T h i s meant t h a t , u n d e r c o n s t a n t l o a d c o n d i t i o n s , K T i n c r e a s e d d u r i n g T a b l e IV C o m p a r i s o n o f C r a c k V e l o c i t y Between D i f f e r e n t B a t c h e s o f S t e e l a t 9.2° C, - 1.00 V „ and K T = 30 - 37 MPa/m. S o l u t i o n C r a c k V e l o c i t y (m/s ) B a t c h A B a t c h B 12.5 m o l / k g NaOH 2.5 x I O " 8 6.9 x I O " 8 3 . 3 5 . m o l / k g NaOH 7.1 x 1 0 " 9 5.7 x 1 0 " 9 cn cn 57 t h e t e s t and c o n s e q u e n t l y , a r a n g e o f Kj i s p l o t t e d f o r e a c h s p e c i m e n . 3 . 3 . 2 12 .5 m o l / k g NaOH 3 . 3 . 2 . 1 The E f f e c t o f S t r e s s I n t e n s i t y . The i n c r e m e n t o f s t r e s s c o r r o s i o n c r a c k g r o w t h , t h e l e n g t h o f t h e t e s t and t h e r e s u l t a n t a v e r a g e c r a c k v e l o c i t y f o r 12 .5 m o l / k g NaOH a t E c o r r a r e t a b u l a t e d i n T a b l e V. Log v i s p l o t t e d v s . Kj i n F i g u r e 14 . The maximum c r a c k v e l o c i t y m e a s u r e d d u r i n g t h i s s e r i e s o f t e s t s was 6.1 x 1 0 " 9 m/s a t a K- o f 38 .3 - 4 4 . 8 MPa/m and - 9 t h e minimum v e l o c i t y m e a s u r e d was 0 ,25 x 10 m/s a t a K- o f 17 .9 - 18.1 MPa/m. A s t r e s s i n d e p e n d e n t , r e g i o n II c r a c k -9 v e l o c i t y o f ~ 2.5 x 10 m/s was o b s e r v e d between a Kj o f 28 MPa/m and 40 MPa/m. A r e g i o n I c r a c k v e l o c i t y can be o b s e r v e d be low 25 MPa/m, but a t h r e s h o l d s t r e s s i n t e n s i t y f o r SCC was n o t f o u n d above 18 MPa/m. The i n c r e a s e i n c r a c k v e l o c i t y above 40 MPa/m may i n d i c a t e t h e o n s e t o f a r e g i o n I I I c r a c k v e l o c i t y . 3 . 3 . 2 . 2 The E f f e c t o f T e m p e r a t u r e and P o t e n t i a l The d e p e n d e n c e o f t h e r e g i o n II c r a c k v e l o c i t y on t e m p e r a t u r e was i n v e s t i g a t e d a t E c o r r a n d a t - 1.00 V , t h e r e s u l t s o f w h i c h a r e t a b u l a t e d i n T a b l e V I . see A d i f f e r e n t b a t c h o f s t e e l was u sed a t e a c h p o t e n t i a l . An A r r h e n i u s r a t e law. o f t h e f o l l o w i n g f o rm was a s s u m e d : 58 T a b l e V E f f e c t o f S t r e s s I n t e n s i t y on C r a c k V e l o c i t y i n 12 .5 m o l / k g NaOH a t £ n n r r and 9 2 ° C. MPa ^m C r a c k Growth (mm) T e s t L e n g t h ( h r s ) C r a c k V e l o c i t y (m/s) x 1 0 J 1 7 . 9 - 18.1 0.4 476 0 .3 2 3 . 0 - 2 3 . 9 1.7 479 1.0 23 .8 - 2 5 . 4 2.7 479 1 .6 2.5.6 - 2 7 . 6 3.1 409 2.2 30.1 - 3 2 . 7 3.1 413 2.1 3 1 . 0 - 33 .7 3.9 386 2.8 35 .0 - 37 .8 3.4 385 2.5 38 .3 - 4 4 . 8 7.0 313 6 i l 59 10 -7 i — : — r r i r CO E 10 - 8 != 10 o o _ j UJ < or o -9 10 -10; 12.5 m o ! / k g N a O H 9 2 ° C , E, ' CORR 10 -II 10 2 0 3 0 4 0 5 0 S T R E S S I N T E N S I T Y ( K , ) , ' M P a - v / n m F igure 14 E f f e c t of s t r e s s i n t e n s i t y on crack v e l o c i t y i n 12.5 mol/kg NaOH at 92° C and E . 3 c o r r 60 v e " Q / R T o where v i s t h e a v e r a g e c r a c k v e l o c i t y , V q i s a c o n s t a n t , Q i s t h e a c t i v a t i o n e n e r g y , R i s t h e gas c o n s t a n t and T i s t he a b s o l u t e t e m p e r a t u r e . The r e s u l t s o f b o t h s e r i e s o f e x p e r i m e n t s a r e p l o t t e d i n F i g u r e 15 as l o g v v s . 1/T. I t i s a p p a r e n t t h a t t h e r e was more s c a t t e r to t h e d a t a a t E _ t h a n a t - 1.00 V c o r r see A ; l e a s t s q u a r e s a n a l y s i s o f t h e d a t a y i e l d e d an a p p a r e n t a c t i v a t i o n e n e r g y o f 23 + 9 k j / m o l a t E r and 24 + 1 k J / m o l a t - 1.00 V . I t was assumed t h a t t h e — see m e a s u r e d c r a c k v e l o c i t i e s r e m a i n e d w i t h i n r e g i o n II a t t h e K- l e v e l s e m p l o y e d . A l t h o u g h i t was d i f f i c u l t t o compare the crack v e l o c i t i e s obtained at d i f f e r e n t , p o t e n t i a l s and the same t e m p e r a t u r e ? due t o d i f f e r e n c e s between t h e two b a t c h e s o f s t e e l , t h e r e was a b o u t an o r d e r o f m a g n i t u d e f a s t e r c r a c k v e l o c i t y a t - 1.00 V _ t h a n a t E „ . J see c o r r 3 . 3 . 3 3.35 m o l / k g NaOH 3 . 3 . 3 . 1 The E f f e c t o f S t r e s s I n t e n s i t y The r e s u l t s o f t he f r a c t u r e m e c h a n i c s t e s t s i n t h e 3.35 m o l / k g NaOH s o l u t i o n a r e t a b u l a t e d i n T a b l e VI I and a p l o t o f l o g v v s . Kj i s shown i n F i g u r e 16. A l l t e s t s were p e r f o r m e d a t 9 2 ° C and t h e p o t e n t i a l was h e l d c o n s t a n t a t - 1.00 V c / , Q , t h e p o t e n t i a l a t w h i c h t h e T a b l e VI E f f e c t o f T e m p e r a t u r e and P o t e n t i a l on C r a c k V e l o c i t y i n 12 .5 m o l / k g NaOH. B a t c h P o t e n t i a l < V sce> T ( ° C ) K I (MPa/m) G rack Growth (mm) T e s t L e n g t h ( h r s ) C r a c k V e l o c i t y Q (m/ s ) x 10 1 / T x 10 3 ( ° K _ 1 ) 55 34.5 - 37 .7 4 .8 499 2.2 3.05 70 34 .0 - 35 .0 3.4 477 2.0 2.92 A ^ c o r r -92 35 .0 - 37 .8 3.4 385 2.5 2.74 105 35.4 - 41 .9 7.7 260 8.2 2.65 115 35.2 - 41 .1 10 .5 479 6.1 2.58 70 31 .3 - 34 .8 6.5 45 40 2.92 B - 1 .00 92 31 .8 - 37 .4 6.8 28 69 2.74 105 31 .3 - 43.1 14 .3 46 87 2.65 62 Figure 15 Arrhenius p lot of the region IT crack v e l o c i t i e s in 12.5 mol/kg NaOH at E c o p r and - 1.00 V s c e , K. = 31 - 43 MPa^m. SSRT showed m i l d s t e e l t o be most s u s c e p t i b l e t o SCC. - 8 The maximum c r a c k v e l o c i t y m e a s u r e d was 1.2 x 10 m/s a t a Kj o f 36 - 43 MPa/m and t h e s l o w e s t c r a c k v e l o c i t y - 9 — m e a s u r e d was 1.9 x 10 m/s a t a Kj o f 15 .5 - 16.1 MPa/m. - 9 A r e g i o n II c r a c k v e l o c i t y o f ~ 7.2 x 10 m/s can be o b s e r v e d between a Kj o f 20 .rarid~ 35 MPa/m. K J C J Q Q w a s n o t d e t e r m i n e d i n t h i s s e r i e s o f e x p e r i m e n t s , a l t h o u g h r e g i o n I c r a c k v e l o c i t y b e h a v i o r can be o b s e r v e d be low ~ 20 MPa/m. 3 . 3 . 3 . 2 The E f f e c t o f P o t e n t i a l A s p e c i m e n was a l s o t e s t e d a t - 0 .88 V s c g , t h e p o t e n t i a l o f maximum s u s c e p t i b i l i t y t o SCC i n t h e s i m u l a t e d w h i t e l i q u o r . A m e a s u r a b l e c r a c k v e l o c i t y was f o u n d , i n c o n t r a s t t o t h e s l ow s t r a i n r a t e t e s t , where SCC was no t o b s e r v e d a t - 0 .88 V e i n t h e same s t r e n g t h s o l u -t i o n . The s t r e s s c o r r o s i o n c r a c k v e l o c i t y a t 9 2 ° C, - 0 . 8 8 - V and Kj = 33 .6 - 35 .3 MPa/m was 1.9 x I O - 9 m/s , compared t o t h e r e g i o n II c r a c k v e l o c i t y o f t 7.2 x 10 m/s a t - 1.00 V s c e and s i m i l a r Kj l e v e l s between 3 0 . 3 MPa/m and 3 7 . 7 MPa/m. A l l s p e c i m e n s were m a c h i n e d f r o m t h e same b a t c h o f s t e e l . 3 . 3 . 4 2.5 m o l / k g NaOH + 0 .42 m o l / k g Na 2 S 3 . 3 . 4 . 1 The E f f e c t o f S t r e s s I n t e n s i t y The r e s u l t s o f t h e f r a c t u r e m e c h a n i c s e x p e r i m e n t s i n t h e s i m u l a t e d w h i t e l i q u o r a r e t a b u l a t e d i n 64 T a b l e VI I E f f e c t o f S t r e s s I n t e n s i t y on C r a c k V e l o c i t y i n 3.35 m o l / k g NaOH a t 9 2 ° C and - 1.00 V K I C r a c k Growth T e s t L e n g t h C r a c k V e l o c i t y MPa/m (mm) ( h r s ) (m/s) x 10 9 10 .3 - 11 . 2 4 .3 381 3.1 15 .5 - 16. 1 2.9 423 1.9 2 0 . 7 - 24. 3 6.7 428 4 .5 25.1 - 28 . 0 5.5 221 6.9 2 9 . 7 - 33 . 3 5.3 258 5.7 35 .7 - 4 3 . 0 8 .3 196 12 65 £ o < r r o 0 -7 10 - 8 t 10 o o - J UJ > -9 10 - 1 0 i 0 " n L 3 . 35 m o l / k g N o O H 9 2 ° C , - 1 . 0 0 V SCE 1 10 2 0 3 0 4 0 5 0 S T R E S S I N T E N S I T Y (K ), M P o ^ Figure 16 Effect of stress in tens i ty on crack v e l o c i t y . i n 3.35 mol/kg NaOH at 92° C and -1.00 V .. T a b l e V I I I . Log v i s p l o t t e d v s . Kj i n F i g u r e 17. A l t h o u g h f e w e r s p e c i m e n s were t e s t e d i n t h i s s o l u t i o n t h a n i n t h e o t h e r s o l u t i o n s , i t i s c l e a r t h a t t h e s t r e s s c o r r o s i o n c r a c k v e l o c i t y a t c o m p a r a b l e s t r e s s i n t e n s i t i e s was l o w e r i n t h e s i m u l a t e d w h i t e l i q u o r t h a n i n t h e c o r r e s p o n d i n g NaOH s o l u t i o n a t t h e r e s p e c t i v e a c t i v e - p a s s i v e t r a n s i t i o n p o t e n t i a l s . R e g i o n I c r a c k v e l o c i t y b e h a v i o r can be o b s e r v e d be low ~21 MPa/m, bu t no t enough d a t a was o b t a i n e d t o c l e a r l y d e l i n e a t e t h e b o u n d a r y between r e g i o n I and r e g i o n II i n F i g u r e 17. A r e g i o n II c r a c k v e l o c i t y o f - 9 4 x 10 m/s was o b s e r v e d , w h i c h i s f a s t e r t h a n t h e c o m p a r a b l e v e l o c i t y o b s e r v e d i n 3.35 m o l / k g NaOH a t - 0 .88 V s c g . The s l o w e s t m e a s u r e d c r a c k v e l o c i t y was 2.2 x 10 " 1 0 m/s a t 15 .5 - 15 .6 MPa/m, and t h e f a s t e s t -9 m e a s u r e d c r a c k v e l o c i t y was 5.0 x 10 m/s a t a Kj o f 31 .4 - 32 .5 MPa/m. The a v e r a g e s u l p h i d e c o n c e n t r a t i o n m e a s u r e d o v e r t h e l e n g t h o f e a c h t e s t i s i n c l u d e d i n T a b l e V I I I ; i t v a r i e d f r o m 0.31 M to 0*46 M. The s u l p h i d e c o n c e n t r a t i o n a l w a y s d e c r e a s e d o v e r t he l e n g t h o f a t e s t . 3.4 F r a c t o g r a p h y 3.4.1 G e n e r a l U n l e s s o t h e r w i s e n o t e d , a l l f r a c t o g r a p h s o f s t r e s s c o r r o s i o n c r a c k s u r f a c e s were t a k e n as c l o s e t o t h e l e a d i n g edge o f t h e c r a c k as was p o s s i b l e , i n o r d e r t o m i n i m i z e t h e i n f l u e n c e o f g e n e r a l c o r r o s i o n on t h e T a b l e V I I I E f f e c t o f S t r e s s I n t e n s i t y on C r a c k V e l o c i t y i n 2.5 m o l / k g NaOH + 0.42 m o l / k g Na 9 S a t 92° C and - 0 .88 V C r a c k Growth T e s t L e n g t h C r a c k V e l o c i t y T s 2 " ] . avg MPa/m (mm) ( h r s ) (m/s ) x 10 9 (ti) 15 .5 - 15.6 0.2 235 0 .2 0 .46 21 .2 - 22.2 2.0 236 2.3 0.31 2 5 . 9 - 26 .7 1 .1 214 1 .4 0 .40 31 .4 - 32 .5 3.0 165 5.0 0 .37 3 6 . 0 - 38.9 3.5 212 4 .6 0 .44 68 >-CJ O _ l LU > < cr CJ 10 - 7 10' 8 I 0 ~ 9 10 -10 10 - I I N a O H + N a 2 S 9 2 ° C , - 0 . 8 8 V SCE 10 2 0 3 0 4 0 5 0 S T R E S S I N T E N S I T Y ( K , ) , M P a V m Figure 17 E f f e c t of s t r e s s i n t e n s i t y on crack v e l o c i t y i n 2.5 mol/kg NaOH + 0.42 mol/kg Na„S at 92° C and - 0.88 V 2 see 69 a p p e a r a n c e o f t h e t o p o g r a p h y . R e p r e s e n t a t i v e f r a c t o g r a p h s o f each s p e c i m e n were c h o s e n f r o m s e v e r a l f r a c t o g r a p h s t a k e n a t r a n d o m l y s c a t t e r e d p o i n t s a c r o s s t h e c r a c k f r o n t . No change i n f r a c t o g r a p h y was o b s e r v e d f o r any s p e c i m e n a c r o s s t h e c r a c k f r o n t . A l l f r a c t o g r a p h s a r e o r i e n t e d w i t h t h e d i r e c t i o n o f c r a c k p r o p a g a t i o n f r om t h e t o p t o t h e b o t t o m o f t h e p a g e . The c r a c k s u r f a c e s o f a l l s p e c i m e n s were c o v e r e d by a b l a c k l a y e r o f c o r r o s i o n p r o d u c t when t h e y were removed f r om t h e t e s t s o l u t i o n s . T h i s l a y e r was o f v a r y i n g t h i c k -ne s s and a p p e a r a n c e , d e p e n d i n g on t h e s o l u t i o n c o m p o s i t i o n , s p e c i m e n p o t e n t i a l and l e n g t h o f t e s t . U s u a l l y , t h e c o r -r o s i o n p r o d u c t was t h i c k e s t on t h e s u r f a c e o f t h e m a c h i n e d c r a c k and f a t i g u e p r e c r a c k , and t h i n n e s t on t h e s u r f a c e o f t h e s t r e s s c o r r o s i o n c r a c k , e s p e c i a l l y a t t h e l e a d i n g edge o f t h e c r a c k . F i g u r e 18 sho»ws t h e c o v e r a g e o f c o r r o s i o n p r o d u c t on t h e l e a d i n g edge o f t h e s t r e s s c o r r o s i o n c r a c k o f s p e c i m e n s t e s t e d u n d e r v a r i o u s c o n d i t i o n s . The l a y e r o f c o r r o s i o n p r o d u c t was q u i t e t h i c k i n 12 .5 m o l / k g NaOH a t E a n d o b s c u r e d t h e f e a t u r e s on t h e c r a c k s u r f a c e , w h i l e c o r r a t t h e a c t i v e - p a s s i v e t r a n s i t i o n i n each s o l u t i o n , i t was much t h i n n e r and d i d no t o b s c u r e t h e f r a c t o g r a p h y . The c o v e r a g e o f c o r r o s i o n p r o d u c t on e x p o s e d s u r f a c e s o f t h e s p e c i m e n s o u t s i d e o f t h e c r a c k was g e n e r a l l y much t h i n n e r t h a n t h a t o b s e r v e d i n t h e c r a c k s . 70 a) 12.5 mol/kg NaOH, Kj = 31.0 - 33.7 MPa>mT b) 12.5 mol/kg NaOH, -1.00 V Kj • 31.3 - 43.1 MPav'm see c) 3.35 mol/kg NaOH, -1.00 V Kj = 20.7 - 24.3 MPav£ see d) 2.5 mol/kg NaOH + 0.42 mol/kg Na 2S,-0.88 V s c e , Kj = 25.9 - 26.7 MPa>4n Figure 18 Comparison of the corrosion product on the stress corrosion crack surface in d i f f e ren t environments. a) before cleaning with i nh i b i t ed acid b) a f t e r cleaning with i nh ib i t ed acid Figure 19 B r i t t l e overload f a i l u r e near the crack t i p . 72 A d e t a i l e d e x a m i n a t i o n o f t h e s u r f a c e o f e a c h s p e c i m e n was c o n d u c t e d a f t e r c l e a n i n g i n t h e i n h i b i t e d a c i d s o l u t i o n . T h i s :• s o l u t i o n p r o d u c e d no a r t i f a c t s , as c o n f i r m e d by e x a m i n -i n g b r i t t l e f r a c t u r e s u r f a c e s b e f o r e and a f t e r e x p o s u r e t o t h e s o l u t i o n , shown i n F i g u r e 1 9 . R e p r e s e n t a t i v e f r a c t o -g r a p h s o f t h e two b a t c h e s o f s t e e l i n 12 .5 m o l / k g NaOH and 3.35 m o l / k g NaOH a t - 1.00 V s c e and 9 2 ° C a r e shown i n F i g u r e 2 0 . The f r a c t o g r a p h y o f b o t h b a t c h e s o f s t e e l i s s i m i l a r , w i t h t h e e x c e p t i o n o f t h e l o n g r i b b o n - l i k e f l a t f e a t u r e s r u n n i n g i n t h e d i r e c t i o n o f c r a c k p r o p a g a t i o n i n b a t c h B. T h e s e f e a t u r e s o c c u r r e d l e s s f r e q u e n t l y i n b a t c h A. O c c a s i o n a l l y , t h e c r a c k p a t h o f s p e c i m e n s u n d e r p o t e n t i o -s t a t i c c o n t r o l c h a n g e d f r o m p r i m a r i l y i n t e r g r a n u l a r t o p r i m a r i l y t r a n s g r a n u l a r , o r v i c e v e r s a . In a l l c a s e s , t h e l e a d i n g edge o f t h e c r a c k a t t h e t e r m i n a t i o n o f t h e t e s t was as shown and d e s c r i b e d i n t h e f o l l o w i n g s e c t i o n s . 3 . 4 . 2 12 .5 m o l / k g NaOH 3 . 4 . 2 . 1 The E f f e c t o f S t r e s s I n t e n s i t y The c r a c k s u r f a c e s o f a l l s p e c i m e n s were b a d l y e t c h e d by g e n e r a l d i s s o l u t i o n and most f i n e f e a t u r e s were o b l i t e r a t e d . L a r g e s e c o n d a r y c r a c k s r a n p a r a l l e l t o t h e d i r e c t i o n o f c r a c k p r o p a g a t i o n . F i g u r e 21 shows r e p r e s e n t a t i v e f r a c t o g r a p h s o f t h e s t r e s s c o r r o s i o n c r a c k s u r f a c e a t E and s e l e c t e d s t r e s s i n t e n s i t i e s . a) Batch A, 12.5 mol/kg NaOH, KT = 31.0 - 33.8 MPav'm b) Batch B, 12.5 mol/kg NaOH, KT = 31.8 - 37.4 MPa^ m c) Batch A, 3.35 mol/kg NaOH, d) Batch B, 3.35 mol/kg NaOH K r = 30.3 - 32.7 MPa/m K- = 29.7 - 33.3 M P a ^ Figure 20 Comparison of fractography between specimens machined from d i f f e r en t batches of steel at - 1.00 V „ and 92° C. a) Kj = 17.9 - 18.1 MPa/m b) Kj = 23.8 - 25.4 MPa/m 1 c) Kj = 30.1 - 32.7 MPa/m d) Kj = 35.0 - 37.8 MPa/m Figure 21 V a r i a t i o n of f ractography with s t re s s i n t e n s i t y in 12.5 mol/kg NaOH at 92° C and E co r r 75 The f r a c t o g r a p h y o f a l l s p e c i m e n s was s i m i l a r , e x c e p t f o r t h e s p e c i m e n t e s t e d .at 18 MPa/m. I t s s u r f a c e was p a r t l y c o v e r e d by d i m p l e s c h a r a c t e r i s t i c o f m i c r o v o i d c o a l e s c e n c e and was no t p e n e t r a t e d by as many s e c o n d a r y c r a c k s as t h e o t h e r s p e c i m e n s . O c c a s i o n a l l o n g , f1 a t * r i b b o n - 1 i k e f e a t u r e s r a n i n t h e d i r e c t i o n o f c r a c k p r o p a g a t i o n i n a l l s p e c i m e n s . T h e s e were somet imes a s s o c i a t e d w i t h t h e s e c o n d a r y c r a c k i n g o b s e r v e d . The c r a c k p a t h i n a l l s p e c i m e n s was o b s e r v e d t o be t r a n s g r a n u l a r . The p r o g r e s s i o n o f t he s t r e s s c o r r o s i o n was uneven a n d , i n many c a s e s , a t o n g u e o f s t r e s s c o r r o s i o n f r a c t u r e was n o t i c e d i n t r u d i n g i n t o t h e b r i t t l e o v e r l o a d f a i l u r e a t t h e c r a c k f r o n t . 3 . 4 . 2 . 2 The E f f e c t o f T e m p e r a t u r e and P o t e n t i a l T h e r e was a l a r g e v a r i a t i o n i n t h e a p -p e a r a n c e o f t h e c r a c k s u r f a c e o f s p e c i m e n s t e s t e d a t F - c o r r and d i f f e r e n t t e m p e r a t u r e s , as shown i n F i g u r e 22 . A t 5 5 ° C, t h e c r a c k s u r f a c e was f o u n d t o be s e v e r e l y : c o r r o d e d and: p i t t e d . The c r a c k p a t h was t r a n s g r a n u l a r , and deep s e c o n d a r y c r a c k s were o b s e r v e d t o run i n t h e d i r e c t i o n o f c r a c k p r o p a g a t i o n . The s p e c i m e n t e s t e d a t 7 0 ° C showed much l e s s g e n e r a l c o r r o s i o n on t h e c r a c k s u r f a c e t h a n t h e o t h e r s p e c i m e n s . The s u r f a c e f e a t u r e s were s h a r p l y d e f i n e d and t h e c r a c k p a t h was p r i m a r i l y t r a n s g r a n u l a r . The f r a c t o -g r a p h y a t 9 2 ° C has a l r e a d y been d i s c u s s e d ( F i g u r e 2 1 ) . 76 Figure 22 V a r i a t i o n o f f ractography with temperature in 12.5 mol/kg NaOH at E r . c o r r 77 78 At 1 0 5 ° C, t h e s u r f a c e o f t h e s p e c i m e n was b a d l y c o r r o d e d . The c r a c k p a t h was t r a n s g r a n u l a r and t h e t o p o g r a p h y s t r o n g l y d i r e c t i o n a l i n t h e d i r e c t i o n o f c r a c k p r o p a g a t i o n . Long s e c o n d a r y c r a c k s r a n down t h e l e n g t h o f t h e s p e c i m e n . The s u r f a c e o f t h e s p e c i m e n t e s t e d a t 1 1 5 ° C was m a r k e d l y d i f -f e r e n t f r o m t h a t o f t h e o t h e r s p e c i m e n s . The f r a c t o g r a p h y was a l m o s t e n t i r e l y i n t e r g r a n u l a r , and s e c o n d a r y c r a c k s were s h o r t , s h a l l o w and f o l l o w e d t h e g r a i n b o u n d a r i e s . The s u r -f a c e o f t h e g r a i n s was g e n e r a l l y smooth and t h e s h a r p edges r o u n d e d o f f . F i g u r e 23 i l l u s t r a t e s t h e f r a c t o g r a p h y o f s p e c i m e n s t e s t e d a t 7 0 ° C, 9 2 ° C a n d . 1 0 5 ° C i n 12 .5 m o l / k g NaOH a t - 1.00 V g . The c r a c k p a t h o f a l l t h r e e s p e c i m e n s was m ixed i n t e r g r a n u l a r - t r a n s g r a n u l a r , i n c o n t r a s t t o t h e t r a n s -g r a n u l a r c r a c k p a t h o b s e r v e d a t E c o r r - The t o p o g r a p h y o f t h e s p e c i m e n s was q u i t e r ough and b r o k e n . O c c a s i o n a l f l a t , r i b b o n - l i k e f e a t u r e s c o u l d be seen r u n n i n g i n t h e d i r e c t i o n o f c r a c k p r o p a g a t i o n . S e c o n d a r y c r a c k i n g was no t as e x -t e n s i v e o r deep i n t h e s e s p e c i m e n s as i n t h o s e t e s t e d a t t h e f r e e c o r r o s i o n p o t e n t i a l . 3 . 4 . 3 3.35 m o l / k g NaOH 3 . 4 . 3 . 1 The E f f e c t o f S t r e s s I n t e n s i t y A l l s p e c i m e n s t e s t e d i n 3.35 m o l / k g NaOH s o l u t i o n s e x h i b i t e d b o t h t r a n s g r a n u l a r and i n t e r g r a n u l a r 79 A) K J = 15.5 - 16.1 MPa/m b) Kj = 20.7 - 24.3 MPa/m C) Kj = 29.7 - 33.3 MPa/m d) Kj - 35.7 - 43.0 MPa/m Figure 24 V a r i a t i o n o f f ractography with s t re s s i n t e n s i t y i n 3.35 mol/kg NaOH at 92° C and - 1.00 V see f e a t u r e s . No p i t t i n g was o b s e r v e d and g e n e r a l c o r r o s i o n d i d no t a p p e a r t o be e x t e n s i v e , e x c e p t f o r a r o u n d i n g o f f o f e x p o s e d edges and r i d g e s . The v a r i a t i o n o f f r a c t o -g r a p h y w i t h s t r e s s i n t e n s i t y i s shown i n F i g u r e 24 . Long , r i b b o n - l i k e l e n g t h s o f t r a n s g r a n u l a r c r a c k i n g can be o b s e r v e d r u n n i n g p a r a l l e l t o t h e d i r e c t i o n o f c r a c k p r o -p a g a t i o n . T h e s e l i n e s o f t r a n s g r a n u l a r c r a c k i n g were o f t e n a s s o c i a t e d w i t h t h e s i d e s o f t h e m a c r o s c o p i c r i d g e s . 3 . 4 . 3 . 2 The E f f e c t o f P o t e n t i a l The c r a c k s u r f a c e o f t h e s p e c i m e n t e s t e d a t a p a s s i v e p o t e n t i a l ( - 0 . 8 8 V g c e ) was b a d l y e t c h e d and p i t t e d by g e n e r a l c o r r o s i o n w h i c h o b s c u r e d t h e f r a c t o -g r a p h y . The s p e c i m e n was no t i m m e d i a t e l y f r o z e n i n l i q u i d N^ and s p l i t a p a r t . I n s t e a d , i t was s e c t i o n e d and t h e f a c e was p o l i s h e d t o r e v e a l t h e c r a c k p a t h . C o n s e q u e n t l y , t h e s p e c i m e n r e s i d e d a t E C Q r r f o r ~ 24 h o u r s b e f o r e b e i n g opened i n l i q u i d N,,. T h i s may be t h e c a u s e o f t h e o b s e r v e d c o r r o s i o n . Few s e c o n d a r y c r a c k s were o b s e r v e d , h o w e v e r , and t h e f e a t u r e s d i d no t a p p e a r t o be as r o u g h as t h o s e a t - 1.00 V . The c r a c k p a t h c o u l d no t be d e t e r m i n e d , see ^ .3 .4 .4 2.5 m o l / k g NaOH + 0 .42 m o l / k g Na 2 S 3 . 4 . 4 . 1 The E f f e c t o f S t r e s s I n t e n s i t y The e f f e c t o f s t r e s s i n t e n s i t y on f r a c t o g r a p h y i n t h e s i m u l a t e d w h i t e l i q u o r i s shown i n 81 c) K, = 31.4 - 32.5 MPa»€ d) KT - 36.0 - 38.9 MPav^ n Figure 25 V a r i a t i o n o f f ractography with s t re s s i n t e n s i t y i n 2.5 mol/kg NaOH + 0.42 mol/kg Na,,S at 92° C and - 0.88 V 82 F i g u r e 25 . The t o p o g r a p h y o f a l l s p e c i m e n s was rough and b r o k e n , and deep s e c o n d a r y c r a c k s were o b s e r v e d on a l l s u r f a c e s . A g a i n , l o n g t r a n s g r a n u l a r r i b b o n s were o b s e r v e d r u n n i n g l o n g i t u d i n a l l y a l o n g t h e f r a c t u r e s u r f a c e . The c r a c k p a t h was a m i x t u r e o f i n t e r g r a n u l a r and t r a n s g r a n u l a r c r a c k -i n g . C h a p t e r 4 DISCUSSION 4.1 G e n e r a l 4 .1 .1 SSRT In t h i s w o r k , t h e SSRT p r o v e d t o be a v a l u a b l e method o f a s s e s s i n g t h e s u s c e p t i b i l i t y o f m i l d s t e e l t o c a u s t i c c r a c k i n g i n p l a i n a l k a l i n e and a l k a l i n e s u l p h i d e s o l u t i o n s . A s e r i e s o f s h o r t e x p e r i m e n t s ( c o m p a r e d t o t h e f r a c t u r e m e c h a n i c s t e s t s ) s e r v e d t o u n e q i v o c a l l y demon-s t r a t e t h a t SCC o f m i l d s t e e l i n b o t h 3.35 m o l / k g NaOH and 2.5 m o l / k g NaOH + 0 .42 m o l / k g Na^S s o l u t i o n s was most s e v e r e a t a p o t e n t i a l a s s o c i a t e d w i t h t h e a c t i v e - p a s s i v e t r a n s i t i o n i n each s o l u t i o n . A l i m i t a t i o n o f t h e SSRT was a l s o r e v e a l e d by t h i s work . SCC was n o t o b s e r v e d a t - 0 .88 V i n t h e 3.35 m o l / k g NaOH s o l u t i o n i n a s l ow see 3 s t r a i n r a t e e x p e r i m e n t , even t h o u g h a f r a c t u r e m e c h a n i c s e x p e r i m e n t u n d e r t h e same c o n d i t i o n s p r o d u c e d a m e a s u r -a b l e i n c r e m e n t o f s t r e s s c o r r o s i o n c r a c k g r o w t h , a l b e i t l e s s t h a n a t - 1.00 V $ c e - T h i s d i s c r e p a n c y i s j e x p l a i n e d b e s t by r e l a t i n g i t t o t h e e f f e c t o f s t r a i n r a t e i n t h e SSRT. 83 P a r k i n s and o t h e r s ' ' have d e m o n s t r a t e d t h a t t h e r a n g e o f c o n d i t i o n s o v e r w h i c h SCC o c c u r s i n a s l ow s t r a i n r a t e t e s t i n c r e a s e s as t he s t r a i n r a t e i s d e c r e a s e d . T h i s i s b e c a u s e t h e f a i l u r e o f a s p e c i m e n i s g o v e r n e d by two c o m p e t i n g p r o c e s s e s ; SCC and d u c t i l e t e a r i n g . I f t h e s t r a i n r a t e i s t o o h i g h i n r e l a t i o n t o t h e r a t e o f s t r e s s c o r r o s i o n c r a c k p r o p a g a t i o n , t h e s p e c i m e n w i l l f a i l by d u c t i l e o v e r l o a d b e f o r e t h e s t r e s s c o r r o s i o n c r a c k s have an o p p o r t u n i t y t o i n i t i a t e o r p r o p a g a t e . I f a s l o w e r s t r a i n r a t e i s u s e d , t he s t r e s s c o r r o s i o n c r a c k s have t i m e t o i n i t i a t e and p r o p a g a t e i n t o t h e s p e c i m e n b e f o r e d u c t i l e o v e r l o a d o c c u r s . The s t r a i n r a t e u sed i n t h i s work was f a s t compared t o t h a t e m p l o y e d by Humphr i e s and P a r k i n s 8 i n NaOH s o l u t i o n s . Under b o r d e r l i n e c o n d i t i o n s f o r SCC, as a t - 0 .88 V s c e i n 3.35 m o l / k g NaOH, t h e use o f a s l o w e r s t r a i n r a t e m i g h t have r e s u l t e d i n t h e o c c u r r e n c e o f SCC a t - 0 .88 V n i n t h e s l ow s t r a i n r a t e t e s t and e l i m i n a t e d see the d i s c r e p a n c y between t h e two t e c h n i q u e s . A f u r t h e r i m p l i c a t i o n o f t h i s r e s u l t i s t h a t w h i l e t h e SSRT p r o v i d e s an e x c e l l e n t measure o f t h e ea se o f i n i t i a t i o n o f SCC i n an e n v i r o n m e n t , i t does no t e s t a -b l i s h w h e t h e r o r n o t t h e c o n d i t i o n s r e q u i r e d f o r c r a c k " p r o p a g a t i o n o c c u r i n t h e p r e s e n c e o f an e x i s t i n g c r a c k . 7 8 T h i s i s i m p o r t a n t i n l i g h t o f t h e work by Brown e t a l . , who showed t h a t t h e e n v i r o n m e n t a t t h e t i p o f a s t r e s s c o r r o s i o n c r a c k can be v e r y d i f f e r e n t f r om t h e b u l k e n v i r o n m e n t . T h u s , t h e SSRT a l o n e c a n n o t p r o v i d e t h e d a t a r e q u i r e d t o u n d e r s t a n d t h e e f f e c t s o f an e n v i r o n m e n t on SCC. A t e c h n i q u e w h i c h can i n v e s t i g a t e t h e p r o p a g a t i o n k i n e t i c s o f an e x i s t i n g c r a c k i s a l s o r e q u i r e d . 4 . 1 . 2 F r a c t u r e M e c h a n i c s T e c h n i q u e The i n t e r p r e t a t i o n o f q u a n t i t a t i v e d a t a o b t a i n e d f r o m f r a c t u r e m e c h a n i c s e x p e r i m e n t s i s much b e t t e r e s t a b l i s h e d t h a n t h a t o f d a t a o b t a i n e d f r o m t h e 17 19 2 1 2 2 SSRT. ' ' ' F o r t h i s r e a s o n , t h e f r a c t u r e m e c h a n i c s t e c h n i q u e p r o v e d t o be an e x c e l l e n t method o f a s s e s s i n g t h e i n f l u e n c e o f e n v i r o n m e n t a l v a r i a b l e s on t h e SCC o f m i l d s t e e l i n a l k a l i n e s o l u t i o n s . The p r i n c i p a l l i m i t a t i o n o f t h e t e c h n i q u e i n t h i s work was t h e l e n g t h o f t h e t e s t s , w h i c h made i t d i f f i c u l t t o o b t a i n t h e amount o f d a t a r e q u i r e d t o a d e q u a t e l y d e f i n e t h e b o u n d a r i e s o f r e g i o n I and r e g i o n II c r a c k v e l o c i t y b e h a v i o r , and t o c o r r e l a t e t h e d a t a o b t a i n e d f r o m d i f f e r e n t b a t c h e s o f s t e e l . A l -t h o u g h i t was i m p o s s i b l e t o d e t e r m i n e . a c c u r a t e l y t h e s c a t t e r be tween t h e d i f f e r e n t b a t c h e s o f s t e e l i n t h i s w o r k , H y a t t and S p e i d e l o b s e r v e d a s c a t t e r o f a b o u t two t i m e s i n t h e m e a s u r e d c r a c k v e l o c i t y be tween d i f f e r e n t b a t c h e s o f a luminum DCB s p e c i m e n s o f t h e same n o m i n a l 21 c o m p o s i t i o n and g i v e n t h e same h e a t t r e a t m e n t . T h i s i s c o n s i s t e n t w i t h t h e r e s u l t s o f t h e p r e s e n t work . The r e s u l t s o f t h e f r a c t u r e m e c h a n i c s e x p e r i m e n t s c o n d u c t e d a t low s t r e s s i n t e n s i t i e s s h o u l d be v i e w e d i n l i g h t o f t h e maximum f a t i g u e s t r e s s i n t e n s i t y a p p l i e d t o t h e s p e c i m e n . Brown has p o i n t e d ou t t h a t t h e g rowth o f a s t r e s s c o r r o s i o n c r a c k m i g h t be i n h i b i t e d , o r c o m p l e t e l y h a l t e d by t h e p l a s t i c zone f o r m e d a t t h e t i p o f a c r a c k i f t he maximum f a t i g u e s t r e s s i n t e n s i t y i s t o o l a r g e compared 1 6 to t he SCC t e s t s t r e s s i n t e n s i t y . The maximum s i z e o f t h e p l a s t i c zone d e v e l o p e d a t t h e t i p o f t h e f a t i g u e 3 c r a c k , r w , may be e s t i m a t e d f r o m e q u a t i o n 6. T h i s f o r m u l a a p p l i e s t o a p l a n e s t r e s s p l a s t i c z o n e . P l a n e s t r a i n p l a s t i c zones a r e a p p r o x i m a t e l y one t h i r d as l a r g e . S u b s t i t u t i n g an a v e r a g e v a l u e o f 643 MPa f o r t h e y i e l d s t r e n g t h o f t h e s t e e l , and a Kj o f 12 MPa/m and 24 MPa/m, r e s p e c t i v e l y , f o r t h e maximum f a t i g u e s t r e s s i n t e n s i t i e s d e v e l o p e d d u r i n g t h e two f a t i g u e c y c l e s u s e d * g i v e s m a x i -mum p l a s t i c zone s i z e s o f 0 .06 mm o r 0 .22mm a t t h e c r a c k t i p due t o f a t i g u e p r e c r a c k i n g . y 6 As a l l c r a c k v e l o c i t i e s m e a s u r e d i n t h i s work were a v e r a g e v a l u e s o v e r a p e r i o d o f t i m e , i t i s d i f f i c u l t t o d e t e r m i n e t h e e f f e c t t h a t t h i s s i z e o f p l a s t i c zone wou ld 87 have had on t h e s t r e s s c o r r o s i o n c r a c k v e l o c i t y . R e a l i s t i -c a l l y , h o w e v e r , i f e i t h e r t h e l e n g t h o f t h e s t r e s s c o r r o s i o n c r a c k were much g r e a t e r t h a n t h e s i z e o f t h e p l a s t i c zone f o r m e d ahead o f t h e f a t i g u e c r a c k o r t h e t e s t s t r e s s i n -t e n s i t y h i g h e r t h a n t h e maximum f a t i g u e s t r e s s i n t e n s i t y , t h e e f f e c t s h o u l d no t have been s i g n i f i c a n t . On t h i s b a s i s , o n l y t h e c r a c k v e l o c i t i e s o f s p e c i m e n s t e s t e d a t 18 MPa/m i n 12 .5 m o l / k g NaOH and a t 15 .5 MPa/m i n t h e s i m u l a t e d w h i t e l i q u o r c o u l d be c o n s i d e r e d s u b j e c t t o s i g n i f i c a n t e r r o r f r o m t h i s s o u r c e . 4 . 1 . 3 K i n e t i c s O n l y a few q u a n t i t a t i v e s t u d i e s have been made o f t h e e f f e c t o f t e m p e r a t u r e on t h e SCC o f m i l d s t e e l i n a l k a l i n e s o l u t i o n s . Bohnenkamp m e a s u r e d an a p p a r e n t a c t i v a t i o n e n e r g y o f 75 k J / m o l f o r Armco i r o n i n 33% 37 NaOH a t t h e a c t i v e p a s s i v e t r a n s i t i o n . M a z i l l e and U h l i g r e p o r t e d a p p a r e n t a c t i v a t i o n e n e r g i e s o f 44 k J / m o l and 73 k J / m o l , r e s p e c t i v e l y , f o r a 0 .09% C m i l d s t e e l and 38 a 0 .24% C m a r t e n s i t i c s t e e l u n d e r s i m i l a r c o n d i t i o n s . Newman f o u n d t h a t t h e a p p a r e n t a c t i v a t i o n e n e r g y o f a 3% Cr -Mo s t e e l i n 8 M NaOH a t t h e a c t i v e - p a s s i v e t r a n s i t i o n v< was 88 k J / m o l . 6 1 T h e s e r e s u l t s a r e somewhat h i g h e r t h a n the r e g i o n II a c t i v a t i o n e n e r g i e s o f 2 3 - 2 4 k J / m o l d e t e r m i n e d i n t h i s work . Based on s i m i l a r r e s u l t s o b t a i n e d f o r t i t a n i u m by Beck 21 and.a luminum by Hya t t and S p e i d e l , Newman d e c i d e d t h a t the a c t i v a t i o n energy he de te rm ined was r e p r e s e n t a t i v e o f r e g i o n I c r a c k v e l o c i t i e s . The v a l u e s found by Bohnenkamp and M a z i l l e and U h l i g a l s o l i k e l y r e p r e s e n t r e g i o n I SCC. Th i s can be s u b s t a n t i a t e d by t a k i n g the r e g i o n II c r a c k v e l o c i t y found i n t h i s work f o r 12.5 mol/kg NaOH a t - 1.00 V ••, and u s i n g i t to e s t i m a t e the t ime to f a i l u r e of t h e i r s pec imen s , assuming the c r a c k s f o l l o w e d r e g i o n II v e l o c i t i e s t h r o u g h o u t . The r e s u l t s of such c a l c u l a t i o n s are much l ower than the t imes to f a i l u r e r e p o r t e d by Bohnenkamp and M a z i l l e and U h l i g . Th i s i n d i c a t e s t h a t a s u b s t a n t i a l f r a c t i o n o f the t ime to f a i l u r e of t h e i r spec imens was spent i n r e g i o n I. The a c t i v a t i o n energy f o r charge t r a n s f e r , or a c t i v a t i o n c o n t r o l l e d r e a c t i o n s can va ry f rom 21-105 6 7 k J / m o l , w h i l e t h a t f o r aqueous d i f f u s i o n o f i on s i s u s u a l l y 17-18 k J / m o l . 6 3 Both B e c k 6 2 and Hya t t and S p e i d e l found r e g i o n II a c t i v a t i o n e n e r g i e s of ~ 21 kJ/mol f o r t i t a n i u m and aluminum a l l o y s r e s p e c t i v e l y . They c o n c l u d e d t h a t t h i s v a l ue was i n d i c a t i v e o f aqueous mass t r a n s p o r t c o n t r o l l e d p r o c e s s e s . There was more s c a t t e r i n the da ta o b t a i n e d i n 12 .5 m o l / k g NaOH a t E t h a n a t - 1 . 0 0 V . The most c o r r see p r o b a b l e e x p l a n a t i o n f o r t h i s s c a t t e r i s t h a t i t r e f l e c t s t h e l a c k o f c o n t r o l o f t he p o t e n t i a l , and t h u s t h e c o r -r o s i o n c u r r e n t d e n s i t y a t E - c o r r ' A l t h o u g h d i f f e r e n t b a t c h e s o f s t e e l were u sed a t t h e two p o t e n t i a l s , i t has been assumed t h a t t h e r e s u l t s w o u l d have been s i m i l a r i f o n l y one b a t c h had been e m p l o y e d . 4 . 1 . 4 F r a c t o g r a p h y T h e r e i s v e r y l i t t l e f r a c t o g r a p h y p u b l i s h e d i n t h e l i t e r a t u r e t o compare w i t h t h e r e s u l t s o f t h i s work . S c u l l y has s u g g e s t e d t h a t t h e l a c k o f f r a c t o g r a p h y o f SCC o f m i l d s t e e l i n a l k a l i n e s o l u t i o n s i s due to t h e t h i c k c o r r o s i o n f i l m w h i c h o b s c u r e s t h e s u r f a c e . ^ 4 B o h n e n k a m p 3 ^ i n c l u d e d m i c r o g r a p h s show ing i n t e r g r a n u l a r c r a c k i n g a t t he a c t i v e - p a s s i v e t r a n s i t i o n and t r a n s g r a n u l a r c r a c k i n g a t t h e f r e e c o r r o s i o n p o t e n t i a l and P e r d i u s e t a l . p u b l i s h e d 52 a few f r a c t o g r a p h s o f SCC i n 35% NaOH. R e i n o e h l and B e r r y o b s e r v e d p r e d o m i n a n t l y i n t e r g r a n u l a r c r a c k s i n t h e i r w o r k , w i t h i n s t a n c e s o f t r a n s g r a n u l a r c r a c k i n g a c r o s s i n d i v i d u a l g r a i n s . The se r e s u l t s a r e i n g e n e r a l a g r e e -ment w i t h t h e f r a c t o g r a p h y o b s e r v e d i n t h i s work . M ixed i n t e r g r a n u l a r - t r a n s g r a n u l a r c r a c k i n g was o b s e r v e d a t t h e a c t i v e - p a s s i v e t r a n s i t i o n i n each s o l u t i o n and t r a n s -g r a n u l a r c r a c k i n g was o b s e r v e d a t E c o r r ">n 12 .5 m o l / k g NaOH. In g e n e r a l , i n t e r g r a n u l a r c r a c k i n g i s r e l a t e d t o a c t i v e o r p r e - e x i s t i n g p a t h s and t r a n s g r a n u l a r c r a c k i n g t o 45 49 52 s t r a i n - g e n e r a t e d p a t h s . ' ' T h i s a p p l i e s r e g a r d l e s s o f t h e mechan i sm o f c r a c k i n g . The p r e - e x i s t i n g p a t h s a r e u s u a l l y c o n s i d e r e d t o be a l o n g g r a i n b o u n d a r i e s . The g r a i n b o u n d a r i e s a r e a c t i v e w i t h r e s p e c t t o t h e s u r r o u n d i n g g r a i n s due e i t h e r t o t h e s e g r e g a t i o n o f p r e c i p i t a t e s o r d i s s o l v e d e l e m e n t s t o t h e g r a i n b o u n d a r i e s , o r b e c a u s e t h e p a s s i v e f i l m i s more e a s i l y d i s r u p t e d a t a g r a i n b o u n d a r y t h a n i n t h e c e n t r e o f a g r a i n . In s u p p o r t o f t h e f o r m e r , B o h n e n -kamp has d e m o n s t r a t e d t h a t c a r b o n r i c h a r e a s w i l l d i s s o l v e 37 p r e f e r e n t i a l l y i n a l k a l i n e s o l u t i o n s . S t r a i n - g e n e r a t e d p a t h s a r e u s u a l l y c o n s i d e r e d t o o c c u r as a r e s u l t o f t h e emergence o f s l i p s t e p s o r d i s l o c a t i o n s 35 49 66 on t h e s u r f a c e o f t h e g r a i n . ' ' They may be i n t e r -g r a n u l a r i f t h e t h i c k n e s s o f t h e p a s s i v e f i l m on t h e s u r -f a c e , o r t h e r a t e o f r e p a s s i v a t i o n , i s s uch t h a t t h e g r a i n b o u n d a r i e s a r e t h e o n l y l o c a t i o n where t h e a c t i o n o f an e m e r g e n t s l i p s t e p / can e x p o s e s u f f i c i e n t b a r e m e t a l f o r d i s s o l u t i o n t o o c c u r . I f t h e p a s s i v e f i l m on t h e g r a i n s u r f a c e i s t h i n compared t o t h e h e i g h t o f t h e s l i p s t e p s , o r t h e r e p a s s i v a t i o n k i n e t i c s s l o w , t h e n c r a c k p r o p a g a t i o n can o c c u r t r a n s g r a n u l a r l y a l o n g t he p l a n e s o f t h e e m e r g i n g s i i p s t e p s . 91 The f r a c t o g r a p h y o f a s t r e s s c o r r o s i o n c r a c k a l s o depends upon o t h e r f a c t o r s . In t h i s w o r k , t h e p r e s e n c e o f t h e l o n g f l a t , r i b b o n l i k e f e a t u r e s o b s e r v e d i n F i g u r e s 20-25 i s n o t c o n s i s t e n t w i t h d i s s o l u t i o n a l o n g e i t h e r p r e - e x i s t i n g o r s t r a i n - g e n e r a t e d c r a c k p a t h s . E x a m i n a t i o n o f u n c o r -r o d e d f a t i g u e c r a c k s u r f a c e s o f s p e c i m e n s m a c h i n e d f r o m b o t h b a t c h A and B s t e e l r e v e a l e d t h e p r e s e n c e o f many i n c l u s i o n s t r i n g e r s o r i e n t e d i n t h e d i r e c t i o n o f c r a c k p r o p a g a t i o n ( i . e . i n t h e r o l l i n g d i r e c t i o n ) . T h i s i s i l -l u s t r a t e d i n F i g u r e 2 6 . The s t r i n g e r s a r e b o t h l o n g e r and more numerous i n b a t c h B t h a n i n b a t c h A. The a p p e a r a n c e o f t h e t r e n c h e s o c c u p i e d by t h e s e s t r i n g e r s on t h e f a t i g u e p r e c r a c k s u r f a c e s i s a l m o s t i d e n t i c a l t o t h a t o f t h e r i b b o n -l i k e f e a t u r e s o b s e r v e d on t h e s t r e s s c o r r o s i o n s u r f a c e s , w i t h t he e x c e p t i o n t h a t t h e s t r i n g e r s were no t o b s e r v e d on t h e s t r e s s c o r r o s i o n s u r f a c e s . T h i s may have been due t o d i s s o l u t i o n o f t he i n c l u s i o n s by t h e a l k a l i n e s o l u t i o n s , o r t o t h e i r r e m o v a l d u r i n g c l e a n i n g i n t h e i n h i b i t e d a c i d s o l u t i o n . S e v e r a l o f t h e t r e n c h e s o c c u p i e d by t h e s t r i n g e r s i n F i g u r e 26 run s i d e - b y - s i d e , s omet imes a t t h e same l e v e l and somet imes a t d i f f e r e n t l e v e l s . A d j a c e n t t r e n c h e s were o f t e n o b s e r v e d t o f o rm t h e s i d e s o f l a r g e r i d g e s , s u c h as t h o s e v i s i b l e i n F i g u r e 13. The b r i d g i n g by SCC o f t h e l i g a m e n t s between g r o u p s o f s t r i n g e r s can a l s o h e l p t o e x p l a i n t h e f o r m a t i o n o f t h e s e m a c r o s c o p i c r i d g e s , and t h e i r d i r e c t i o n a l i t y . a) Batch A b) Batch B Figure 26 Comparison of uncorroded fa t i gue p re -c rack sur faces of Batch A and Batch B s t e e l . 93 The t r a n s i t i o n f r o m t r a n s g r a n u l a r c r a c k i n g a t E c o r r i n 12 .5 m o l / k g NaOH to m i x e d i n t e r g r a n u l a r - t r a n s g r a n u l a r c r a c k i n g a t - 1.00 V ^ c e can be e x p l a i n e d i n one o f two ways . One e x p l a n a t i o n i s t h a t t h e t r a n s i t i o n o c c u r r e d as a r e s u l t o f a change i n t h e p r o p e r t i e s o f t he p a s s i v e f i l m . A t F_corr ' t h e f i l m on t h e m e t a l s u r f a c e was t h i n o r p o o r l y f o r m e d , and e a s i l y b r o k e n by t h e e m e r g i n g s l i p s t e p s a t t he c r a c k t i p . T h u s , t h e c r a c k f o l l o w e d a p r e d o m i n a n t l y t r a n s -g r a n u l a r , s t r a i n - g e n e r a t e d p a t h . As t h e p o t e n t i a l was i n c r e a s e d t o - 1.00 V , t h e p a s s i v e f i l m became more see r s t a b l e and a d h e r e n t . The e m e r g i n g s l i p s t e p s were l e s s a b l e to d i s r u p t t h e f o r m a t i o n o f t h e f i l m a c r o s s t h e g r a i n s and d i s s o l u t i o n o c c u r r e d a l o n g t h e g r a i n b o u n d a r i e s , where t h e f i l m was more e a s i l y b r o k e n . T h i s e x p l a n a t i o n can a l s o a c c o u n t f o r t he m i x e d i n t e r g r a n u l a r - t r a n s g r a n u l a r c r a c k p a t h o b s e r v e d a t t h e a c t i v e - p a s s i v e t r a n s i t i o n i n t h e o t h e r two s o l u t i o n s . An a l t e r n a t i v e e x p l a n a t i o n f o r t h e change i n c r a c k p a t h f r om t r a n s g r a n u l a r to m ixed i n t e r g r a n u l a r - t r a n s g r a n u l a r i s t h a t a change i n t h e mechan i sm o f c r a c k i n g o c c u r r e d f r om E to - 1.00 V . A mechan i sm such as h y d r o g e n em-c o r r see J 3 b r i t t l e m e n t , w h i c h f a v o u r s s t r a i n g e n e r a t e d p a t h s , c o u l d a c c o u n t f o r t he t r a n s g r a n u l a r c r a c k i n g o b s e r v e d a t E c o r r -One w h i c h f a v o u r e d p r e - e x i s t i n g p a t h s c o u l d t h e n a c c o u n t 94 f o r t h e m ixed i n t e r g r a n u l a r - t r a n s g r a n u l a r c r a c k i n g o b s e r v e d a t t h e a c t i v e - p a s s i v e t r a n s i t i o n i n a l l t h r e e s o l u t i o n s . The f r a c t o g r a p h y o b s e r v e d a t 1 1 5 ° C i n 12 .5 m o l / k g NaOH a t E i s anoma lous and c a n n o t be e x p l a i n e d i n r e l a t i o n t o c o r r r t h e o t h e r s p e c i m e n s . 4 .2 Mechan i sms Any p r o p o s e d mechan i sm must be a b l e t o : e x p l a i n.. s u c c e s s -f u l l y t h e f o l l o w i n g o b s e r v a t i o n s : 1) In t h e f r a c t u r e m e c h a n i c s e x p e r i m e n t s , t h e m e a s u r e d c r a c k v e l o c i t y showed r e g i o n s o f s t r e s s d e p e n d e n c e and s t r e s s i n d e p e n d e n c e . T h i s a p p l i e d t o a l l t h r e e s o l u t i o n s . 2) As d e f i n e d by t h e SSRT, SCC i n t h e 3.35 m o l / k g NaOH and 2.5 m o l / k g NaOH + 0 .42 m o l / k g Na 2 S s o l u t i o n s was l i m i t e d t o a na r row r a n g e o f p o t e n t i a l s a b o u t t he a c t i v e - p a s s i v e t r a n s i t i o n i n e a c h s o l u t i o n . 3) A l t h o u g h SCC was o b s e r v e d a t b o t h E c Q r r and - 1 . 0 0 V i n 12 .5 m o l / k g NaOH, t he m e a s u r e d c r a c k see v e l o c i t y was an o r d e r o f m a g n i t u d e h i g h e r a t t h e l a t t e r . 4) The s t r e s s c o r r o s i o n c r a c k v e l o c i t y i s a b o u t an o r d e r o f m a g n i t u d e f a s t e r i n t h e 12 .5 m o l / k g NaOH s o l u t i o n t h a n i n t h e 3.35 m o l / k g NaOH s o l u t i o n a t 9 2 ° C and - 1.00 V see 5) The s t r e s s c o r r o s i o n c r a c k v e l o c i t y i n t h e 2.5 m o l / k g NaOH + 0 .42 m o l / k g Na,,S s o l u t i o n i s s l o w e r t h a n i n t h e 3.35 m o l / k g NaOH s o l u t i o n a t t h e r e s p e c t i v e a c t i v e - p a s s i v e t r a n s i t i o n i n e a c h s o l u t i o n . 6) The s t r e s s c o r r o s i o n c r a c k v e l o c i t y i n 3 .35 m o l / k g NaOH was s l o w e r a t - 0 .88 V t h a n a t - 1.00 V„ . see see 7) SCC i n t h e 12 .5 m o l / k g NaOH s o l u t i o n i s t e m p e r a t u r e d e p e n d e n t , w i t h an a p p a r e n t a c t i v a t i o n e n e r g y o f ~ 23 k J / m o l a t E c o r r and ~ 24 k J / m o l a t - 1.00 V see 8) T h e r e was no a p p a r e n t change i n t h e c r a c k t i p s o l u t i o n pH f rom t h e b u l k v a l u e i n any o f t he s o l u t i o n s . 9) The f r a c t o g r a p h y o f t he s p e c i m e n s t e s t e d i n 12 .5 m o l / k g NaOH a t E c Q r r was t r a n s g r a n u l a r , w h i l e a t - 1.00 V s c g , i t was m ixed i n t e r g r a n u l a r - t r a n s g r a n u l a r 10) The f r a c t o g r a p h y o f s p e c i m e n s t e s t e d i n b o t h t h e 3.35 m o l / k g NaOH s o l u t i o n and t h e 2.5 m o l / k g NaOH + 0 .42 m o l / k g Na^S s o l u t i o n was m i x e d i n t e r g r a n u l a r -t r a n s g r a n u l a r .at the ac t i ve -pass i ve t r a n s i t i o n in each so lu t i on . 4 .2 .1 The R o l e o f S t r e s s I n t e n s i t y and P a s s i v a t i o n S t r e s s o r s t r a i n i n SCC i s u s u a l l y p o s t u l a t e d t o p r o v i d e a b a r e m e t a l s u r f a c e , f r e e f r om p a s s i v e f i l m s , u • u o . u . • 35 ,48 ,50 ,51 on w h i c h t h e c r a c k i n g p r o c e s s can o c c u r . ' ' ' 96 C r a c k a d v a n c e i s o f t e n c o n s i d e r e d t o o c c u r v i a a c o n t i n u o u s 4 8 5 0 51 c y c l e o f f i l m r u p t u r e , d i s s o l u t i o n and f i l m r e p a i r . ' ' In f r a c t u r e m e c h a n i c s e x p e r i m e n t s SCC' can be s t u d i e d i n b o t h s t r e s s d e p e n d e n t and s t r e s s i n d e p e n d e n t r e g i o n s . F o r e x a m p l e , Newman has s t u d i e d t h e r e g i o n I SCC k i n e t i c s o f a 3%Cr-Mo s t e e l i n 8 M NaOH a t 100° C . 6 1 He c o n c l u d e d t h a t h i s r e s u l t s were c o n s i s t e n t w i t h a mechan i sm i n v o l v i n g r e p e t i t i v e f i l m r u p t u r e a t t h e c r a c k t i p . In t h i s w o r k , t h e e f f e c t s o f ; t e m p e r a t u r e , s o l u t i o n c o m p o s i t i o n and p o t e n t i a l were i n v e s t i g a t e d i n r e g i o n II o f t h e a p p r o p r i a t e l o g v - Kj c u r v e . W i t h i n t h i s r e g i o n , i t was as sumed t h a t t h e r a t e c o n t r o l l i n g s t e p was not s t r e s s a c t i v a t e d and t h a t s u f f i c i e n t b a r e m e t a l f o r d i s s o l u t i o n was a l w a y s a v a i l a b l e a t t h e c r a c k t i p . T h u s , t h e c r a c k r a t e was e s t a b l i s h e d by t h e e n v i r o n m e n t a l c o n d i t i o n s and f i l m r u p t u r e and r e p a i r was no t a c o n s i d e r a t i o n i n t h e d e t e r m i n a t i o n o f a m e c h a n i s m . I f t h e s t r e s s i n t e n s i t y a t t h e c r a c k t i p c o n t r o l s t h e amount o f b a r e s u r f a c e e x p o s e d a t any one t i m e by d i s r u p t i n g , t h e f o r m a t i o n o f a p a s s i v e f i l m , t h e n t h e s t a b i l i t y and r a t e o f g rowth o f t h e f i l m w i l l be a c r i t i c a l f a c t o r i n d e t e r m i n i n g b o t h t he o n s e t and e x t e n t o f r e g i o n II i n a l o g v - Kj. c u r v e . As t h e e l e c t r o c h e m i c a l p o t e n t i a l o f m i l d s t e e l i n a l k a l i n e s o l u t i o n s i s i n c r e a s e d f r o m t h e a c t i v e - p a s s i v e t r a n s i t i o n , t h e p a s s i v e f i l m becomes more 43 44 45 46 a d h e r e n t , and t h e r a t e o f r e p a s s i v a t i o n i n c r e a s e s . ' • * ' A t some p o i n t , a g i v e n r e g i o n II s t r e s s i n t e n s i t y w i l l be u n a b l e t o m a i n t a i n t h e amount o f b a r e s u r f a c e a t t h e c r a c k t i p r e q u i r e d f o r maximum c r a c k p r o p a g a t i o n and t h e c r a c k v e l o c i t y w i l l f a l l t o a r e g i o n I, s t r e s s i n t e n s i t y c o n -t r o l l e d v a l u e . T h u s , i n c r e a s i n g t h e p o t e n t i a l f r o m t h e a c t i v e - p a s s i v e t r a n s i t i o n o f m i l d s t e e l • i n a l k a l i n e s o l u t i o n s has t h e e f f e c t o f s h i f t i n g t h e l o g v - Kj c u r v e t o w a r d s h i g h e r s t r e s s i n t e n s i t i e s . E v e n t u a l l y , t h e s t a -b i l i t y and r e p a s s i v a t i o n r a t e o f t h e s u r f a c e f i l m w i l l be such t h a t a p p r o a c h e s b e f o r e s u f f i c i e n t b a r e m e t a l i s e x p o s e d f o r a s t r e s s c o r r o s i o n c r a c k t o p r o p a g a t e . T h i s e x p l a i n s why t h e c r a c k v e l o c i t y m e a s u r e d a t - 0 .88 V i n 3.35 m o l / k g NaOH was s l o w e r t h a n t h o s e see 3 m e a s u r e d a t - 1 . 0 0 V s c e a t t h e same t e m p e r a t u r e . The i n -c r e a s e d s t a b i l i t y and r e p a s s i v a t i o n r a t e o f t h e s u r f a c e f i l m w i t h i n c r e a s e d p o t e n t i a l f r om the a c t i v e - p a s s i v e t r a n s i t i o n a l s o e x p l a i n s why t h e r e s u l t s o f t h e SSRT showed t h a t SCC o f m i l d s t e e l i n c a u s t i c s o l u t i o n s does no t o c c u r a t p o t e n t i a l s much g r e a t e r t h a n t h e a c t i v e - p a s s i v e t r a n s i -t i o n . 98 4 . 2 . 2 A n o d i c D i s s o l u t i o n A n o d i c d i s s o l u t i o n o f m a t e r i a l a t t he c r a c k t i p has l o n g been c o n s i d e r e d t h e method o f p r o p a g a t i o n o f SCC i n many s y s t e m s . In a l k a l i n e s o l u t i o n s , t he p r o p o s e d s e q u e n c e o f c h a r g e t r a n s f e r and c h e m i c a l r e a c t i o n s l e a d i n g t o t h e d i s s o l u t i o n o f i r o n , and f o r m a t i o n o f a p a s s i v e 44 f i l m a t 2 5 ° C i s as f o l l o w s : Fe + OH" [ F e O H ] g d + e " 7a [ F e O H ] a d = [ F e O H ] ^ d + e " 7b [ F e O H ] a d + OH" F e ( 0 H ) 2 7c ; F e ( 0 H ) 2 + OH" ^ H F e 0 2 + H 2 0 7d F e ( 0 H ) 2 + OH" FeOOH + HgO •+ e " 7e At h i g h e r t e m p e r a t u r e s , t h e c o m p o s i t i o n o f t h e p a s s i v e f i l m i s d e p e n d e n t upon k i n e t i c f a c t o r s and F ^ O ^ may be f o r m e d i n s t e a d o f FeOOH. To be c o n s i s t e n t w i t h d a t a p r e s e n t e d by B i e r n a t and R o b i n s a t 1 0 0 ° C , 7 1 t h e above mechan i sm w i l l be used i n t h e f o l l o w i n g d i s c u s s i o n . The c o n s i d e r a t i o n o f FeOOH o v e r F e ^ ^ , o r v i c e v e r s a , does no t a f f e c t t h e a n a l y s i s . I f c o n t i n u o u s d i s s o l u t i o n o f m a t e r i a l a t t h e c r a c k t i p o c c u r s , t h e n the c r a c k v e l o c i t y , v , may be r e l a t e d t o t h e a n o d i c c u r r e n t d e n s i t y a t t h e c r a c k t i p , i , v i a e q u a t i o n 8 a v = i w . . . . 8 a Fd _ 3 where w i s t h e e q u i v a l e n t w e i g h t o f i r o n ( 2 7 . 9 x 10 k g , d i v a l e n t s p e c i e s ) , F i s t h e F a r a d a y ( 9 . 6 5 x 1 0 4 A . s ) and 3 3 d i s t h e d e n s i t y o f i r o n ( 7 . 8 6 x 10 kg/m ). I t i s no t c l e a r , h o w e v e r , what t o s u b s t i t u t e f o r i i n o r d e r t o be a b l e t o p r e d i c t v . The p o t e n t i a l d i f f e r e n c e between the t e s t p o t e n t i a l , and t h e e q u i l i b r i u m p o t e n t i a l o f t h e M / M n + e l e c t r o d e , E r e v > i s t h e t o t a l o v e r v o l t a g e , n T . T h i s o v e r -v o l t a g e i s a c t u a l l y t h e sum o f s e v e r a l d i f f e r e n t com-4. 70 p o n e n t s : T A . D 99 35 n n + n + iR 9 A D where n i s t h e a c t i v a t i o n o v e r p o t e n t i a l , n i s t h e d i f -f u s i o n o v e r p o t e n t i a l and iR i s t h e p o t e n t i a l d r o p t h r o u g h t h e s o l u t i o n . The v a l u e o f i w i l l depend upon t h e r e -A D l a t i v e v a l u e s o f n , n and i R . In h i g h l y c o n d u c t i v e s o l u t i o n s , such as t h e ones e m p l o y e d i n t h i s w o r k , t h e iR d r o p i s u s u a l l y c o n s i d e r e d t o be s m a l l i n r e l a t i o n t o n , n H D 55 and n . I f t h e m e t a l i s d i s s o l v i n g u n d e r a c t i v a t i o n c o n t r o l 6 7 e q u a t i o n 10 i s o b e y e d : A b a logf]A ••"10 100 where i i s t h e e x c h a n g e c u r r e n t d e n s i t y f o r t h e a n o d i c r e a c t i o n , and b i s t h e T a f e l s l o p e . S i n c e v i s p r o -a p o r t i o n a l t o i > t h e r a t i o o f t h e c r a c k v e l o c i t y p r o d u c e d a a t an a n o d i c o v e r p o t e n t i a l n-j , to t h a t p r o d u c e d a t an A A a n o d i c o v e r p o t e n t i a l n 2 c a n D e e x p r e s s e d i n t e rms o f n-j , A and n 2 as .shown i n e q u a t i o n 1 1 : / \ - < A A l o g v 0 -= l o g / i a \ < r ; n9 - n T 11 T h i s e q u a t i o n assumes t h a t i i s t h e same a t b o t h p o t e n t i a l s . t i o n t e c h n i q u e a r e s u b s t i t u t e d i n t o e q u a t i o n 11... i n p l a c e o f i , an e s t i m a t e ..of t h e . c u r r e n t d e n s i t i e s r e q u i r e d f o r a l a c t i v a t i o n c o n t r o l i n a l l t h r e e s o l u t i o n s may be o b t a i n e d . The r e s u l t s a r e shown i n T a b l e IX, a l o n g w i t h t h e c u r r e n t d e n s i t i e s r e q u i r e d t o s u p p o r t t h e m e a s u r e d r e g i o n II c r a c k v e l o c i t i e s i n e a c h s o l u t i o n ( c a l c u l a t e d f r o m e q u a t i o n 8 ) . C l e a r l y , more t h a n enough c u r r e n t i s a v a i l a b l e t o s u p p o r t t h e c r a c k v e l o c i t i e s m e a s u r e d a t t h e a c t i v e - p a s s i v e t r a n s i -t ions^ ' , ! ! 12 .5 m o l / k g NaOH and t h e s i m u l a t e d w h i t e l i q u o r . The c r a c k v e l o c i t i e s o b s e r v e d a t E - c o r r i n 12 .5 m o l / k g NaOH and a t - 1.00 V s c g i n 3.35 m o l / k g NaOH c a n n o t be a c c o u n t e d f o r by t he above a n a l y s i s . I f t h e v a l u e s o f i c o r r o b t a i n e d by t h e l i n e a r p o l a r i z a -T a b l e J X -\l a 1 ue s o f i R e q u i r e d t o S u p p o r t t h e M e a s u r e d C r a c k V e l o c i t i e s and i P r e d i c t e d From i a c o r r Solut ion Batch E ( V sce) ' V .' (m/s) i required a ? (A/irr) i predicted (A/m 2) 12.5 mol/kg NaOH A A .B -1.00 -1.00 2.4 x 1 0 " 9 2.5 x 1 0 " 8 6.8 x 10 8 .65 680 1800 2.0 + 0.6 4300 + 1300 4500 i 600 3.35 mol/kg NaOH A B -1.00 -1.Q0 7.1 x 1 0 " 9 7.2 x 1 0 " 9 190 200 95 + 42 120 + 2 0 2.5 mol/kg NaOH + 0.42 mol/kg Na2"s B , -0.88 -9 4 x 10 y 110 5700 + 1800 102 Two p o s s i b l e i n t e r p r e t a t i o n s o f t h e r e s u l t s o f T a b l e IX e x i s t . The f i r s t i s t h a t t h e v a l u e o f i „ „ „ c o r r m e a s u r e d by t h e l i n e a r p o l a r i z a t i o n t e c h n i q u e i s t h a t o f a f i l m c o v e r e d s u r f a c e and no t t h a t o f b a r e m e t a l , and t h e d i f f e r e n c e i s g r e a t enough t o a c c o u n t f o r t h e d i s c r e p a n c i e s i n T a b l e IX. E v i d e n c e f o r t h e p r e s e n c e o f a f i l m on t h e m e t a l s u r f a c e a t E e x i s t s . A t a r n i s h f i l m was o b s e r v e d c o r r t o f o rm on t h e s u r f a c e o f t h e l i n e a r p o l a r i z a t i o n s p e c i m e n s as soon as t h e s p e c i m e n p o t e n t i a l was a l l o w e d - t o r e t u r n t o E f r o m - 1.40 V . I f t h e t i p o f a s t r e s s c o r r o s i o n c o r r see c r a c k i s m a i n t a i n e d more f i l m f r e e t h a n t h e m e t a l s u r f a c e a t E , h i g h e r c u r r e n t d e n s i t i e s t h a n p r e d i c t e d wou ld be c o r r 3 r a r e a s o n a b l e r e s u l t . The d i f f e r e n c e between t h e r e q u i r e d and p r e d i c t e d v a l u e s o f i i n 12 .5 m o l / k g NaOH and t h e s i m u l a t e d w h i t e l i q u o r a t t h e a c t i v e - p a s s i v e t r a n s i t i o n can be a c c o u n t e d f o r i f d i s s o l u t i o n a t t h e c r a c k t i p was u n d e r m ixed a c t i v a t i o n - d i f f u s i o n c o n t r o l i n s t e a d o f pu re a c t i v a t i o n c o n t r o l . T h i s i s i l l u s t r a t e d s c h e m a t i c a l l y i n F i g u r e 2 7 . An a n o d i c p o l a r i z a t i o n c u r v e i s drawn as i t wou ld a p p e a r u n d e r a c t i v a t i o n c o n t r o l ( d a s h e d l i n e ) and as m o d i f i e d by a m ixed a c t i v a t i o n - d i f f u s i o n c o n t r o l l e d mechan i sm ( s o l i d l i n e ) . F o r c l a r i t y , t h e axes a r e no t drawn t o s c a l e and E and i a r e shown to be t h e same f o r b o t h E, and E 0 . r e v o 1 c 1 0 3 Figure 27 Anodic p o l a r i z a t i o n curve of bare metal surface under a c t i v a t i o n c o n t r o l and mixed a c t i v a t i o n - d i f f u s i o n c o n t r o l . 104 In t h e p r e s e n t w o r k , and E^ wou ld c o r r e s p o n d t o E c Q r r and - 1.00 V » r e s p e c t i v e l y , i n 12 .5 m o l / k g NaOH. The a n o d i c c u r r e n t d e n s i t y u n d e r m i x e d a c t i v a t i o n - d i f f u s i o n c o n t r o l wou ld be much l o w e r t h a n t h a t p r e d i c t e d by e q u a t i o n 12 f o r a c t i v a t i o n c o n t r o l . In p r i n c i p a l , t h e d i f f u s i o n o v e r v o l t a g e , n°, may be c a l c u l a t e d f r o m e q u a t i o n 1 2 : 7 0 n D 2.303RT 1 og / c n „ D \ 1 2 OHP zF \ c b where R i s t h e gas c o n s t a n t ( 8 . 3 1 4 J / m o l . d e g ) , T i s t h e a b s o l u t e t e m p e r a t u r e , z i s t h e number o f e q u i v a l e n t s e x -c h a n g e d i n t he r e a c t i o n , C g ^ p i s t h e c o n c e n t r a t i o n o f d i f -f u s i n g s p e c i e s a t t he o u t e r H e l m h o l z p l a n e (OHP) and c^ i s t h e c o n c e n t r a t i o n o f d i f f u s i n g s p e c i e s i n t h e b u l k s o l u t i o n ( i n s i d e t h e c r a c k ) . A l t h o u g h i t i s p o s s i b l e f o r e i t h e r d i f f u s i o n o f OH" t o t h e OHP o r HFeO" f r o m t h e OHP t o be r a t e c o n t r o l l i n g ( s e e e q u a t i o n s 7a - 7 e ) , t h e l a t t e r i s more l i k e l y t o o c c u r . A v a l u e o f c^ f o r HFeO" may be c a l c u l a t e d by a s s u m i n g t h a t HFeO,, i n t h e c r a c k i s i n e q u i l i b r i u m w i t h t h e p a s s i v e f i l m c o v e r i n g t h e s i d e s o f t h e c r a c k , bu t a v a l u e f o r i s more d i f f i c u l t t o d e t e r m i n e . Bo th c, and c n u n a r e r e l a t e d t o t h e a n o d i c b OHP c u r r e n t d e n s i t y due t o d i f f u s i o n , i ^ , by e q u a t i o n 1 3 : 7 ^ 105 1 P = i^. (COHP - V •••• 1 3 6 where D i s t h e d i f f u s i o n c o e f f i c i e n t o f t h e i o n and 6 i s t h e d i f f u s i o n l a y e r t h i c k n e s s . D and 6 a r e comp lex f u n c t i o n s o f t e m p e r a t u r e , s o l u t i o n c o m p o s i t i o n , s o l u t i o n 63 72 73 v i s c o s i t y and t r a n s f e r e n c e number o f t h e i o n ' ' and i t was no t p o s s i b l e t o d e r i v e m e a n i n g f u l v a l u e s f o r t he p r e s e n t work . T h u s , v a l u e s o f C Q H P , and t h e r e f o r e n^ f o r t h e t e s t c o n d i t i o n s c o u l d no t be d e t e r m i n e d by t h i s m e t h o d . The s e c o n d i n t e r p r e t a t i o n o f t h e r e s u l t s i n T a b l e IX i s t h a t t h e p r e s e n c e o f a f i l m on t h e s u r f a c e o f t h e l i n e a r p o l a r i z a t i o n s p e c i m e n s o n l y had a s l i g h t e f f e c t on t h e v a l u e o f i and t h a t t h e c r a c k v e l o c i t i e s o b s e r v e d a t c o r r E i n 1 2 . 5 m o l / k g NaOH were due t o a p r o c e s s o t h e r t h a n c o r r 3 r a n o d i c d i s s o l u t i o n o f m a t e r i a l a t t h e c r a c k t i p . The c r a c k v e l o c i t i e s o b t a i n e d a t t h e a c t i v e - p a s s i v e t r a n s i t i o n i n a l l t h e s o l u t i o n s can s t i l l be e x p l a i n e d by a m i x e d a c t i v a t i o n - d i f f u s i o n c o n t r o l l e d d i s s o l u t i o n mechan i sm ( F i g u r e 2 7 ) . E v i d e n c e f o r t h i s i n t e r p r e t a t i o n comes f r o m t h e work 46 o f Hoar and J o n e s . They u s e d s t r a i n i n g e l e c t r o d e e x p e r i -ments t o show t h a t t h e c u r r e n t d e n s i t y on 0 .1% C i n 10 M NaOH a t 1 2 0 ° C r o s e l e s s t h a n 5 t i m e s upon s t r a i n i n g a t 106 p o t e n t i a l s l o w e r t h a n - 1.1 V c e . T h i s shows t h a t w h i l e t he p a s s i v e f i l m f o r m e d a t a c t i v e p o t e n t i a l s i s p o o r l y p r o t e c t i v e , i t can r e d u c e i f r o m t h e v a l u e o b t a i n e d on cl b a r e m e t a l . An i n c r e a s e i n i o f t h e m a g n i t u d e c o r r 3 o b s e r v e d by Hoar and J o n e s i s enough t o a c c o u n t f o r t h e c r a c k r a t e i n 3.35 m o l / k g NaOH a t - 1 . 0 0 V , but no t i n 3 see 12 .5 m o l / k g NaOH a t E c Q r r . A D The v a l u e s o f n and n f o r t h e e x p e r i m e n t s a t t h e a c t i v e - p a s s i v e t r a n s i t i o n i n a l l t h e s o l u t i o n s can be e s t i m a t e d by a s s u m i n g t h a t i ' „ i s i n c r e a s e d by f i v e t i m e s 3 c o r r J on b a r e m e t a l o v e r t h e m e a s u r e d v a l u e . The f o l l o w i n g a d d i t i o n a l a s s u m p t i o n s were made: 1.) t h e v a l u e o f c^ f o r HFeO^ i s t h e v a l u e i n e q u i l i b r i u m w i t h t h e p a s s i v e f i l m on t h e s i d e s o f t h e c r a c k (FeOOH) and 2) E r e v f o r t h e F e / H F e O ^ r e a c t i o n i s d i c t a t e d by t h e v a l u e o f c b d e t e r -mined a b o v e . The e q u i l i b r i u m c o n c e n t r a t i o n o f HFeO^ i n t h e c r a c k a t 9 2 ° C was c a l c u l a t e d f r o m : 7 1 FeOOH + e " ^ HFeO~ 14a EFe00H/HFe0: = "1-314 - 0.072 log [HFeO"], . . . . 14b E f o r e a c h s o l u t i o n a t t he a c t i v e - p a s s i v e t r a n s i t i o n and r e v 9 2 ° C was c a l c u l a t e d f r o m : 7 1 107 HFeO" + H 2 0 + 2 e " =^ Fe + 30H 15a see 15b The pH o f each s o l u t i o n was t a k e n f r o m A p p e n d i x B. The e x c h a n g e c u r r e n t d e n s i t y , i , was t h e n c a l c u l a t e d f o r e a c h e q u a t i o n 10 . An a n o d i c T a f e l s l o p e o f 0 .072 V / d e c a d e a t 68 9 2 ° C was a s s u m e d . T a b l e X shows t h e c a l c u l a t e d v a l u e s A o f c ^ , F _ r e v » and i Q f o r e a c h s o l u t i o n . V a l u e s o f n and n D c a l c u l a t e d f r o m e q u a t i o n s 9 and 10 a r e a l s o t a b u l a t e d . C l e a r l y , b o t h a c t i v a t i o n and d i f f u s i o n p r o c e s s e s a r e i m p o r t a n t t o t h e t o t a l o v e r p o t e n t i a l and t h e a s s u m p t i o n o f a m ixed a c t i v a t i o n - d i f f u s i o n c o n t r o l l e d mechan i sm a t t h e a c t i v e - p a s s i v e t r a n s i t i o n i s j u s t i f i e d . T h i s i s c o n -s i s t e n t w i t h t h e r e s u l t s o f t h e a c t i v a t i o n e n e r g y a n a l y s i s . An a p p a r e n t a c t i v a t i o n e n e r g y o f 24 k J / m o l i s h i g h f o r a 6 3 d i f f u s i o n c o n t r o l l e d r e a c t i o n , and on t h e low end o f t h e r ange o f a c t i v a t i o n e n e r g i e s e x p e c t e d f o r c h a r g e t r a n s f e r c o n t r o l l e d r e a c t i o n s . ^ 7 4 . 2 . 3 Hyd rogen E m b r i t t 1 e m e n t W h i l e h y d r o g e n e m b r i t t l e m e n t has been c o n -s i d e r e d a mechan i sm f o r t h e SCC o f many m e t a l s i n n e u t r a l and a c i d i c s o l u t i o n s , i t has n o t r e c e i v e d much s u p p o r t as r e a c t i o n f r o m t h e assumed v a l u e o f i c o r r (5 i c o r r ) u s i n g Tab le X C a l c u l a t e d O v e r p o t e n t i a l s Based on E s t i m a t e d Va lue s o f i on Bare M e t a l . S o l u t i o n t e s t ( V sce> C b (mol/dm 3 ) ^rev i 0 (A/m 2 ) A n (V) D n. (V) 12 .5 mol/kg NaOH -1 .00 4.4 x 1 0 " 5 -1 .35 3.0 x 10 " I 0. 27 0.08 3.35 mol/kg NaOH -1 .00 4.4 x 1 0 " 5 - 1.28 7.6 X 1 0 " 2 0. 25 0.03 2 .5 mol/kg NaOH + 0.42 mol/kg Na 2S -0 .88 9.4 x 1 0 " 7 -1 .32 2.2 X 1 0 "3 0. 27 0.17 109 an e x p l a n a t i o n f o r t h e c a u s t i c c r a c k i n g o f m i l d s t e e l . R e i n o e h l and B e r r y have s u g g e s t e d t h a t h y d r o g e n e m b r i t t l e -ment may be r e s p o n s i b l e f o r t h e o c c a s i o n a l i n s t a n c e s o f t r a n s g r a n u l a r c r a c k i n g o b s e r v e d i n m i l d s t e e l e x p o s e d t o 2 6 a l k a l i n e s o l u t i o n s . Dahl e t a l . o b s e r v e d an u p t a k e o f h y d r o g e n i n t h e m e c h a n i c a l l y s t r e s s e d z o n e s o f s p e c i m e n s o f a 3%Cr-Mo s t e e l e x p o s e d t o 20% NaOH a t 2 2 5 ° C , b u t d i d n o t f e e l t h a t t h e r e was s u f f i c i e n t e v i d e n c e t o p o s t u l a t e a 74 h y d r o g e n e m b r i t t l e m e n t m e c h a n i s m . P e r d i u s e t a l . a l s o o b s e r v e d an i n c r e a s e i n h y d r o g e n i n t h e m e t a l n e a r t h e c r a c k t i p o f a s t e e l e x p o s e d to 33% NaOH a t 9 0 ° C and a 52 p o t e n t i a l n e a r t h e a c t i v e - p a s s i v e t r a n s i t i o n . They assumed t h a t t h e pH o f t h e s o l u t i o n i n t h e c r a c k f e l l s u f f i c i e n t l y , v i a e q u a t i o n s 7a - 7e , t o a l l o w t h e r e -d u c t i o n o f H + t o a t o m i c h y d r o g e n i n t h e c r a c k , w h i c h s u b s e q u e n t l y e n t e r e d t h e m e t a l m a t r i x and e m b r i t t l e d t h e m a t e r i a l a t t h e c r a c k t i p . The r e s u l t s o f t he p r e s e n t work' show t h a t t h e pH o f t h e c r a c k t i p s o l u t i o n does n o t change f r o m t h e b u l k v a l u e , so t h a t t h e e v o l u t i o n o f h y d r o g e n i n t h e c r a c k must be t h e r m o d y n a m i c a l 1 y p o s s i b l e a t t h e b u l k pH o f t h e s o l u t i o n f o r h y d r o g e n e m b r i t t l e m e n t t o o c c u r . At 9 2 ° C and 1 a t m . , B i e r n a t and R o b i n s have shown t h a t t h e t h e r m o d y n a m i c a l 1 y r e v e r s i b l e p o t e n t i a l f o r h y d r o g e n e v o l u t i o n v i a t he r e d u c t i o n o f H + i s g i v e n by e q u a t i o n 1 6 : 7 1 n o E H + / H 2 = " ° - 1 8 4 6 - 0.072 pH , V s c e . . . 16 T a b l e XI t a b u l a t e s t h e v a l u e o f E u + ' / u and pH o f e a c h o f t h e t h r e e s o l u t i o n s a t 9 2 ° C. The d e t e r m i n a t i o n o f t h e pH o f a l l t h r e e s o l u t i o n s i s shown i n A p p e n d i x B. The p o t e n t i a l o f e v e r y s p e c i m e n i n e x p e r i m e n t s c a r r i e d o u t a t t h e a c t i v e - p a s s i v e t r a n s i t i o n was n o b l e t o t h e a p p r o p r i a t e v a l u e o f E j^+^ ., t h u s e l i m i n a t i n g t h e p o s s i b i l i t y o f h y d r o g e n e v o l u t i o n a t t h e c r a c k t i p o f t h o s e s p e c i m e n s on t h e r m o d y n a m i c g r o u n d s . H y d r o g e n e v o l u t i o n i n t h e c r a c k i s p o s s i b l e i n : 12 .5 m o l / k g NaOH a t E C Q r r , h o w e v e r , and h y d r o g e n e m b r i t t l e -ment c a n n o t be r u l e d o u t as a mechan i sm o f SCC u n d e r t h o s e c o n d i t i o n s . I f t h e SCC a t E _ was due t o h y d r o g e n em-c o r r J 3 b r i t t l e m e n t , i t w o u l d e x p l a i n t h e r a p i d c r a c k r a t e i n r e l a t i o n t o t h e m e a s u r e d v a l u e o f i - c o r r » and t h e change i n f r a c t o g r a p h y f r o m t r a n s g r a n u l a r a t E C Q r r t o a m ixed i n t e r -g r a n u l a r - t r a n s g r a n u l a r a t - 1.00 V s c e - W h i l e t h e a p p a r e n t a c t i v a t i o n e n e r g y o f 23 k J / m o l d e t e r m i n e d a t E C Q r r i n 12 .5 m o l / k g NaOH i s l o w e r t h a n t h e 3 3 - 3 8 k J / m o l o b s e r v e d f o r t he h y d r o g e n e m b r i t t l e m e n t o f s t e e l s i n n e u t r a l aqueous 75 c o n d i t i o n s , h y d r o g e n e m b r i t t l e m e n t s t i l l c a n n o t be r e j e c t e d . The r a t e o f h y d r o g e n e v o l u t i o n w i l l be c o n t r o l l e d by t h e r a t e o f t h e e l e c t r o c h e m i c a l r e a c t i o n s a t t h e c r a c k T a b l e XI C a l c u l a t e d pH and E u + / U a t 92° C. Solut ion pH E H + / H 2 ( V s c e ) 12.5 mol/kg NaOH 13.5 - 1.18 3.35 mol/kg NaOH 12.8 - 1.13 2.5 mol/kg NaOH + 0.42 mol/kg Na2S 12.6 - 1.11 11 2 t i p . Thus t h e c r a c k r a t e w i l l be c o n t r o l l e d by t h e r a t e o f t he e l e c t r o c h e m i c a l r e a c t i o n s , w h i c h i s r e f l e c t e d i n t h e m e a s u r e d a c t i v a t i o n e n e r g y . 4 . 2 . 4 A d s o r p t i o n o f Damaging A n i o n s M a z i l l e and U h l i g have c o n t e n d e d t h a t t he SCC o f m i l d s t e e l i n a l k a l i n e s o l u t i o n s i s due to t h e a d s o r p t i o n o f damag ing a n i o n s o n t o t he m e t a l s u r f a c e a t 38 t h e c r a c k t i p . T h e s e a d - i o n s weaken t h e i n t r a m e t a l 1 i c bonds i n t h e i r o n l a t t i c e and f a c i l i t a t e f a i l u r e o f t h e a l r e a d y s t r e s s e d bonds by m e c h a n i c a l r u p t u r e . S i n c e t h e mechan i sm p r o p o s e d i n e q u a t i o n s 7 a - 7 e f o r a n o d i c d i s s o l u t i o n r e q u i r e s t h e a d s o r p t i o n o f 0H~ o n t o t h e m e t a l s u r f a c e and t h e s u b s e q u e n t r e m o v a l o f Fe by s o l v a t i o n , a d s o r p t i o n p r o c e s s e s a r e o b v i o u s l y i m p o r t a n t to t he SCC o f m i l d s t e e l i n a l k a l i n e s o l u t i o n s . T h e s e e f f e c t s a r e d i f f i c u l t t o q u a n t i f y , h o w e v e r , and i t does n o t a p p e a r n e c e s s a r y t o i n v o k e a m e c h a n i c a l r u p t u r i n g o f bonds when i t can be shown t h a t i t i s p o s s i b l e f o r Fe t o be removed f r om t h e l a t t i c e by d i s s o l u t i o n u n d e r t h e t e s t c o n d i t i o n s . F o r t h e s e r e a s o n s , a mechan i sm i n v o l v i n g t h e a d s o r p t i o n o f a damag ing i o n was n o t c o n s i d e r e d f u r t h e r . 4 . 2 . 5 . A s s e s s m e n t o f Mechan i sms Of t h e t h r e e mechan i sms d i s c u s s e d , o n l y h y d r o g e n e m b r i t t l e m e n t and a n o d i c d i s s o l u t i o n a p p e a r 1 1 3 r e l e v a n t t o t h i s work . In t h e c a s e o f c a u s t i c c r a c k i n g o f m i l d s t e e l , a mechan i sm b a s e d on t h e a d s o r p t i o n o f damag ing a n i o n s does n o t d i f f e r g r e a t l y f r o m one i n v o l v i n g a n o d i c d i s s o l u t i o n . F o r t h i s r e a s o n , i t was no t p u r s u e d i n d e p t h . N e i t h e r h y d r o g e n e m b r i t t l e m e n t no r a n o d i c d i s s o l u t i o n was c a p a b l e o f s o l e l y e x p l a i n i n g a l l t h e o b s e r v a t i o n s o f t h i s work . Hyd rogen e m b r i t t l e m e n t was e l i m i n a t e d as a p o s s i b l e mechan i sm f o r SCC w h i c h o c c u r r e d a t t h e a c t i v e -p a s s i v e t r a n s i t i o n o f a l l t h r e e s o l u t i o n s , bu t c o u l d n o t be r e j e c t e d as a c a u s e o f SCC a t E i n 12 .5 m o l / k g NaOH. c o r r On t h e o t h e r h a n d , even t h o u g h an a n o d i c d i s s o l u t i o n mechan i sm b a s e d on m ixed a c t i v a t i o n - d i f f u s i o n c o n t r o l was f i t t e d t o t he c r a c k v e l o c i t y d a t a i n a l l t h r e e s o l u t i o n s , i t d i d no t p r o v i d e a c o m p l e t e l y s a t i s f a c t o r y a n s w e r . I f an a n o d i c d i s s o l u t i o n mechan i sm was o p e r a t i v e a t E c o r r l n 12.5 m o l / k g NaOH, i t must be a c c e p t e d t h a t t h e p r e s e n c e o f a p a s s i v e f i l m r e d u c e d the m e a s u r e d i c o r r s i g n i f i c a n t l y f r o m t h e v a l u e f o r b a r e m e t a l . Hoar and J o n e s have shown 4 6 t h a t t h i s a s s u m p t i o n i s p r o b a b l y i n c o r r e c t . An a n o d i c d i s s o l u t i o n mechan i sm ba sed on m i x e d a c t i v a t i o n - d i f f u s i o n c o n t r o l can be u sed t o e x p l a i n t h e r e s u l t s o b t a i n e d a t t h e a c t i v e - p a s s i v e t r a n s i t i o n i n a l l t h r e e s o l u t i o n s . The most c o n s i s t e n t i n t e r p r e t a t i o n o f t h e d a t a i s t o 114 a p p l y a h y d r o g e n e m b r i t t l e m e n t mechan i sm a t E c Q r r i n 12 .5 m o l / k g NaOH and a m ixed a c t i v a t i o n - d i f f u s i o n c o n t r o l l e d d i s s o l u t i o n mechan i sm a t t h e a c t i v e - p a s s i v e t r a n s i t i o n i n a l l t h r e e s o l u t i o n s . D i s s o l u t i o n p r o c e s s e s may a l s o c o n t r i -b u t e t o a p o r t i o n o f t h e c r a c k v e l o c i t y a t E C Q r r i n 12 .5 m o l / k g NaOH. A change i n mechan i sm f rom h y d r o g e n e m b r i t t l e m e n t i n t h e a c t i v e r e g i o n t o a n o d i c d i s s o l u t i o n a t t h e a c t i v e - p a s s i v e t r a n s i t i o n has p r e v i o u s l y . . . . been . o b s e r v e d f o r m i l d s t e e l i n 1 M sod ium p r o p r i d h a t e . a t pH 6, and 9 0 ° C 7 6 and i n a C 0 3 - H C 0 3 s o l u t i o n a t pH 9 . 7 9 T h i s i n t e r p r e t a t i o n i s c o n s i s t e n t w i t h t h e a p p a r e n t a c t i v a t i o n e n e r g y d e t e r m i n e d a t - 1.00 V- i n 12 .5 m o l / k g NaOH. The m e a s u r e d v a l u e o f 24 k J / m o l i s h i g h e r t h a n e x p e c t e d f o r a mechan i sm c o n t r o l l e d by aqueous d i f f u s i o n , bu t w i t h i n t h e * 63 67 r a n g e o b s e r v e d f o r c h a r g e t r a n s f e r c o n t r o l l e d r e a c t i o n s . ' The a g r e e m e n t between t h e m e a s u r e d a c t i v a t i o n e n e r g i e s a t - 1.00 V and E i n 12 .5 m o l / k g NaOH may have been see c o r r f o r t u i t o u s , o r i t may i n d i c a t e t h a t t h e r a t e o f h y d r o g e n e v o l u t i o n , and t h u s t h e c r a c k r a t e , i s c o n t r o l l e d by t h e r a t e o f t h e e l e c t r o c h e m i c a l r e a c t i o n s o c c u r r i n g a t t h e t i p o f t he c r a c k . The t r a n s g r a n u l a r f r a c t o g r a p h y o b s e r v e d a t E c Q r r i s c o n s i s t e n t w i t h a h y d r o g e n e v o l u t i o n m e c h a n i s m , w h i l e t h e m i x e d i n t e r g r a n u l a r - t r a n s g r a n u l a r f r a c t o g r a p h y o b s e r v e d a t t h e a c t i v e - p a s s i v e t r a n s i t i o n i s c o n s i s t e n t w i t h a d i s s o l u t i o n m o d e l . 115 The s i m i l a r c r a c k v e l o c i t i e s o b s e r v e d i n t h e 3.35 m o l / k g NaOH s o l u t i o n and t h e s i m u l a t e d w h i t e l i q u o r s u p p o r t a m ixed a c t i v a t i o n - d i f f u s i o n c o n t r o l l e d m e c h a n i s m . The 2-r o l e o f S i o n s i n t h e c a u s t i c c r a c k i n g o f m i l d s t e e l a p p e a r s p r i m a r i l y t o be t o d e l a y t h e o n s e t o f p a s s i v i t y and t h e f o r m a t i o n o f a p r o t e c t i v e f i l m , as p o s t u l a t e d by 41 T r o m a n s . T h i s r e s u l t s i n t h e o c c u r r e n c e o f SCC a t h i g h e r p o t e n t i a l s t h a n o b s e r v e d i n p l a i n NaOH s o l u t i o n s . The s m a l l d i f f e r e n c e i n i c o r r o b s e r v e d between t h e s i m u l a t e d w h i t e l i q u o r and 3.35 m o l / k g NaOH can be a t t r i b u t e d t o e i t h e r t h e change i n OH" c o n c e n t r a t i o n , o r t o t h e p r e s e n c e o f i o n s . The l a c k o f an o b s e r v e d pH change down t h e c r a c k i s a l s o c o n s i s t e n t w i t h t h e above i n t e r p r e t a t i o n o f t h e r e s u l t s . The m o b i l i t y o f OH i s g r e a t e r t h a n t h a t o f 72 o t h e r i o n s , and c o m b i n e d w i t h t h e l a r g e e x c e s s o f OH compared t o HFeO" i n t h e s o l u t i o n i n t h e c r a c k ( T a b l e X ) , makes i t u n l i k e l y t h a t t h e OH" c o n c e n t r a t i o n wou ld have been m e a s u r a b l y d e p l e t e d i n t h e c r a c k . A l t h o u g h the m e a s u r e d v a l u e s o f i c o r r were s y s t e m a t i c a l l y l o w e r f o r B a t c h A s t e e l t h a n f o r B a t c h B s t e e l , a s i g n i f i c a n t d i f f e r e n c e was no t o b s e r v e d w i t h i n t h e e r r o r o f t h e m e a s u r e m e n t s . L i k e w i s e , no f i r m c o n c l u s i o n s c o u l d be r e a c h e d a b o u t t h e d i f f e r e n c e s i n . c r a c k v e l o c i t y o f t h e two b a t c h e s o f s t e e l . Any d i f f e r e n c e s o b s e r v e d i n i c o r r o r c r a c k v e l o c i t y between t h e two b a t c h e s o f s t e e l can be a t t r i b u t e d t o c o m p o s i t i o n a l d i f f e r e n c e s i n t h e s t e e l s , 35 and t h e i r e f f e c t on g r a i n b o u n d a r y s e g r e g a t i o n . The f o r m a t i o n and d i s t r i b u t i o n o f i n c l u s i o n s i n t h e s t e e l s may a l s o have had an e f f e c t on t he c r a c k r a t e s . 4 .3 I n d u s t r i a l I m p l i c a t i o n s The m a j o r i m p l i c a t i o n o f t h i s work f o r t h e p u l p and p a p e r i n d u s t r y l i e s i n t h e a s s o c i a t i o n o f s t r e s s c o r r o s i o n c r a c k i n i t i a t i o n w i t h a p o t e n t i a l r ange a b o u t t h e a c t i v e -p a s s i v e t r a n s i t i o n , and t h e change i n p o t e n t i a l o f t h i s t r a n s i t i o n w i t h s o l u t i o n c o m p o s i t i o n , as shown i n F i g u r e 6. The c o m p o s i t i o n o f t h e w h i t e l i q u o r v a r i e s between m i l l s , 7 7 and t h u s , so w i l l t h e p o t e n t i a l o f maximum s u s c e p t i b i l i t y o f t h e m i l d s t e e l s t r u c t u r e t o SCC. A l t h o u g h t h e E c o r r o f t h e s t e e l u n d e r t h e c o n d i t i o n s o f t h i s work was o u t s i d e t h e m e a s u r e d l i m i t s o f s u s c e p t i - ' b i l i t y t o SCC i n t h e s i m u l a t e d w h i t e l i q u o r , t h e v a l u e o f F-corr u n d e r m i l l c o n d i t i o n s may v a r y , p a r t i c u l a r l y i f o x i d a n t s a r e p r e s e n t i n t h e p r o c e s s s t r e a m . One such 2 - 58 o x i d a n t i s t h e p o l y s u l p h i d e i o n , S (where x > l ) , X w h i c h i s u s u a l l y p r e s e n t i n t h e w h i t e l i q u o r i n s m a l l 117 q u a n t i t i e s as a r e s u l t o f i n a d v e r t e n t a i r o x i d a t i o n o f t h e w h i t e l i q u o r d u r i n g p r e p a r a t i o n o r s t o r a g e , v i a e q u a t i o n 1 7 a , o r i n l a r g e r q u a n t i t i e s by d e l i b e r a t e a i r o x i d a t i o n , o r by t h e a d d i t i o n o f e l e m e n t a l s u l p h u r t o t h e w h i t e l i q u o r a c c o r d i n g t o e q u a t i o n 17b. 2 x S 2 ' + ( x - l ) 0 2 + 2 (x - l )H 2 0 - 2S 2~ + 4 (x - l )0H" ....... 17a (x- l )S + S 2 " S 2 " 17b X 2-In p r a c t i c e , an e q u i l i b r i u m w h i c h c o n t r o l s t h e S c o n c e n t r a -A 2- 2- 2-t i o n i s e s t a b l i s h e d be tween t h e S.x , ,. S and $2^3 i o n s i n s o l u t i o n , as shown i n e q u a t i o n 18. 4S 2 " + 6 ( x - l ) 0 H ' ^ 2 ( x + l ) S 2 " + ( x - l ) S 2 0 3 2 " + 3 (x - l )H 2 0 . . . . 18 2-The e f f e c t o f a d d i n g S v t o w h i t e l i q u o r i s t o r a i s e t h e c o r r o s i o n p o t e n t i a l . ^ 8 T h i s i s shown i n F i g u r e 2 8 , where E c o r r o f m i l d s t e e l i n s i m u l a t e d w h i t e l i q u o r , 60 s a f t e r t h e a d d i t i o n o f t h e g i v e n amount o f e l e m e n t a l s u l p h u r , i s s u p e r i m p o s e d on t h e a n o d i c p o l a r i z a t i o n c u r v e 2-f o r t h e s u l p h u r f r e e s o l u t i o n . A l t h o u g h t h e S i o n c o n -A c e n t r a t i o n was no t d e t e r m i n e d , F i g u r e 28 c l e a r l y shows t h a t t h e f r e e c o r r o s i o n p o t e n t i a l o f m i l d s t e e l may l i e i n t h e a c t i v e o r p a s s i v e r e g i o n s , d e p e n d i n g on t h e c o n -c e n t r a t i o n o f S x p r e s e n t . C o n s e q u e n t l y , E c o r r ° f t h e 118 0.5 Figure 28 E f f ec t of sulphur add i t ion upon E in 2.5 mol/kg NaOH + 0.42 mol/kg Na^S superimposed on the anodic p o l a r i z a t i o n curve fo r the sulphur free s o l u t i on . s t e e l s t r u c t u r e may f a l l w i t h i n t h e p o t e n t i a l r a n g e o f 2-maximum s u s c e p t i b i l i t y t o SCC. I f t h e S i o n c o n c e n t r a -X t i o n change s so t h a t E v a r i e s f r o m a c t i v e t o p a s s i v e c o r r r p o t e n t i a l s , t h e n t h e l e n g t h o f t i m e t a k e n by t h e t r a n s i -p c t i o n may be c r i t i c a l t o t h e o c c u r r e n c e o f SCC. A n o t h e r a p p l i c a t i o n o f t h i s work o f use t o t h e p u l p and p a p e r i n d u s t r y i s t h e c o r r e l a t i o n be tween c r a c k v e l o c i t y and s t r e s s i n t e n s i t y i n 3.35 m o l / k g NaOH and t h e s i m u l a t e d w h i t e l i q u o r . W h i l e t h e r e s u l t s o f t h i s work a r e n o t d i r e c t l y a p p l i c a b l e t o t h e i n d u s t r y , w h i c h i s u n l i k e l y t o use c o l d worked m a t e r i a l f o r c o n s t r u c t i o n p u r p o s e s , t h e y do p r o v i d e g u i d e l i n e s f o r t h e m a g n i t u d e o f s t r e s s c o r r o s i o n c r a c k v e l o c i t y e n c o u n t e r e d i n d u s t r i a l l y , and a l s o t he s t r e s s i n t e n s i t i e s a t w h i c h SCC i s l i k e l y t o o c c u r . In g e n e r a l t e r m s , t h e r e s u l t s o f t h i s work a l s o c l e a r l y show t h a t c a u s t i c c r a c k i n g o f m i l d s t e e l o c c u r s q u i t e r e a d i l y i n c o n d i t i o n s s i m i l a r t o t h o s e e n c o u n t e r e d i n t h e K r a f t p r o c e s s , and i n p a r t i c u l a r , when t h e m e t a l i s s u b j e c t e d t o a l a r g e s t r e s s i n t e n s i t y . An a w a r e n e s s o f t h e s e r e s u l t s may have p r e v e n t e d t h e c a t a s t r o p h i c 3 3 f a i l u r e o f a c o n t i n u o u s d i g e s t o r i n A l a b a m a . The p o s s i b i l i t y t h a t SCC, by a n o d i c d i s s o l u t i o n o r h y d r o g e n e m b r i t t l e m e n t , may o c c u r a t E c o r r i n w h i t e l i q u o r c a n n o t be e l i m i n a t e d . J u d g i n g by t h e r e s u l t s o b t a i n e d i n t h e 12 .5 m o l / k g NaOH s o l u t i o n , t h e e x p e c t e d c r a c k v e l o c i t i e s s h o u l d be a t l e a s t an o r d e r o f m a g n i t u d e l o w e r t h a n t h o s e m e a s u r e d a t t he a c t i v e - p a s s i v e t r a n s i t i o n . C h a p t e r 5 CONCLUSION 5.1 C o n c l u s i o n s The r e s u l t s o f t h e p r e s e n t work on SCC o f A IS I C -1018 s t e e l i n a 12 .5 m o l / k g NaOH s o l u t i o n , a 3.35 m o l / k g NaOH s o l u t i o n and a 2.5 m o l / k g NaOH + 0 .42 m o l / k g N a ? S ; s o l u t i o n s u p p o r t t h e f o l l o w i n g c o n c l u s i o n s . A l l d i r e c t c o m p a r i s o n s a r e between s p e c i m e n s m a c h i n e d f r om t h e same b a t c h o f s t e e l . i ) The p o t e n t i a l r e g i m e o f maximum s u s c e p t i b i l i t y t o SCC i n 3.35 m o l / k g NaOH and 2.5 m o l / k g NaOH + 0 .42 m o l / k g N a 2 S a t 9 2 ° C o c c u r r e d a t p o t e n t i a l s n e a r t h e a c t i v e - p a s s i v e t r a n s i t i o n i n each s o l u -t i o n . Maximum s u s c e p t i b i l i t y was o b s e r v e d a t - 1.00 V i n 3.35 m o l / k g NaOH and - 0 .88 V see a see i n 2.5 m o l / k g NaOH + 0 .42 m o l / k g N a 2 S . i i ) The s t e e l e x h i b i t e d b o t h s t r e s s i n t e n s i t y d e p e n d e n t and s t r e s s i n t e n s i t y i n d e p e n d e n t c r a c k -i n g i n e a c h o f t h e t h r e e s o l u t i o n s . 121 122 i i i ) The r e g i o n II c r a c k v e l o c i t y i n 12 .5 m o l / k g NaOH a t 9 2 ° C, was f a s t e r a t - 1.00 V ( see (- 2.5 x 1 0 " 8 m/s) t h a n a t E ( , Q r r (~ 2.4 x 1 0 " 9 m / s ) . i v ) The r e g i o n II c r a c k v e l o c i t y a t - 1 . 0 0 V S 0 6 and 9 2 ° C was f a s t e r i n 12 .5 m o l / k g NaOH (~ 6.9 x 1 0 " 8 m / s ) , t h a n i n 3.35 m o l / k g NaOH (~ 7.2 x 1 0 " 9 m / s ) . v) The r e g i o n II c r a c k v e l o c i t y a t 9 2 ° C and a t t he most s u s c e p t i b l e p o t e n t i a l f o r SCC i n e a c h s o l u t i o n ( - 1 . 0 0 V „ and - 0 .88 V ) was f a s t e r see see i n 3.35 m o l / k g NaOH (~ 7.2 x 1 0 " 9 m/s) t h a n i n 2.5 m o l / k g NaOH + 0 .42 m o l / k g N a 2 S (~ 3.3 x 1 0 " 9 m / s ) . v i ) An A r r h e n i u s r a t e l a w , w i t h an a p p a r e n t a c t i v a -t i o n e n e r g y o f 24 + 1 k J / m o l , was o b e y e d by t h e c r a c k v e l o c i t y i n 12 .5 m o l / k g NaOH a t - 1.00 V $ c e The c r a c k v e l o c i t y i n 12 .5 m o l / k g NaOH a t E c o r r f o r a d i f f e r e n t b a t c h o f s t e e l was a l s o f i t t e d t o an A r r h e n i u s r a t e law and an a p p a r e n t a c t i v a -t i o n e n e r g y o f 23 + 9 k J / m o l was o b t a i n e d . 1 23 v i i ) C r a c k i n g f o l l o w e d a t r a n s g r a n u l a r p a t h i n 12 .5 m o l / k g NaOH a t E , w i t h one e x c e p t i o n . 3 c o r r r A m i x e d t r a n s g r a n u l a r - i n t e r g r a n u l a r p a t h was f o l l o w e d a t - 1.00 V i n 12 .5 m o l / k g NaOH and see 3 3.35 m o l / k g NaOH, and a t - 0 .88 V g c e i n 2.5 m o l / k g NaOH + 0 .42 m o l / k g N a 2 S . v i i i ) No d e t e c t a b l e change i n pH f r o m t h e b u l k v a l u e o c c u r r e d i n t h e l i q u i d t r a p p e d a t t h e c r a c k t i p . i x ) The r e s u l t s a r e b e s t e x p l a i n e d by p o s t u l a t i n g a mechan i sm i n v o l v i n g b o t h h y d r o g e n e m b r i t t l e -ment and m ixed a c t i v a t i o n - d i f f u s i o n c o n t r o l l e d a n o d i c d i s s o l u t i o n a t E „ i n 12 .5 m o l / k g NaOH c o r r 3 and a mechan i sm i n v o l v i n g m ixed a c t i v a t i o n -d i f f u s i o n c o n t r o l l e d a n o d i c d i s s o l u t i o n a t t h e a c t i v e - p a s s i v e t r a n s i t i o n i n each s o l u t i o n . The p o s s i b i l i t y o f h y d r o g e n e m b r i t t l e m e n t can be e l i m i n a t e d a t t h e a c t i v e - p a s s i v e t r a n s i t i o n i n each s o l u t i o n on t h e r m o d y n a m i c g r o u n d s . 5.2 1) S u g g e s t i o n s f o r F u t u r e Work I n v e s t i g a t e t h e e f f e c t s o f h e a t t r e a t m e n t on SCC, i n p a r t i c u l a r w i t h . r e s p e c t t c t h e m i c r o s t r u c t u r e 124 o f t h e h e a t a f f e c t e d z o n e s o f w e l d s . i i ) I n v e s t i g a t e t h e SCC o f m i l d s t e e l i n t h e 3.35 m o l / k g NaOH and 2.5 m o l / k g NaOH + 0 .42 m o l / k g Na^S unde r p r e s s u r e and t e m p e r a t u r e c o n d i t i o n s s i m u l a t i n g t h e e n v i r o n m e n t i n s i d e a K r a f t p r o c e s s d i g e s t o r . i i i ) E x t e n d t h e f r a c t u r e m e c h a n i c s s t u d y t o i n c l u d e a r a n g e o f p o t e n t i a l s up t o t h e a c t i v e - p a s s i v e t r a n s i t i o n i n each s o l u t i o n i n o r d e r t o b e t t e r d e t e r m i n e w h e t h e r t h e mechan i sm o f SCC i s h y d r o g e n e m b r i t t l e m e n t , o r a n o d i c d i s s o l u t i o n , and t h e p o t e n t i a l a t w h i c h t h e t r a n s i t i o n o c c u r s . 1 25 B I B L I O G R A P H Y S p e i d e l , M . 0. and F o u r t , P. M. , S t r e s s C o r r o s i o n  C r a c k i n g and Hyd rogen E m b r i t t l e m e n t o f I r o n Based  A l T o y s , e d . R.W. S t a e h l e , J . Hochman, R. D. M c C r i g h t and J . E. S l a t e r , NACE, H o u s t o n , ( 1 9 7 7 ) , p p . 5 7 - 6 0 . S t a e h l e , R. W. , F u n d a m e n t a l A s p e c t s o f S t r e s s  C o r r o s i o n C r a c k i n g , e d . R. W. S t a e h l e , A. J . F o r t y , and D. van R o o y e n , NACE, H o u s t o n , ( 1 9 6 9 ) , pp . 3 - 1 4 . H e r t z b e r g , R. W., D e f o r m a t i o n and F r a c t u r e M e c h a n i c s  o f E n g i n e e r i n g M a t e r i a l s , J . W i l e y & S o n s , New Y o r k , ( 1 9 7 6 ) . P a r k i n s , R. N . , B r i t . C o r r . J . , 7 , 1 5 , ( 1 9 7 2 ) . A m b r o s e , J . R. and K r u g e r , J . , C o r r o s i o n , 2 8 , 3 0 , ( 1 9 7 2 ) . P o u r b a i x , M . , T h e o r y o f S t r e s s C o r r o s i o n C r a c k i n g  i n A l l o y s , e d . J . C. S c u l l y , NATO, B r u s s e l s , ( 1 9 7 1 ) , pp . 4 4 2 - 4 4 8 . P a r k i n s , R . N . , T h e o r y o f S t r e s s C o r r o s i o n C r a c k i n g  i n A l l o y s , e d . J . C . S c u l l y , NATO, B r u s s e l s , ( 1 9 7 1 ) , pp . 4 4 9 - 4 6 8 . H u m p h r i e s , M . J . and P a r k i n s , R . N . , C o r r o s i o n S c i e n c e , 7, 7 4 7 , ( 1 9 6 7 ) . S t r e s s C o r r o s i o n C r a c k i n g : The S low S t r a i n Ra te  T e c h n i q u e , STP 665 , e d . G. M. U g i a n s k y and J . H. P a y e r , ASTM, P h i l a d e l p h i a , ( 1 9 7 9 ) . P a r k i n s , R. N . , S t r e s s C o r r o s i o n C r a c k i n g : The S low  S t r a i n Ra te T e c h n i q u e , STP 6 6 5 , e d . G. M. U g i a n s k y and J . H. P a y e r , ASTM, P h i l a d e l p h i a , ( 1 9 7 9 ) , p p . 5 - 2 5 . P a y e r , J . H . , B e r r y , W. E. and B o y d , W. K., S t r e s s  C o r r o s i o n C r a c k i n g : The S low S t r a i n Rate T e c h n i q u e , STP 6 6 5 , e d . G. M. U g i a n s k y and J . H. P a y e r , ASTM, P h i l a d e l p h i a , ( 1 9 7 9 ) , pp . 6 1 - 7 7 . P o w e l l , D. T . and S c u l l y , J . C , C o r r o s i o n , 2 4 , 1 5 1 , ( 1 9 6 8 ) . 126 13 D i e g l e , R.B. and B o y d , W. K. , S t r e s s C o r r o s i o n C r a c k i n g : The S low S t r a i n Ra te T e c h n i q u e , STP 6 6 5 , e d . G. M. U g i a n s k y and J . H. P a y e r , ASTM, P h i l a d e l p h i a , ( 1 9 7 9 ) , pp . 2 1 - 4 6 . 1.4 Mom, A . J . , D e n c h e r , R. T . , v . d . Wekken, C. J . and S h u l t z e , W. A . , S t r e s s C o r r o s i o n C r a c k i n g : The  S low S t r a i n Rate T e c h n i q u e , STP 6 6 5 , e d . G. M. U g i a n s k y and J . H. P a y e r , ASTM, P h i l a d e l p h i a , ( 1 9 7 9 ) , pp . 3 0 5 - 3 1 9 . 15 S c u l l y , J . C , S t r e s s C o r r o s i o n C r a c k i n g : The S low  S t r a i n Rate T e c h n i q u e , STP 6 6 5 , e d . G. M. U g i a n s k y and J . H. P a y e r , ASTM, P h i l a d e l p h i a , ( 1 9 7 9 ) , pp. 2 3 7 - 2 5 3 . 16 B r o w n , B. F. , Met . Rev . , j_3, 1 71 , (1 9 6 8 ) . 17 W e i , R. P . , F u n d a m e n t a l A s p e c t s o f S t r e s s C o r r o s i o n  C r a c k i n g , e d . R. W. S t a e h l e , A. J . F o r t y and D. van R o o y e n , NACE, H o u s t o n , ( 1 9 6 9 ) , pp . 1 0 4 - 1 1 1 . 18 K n o t t , J . F. , F u n d a m e n t a l s o f F r a c t u r e M e c h a n i c s , J . W i l e y and S o n s , New Y o r k , ( 1 9 7 3 ) . 19. B r o w n , B. F. and S r a w l e y , J . E . , P l a n e S t r a i n C r a c k  T o u g h n e s s T e s t i n g , STP 4 1 0 , ASTM, P h i l a d e l p h i a , ( 1 9 6 6 ) . 20 ASTM E 3 9 9 - 7 8 a , ASTM, P h i l a d e l p h i a , 1978 . 21 H y a t t , M. V. and S p e i d e l , M. 0. , A d v a n c e s i n  C o r r o s i o n S c i e n c e and T e c h n o l o g y , V o l . 2, e d . M. G. F o n t a n a and R. W. S t a e h l e , P lenum P r e s s , New Y o r k , (1 972) , pp . 1 1 5 - 3 3 5 . 22 B l a c k b u r n , M. J . , F e e n y , J . A . , and B e c k , T . R. , A d v a n c e s i n C o r r o s i o n S c i e n c e and T e c h n o l o g y , V o l . 3 , e d . M. G. F o n t a n a and R. W. S t a e h l e , P lenum P r e s s , New Y o r k , (1 973) , pp . 6 2 - 2 9 2 . 23 M c K i n t y r e , P . , S t r e s s C o r r o s i o n C r a c k i n g and  H y d r o g e n E m b r i t t l e m e n t o f I r o n Ba sed A l l o y s , e d . R. W. S t a e h l e , J . Hockmann, R. D. M c C r i g h t and J . E. S l a t e r , NACE, H o u s t o n , (1977) pp . 7 8 8 - 7 9 7 . 127 24 L e e , L . P . and T r o m a n s , D., E n v i r o n m e n t S e n s i t i v e  F r a c t u r e o f E n g i n e e r i n g M a t e r i a l s , e d . Z. A. F o u r o u l i s , A IME, New Y o r k , ( 1 9 7 9 ) , pp . 2 3 2 - 2 4 9 . 25 R u s s e l l , A. and T r o m a n s , D., Met . T r a n s . A , 1 OA, 1 2 2 9 , ( 1 9 7 9 ) . 26 R e i n o e h l , J . E. and B e r r y , W. E. , C o r r o s i o n , 2 8 , 1 5 1 , ( 1 9 7 2 ) . 27 S c h m i d t , H. W. , G a g n e r , P. J . , H e i n e m a n n , G. , P o g a c a r , C. F. and Wyche, E. H. , C o r r o s i o n , 7_, 2 9 5 , ( 1 9 5 1 ) . 28 C h a m p i o n , F. A . , Chem. and I nd . , 967 , ( 1 9 5 7 ) . 29 B e r k , A. A. and W a l d e c k , W. F. , Chem. Engng . 5_7, 2 3 5 , ( 1 9 5 0 ) . 30 P o u l s o n , B . , H e n n i k s o n , L. and A r u p , H . , B r i t . C o r r . J . , 9 , 91 , ( 1 9 7 4 ) . 31 T o w n s e n d , H. E . , Mat . P r o t . , V\_, 3 3 , (1 9 7 2 ) . 32 C h a r l t o n , R . S . , Mat . P e r f . , 1_7 , 27 , (1 9 7 8 ) . 33 M a c M i l l a n B l o e d e l I n c . , P i n e H i l l s , A l a b a m a , ( 1 9 8 0 ) . 34, P a r k i n s , R . N . , F u n d a m e n t a l A s p e c t s o f S t r e s s C o r r o s i o n C r a c k i n g , e d . R. 'W. S t a e h l e , A. J . F o r t y , and D. van R o o y e n , NACE, H o u s t o n , ( 1 9 6 9 ) , pp . 3 6 1 -373 . 35 P a r k i n s , R. N . , S t r e s s C o r r o s i o n C r a c k i n g and  H y d r o g e n E m b r i t t l e m e n t o f I r o n Based A l l o y s , e d . R. W. S t a e h l e , J . Hochmann, R. D. M c C r i g h t and J . E. S l a t e r , NACE, H o u s t o n , ( 1 9 7 7 ) , p p . 6 0 1 - 6 2 4 . 36 C a r t e r , C. S. and H y a t t , M. V . , S t r e s s C o r r o s i o n  C r a c k i n g and H y d r o g e n E m b r i t t l e m e n t o f I r o n Ba sed  A l 1 o y s , e d . R. W. S t a e h l e , J . Hochmann, R. D. M c C r i g h t and J . E. S l a t e r , NACE, H o u s t o n , ( 1 977 ), pp . 5 2 4 - 6 0 0 . 37 Bohnenkamp, K. , F u n d a m e n t a l A s p e c t s o f S t r e s s  C o r r o s i o n C r a c k i n g , e d . R. W. S t a e h l e , A. J . F o r t y , and D. van R o o y e n , NACE, H o u s t o n , ( 1 9 6 9 ) , pp . 3 7 4 - 3 8 3 . 128 38 M a z i l l e , H. and U h l i g , H. H . , C o r r o s i o n , 2 8 , 4 2 7 , ( 1 9 7 2 ) . 39 H u m p h r i e s , M. J . a n d - P a r k i n s , R. N . , F u n d a m e n t a l  A s p e c t s o f S t r e s s C o r r o s i o n C r a c k i n g , e d . R. W. S t a e h l e , A. J . F o r t y and D. van R o o y e n , NACE, H o u s t o n , ( 1 9 6 9 ) , pp . 3 8 4 - 3 9 5 . 40 G r a f e n , H. , C o r r o s i o n S c i e n c e , 7_, 1 77 , ( 1 9 6 7 ) . 41 T r o m a n s , D. , J . E l e c t r o c h e m . S o c . , 127 , 1 2 5 3 , ( 1 9 8 0 ) . 42 B r u s i c , V. O x i d e s and O x i d e F i l m s , V o l . I, e d . J . W. D i g g l e , M a r c e l D e k k e r , I n c . , New Y o r k , ( 1 9 7 2 ) , pp . 2 - 9 1 . 43 M a c D o n a l d , D. D. and R o b e r t s , B . , E l e c t r o c h e m i c a  A c t a , 2_3, 781 , ( 1 9 7 8 ) . 44 S c h r e b l e r Guzman, R. S . , V i l c h e , J . R. and A r v i a , A. J . , E l e c t r o c h e m i c a A c t a , 2 4 , 3 9 5 , (1 9 7 9 ) . 45 D i e g l e , R . .B . and V e r m i l y e a , D. A . , C o r r o s i o n , 3 2 , 4 1 1 , ( 1 9 7 6 ) . 46 H o a r , T . P. and J o n e s , R. W. , C o r r o s i o n S c i e n c e , 1 3 , 7 2 5 , ( 1 9 7 3 ) . 47 D i e g l e , R. B. and V e r m i l y e a , D. A . , J . E l e c t r o c h e m . S o c . , 1_2^, 1 8 0 , (1 9 7 5 ) . 48 S c u l l y , J . C , M e t a l S c i e n c e , 1_7 , 2 9 0 , (1 9 7 8 ) . 49 S c u l l y , J . C , C o r r o s i o n S c i e n c e , 20 , 997 , (1 9 8 0 ) . 50 S t a e h l e , R. W. , S t r e s s C o r r o s i o n C r a c k i n g and  H y d r o g e n E m b r i t t l e m e n t o f I r o n Based A l l o y s , e d . R. W. S t a e h l e , J . Hochmann, R. D. M c C r i g h t , and J . E. S l a t e r , NACE, H o u s t o n , ( 1 9 7 7 ) , pp . 1 8 0 -2 0 7 . 51 V e r m i l y e a , D. A. and D i e g l e , R. B . , C o r r o s i o n , 3 2 , 2 6 , ( 1 9 7 6 ) . 52 P e r d i u s , F. , B r a b e r s , M. and Van H a u t e , A . , Mechan i sms o f E n v i r o n m e n t S e n s i t i v e C r a c k i n g o f  E n g i n e e r i n g M a t e r i a l s , e d . P. R. Swann, F. P. F o r d , and A. R. C. Westwood, The M e t a l s S o c i e t y , L o n d o n , ( 1 9 7 7 ) , pp . 5 3 - 6 5 . 129 53.. B i g n o l d , G. J . , C o r r o s i o n , 2 8 , 307 , ( 1 9 7 2 ) . 54 D o i g , P. and F l e w i t t , P. E. J . , Met . T r a n s . A. , 9A, 3 5 7 , ( 1 9 7 8 ) . 55 M e l v i l l e , P. H. , B r i t . C o r r . J . , 1_4, 1 5 , (1 9 7 9 ) . 56 M a g e n s e n , M. , Maahn, E. , and B e c h - N i e l s o n , G . , B r i t . C o r r . J . , ]_]_, 181 , (1 9 7 6 ) . 57 P u l p and P a p e r M a n u f a c t u r e , V o l . 1, 2nd e d . , e d . R. G. M a c d o n a l d , M c G r a w - H i l l , 1 9 6 9 . 58 W e n s l e y , D. A. and C h a r l t o n , R. S . , C o r r o s i o n , 3 6 , 3 8 5 , ( 1 9 8 0 ) . 59 P a p p , J . , C e l l u l o s e C h e m i s t r y and T e c h n o l o g y , 5_, 1 4 7 j ( 1 9 7 1 - j - ; 60 R a u d s e p p , R., M . A p . S c . T h e s i s , U n i v e r s i t y o f B r i t i s h C o l u m b i a , ( 1 9 8 1 ) . 61 Newman, J . F. , Mechan i sms o f E n v i r o n m e n t S e n s i t i v e  C r a c k i n g o f M a t e r i a l s , e d . P. R. Swann, F. P. F o r d , and A. R. C. Wes twood, The M e t a l s S o c i e t y , L o n d o n , ( 1 9 7 7 ) , pp. 1 9 - 3 1 . 62 B e c k , T . R., The T h e o r y o f S t r e s s C o r r o s i o n  C r a c k i n g i n A l l o y s , e d . J . C. S c u l l y , NATO, B r u s s e l s , (1971) , pp . 6 4 - 8 5 . 63 G l a s s t o n e , S . , La i d l e r , K. J . , and E y r i n g , H . , The T h e o r y o f Ra te P r o c e s s e s , M c G r a w - H i l l , New Y o r k , ( 1 9 4 1 ) . 64 S c u l l y , J . C , S t r e s s C o r r o s i o n C r a c k i n g and  Hyd rogen E m b r i t t l e m e n t o f I r o n Based A l l o y s , e d . R. W. S t a e h l e , J . Hochmann, R . -D. M c C r i g h t and J . E. S l a t e r , NACE, H o u s t o n , 1 9 7 7 , pp . 4 9 6 - 5 0 8 . 65 L e e s , D. J . , Mechan i sms o f E n v i r o n m e n t S e n s i t i v e  C r a c k i n g o f M a t e r i a l s , e d . P. R. Swann, F. P. F o r d , and A. R. C. Wes twood, The M e t a l s S o c i e t y , L o n d o n , ( 1 9 7 7 ) , pp . 5 5 7 - 5 7 3 . 66 S t a e h l e , R. W., S t r e s s C o r r o s i o n C r a c k i n g and  H y d r o g e n E m b r i t t l e m e n t o f I r on Ba sed A l l o y s , e d . R. W. S t a e h l e , J . Hochmann, R. D. M c C r i g h t and J . E. S l a t e r , NACE, H o u s t o n , 1 9 7 7 , pp . 1 8 0 - 2 0 7 . 1 30 67 W e s t , J . M. , E l e c t r o d e p o s i t i o n and C o r r o s i o n  P r o c e s s e s , D. van N o s t r a n d , L o n d o n , ( 1 9 6 5 ) . 68 D o i g , P. and F l e w i t t , P . E . J . , C o r r . S c i . , 1 7 , 3 6 9 , ( 1 9 7 7 ) . 69 S t e r n , M. and G e a r y , A . L . , J . E l e c t r o c h e m . S o c . , 1 0 4 , 5 6 , ( 1 9 5 7 ) . 70 B o c k r i s , J . O 'M. and R e d d y , A . K . N . , Modern  E T e c t r o c h e m i s t r y , V o l . 2, P l e n u m , New Y o r k , (1 9 7 0 ) . 71 B i e r n a t , R. J . and R o b i n s , R. G. , E l e c t r o c h e m i c a  A c t a , 1 7., 1261 , (1 9 7 2 ) . 72 M o o r e , W. J . , P h y s i c a l C h e m i s t r y , 4 th E d . , P r e n t i c e - H a l l , L o n d o n , ( 1 9 7 2 ) . 73 V e t t e r , K . J . , E l e c t r o c h e m i c a l K i n e t i c s , A c a d e m i c P r e s s , New Y o r k , ( 1 9 6 7 ) . 74 D a h l , L . , D a h l g r e n , T . , and L a g m y r , N . , H i gh  T e m p e r a t u r e H i gh P r e s s u r e E l e c t r o c h e m i s t r y i n  Aqueous S o l u t i o n s , e d . R. W. S t a e h l e , D. de G. J o n e s and J . E. S l a t e r , NACE, H o u s t o n , ( 1 9 7 6 ) , pp . 5 3 3 - 5 4 5 . 75 K e r n s , G. E . , Wang, M. T . and S t a e h l e , R. W. , S t r e s s C o r r o s i o n C r a c k i n g and H y d r o g e n E m b r i t t l e m e n t  o f I r o n Ba sed A l l o y s , e d . R. W. S t a e h l e , J . Hochmann, R. D. M c C r i g h t and J . E. S l a t e r , NACE, H o u s t o n , ( 1 9 7 7 ) , pp . 7 0 0 - 7 3 5 . 76 J o n e s , de G. D., Newman, J . F. and H a r r i s o n , R. P . , P r o c e e d i n g s o f t h e 5 th I n t e r n a t i o n a l C o n g r e s s on  M e t a l l i c C o r r o s i o n , e d . N. S a t o , NACE, H o u s t o n , ( 1 9 7 4 ) , pp . 4 3 4 - 4 3 8 . 77 M u e l l e r , W. A . , TAPPI , 4 0 , 1 29 , (1 9 5 7 ) . 78 B r o w n , B. F . , Fu j i i , C. T . , D a h l b e r g , E. P . , J . E l e c t r o c h e m . S o c , 1 16 , 21 8, (1 9 6 9 ) . 79 P a r k i n s , R. N. , w r i t t e n d i s c u s s i o n o f R e f . 52 . 131 APPENDIX A The E f f e c t o f R e s i d u a l S t r e s s e s on t h e Shape o f t h e C r a c k f r o n t The t e n d e n c y o f t h e s t r e s s c o r r o s i o n c r a c k s t o l e a d s e v e r a l m i l l i m e t e r s f r o m the edges o f t h e s p e c i m e n s , and l a g on t h e s u r f a c e c a u s e d s p e c u l a t i o n as t o w h e t h e r t h i s c h a r a c t e r i s t i c shape was due t o e n v i r o n m e n t a l i n -f l u e n c e s , o r t o r e s i d u a l s t r e s s e s l e f t i n t h e b a r s f r o m t h e f o r m i n g p r o c e s s . T h i s was i n v e s t i g a t e d by e q u a l l y m i l l i n g t h e s i d e s o f a 31 .8 mm x 31 .8 mm c r o s s - s e c t i o n , c o l d r o l l e d C -1018 s t e e l b a r t o a s q u a r e c r o s s - s e c t i o n o f 25 .4 mm x 25 .4 mm. DCB s p e c i m e n s were t h e n m a c h i n e d f r om t h e b a r as p r e v i o u s l y d e s c r i b e d . The c h e m i c a l c o m p o s i t i o n o f t h e b a r was s i m i l a r to t h e c o m p o s i t i o n o f t h e o t h e r b a r s r e p o r t e d i n T a b l e I, and t h e y i e l d s t r e n g t h a t 2 0 ° C was 595 MPa. T h r e e o f t h e s e s p e c i a l l y m a c h i n e d s p e c i m e n s were t e s t e d i n 3.35 m o l / k g NaOH a t - 1.00 V s c e and 9 2 ° C. F i g u r e A l shows a m a c r o p h o t o g r a p h o f t h e s t r e s s c o r r o s i o n c r a c k s u r f a c e o f one o f t h e s e s p e c i m e n s . A l t h o u g h t h e c r a c k f r o n t o f t h e s p e c i m e n s t i l l l a g g e d a t t h e e d g e s , t h e c r a c k has o b v i o u s l y p r o p a g a t e d some d i s t a n c e on t h e 1 3 2 R g u r e Al S t ress co r ros ion crack surface of specimen machined from a 32 mm x 32 mm bar. Tested in 3.35 mol/kg NaOH at -1.00 V „ and 92° C. KT = 48-53 MPa/m. 133 s u r f a c e o f t h e s p e c i m e n . T h i s i s i n c o n t r a s t t o s p e c i m e n s m a c h i n e d f r om t h e 2 5 . 4 mm x 25 .4 mm b a r s ( F i g u r e 1 3 ) , f o r w h i c h t h e s t r e s s c o r r o s i o n c r a c k s showed no e v i d e n c e o f p r o p a g a t i n g on t he f r e e s u r f a c e s . T h u s , t h e i n a b i l i t y o f t h e s t r e s s c o r r o s i o n c r a c k s i n t h e c o l d r o l l e d m a t e r i a l u sed i n t h i s work t o p r o p a g a t e a l o n g t h e s u r f a c e o f t h e s p e c i m e n s can be shown t o be a t l e a s t p a r t l y due to t he e f f e c t o f r e s i d u a l s t r e s s e s l e f t i n t h e ba r by t h e f o r m i n g p r o c e s s . The f a c t t h a t t h e c r a c k f r o n t . o f t h e s p e c i a l b a r s t i l l l a g g e d a t t h e f r e e s u r f a c e s i n d i c a t e s t h a t s t r e s s s t a t e c o n s i d e r a t i o n s o r e n v i r o n m e n t a l e f f e c t s a r e i m -p o r t a n t a l s o . 134 APPENDIX B C a l c u l a t i o n o f pH a t 9 2 ° C The pH o f e a c h s o l u t i o n a t 9 2 ° C was e s t i m a t e d f r o m d a t a s u p p l i e d by Conway . * The d i s s o c i a t i o n c o n s t a n t o f w a t e r and t h e a c t i v i t y c o e f f i c i e n t s o f 0H~ a t 9 2 ° C were e x t r a p o l a t e d f r om d a t a o b t a i n e d a t l o w e r t e m p e r a t u r e s . The a c t i v i t y o f w a t e r i n c o n c e n t r a t e d NaOH s o l u t i o n s i s 0 .35 a t 2 5 ° C. I t was assumed t o i n c r e a s e w i t h t e m p e r a -t u r e t o w a r d s t h e i d e a l v a l u e o f 1 .0 . The v a l u e s o f t h e a c t i v i t y c o e f f i c i e n t s o f 0 H ~ , t h e 0H~ a c t i v i t y , and the r a n g e o f pH f o r an a c t i v i t y o f w a t e r between 0 .35 and 1.0 a r e shown i n T a b l e A l . The pH o f each s o l u t i o n was t a k e n as t h e m i d p o i n t o f each range." *Conway , B . E . , E l e c t r o c h e m i c a l D a t a , E l s e v i e r , A m s t e r d a m , (1 9 5 2 ) . T a b l e BI A c t i v i t y C o e f f i c i e n t s (YQH-) and A c t i v i t i e s ( a g ^ - ) f o r 0H~ and C a l c u l a t e d pH f o r a l l T h r e e S o l u t i o n s . Solut ion Y OH" a 0H~ pH 12.5 mol/kg NaOH 1.0 12.5 13. 3 - 13.7 3.35 mol/kg NaOH 0.7 2.4 12. 5 - 13.0 2.5 mol/kg NaOH + 0.42 mol/kg Na2S 0.6 1.5 12. 3 - 12.8 OJ on 

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