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Stress corrosion cracking of 316 stainless steel in caustic solutions 1982

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STRESS CORROSION CRACKING OF 316 STAINLESS STEEL IN CAUSTIC SOLUTIONS by DAVID CHARLES CROWE B . S c , ( M e c h a n i c a l E n g i n e e r i n g ) , The U n i v e r s i t y o f M a n i t o b a , 1977 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE : " i n THE FACULTY OF GRADUATE STUDIES Depar tmen t o f M e t a l l u r g i c a l E n g i n e e r i n g We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRIT ISH COLUMBIA May 1982 0 D a v i d C h a r l e s C r o w e , 1 9 8 2 . I n 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 o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e 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 , I a g r e e t h a t t h e 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 a n d s t u d y . I f u r t h e r a g r e e 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 c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e h e a d o f my d e p a r t m e n t o r by h i s o r h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d 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 n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t o f M&M/fu'ry/cq-l E^/i/ie<?r/'/?Q The 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 2075 Wesbrook P l a c e V a n c o u v e r , Canada V6T 1W5 D a t e ABSTRACT 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 t y p e 316 s t a i n l e s s s t e e l was s t u d i e d i n hot (92°C) s o l u t i o n s o f 3.35 mol/kg NaOH and 2.5 mol/kg NaOH + 0.423 mol/kg Na 2S by means o f p o t e n t i o - s t a t i c a l l y c o n t r o l l e d slow s t r a i n r a t e t e s t i n g t e c h n i q u e s ( S S R 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 a l s o d e t e r m i n e d f o r ; the s t e e l , t o g e t h e r w i t h t h o s e f o r N i , Cr and Fe. SCC o c c u r - r e d i n t h e t r a n s p a s s i v e r e g i o n i n 3.35 mol/kg NaOH, w i t h no d e t e c t a b l e t e n d e n c y t o c r a c k i n t h e a c t i v e - p a s s i v e r e g i o n , u n l e s s i n the s e n s i t i z e d c o n d i t i o n . In t h e NaOH + Na^S s o l u - t i o n , SCC was d e t e c t e d i n t h e a c t i v e - p a s s i v e r e g i o n . 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 were used t o s t u d y t h e k i n e t i c s 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 i n 3.35, 8 and 12 mol/kg NaOH, and 12 mol/kg NaOH + 0 . 4 2 3 mol/kg Na 2S. C r a c k i n g was s t u d i e d as a f u n c t i o n o f s t r e s s i n t e n s i t y ( K j ) , t e m p e r a t u r e (T) and p o t e n t i a l ( E ) . Crac k f r a c t o g r a p h y was s t u d i e d by 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 y and c o r r o s i o n f i l m s i n v e s t i g a t e d by e l e c t r o n d i f f r a c t i o n . Region I ( K j - d e p e n d e n t ) and R e g i o n II ( K - j - i n d e p e n d e n t ) c r a c k b e h a v i o r were o b s e r v e d . The r e s u l t s i n d i c a t e d t h a t SCC was a s s o c i a t e d w i t h p o t e n t i a l s a t . w h i c h i n s t a b i l i t i e s o c c u r r e d i n p a s s i v e f i l m s and t h a t t h e b a s i c mechanism o f c r a c k i n g i n v o l v e d a f i l m r u p t u r e and d i s s o l u t i o n p r o c e s s , w i t h d i s s o l u t i o n p r o c e s s e s i i e x e r t i n g p r e d o m i n a n t r a t e c o n t r o l i n Region I I . In t h e c a s e o f t h e s u l f i d e c o n t a i n i n g s o l u t i o n , h y drogen e m b r i t t l e m e n t p r o c e s s e s c o u l d n o t be e l i m i n a t e d as a c o n t r i b u t i n g f a c t o r . i i i TABLE OF CONTENTS Page Abstract • • n Table of Contents i v L i s t of Tables , v i i L i s t of Figures v i i l L i s t of Symbols and Abbreviations xi Acknowledgement x i n 1. INTRODUCTION 1 1.1 Temperature 2 1.2 Electrochemical Potential 12 1.3 Composition and Concentration of the Environment 4 1.4 A l l o y Composition — 5 1.5 Thermomechanical E f f e c t s 7 1.6 Stress Intensity 8 1.7 Mechanisms of SCC 8 1.8 Present Objectives 11 2. EXPERIMENTAL 12 2.1 P o l a r i z a t i o n Curves ... 12 2.1.0 Introduction 12 2.1.1 Materials and Preparation 12 2.1.2 Procedure 16 2.2 Slow S t r a i n Rate Test 17 2.2.0 Introduction 17 2.2.1 Materials and Preparation 18 2.2.2 Procedure . 20 iv. Page 2.3 Fracture Mechanics Testing 23 2.3.0 Introduction 23 2.3.1 Materials and Preparation 27 2.3.2 Procedure 30 2.4 Electron D i f f r a c t i o n Analysis of.Surface.Films 32 2.4.0 Introduction 32 2.4.1 Sampling f o r Corrosion Film Analysis 32 2.4.2 Procedure 33 3. RESULTS • 35 3.1 Anodic P o l a r i z a t i o n Curves 35 3.1.0 NaOH 35 3.1.1 NaOH + Na 2S 39 3.2 Slow S t r a i n Rate Tests 45 3.2.0 NaOH 45 3.2.1 NaOH + NagS 52 3.3 Fracture Mechanics Testing ........ 52 3.3.0 NaOH ... 52 3.3.0.0 E f f e c t of Stress Intensity .. 52 3.3.0.1 Temperature E f f e c t 56 3.3.0.2 E f f e c t of NaOH Concentration 60 3.3.0.3 E f f e c t of Applied Potential 60 3.3.0.4 E f f e c t of Cold Work 63 3.3.0.5 Fractography 63 3.3.0.6 pH Measurement 71 v Page 3.3.1 NaOH + Na2S 77 3.3.1.0 2.5 mol/kg NaOH + 0.423 mol/kg Na 2S (Simulated White Liquor) 77 3.3.1.1 12 mol/kg NaOH + 0.423 mol/kg Na 2S ..... 77 3.3.1.2 Fractography 79 3.3.1.3 Solution Appearance 79 3.4 Electron D i f f r a c t i o n Analysis of Surface Films i n NaOH.. 82 4. DISCUSSION 86 4.1 Interpretation of Anodic P o l a r i z a t i o n Curves 86 4.2 SCC S u s c e p t i b i l i t y . 92 4.3 Crack Growth Rates and the Mechanism of Cracking 97 4.3.1 Potential Dependence of Crack Growth Rate 97 4.3.2 Dissolution Rate and Crack Growth Rate 98 4.3.3 Kinetics of Crack Growth Rates 101 4.4 Fractography and the Dissolution Mechanism ......... 103 4.4.0 Corrosion Deposits — . 103 4.4.1 Fracture Mode .. 105 4.5 Electron D i f f r a c t i o n Analysis of Surface Films in NaOH.. 109 5. SUMMARY -.. H I BIBLIOGRAPHY 113 vi LIST OF TABLES Table Page I Chemical Composition of Steels 14 II Mechanical Properties of SSRT Specimens 19 III Summary of Fracture Mechanics Test Data 57 IV Electron D i f f r a c t i o n Pattern Data 85 v i i LIST OF FIGURES Figure Page 1 Test c e l l f o r p o l a r i z a t i o n studies a) t e s t electrode b) Luggin c a p i l l a r y c) counter electrode d) temperature probe 3) nitrogen purge f ) l i d g) beaker 15 2 Slow s t r a i n rate t e s t c e l l a) specimen b) Luggin c a p i l l a r y c) c e l l d) l i d e) re f l u x condenser f.) temperature probe g) nitrogen purge h) Teflon c e l l bottom i ) counter electrode 21 3 T-notch double c a n t i l e v e r beam specimen 25 4 Fracture mechanics t e s t i n g c e l l a) specimen b) grips c) pins d) c e l l l i d e) beaker . 29 5. Anodic p o l a r i z a t i o n curve, 316 s t a i n l e s s steel rod, 3.35 mol/kg NaOH, 92 °C 36 6 Anodic p o l a r i z a t i o n curve, 316 s t a i n l e s s steel p l a t e , 3.35 mol/kg NaOH, 92 °C 37 7 Anodic p o l a r i z a t i o n curves at selected NaOH concentrations, 316 s t a i n l e s s steel rod, 92 °C 38 8 Anodic polarization, curves at selected temperatures, 316 st a i n l e s s steel rod, 3.35. mol/kg NaOH 40 9, Anodic p o l a r i z a t i o n curves, chromium, 3.35 and 8 mol/kg NaOH, 92 °C 41 10 Anodic p o l a r i z a t i o n curve, n i c k e l , 3.35 mol/kg NaOH, 92 ° C . 42 11 Anodic p o l a r i z a t i o n curve, i r o n , 3.35 mol/kg NaOH, 92 °C ... 43 12 Anodic p o l a r i z a t i o n curve, 316 s t a i n l e s s steel rod, 2.5 mol/kg NaOH + 0.423 mol/kg Na 2S, 92 °C 44 13 Anodic p o l a r i z a t i o n curves i n solutions of selected NaOH concentration with Na^S, 316 st a i n l e s s steel rod, 92 °C 46 14 Anodic p o l a r i z a t i o n curves at selected temperatures, 316 s t a i n l e s s steel plate, 2.5 mol/kg NaOH + 0.423 mol/kg Na 2S 47 15 E f f e c t of potential on percent reduction i n area during slow s t r a i n rate t e s t s , 3.35 mol/kg NaOH, 92 °C. Anodic p o l a r i z a t i o n curve f o r 316 s t a i n l e s s steel rod 48 v i i i Figure Page 16 SSRT specimen a f t e r t e s t i n g at -0.10 V S C E in NaOH s o l u t i o n . . 50 17 SSRT specimen a f t e r t e s t i n g at -0.95'. V S C E i n NaOH s o l u t i o n . . 50 18 Surface f i l m on SSRT specimens tested i n NaOH so l u t i o n a) 0.25 V S C E , b) -1.00 V $ C E 51 19 E f f e c t of potential on percent reduction in area during slow s t r a i n rate tests 2.5 mol/kg NaOH + 0.423 mol/kg Na 2S, 92 °C. Anodic p o l a r i z a t i o n curve f o r 316 s t a i n l e s s steel rod ........ 53 20 SSRT specimen a f t e r t e s t i n g at -1.15 V_ r F in NaOH + Na 2S solu t i o n 54 21 Crack growth rate versus stress i n t e n s i t y at selected temperatures i n 3.35 mol/kg NaOH, -0.10 55 22 Crack growth rate versus stress i n t e n s i t y at selected temperatures i n 12 mol/kg NaOH,,-0.10 V S C E -.. 58 23 Arrhenius p l o t of the Region II crack growth rates i n 3.35 and 12 mol/kg NaOH, -0.10 V $ C E 59 24 Crack growth rate versus stress i n t e n s i t y at selected NaOH concentrations, 92 °C, -0.10 V $ C E 61 25 Crack growth rate versus stress i n t e n s i t y at selected potentials i n 3.35 mol/kg NaOH 92 °C 62 26 Fracture surface a f t e r t e s t i n g in 3.35 mol/kg NaOH, 92 °C, -0.10 V S C E , 45 - 47 MPa*¥ 64 27 Fracture surfaces a f t e r t e s t i n g in 3.35 mol/kg NaOH, 92 °C, -O.lO.V-pp-at a) 38-40 MPav̂ m, b) 46-48 MPavfii", c) 55-60 MPa^mbr.... 65,66 28 Crack branching. Stress i n t e n s i t y r i s e s from 75 to 105 MPavrn in t h i s view 68 29 Deposits on fracture surface a f t e r t e s t i n g i n 3.35 mol/kg NaOH, 92 °C, -0.10 V $ C E 69 30 Fracture surfaces a f t e r t e s t i n g in 3.35 mol/kg NaOH, -0.10 V S C E , a) 82 °C, 33-34 MPavrrT, b) 72 °C, 38-40 MPa^.. 70 i x Figure Page 31 Fracture surface a f t e r t e s t i n g i n 12 mol/kg NaOH, 92 °C, -0.10 V $ C E , 28-29 MPavfiT 72 32 Intergranular facet d i s p l a y i n g i n t e r s e c t i n g transgranular cracking. This i s a magnification of the center of Figure 31 73 33 Corrosion deposits a f t e r t e s t i n g in 3.35 mol/kg NaOH, 92 °C, a) -0.175 V c . r F , 36 MPaVm, b) -0.10 42-44 MPav¥, c) 0.00 V $ e E , 41-43 MPav̂ m .... 74,75 34 Fracture surfaces a f t e r t e s t i n g in 3.35 mol/kg NaOH, 92 °C, a) -0.175 V S C E , 33-35 MPâ /m, b) 0.00 V s e E , 43-44 MPav€ ... 76 35 Crack growth rate versus stress i n t e n s i t y i n 12 mol/kg NaOH + 0.423 mol/kg Na 2S, 92 °C, -1.175 V $ C E 78 36 Fracture surfaces a f t e r t e s t i n g i n 12 mol/kg NaOH + 0.423 mol/kg Na 9S, 92 °C, -1.175 V Q r F a) 37-39 MPav¥, b) 50-54^MPa^rT ... 80 37 Corrosion deposits a f t e r t e s t i n g in 12 mol/kg NaOH + 0.423 mol/kg Na 2S, 92 °C, -1.T75 V $ C E , 50-54 MPa^rrT .. 81 38 Electron d i f f r a c t i o n pattern from co r r o s i o n , f i l m 83 39 Diameter of d i f f r a c t i o n rings versus /h^ + kc +• 1L 84 40 Anodic p o l a r i z a t i o n curves, 316 s t a i n l e s s s t e e l , 3.35 mol/kg NaOH, 92 °C. I d e n t i f i c a t i o n of reactions 88 41 Anodic p o l a r i z a t i o n curves, 316 s t a i n l e s s s t e e l , NaOH + Na QS, 92 °C. I d e n t i f i c a t i o n of reactions 91 x L I S T OF SYMBOLS AND A B B R E V I A T I O N S Symbo l a a ( d D E E ( F h , k , l i _ •CORR ISCC P Q p R r T v V SHE c r a c k l e n g t h l a t t i c e p a r a m e t e r c r y s t a l d - s p a c i n g d i a m e t e r o f d i f f r a c t i o n p a t t e r n r i n g e l e c t r o c h e m i c a l p o t e n t i a l c o r r o s i o n p o t e n t i a l F a r a d a y (9.85 x 1 0 4 A - s ) M i l l e r i n d i c e s a n o d i c c u r r e n t d e n s i t y c a m e r a c o n s t a n t f o r e l e c t r o n d i f f r a c t i o n s t r e s s i n t e n s i t y f o r mode I o p e n i n g t h r e s h o l d s t r e s s i n t e n s i t y 1 oad 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 n s i t y gas c o n s t a n t (8v314 k J / m o l d e g ) p l a s t i c z o n e s i z e y i e l d s t r e s s t e m p e r a t u r e °K c r a c k v e l o c i t y v o l t s w i t h r e s p e c t t o t h e s t a n d a r d h y d r o g e n e l e c t r o d e x i V $ C E v o l t s w i t h r e s p e c t to t h e s t a n d a r d c a l o m e l e l e c t r o d e W e q u i v a l e n t w e i g h t A b b r e v i a t i o n SCC s t r e s s c o r r o s i o n c r a c k i n g SSRT slow s t r a i n r a t e t e s t TN-DCB T - n o t c h d o u b l e c a n t i l e v e r beam x i i ACKNOWLEDGEMENT I am i n d e b t e d t o my r e s e a r c h s u p e r v i s o r , Desmond Tromans, f o r h i s u n f a i l i n g e n c o u r a g e m e n t and p a t i e n c e . . The s t a f f has a l s o been v e r y g e n e r o u s i n h e l p i n g me. L. F r e d e r i c k , R. MacLeod, E. K l a s s e n , M. Mager, H. Tump, E. A r m s t r o n g and K. Kent d e s e r v e s p e c i a l t h a n k s . My p a r e n t s have g i v e n me a g r e a t amount o f u n d e r s t a n d i n g and s u p p o r t t h r o u g h o u t t h i s work. F i n a n c i a l s u p p o r t has been p r o v i d e d by A l c a n as a f e l l o w - s h i p w hich I r e c e i v e d f o r two y e a r s . A d d i t i o n a l s u p p o r t was p r o v i d e d by 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 . T h e i r c o n t r i b u t i o n s have been g r a t e f u l l y r e c e i v e d . x i i i 1 1 . INTRODUCTION Stress corrosion cracking (SCC) is a time dependent f a i l u r e process caused by the conjoint action of a t e n s i l e stress and corrosion. It occurs in many di f f e r e n t a l l o y - environment combinations and is of p a r t i c u l a r concern in in d u s t r i a l situations wherever steels are exposed to hot, caustic solutions. Such steels include type 316 austenitic stainless s t e e l , which is the subject of this thesis. Sodium hydroxide solutions are used in a variety of chemical processes such as wood pulping, alumina production, or a l k a l i n i z a t i o n of chemical process streams. Unwanted caustic deposits also build up in crevices or splash zones in b o i l e r s , tanks or other equipment where pH control is practiced. The effects of caustic solutions have been i n - vestigated extensively and a better understanding has devel- oped hand in hand with better techniques of investigation. Several techniques have been employed for study of SCC. Early studies of stress corrosion of stainless steel in caustic solutions were done using loaded te n s i l e specimens^ 1-3 or pressurized tube specimens. U-bend,. specimens and 4 - 7 loaded longitudinal sections of pipe also have been used. In most of these tests the electrochemical potential was not g measured or controlled. More recently, straining electrode 2 o r slow s t r a i n r a t e t e s t i n g has been e m p l o y e d . ^ ' ^ The slow s t r a i n r a t e t e s t has been used p r i m a r i l y as a r e l a t i v e l y r a p i d t e s t f o r d e t e r m i n i n g p o t e n t i a l r e g i o n s f o r s u s c e p t - i b i l i t y t o SCC. A n o t h e r r e c e n t method employs 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 t o a l l o w s t u d y o f growth o f a c r a c k o f 12-15 w e l l c h a r a c t e r i z e d g e ometry. The c r a c k growth r a t e i s r e l a t e d t o t h e s t r e s s i n t e n s i t y , K j , a t t h e c r a c k t i p . Time r e q u i r e d f o r c r a c k i n i t i a t i o n i s m i n i m i z e d . S t u d i e s o f SCC o f s t a i n l e s s s t e e l s i n c a u s t i c s o l u t i o n s have shown t h a t s e v e r a l v a r i a b l e s a r e i m p o r t a n t . These i n - c l u d e t e m p e r a t u r e , c o m p o s i t i o n and c o n c e n t r a t i o n o f t h e e n v i r o n - ment, e l e c t r o c h e m i c a l p o t e n t i a l , a l l o y c o m p o s i t i o n , thermo- m e c h a n i c a l e f f e c t s , and s t r e s s i n t e n s i t y . Much o f t h e p r e - s e n t knowledge o f SCC i n F e - C r - N i a l l o y s has been summarized , . 16-21 i n s e v e r a l r e v i e w s . 1 .1 T e m p e r a t u r e The e f f e c t o f t e m p e r a t u r e was i n v e s t i g a t e d s y s t e m a t i c a l l y 3 by Snowden who showed t h a t t i m e - t o - f a i l u r e o f s p e c i m e n s i n - c r e a s e d d r a m a t i c a l l y as t h e t e m p e r a t u r e was d e c r e a s e d . Agrawal 2 2 and S t a e h l e a l s o o b s e r v e d t h i s . 1.2 E l e c t r o c h e m i c a l P o t e n t i a l The i m p o r t a n c e o f e l e c t r o c h e m i c a l p o t e n t i a l t o SCC was 3 not s t u d i e d u n t i l r e c e n t l y . T h e u s , ^ i n a s t u d y o f I n c o n e l 600, I n c o l o y 800 and t y p e 304, o b s e r v e d t h a t t h e 304 s t a i n l e s s s t e e l c r a c k e d i f t h e p o t e n t i a l was more t h a n 30 mV above t h e open c i r c u i t c o r r o s i o n p o t e n t i a l . M o r r i s has i n v e s t i g a t e d t h e p o t e n t i a l dependence o f SCC o f A l l o y 6 0 0 . 2 3 8 Park e t a l . i n a s t u d y o f 304 s t a i n l e s s s t e e l s t r a i n i n g e l e c t r o d e s i n b o i l i n g 20 N NaOH, f o u n d a s h o r t t i m e - t o - f a i l u r e n e a r t h e c o r r o s i o n p o t e n t i a l , no f a i l u r e i n t h e p a s s i v e r e g i o n , f a i l u r e t i m e s d e c r e a s i n g i n t h e p r i m a r y t r a n s p a s s i v e r e g i o n and v e r y s h o r t t i m e t o f a i l u r e i n t h e s e c o n d a r y p a s s i v e r e g i o n . ?4 Dahl e t a l . f o u n d t h a t i n 20% NaOH a t 2 2 5 ° C , 18 C r - 9 N i s t a i n l e s s s t e e l was s u s c e p t i b l e t o c r a c k i n g a t t h e a c t i v e a n o d i c c u r r e n t peak and a t c a t h o d i c c u r r e n t s . 25 26 Long e t a l . and Agrawal e t a l . have c o n d u c t e d p o l a r - i z a t i o n s t u d i e s o f F e - C r - N i a l l o y s i n NaOH s o l u t i o n s t o g a i n a b e t t e r u n d e r s t a n d i n g o f how t h e p o l a r i z a t i o n b e h a v i o r o f the a l l o y i s a f f e c t e d by t h a t o f i t s c o n s t i t u e n t s . 27 Okada e t a l . q u o t e d u n p u b l i s h e d work by Subramanyam and S t a e h l e which i n d i c a t e d t h a t t h e p o t e n t i a l a f f e c t e d t h e c r a c k i n g mode. I t was i n t e r g r a n u l a r a t t h e t r a n s p a s s i v e p o t e n t i a l and t r a n s g r a n u l a r a t t h e a c t i v e - p a s s i v e p o t e n t i a l s f o r 304 s t a i n l e s s s t e e l i n 70% NaOH. Park e t a l . o b s e r v e d t h a t c r a c k i n g mode was a f u n c t i o n o f p o t e n t i a l . 1.3 C o m p o s i t i o n and C o n c e n t r a t i o n o f t h e E n v i o r n m e n t H i g h e r c o n c e n t r a t i o n s o f NaOH have been shown t o r e d u c e 3 7 22 t i m e - t o - f a i l u r e . The c o n c e n t r a t i o n o f c a u s t i c was o b s e r v e d t o a f f e c t the f r a c t o g r a p h y , c h a n g i n g i t from i n t e r - g r a n u l a r t o 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 i n 304 s t a i n l e s s 2 28 s t e e l as t h e NaOH c o n c e n t r a t i o n was i n c r e a s e d . ' The a d d i t i o n o f o t h e r s p e c i e s t o t h e s o l u t i o n may s h i f t 9 29 30 the f r e e c o r r o s i o n p o t e n t i a l . ' ' In t h i s way many a d d i - t i v e s may i n h i b i t c o r r o s i o n by s h i f t i n g 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 i n t o a r e g i o n where t h e r e i s no s u s c e p t i b i l i t y t o 9 29 SCC. Park e t a-l. ' c o n c l u d e d t h a t t h e i n h i b i t o r s t h e y t e s t e d s h i f 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 f o r 304 s t a i n l e s s s t e e l i n a n o b l e d i r e c t i o n i n 20 N NaOH. E a r l y o b s e r v a t i o n s 3 o f i n h i b i t o r s d i d not c o n s i d e r t h i s e f f e c t on p o t e n t i a l . A d d i t i o n o f s u l f u r t o s i m u l a t e d w h i t e l i q u o r (NaOH + Na^S s o l u t i o n ) has been shown t o s h i f t t h e f r e e c o r r o s i o n 31 p o t e n t i a l f o r m i l d s t e e l . More s u l f u r i s needed as t h e 32 Na^S^O^ c o n c e n t r a t i o n i s i n c r e a s e d . P o l y s u l f i d e a c t s as a p a s s i v a t i n g i n h i b i t o r f o r m i l d and s t a i n l e s s s t e e l s . ^ Theus and S t a e h l e 1 ' have o b s e r v e d t h a t s u l f i d e may a f f e c t t h e f r a c t u r e mode. They q u o t e d work on 304 s t a i n l e s s 34 s t e e l O H i n 50% NaOH w i t h 0.03 m o l / l i t e r Na 2S a t 180°C i n which i n t e r 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 . In o t h e r work w i t h 50% NaOH a t h i g h e r t e m p e r a t u r e , 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 . The f r a c t o g r a p h y was a l s o o b s e r v e d t o change when K0H 2 was used i n s t e a d o f NaOH. NaCl has been o b s e r v e d t o i n c r e a s e t h e t i m e - t o - f a i l u r e f o r 321 s t a i n l e s s s t e e l i n 3% NaOH. 3 5 1 .4 A l l o y C o m p o s i t i o n The e f f e c t o f c o m p o s i t i o n o f t h e a l l o y has been, i n - v e s t i g a t e d . 1 I n c r e a s i n g amounts o f chromium i n t h e a l l o y i m p r o ved r e s i s t a n c e t o SCC i n 50% NaOH a t 300 °C. At 10-15% chromium, a h i g h e r n i c k e l c o n t e n t a l s o was f o u n d t o be bene- f i c i a l but the s t i l l h i g h e r n i c k e l c o n t e n t used w i t h 20-25% chromium r e d u c e d r e s i s t a n c e t o c r a c k i n g . M c l l r e e and M i c h e l s 6 f o u n d t h a t i n d e a e r a t e d 50% NaOH a t 300 °C, n i c k e l i n c r e a s e d t h e r e s i s t a n c e t o SCC but i n an a e r a t e d 50% NaOH s o l u t i o n b o t h h i g h chromium and h i g h n i c k e l c o n t e n t were n e c e s s a r y t o i n c r e a s e SCC r e s i s t a n c e . P o t e n t i a l 6 was n o t measured o r c o n t r o l l e d . The i n f l u e n c e o f n i c k e l has been i n v e s t i g a t e d t h o r o u g h l y 36 37 36 i n MgCl^ s o l u t i o n s . ' Cops on p r o d u c e d a c u r v e r e l a t i n g n i c k e l c o n t e n t t o t i m e - t o - f a i l u r e i n Mg C l ^ • A minimum t i m e - 1 3 t o - f a i l u r e o c c u r r e d f o r about 8% n i c k e l . S p e i d e l o b s e r v e d f o r s t a i n l e s s s t e e l i n NaCl s o l u t i o n a t 105 °C t h a t t h e Copson c u r v e i s r e p r o d u c e d by p l o t t i n g t h e s t r e s s c o r r o s i o n t h r e s h - h o l d s t r e s s i n t e n s i t y ( K J ^ Q Q ) v e r s u s n i c k e l c o n t e n t i n a s t a i n l e s s s t e e l o f 18% chromium. K j i s t h e s t r e s s i n - 4 t e n s i t y below w h i c h c r a c k i n g i s n o t d e t e c t e d . S e d n k s e t a l . f o u n d t h a t K J ^ Q Q i n c r e a s e d w i t h i n c r e a s i n g n i c k e l c o n t e n t f o r s t a i n l e s s s t e e l i n NaOH s o l u t i o n s . As t h e n i c k e l c o n t e n t was i n c r e a s e d t h e c r a c k p a t h changed from t r a n s g r a n u l a r t o i n t e r g r a n u l a r . The amount o f n i c k e l and chromium i n t h e a l l o y has been i d e n t i f i e d as i m p o r t a n t i n the s t a i n l e s s o v e r l a y used i n 38 k r a f t p u l p d i g e s t e r s . C r o o k s and L i n n e r t a t t r i b u t e d o v e r l a y c o r r o s i o n t o a t t a c k o f s m a l l a r e a s o f low a l l o y c o n t e n t d i s - p e r s e d t h r o u g h o u t t h e o v e r l a y l i n i n g and f r e q u e n t l y a low a v e r a g e l e v e l o f a l l o y i n g e l e m e n t s i n t h e l i n i n g . M a r t e n s i t e was o b s e r v e d i n p l a c e s where a l l o y c o n t e n t was below a p p r o x i - m a t e l y 13% chromium and 8% n i c k e l . R a p i d a t t a c k o c c u r r e d t h e r e 39 40 O t h e r s t u d i e s have c o n s i d e r e d t h e e f f e c t o f c a r b o n , ' 7 1 6 39 41 42 43 40 molybdenum.,' s i l i c o n , aluminum, and p h o s p h o r u s . 1.5 T h e r m o m e c h a n i c a l E f f e c t s S e n s i t i z a t i o n ( i . e . c a r b i d e p r e c i p i t a t i o n ) o c c u r s when the a l l o y i s h e a t e d i n t h e range 500 t o 800 °C. C a r b i d e s a r e formed a t t h e g r a i n b o u n d a r i e s , and t h e s e p r o d u c e a c o n t i n u o u s 44 chromium denuded p a t h f o r i n t e r g r a n u l a r c o r r o s i o n . The e f f e c t o f s e n s i t i z a t i o n on SCC o f s t a i n l e s s s t e e l i n NaOH 5 s o l u t i o n s has been i n v e s t i g a t e d . W i l s o n and Aspden con- c l u d e d t h a t s e n s i t i z a t i o n i s not damaging t o 304 s t a i n l e s s s t e e l i n 10% NaOH a t 316 °C and 332 °C o r i n 50% NaOH a t 316 ° C 39 W i l s o n e t a l . f o u n d t h a t f o r 304 s t a i n l e s s s t e e l i n 50% NaOH a t 371 °C s e n s i t i z a t i o n had no e f f e c t on c r a c k i n g . I t a l s o had no e f f e c t on t h e l i f e o f 316L o r 316 s t a i n l e s s s t e e l i n 10% NaOH a t 316 °C. In b o t h c a s e s c r a c k i n g was t r a n s - g r a n u l a r . They c o n c l u d e d t h a t g r a i n b oundary c a r b i d e s p e r se d i d n o t i n d u c e s u s c e p t i b i l i t y i n 10-50% NaOH a t 149-371 °C. A l s o , t h e y f o u n d t h a t i n 10% NaOH a t 316 °C t h e 304 s t a i n l e s s s t e e l was r e s i s t a n t t o SCC but t y p e 316 was n o t . 6 45 The e f f e c t o f c o l d work has a l s o been i n v e s t i g a t e d . ' 46 C o l d work may i n d u c e p a r t i a l m a r t e n s i t i c t r a n s f o r m a t i o n . A s a r o e t a l . ^ i d e n t i f i e d m a r t e n s i t e f o r m a t i o n w i t h t r a n s - g r a n u l a r SCC o f 304 s t a i n l e s s s t e e l i n hot c a u s t i c s o l u t i o n s . 6 M c l l r e e and M i c h e l s o b t a i n e d e v i d e n c e t h a t s t r e s s r e l i e f o f 304 s t a i n l e s s s t e e l f o r f o u r hours a t 593 °C d i d n o t improve 45 r e s i s t a n c e i n 50% NaOH a t 294 and 332 °C.Wilson and Aspden f o u n d t h a t c o l d r o l l i n g had no e f f e c t on t h e c r a c k i n g o f 304 and 304L s t a i n l e s s s t e e l i n both 10% and 50% NaOH. 39 G r a i n s i z e was f o u n d t o have no e f f e c t on f a i l u r e t i m e . 1.6 S t r e s s I n t e n s i t y The i n f l u e n c e o f s t r e s s i n t e n s i t y on c r a c k i n g r a t e o f s t a i n l e s s s t e e l i n h y d r o x i d e s o l u t i o n s has not been examined even though i t has been shown t o be i m p o r t a n t i n o t h e r systems. , 2-' 4' 4 8 - 4 9 1.7 Mechanisms o f SCC In t h e r e s e a r c h d e s c r i b e d i n t h e l i t e r a t u r e , t h e v a r i a b l e s i n . 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 have not been f u l l y c o n t r o l l e d or d e f i n e d . In p a r t i c u l a r , t h e p o t e n t i a l may not have been measured o r c o n t r o l l e d ; o n l y r e c e n t l y has i t s i m p o r t a n c e been r e c o g n i z e d . W i t h i n t h e s e l i m i t a t i o n s , s e v e r a l mechanisms have been p r o p o s e d t o a c c o u n t f o r t h e c r a c k i n g p r o c e s s . These mechanisms have been e s s e n t i a l l y q u a l i t a t i v e r a t h e r t h an q u a n t i t a t i v e l y p r e d i c t i v e . Most m e c h a n i s t i c d e s c r i p t i o n s o f SCC i n c a u s t i c e n v i r o n - ments have c e n t e r e d on t h e f i l m r u p t u r e and d i s s o l u t i o n 9 21 mechanism. Th i s has been d e s c r i b e d by S t a e h l e . Accord ing to t h i s model , a p r o t e c t i v e su r f a ce f i l m i s rup tured by s l i p s t e p s . L o c a l i z e d d i s s o l u t i o n occurs at the newly exposed s u r f a c e u n t i l r e p a s s i v a t i o n i s comp le te , with the amount o f d i s s o l u t i o n o c c u r r i n g be fo re r e p a s s i v a t i o n i n f l u e n c i n g the geometry of the a t t acked a r e a . Subsequent f i l m rupture and d i s s o l u t i o n c y c l e s at the l o c a l i z e d area c o n s t i t u t e c r a c k i n g . In t h i s model , the c r a c k i n g r a t e i s r e l a t e d to the d i s s o l u - t i o n r a t e and time spent i n the d i s s o l u t i o n s t age . F i lm i n s t a b i l i t y in va r i ous p o t e n t i a l ranges and t r a n s i e n t d i s - s o l u t i o n c u r r e n t s are the p r i n c i p a l f a c t o r s in de termin ing s u s c e p t i b i l i t y . In some in s t ances the d e t a i l e d d i s s o l u t i o n morphology o f c rack propagat ion i s b e l i e v e d to a r i s e v i a t u n n e l i n g at the crack t i p . 2 ^ ' ^ 51 Vermi lyea has fo rmula ted a d e t a i l e d f i l m rupture model in which the crack advances by d i s s o l u t i o n be fo re repas - s i v a t i o n o c c u r s . Fu r the r d i s s o l u t i o n would occur when the 52 su r f ace f i l m i s ruptured aga in . D ieg le and Vermi lyea have presented ev idence that the r a t i o of crack t i p c o r r o s i o n r a te to crack t i p s t r a i n r a te must exceed a c r i t i c a l v a l u e , depend- ing on the s t r a i n g r a d i e n t ahead o f the crack t i p , to promote SCC of s t e e l i n NaOH. 53 S c u l l y has d e s c r i b e d a model f o r d i s s o l u t i o n c o n t r o l l e d c r a c k i n g , r e q u i r i n g a c r i t i c a l de lay in r e p a s s i v a t i o n time 10 54 d u r i n g w h i c h d i s s o l u t i o n o c c u r s . I n a l a t e r p a p e r , S c u l l y 5 5 h a s q u o t e d w o r k b y Newman s h o w i n g t h a t t h e c h a r g e p a s s e d d u r i n g t h e r e p a s s i v a t i o n e v e n t f i t t e d t h e a n a l y s i s o f c r a c k - i n g i n a C r - M o s t e e l e x p o s e d t o 8M NaOH a t 1 0 0 ° C . S c u l l y h a s n o t e d t h a t V e r m i l y e a ' s m o d e l may b e m o r e a p p l i c a b l e t o s y s t e m s l i k e s t e e l i n h y d r o x i d e s o l u t i o n w h e r e t h e c r a c k t i p s o l u t i o n may n o t be m u c h d i f f e r e n t t h a n t h e b u l k s o l u t i o n a n d c o m m e n t e d t h a t e v i d e n c e a b o u t t h e c r a c k t i p s o l u t i o n c o m p o s i t i o n i s s u r p r i s i n g l y l a c k i n g . 8 P a r k e t a l . u s i n g a h i g h s t r a i n r a t e l o a d i n g t e c h n i q u e w h e r e t h e s u r f a c e w a s r u p t u r e d , h a v e r e l a t e d t h e S C C s u s c e p t - i b i l i t y o f s t a i n l e s s s t e e l i n h y d r o x i d e s o l u t i o n t o t h e r a t i o o f c u r r e n t o n a b a r e m e t a l s u r f a c e t o t h a t o n a f i l m e d e l e c - , t r o d e . T h e i r w o r k s u p p o r t e d t h e f i l m r u p t u r e a r i d d i s s o l u t i o n m o d e l . 56 B i g n o l d d e v e l o p e d a m o d e l t o e x p l a i n t h e o b s e r v e d p o t e n t i a l d e p e n d e n c e o f c r a c k i n g r a t e a n d a s p e c t r a t i o , a n d t h e e x i s t e n c e o f a n a n o d i c p o t e n t i a l a b o v e w h i c h c r a c k i n g c e a s e s . H i s m o d e l i s b a s e d o n p o t e n t i a l a n d c u r r e n t d i s t r i - 57 b u t i o n s i n a c r a c k . D o i g a n d F l e w i t t h a v e a l s o c o n s i d e r e d t h e d i s t r i b u t i o n o f p o t e n t i a l i n a s t r e s s c o r r o s i o n c r a c k , a n d how t h i s may a f f e c t c r a c k p r o p a g a t i o n . V e r y l i t t l e q u a n t i t a t i v e i n f o r m a t i o n i s a v a i l a b l e t o 11 s u p p o r t o r d i s p r o v e t h e mechanisms s u g g e s t e d . The r o l e o f hydrogen e m b r i t t l e m e n t i n c a u s t i c SCC o f 58 s t a i n l e s s s t e e l s has been i n v e s t i g a t e d by H o l z w o r t h . The l o s s o f d u c t i l i t y a f t e r c a t h o d i c c h a r g i n g was r e l a t e d t o t h e 59 amount o f m a r t e n s i t i c phases p r e s e n t . O ' B r i e n and S e t o have s t u d i e d t h e mechanism o f hy d r o g e n e v o l u t i o n a t a s t a i n - l e s s s t e e l e l e c t r o d e i n NaOH s o l u t i o n . They have t a b u l a t e d r e v e r s i b l e p o t e n t i a l s , exchange c u r r e n t d e n s i t i e s and T a f e l s l o p e s on 304L s t a i n l e s s s t e e l f o r a ran g e o f NaOH c o n c e n t r a - t i o n s . However no one has e s t a b l i s h e d t h a t thermodynamic c o n d i t i o n s e x i s t w i t h i n a c r a c k under 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 which c o u l d g e n e r a t e h y d r o g e n and promote h y d r o g e n e m b r i t t l e m e n t . 1.8 P r e s e n t O b j e c t i v e s The p r e s e n t i n v e s t i g a t i o n has employed improved t e c h n i - ques, p a r t i c u l a r l y SSRT and f r a c t u r e m e c h a n i c s , t o o b t a i n new q u a n t i t a t i v e i n f o r m a t i o n on SCC i n s o l u t i o n s o f NaOH and s o l u t i o n s o f NaOH w i t h Na,,S added. The s o l u t i o n c o n t a i n i n g s u l f i d e s i m u l a t e s w h i t e l i q u o r w o o d - p u l p i n g s o l u t i o n used i n the K r a f t p r o c e s s . D u r i n g t h e e x p e r i m e n t a l i n v e s t i g a t i o n t h e t e m p e r a t u r 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 , e n v i r o n m e n t c o m p o s i t i o n and 12 c o n c e n t r a t i o n , a l l o y c o m p o s i t i o n , t h e r m o m e c h a n i c a l p r o p e r t i e s , and s t r e s s i n t e n s i t y have been c o n t r o l l e d i n o r d e r t o p r o v i d e q u a n t i t a t i v e d a t a u s e f u l f o r u n d e r s t a n d i n g r a t e c o n t r o l l i n g mechanisms and f o r d e s i g n i n g e q u i p m e n t . 2. EXPERIMENTAL 2.1 P o l a r i z a t i o n C u r v e s 2.1.0 I n t r o d u c t i on 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 on m e tal e l e c t r o d e s when submersed i n aqueous s o l u t i o n . The r a t e and d i r e c t i o n 6 0 o f each r e a c t i o n depends on t h e p o t e n t i a l o f t h e e l e c t r o d e . To a p p l y a p o t e n t i a l d i f f e r e n t from t h a t f o u n d under 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 r e q u i r e s t h a t c u r r e n t be s u p p l i e d v i a an e x t e r n a l c i r c u i t . In a p o t e n t i o d y n a m i c t e s t , the p o t e n t i a l ( w i t h r e s p e c t t o a s t a n d a r d r e f e r e n c e e l e c t r o d e ) i s v a r i e d c o n t i n u o u s l y t h r o u g h a range and t h i s c u r r e n t i s r e c o r d e d . The r e c o r d o f t h e c u r r e n t v e r s u s p o t e n t i a l i s a p o l a r i z a t i o n c u r v e . ^ The c u r v e may be used t o o b t a i n i n f o r m a t i o n on t h e r e a c t i o n s which t a k e p l a c e on t h e s u r f a c e . ^ 5 ^ 2.1.1 M a t e r i a l s and P r e p a r a t i o n P o l a r i z a t i o n c u r v e s were d e t e r m i n e d f o r f i v e m a t e r i a l s . Two AT SI Type 316 s t a i n l e s s s t e e l s , pure i r o n , n i c k e l and chromium were examined. 13 Type 316 s t a i n l e s s s t e e l was r e c e i v e d as 9.5 mm d i a m e t e r r o d and as 10.6 mm t h i c k hot r o l l e d , a n n e a l e d and p i c k l e d p l a t e . The c o m p o s i t i o n s were d e t e r m i n e d by s p e c t r o s c o p i c a n a l y s i s , c o u r t e s y o f CAE M a c h i n e r y . Carbon c o n t e n t was d e t e r m i n e d s e p a r a t e l y by t h e more a c c u r a t e LECO method. The r e s u l t s o f t h e a n a l y s e s a r e g i v e n i n T a b l e I. Armco i r o n was r e c e i v e d as 0.46 mm s h e e t . High p u r i t y vacuum a r c n i c k e l d i s c s were 15 mm d i a m e t e r and 6.5 mm t h i c k . P u r i f i e d c a r b o n f r e e f u s e d chromium metal was i n t h e form o f i r r e g u l a r chunks l e s s t h a n 1 cm l o n g . 2 E l e c t r o d e s v l cm i n a r e a were c u t from each s t o c k . A pure n i c k e l w i r e was s p o t w e l d e d t o each e l e c t r o d e and t h e w i r e was p a s s e d t h r o u g h 3 mm PTFE t u b i n g . The e l e c t r o d e and tube were s e t i n a d i s c o f "Quiekmount" s e l f s e t t i n g a c r y l i c p l a s t i c . The e l e c t r o d e f a c e , i n i t s m o u n t i n g , was p o l i s h e d t o 600 g r i t , w i t h f i n a l p o l i s h i n g b e i n g done on a c l e a n p a p e r w i t h d i s t i l l e d w a t e r j u s t b e f o r e p l a c i n g i n t o the t e s t c e l 1 . The t e s t c e l l , as shown s c h e m a t i c a l l y i n F i g u r e 1, was a 600 ml \ ( p o l y t e t r a f 1 u o r e t h y l e n e ) T e f l o n b e a k e r f i t t e d w i t h a T e f l o n l i d . The t e m p e r a t u r e o f t h e s o l u t i o n was measured by a T e f l o n - c o a t e d t h e r m i s t o r t e m p e r a t u r e probe c o n n e c t e d t o T a b l e I Chemical C o m p o s i t i o n o f Type 316 S t e e l s Rod M a t e r i a l C Mn S i Ni Cr Mo P S Fe B a t c h 1 0.09 1 .67 0.4 11 .33 17.33 2.57 0.012 0.021 b a l .wt% B a t c h 2 0.09 1 .69 0.38 11.52 16.6 2 0.014 0.014 t i a l . wt% P l a t e M a t e r i a l : C Mn Si Ni Cr Mo P S Fe -0.09 1 .75 0.38 •10.29 15.7 2.0 0.015 0.015 b a l wt% 1 5 rtltt Ti- ll M-L' i 11 11 c ! L. U — J J 'dL • i Hi LJ e it—t - r — 1 L T~l J F i g u r e 1: T e s t c e l l f o r p o l a r i z a t i o n s t u d i e s a) t e s t e l e c t r o d e (b) L u g g i n c a p i l l a r y c) c o u n t e r e l e c t r o d e (d) t e m p e r a t u r e probe e) n i t r o g e n purge ( f ) 1 i d g) b e a k e r . 16 a t e m p e r a t u r e c o n t r o l l e r . The c o n t r o l l e r r e g u l a t e d t h e c u r - r e n t t o a h e a t i n g m a n t l e i n wh i c h t h e t e s t c e l l was e n c l o s e d . T e m p e r a t u r e was r e g u l a t e d t o w i t h i n + 1 °C. Two 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 , t h e w o r k i n g e l e c t r o d e and t h e L u g g i n c a p i l - l a r y r e f e r e n c e e l e c t r o d e were i n s e r t e d t h r o u g h t h e l i d . T h e r e were o p e n i n g s i n t h e l i d f o r t h e t e m p e r a t u r e p r o b e , r e f l u x c o n - d e n s e r , T e f l o n c o a t e d t h e r m o m e t e r , and n i t r o g e n purge l i n e . The T e f l o n L u g g i n c a p i l l a r y c o n t a i n e d a c o t t o n t h r e a d and was f i l l e d w i t h s a t u r a t e d ( a t 24 °C) KCl s o l u t i o n . The c o t t o n t h r e a d m i n i m i z e d v a p o r b u b b l e f o r m a t i o n . The L u g g i n c a p i l - l a r y c o n n e c t e d v i a a s a t u r a t e d KCl s a l t b r i d g e t o an e x t e r n a l s t a n d a r d c a l o m e l e l e c t r o d e a t 24°°C. The s o l u t i o n s were made w i t h r e a g e n t g r a d e NaOH p e l l e t s and N a £ S ' 9 H 2 0 h y d r a t e . D i s t i l l e d w a t e r was employed which was f i r s t b o i l e d and p u r g e d w i t h USP n i t r o g e n b e f o r e a d d i n g the c h e m i c a l s . The purge was c o n t i n u e d t h r o u g h o u t t h e t e s t s . The p o t e n t i a l was s c a n n e d d u r i n g the t e s t by u s i n g 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 p o t e n t i o s t a t (Model 173) e q u i p p e d w i t h an e l e c t r o m e t e r probe (Model 1 7 8 ) , 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 (Model 376) and programmer (Model 1 7 5 ) . The p o t e n - t i a l and l o g a r i t h m o f c u r r e n t were r e c o r d e d on an X-Y r e c o r d e r . 2.1.2 P r o c e d u r e The t e s t s o l u t i o n was p l a c e d i n the c e l l , t h e n b r o u g h t 17 t o t h e t e s t t e m p e r a t u r e . The s p e c i men was i n s e r t e d and a c a t h o d i c p o t e n t i a l (< 1 . 2 5 V$QIT) w a s a p p l i e d f o r 30 m i n u t e s to remove any o x i d e f i l m s . A f t e r w a r d s , the p o t e n t i a l was s c a n n e d i n t h e a n o d i c d i r e c t i o n a t 1 mV/see. C u r r e n t and p o t e n t i a l were r e c o r d e d a u t o m a t i c a l l y . 2 . 2 Slow S t r a i n Rate T e s t s 2 . 2 . 0 I n t r o d u c t i o n S t r e s s c o r r o s i o n t e s t s were c o n d u c t e d a t known p o t e n t i a l w i t h r e s p e c t t o 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 i n o r d e r t o c o r r e l a t e SCC w i t h e l e c t r o c h e m i c a l b e h a v i o r . The slow s t r a i n r a t e t e s t (SSRT) p r o v i d e s a q u i c k t e s t t o d e t e r m i n e s u s c e p t i b i l i t y t o SCC. ' A w a i s t e d , c y l i n d r i - c a l t e n s i l e s p e c i m e n , s u r r o u n d e d by t e s t s o l u t i o n , i s p u l l e d a t a v e r y s l o w r a t e u n t i l i t f a i l s . P o t e n t i a l i s c o n t r o l l e d d u r i n g th e t e s t . I f t h e t e s t i s c o n d u c t e d under i n e r t c o n d i t i o n s , t h e f a i l u r e i s d u c t i l e , and t h e s p e c i m e n necks b e f o r e f r a c t u r e so t h a t t h e f i n a l a r e a o f t h e f r a c t u r e d s u r f a c e i s s m a l l . I f t h e t e s t i s c o n d u c t e d under c o n d i t i o n s o f s u s c e p t i b i l i t y , c r a c k i n g o c c u r s . The t e s t i s s h o r t e r b e c a u s e the s p e c i men f r a c t u r e s b e f o r e i t has n ecked p a r t l y or f u l l y and t h e f i n a l a r e a o f t h e f r a c t u r e i s l a r g e r . By m e a s u r i n g the a r e a s o f the f r a c t u r e s 18 as a f r a c t i o n o f t h e o r i g i n a l c r o s s - s e c t i o n a l a r e a o f t h e s p e c i m e n , s u s c e p t i b i 1 i t y can be d e t e r m i n e d . I f s u s c e p t i b l e , t h e p e r c e n t r e d u c t i o n i n a r e a w i l l be l e s s t h a n i t would be i n i n e r t c o n d i t i o n s . The p e r c e n t r e d u c t i o n i n a r e a i s p l o t - t e d v e r s u s p o t e n t i a l t o d e t e r m i n e r e g i m e s o f SCC s u s c e p t - i b i 1 i t y . 2.2.1 M a t e r i a l s and P r e p a r a t i o n Type 316 s t a i n l e s s s t e e l , r e c e i v e d as 9.5 mm d i a m e t e r r o d , was used t o f a b r i c a t e t h e SSRT s p e c i m e n s . The m a t e r i a l c o m p o s i t i o n has been l i s t e d i n T a b l e I. The s p e c i m e n s were 25.4 cm l o n g , t h r e a d e d a t each end. A c e n t r a l 25.4.mm c y l i n - d r i c a l gage s e c t i o n 4 mm i n d i a m e t e r was machined i n each s p e c i m e n . Most s p e c i m e n s were t e s t e d as r e c e i v e d . One was t e s t e d a f t e r b e i n g a n n e a l e d a t 1050 °C f o r 1 h o u r , t h e n quenched i n w a t e r . T h r e e were t e s t e d a f t e r b e i n g • a n n e a l e d f o r T h o u r , s e n s i t i z e d a t 650 °C f o r 2 h o u r s * and t h e n q u e n c h e d . i n w a t e r . S e n s i t i z a t i o n t r e a t m e n t was s i m i l a r t o t h a t used by o t h e r s . ^ > 6 4 D u r i n g h e a t t r e a t i n g t h e s p e c i m e n s were e n c l o s e d i n s t a i n l e s s s t e e l (Sen Pak) e n v e l o p e s . The m e c h a n i c a l p r o p e r t i e s f o r the 3 m a t e r i a l h i s t o r i e s a r e l i s t e d i n T a b l e I I . The s p e cimen gage s e c t i o n s were p o l i s h e d w i t h 3/0 emery paper and d e g r e a s e d wi t h c h l o r e t h a n e . Each specimen was wrap- ped w i t h T e f l o n t a p e , l e a v i n g o n l y t h e gage s e c t i o n e x p o s e d . T a b l e II M e c h a n i c a l P r o p e r t i e s o f SSRT Specimens - M a t e r i a l Hi s t o r y Y i e l d S t r e n g t h a t 92 °C MPa U l t i m a t e T e n s i Te S t r e n g t h a t 92 °C MPa' Hardness HRB A s - r e c e i ved 283 530 87 A n n e a l e d 170 - 72 A n n e a l e d & S e n s i t i z e d 170 - 70 20 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 n eeded. The c e l l used f o r t h e SSRT 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. I t s c o n s t r u c t i o n was s i m i l a r t o t h a t o f t h e c e l l used i n t h e p o l a r i z a t i o n s t u d i e s . The s p e c i m e n f i t t e d t i g h t l y i n t o the c e l l bottom t o p r e v e n t l e a k a g e o f t h e s o l u t i o n . The t h r e a d e d ends o f t h e s p e c i m e n screwed i n t o g r i p s w hich p i n n e d i n t o a f l o o r model I n s t r o n t e s t i n g m a chine. H e a t i n g t a p e , wrapped a r o u n d the c e l l , was r e g u l a t e d w i t h a t h e r m i s t o r probe and t e m p e r a t u r e c o n t r o l l e r as d e s c r i b e d i n S e c t i o n 2.1.1. The p o t e n t i a l was measured w i t h r e s p e c t t o a room tempera- t u r e . 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 (SCE) and c o n t r o l l e d t o + 0.005 V d u r i n g t h e t e s t w i t h t h e same p o t e n t i o s t a t used i n t h e p o l a r i z a t i o n s t u d i e s . A r o l l t y p e c h a r t was used t o r e c o r d l o a d v e r s u s e l o n g a t i o n d u r i n g t h e t e s t . S o l u t i o n s were made as d e s c r i b e d i n S e c t i o n 2.1.1. 2.2.2 P r o c e d u r e The SSRT s p e c i m e n was i n s e r t e d i n t h e t e s t c e l l and p l a c e d i n t h e I n s t r o n . The s o l u t i o n f o r t h e t e s t , f r e s h l y mixed and h e a t e d t o t e m p e r a t u r e was p oured i n t o t h e c e l l . N i t r o g e n purge was begun i m m e d i a t e l y t o p r o v i d e s t i r r i n g and t o p r e v e n t o x i d a t i o n o f s u l f i d e i n t h o s e t e s t s u s i n g s u l f i d e . H e a t i n g t a p e was wrapped around t h e c e l l and h e a t a p p l i e d . Slow strain rate test c e l l a) specimen b) Luggin c a p i l l a r y c) c e l l d) l i d e) reflux condenser f) temperature probe g) nitrogen purge h) Teflon c e l l bottom i) counter electrode 22 A c a t h o d i c p o t e n t i a l o f - 1 . 2 5 w a s a p p l i e d t o t h e s p e c i m e n f o r 3 0 m i n u t e s p r i o r t o t h e t e s t t o r e d u c e a n y s u r - f a c e f i l m . T h e p o t e n t i a l w a s t h e n s e t t o t h e t e s t p o t e n t i a l . A f t e r a n o t h e r 30 m i n u t e s , t h e c r o s s h e a d o f t h e I n s t r o n w a s - 6 -1 s e t i n m o t i o n t o g i v e a s t r a i n r a t e o f 3 . 3 x 1 0 " S~.•; a n d t h e l o a d r e c o r d e d . E a c h t e s t l a s t e d ; -\. 4 8 h o u r s , e n d i n g w i t h f r a c t u r e . I f S C C o c c u r r e d , t h e t e s t t i m e w a s s h o r t e r . When t h e s p e c i m e n w a s r e m o v e d f r o m t h e c e l l , t h e d i a m e t e r o f t h e f r a c t u r e s u r - f a c e s w a s m e a s u r e d w i t h a t r a v e l l i n g m i c r o s c o p e a n d t h e p e r - ; c e n t r e d u c t i o n i n a r e a c a l c u l a t e d . T h e f r a c t u r e s u r f a c e s o f t h e s p e c i m e n s w e r e e x a m i n e d a n d p h o t o g r a p h e d i n a 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 . I n s o m e o f t h e t e s t s , o n e - h a l f o f t h e f a i l e d g a g e s e c t i o n w a s m o u n t e d i n ' Q u i c k m o u n t ' e p o x y a n d p o l i s h e d t o 1 ym w i t h d i a m o n d p a s t e . T h e m o u n t e d s p e c i m e n w a s t h e n e t c h e d w i t h a m i x t u r e o f 10 ml H N 0 3 , 10 ml C H 3 C 0 0 H , a n d 1 8 ml H C l w i t h 4 d r o p s o f g l y c e r o l . E x a m i n a t i o n a n d p h o t o g r a p h y w e r e c o n d u c t e d w i t h a Z e i s s U l t r a - p h o t o p t i c a l m i c r o s c o p e . A n a l i q u o t o f s o l u t i o n w a s t a k e n f r o m o n e t e s t t o d e t e r - m i n e c h l o r i d e c o n c e n t r a t i o n r e s u l t i n g f r o m l e a k a g e o f KC1 f r o m t h e L u g g i n c a p i l l a r y . A n a u t o m a t i c t i t r a t o r ( R a d i o - m e t e r T T T 8 0 ) a n d a n a u t o m a t i c b u r e t t e ( R a d i o m e t e r A B U - 8 0 ) 23 were employed i n t h e a n a l y s i s . 2. 3 F r a c t u r e M e c h a n i c s T e s t i n g ' 2.3.0 I n t r o d u c t i o n P o l a r i z a t i o n s t u d i e s and s l o w s t r a i n r a t e t e s t s t o g e t h e r p r o v i d e i n f o r m a t i o n on t h e p o t e n t i a l a t w h i c h SCC may o c c u r i n a g i v e n e n v i r o n m e n t , and on 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 p r o m o t i n g s u s c e p t i b i l i t y . F r a c t u r e m e c h a n i c s t e s t i n g can add t o t h i s u n d e r s t a n d i n g by g i v i n g i n f o r m a t i o n on k i n e t i c f a c t o r s and p r o v i d i n g u s e f u l i n f o r m a t i o n f o r e q u ipment d e s i g n e r s and o p e r a t o r s . Specimens used i n f r a c t u r e m e c h a n i c s t e s t i n g a r e p r e - n o t c h e d and f a t i g u e p r e - c r a c k e d t o overcome i n i t i a t i o n p r o b - lems and p r o v i d e d a t a on c r a c k growth k i n e t i c s . The s p e c i m e n s a r e d e s i g n e d so t h a t l o a d , c r a c k l e n g t h , and s t r e s s i n t e n s i t y may be r e l a t e d by a known K j - c a l i b r a t i o n r e l a t i o n . The s t r e s s i n t e n s i t y , K j , can t h e n be c a l c u l a t e d f o r a g i v e n l o a d and c r a c k l e n g t h t o g i v e a measure o f t h e i n t e n s i t y o f s t r e s s a t 4. ft fi R the c r a c k t i p . ' Rate o f c r a c k growth may be r e l a t e d t o s t r e s s i n t e n s i t y t h r o u g h o u t th e t e s t . In c o r r o s i v e e n v i r o n - ments, t h e r e i s o f t e n a s t r e s s i n t e n s i t y below which no 6 5 c r a c k i n g can be d e t e c t e d ; t h i s i s c a l l e d K ^ ^ Q . A S KJ. I S r a i s e d above t h i s v a l u e , c r a c k growth r a t e r i s e s t o o . T h i s r e g i o n o f K j - d e p e n d e n t v e l o c i t y i s c a l l e d R e g i o n I. A r e g i o n o f K T - i n d e p e n d e n t c r a c k growth r a t e e x t e n d s t h r o u g h 24 i n t e r m e d i a t e v a l u e s o f K^; t h i s i s Region I I . F i n a l l y t h e growth r a t e b e g i n s t o i n c r e a s e a g a i n u n t i l f a i l u r e o c c u r s a t KI(.; t h i s i s R e g i o n I I I . T h i s b e h a v i o r has been o b s e r v e d by a number o f i n v e s t i g a t o r s f o r a v a r i e t y o f a l l o y s and s o l u - t i c s . 1 3 ' 4 8 ' 4 9 ^ISCC l s d i f f i c u l t t° d e t e r m i n e b e c a u s e c r a c k i n g r a t e becomes i n f i n i t e l y slow as Kj i s d e c r e a s e d t oward ^ j 5 Q Q * I n i t i a t i o n would have t o t a k e p l a c e j u s t above K J C . Q C b e c a u s e the Kj v a l u e i n c r e a s e s d u r i n g c r a c k i n g f o r t h i s s p e c i m e n d e s i g n R e g i o n I i s o f i n t e r e s t t o d e s i g n e r s b e c a u s e equipment which i s c r a c k i n g spends a l a r g e p o r t i o n o f i t s l i f e t h e r e . R e g i o n II i s a l s o i m p o r t a n t and i t i s o f s p e c i a l i n t e r e s t t o r e s e a r c h e r s s t u d y i n g t h e c a u s e s o f SCC because t h e c r a c k growth k i n e t i c s i n t h a t r e g i o n a r e c o n t r o l l e d s o l e l y by s t r e s s i n d e p e n d e n t p r o c e s s e s . The T - n o t c h d o u b l e c a n t i l e v e r beam (TN-DCB) was o r i g i - 1 4 n a t e d by R u s s e l and Tromans. I t i s i l l u s t r a t e d i n F i g u r e 3. The Kj c a l i b r a t i o n f o r the TN-DCB specimen i s g i v e n by: Kj'= (T.172. x 1 0 5 ) P ( a ) 0 ' 5 ' [2 . 43-3 . 62 ( a / 0 . 032 ) + 1 4 . 5 ( a / 0 . 0 3 2 ) 2 - 24. 6(a/0 .03'2) 3 + 2 6 . 5 ( a / 0 . 0 3 2 ) 4 ] . . . (1 ) 2 5 F i g u r e 3 T - n o t c h d o u b l e c a n t i l e v e r b e a m s p e c i m e n . 26 E r r o r s due t o t h e downward growth o f c r a c k s have been c a l c u - 48 l a t e d by R u s s e l , but have been c o n s i d e r e d t o be n e g l i g i b l e i n t h i s p r e s e n t s t u d y . The TN-DCB has been used h e r e b e c a u s e : 1. I t i s t h i n , so t h e c r a c k l e n g t h measured i n t h e s u r f a c e w i l l n o t d i f f e r g r e a t l y from th e c r a c k f r o n t a t t h e c e n - t e r . 2. R e s e a r c h has been done on t h i s m a t e r i a l i n M g C ^ u s i n g 14 48 t h i s s p e c i men d e s i g n . T h i s s i m i l a r i t y w i l l a i d c o m p a r i s o n o f b e h a v i o r i n t h e s e d i f f e r e n t e n v i r o n m e n t s . The specimen has some d i s a d v a n t a g e s : 1. The v e r y low l o a d s r e q u i r e d t o p r o d u c e a f a t i g u e p r e - c r a c k a r e d i f f i c u l t t o a p p l y a c c u r a t e l y and t h e s t r e s s i n t e n s i t i e s r e q u i r e d t o p r e - c r a c k p r e c l u d e SCC t e s t i n g below t h a t s t r e s s i n t e n s i t y . 2. The s p e c i m e n i s t h i n , so p l a n e s t r a i n c o n d i t i o n s would n o t be s a t i s f i e d a t h i g h s t r e s s i n t e n s i t i e s , d e p e n d i n g on y i e l d s t r e n g t h . To f u l f i l t h e c r i t e r i o n f o r p l a n e - s t r a i n t e s t i n g t h e 2 66 s p e c i m e n t h i c k n e s s must be >'2.5 ( K / a y ) . For t h e m a t e r i a l u s e d , p l a n e s t r a i n c o n d i t i o n s a r e s a t i s f i e d o n l y t o a s t r e s s 27 i n t e n s i t y v a l u e o f . 17 MPa/nT (a s s u m i n g -a = 475 MPa). In s p i t e o f t h i s , t e s t i n g was c o n d u c t e d up t o 100 •-.MRa/m" a t which p o i n t g r o s s d e f o r m a t i o n began. The c r a c k s were s t r a i g h t and d i d n o t seem t o a v o i d p r o p a g a t i o n 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 zones as has been s u g g e s t e d . ^ 2.3.1 M a t e r i a l s and P r e p a r a t i o n Type 316 s t a i n l e s s s t e e l from a s i n g l e p l a t e was u s e d . I t s c o m p o s i t i o n has been l i s t e d i n T a b l e I . The p l a t e was c o l d - r o l l e d t o an i n t e r m e d i a t e t h i c k n e s s , t h e n ?. .a itn.e.a l e d:. .'at. 1050 °C f o r 30 m i n u t e s i n a s t a i n l e s s s t e e l f o i l e n v e l o p e (Sen P a k ) . I t was t h e n quenched i n w a t e r . F u r t h e r c o l d r o l l i n g l e f t t h e m a t e r i a l w i t h a t h i c k n e s s o f 3.2 mm and 25% c o l d work. The c o l d worked p l a t e had a y i e l d s t r e n g t h o f 475 MPa and u l t i m a t e t e n s i l e s t r e n g t h o f 830 MPa. Ha r d n e s s measured HRC 25. The p l a t e m a t e r i a l and worked 14 48 c o n d i t i o n were i d e n t i c a l t o t h o s e employed by R u s s e l . ' One s p e c i m e n was made o f m a t e r i a l w hich was r o l l e d t o i t s f i n a l t h i c k n e s s t h e n a n n e a l e d .at 1050 °C f o r 1 h o u r . T h u s , i t was f r e e b f c o l d work. Ha r d n e s s measured HRB 76. Specimens were machined from t h e r o l l e d m a t e r i a l so t h a t c r a c k s would p r o p a g a t e i n t h e r o l l i n g d i r e c t i o n . The s p e c i - mens were p o l i s h e d t o 600 g r i t on one s i d e and t h e s u r f a c e 2 8 was s c r i b e d w i t h f i d u c i a l l i n e s 1 mm a p a r t t o be used t o measure c r a c k v e l o c i t y . S t a r t e r c r a c k s were sawn i n each s p e c i m e n w i t h a j e w e l - l e r s saw. The 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 t o r e d u c e i n i t i a t i o n t i m e . The f a t i g u i n g was c a r r i e d out a t a maximum s t r e s s i n t e n s i t y v a l u e l o w e r than t h a t t o be used i n t h e t e s t . A S o n n t a g f a t i g u e machine was u t i l i z e d f o r p r e - c r a c k i n g . F i n a l l y t h e s p e c i m e n s were wrapped w i t h T e f l o n t a p e l e a v i n g a band o f e x p o s e d a r e a i n t h e v i c i n i t y o f t h e c r a c k and i t s r e g i o n o f p r o p a g a t i o n . The c o r r o s i o n c e l l s used i n t h e t e s t s were o f a s i m i l a r c o n s t r u c t i o n t o t h a t used i n t h e p o l a r i z a t i o n s t u d i e s ( S e c t i o n 2 . 1 . 1 ) . The s o l u t i o n s were mixed i n t h e same way, n i t r o g e n p u r g i n g c o n t i n u e d t h r o u g h o u t t h e t e s t s , t e m p e r a t u r e was co n - t r o l l e d t o + 1 °C, and p o t e n t i a l was c o n t r o l l e d t o + 0.005 V w i t h r e s p e c t t o an e x t e r n a l s t a n d a r d c a l o m e l e l e c t r o d e . Each s p e c i m e n was mounted i n i t s t e s t c e l l and l o a d e d as shown i n F i g u r e 4. T e n s o m e t e r s ( H o u n s f i e l d and Monsanto) were used m o s t l y , w i t h a s p r i n g i n s e r i e s w i t h t h e specimen t o m i n i m i z e l o a d drop r e s u l t i n g from c r a c k i n g . Some s p e c i - mens were l o a d e d by a c a l i b r a t e d homemade d e v i c e which used a F r a c t u r e m e c h a n i c s t e s t i n g c e l a) specimen b) g r i p s c ) p i n s d ) c e l l l i d e) b e a k e r . 30 w e i g h t e d l e v e r t o a p p l y the l o a d . The p o t e n t i a l was a p p l i e d i n t h e same manner as f o r the sl o w s t r a i n r a t e t e s t s . A v a r i e t y o f p o t e n t i o s t a t s was u s e d , i n c l u d i n g ECO model 549 and Wenking models OPA 69 and 68TS10. 2.3.2 P r o c e d u r e The s p e c i m e n and c e l l were p l a c e d i n t h e t e n s o m e t e r , the s o l u t i o n was added, and p o t e n t i a l was a p p l i e d . No c a t h o - d i c p o t e n t i a l was a p p l i e d t o r e d u c e s u r f a c e f i l m s p r i o r t o t h e t e s t . Load was a p p l i e d 30 m i n u t e s a f t e r e s t a b l i s h i n g t h e p o t e n t i a l c o n t r o l . The L u g g i n c a p i l l a r y had t o be r e p l a c e d a b out once p e r week b e c a u s e t h e c o t t o n t h r e a d d i s s o l v e d o u t o f i t s t i p and a b u b b l e f o r m e d , b r e a k i n g t h e c i r c u i t . . The t e s t s o l u t i o n was r e p l a c e d p e r i o d i c a l l y w i t h f r e s h s o l u t i o n . The l e v e l o f s o l u t i o n i n t h e c e l l was m a i n t a i n e d d a i l y by a d d i n g a few m i l l i l i t e r s o f b o i l e d d i s t i l l e d w a t e r . The c e l l was opened e v e r y few days t o measure t h e c r a c k l e n g t h . T h i s was done v i s u a l l y w i t h t h e a i d o f t h e . s c a l e s c r i b e d on the specimen s u r f a c e . Time, l o a d , c r a c k l e n g t h and c a l c u l a t e d s t r e s s i n t e n s i t y were r e c o r d e d . In t h o s e c a s e s where c r a c k i n g o c c u r r e d on one s i d e o f t h e specimen o n l y , the o t h e r s i d e was sawed p e r i o d i c a l l y to a m a t c h i n g l e n g t h t o m a i n t a i n p r o p e r l o a d i n g g e ometry. 31 At t h e c o m p l e t i o n o f some o f t h e t e s t s , t h e c o r r o s i o n c e l l was l o w e r e d away from t h e s p e c i m e n and t h e s u r f a c e o f t h e s p e c i m e n was d r i e d q u i c k l y w i t h a b s o r b e n t p a p e r t i s s u e . pH i n d i c a t o r p a p e r ( p H y d r i o n ) w i t h pH i n t e r v a l s o f 0.5, was p r e s s e d a g a i n s t t h e specimen a t t h e c r a c k , so t h a t t h e p a p e r a b s o r b e d s o l u t i o n d r a i n i n g from t h e c r a c k . The i n d i c a t o r p a p e r showed t h a t t h i s c r a c k s o l u t i o n has a pH o f <\* 14 a t room t e m p e r a t u r e . The T e f l o n t a p e was removed and t h e s p e c i m e n was r i n s e d w i t h d i s t i l l e d w a t e r and e t h a n o l b e f o r e b e i n g s e c t i o n e d . Specimens w i t h heavy c o r r o s i o n d e p o s i t s were c l e a n e d u l t r a - s o n i c a l l y w i t h an i n h i b i t e d a c i d s o l u t i o n composed o f 3 ml H C l , 4 ml 2 - B u t y n e - l , 4 d i o l ( 3 5 % aqueous s o l u t i o n ) p l u s 50 ml 6 8 d i s t i l l e d w a t e r which p r o d u c e d no a r t i f a c t s . The f r a c t o g r a p h y was examined i n an ETEC 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 a t 20 keV. The c r a c k l e n g t h s measured d u r i n g t h e t e s t were p l o t t e d v e r s u s t i m e . A l i n e was f i t t e d to t h i s d a t a u s i n g t h e l e a s t s q u a r e s method i f t h r e e a p p r o x i m a t e l y c o l i n e a r p o i n t s were a v a i l a b l e . O t h e r w i s e t h e l i n e t h r o u g h two p o i n t s was c a l c u l a - t e d . T h i s was t h e c a s e f o r most R e g i o n I d a t a . D i f f e r e n t i a - t i o n o f t h e e q u a t i o n f o r t h e l i n e y i e l d e d t h e v e l o c i t y . The v e l o c i t y has been p l o t t e d as a c o n s t a n t v a l u e f o r R e g i o n II ( 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 ) . E r r o r was c a l c u l a - t e d f o r 95% c o n f i d e n c e l i m i t s u s i n g c o n s t a n t s from S t u d e n t ' s 69 t - d i s t r i b u t i o n as d e s c r i b e d by S t a n t o n . V a l u e s f o r a p p a r e n t a c t i v a t i o n e n e r g y were c a l c u l a t e d i n t h e same manner u s i n g 90% c o n f i d e n c e l i m i t s . 2.4 E l e c t r o n D i f f r a c t i o n ; A n a l y s i s o f S u r f a c e F i l m s 2.4.0 I n t r o d u c t i o n A n a l y s i s o f the c o r r o s i o n f i l m s r e m a i n i n g on t h e f r a c t u r e s u r f a c e s ^ may p r o v i d e i m p o r t a n t c l u e s about the mechanism o f c r a c k i n g . D i f f e r e n c e s i n c o m p o s i t i o n between the f i l m and t h e m a t r i x may show which e l e m e n t s have been d i s s o l v e d d u r i n g c r a c k i n g . The f i l m s t r u c t u r e may be formed o f t h e p r o d u c t s o f v a r i o u s d i s s o l u t i o n r e a c t i o n s . Examina- t i o n o f the c r y s t a l s t r u c t u r e may p r o v i d e i n s i g h t i n t o t h e s e r e a c t i o n s . 2.4.1 S a m p l i n g f o r C o r r o s i o n F i l m A n a l y s i s F r a c t u r e m e c h a n i c s (TN-DCB) s p e c i m e n s from t e s t s i n 3.35 mol/kg NaOH a t 92 °C were s e c t i o n e d w i t h a j e w e l l e r s saw t o e x p o s e the f r a c t u r e s u r f a c e s . S e c t i o n s were p l a c e d i n s h a l l o w d i s h e s and submerged i n a s o l u t i o n o f 1 volume p e r c e n t bromine i n methanol as d e s c r i b e d by N i k i f o r u k . ^ A f t e r 2 'days, th e s e c t i o n s were t a p p e d l i g h t l y t o d i s l o d g e 3 3 the f i l m and t h e n removed. S m a l l p i e c e s o f t h e s u r f a c e f i l m were l e f t f l o a t i n g i n the m e t h a n o l - b r o m i n e m i x t u r e . These were p i c k e d up w i t h a f i n e c o p p e r e l e c t r o n m i c r o s c o p e s p e c i m e n g r i d and p l a c e d i n methanol t o remove t r a c e s o f b r o m i n e . Sub- s e q u e n t l y , t h e f i l m s were p i c k e d up w i t h a f i n e c o p p e r g r i d c o v e r e d w i t h a s u p p o r t f i l m o f c a r b o n and p l a c e d on a b s o r b e n t f i l t e r p a p e r t o d r y . A f t e r w a r d s t h e g r i d w i t h f i l m s was s t o r e d i n a d e s i c c a t o r . 2.4.2 P r o c e d u r e The g r a t i n g w i t h f i l m on i t , was i n s e r t e d i n a sample h o l d e r and p l a c e d i n t h e t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e , H i t a c h i model HU-11A, f o r e x a m i n a t i o n a t 100 keV. A f i e l d l i m i t i n g a p e r t u r e was used t o s e l e c t t h e a r e a from which an e l e c t r o n d i f f r a c t i o n r i n g p a t t e r n would be o b t a i n e d . A f t e r t h e p h o t o g r a p h i c p l a t e was e x p o s e d t o t h e e l e c t r o n d i f f r a c t i o n p a t t e r n , t h e d i f f r a c t i o n r i n g p a t t e r n o f an e v a p o r a t e d g o l d f i l m s t a n d a r d was p h o t o g r a p h e d . T h i s s e r v e d as a c a l i b r a t i o n s t a n d a r d . The d i s t a n c e s , d, between c r y s t a l p l a n e s i n t h e l a t t i c e were known f o r t h e g o l d s t a n d a r d . The d i a m e t e r , D, o f the c o r r e s p o n d i n g d i f f r a c t i o n r i n g s were measured on t h e d i f - f r a c t i o n p a t t e r n . U s i n g e q u a t i o n 2, i t was p o s s i b l e t o d e t e r m i n e t h e camera c o n s t a n t , . <: 34 Dd = . . . ( 2 ) The d i a m e t e r s o f t h e r i n g s were measured on t h e d i f f r a c - t i o n p a t t e r n o f t h e u n i d e n t i f i e d s u r f a c e f i l m , and u s i n g the camera c o n s t a n t , t h e , d - s p a c i n g was c a l c u l a t e d . The d - s p a c i n g and t h e r e l a t i v e i n t e n s i t i e s were matched w i t h t h o s e l i s t e d i n t h e powder d i f f r a c t i o n c a t a l o g ^ 1 t o i d e n t i f y t h e compound i n t h e f i l m . These were c o n f i r m e d by c o m p a r i s o n w i t h t h e o r e t i - c a l e s t i m a t e s o f r e l a t i v e i n t e n s i t i e s f o r e l e c t r o n d i f f r a c - 72 t i o n i n FegO^ as c a l c u l a t e d by B i r l e y . The p a t t e r n s were f o u n d t o be c h a r a c t e r i s t i c o f s p i n e l s t r u c t u r e s e x h i b i t i n g c u b i c symmetry f o r w h i c h : d h k l = a o / ( h 2 + k 2 + l 2 ) 0 - 5 , . . . ( 3 ) where a i s t h e l a t t i c e p a r a m e t e r and h, k and 1 a r e the M i l l e r i n d i c e s o f t h e p l a n e o f i n t e r e s t . The f a c t o r / h +k +1 ; was p l o t t e d v e r s u s t h e d i a m e t e r o f the r i n g s o f t h e d i f f r a c t i o n p a t t e r n and a l i n e was f i t t e d to the d a t a . From t h i s , t h e l a t t i c e p a r a m e t e r , a Q , c o u l d be c a l c u l a t e d u s i n g e q u a t i o n 3. 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 3 . 1 . 0 NaOH 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 have been drawn w i t h r e s p e c t t o two d i f f e r e n t p o t e n t i a l s c a l e s : t h e s t a n d a r d c a l o m e l e l e c t r o d e , V $ C E ' a n d t h e s t a n d a r c l h y d r o g e n e l e c t r o d e , ^SHE" These a r e r e l a t e d by: VSHE = - ° ' 2 4 1 6 V S C E • • ' ( 4 ) No c o r r e c t i o n was made t o t h e p o t e n t i a l t o a c c o u n t f o r t h e r m a l g r a d i e n t e f f e c t s and l i q u i d j u n c t i o n s . F i g u r e 5 shows t h e c u r v e f o r 3 1 6 s t a i n l e s s s t e e l r o d i n 3 . 3 5 mol/kg NaOH a t 9 2 °C. The a c t i v e - p a s s i v e t r a n s i t i o n " nose" l o c a t e d a t - 1 . 0 0 V S C E was f l a t and d o u b l e peaked. The p r i m a r y p a s s i v e range e x t e n d e d from - 0 . 7 5 t o - 0 . 2 5 V ^ F . A N C ' A p r i m a r y t r a n s p a s s i v e r e g i o n was o b s e r v e d a t - 0 . 1 0 V ^ r ^ . From 0 t o 0 . 2 5 V S C E t h e r e was a s e c o n d a r y p a s s i v e r e g i o n . F i g u r e 6 shows t h e p o l a r i z a t i o n c u r v e f o r t h e p l a t e m a t e r i a l . I t was s i m i l a r t o F i g u r e 5 e x c e p t t h a t t h e a c t i v e - p a s s i v e "nose" e x h i b i t e d a s i n g l e peak. The e f f e c t o f NaOH i s shown i n F i g u r e 7 . I n c r e a s i n g t h e c o n c e n t r a t i o n i n c r e a s e d t h e c u r r e n t d e n s i t y , and d i s p l a c e d 36 0.50 CURRENT DENSITY A/m2 F i g u r e 5 A n o d i c p o l a r i z a t i o n c u r v e , 316 s t a i n l e s s s t e e l r o d , 3.35 mol/kg NaOH, 92 °C. ure 6 A n o d i c p o l a r i z a t i o n c u r v e , 316 s t a i n l e s s s t e e l p l a t e , 3.35 mol/kg NaOH, 92 °C. 38 0.50 CURRENT DENSITY A/m2 • i F i g u r e 7 A n o d i c p o l a r i z a t i o n c u r v e s a t s e l e c t e d NaOH c o n c e n t r a t i o n s , 316 s t a i n l e s s s t e e l r o d , 92 °C. t h e p r i m a r y t r a n s p a s s i v e ( m o r e a c t i v e ) v a l u e s . p e a k a n d c o r r o s i o n p o t e n t i a l 3 9 t o l o w e r R a i s i n g t h e t e m p e r a t u r e i n c r e a s e d t h e c u r r e n t d e n s i t y i n 3 . 3 5 m o l / k g NaOH a n d l o w e r e d t h e p o t e n t i a l o f t h e t r a n s p a s s i v e p e a k a s i l l u s t r a t e d i n F i g u r e 8 . F i g u r e 9 i l l u s t r a t e s t h e p o l a r i z a t i o n c u r v e s f o r c h r o m i u m i n t w o c o n c e n t r a t i o n s o f N a O H . T h e t r a n s p a s s i v e c u r r e n t d e n s i t y w a s s h i f t e d t o l o w e r p o t e n t i a l i n t h e m o r e c o n c e n t r a t e d s o l u - t i o n . T h e c o r r o s i o n p o t e n t i a l , E ^ O R R ' w a s i n t h e p a s s i v e r e g i o n i n 3 . 3 5 m o l / k g NaOH a n d i n t h e a c t i v e r e g i o n i n 8 m o l / k g N a O H , a c c o u n t i n g f o 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 a t - 1 . 1 0 V ^ r f n t h e l a t t e r . T h e s o l u t i o n w a s y e l l o w a f t e r c o m p l e t i o n o f t h e a n o d i c s c a n . T h e p o l a r i z a t i o n b e h a v i o r o f n i c k e l i s s h o w n i n F i g u r e 1 0 . T h e r e w a s a n a c t i v e - p a s s i v e p e a k a t - 0 . 8 5 V ^ ^ ' a n d a p a s s i v e r e g i o n a t m o r e n o b l e p o t e n t i a l s . I r o n h a d a n a c t i v e - p a s s i v e c u r r e n t p e a k a t - 1 . 0 5 V < - C E a s s h o w n i n F i g u r e 1 1 . 3 . 1 . 1 NaOH + N a 2 S F i g u r e 1 2 s h o w s t h e p o l a r i z a t i o n c u r v e f o r t h e 3 1 6 s t a i n l e s s s t e e l r o d i n 2 . 5 m o l / k g NaOH + 0 . 4 2 3 m o l / k g N a 2 S 4 0 F i g u r e 8 A n o d i c p o l a r i z a t i o n c u r v e s a t s e l e c t e d t e m p e r a t u r e s , 316 s t a i n l e s s s t e e l r o d , 3.35 mol/kg NaOH. 41 F i g u r e 9 A n o d i c p o l a r i z a t i o n c u r v e s , chromium, 3.35 and 8 mol/kg NaOH, 92 °C. F i g u r e 10 A n o d i c p o l a r i z a t i o n c u r v e , n i c k e l , 3.35 mol/kg NaOH, 92 °C. F i g u r e 11 A n o d i c p o l a r i z a t i o n c u r v e , i r o n , 3.35 mol/kg NaOH, 92 °C. 44 F i g u r e 12 A n o d i c p o l a r i z a t i o n c u r v e , : ! ;i 316 s t a i n l e s s s t e e l r o d , 2.5 mol/kg NaOH + 0.423 mol/kg Na 2S, 92 °C. 45 a t 92 °C. The c u r r e n t d e n s i t y between -1.15 and -0.75 V^^. was o f t h e same o r d e r o f m a g n i t u d e as t h a t o f t h e a c t i v e - p a s s i v e c u r r e n t peak i n the s o l u t i o n w i t h o u t s u l f i d e . F i g u r e 1 3 i 11 u s t r a t e s t h e e f f e c t o f i n c r e a s i n g c o n c e n t r a - i <> t i o n s o f NaOH. The' c o r r o s i o n , p o t e n t i a l , E Q O R R ' was i n t h e a c t i v e r e g i o n and an a c t i v e - p a s s i v e t r a n s i t i o n was e v i d e n t . The c u r r e n t peak became l a r g e r as t h e NaOH c o n c e n t r a t i o n was i n c r e a s e d , and was much l a r g e r t h a n t h a t i n t h e s u l f i d e f r e e s o l u t i ons. The e f f e c t o f t e m p e r a t u r e i n 2.5 mol/kg NaOH + 0.423 mol/kg Na 2S i s i l l u s t r a t e d i n F i g u r e 14 f o r p l a t e m a t e r i a l . The c u r r e n t d e n s i t y i n c r e a s e d and E Q Q R R d e c r e a s e d as t h e t e m p e r a t u r e was i n c r e a s e d . Two s m a l l c u r r e n t peaks were o b s e r v e d 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 , s i m i l a r t o t h o s e ii seen i n t h e NaOH s o l u t i o n s . The c u r r e n t peaks were i n s i g n i - f i c a n t compared t o t h o s e shown i n F i g u r e 13. 3.2 Slow S t r a i n Rate T e s t s 3.2 . 0 NaOH The p e r c e n t r e d u c t i o n s i n c r o s s - s e c t i o n a l a r e a o f t h e SSRT s p e c i m e n s a r e p l o t t e d v e r s u s p o t e n t i a l i n F i g u r e 15. 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 at -0.50 t o -1.50 V c r c . T h i s i n d i c a t e d t h a t t h e g r e a t e s t s u s c e p t i b i l i t y t o SCC 46 F i g u r e 13 A n o d i c p o l a r i z a t i o n c u r v e s i n s o l u t i o n s o f s e l e c t e d NaOH c o n - c e n t r a t i o n w i t h Na^S, 316 s t a i n l e s s s t e e l r o d , 92 °C. F i g u r e 14 A n o d i c p o l a r i z a t i o n c u r v e s a t s e l e c t e d t e r m p e r a t u r e s , 316 s t a i n l e s s s t e e l p l a t e . 2.5 mol/kg NaOH + 0.423 mol/kg Na 9S. 48 F i g u r e 15 E f f e c t o f p o t e n t i a l on p e r c e n t r e d u c t i o n i n a r e a d u r i n g s l o w s t r a i n r a t e t e s t s , 3.35 mol/kg NaOH, 92 °C. A n o d i c p o l a r i z a t i o n c u r v e f o r 316 s t a i n l e s s s t e e l r o d . 49 was n e a r t h e p r i m a r y t r a n s p a s s i v e peak. F i g u r e 16 shows t h e a p p e a r a n c e o f t h e f r a c t u r e r e g i o n o f th e s p e c i m e n t e s t e d a t -0.10 V S Q E - The s u r f a c e was s e v e r e l y a t t a c k e d and e x h i b i t e d i n t e r g r a n u l a r c r a c k s . The s u r f a c e s o f s p e c i m e n s t e s t e d a t 0.10, 0, -0.50 and -0.15 V s c £ , w e r e c r a c k e d s i m i l a r l y . The f r a c t u r e s e c t i o n s i n d i c a t e d t h a t c r a c k i n g o c c u r r e d i n t h e s e c o n d a r y p a s s i v e r e g i o n b u t not i n t h e p r i m a r y p a s s i v e r e g i o n . S u r f a c e c r a c k i n g was a b s e n t a t l o w e r 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 , as shown a t -0.95 V"sCE i n F i g u r e 17. An a n n e a l e d s p e c i m e n t e s t e d a t -0.10 V S C E showed s l i g h t l y l e s s s u s c e p t i b i l i t y t o SCC t h a n the a s - r e c e i v e d m a t e r i a l . A s e n s i t i z e d s p e c i m e n t e s t e d a t -1.15 V , , ^ was s l i g h t l y more s u s c e p t i b l e . One t e s t e d a t -0.85 V ^ i r showed no d i f - f e r e n c e from t h e a s - r e c e i v e d m a t e r i a l . The s u r f a c e f i l m o b s e r v e d on t h e s p e c i m e n s v a r i e d w i t h the p o t e n t i a l . I t was dark red-brown above about -0.50 V ^ ^ ^ At 0.25 V < . £ £ , i t a p p e a r e d t o be t h i c k and f a i r l y b r i t t l e , as seen i n F i g u r e 18a. At -1.0 V j - ^ , t h e s u r f a c e f i l m a p p e a r e d t o be v e r y much t h i n n e r w i t h a d i s t o r t e d a p p e a r a n c e as shown i n F i g u r e 18b. 50 F i g u r e 16 SSRT specimen a f t e r t e s t i n g a t -0.10 V - r F O L C i n NaOH s o l u t i o n . F i g u r e 17 SSRT s p e c i m e n a f t e r t e s t i n g a t -0.95 V i n NaOH s o l u t i o n . b F i g u r e 1 8 S u r f a c e f i l m on SSRT speci m e n s t e s t e d i n NaOH s o l u t i o n a) 0.25 V,,ri-, b) - 1.00 V - r F . 52 The s o l u t i o n from one t e s t (-0.25 V c ^ r r ) was a n a l y z e d f o r [ C l " ] . The c o n c e n t r a t i o n was ^ 0.03 mol/kg a f t e r a 2 day t e s t . 3.2.1 NaOH + Na^S; The p e r c e n t r e d u c t i o n s i n c r o s s - s e c t i o n a l a r e a o f t h e SSRT s p e c i m e n s a r e i l l u s t r a t e d i n F i g u r e 19. Some s u s c e p t i - b i l i t y t o SCC i n t h e a c t i v e r e g i o n was i n d i c a t e d by t h e de- c r e a s e i n r e d u c t i o n i n a r e a a t -1.15 Vc-^^. T e s t s were not c o n d u c t e d a t h i g h e r p o t e n t i a l s due t o o x i d a t i o n o f s u l f i d e i o n s as e v i d e n c e d by t h e i n c r e a s e i n c u r r e n t above -0.75 V S C E . The f r a c t u r e d r e g i o n o f t h e specimen t e s t e d a t -1.15 ^SCE ^ s "* 1 1 u s t r a t e d i n F i g u r e 20. T h e r e were no c r a c k s i n the s u r f a c e f i l m between -0.85 and -0.95 V ^ Q ^ . No e f f e c t o f s e n s i t i z a t i o n was o b s e r v e d i n sp e c i m e n s t e s t e d a t -1.00 and -1.15 V$CE" 3.3. F r a c t u r e M e c h a n i c s T e s t i n g 3.3.0 NaOH 3.3.0.0 E f f e c t o f S t r e s s I n t e n s i t y F i g u r e 21 i s a p l o t o f c r a c k growth r a t e v e r s u s Kj i n 3.35 mol/kg NaOH, 92 °C, -0.10 V $ C E . The d a t a show Region I 53 % REDUCTION IN AREA 50 60 70 80 90 -i 1 1 1 1 CURRENT DENSITY A/m 2 F i g u r e 19 E f f e c t o f p o t e n t i a l on p e r c e n t r e d u c t i o n i n a r e a d u r i n g slow s t r a i n r a t e t e s t s , 2.5 mol/kg NaOH + 0 . 4 2 3 mol/kg N a 2S, 92 °C. A n o d i c p o l a r i z a t i o n c u r v e f o r 316 s t a i n l e s s s t e e l r o d . Figure 20 SSRT specimen after testing at -1.15 V $ C E in NaOH + Na2S solution 10 >—• t> r • e—e— T°C e- 92 A - 8 2 B - 7 2 10 20 30 40 50 60 70 80 STRESS INTENSITY MPavTTT C r a c k g r o w t h r a t e v e r s u s s t r e s s i n t e n s i t y a t s e l e c t e d t e m p e r a t u r e s i n 3 . 3 5 m o l / k g N a O H , - 0 . 1 0 V c r c . 56 (Kj. d e p e n d e n t ) and R e g i o n I I (Kj. i n d e p e n d e n t ) b e h a v i o r . Region I d a t a were d i f f i c u l t t o o b t a i n b e c a u s e c r a c k i n g was so slow. K i s c C a P P e a r e c ! t 0 D e l e s s t h an TO MPa/m . The d a t a used t o c o n s t r u c t F i g u r e 21 a r e summarized i n T a b l e I I I . 3.3.0.1 T e m p e r a t u r e E f f e c t C r a c k growth r a t e d a t a i n 3.35 mol/kg NaOH a t 92, 82 and 72 °C a r e l i s t e d i n T a b l e I I I and i l l u s t r a t e d i n F i g u r e 21. R e s u l t s o b t a i n e d i n 12 mol/kg NaOH a t 92 and 82 °C a r e a l s o l i s t e d i n T a b l e I I I and a r e shown i n F i g u r e 22. The p o t e n t i a l was m a i n t a i n e d a t -0.10 V^^^ i n a l l t h e s e t e s t s . C r a c k growth r a t e d e c r e a s e d w i t h d e c r e a s i n g t e m p e r a t u r e . The e f f e c t o f t h e t e m p e r a t u r e on Re g i o n II c r a c k ! growth r a t e was e v a l u a t e d assuming a s i m p l e A r r h e n i u s : r a t e law: V = V Q exp (-Q/RT) .. . ( 5 ) where V = c r a c k growth r a t e V Q= e x p e r i m e n t a l c o n s t a n t Q = a p p a r e n t a c t i v a t i o n e n e r g y R = gas c o n s t a n t T = t e m p e r a t u r e (°K) The l o g a r i t h m o f c r a c k growth r a t e was p l o t t e d v e r s u s 1/T as shown i n F i g u r e 23. A l i n e was f i t t e d t o t h e 3.35 5 7 TABLE I I I Summary of F r a c t u r e Mechanics Test Data NaOH Concentration mol/kg 3.35 3.35 3.35 12 12 3.35 3.35 T 92 82 72 92 92 82 92 92 SCE -0.10 -0.10 -0.10 -0.10 -0.10 -0.10 0.0 -0.175 Kj Range MPa^n 10 - 10.5 18 - 22.6 22.6 - 27 27 33 50 40 40 30 - 30.7 - 52.7 - 39.3 - 98.3 - 110.3 - 45.7 26 35.6 35.5 72.1 20 - 23.5 23.5 - 34 34 - 75.4 15 27.6 27.6 73.1 16 - 52 15 - 28.8 36.3 - 63.1 25.8 - 30.9 30.9 - 94.4 30 - 35.2 35.2 - 64.8 25 46 Crack Growth Rate m/s 4.29 + 2.15)xlC' 4.04 + 0.81)x!0 6.17 + 1.23)xlO" 7.26 + 1.82)xl0" 1.02 + 0.08)xl0" 0.94 + 0.27)xl0 1.02 + 0.34)xl0 1.04 + 0.17)xl0 0.90 + 0.18)xl0 •10 -9 -8 -8 -8 -8 2.5 +0.19)xl0" 5.39 + 0.90)xl0" 1.05 + 0.55)xl0 2.09 + 0.22)xl0 3.09 + 0.25)x1O -9 0.82 1.50 + 0.16)xl0 + 0.55)xl0 -8 1.36 0.99 1.35 + 0.16)xl0 + 0.09)xl0 + 0.11 )xl0 -8 -8 -8 i 2.29 + 0.02)xl0 0.94 + 0.13)xl0 -9 2.23 3.01 + 0.29)xl0" + 0.17)xlC" (0.31 + 0.04)xl0" Number of Observations 6 17 58 10 20 30 AO 50 60 70 STRESS INTENSITY (K,) MPa/m" F i g u r e 22 C r a c k g r o w t h r a t e v e r s u s s t r e s s i n t e n s i t y a t s e l e c t e d t e m p e r a t u r e s i n 12 m o l / k g NaOH, -0.10 V c r r . [ NaOH ] mol/kg 0 - 3 . 3 5 A-12.0 _l I I I L_ 2 7 2.8 2.9 1 / T x 10 3 Arrhen iu s p l o t of the Region II c rack growth ra te s in 3.35. and 12 mol/kg NaOH, -0.10 V,,-,-. 60 mol/kg NaOH d a t a u s i n g t h e l e a s t s q u a r e s method. The ap- p a r e n t a c t i v a t i o n e n e r g y , c a l c u l a t e d from t h e g r a d i e n t o f the l i n e , was 60 ± 8 kJ/mol ( 9 0 % c o n f i d e n c e ) . The a c t i v a - t i o n e n e r g y i n 12 mol/kg NaOH was e s t i m a t e d t o be ^ 37 k J / m o l . 3.3.0.2 E f f e c t o f NaOH C o n c e n t r a t i o n The e f f e c t o f NaOH c o n c e n t r a t i o n i s i l l u s t r a t e d i n F i g u r e 24 and t a b u l a t e d i n T a b l e I I I . The e f f e c t o f c o n c e n t r a - t i o n a p p e a r e d t o be s m a l l and w i t h i n t h e e r r o r s o f measurement a t 8 and 12 mol/kg NaOH. The d a t a o f F i g u r e 23 s u g g e s t s t h a t , a l t h o u g h t h e e f f e c t o f c o n c e n t r a t i o n was s m a l l a t 92 °C, i t had a g r e a t e r e f f e c t a t 82 °C. 3.3.0.3 E f f e c t o f A p p l i e d P o t e n t i a l T e s t r e s u l t s o b t a i n e d a t 0, -0.110 and -0.175 i n 3.35 mol/kg NaOH, 92 °C a r e shown i n T a b l e I I I and F i g u r e 25. The c r a c k growth r a t e was f a s t e s t a t -0.10 V ^ Q ^ , i n agreement w i t h 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 i n t h e SSRT. In a d d i t i o n , f r a c t u r e m e c h a n i c s t e s t s were c o n d u c t e d a t a c t i v e - p a s s i v e p o t e n t i a l s . A t e s t c o n d u c t e d i n 3.35 mol/kg NaOH a t 92 °C, -1.15 V $ C E and 30 MPa/jrT, showed no c r a c k i n g i n 10 d a y s . A n o t h e r t e s t , a t -0.85 V $ C E and 30 MPa/IT, showed no c r a c k i n g a f t e r 15 d a y s . C r a c k growth r a t e s would have been < 2.89 x 1 0 " 1 0 m/s and < 1.92 x 1 0 - 1 0 m/s r e s p e c t i v e l y t o be i n d e t e c t a b l e i n t h e t e s t p e r i o d . 10 o x 1.0 LU 5 o or ID 0.1 i£ o < or o WW P T t NaOH ] m o l / k g 0 - 3.35 A - 8.0 B - 12.0 61 10 20 30 40 50 60 70 STRESS INTENSITY <Kj) MPavAm" 80 F i g u r e 24 C r a c k g r o w t h r a t e v e r s u s s t r e s s i n t e n s i t y a t s e l e c t e d NaOH c o n c e n t r a t i o n s , 9 2 ° C , - ° - 1 0 V S C E : 62 10 £ < or o or ID 0.1 it o < or o fcfr ĵ a oft (a • • • 10 - i 1 i i_ V S C E A - - 0 . 1 7 5 G - -0 .10 B- 0 .0 G - -1.15 V - - 0 . 8 5 _l i_ 20 30 40 50 60 STRESS INTENSITY (K,) MPaVm" 70 80 F i g u r e 25 C r a c k growth r a t e v e r s u s s t r e s s i n t e n s i t y a t s e l e c t e d p o t e n t i a l s i n 3.35 mol/kg NaOH, 92 °C. 63 3.3.0.4 E f f e c t o f C o l d Work A l l o f t h e c r a c k growth r a t e d a t a i n T a b l e I I I were o b t a i n e d w i t h 25% c o l d worked m a t e r i a l . A n n e a l e d m a t e r i a l (nb c o l d work) was a l s o t e s t e d a t -0.10 V $ C E and 92 °C but c r a c k i n g c o u l d not be i n i t i a t e d . The s t r e s s i n t e n s i t y c o u l d not be i n - c r e a s e d above 28 MPa/nT w i t h o u t c o n s i d e r a b l e d e f o r m a t i o n o f t h e m a t e r i a l . Even a f t e r a month, no c r a c k i n g was o b s e r v e d . C r a c k growth r a t e would have been < 1 x 10 ^ m/s t o be un- d e t e c t a b l e i n t h e t e s t p e r i o d . 3.3.0.5 F r a c t o g r a p h y The c r a c k f r o n t was n o t s t r a i g h t d u r i n g t h e t e s t s . Sometimes i t l e d on one f a c e o f t h e s p e c i m e n , sometimes on t h e o t h e r f a c e , i n an a p p a r e n t l y random manner. In a l l f r a c t o g r a p h s t h e c r a c k i n g d i r e c t i o n i s from the t o p t o t h e bottom o f t h e p h o t o g r a p h . C r a c k i n g i n 3.35 mol/kg NaOH was 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 . F i g u r e 26 shows an u n c l e a n e d f r a c t u r e s u r f a c e o f a s p e c i m e n c r a c k e d a t -0.10 V ^ Q ^ and 92 °C. F i g u r e 27 i l l u s t r a t e s t h e e f f e c t o f i n c r e a s i n g s t r e s s i n t e n s i t y on t h e f r a c t o g r a p h y ( c l e a n e d ) . S m a l l a r e a 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 c c u r r e d a t h i g h e r K - l e v e l s as shown i n F i g u r e 27c. In F i g u r e 27a t h e r e were remnants o f c o r r o s i o n f i l m a d h e r i n g 64 F i g u r e 26 F r a c t u r e s u r f a c e a f t e r t e s t i n g i n 3.35 mol/kg NaOH, 92 °C, -0.10 V $ C E 45-47 MPa/m".  I 66 c F i g u r e 27 F r a c t u r e s u r f a c e s a f t e r t e s t i n g i n 3.35 mol/kg NaOH, 92 °C, -0.10 V $ C E a t a) 38-40 MPa/m, b) 46-48 MPa/m, c) 55-60 MPa/m. 67 t o t h e g r a i n s ; t h e s e d i d n o t come o f f d u r i n g t h e c l e a n i n g (as d e s c r i b e d i n S e c t i o n 2 . 3 . 2 ) . At s t r e s s i n t e n s i t y , o f a b o u t 60 MPa/m crack:, b r a n c h i n g o c c u r r e d , as seen i n F i g u r e 28. C r a c k s a p p e a r e d t o i n i t i a t e on t h e s u r f a c e i n some c a s e s but a c t u a l l y were formed where i n t e r n a l c r a c k s t u n n e l l e d b e n e a t h t h e s u r f a c e and emerged ahead o f t h e main c r a c k . An i n t e r e s t i n g a s p e c t o f t h e f r a c t o g r a p h y was t h e f o r m a - t i o n o f d e p o s i t s and s u r f a c e f i l m s . At -0.10 V ^ r r * a f i n e s u r f a c e f i l m w i t h the a p p e a r a n c e o f a s t r a w mat c o v e r e d t h e s u r f a c e , becoming p r o g r e s s i v e l y t h i c k e r f u r t h e r from t h e c r a c k f r o n t . I t was r u s t r e d i n c o l o r . F u r t h e r a l o n g t h e c r a c k , d ark mounds a p p e a r e d as i n F i g u r e 29a. In t h a t a r e a t h e f r a c t u r e s u r f a c e was b l a c k . F i n a l l y , d e p o s i t s i n a n e e d l e - l i k e form were f o u n d i n t h e o l d e s t p o r t i o n o f t h e c r a c k , as shown i n F i g u r e 29b. Com p a r i s o n o f F i g u r e 30 ( c l e a n e d s u r f a c e s ) w i t h F i g u r e 27b shows t h a t 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 i n f r a c t o - g r a p h y between 72 and 92 °C when o t h e r c o n d i t i o n s were h e l d t h e same. The f r a c t o g r a p h y was a f f e c t e d by t h e c a u s t i c c o n c e n t r a t i o n . 68 F i g u r e 28 C r a c k b r a n c h i n g . S t r e s s i n t e n s i t y r i s e s from 75 to 105 MPa/m i n t h i s v iew. F i g u r e 29 D e p o s i t s on f r a c t u r e s u r f a c e a f t e r t e s t i n g i n 3.35 mol/kg NaOH, 92 °C, - 0 . 1 0 V c r r . F i g u r e 30 F r a c t u r e s u r f a c e s a f t e r t e s t i n g i n 3.35 mol/kg NaOH, -0.10 V $ C E , a) 82 °C 33-34 MPa/m, b) 72 °C, 38-40 MPa/m. 71 F i g u r e 31 shows a r e a o f t r a n s g r a n u l a r f r a c t u r e on a sp e c i m e n t e s t e d i n 12 mol/kg NaOH. F i g u r e 32, a m a g n i f i e d view o f the c e n t e r o f F i g u r e 31, i l l u s t r a t e s what may have been a c r o s s - s e c t i o n t h r o u g h s e v e r a l t r a n s g r a n u l a r c r a c k s . C o r r o s i o n d e p o s i t s on t h e f r a c t u r e s u r f a c e were d i f f e r - e n t i n t h e s t r o n g e r c a u s t i c s o l u t i o n . They were a b r o n z e c o l o r n e a r t h e c r a c k t i p and d i d not e x h i b i t c r y s t a l l i n e f a c e t s a t h i g h m a g n i f i c a t i o n . The d e p o s i t s became brown t h e n b l a c k f u r t h e r from t h e c r a c k t i p . F i g u r e 33 shows t h e e f f e c t o f p o t e n t i a l on c o r r o s i o n d e p o s i t s . As t h e p o t e n t i a l was r a i s e d , t h e d e n s i t y o f t h e d e p o s i t s i n c r e a s e d . When t h e s u r f a c e s were c l e a n e d , the mode o f f a i l u r e was shown t o be e s s e n t i a l l y t h e same, as can be seen by c o m p a r i n g F i g u r e s 3 4 a , 34b and 27a. 3.3.0.6 pH Measurement Measurements o f pH, v i a i n d i c a t o r p a p e r , showed t h a t t h e s o l u t i o n which d r a i n e d o u t o f t h e c r a c k was t h e same as t h a t o f t h e b u l k s o l u t i o n \(ite. ̂  14 a t 25 ° C ) . 72 F i g u r e 31 F r a c t u r e s u r f a c e a f t e r t e s t i n g i n 12 mol/kg NaOH, 92 °C, -0.10 V $ C E , 28-29 MPa/m. 73 F i g u r e 32 I n t e r g r a n u l a r f a c e t d i s p l a y i n g i n t e r - s e c t i n g t r a n s g r a n u l a r c r a c k i n g . T h i s i s a m a g n i f i c a t i o n o f t h e c e n t e r o f F i g u r e 31. 74 b 75 F i g u r e 33 C o r r o s i o n d e p o s i t s a f t e r t e s t i n g i n 3.35 mol/kg NaOH, 92 °C a) -0.175 V S C E , 36 MPa/m b) -0.10 V $ C E 42-44 MPa/m. c) 0.00 V c r c , 41-43 MPa/m.  77 3.3.1 NaOH + Na 2S 3.3.1.0 2.5 mol/kg NaOH + 0.423 mol/kg Na 2S ( S i m u l a t e d White L i q u o r ) F r a c t u r e m e c h a n i c s t e s t s were c o n d u c t e d i n 2.5 mol/kg NaOH + 0.423 mol/kg N a 2S, 92 °C, a t a c t i v e - p a s s i v e p o t e n t i a l s . One t e s t s p e c i m e n , a t -1.00 V^^^ and 30 MPa/m, d i d not c r a c k w i t h i n 7 days ( v < 4 . 1 x 1 0 - 1 ^ m/s). A n o t h e r t e s t was s t a r t e d a t -1.10 V<*££ and 20 MPa/m, but c r a c k i n g d i d not o c c u r w i t h i n 11 days (v< 2.6 x 1 0 " ^ m/s). The s t r e s s i n t e n s i t y was r a i s e d t o 30 MPa/m f o r 13 d a y s , w i t h no e f f e c t (v<<2.2 x 1 0 " 1 0 m/s). F i n a l l y , t h e t e m p e r a t u r e was r a i s e d to 100 °C f o r 8 days but no c r a c k i n g was o b s e r v e d (v^-3.6 x 1 0 - 1 ^ m/s). 3:3:1:1 12 mol/kg NaOH + 0.423 mol/kg Na 2S F i g u r e 35 i l l u s t r a t e s t h e r e l a t i o n between s t r e s s i n t e n s i t y and c r a c k growth r a t e i n 12 mol/kg NaOH + 0.423 mol/kg Na 2S a t 92 °C and -1.175 V^^^. The c r a c k growth r a t e - 9 was f o u n d t o be (1.48 + 0.48) x 10 m/s f o r 6 d a t a p o i n t s between 30 and 37.1 MPa/m, and (2.57 + 0.59) x 1 0 ~ 9 m/s f o r 8 d a t a p o i n t s between 37.1 and 57.9 MPav^m. D u r i n g t h e t e s t , the p o t e n t i a l was r a i s e d t o -1.15 f o r a few days and c r a c k i n g c e a s e d d u r i n g t h a t p e r i o d . C r a c k i n g resumed when the p o t e n t i a l was r e s e t t o -1.175 V . T h i s o b s e r v a t i o n s u g g e s t e d t h a t c r a c k i n g may o c c u r i n t h e 2.5 mol/kg NaOH + 0.423 mol/kg Na 2S a t -1.175 V $ C E , even though i t d i d not a t 78 0 ) £ 10 > i-ob- < or o cr „ ID 0.1 < cr o -J 1 1 1 i i i I 10 20 30 40 50 60 70 80 STRESS INTENSITY (K,) MPa^rfT f i g u r e 35 C r a c k growth r a t e v e r s u s s t r e s s i n t e n s i t y i n 12 mol/kg NaOH + 0.423 mol/kg N a 2S, 92 °C, -1.175 V c r c . 79 -1.15 V S C E . 3.3.1.2 F r a c t o g r a p h y In a l 1 . f r a e t o g r a p h s 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 s from t h e top t o t h e bottom o f the p h o t o g r a p h . The f r a c t o g r a p h y o f t h e sp e c i m e n c r a c k e d i n t h e NaOH + Na 2S s o l u t i o n was mixed 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 . I t i s i l l u s t r a t e d i n F i g u r e 36. Th 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 i n c r a c k i n g m o r p h o l o g y w i t h i n the range o f s t r e s s i n t e n s i t y t e s t e d . The s u r f a c e s were c o n s i d e r a b l y r o u g h e r t h a n t h o s e o b t a i n e d i n NaOH. C o r r o s i o n d e p o s i t s shown i n F i g u r e 37 were b l a c k i n c o l o r , w i t h a t i n g e o f g r e e n when r i n s e d w i t h d i s t i l l e d w a t e r . D u r i n g removal o f d e p o s i t s 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 , a hydrogen s u l f i d e s m e l l was p r o d u c e d i n d i c a t i n g the d e p o s i t s were metal s u l f i d e s . 3.3.1.3 S o l u t i o n A p p e a r a n c e The s o l u t i o n t a k e n from t h e c e l l d u r i n g t h e t e s t was brown i n c o l o r . When d i l u t e d w i t h t h e a d d i t i o n o f d i s t i l - l e d w a t e r ( i . e . r e d u c t i o n o f pH), the s o l u t i o n t u r n e d g r e e n . D i l u t i o n w i t h f r e s h s t r o n g c a u s t i c s o l u t i o n d i d n o t have t h i s e f f e c t . b F i g u r e 36 F r a c t u r e s u r f a c e s a f t e r t e s t i n g i n 12 mol/kg NaOH + 0.423 mol/kg N a 2S, 92 °C, -1.175 V S C E a) 37-39 MPa/m b) 50-54 MPa/m. 81 F i g u r e 37 C o r r o s i o n d e p o s i t s a f t e r t e s t i n g i n 12 mol/kg NaOH + 0.423 mol/kg N a 2 S , 92 °C, -1.175 V Q p p j 50-54 MPa/m. 82 3.4 E l e c t r o n D i f f r a c t i o n A n a l y s i s o f S u r f a c e F i l m s i n NaOH An example o f a t y p i c a l e l e c t r o n d i f f r a c t i o n p a t t e r n i s shown i n F i g u r e 38. T a b l e IV i s an a n a l y s i s , o f t h e d a t a f o r t h i s sample. F i g u r e 39 i s a p l o t o f . D, t h e d i a m e t e r o f t h e d i f f r a c t i o n p a t t e r n r i n g s , v e r s u s v h +k +1 o b t a i n e d from t h e d a t a i n T a b l e IV. The l a t t i c e p a r a m e t e r , a Q , f o r t h e o c u b i c s p i n e l was 8 . 5 0 + 0.01 A a c c o r d i n g t o e q u a t i o n 3. The i n t e n s i t i e s o f t h e d i f f r a c t i o n r i n g s were c o n s i s t e n t w i t h Fe^O i n the x - r a y powder d i f f r a c t i o n f i l e 7 1 and w i t h t h e i n t e n s i - 72 t i e s c a l c u l a t e d f o r e l e c t r o n d i f f r a c t i o n i n Fe^O^. O t h e r v a l u e s o b t a i n e d f o r t h e l a t t i c e p a r a m e t e r were 8.5 + 0.1, 8.8 + 0.2, 9.3 + 0.3 and 9.3 + 0.2 A. F i g u r e 38 E l e c t r o n d i f f r a c t i o n p a t t e r n from c o r r o s i o n f i l m . 84 / h 2 + k 2 + l 2 F i g u r e 3 9 D i a m e t e r o f d i f f r a c t i o n r i n g s v e r s u s A W . T a b l e IV E l e c t r o n - Di f f r a c t i o n P a t t e r n ' Data 1 R i n g Number Di ameter Inches d 6 A I n t e n s i t y h, k, 1 / h 2 + k 2 + l 2 1 1.1.2 2.97 weak 2 2 0 2.83 2 1 .22 2.73 s p o t s c - - 3 1 .31 2.54 weak 3 1 1 3.32 4 1 .56 2.13 wea k 4 0 0 4. 5 1.72 1 .94 s p o t 3 3 1 4.36 6 1 .95 1 .71 v e r y weak 4 2 2 4.90 7 2.10 1 .60 v.v. weak 3 3=: 3 5.20 5 1 1 8 2.25 1 .48 medi um 4 4 0 5.66 9 2.44 1 .36 s p o t 6 2 0 6.32 10 2.62 1 .27 v.v. weak 5 3 3 6.56 11 2.72 1 .22 medi um 4 4 4 6.93 12 3.00 1.11 v.v. weak 7 3 1 7.68 6 4 2 13 3.16 1 .05 v.v. weak 8 0 0 8.00 86 4. DISCUSSION 4.1 I n t e r p r e t a t i o n o f A n o d i c ' P o l a r i z a t i o n C u r v e s 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 on the s u r f a c e may be i d e n t i f i e d by e x a m i n i n g t h e p o l a r i z a t i o n c u r v e s . A n o d i c c u r r e n t peaks r e s u l t from r e a c t i o n s which t a k e p l a c e t o change a metal s p e c i e s t o a form more t h e r m o d y n a m i c a l l y s t a b l e a t t h a t p o t e n t i a l . The thermodynamic s t a b i l i t y o f a s p e c i e s may be d e t e r m i n e d from an E-pH d i a g r a m , and t h u s t h e p o s s i b l e r e a c t i o n s w hich c a u s e t h e c u r r e n t peaks may be i d e n t i f i e d . The p o l a r i z a t i o n c u r v e f o r the 316 s t a i n l e s s s t e e l r o d m a t e r i a l i n 3.35 mol/kg.NaOH a t 92'°C has been p r e s e n t e d i n F i g u r e 5. I t i s b e l i e v e d t o be a c o m p o s i t e c u r v e which may be i n t e r p r e t e d i n terms o f t h e b e h a v i o r o f t h e major a l l o y c o n - s t i t u e n t s . The p o l a r i z a t i o n b e h a v i o r o f chromium was c o n s i d e r e d w i t h r e f e r e n c e t o t h e E-pH e q u i l i b r i u m d i a g r a m c o n s t r u c t e d by L e e 7 3 f o r t h e C r - H 2 0 s y s t e m a t 100 °C. The pH i s -v 12.5-13 i n 31 3.35 mol/kg NaOH a t 92 °C. The t r a n s p a s s i v e r e g i o n i n F i g u r e 9 c o r r e s p o n d e d t o t h e d i s s o l u t i o n o f chromium o x i d e t o 2 - 7 3 C r 0 4 . The y e l l o w c o l o r o f t h e s o l u t i o n a f t e r p o t e n t i o - 2- dynamic s c a n n i n g i n NaOH c o n f i r m e d t h e p r e s e n c e o f CrO^ i o n s . By i n s p e c t i o n , i t was c o n c l u d e d t h a t the p r i m a r y t r a n s - p a s s i v e r e g i o n o b s e r v e d above -0.25 V^r i n F i g u r e 40 r e s u l t e d from f o r m a t i o n o f CrO^ 87 2- The p o l a r i z a t i o n b e h a v i o r o f n i c k e l was a n a l y z e d u s i n g 74 an E-pH d i a g r a m c o n s t r u c t e d by Cowan and S t a e h l e f o r t h e N i - H 2 0 s y s t e m a t 100 °C. The i n c r e a s i n g c u r r e n t o b s e r v e d above t h e c o r r o s i o n p o t e n t i a l i n F i g u r e 10 may have been due 74 to f o r m a t i o n o f HNi0^- v i a e q u a t i o n 7. HNi0 2- + 3H + + 2e" Ni + 2H 20 ...(7) EHNiO-/Ni = -°-837-0.074 pH + 0.037 log [HNi0~], V $ H E ...(7a) The p o t e n t i a l o f t h e c u r r e n t peak, -0.85 V < , ^ E was a t t r i b u t e d t o f o r m a t i o n o f a p a s s i v a t i n g f i l m . I t was n o t e d t h a t t h e a c t i v e - p a s s i v e peak a t -0.85 V ^ ^ E f o r n i c k e l ( F i g u r e 10) c o r - r e s p o n d e d to t h e peak a t -0.9 V<jCr£ f o r t h e a l l o y ( F i g u r e s 5 and 4 0 ) . 75 D i s s o l u t i o n o f i r o n t o HFeO^ v i a e q u a t i o n 8 may have been t h e m a j o r r e a c t i o n c o n t r i b u t i n g t o t h e a c t i v e - p a s s i v e c u r r e n t peak seen i n F i g u r e 11. HFe0 2- + 3H + + 2e" = ^ Fe + 2H 20 ...(8) EHFe0 - / F e = 0.503-0.111 pH + 0.037 log [HFeO"], V $ H E ...(8a) The 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 would be d e f i n e d by t h e f o r m a t i o n o f an o x i d e f i l m . The c u r r e n t peak o b s e r v e d F i g u r e 4 0 A n o d i c p o l a r i z a t i o n c u r v e s , 3 1 6 s t a i n l e s s s t e e l , 3 . 3 5 m o l / k g N a O H , 9 2 ° C . I d e n t i f i c a t i o n o f r e a c t i o n s . 89 a t -1.05 V SCE on F i g u r e 11 c o r r e s p o n d e d t o t h e peak a t -1.05 i n F i g u r e 40. Thus, each o f t h e c u r r e n t peaks has been i d e n t i f i e d w i t h a r e a c t i o n o f one o f t h e m a j o r a l l o y c o n s t i t u e n t s . F i g u r e 40 i l l u s t r a t e s t h a t t h e a c t i v e - p a s s i v e peak due t o i r o n was more p r o n o u n c e d f o r t h e p l a t e m a t e r i a l . I r o n d i s s o l u t i o n may have been enhanced by t h e l o w e r chromium con- t e n t i n t h e p l a t e m a t e r i a l . The i n c r e a s e i n peak c u r r e n t d e n s i t i e s as c a u s t i c c o n - c e n t r a t i o n was i n c r e a s e d ( F i g u r e 7) may be e x p l a i n e d u s i n g E-pH d i a g r a m s . A c c o r d i n g t o t h e d i a g r a m s , i r o n , n i c k e l and chromium may n o t p a s s i v a t e a t pH 13. However, i f t h e c o n c e n t r a t i o n o f d i s s o l v e d s p e c i e s i s i n c r e a s e d above th e 10~ mol/kg c o n c e n t r a - t i o n assumed f o r t h e d i a g r a m s t h e n t h e p a s s i v i t y r e g i o n may e x t e n d t o h i g h e r pH. To i n c r e a s e t h e c o n c e n t r a t i o n o f d i s - s o l v e d s p e c i e s , more d i s s o l u t i o n must o c c u r . Thus, i n the h i g h e r pH s o l u t i o n s h i g h e r c u r r e n t d e n s i t i e s must be r e a c h e d b e f o r e enough d i s s o l u t i o n has o c c u r r e d t o p r o v i d e a c o n c e n t r a - t i o n o f d i s s o l v e d s p e c i e s a d e q u a t e t o c a u s e p a s s i v i t y . I n c r e a s e d t e m p e r a t u r e d i d not i n c r e a s e t h e c u r r e n t d e n s i t y s i g n i f i c a n t l y ( F i g u r e 8) b e c a u s e t h e thermodynamic s t a b i l i t i e s do n o t change s i g n i f i c a n t l y o v e r t h i s r ange o f t e m p e r a t u r e 90 as shown by t h e E-pH d i a g r a m s . 7 3 - 7 5 The p o l a r i z a t i o n b e h a v i o r o f t h e s t e e l i n t h e NaOH + Na 2S s o l u t i o n has been a n a l y z e d by a n a l o g y w i t h t h e b e h a v i o r i n s u l f i d e f r e e s o l u t i o n s due t o l a c k o f d a t a on thermo- dynamics i n s u l f i d e s o l u t i o n s . F i g u r e 41 i l l u s t r a t e s how the a c t i v e - p a s s i v e c u r r e n t peak o b s e r v e d i n t h e NaOH s o l u t i o n was h i d d e n i n t h e s u l f i d e c o n t a i n i n g s o l u t i o n . The p o l a r i z a t i o n c u r r e n t i n t h e s u l f i d e s o l u t i o n was t h e sum o f t h e c u r r e n t o f p o l a r i z a t i o n o f t h e s t e e l i n NaOH p l u s c u r r e n t f o r t h e o x i d a t i o n o f s u l f i d e . The 7 6 s u l f i d e may o x i d i z e t o t h i o s u l f a t e v i a e q u a t i o n 9. S 2 0 3 2 " + 6H + + 8e" 2S 2~ + 3H 20 ...(9) E 2 " 2" = ° - 0 3 4 - ° - 0 5 6 P H + 0.0093 l o g ^ [ S 2 0 3 2 ]/[S 2 | » V $ H E ..(9a) The r e v e r s i b l e p o t e n t i a l o f e q u a t i o n 9, -0.959 V S C E , was fi i c a l c u l a t e d a f t e r Tromans u s i n g t h e a v e r a g e pH v a l u e g i v e n 49 by S i n g b e i l . Hence, t h e p o l a r i z a t i o n b e h a v i o r o f t h e s t e e l was h i d d e n a t > -0.75 V<.£ E by o x i d a t i o n o f s u l f i d e . 75 76 B i e r n a t and Robins ' have c o n s t r u c t e d E-pH d i a g r a m s f o r t h e Fe-S-H 20 and S-H 20 systems a t 100 °C. A c c o r d i n g t o t h e i r d a t a , i r o n s u l f i d e s a r e s t a b l e a t low p o t e n t i a l . 91 F i g u r e 41 A n o d i c p o l a r i z a t i o n c u r v e s , 316 s t a i n l e s s s t e e l , NaOH + Na 2S, 92 °C. I d e n t i f i c a t i o n o f r e a c t i o n s . S i m i l a r l y , MacDonald and S y r e t t have c o n s t r u c t e d an E-pH d i a g r a m f o r t h e Ni-S-H^O s y s t e m a t 25 °C, i n d i c a t i n g p o s s i b l e f o r m a t i o n o f n i c k e l s u l f i d e s a t low p o t e n t i a l . No E-pH d i a - gram was a v a i 1 a b l e f o r t h e Cr-S-H^O s y s t e m . For h i g h e r c o n c e n t r a t i o n s o f NaOH, l a r g e r peak a n o d i c c u r r e n t d e n s i t i e s were o b s e r v e d ( F i g u r e 1 3 ) . S i m i l a r peaks 49 61 62 have been o b s e r v e d f o r m i l d s t e e l . ' ' The p o s i t i o n o f th e m a j o r c u r r e n t peak has been a t t r i b u t e d t o d e p o s i t i o n o f fi l FeS when i t s s o l u b i l i t y p r o d u c t i s e x c e e d e d . 4.2 SCC S u s c e p t i b i l i t y C o m p a r i s o n o f t h e slow s t r a i n r a t e t e s t s w i t h p o l a r i z a - t i o n b e h a v i o u r i n d i c a t e d t h a t SCC s u s c e p t i b i l i t y i n NaOH s o l u t i o n was a s s o c i a t e d w i t h i n s t a b i l i t y o f t h e p a s s i v e f i l m . The p o l a r i z a t i o n s t u d y showed t h a t breakdown o f p a s s i v e f i l m and d i s s o l u t i o n t o chromate o c c u r r e d i n t h e p r i m a r y t r a n s p a s ^ s i v e r e g i o n where SCC s u s c e p t i b i l i t y was g r e a t e s t ( F i g u r e 1 5 ) . The s u r f a c e o f a sp e c i m e n t e s t e d i n t h e s u s c e p t i b l e p o t e n t i a l range was d e e p l y c r a c k e d ( F i g u r e 1 6 ) . The c r a c k s may have formed where t h e s u r f a c e f i l m was b r o k e n , t h u s a l l o w i n g f a s t l o c a l i z e d d i s s o l u t i o n . R u p t u r e o f t h e f i l m may have o c c u r r e d by t h e sudden r e l e a s e o f d i s l o c a t i o n s p i l e d up b e h i n d t h e f i l m . A c c o r d i n g t o t h e f i l m r u p t u r e and d i s s o - 1 9 l u t i o n m o d e l, t h e r e l e a s e d d i s l o c a t i o n s would form a s l i p 93 s t e p o f b a r e metal which would th e n be d i s s o l v e d . F o r m a t i o n o f chromate i o n s would have c a u s e d r a p i d d i s s o l u t i o n r e s u l t - i n g i n f o r m a t i o n o f deep c r a c k s b e f o r e r e p a s s i v a t i o n w i t h an i r o n - n i c k e l f i l m c o u l d o c c u r . A SCC mechanism i n v o l v i n g h y d r o g e n e m b r i t t l e m e n t would be r u l e d o u t a t p r i m a r y t r a n s p a s s i v e p o t e n t i a l s . In pH 12.5 75 s o l u t i o n s , h y d r o g e n e v o l u t i o n v i a e q u a t i o n 10 would o c c u r o n l y below -1.17 Vc,^. 2 H + +• 2e" H 2 _ ( 1 0 ) E = 0.057-0.074 pH - 0.037 l o g a . ..(10a) H /H 2 H 2 A pH measurement o f s o l u t i o n d r a i n i n g out o f a s t r e s s c o r r o s i o n c r a c k i n a f r a c t u r e m e c h a n i c s specimen showed t h a t pH was not m e a s u r a b l y l o w e r i n t h e c r a c k s o l u t i o n t h a n i n t h e b u l k s o l u - t i o n . Thus l o c a l h y drogen e v o l u t i o n was u n l i k e l y even i n s i d e c r a c k s . 49 In c o n t r a s t t o b e h a v i o r o f m i l d s t e e l i n NaOH s o l u t i o n s , t h e 316 s t a i n l e s s s t e e l showed no s u s c e p t i b i l i t y t o SCC a t t h e a c t i v e - p a s s i v e p o t e n t i a l . The a b s e n c e o f s u s c e p t i b i l i t y a t t h e s e p o t e n t i a l s may have r e s u l t e d from a number o f f a c t o r s . No s i g n i f i c a n t f i l m i n s t a b i l i t y may o c c u r i n t h i s r a n g e . A s t a b l e chromium f i l m may m i n i m i z e d i s s o l u t i o n o f i r o n and 94 n i c k e l and t h u s p r e v e n t i n s t a b i l i t y . An a l t e r n a t e f a c t o r c a u s i n g t h e a bsence o f s u s c e p t i b i l i t y may have been t h a t t h e s t r a i n r a t e was t o o h i g h . In t h a t c a s e , f a i l u r e would have o c c u r r e d b e f o r e c r a c k i n g had had t i m e t o p r o g r e s s . I f c r a c k i n g i n t h e slow s t r a i n r a t e t e s t s o c c u r r e d by c y c l e s 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 r e p a s s i v a t i o n t h e n th e r a t e and d u r a t i o n o f 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 c y c l e s would have a f f e c t e d t h e c r a c k i n g r a t e . The low c u r r e n t d e n s i t y a t t h e a c t i v e - p a s s i v e peak i n d i c a t e d t h a t d i s s o l u t i o n r a t e was r e l a t i v e l y low a t t h i s p o t e n t i a l . V ery l i t t l e d i s - s o l u t i o n m i ght have t a k e n p l a c e b e f o r e r e p a s s i v a t i o n was c o m p l e t e . Time i n t h e d i s s o l u t i o n s t a g e would have been de- c r e a s e d by r a p i d r e p a s s i v a t i o n k i n e t i c s , p e r h a p s r e d u c i n g d i s - s o l u t i o n below t h a t r e q u i r e d t o s u s t a i n c r a c k i n g . A d d i t i o n - a l l y , i f h y drogen e m b r i t t l e m e n t were i m p o r t a n t i n t h e c r a c k i n g mechanism a t t h i s p o t e n t i a l , t h e n r a p i d r e p a s s i v a t i o n m ight r e d u c e t h e p e r i o d d u r i n g w hich hydrogen i s a b s o r b e d i n t o t h e l a t t i c e . o Park e t a l . have e x p l a i n e d SCC o f 304 s t a i n l e s s s t e e l i n 20N NaOH i n terms o f t h e r a t i o o f c u r r e n t d e n s i t y on a b a r e m e t a l s u r f a c e t o t h a t on a f i l m e d m e t a l . T h i s r a t i o was l o w e s t i n t h e p r i m a r y t r a n s p a s s i v e r e g i o n . A l t h o u g h t h e y o b s e r v e d c r a c k i n g n e a r t h e c o r r o s i o n p o t e n t i a l ( / v-0.9 V S H E ) , t h e r a t e o f c r a c k i n g was l o w e r f o r a l l o y s w i t h h i g h e r chromium 95 c o n t e n t . The p r e s e n t i n v e s t i g a t i o n , d i f f e r e d from t h a t o f Park e t a l . i n t h a t c r a c k i n g was not o b s e r v e d i n NaOH s o l u - t i o n s a t t h e c o r r o s i o n p o t e n t i a l . The d i f f e r e n c e may have been due t o t h e i r more c o n c e n t r a t e d s o l u t i o n s , f a s t e r s t r a i n r a t e , and d i f f e r e n t a l l o y c o m p o s i t i o n . S a n t a r i n i 7 8 t e s t e d a 17 C r - 1 3 N i s t e e l i n 50% NaOH a t 130 °C. D u r i n g t h e t e s t , t h e open c i r c u i t p o t e n t i a l r o s e from -1.17 t o -1.13 t h e n -1.00 V$QI£- T h i s was i n t h e p o t e n t i a l r e gime where i r o n i n t h e a l l o y would be d i s s o l v i n g , chromium would form a p a s s i v e f i l m and n i c k e l would be thermodynamic- a l l y s t a b l e . S a n t a r i n i i d e n t i f i e d t h i s r e g i o n as d a n g e r o u s f o r SCC. The same r e s u l t was n o t o b s e r v e d i n t h e p r e s e n t s t u d y , p e r h a p s b e c a u s e o f t h e l o w e r 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 h e r e . A l s o , t h e p r e s e n t s t u d y was p o t e n t i o s t a t i c and so would n o t a l l o w a s h i f t i n p o t e n t i a l , as was o b s e r v e d by S a n t a r i n i and i n c l u d e d i n t h e model he p r o p o s e d . In t h e 2.5 mol/kg NaOH + 0.423 mol/kg Na 2S ( 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 u s c e p t i b i l i t y t o SCC was d e t e c t e d by SSRT a t -1.15 t o -1.175 V ^ ^ ^ . A c c o r d i n g t o e q u a t i o n 10, H^ 2- may be e v o l v e d a t t h i s p o t e n t i a l . The p r e s e n c e o f S i o n s may r e t a r d t h e hydrogen e v o l u t i o n and so promote a b s o r p t i o n o f a d s o r b e d h y d r o g e n . T h i s d i s s o l v e d h y d r o g e n may t h e n d i f - f u s e i n t o t h e metal t o c a u s e h y d r o g e n e m b r i t t l e m e n t . The e f f e c t o f s u l f i d e (H 9S s o l u t i o n ) i n p o i s o n i n g t h e r e c o m b i n a t i o n 96 79 r e a c t i o n 10 has been shown e x p e r i m e n t a l l y . The s u l f i d e a l s o may have weakened t h e p a s s i v e s u r f a c e f i l m formed a t t h i s p o t e n t i a l , making i t l e s s p r o t e c t i v e and t h u s a l l o w i n g d i s s o l u t i o n t o o c c u r . A n o t h e r p o s s i b i l i t y may be t h a t 2- the s u l f i d e a f f e c t e d t h e r e p a s s i v a t i o n r a t e . S may have a d s o r - bed o n t o t h e s u r f a c e i n s t a d o f 0H~, t h u s r e d u c i n g the r a t e o f f o r m a t i o n o f p a s s i v e f i l m and s l o w i n g r e p a s s i v a t i o n . The s l o w e r r e p a s s i v a t i o n r a t e may l e a v e the s u r f a c e bare f o r a l o n g e r p e r i o d o f t i m e . T h i s bare s u r f a c e may c a t a l y z e e v o l u t i o n o f a d s o r b e d hydrogen t h e r e b y l e a d i n g t o g r e a t e r h y d r o g e n a b s o r p t i o n 79 as shown by B e r k o w i t z and H o r o w i t z . S e n s i t i z a t i o n c a u s e d some s u s c e p t i b i l i t y i n 3.35 mol/kg NaOH a t -1.15 i n t h e slow s t r a i n r a t e t e s t s ( F i g u r e 1 5 ) . The s e n s i t i z a t i o n would have t i e d up some o f t h e chromium i n i n - t e r g r a n u l a r c a r b i d e s and hence t h e f i l m may n o t have formed p r o - p e r l y a d j a c e n t t o g r a i n b o u n d a r i e s . Lower chromium c o n t e n t may a l s o have r e s u l t e d i n s l o w e r r e p a s s i v a t i o n t h u s a l l o w i n g more r d i s s o l u t i o n b e f o r e r e p a s s i v a t i o n was c o m p l e t e . A l t e r n a t e l y , s l o w e r r e p a s s i v a t i o n k i n e t i c s m i g h t a l l o w l o n g e r t i m e f o r a ! hydrogen e m b r i t t l e m e n t mechanism t o be o p e r a t i v e . S e n s i t i z a t i o n d i d n o t a f f e c t s u s c e p t i b i l i t y i n the NaOH + Na 2S s o l u t i o n . Perhaps i t showed no e f f e c t b e c a u s e s u l f i d e had a l r e a d y s l o w e d t h e r e p a s s i v a t i o n k i n e t i c s . An a n n e a l e d SSRT specimen t e s t e d a t t h e t r a n s p a s s i v e p o t e n t i a l i n NaOH s o l u t i o n had a l a r g e r p e r c e n t r e d u c t i o n i n a r e a a t f a i l u r e t h a n d i d an a s - r e c e i v e d s p e c i m e n . The a n n e a l e d m a t e r i a l was s o f t e r t h an t h e wo r k - h a r d e n e d a s - r e c e i v e d m a t e r i a l . C r i t i c a l s t r a i n s would n o t have d e v e l o p e d i n t h e m a t e r i a l u n t i l l a t e r i n t h e t e s t when e l o n g a t i o n was g r e a t e r . A n n e a l e d s p e c i e mens t e s t e d a t n o n - s u s c e p t i b l e p o t e n t i a l s m i g h t have. v . . 97 showed h i g h e r p e r c e n t r e d u c t i o n s i n a r e a , t o o . The r e s u l t w i t h t h e a n n e a l e d s p e c imen i n d i c a t e d t h a t s t r e s s r e l i e f may r e d u c e s u s c e p t i b i l i t y but does n o t n e c e s s a r i l y p r e v e n t SCC. 4.3 C r a c k Growth Rates and t h e Mechanism o f C r a c k i n g SCC s u s c e p t i b i l i t y was g r e a t e s t i n t h e p o t e n t i a l r e g i o n s i n d i c a t e d by t h e slow s t r a i n r a t e t e s t s , and t h e s e r e g i o n s were c o r r e l a t e d t o p o t e n t i a l r a n g e s f o r f i l m i n s t a b i l i t y by means o f t h e p o l a r i z a t i o n c u r v e s . C r a c k i n g was a t t r i b u t e d t o the o p e r a t i o n o f a f i l m r u p t u r e and d i s s o l u t i o n mechanism. As d e s c r i b e d below, a s t u d y o f t h e c r a c k growth r a t e s under a v a r i e t y o f c o n d i t i o n s p r o v i d e d c o n c l u s i v e e v i d e n c e r e g a r d i n g t h e n a t u r e o f t h e d i s s o l u t i o n mechanism. 4.3.1 P o t e n t i a l Dependence o f C r a c k Growth Rate F r a c t u r e m e c h a n i c s t e s t i n g c o n f i r m e d t h e SSRT r e s u l t t h a t SCC s u s c e p t i b i l i t y was p o t e n t i a l d e p e n d e n t . In t h e 3.35 mol/kg NaOH, Re g i o n II c r a c k growth was s l o w e r a t 0.0 and -0.175 V $ C E t h a n a t -0.10 V $ C E ( F i g u r e 2 5 ) . T h i s r e s u l t was e x p e c t e d b ecause 0.0 and -0.175 V S C E were a t t h e e x t r e m i t i e s o f t h e r e g i o n o f SCC s u s c e p t i b i l i t y d e t e r m i n e d by SSRT. F r a c t u r e m e c h a n i c s t e s t s a t -0.85 and -1.15 V c o n f i r m e d t h e ab s e n c e o f c r a c k i n g a t a c t i v e - p a s s i v e p o t e n t i a l s i n t h e 3.35 mol/kg NaOH. 98 The c r a c k i n g i n s u l f i d e c o n t a i n i n g s o l u t i o n s was p o t e n t i a l d e p e n d e n t , a l s o . The c r a c k i n g o b s e r v e d i n 12 mol/kg NaOH + 0.423 mol/kg Na 2S a t -1.175 V $ C E was s t o p p e d when the p o t e n - t i a l was r a i s e d t o -1.15 V S C E > Specimens t e s t e d i n 2.5 mol/kg NaOH + 0.423 mol/kg Na 2S d i d n o t c r a c k a t -1.00 o r -1.10 V $ C E . The SSRT r e s u l t s i n d i c a t e d some SCC s u s c e p t i b i l i t y a t ^ - 1.15 V S C E b u t t n i s w a s n o t o b s e r v e d f o r t h e 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 made from p l a t e m a t e r i a l . P e rhaps t h e d i f f e r e n c e was due t o d i f f e r e n t c o m p o s i t i o n s o f t h e r o d and p l a t e , m a i n l y o f chromium and n i c k e l . A l t e r n a t e l y , t h e r e l a t i v e l y h i g h p e r - c e n t r e d u c t i o n i n a r e a a t a c t i v e - p a s s i v e p o t e n t i a l s may have i n d i c a t e d m a r g i n a l SCC s u s c e p t i b i l i t y , w h i c h was i n s u f f i c i e n t t o i n i t i a t e c r a c k i n g i n t h e 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 . Thus, t h e i n v e s t i g a t i o n o f p o t e n t i a l dependence o f c r a c k growth r a t e i n d i c a t e d t h a t t h e slow s t r a i n r a t e t e s t s and f r a c t u r e m e c h a n i c s t e s t s were c o n s i s t e n t and t h a t c r a c k growth r a t e was f a s t e s t i n t h o s e p o t e n t i a l r a n g e s o f g r e a t e s t s u s c e p t - i b i l i t y t o SCC as p r e d i c t e d by SSRT. 4.3.2 D i s s o l u t i o n Rate and C r a c k Growth Rate F r a c t u r e m e c h a n i c s t e s t s p r o v i d e d c r a c k growth r a t e s i n R e g i o n s I and II o f t h e v - Kj. p l o t s . Kj i n d e p e n d e n t c r a c k i n g k i n e t i c s i n R e g i o n II were th e r e s u l t o f s t r e s s i n - d e p e n d e n t p r o c e s s e s o n l y , o f which d i s s o l u t i o n i s t h e most p r o b a b l e . 99 An a t t e m p t was made t o r e l a t e t h e c r a c k growth r a t e t o 80 t h e d i s s o l u t i o n r a t e v i a F a r a d a y ' s law. i W ...(11) a where v = c r a c k growth r a t e i = a n o d i c c u r r e n t d e n s i t y a J W = e q u i v a l e n t w e i g h t o f t h e m e tal F = F a r a d a y (9.65 x TO 4 A«S) p = d e n s i t y o f m e tal The a n o d i c c u r r e n t d e n s i t i e s , i , were o b t a i n e d from t h e a p o l a r i z a t i o n c u r v e s . At -0.10 V<- C E t h e c u r r e n t d e n s i t y f o r d i s s o l u t i o n o f chromium was much l a r g e r t h a n t h a t o f i r o n o r n i c k e l ( C r : 14.3 A/m 2, Fe: 2.0 A/m 2, N i : 0.3 A/m 2). The c r a c k v e l o c i t y c a l c u l a t e d from t h e a n o d i c c u r r e n t d e n s i t y f o r d i s s o l u t i o n t o C r ( V I ) was: 1.79 x 1 0 ~ 1 0 m/S. S i m i l a r l y f o r i r o n t o F e ( I I I ) t h e c a l c u l a t e d c r a c k growth r a t e was 0.49 x 1 0 " 1 0 m/s,and f o r n i c k e l t o N i ( I I ) i t was 1.02 x 1 0 " 1 1 m/s. C r a c k growth r a t e i n s u l f i d e s o l u t i o n was not compared w i t h e x p e c t e d c r a c k growth r a t e c a l c u l a t e d from d i s s o l u t i o n r a t e b e c a u s e o f u n c e r t a i n t y a b o u t what s p e c i e s would be f o r m e d ; o r what t h e i r d i s s o l u t i o n r a t e s would be. None of t h e c a l c u l a t e d c r a c k growth r a t e s was l a r g e enough t o a c c o u n t f o r t h e o b s e r v e d r a t e i n NaOH (^1 x l O m/s). 1 00 However, d u r i n g c r a c k i n g , d i s s o l u t i o n would not have been o c c u r - r i n g u n i f o r m l y o v e r t h e s u r f a c e , but would have been c o n c e n t r a - t e d a t d e f e c t s i n t h e o x i d e f i l m . T h e r e f o r e , t h e t r u e c u r r e n t d e n s i t y a t d e f e c t s i t e s would have been much h i g h e r t h a n t h a t i n d i c a t e d by t h e p o l a r i z a t i o n d i a g r a m s . 8 Park e t a l . have shown t h a t c u r r e n t d e n s i t y on a ba r e s t a i n l e s s s t e e l e l e c t r o d e i n NaOH i s about 100 t i m e s t h a t on a 91 f i l m e d e l e c t r o d e . T h i s i s c o r r o b o r a t e d by Hoar and Jon e s 82 f o r m i l d s t e e l s t r a i n i n g i n NaOH, and by D i e g l e and V e r m i l y e a f o r i r o n i n NaOH u s i n g a drop w e i g h t a p p a r a t u s . I f c u r r e n t d e n s i t i e s on s t r a i n i n g e l e c t r o d e s a r e 100 t i m e s h i g h e r t han on s t a t i c e l e c t r o d e s , t h e n c r a c k i n g r a t e s may be a l s o . T h i s would a c c o u n t f o r t h e o b s e r v e d c r a c k i n g r a t e s . I t has been c o n c l u d e d t h a t t h e ma g n i t u d e o f c r a c k growth r a t e c a n n o t be c a l c u l a t e d f r o m t h e magni t u d e o f the c u r r e n t d e n s i t y on t h e p o l a r i z a t i o n d i a g r a m . However, i n c r e a s e s i n ;i. a n o d i c c u r r e n t d e n s i t y s t i l l may c o r r e l a t e w i t h i n c r e a s e s i n c r a c k growth r a t e . In 3.35 mol/kg NaOH a t -0.10 V S C E , i n - c r e a s e s i n c r a c k growth r a t e o v e r a range o f t e m p e r a t u r e ( F i g u r e 21) c o r r e l a t e d w i t h i n c r e a s e s i n c u r r e n t d e n s i t y ( F i g u r e 8 ) . At 92 °C, c r a c k growth r a t e ( F i g u r e 24) c o r - r e l a t e d w i t h i n c r e a s e i n c u r r e n t d e n s i t y o v e r a range o f p o t e n t i a l s ( F i g u r e 7 ) . 101 I t a l r e a d y has been shown t h a t t h e c r a c k growth r a t e s a r e q u a l i t a t i v e l y 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 mechanism. Un- f o r t u n a t e l y , c o m p a r i s o n o f c u r r e n t d e n s i t i e s on p o l a r i z a t i o n c u r v e s have been shown t o be i n a d e q u a t e f o r q u a n t i t a t i v e l y p r e d i c t i n g c r a c k growth r a t e s . C u r r e n t d e n s i t i e s were co n - s i s t e n t w i t h a l o c a l i z e d d i s s o l u t i o n mechanism but n e i t h e r c o n f i r m e d i t n o r r e v e a l e d t h e n a t u r e o f t h e d i s s o l u t i o n p r o c e s s . 4.3.3 K i n e t i c s o f C r a c k Growth Rates The a p p a r e n t a c t i v a t i o n e n e r g y f o r c r a c k growth r a t e was f o u n d t o be ^ 60 kJ/mol i n 3.35 mol/kg NaOH and ^ 37 kJ/mol i n 12 mol/kg NaOH ( b o t h a t -0.10 V S C E ) . These 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 were 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 p r o c e s s c o n - t r o l l e d by c h a r g e t r a n s f e r , f o r which v a l u e s may range from 21- o o 105 k J / m o l . The 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 were t o o l a r g e t o r e s u l t s o l e l y from c o n t r o l by d i f f u s i o n ( t r a n s p o r t ) i n t h e 1 i q u i d p h a s e . An a t t e m p t was made t o c o r r e l a t 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 f o r c r a c k i n g w i t h t h a t o f t h e d i s s o l u t i o n c u r r e n t measured on t h e p o l a r i z a t i o n d i a g r a m ( F i g u r e 8 ) . The a c t i v a - t i o n e n e r g y v a r i e d from 93.8 + 33.1 kJ/mol a t -0.10 V t h r o u g h 66.8 + 1 9 . 9 kJ/mol a t t h e p r i m a r y t r a n s p a s s i v e c u r r e n t peak t o 33.9 + 3.8 kJ/mol a t t h e s e c o n d a r y p a s s i v e minimum. The f a c t t h a t t h e a c t i v a t i o n e n e r g y o f t h e c u r r e n t peak gave t h e b e s t c o r r e l a t i o n 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 f o r c r a c k growth r a t e s u p p o r t s t h e f i l m r u p t u r e and d i s s o l u t i o n 102 model i n which t h e m agnitude o f t h e t r a n s i e n t d i s s o l u t i o n c u r - r e n t peak may g r e a t l y a f f e c t c r a c k growth r a t e . The a p p a r e n t a c t i v a t i o n e n e r g y i n t h e 3.35 mol/kg NaOH s o l u t i o n was i n d i c a t i v e o f c h a r g e t r a n s f e r w h i l e t h a t i n t h e 12 mol/kg s o l u t i o n was more r e p r e s e n t a t i v e o f t h e l o w e r v a l u e s 49 o b t a i n e d i n mixed c h a r g e t r a n s f e r - d i f f u s i o n c o n t r o l . In the 12 mol/kg s o l u t i o n , d i f f u s i o n may p l a y a g r e a t e r p a r t i n t h e mixed c h a r g e t r a n s f e r - d i f f u s i o n c o n t r o l o f d i s s o l u t i o n . I n s u f f i c i e n t d a t a p r e c l u d e d c a l c u l a t i o n o f an a c t i v a - t i o n e n e r g y f o r c r a c k i n g i n s u l f i d e s o l u t i o n s . The v a l u e s o f a c t i v a t i o n e n e r g y o b t a i n e d were c o n s i s t e n t w i t h r e s u l t s o b t a i n e d i n o t h e r e n v i r o n m e n t s by o t h e r i n - v e s t i g a t o r s . R u s s e l f o u n 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 67 kJ/mol f o r 25% c o l d worked 316 s t a i n l e s s s t e e l from the same p l a t e used i n t h e p r e s e n t s t u d y . 1 4 ' 4 8 S p e i d e l , t e s t i n g 304 s t a i n l e s s s t e e l i n 22% NaCl s o l u t i o n f o u n d an a p p a r e n t 1 3 a c t i v a t i o n e n e r g y o f 72 k J / m o l . S t a e h l e o b t a i n e d 42-75.3 kJ/mol f o r n i c k e l s t r a i n i n g and s t a t i c e l e c t r o d e s i n 1 N H 2 S 0 4 . The l o w e r a c t i v a t i o n e n e r g y o f s t r a i n i n g e l e c t r o d e s was 85 a t t r i b u t e d t o ' l a t t i c e d i s a r r a y ' a f t e r Hoar who p r e d i c t e d t h a t t h e r e m i g h t be a r e d u c t i o n i n t h e a c t i v a t i o n e n e r g y f o r m e t a l d i s s o l u t i o n due t o an i n c r e a s e i n t h e i n t e r n a l e n e r g y o f atoms i n r a p i d l y e m e r g i n g s l i p " p o c k e t s . " 1 0 3 8 6 P e t i t s t u d i e d t h e t r a n s p a s s i v e b e h a v i o r o f n i c k e l i n s o l u t i o n s o f s u l f u r i c a c i d a n d s o d i u m s u l f a t e . He d e t e r m i n e d t h e a c t i v a t i o n e n e r g i e s o f t h e r e a c t i o n r a t e s ( e q u i v a l e n t t o c u r r e n t s a t t h e t r a n s p a s s i v e p e a k o r i n t h e s e c o n d a r y p a s s i v e r e g i o n ) . I m p e d a n c e m e a s u r e m e n t s s h o w e d t h a t t h e r e a c t i o n s o c c u r r e d i n s e v e r a l s t a g e s . T h e a c t i v a t i o n e n e r g i e s q u o t e d w e r e 5 0 . 5 - 7 0 . 0 k J / m o l a t a p o l a r i z a t i o n r a t e o f 3 3 m V / m i n . 1 c S t a e h l e h a s s u g g e s t e d t h a t t h e c u r r e n t t r a n s i e n t a s s o c i - a t e d w i t h c o r r o s i o n o f f i l m - f r e e m e t a l f o l l o w i n g f i l m r u p t u r e i s r e l a t e d t o t h e r a t e o f c r a c k p r o p a g a t i o n . He p r e s e n t e d r e s u l t s t h a t s h o w e d a c t i v a t i o n e n e r g i e s f o r . ' p a r t i a l c u r r e n t s ' , a n d t h a t t h e a c t i v a t i o n e n e r g y i n c r e a s e d w i t h n i c k e l c o n t e n t . T h u s n i c k e l w o u l d r e d u c e c r a c k i n g r a t e b y r e d u c i n g t h e s i z e o f t h e c u r r e n t t r a n s i e n t f o l l o w i n g r u p t u r e , a n d c h r o m i u m w o u l d i n c r e a s e t h e c r a c k i n g r a t e b y i n c r e a s i n g t h e s i z e o f t h e c u r - r e n t t r a n s i e n t . 4 . 4 F r a c t o g r a p h y a n d t h e D i s s o l u t i o n M e c h a n i s m 4 . 4 . 0 C o r r o s i o n D e p o s i t s T h e c o r r o s i o n d e p o s i t s s h o w n i n F i g u r e 2 9 a p p e a r t o h a v e p r e c i p i t a t e d f r o m s o l u t i o n . T h e p r e s e n c e o f t h e s e c r y s t a l l i n e d e p o s i t s i n d i c a t e d t h a t t h e s o l u t i o n w i t h i n t h e c r a c k w a s s a t u r a t e d w i t h m e t a l s p e c i e s . U n f o r t u n a t e l y t h e _ c o m p o s i t i o n o f t h e d e p o s i t s c o u l d n o t be d e t e r m i n e d . T h e c r y s t a l l i n e d e p o s i t s i n F i g u r e 2 9 b r e s e m b l e d h e m a t i t e 104 p l a t e l e t s o b s e r v e d on i r o n i n NaOH s o l u t i o n s . At h i g h e r t e m p e r a t u r e s , where c r a c k i n g was f a s t e r , t h e d e p o s i t s were s l i g h t l y t h i c k e r . The f a s t e r d i s s o l u t i o n a t th e h i g h e r t e m p e r a t u r e s may have c a r r i e d more i o n s i n t o s o l u - t i o n , and t h e s e s u b s e q u e n t l y may have p r e c i p i t a t e d t o form t h e t h i c k e r d e p o s i t . These p r e c i p i t a t e s a p p e a r e d t o have formed d u r i n g c r a c k i n g . They were t h i c k e r n e a r t h e c r a c k mouth where t h e f r a c t u r e s u r f a c e had been e x p o s e d l o n g e r , t h u s a l l o w i n g a l o n g e r t i m e f o r p r e c i p i t a t i o n t o o c c u r . The c o r r o s i o n d e p o s i t s were a f f e c t e d s i g n i f i c a n t l y by p o t e n t i a l . D e p o s i t i o n was g r e a t e r as t h e p o t e n t i a l was i n - c r e a s e d from -0.175 t o 0.0 V S C E ( F i g u r e 3 3 ) . T h i s may have been t h e r e s u l t o f t r a n s i t i o n from p a s s i v e t o s e c o n d a r y p a s s i v e f i l m . I t may a l s o have r e f l e c t e d d i f f e r i n g d e p o s i t i o n r e s u l t i n g from t h e d i f f e r e n t e q u i l i b r i u m c o n c e n t r a t i o n s f o r s o l u b l e s p e c i e s a t t h e s e p o t e n t i a l s . The heavy d e p o s i t s a t 0.0 V m a y have i n t e r f e r e d w i t h d i f f u s i o n p r o c e s s e s o v e r d i s t a n c e s s m a l l e r t h a n a g r a i n . Heavy d e p o s i t i o n was f o u n d i n t h e 12 mol/kg NaOH s o l u t i o n . The c o n t r o l p o t e n t i a l was i n t h e s e c o n d a r y p a s s i v e r e g i o n . C o r r o s i o n d e p o s i t s were t h i c k e r t h a n t h o s e f o u n d a t t h e same p o t e n t i a l i n t h e 3.35 mol/kg NaOH, t h u s i n d i c a t i n g t h a t d i f - f u s i o n may have been a f f e c t e d i n t h e s t r o n g e r NaOH s o l u t i o n . 105 Indeed, 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 c o n f i r m e d t h i s , b e i n g a low v a l u e which was c o n s i s t e n t w i t h 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 . C o r r o s i o n d e p o s i t s were a l s o f o u n d i n t h e s u l f i d e c o n - t a i n i n g s o l u t i o n s . The o b s e r v a t i o n o f metal s u l f i d e s on the f r a c t u r e s u r f a c e s was i n agreement w i t h p r e v i o u s o b s e r v a t i o n s o f i r o n s u l f i d e f o r m a t i o n on i r o n i n NaOH + Na^S s o l u t i o n . 5 1 As p r e v i o u s l y s t a t e d , s o l u t i o n t a k e n from t h e c e l l d u r i n g t h e t e s t t u r n e d g r e e n when n e u t r a l i z e d w i t h t h e a d d i t i o n o f d i s t i l - 88 l e d w a t e r . T a y l o r and S h o e s m i t h have s t u d i e d g r e e n a l k a l i n e s u l f i d e s o l u t i o n s o f pH 12-13 and have d e t e r m i n e d t h a t t h e g r e e n c o l o r i s d e r i v e d from c o l l o i d a l NaFeS^ i n s o l u t i o n . T h i s was e v i d e n c e t h a t i r o n d i s s o l v e d d u r i n g 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 . The f o r m a t i o n o f t h e s e d e p o s i t s would how- e v e r n o t r u l e o ut hy d r o g e n e m b r i t t l e m e n t a t t h i s p o t e n t i a l as th e d e p o s i t s may have formed by d i s s o l u t i o n from the c r a c k w a l 1 s . 4.4.1 F r a c t u r e Mode The c r a c k i n g p r o g r e s s e d m o s t l y i n an i n t e r g r a n u l a r mode. I n t e r g r a n u l a r s e g r e g a t i o n may have been a f a c t o r i n t h e f a s t e r 89 d i s s o l u t i o n a t g r a i n b o u n d a r i e s . A n o t h e r f a c t o r c a u s i n g t h e i n t e r g r a n u l a r c r a c k i n g may have been t h a t s t r a i n was c o n c e n t r a - t e d t h e r e . D i s l o c a t i o n p i l e - u p s a t t h e g r a i n b o u n d a r i e s may have r e s u l t e d i n a g r e a t e r s t r a i n r a t e i n t h e a d j a c e n t a r e a 106 when r e l e a s e d . S l i p bands were o b s e r v e d on t h e g r a i n s u r f a c e s i n F i g u r e s 26-33, o f f e r i n g e v i d e n c e t h a t s u b s t a n t i a l s t r a i n i n g t ook p l a c e t h e r e . L a r g e r and more numerous s l i p s t e p s r e s u l t - i n g f r o m t h i s s t r a i n i n g would have p r o v i d e d s i t e s f o r g r e a t e r d i s s o l u t i o n a t t h e 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 t h e m s e l v e s may have i n c r e a s e d t h e d i s s o l u t i o n r a t e due t o t h e i r i n h e r e n t l o c a l a t o m i c d i s a r r a y . The d i s a r r a y . m a y have i n - c r e a s e d t h e r a t e o f a d s o r p t i o n o f damaging s p e c i e s by o f f e r i n g an i n c r e a s e d number o f s u r f a c e i m p e r f e c t i o n s a t which adsorp4r" t t o n c o u l d t a k e p l a c e . T h i s a d s o r p t i o n may have s t i m u l a t e d f a s t e r d i s s o l u t i o n . At t h e p r i m a r y t r a n s p a s s i v e p o t e n t i a l s , a d s o r p t i o n may have been e s s e n t i a l t o t h e d i s s o l u t i o n p r o c e s s . K n o e d l e r and 90 H e u s l e r have s u g g e s t e d t h a t t h e o x i d a t i o n o f chromium t o 2- CrO^ p r o c e e d s v i a s i x c o n s e c u t i v e c h a r g e t r a n s f e r r e a c t i o n s . The i n t e r m e d i a t e s c o v e r t h e s u r f a c e w i t h an a d s o r b e d m o n o l a y e r . Most o f t h e s u r f a c e i s c o v e r e d w i t h C r ( I V ) , i t s o x i d a t i o n b e i n g t h e r a t e d e t e r m i n i n g s t e p . The r e a c t i o n s a r e summarized by e q u a t i o n 12. C r ( 0 H ) x + 1 + e" =F=*Cr(0H) x + OH" ...(12) The f r a c t o g r a p h y t h a t was o b s e r v e d i n t h i s s t u d y was c o n s i s t e n t w i t h 0H~ a d s o r p t i o n , and t h u s s u p p o r t e d t h e d i s s o l u t i o n mechanism s u g g e s t e d by K n o e d l e r and H e u s l e r . 107 F i g u r e 27 i l l u s t r a t e s t h e 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 . 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 o n l y a t h i g h s t r e s s i n t e n s i t y i n 3.35 mol/kg NaOH. The g r e a t e r amount o f d e f o r m a t i o n a t t h i s h i g h s t r e s s i n t e n s i t y may have p r o v i d e d c o m p e t i n g s i t e s w i t h i n t h e g r a i n e i t h e r by s e v e r e d i s r u p t i o n o f o x i d e f i l m o r h i g h s t r a i n e n e r g y s i t e s f o r a d s o r p t i o n . 48 T h i s o b s e r v e d e f f e c t was o p p o s i t e t o t h e r e s u l t s o f R u s s e l f o r 316 s t a i n l e s s s t e e l i n MgCL,,. A d e c r e a s i n g f r a c t i o n o f t r a n s g r a n u l a r SCC and i n c r e a s i n g i n t e r g r a n u l a r SCC were o b s e r v e d as Kj was i n c r e a s e d i n M g C ^ . The d i f f e r e n t b e h a v i o r o b s e r v e d by R u s s e l may have r e s u l t e d from d i f f e r e n t t e m p e r a - t u r e , p o t e n t i a l , pH, or by t h e p r e s e n c e o f C l " . M e t a l l u r g i c a l d i f f e r e n c e s c o u l d be r u l e d o u t as t h e p l a t e and p r e p a r a t i o n o f spe c i m e n s were t h e same as f o r t h e p r e s e n t s t u d i e s . I f g r a i n b oundary s e g r e g a t i o n e f f e c t s were i m p o r t a n t t o t h e i n t e r - g r a n u l a r f a i l u r e i n c a u s t i c , t h e y o b v i o u s l y had a d i f f e r e n t e f f e c t i n MgCl,,. In t h e NaOH s o l u t i o n s , t h e r e was no change i n t h e mode o f c r a c k i n g o v e r t h e range o f p r i m a r y t r a n s p a s s i v e , t r a n s p a s s i v e peak, and s e c o n d a r y p a s s i v e p o t e n t i a l s i n v e s t i g a t e d . Beyond t h i s r a n g e , c r a c k i n g mode may change. Some e v i d e n c e i n t h e 27 l i t e r a t u r e s u p p o r t s t h i s p o s s i b i l i t y . Okada e t a l . have q u o t e d a s t u d y by Subramanyam and S t a e h l e w hich showed t h a t c r a c k i n g mode depended on e l e c t r o d e p o t e n t i a l . At t r a n s p a s s i v e p o t e n t i a l s , i n t e r g r a n u l a r c r a c k i n g was f a v o r e d i n 304 s t a i n l e s s 108 s t e e l i n 70% NaOH but a t t h e a c t i v e - p a s s i v e p o t e n t i a l t r a n s g r a n u l a r c r a c k i n g p r e d o m i n a t e d . The c r a c k i n g mode, i n NaOH s o l u t i o n s d i d not change w i t h t e m p e r a t u r e ( F i g u r e s 26 and 30) i n s p i t e o f t h e s u b s t a n t i a l r ange o f c r a c k growth r a t e . The ab s e n c e o f an e f f e c t i n d i ^ c a t e d t h a t change i n c r a c k i n g mode a t h i g h Kj v a l u e s a t 92 °C was n o t due t o h i g h e r c r a c k growth r a t e a t t h o s e h i g h e r s t r e s s i n t e n s i t i e s . C a u s t i c c o n c e n t r a t i o n a f f e c t e d t h e mode o f c r a c k i n g . F i g u r e 31 i l l u s t r a t e s t h a t some t r a n s g r a n u l a r c r a c k i n g o c c u r - r e d i n 12 mol/kg NaOH pe r h a p s when the o r i e n t a t i o n was f a v o r - a b l e . A c l u e t o u n d e r s t a n d i n g t h e change i n f r a c t o g r a p h y w i t h c o n c e n t r a t i o n s m i g h t be f o u n d i n t h e v a l u e o f t h e a c t i v a t i o n e n e r g y . T h a t v a l u e s u g g e s t e d t h a t i n t h e c o n c e n t r a t e d s o l u - t i o n , d i f f u s i o n was p l a y i n g a l a r g e r r o l e i n mixed c o n t r o l o f d i s s o l u t i o n . T h i s may i n t u r n have a f f e c t e d t h e f r a c t o g r a p h y . S u r f a c e i m p e r f e c t i o n s a t g r a i n b o u n d a r i e s w h i c h may have been i m p o r t a n t i n i n c r e a s i n g d i s s o l u t i o n r a t e f o r a c t i v a t i o n c o n - t r o l l e d d i s s o l u t i o n may have been l e s s i m p o r t a n t i n a c r a c k i n g p r o c e s s a l r e a d y s l o w e d down by d i f f u s i o n i n f l u e n c e s . Thus t r a n s g r a n u l a r c r a c k i n g c o u l d be as f a v o r a b l e as i n t e r g r a n u l a r . In c o n t r a s t t o t h e i n t e r g r a n u l a r c r a c k i n g i n t h e NaOH, th e f r a c t u r e s u r f a c e s i n t h e 12 mol/kg NaOH + 0.423 mol/kg Na^S s o l u t i o n e x h i b i t e d 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 109 c r a c k i n g . The l a r g e r p r o p o r t i o n o f t r a n s g r a n u l a r c r a c k i n g i n the s u l f i d e c o n t a i n i n g s o l u t i o n t h a n i n t h e s t r a i g h t NaOH s o l u t i o n may have r e s u l t e d from t h e p r e s e n c e o f t h e s u l f i d e i o n s . The s u l f i d e may have a d s o r b e d on t h e l a t t i c e imper- f e c t i o n s a t the g r a i n b o u n d a r i e s t h e r e b y d e c r e a s i n g t h e a d s o r p - t i o n o f 0H~ and l e s s e n i n g t h e d i s s o l u t i o n r a t e a t t h e g r a i n 2- b o u n d a r i e s . I f a d s o r p t i o n o f S r e d u c e s d i s s o l u t i o n r a t e t h e n , s i m i l a r l y , i t may be r e s p o n s i b l e f o r the s l o w e r c r a c k growth r a t e i n t h e NaOH + Na 2S s o l u t i o n . A l t e r n a t e l y , t h e d i f f e r e n c e i n f r a c t o g r a p h y may have r e s u l t e d from t e s t i n g i n a much d i f f e r e n t p o t e n t i a l r a n g e . Hydrogen e m b r i t t l e m e n t was p o s s i b l e a t t h i s p o t e n t i a l and 2_ may have c a u s e d t h e t r a n s g r a n u l a r c r a c k i n g . S may have been i m p o r t a n t i n c a u s i n g h y drogen e m b r i t t l e m e n t by p o i s o n i n g h y d r o - gen r e c o m b i n a t i o n . Lack o f e x p e r i m e n t a l r e s u l t s a t t h e s e c o n - d i t i o n s has p r e c l u d e d e v a l u a t i o n o f t h i s p o s s i b i l i t y . In c o n t r a s t to the p r e s e n t r e s u l t , A s a r o 3 4 found t h a t 304 s t a i n l e s s s t e e l showed m o s t l y i n t e r g r a n u l a r c r a c k i n g i n 50% NaOH w i t h 0.03 m o l / l i t e r Na 2S a t 180 °C. The d i f f e r e n c e 2- may have r e s u l t e d from e i t h e r l o w e r S c o n c e n t r a t i o n o r d i f - f e r e n t p o t e n t i a l (unknown). 4.5 E l e c t r o n D i f f r a c t i o n A n a l y s i s o f S u r f a c e F i l m s i n NaOH The e l e c t r o n d i f f r a c t i o n r i n g p a t t e r n s were c o n s i s t e n t n o 71 72 w i t h a s p i n e l t y p e o f s t r u c t u r e ' but t h e l a t t i c e para- m e t e r s were g e n e r a l l y a l i t t l e h i g h e r t h a n t h a t e x p e c t e d from 91 ° p u b l i s h e d d a t a f o r Fe-b.ased s p i n e l s e.g. 8.50 A v e r s u s o ^ 8.38 A f o r Fe^O^. The d i f f e r e n c e may have been due t o a d i f - f e r e n t v a l e n c y s t a t e o r an i r r e g u l a r l a t t i c e a r r a n g e m e n t h e r e . N i k i f o r u k 7 0 o b t a i n e d r e a s o n a b l e v a l u e s f o r s p i n e l l a t t i c e p a r a m e t e r s formed on s t a i n l e s s s t e e l s i n MgCl^ u s i n g t h e same method. T h e r e were some f u n d a m e n t a l p r o b l e m s w i t h t h i s method o f a n a l y s i s . The f i l m a n a l y z e d was not a r e p r e s e n t a t i v e sample o f t h e f i l m on t h e f r a c t u r e s u r f a c e . Much o f t h e s u r f a c e d e p o s i t was t h i c k and as su c h was opaque t o e l e c t r o n s . The p a t t e r n s may n o t have been f o r m a t e r i a l from t h e f r a c t u r e s u r f a c e but from t h e s i d e s o f t h e s p e c i m e n . The method 72 d e s c r i b e d by B i r l e y f o r o b t a i n i n g e l e c t r o n d i f f r a c t i o n p a t t e r n s from t h e u n s t r i p p e d f r a c t u r e s u r f a c e would a v o i d t h e s e p r o b l e m s . In t h a t method, t h e e l e c t r o n beam p a s s e s t h r o u g h o x i d e s on s u r f a c e a s p e r i t i e s . The s u r f a c e f i l m was t e n t a t i v e l y i d e n t i f i e d as Fe^P^. E-pH d i a g r a m s i n d i c a t e t h a t FeOOH m i g h t be formed a t p r i m a r y t r a n s p a s s i v e p o t e n t i a l s . The FeOOH may have d e h y d r a t e d on removal from t h e s o l u t i o n o r i n s u b s e q u e n t h a n d l i n g t o form H F e 5 0 8 ( Y - F e 2 0 3 ) ^ v i a : 5 FeOOH H F e 5 0 8 + 2H 20 ...(13) I l l 7 2 B i r l e y has n o t e d t h a t i t i s a l m o s t i m p o s s i b l e t o d i s t i n g u i s h between y-Fe^O^ ( H F e 5 0 g ) and F e 3 0 4 by t h e e l e c t r o n d i f f r a c t i o n t e c h n i q u e b e c a u s e o f t h e l i m i t a t i o n s i n t h e a c c u r a c y o f t h e t e c h n i que. 5. SUMMARY P o l a r i z a t i o n s t u d i e s , s l o w s t r a i n r a t e t e s t s , f r a c t u r e m e c h a n i c s t e s t s , e l e c t r o n d i f f r a c t i o n , and f r a c t o g r a p h i c t e c h n i q u e s have been employed t o o b t a i n e n g i n e e r i n g d a t a and to r e v e a l t h e mechanism o f cr.ackihgj o f 316 s t a i n l e s s s t e e l i n c a u s t i c s o l u t i o n s . In NaOH s o l u t i o n , c r a c k i n g o c c u r r e d i n t h e p r i m a r y t r a n s p a s s i v e p o t e n t i a l range where t h e chromium p a s s i v e f i l m 2 - was u n s t a b l e and d i s s o l u t i o n o f chromium t o CrO^ was o c c u r - r i n g . At a c t i v e - p a s s i v e p o t e n t i a l s , o n l y s e n s i t i z e d m a t e r i a l showed some s u s c e p t i b i l i t y , and t h a t may have r e s u l t e d f r o m f i l m i n s t a b i l i t y and i r o n d i s s o l u t i o n . The mechanism o f c r a c k i n g a t p r i m a r y t r a n s p a s s i v e p o t e n t i a l s i n NaOH s o l u t i o n a p p e a r e d t o be d i s s o l u t i o n , which may i n v o l v e an a d s o r p t i o n s t e p . T h e r e was e v i d e n c e t h a t d i f f u s i o n i n t h e l i q u i d phase a l s o a f f e c t e d c r a c k i n g k i n e t i c s a t p o t e n t i a l s i n t h e s e c o n d a r y p a s s i v e r e g i o n . In t h e NaOH + Na 2S s o l u t i o n s , c r a c k i n g o c c u r r e d a t a c t i v e p o t e n t i a l s . F i l m i n s t a b i l i t y a t t h e s e - p o t e n t i a l s may have r e s u l t e d from the p re sence o f s u l f i d e . Hydrogen e m b r i t t l e m e n t c o u l d not be r u l e d out at a c t i v e - p a s s i v e p o t e n t t i a 1 s . BIBLIOGRAPHY 11 3 1. J . E. Truman and R. P e r r y , B r i t . C o r r . J . 1 pp.60-66 ( 1 9 6 6 ) . 2. P. P. Snowden, J . I . S . I . . 197 pp. 1 36-141 (1 961 ). 3. P. P. Snowden, J . 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