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Cracking of mild steel in NaOH and caustic aluminate solutions Sriram, Rajagopal 1984

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CRACKING OF MILD STEEL IN NaOH AND CAUSTIC ALUMINATE SOLUTIONS By RAJAGOPAL SRIRAM B.Sc. Bangalore Un i v e r s i t y , India, 1977 B.E. Indian I n s t i t u t e of Science, India, 1980 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of M e t a l l u r g i c a l Engineering) We accept t h i s thesis as conforming to the reauired standard THE UNIVERSITY OF BRITISH COLUMBIA January 1984 © Rajagopal Sriram, 1984 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements fo r an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e for reference and study. I further agree that permission for extensive copying of t h i s thesis f o r scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date DE-6 (3/81) ABSTRACT Stress corrosion cracking (SCC) of ASME SA 516 Grade 70 s t e e l 0 was s t u d i e d i n hot (92 C) NaOH s o l u t i o n s of d i f f e r e n t c a u s t i c concentrations, caustic aluminate solutions and an I n d u s t r i a l Bayer s o l u t i o n (IBS). The p o t e n t i a l regimes of stress corrosion cracking s u s c e p t i b i l i t y were assessed using the slow s t r a i n rate technique (SSRT). At anodic p o t e n t i a l s , SCC tendency was most pronounced i n the active-passive transion region. Under these conditions, the largest number of secondary cracks and highest apparent crack v e l o c i t i e s were observed i n the IBS. The i n h i b i t i o n of corrosion of s t e e l i n caustic solutions by the addition of alumina trihydrate was studied by using anodic p o l a r i s a t i o n t e s t s , c y c l i c voltammetry, electron d i f f r a c t i o n and energy dispersive X-ray spectroscopy (EDS) techniques. The re s u l t s were consistent with the formation of an amorphous iron-aluminate f i l m on the electrode surface. Fracture mechanics techniques were used to study the k i n e t i c s of stress corrosion crack propagation i n caustic solutions of d i f f e r e n t concentrations ( 2, 4 and 8 mols/Kg NaOH), 4 mols/Kg NaOH + 1 mol/KG of alumina and IBS. Both stress i n t e n s i t y dependent (region I) and stress i n t e n s i t y independent (region II) behaviour were observed. Crack fractography was studied by scanning electron microscopy. The r e s u l t s suggested that cracking at anodic potentials i i appears to be consistent with the general p r i n c i p l e s of the f i l m -rupture d i s s o l u t i o n model and that the d i s s o l u t i o n processes i n the crac k - t i p region were under 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 . i i i TABLE OF CONTENTS Page Abstract II Table of Contents. IV L i s t of Tables VII L i s t of figures IX L i s t of Symbols and Abbreviations XII Acknowledgement XIV Chapter 1 INTRODUCTION 1 1.1 SCC of Mild S t e e l i n Caustic Environments.... 2 1.2 Electrochemical P o t e n t i a l 2 1.3 Composition & Concentration of Environment... 3 1.4 Temperature 4 1.5 E f f e c t of Chemical Composition and Structure of Steel 4 1.6 Stress Intensity 5 1.7 Mechanisms of SCC 5 1.8 Testing Techniques 10 1.8.1 Slow S t r a i n Rate Testing 10 1.8.2 Fracture Mechanics Testing 12 1.8.3 C y c l i c Voltammetry 15 1.9 Origins of Present Work 16 2 EXPERIMENTAL 20 2.1 P o l a r i s a t i o n curve 20 i v 2.1.1 Procedure 24 2.2 Slow S t r a i n Rate Testing 24 2.2.1 Procedure 28 2.3 Electron D i f f r a c t i o n and Composition Analysis of Corrosion and Films 29 2.3.1 Procedure 30 2.4 Corrosion P o t e n t i a l Measurements 31 2.4.1 Procedure 33 2.5 Fracture Mechanics Experiments 33 2.5.1 Procedure 37 2.6 C y c l i c Voltammetry 38 2.6.1 Procedure 39 3 RESULTS 41 3.1 P o l a r i s a t i o n Behaviour 41 3.2 SSRT Results 46 3.3 Corrosion Potentials 55 3.4 E l e c t r o n D i f f r a c t i o n 55 3.5 C y c l i c Voltammetry 58 3.6 Fracture Mechanics 66 3.7 Fractography of SCC 75 4 DISCUSSION 85 4.1 P o l a r i s a t i o n Behaviour 85 4.1.1 Mechanism of Alumina I n h i b i t i o n 85 4.1.2 E f f e c t of Carbonate & Aluminate Additions to NaOH 87 4.1.3 Organic Acid Additions 88 v 4.2 Slow S t r a i n Rate Testing 89 4.2.1 Cracking at Cathodic Potentials 89 4.2.2 Cracking at Anodic Potentials 91 4.3 Fracture mechanics Tests 92 4.3.1 E f f e c t of Sodium Hydroxide Concentrations on Crack V e l o c i t y .... 92 4.3.2 Comparison of Crack V e l o c i t i e s from Slow S t r a i n Rate Test With Fracture Mechanics Tests 93 4.4 Estimation of Concentration Overvoltage ... 95 4.5 Coalescence of Cracks 98 5 CONCLUSIONS 99 BIBLIOGRAPHY 101 APPENDIX - 1 106 APPENDIX - 2 107 v i LIST OF TABLES Table Page 1 Chemical Composition of Steel 21 2 Solution Compositions Used for P o l a r i s a t i o n Tests 21 3 Solution Compositions Used for Corrosion P o t e n t i a l Measurements 32 4 Solution Compositions Used for C y c l i c Voltammetry. 40 5 Results of SSRT 51 6 Corrosion P o t e n t i a l s 56 7 Ele c t r o n D i f f r a c t i o n Data i n 4m NaOH Solution .... 57 8 Ele c t r o n D i f f r a c t i o n Data i n 4m NaOH + lm A^Og .. 57 9 E f f e c t of Stress Intensity on Crack V e l o c i t y i n . . . 0 2m NaOH s o l u t i o n at 92 C & -0.980 V g C E 72 10 E f f e c t of Stress Intensity on Crack V e l o c i t y i n . . . 0 4m NaOH so l u t i o n at 92 C & -1.070 V c „ 72 SCE 11 E f f e c t of Stress Intensity on Crack V e l o c i t y i n 0 8m NaOH so l u t i o n at 92 C & -1.030 V g C E 73 12 E f f e c t of Stress Intensity on Crack V e l o c i t y i n 0 4m NaOH + lm A l o 0 o s o l u t i o n at 92 C & -0.960 V . „ 73 13 E f f e c t of Stress Intensity on Crack V e l o c i t y i n 0 I n d u s t r i a l Bayer Solution (IBS) at 92C & -1.020V S C E 77 14 Region II Crack V e l o c i t i e s 77 15 Comparison of Apparent Crack V e l o c i t i e s From SSRT.. with Region II Crack V e l o c i t i e s 93 16 Calculated and Observed Crack V e l o c i t i e s i n Solutions 93 v i i I 1 17 Calculated Overpotentials Based on Estimated Values of i Q 0 r r Using Faraday's Law 93 Al Compliance of the Specimen as a Function of Crack Length 110 A2 Compliance Data from F i t t e d Curve I l l t v i i i LIST OF FIGURES Figure No. Page 1 Schematic of a Ty p i c a l log V-K Plot 14 2 Schematic of the Bayer Process Cycle 18 3 P o l a r i s a t i o n Test C e l l 23 4 SSRT Specimen Geometry 25 5 SSRT Apparatus 26 6 SSRT C e l l 28 7 DCB Specimen Geometry 34 8 Schematic of DCB Test C e l l 35 9 Anodic P o l a r i s a t i o n Curves i n NaOH Solutions.. 42 10 Anodic P o l a r i s a t i o n Curves i n NaOH + A^O-j ... Solutions (Constant 'free NaOH Concentration). 43 11 Anodic P o l a r i s a t i o n Curves i n NaOH + A^O^ • • • Solutions (Variable 'free NaOH Concentration) 44 12 Anodic P o l a r i s a t i o n Curves i n NaOH + AL^Og ... Solutions ( C a u s t i c i t y Corresponding to IBS)... 45 13 Anodic P o l a r i s a t i o n Curves i n NaOH + Na2C0g... Solution 47 14 Anodic P o l a r i s a t i o n Curves i n NaOH + O r g a n i c . Acids 48 15 Anodic P o l a r i s a t i o n Curves i n IBS & SB Solutions 49 16 % Reduction i n Area, No. of Cracks(N) & Apparent Crack V e l o c i t y (V ) i n 2 m NaOH Solutions .. 50 17 % Reduction i n Area, No. of Cracks(N) & Apparent Crack V e l o c i t y (V ) i n 4m NaOH + lm A l o 0 - .. 50 J app 2 3 ix 18 % Reduction i n Area, No. of Cracks (N) & Apparent Crack V e l o c i t y i n IBS 50 19 T y p i c a l Metallograph of Sectioned & Etched ... SSRT Specimen Showing Intergranular Cracking.. 53 20 Fractured SSRT Specimen i n IBS E = - 1 - 0 1 0 V S C E 5 4 21 Fractured SSRT Specimen i n IBS E - -1.200 V g C E 54 22 C y c l i c Voltammogram i n 2m NaOH 59 23 C y c l i c Voltammogram i n 3m NaOH 60 24 C y c l i c Voltammogram i n 4m NaOH 61 25 C y c l i c Voltammogram i n 3.0m NaOH + 0.5m A1 20 3 63 26 C y c l i c Voltammogram i n 4.0m NaOH + 1.0m A1 20 3 64 27 C y c l i c Voltammogram i n 2.0m NaOH + 0.42m Na 2C0 3 65 28 C y c l i c Voltammogram i n 2.0m NaOH +2.3 g/1 Formic 67 29 C y c l i c Voltammogram i n 2.0m NaOH +4.4 g/1 Acetic 68 30 C y c l i c Voltammogram i n IBS 69 31 E f f e c t of Stress Intensity on Crack V e l o c i t y i n NaOH Solution of D i f f e r e n t Concentrations 71 32 E f f e c t of Stress Intensity on Crack V e l o c i t y i n 4m NaOH + lm A1 20 3 Solution 74 33 E f f e c t of Stress Intensity on Crack V e l o c i t y i n IBS 76 34 Fracture surfaces a f t e r t e s t i n g i n 2m NaOH ... E = - ° ' 9 8 0 VSCE K = 32 MPa/m 78 35 Fracture surface a f t e r t e s t i n g i n 4m NaOH .... (a) K = 30 MPa/m 80 (b) K = 40 MPa/m 80 36 Fracture surface a f t e r testing i n 8m NaOH .... E = -1.030 V S C E (a) K = 26 MPa/m 81 (b) K = 38 MPa/m 81 37 Fracture surface a f t e r t e s t i n g i n 4m NaOH + .. lm alumina E = -0.960 V S C E (a) K = 34 MPa/m 82 (b) K = 42 MPa/m 82 38 Fracture surface a f t e r testing i n IBS E = -1.020 V S C E (a) K = 22 MPa/m 83 (b) K = 22 MPa/m 83 (c) K = 38 MPa/m 84 A- l Load-deflection curves 108 A-2 Compliance curve 109 x i LIST OF SYMBOLS AND ABBREVIATIONS Symbols a B S u l k COHP D E F H i a --D *I K I C I SCC N P R T V V app SCE W Crack length Thickness of fracture mechanics specimen Bulk concentration Concentration at the outer Helmholtz plane D i f f u s i o n c o e f f i c i e n t P o t e n t i a l Faraday ( 96500 coulombs) DCB specimen beam Height Anodic current density at the crack t i p D i f f u s i o n current density Stress i n t e n s i t y f actor for mode I opening C r i t i c a l stress i n t e n s i t y f actor SCC threshold stress i n t e n s i t y factor Number of cracks Load Gas constant (8.314 KJ/mol.deg) Absolute temperature Crack v e l o c i t y Apparent crack v e l o c i t y Volts with respect to saturated calomel electrode. Equivalent weight of i r o n D i f f u s i o n layer thickness D i f f u s i o n (concentration) overpotential. x i i Abbreviations DCB Double c a n t i l e v e r beam IBS I n d u s t r i a l bayer s o l u t i o n LEFM Linear e l a s t i c fracture mechanics OHP Outer helmholtz plane SB Simulated bayer SCC Stress corrosion cracking SSRT Slow s t r a i n rate t e s t ( i n g ) x i i i ACKNOWLEDGEMENT I would l i k e to extend my sincere thanks and appreciation to Dr. Desmond Tromans for his advice and assistance throughout t h i s project. I would also l i k e to thank Mr. Dave Crowe for getting me i n i t i a t e d into experimental work, for his constructive c r i t i c i s m and encouragement during t h i s p r o j e c t . Thanks are extended to fellow graduate students and s t a f f of Department of M e t a l l u r g i c a l Engineering for t h e i r help i n t h i s work. My parents deserve a s p e c i a l note of thanks f o r t h e i r understanding and encouragement. The f i n a n c i a l support for t h i s project provided by NRC/DSS (Contract no. OSD81-00174) and the ass i s t a n t s h i p provided by the Department of M e t a l l u r g i c a l Engineering are greatly appreciated. x i v 1. INTRODUCTION 1 Stress corrosion cracking (SCC) i s defined as a cracking process occuring due to the conjoint action of a corrodent and a sustained t e n s i l e stress.The i n s i d i o u s nature of t h i s process leads to unforseen f a i l u r e of components and loss of production due to downtime. In the extreme case, i n j u r i e s may occur to personnel working near the premises of the equipment. SCC became a serious i n d u s t r i a l problem only a f t e r the development of steam engines, although i t was recognised e a r l i e r because of the season cracking of brass c a r t r i d g e s . With the rapid growth of chemical i n d u s t r i e s , the number of systems i n which f a i l u r e occurs has increased dramatically, e s p e c i a l l y with increased t e n s i l e loading demands placed on materials. Accompanying the increased i d e n t i f i c a t i o n of SCC as a major cause of f a i l u r e , there i s a s i g n i f i c a n t increase i n the amount of r e s e a r c h , both academic and i n d u s t r i a l , d i r e c t e d towards understanding the problem from the mechanistic and p r e d i c t i v e viewpoints. This has resulted i n development of techniques to predict and prevent future f a i l u r e s and the i d e n t i f i c a t i o n of i n c r e a s i n g l y large numbers of a l l o y - environment combinations i n which SCC can occur, providing an a d d i t i o n a l impetus and urgency to solve the problem. Contrary to i n t i a l b e l i e f , there i s no s i n g l e uni v e r s a l mechanism to explain the phenomenon. 2 1.1 SCC o f Mild S t e e l i n Caustic "Environments SCC of s t e e l i n a l k a l i n e solutions i s well known, caustic cracking of r i v e t t e d locomotive b o i l e r s being an early example. I t 1 2 3 4 i s a p r o b l e m i n power p l a n t equipment ' ' ' , k r a f t p u l p 5 6 7 8 d i g e s t e r s ' and Bayer p l a n t s * p r o c e s s i n g raw bauxite ore to produce pure alumina. Much of the present knowledge about caustic 9 10 11 12 c r a c k i n g has been reviewed previously ' ' ' . A large number of v a r i a b l e s a f f e c t c a u s t i c SCC of m i l d s t e e l s i n c l u d i n g electrochemical p o t e n t i a l , composition and concentration of environment, stress i n t e n s i t y , carbon content of s t e e l , temperature and cold work. 1.2 Electrochemical P o t e n t i a l 13 14 13 16 Several studies ' ' ' have shown that caustic cracking of mild s t e e l can be associated with a range of potentials near the a c t i v e peak. O l s o n , i n h i s study of SCC i n k r a f t d i g e s t e r systems, reports that cracking was found to occur when the p o t e n t i a l was i n active or active-passive t r a n s i t i o n zones, g Artem'ev , i n h i s study on SCC of carbon s t e e l i n Bayer plants, has found that potentials at the active-passive region are the most dangerous. 3 18 S i n g b e i l and Tromans have found that p o t e n t i a l a f f e c t s the cracking mode and the rate of cracking. The cracking was mostly intergranular at -0.76 V.^ ,, and predominantly transgranular at -1.0 V e , i r r e s p e c t i v e of stress i n t e n s i t y and temperature. 1.3 Composition and Concentration of Environment Caustic cracking has been found to occur at concentrations as 13 low as 4% and as high as 75% , with severest cracking occuring at 3 3 % ^ . K a r p e n k o ^ e t . a l . quote from a work of V.Noev to show that there i s a reduction i n time to f a i l u r e with increase i n caustic c o n c e n t r a t i o n . A s i m i l a r r e s u l t has been found by ot h e r w o r k e r s ^ ' ^ ' R e i n o e h l and Berry^"* have quoted from the work of others to show that additions of other species to caustic solutions may either aggravate or i n h i b i t cracking under f r e e l y corroding conditions, depending on the d i r e c t i o n and magnitude by which the natural p o t e n t i a l i s s h i f t e d . Substances l i k e s i l i c a , permanganate ions and chromate ions, i f present i n c r i t i c a l amounts,can stimulate cracking by moving the p o t e n t i a l s l i g h t l y i n the noble d i r e c t i o n into the c r i t i c a l range where cracking i s p o s s i b l e . However, i f these substances are present i n higher concentrations they act as i n h i b i t o r s by s h i f t i n g the p o t e n t i a l past the c r i t i c a l range into the passive region. Additions of hydrogen peroxide or potasium persulfate lead to a rapid s h i f t of the metal p o t e n t i a l into the passive region and g prevent s t r e s s c o r r o s i o n c r a c k i n g of equipment .Caustic cracking 4 i s i n h i b i t e d by additions of tannin, quebracho and, to a l e s s e r extent, by additions of sodium silicate"""". 1.4 Temperature 18 19 SCC i s more severe as temperature increases ' . In a study on mild s t e e l and 0.24% C s t e e l i n caustic solutions, M a z i l l e et. 20 a l . , found that f a i l u r e times were longer at lower temperatures, which they ascribed to lower crack growth rates and extended crack i n d u c t i o n t i m e s . D i e g l e e t . a l . " " ^ , i n a study of i r o n - c a u s t i c systems, found a ten f o l d increase i n current density. Within the 0 0 IQ temperature range of 50 C to 105 C, S i n g b e i l and Tromans found a l i n e a r r e l a t i o n s h i p between log(V) and 1/T, where V i s the region II (stress independent)velocity of V-K pl o t and T i s the absolute temperature. 1.5 E f f e c t of Chemical Composition and Structure of Steel 2 0 M a z i l l e and U h l i g have f o u n d t h a t i n c a r b o n s t e e l s u s c e p t i b i l i t y to f a i l u r e i s s e n s i t i v e to both structure and composition. However, there i s considerable disagreement regarding the e f f e c t of carbon c o n t e n t . Karpenko and V a s i l e n k o ^ reported from the work of Podgorny that an increase i n carbon content from 0.12% to 0.23% reduced the resistance to SCC. A s i m i l a r r e s u l t has 20 2 been r e p o r t e d by M a z i l l e and U h l i g . Weier r e p o r t e d t h a t resistance to SCC increased with increase i n carbon content. Wilson 21 e t . a l . , observed th a t carbon content had l i t t l e e f f e c t on SCC 5 22 s u s c e p t i b i l i t y . Robinson found that a decrease i n carbon content improved resistance to SCC. 2 3 A d d i t i o n s o f t i t a n i u m and o t h e r c a r b i d e f o r m i n g 2 1 e l e m e n t s 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 o f low c a r b o n s t e e l s . K a r p e n k o and Vasilenko""^ have quoted from the work of S h u l ' t e e t . a l . , t h a t a d d i t i o n s o f 0.2% l a n t h a n u m , aluminum, titanium or vanadium increase the resistance to SCC i n a l k a l i n e s o l u t i o n s . P l a s t i c d e f o r m a t i o n o f s t e e l i n c r e a s e s t h e 24 s u s c e p t i b i l i t y t o SCC i n a l k a l i n e s o l u t i o n s . One of the reasons being a t t r i b u t e d to the s t a b i l i s a t i o n of the steady state corrosion p o t e n t i a l i n the a c t i v e r e g i o n ^ . 1.6 Stress I n t e n s i t y The e f f e c t of stress i n t e n s i t y on cracking rate of mild 18 s t e e l i n c a u s t i c s o l u t i o n s has been examined only r e c e n t l y No i n f o r m a t i o n e x i s t s i n the p u b l i s h e d l i t e r a t u r e about the e f f e c t of s t r e s s i n t e n s i t y i n c a u s t i c aluminate and i n d u s t r i a l s o l u t i o n s . 1.7 Mechanisms of SCC C o n f l i c t i n g views e x i s t regarding the mechanisms of SCC 6 i n a l k a l i n e m e d i a . Most of t h e mechanisms have an electrochemical basis, although hydrogen embrittlement i s not completely o v e r r u l e d i n some ca s e s . A l l the mechanisms i n v o l v e c o m p l e x i n t e r a c t i o n s b e t w e e n m e t a l l u r g y , electrochemistry and mechanics of the system. The p r e d i c t i v e values of these mechanisms are q u a l i t a t i v e . Most of the models developed are based on the f i l m r u p t u r e and d i s s o l u t i o n mechanism o r i g i n a l l y proposed by 2 5 26 L o gan . V e r m i l y e a has proposed a d e t a i l e d theory based on t h i s mechanism. I t provides a conceptual framework which i s amenable to l a t e r modifications and i d e n t i f i e s the relevant material parameters which influence the mechanism. According to t h i s theory, both metal s t r a i n and corrosion are mutually stimulative. At the crack t i p , s t r a i n accelerates corrosion by causing periodic f i l m rupture and corrosion accelerates s t r a i n by removing the most s t r a i n hardened material at the crack t i p . This theory has been further developed i n a l a t e r 27 paper by Vermilyea using fr a c t u r e mechanics models. 2 8 S t a e h l e has argued th a t s l i p p rocesses are c r u c i a l i n breaking the protective f i l m and producing SCC. According to h i s model, the amount of c r a c k t i p metal which d i s s o l v e s between b r e a k i n g and r e f o r m a t i o n c y c l e s o f t h e f i l m constitutes the crack advance stage. The model requires that neither repassivation (reformation of film) i s very f a s t nor 7 the d i s s o l u t i o n i s very great. 29 F o r t y and Humble have g i v e n a d i f f e r e n t i n t r e p r e t a t i o n to the f i l m rupture concept. According to them, a thick corrosion f i l m i s ruptured by b r i t t l e processes. The crack propagates by a sequence of f i l m formation and b r i t t l e f i l m rupture, so that rupture of the f i l m represents crack advance stage. 30 B i g n o l d considers that cracking involves active d i s s o l u t i o n at the crack t i p and passivation near the mouth of the crack. In the i r o n - a l k a l i system, the pH properties of the crack t i p s o l u t i o n saturated with corrosion products w i l l be very s i m i l a r to that of bulk s o l u t i o n . He contends that although there i s a concentration gradient down the crack, the rate of d i s s o l u t i o n at the f i l m free crack t i p i s not under d i f f u s i o n c o n t r o l because crack rates respond to an applied p o t e n t i a l . In Bignold's model, cracking due to hydrogen embrittlement i s not considered s i g n i f i c a n t because hydrogen evolution i s not l o c a l i s e d at the crack t i p . The presence of a concentration gradient down the crack 31 i m p l i e s a p o t e n t i a l g r a d i e n t . Doig and Fl e w i t t have analysed t h e o r e t i c a l l y the electrode p o t e n t i a l d i s t r i b u t i o n down the crack a n d h a v e c o n c l u d e d t h a t e x t e r n a l p o l a r i s a t i o n 8 can be q u a l i t a t i v e l y correlated with changes i n the anodic d i s s o l u t i o n k i n e t i c s at crack t i p . However, changes i n stress c o r r o s i o n c r a c k growth r a t e s are not d i r e c t l y r e l a t e d to 3 2 m a g n i t u d e o f p o l a r i s a t i o n . P o u l s o n has r e p u d i a t e d t h e 31 arguements of Doig and F l e w i t t . He p o i n t s out t h a t based on the model, p o t e n t i a l drops anywhere between ten to a few hundred m i l l i v o l t s can be calculated and there i s very l i t t l e c o r r e l a t i o n between experimental r e s u l t s and predicted r e s u l t s . In addition, a po t e n t i a l drop down the crack may not be as important as 33 suggested 34 U h l i g has proposed an e n t i r e l y d i f f e r e n t mechanism f o r SCC, based on the existence of a c r i t i c a l p o t e n t i a l f o r c r a c k i n g i n some systems. The c r i t i c a l p o t e n t i a l i s t h a t value above which damaging ions are believed to adsorb on defect s i t e s of stressed metal and below which desorption occurs. When the corrosion p o t e n t i a l s of the system are noble to t h i s c r i t i c a l p o t e n t i a l , cracking occurs. This p o t e n t i a l can be s p e c i f i c to the anion and the metal. Addition of extraneous species can s h i f t the c r i t i c a l p o t e n t i a l either i n the noble d i r e c t i o n or i n the active d i r e c t i o n . Cold working usually s h i f t s the c r i t i c a l p o t e n t i a l i n the active d i r e c t i o n . A number of drawbacks f or t h i s 35 36 p a r t i c u l a r mechanism have been pointed out ' , eg., the c r i t i c a l p o t e n t i a l can be e x p l a i n e d i n terms of c o r r o s i o n f i l m i n s t a b i l i t i e s . 9 K a r p e n k o q u o t e s from t h e work of Podgorny r e g a r d i n g the r o l e of hydrogen embrittlement i n SCC of low carbon s t e e l s i n h o t a l k a l i n e s o l u t i o n s . P o d g o r n y f o u n d a c o r r e l a t i o n between the time to f a i l u r e and the amount of hydrogen absorbed by the metal. An increase i n the amount of absorbed hydrogen resulted i n reduced time to f a i l u r e . In t h i s mechanism, fracture r e s u l t s from formation of a b r i t t l e region at the crack t i p because of the introduction of atomic hydrogen into the a l l o y v i a cathodic reactions. The mechanism of hydrogen embrittlement has been a t t r i b u t e d to a number of 37 38 39 causes, including decohesion , formation of martensite ' i n fee 40 steels and d i s l o c a t i o n pinning In a l l the hydrogen embrittlement arguments, the main requirement i s that the crack t i p chemistry produces a p o t e n t i a l and pH at which production of hydrogen i s thermodynamically f e a s i b l e . Also, the rate of production of hydrogen w i l l influence i t s entry i n t o the metal l a t t i c e . Techniques for measuring 41 hydrogen entry into the l a t t i c e have been discussed 42 Preece has argued that cathodic protection, which a c t u a l l y promotes production of hydrogen, retards SCC for a number of a l l o y s i n neutral or acid s o l u t i o n s . This can be a t t r i b u t e d to changes i n pH of the crack t i p s o l u t i o n . 10 1.8 Testing Techniques The following experimental techniques were used i n the present program. 1.8.1 Slow S t r a i n Rate'Testing (SSRT) This test i s a rapid method to assess the s u s c e p t i b i l i t y of metals and a l l o y s to SCC. A r e l a t i v e l y slow s t r a i n rate, eg. e = 10 ^ / s e c i s a p p l i e d to a waisted t e n s i l e specimen immersed i n 43 an environment. Reviews have been p u b l i s h e d on t h i s method . The importance of t h i s test method l i e s i n the fa c t that i t allows c o r r e l a t i o n of a c o n t r o l l e d rate of bare metal exposure, r e s u l t i n g from f i l m rupture, to the process of repassivation. At very low s t r a i n rates, the rate of repassivation i s rapid enough to prevent attack of the exposed bare metal by the environment. At high s t r a i n rates the rate at which metal f a i l s by d u c t i l e fracture exceeds the rate, at which environment can a f f e c t the fracture process. I t i s i n the intermediate range of s t r a i n rates only that the rate of repassivation i s matched by rate of bare metal formation, allowing the environment to gain access to the bare metal. The main advantage of SSRT i s that i t concludes i n a p o s i t i v e manner with r e l a t i v e r a p i d i t y and provides a rapid sorting test for 44 a s s e s s i n g SCC s u s c e p t i b i l i t y . The main disadvantage i s that i t 11 provides no quantitative information which a designer can use. The s t r a i n r a t e u s e d f o r SSRT d e p e n d s on t h e 45 p a r t i c u l a r system under c o n s i d e r a t i o n . D i e g l e and Boyd have quoted s t r a i n rates employed f or various metal-environment 44 combinations from d i f f e r e n t sources. Parkins has noted that f o r — 5 —6 many systems s t r a i n r a t e s i n the range of 10 to 10 /sec 43 -6 promote SCC. Kim e t . a l . , have used a s t r a i n rate of 1X10 /sec for t e s t i n g i n d i f f e r e n t environments and have noted that steels show severe SCC at t h i s s t r a i n rate regardless of environment. —6 Payer et a l . , have used a s t r a i n r a t e of 2.5X10 /sec i n t h e i r tests and found no s i g n i f i c a n t change i n crack v e l o c i t i e s when —6 —6 s t r a i n r a t e s were changed from 2.5X10 to 1.0X10 /sec. Hishida 47 -6 e t . a l . , found that a s t r a i n rate of 4.0X10 /sec was optimum for th e i r tests on s t a i n l e s s s t e e l i n acid chloride s o l u t i o n s . Poulson 4 8 -6 e t . a l employed a s t r a i n r a t e of 2X10 /sec i n t e s t i n g 9Cr-lMo s t e e l i n caustic solutions at high temperature. The cracking response may be assessed on :-1. time to f a i l u r e and 2. percentage reduction i n area. Metallographic techniques may also be used. The specimens can be sectioned l o n g i t u d i n a l l y i n the diametral plane and the longest detectable crack can be measured and an apparent crack v e l o c i t y can be computed. In addition, the number of secondary cracks can be counted as a measure of crack i n i t i a t i o n s u s c e p t i b i l i t y . 12 Time to f a i l u r e includes time f o r i n i t i a t i o n of cracks and time for propagation of cracks. In some alloy-environment systems, 49 i n i t i a t i o n i s a major f r a c t i o n of the t o t a l time , whereas i n others i t i s the propagation. Hence, using time to f a i l u r e as a measure of SCC s u s c e p t i b i l i t y i s not r e l i a b l e f o r p r e d i c t i n g l i f e t i m e of engineering components. 1.8.2 Fracture Mechanics Testing The most u s e f u l data f o r engineers and d e s i g n e r s i s obt a i n e d from l a b o r a t o r y t e s t s t h a t adequately s i m u l a t e service conditions. Tests based on the concepts of l i n e a r e l a s t i c f r a c t u r e mechanics (LEFM) come closest to s a t i s f y i n g t h i s requirement. I t i s based on the concept that defects are always pr e s e n t i n e n g i n e e r i n g s t r u c t u r e s . The two gr e a t advantages of LEFM tests are:-1. that the of stress f i e l d geometry around the crack t i p i s we l l defined and 2. t h e time t o f a i l u r e i n v o l v e s j u s t t h e t i m e f o r propagation. Fracture mechanics tests have contributed s i g n i f i c a n t l y to characterize and quantify stress corrosion crack growth r a t e s ' ^ ' ^ " . Specimens used i n f r a c t u r e mechanics t e s t i n g are f a t i g u e p r e - c r a c k e d to e l i m i n a t e i n i t i a t i o n problems. The 13 stress f i e l d around the crack t i p i s characterized by a parameter c a l l e d the s t r e s s i n t e n s i t y f a c t o r K^(I for opening mode). I t can be r e l a t e d to load on the specimen and crack length by a known function ; Kj = P/B Y(a) (1) where P i s the a p p l i e d l o a d , B i s the t h i c k n e s s of the specimen and Y(a) i s a function of crack length(a) and loading geometry. For a p a r t i c u l a r alloy-environment combination, the stress corrosion crack growth rate da/dt (V) depends uniquely upon stress i n t e n s i t y and the generalised r e l a t i o n s h i p between the two i s shown schematically i n Fig-1. This i s characterized by three regions, I and III being strongly stress i n t e n s i t y dependent and II b e i n g independent. The upper l i m i t of the curve occurs at c r i t i c a l u n s t a b l e crack growth when K ^ K ^ and the lower l i m i t i s reached when K = K T , the threshold stress i n t e n s i t y below which I Iscc s u b - c r i t i c a l crack growth i s not detectable"^. A number of studies have been conducted^^'^^'^^'^^'^^to determine crack v e l o c i t y - s t r e s s i n t e n s i t y r e l a t i o n s h i p s i n various alloy-environment combinations. The complex i n t e r a c t i o n s that occur between the stressed specimen and environment i n region I and III make i t d i f f i c u l t f o r a researcher to separate stress independent and stress dependent mechanisms. Hence region II i s used to f i n d the e f f e c t of temperature, v i s c o s i t y , p o t e n t i a l etc., on crack v e l o c i t y . However, since the crack spends most of i t s l i f e t i m e i n region I, t h i s region i s of greater importance to designers. c r a c k ve loc i ty Rttgion XI I Rtgion I I Rtgion I K i s C C s tress intensity Fig - 1 Schematic of a typical logV - K plot. 15 1.8.3 C y c l i c Voltammetry Since the properties of passive films have been correlated 78 with SCC of a l l o y s , i t i s necessary to characterize the films and mechanisms of oxide growth. C y c l i c voltammetry has been recognised as a powerful technique for studying electrode k i n e t i c s and 58 59 mechanisms of r e a c t i o n s on the electrode ' . This technique i s used to obtain an electrochemical spectrum by c y c l i c a l l y sweeping the electrode p o t e n t i a l at constant rate between l i m i t i n g values. The c y c l i c sweeping i s repeated between anodic and cathodic l i m i t s u n t i l a reproducible and stable trace i s obtained. In the forward scanning d i r e c t i o n , anodic current peaks and plateaus are obtained as a function of voltage due to each electrochemical process. Upon reversing the scan d i r e c t i o n , conjugate peaks are obtained associated with a r e v e r s i b l e or quasi-reversible process t h a t does not i n v o l v e d i s s o l v e d s p e c i e s . The degree of r e v e r s i b i l i t y and mechanisms of oxide growth have been rel a t e d to 60 peak h e i g h t s and peak s e p a r a t i o n s by mathematical models . The scan rate employed i n the forward scan and reverse scan can be ei t h e r the same or d i f f e r e n t . An important v a r i a t i o n of t h i s technique involves r o t a t i n g r i n g d i s c electrode (RRDE) experiments which provide the a b i l i t y to control the e l e c t r o l y t e convection i n order to discriminate between processes that involve mass transport l i m i t e d reactions and other processes. 16 C y c l i c voltammetry can be used to diagnose d i f f e r e n t e l e c t r o d e mechanisms. I f the product of the e l e c t r o d e process i s stable, then on reversing the scan d i r e c t i o n t h i s material can be reduced back to s t a r t i n g material, i e . , a c h e m i c a l l y r e v e r s i b l e system i s c h a r a c t e r i z e d by equal conjugate peak h e i g h t s f o r the r e d u c t i o n and o x i d a t i o n processes. I f the product i s unstable and reacts before the reverse scan can take place, then no wave(peak) w i l l be seen on the reverse scan. Adsorption of a product or reactant can also perturb the system by s p l i t t i n g the peaks. 1.9 Origins Of The Present Work C a u s t i c s o l u t i o n s are handled r o u t i n e l y i n the Bayer p r o c e s s f o r the p r o d u c t i o n of alumina. Almost a l l the aluminum produced today i s manufacured from bauxite. The main c o n s t i t u e n t s of the ore are h y d r a t e d alumina, hydrated i r o n oxides, s i l i c a and sometimes t i t a n i a . The bauxite i s f i r s t digested under a pressure of 345-480 KPa for 2 to 8 hours at a 0 temperature of 150-160 C i n c a u s t i c soda s o l u t i o n s to e x t r a c t alumina as soluble sodium aluminate. When digestion i s complete, the i n s o l u b l e residues, i e . , oxides of i r o n , s i l i c o n and titanium, are removed by pressure f i l t r a t i o n . The aluminate s o l u t i o n i s then pumped into p r e c i p i t a t i n g tanks c a l l e d decomposers at a working 0 t e m p e r a t u r e of 45 t o 60 C. The alumina i s p r e c i p i t a t e d by d i l u t i o n , cooling and seeding of the l i q u o r . The alumina i s removed by vacuum f i l t r a t i o n and the l i q u o r , a f t e r concentration i n 17 an evaporator, i s returned to digesters for a new c y c l e . The l i q u o r i s r e c i r c u l a t e d i n d e f i n i t e l y . The p r e c i p i t a t e d alumina i s f i n a l l y f i l t e r e d o f f , washed and sent to c a l c i n e r s where i t i s converted i n t o anhydrous and non- hygroscopic alumina suitable for use i n the e l e c t r o l y t i c reduction process. Fig.2 gives the diagram of Bayer process. The caustic s o l u t i o n comes i n contact with mild s t e e l i n d i g e s t e r s , decomposers and p i p i n g . Champion^reports that cracking was observed i n evaporators and digesters. He also reports that cracking i s unpredictable, i e . , may not occur even when a l l the g anticipated conditions appear to be present. Artem'ev reports that i n Soviet Bayer plants, welded equipment made of low carbon s t e e l i s s u b j e c t to SCC. A l c a n ^ r e p o r t s c r a c k i n g i n Bayer p l a n t s at several temperatures and compositions. There seems to be a dearth of information In the published caustic cracking l i t e r a t u r e concerning the corrosion p o t e n t i a l s that e x i s t i n tanks and importance of sodium carbonate and organics i n SCC of equipment. Normal p r a c t i c e i n the Bayer process i s to d i s s o l v e as much bauxite as possible i n the shortest possible time and simultaneously produce a s o l u t i o n from which a maximum amount of alumina can be p r e c i p i t a t e d per unit volume. Increasing 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 a n d t h e c o n c e n t r a t i o n 18 Bauxite (dry) Percent Total water 12.5 A 1 2 ° 3 57.8 F e 2 ° 3 24.3 S i 0 2 3.5 TiO 2.5 2 Digested with caustic s o l u t i o n under pressure Pressure : 345-480 KPa Temperature: 150-160 C Time : 2-8 Hrs. Red Mud Aluminate Solution Evoparation of F i l t r a t e P r e c i p i t a t i o n of Alumina Alumina tri- h y d r a t e Percent Total water 42.60 57.27 0.01 A1 20 3 F e 2 ° 3 S i 0 2 Na 20 0.01 0.10 Ca l c i n a t i o n I—\ Calcined Alumina Percent To t a l water 0.18 99.55 0.04 A1 20 3 F e 2 ° 3 S i 0 2 NaoO 0.05 0.18 F i g - 2 Schematic of the Bayer process cycle 19 of sodium hydroxide increases the amount of alumina which can be dissolved. The e f f e c t of i n c r e a s e d c o n c e n t r a t i o n and i n c r e a s e d temperature of Bayer s o l u t i o n s on s t r e s s c o r r o s i o n c r a c k v e l o c i t i e s i s unknown. The present study was i n i t i a t e d to p a r t i a l l y r e c t i f y t h i s s i t u a t i o n . I t i s hoped t h a t the r e s u l t s w i l l contribute to o v e r a l l understanding of ca u s t i c cracking i n Bayer plants and that the data obtained w i l l be of use i n a n t i c i p a t i n g , solving and preventing the problem i n present and future plants. 20 2. EXPERIMENTAL 2.1 P o l a r i s a t i o n Curve A p o l a r i s a t i o n c u r v e p r o v i d e s a u n i q u e means f o r d e s c r i b i n g and p r e d i c t i n g the d i s s o l u t i o n or c o r r o s i o n behaviour of a metal which exhibits an active-passive t r a n s i t i o n . By t h i s curve, the passive p o t e n t i a l region, the passive corrosion rate and conditions necessary f o r achieving p a s s i v i t y can be obtained. I t i s also possible to compare the behaviour of an a l l o y i n solutions of d i f f e r e n t compositions. A seri e s of p o l a r i s a t i o n experiments were conducted on ASME SA 516 Grade 70 s t e e l plate of composition given i n Table-1. Tests were conducted i n as received, hot r o l l e d condition. A rectangular piece (16X12Xl0mm) was machined out of the 12.5mm thick plate and a n i c k e l wire was spot welded to the specimen for e l e c t r i c a l connection. The wire was covered with a c l o s e l y f i t t i n g T eflon tube. The specimen was mounted i n cold set mount and ground 2 so that an area of 192 mm was exposed. Anodic p o l a r i s a t i o n tests were conducted i n d i f f e r e n t s o l u t i o n compositions l i s t e d i n Table-2 ( In the subsequent sections,the terms caustic solutions r e f e r to simple NaOH solutions, caustic aluminate to solutions containing NaOH + alumina and IBS to spent i n d u s t r i a l Bayer solutions containing carbonate, sulphates and 21 Table-1 Chemical Composition of Steel Plate Thickness Chemical composition (wt%) Y i e l d stress MPa C S i Mn P S 12.5 mm 0.18 0.22 1.12 0.021 0.007 -340 25.4 mm 0.19 0.23 1.16 0.021 0.008 -400 Table-2 Solution Compositions Used f or P o l a r i s a t i o n Tests SI.No. NaOH Alumina Na2C0-j Na 2S0^ Organics 1 1.0m — — - — 2 2.0m - - - -3 2.0m 0.5m - - -4 3.0m - - - -5 3.0m 0.5m - - -6 4.0m - - - -7 4.0m 0.5m - - -8 4.0m 1.0m - - -9 5.0m - - - -10 5.0m 0.5m - - -11 5.0m 1,0m - - -12 5.0m 1.5m - - -13 2.0m - 0.45m - -14 2.0m 0.5m 0.45m - -15 2.0m - - - 2.3 g/1 Formic 1 6 * 2.0m - - - 4.4 g/1 Acetic 17* 2.0m 0.5m 0.45m 2.7g/l -18 2.0m 0.5m 0.45m 2.7g/l 2.3 g/1 Formic 19 2.0m 0.5m 0.45m 2.7g/l 4.4 g/1 Acetic 20 2.0m 0.5m 0.45m 2.7g/l Acetic+Formic 21" I n d u s t r i a l Bayer s o l u t i o n (IBS) * Referred as Simulated bayer (S.B) i n the f i g u r e s . + Contains approximately 2.0m NaOH,0.8m Alumina, 0.45m Sodium carbonate, 2.7 g/1 sodium sulphate and 9 g/1 of organics 22 o r g a n i c s ) . B e f o r e e a c h p o l a r i s a t i o n t e s t t h e exposed area was polished to 600 g r i t and cleaned with ethanol and d r i e d . The solutions were prepared from d i s t i l l e d water containing 10 of i r o n species obtained by d i s s o l v i n g FeSO^. The d i s t i l l e d water was b o i l e d and cooled and a n a l y t i c a l grade NaOH p e l l e t s were added. Subsequently, other compounds l i k e alumina t r i - h y d r a t e , sodium carbonate and sodium sulphate were added. Nitrogen purging was done throughout the t e s t , i n c l u d i n g s o l u t i o n preparation and t e s t i n g stages. In these, once the d i s s o l u t i o n of alumina and sodium carbonate and sodium s u l f a t e were 0 c o m p l e t e , th e s o l u t i o n was c o o l e d down t o 60 C to prevent v o l a t i l i z a t i o n of organics upon t h e i r addition. The nitrogen purge was stopped and organics added. 0 A l l p o l a r i s a t i o n t e s t s were done a t 92 C i n f r e s h l y prepared s o l u t i o n . A T e f l o n c e l l , s i m i l a r to one used by 53 Crowe , a s c h e m a t i c of which i s shown i n F i g - 3 , along w i t h platinum counter electrodes was used. The c e l l was heated by an e l e c t r i c heating mantle using a YSI model 71A temperature c o n t r o l l e r along with YSI model 403 thermistor probe with Teflon coating to control the temperature. The p o t e n t i a l of the corroding metal was measured with respect to a saturated calomel electrode 0 (SCE) at 25 C. The l a t t e r was connected to the c e l l through a s a l t bridge containing saturated KC1. The potentials reported herein 23 l l • l i rtftt i - 1 4 — III HI HI in Iii L J J i II M ! i ! !1 u e i i i i i i •it ::: 1 r - 4 ~ . i a , 1 L / / —Hn i i: I j } i *"i ' i i i i i » i i i i i Ca) test electrode (b) Luggin capillary (cj counter electrode (4) temperature probe (e) nitrogen purge (f) l i d (g) beaker Fig - 3 Polarisation test c e l l . 24 include any l i q u i d junction p o t e n t i a l , and potentials due to thermal gradients along the s a l t bridge. No attempt was made to measure the small IR drop and uncompensated resistance i n the so l u t i o n . A l l anodic p o l a r i s a t i o n s were done with EG&G PAR model 350A potentiostat and the scan rate employed was 1 mv/sec. 2.1.1 Procedure The test s o l u t i o n was heated upto the required temperature. The specimen was inserted and ca t h o d i c a l l y p o l a r i s e d to -1.20 V c_„ for about 10 minutes to reduce any surface oxide f i l m . Then the po t e n t i a l scan was started i n the anodic d i r e c t i o n . Both the po t e n t i a l and corresponding current were p l o t t e d . 2.2 Slow S t r a i n Rate Testing (SSRT) In t h i s study, 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 SSRT were used to detect the p o t e n t i a l regimes under which SCC s u s c e p t i b i l i t y i s maximum. A waisted t e n s i l e specimen of dimensions shown i n Fig-4, made out of 12.5mm thick ASME SA 516 Grade 70 s t e e l was used. Fig-5 i s a general view of the slow s t r a i n rate test apparatus. I t i s a modified Hounsfield Tensometer with a gear reducer and 12 RPH synchronous motor. With t h i s s e t up, s t r a i n rates of 2X10 ^/sec could be r e a l i s e d with a t e n s i l e specimen of 25.4mm gage length. The c a l c u l a t i o n s are shown i n Appendix-1. Solution preparation 2 8 0 is . A l l Dimensions i n millimeters, F i g - A SSRT specimen geometry. i F i g " 5 SSRT Apparatus 27 was s i m i l a r to that for the anodic p o l a r i s a t i o n t e s t s . The t e n s i l e specimens were i n i t i a l l y e l ectropolished and stored i n a desiccator. Before the s t a r t of each t e s t , Teflon tape was wrapped around the specimen, except near the gage length,to reduce current requirements. The gage length was successively cleaned with alcohol, acetone and chlorethane* The specimen was inserted into the c e l l and mounted on to the tensometer. Fig-6 shows a schematic of specimen and c e l l . The s o l u t i o n i n the c e l l was heated by wrapping a heating tape around the c e l l w a l l . A temperature c o n t r o l l e r along with YSI model 403 thermistor probe with Teflon coating was used to control the temperature. A l l the 0 t e s t s were conducted at a temperature of 92 C and with nitrogen purging. The p o t e n t i a l was maintained with respect to a room temperature saturated calomel electrode. ECO model 549 potentiostat was used to co n t r o l the p o t e n t i a l . I n i t i a l l y , graphite rods were used as counterelectrodes and were replaced by platinum i n the l a t e r t e s t s . 2.2.1 Procedure Before the s t a r t of the t e s t , slack i n the specimen-grip combination was removed by manually turning the cross-head c o n t r o l . For 30 minutes, the specimen was p o l a r i s e d c a t h o d i c a l l y to -1.20 V„ r„ to reduce any oxide f i l m on the gage 28 1 9 \ f-LZ !e-S fcr 3 Fig - 6 SSRT c e l l geometry. (a) specimen (b) Luggin capillary (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 29 surface. Then the specimen was slowly p u l l e d . The time required for the completion of the test was around 30 hours. After the completion of the t e s t , each ha l f of the specimen was removed, washed with water and alcohol, dried and stored i n a desiccator. The reduction i n area was calculated by measuring the f i n a l d i a m e t e r a n d c o m p a r i n g w i t h t h e i n i t i a l d i a m e t e r ; 2 2 V r2 % Reduction i n Area = X 100 (2) 2 r l where r ^ and a r e i n i t i a l and f i n a l r a d i u s r e s p e c t i v e l y . One h a l f o f t h e b r o k e n t e s t s p e c i m e n was m o u n t e d l o n g i t u d i n a l l y i n epoxy, ground to the mid-plane and polished. A f t e r p o l i s h i n g , the number of cracks and length of the longest were measured. An apparent crack v e l o c i t y was established by d i v i d i n g the longest crack by the duration of t e s t . The specimen was etched with 2% n i t a l s o l u t i o n to reveal the crack mode. 2.3 E l e c t r o n D i f f r a c t i o n and Composition Analysis of Corrosion  Films In t h i s study electron d i f f r a c t i o n was used to characterize the nature of the t h i n corrosion f i l m i n caustic and caustic aluminate s o l u t i o n s . A fractured t e n s i l e specimen of the s t e e l was used and 30 a corrosion f i l m allowed to form on the rough fracture surface at con t r o l l e d p o t e n t i a l . I t was then possible to conduct i n s i t u e l ectron d i f f r a c t i o n of the f i l m by transmission through the protuberences on the rough surface. Because of t h i s , i t was possible to look at the f i l m while s t i l l on the metal, without compromising on s e n s i t i v i t y . The f i l m was analysed f o r two s o l u t i o n s ; 4m NaOH and 4m NaOH+lm A^O-j and the c o n t r o l l e d potentials were -0.980 V g c E and -1.040 V g C E r e s p e c t i v e l y . 2.3.1 Procedure The s o l u t i o n preparation f o r the experiment was very s i m i l a r to that done for anodic p o l a r i s a t i o n studies. A f t e r the so l u t i o n had attained the required temperature the fracture specimen was introduced into the so l u t i o n and ca t h o d i c a l l y p o l a r i s e d for 30 minutes to reduce any oxide f i l m on the surface. Then i t was p o l a r i s e d i n t h e p o s i t i v e d i r e c t i o n t o t h e r e q u i r e d p o t e n t i a l and held there f o r 4 hours. The specimen was l a t e r removed and washed with hot d i l u t e NaOH, to dissolve any alumina p r e c i p i t a t e d on the surface, and then washed with hot water to remove NaOH. Subsequently the specimen was washed with a l c o h o l , dried and stored i n desiccator. The sample was l a t e r placed i n the high r e s o l u t i o n d i f f r a c t i o n chamber of a HU-11A Hi t a c h i model transmission electron microscope for examination at 100 KeV. Once a d i f f r a c t i o n pattern was 31 obtained, a photographic plate was exposed. Gold d i f f r a c t i o n rings were used as a standard for c a l i b r a t i o n . For the gold, interplanar c r y s t a l spacing were known and the diameter 'D' for the corresponding d i f f r a c t i o n rings were measured. Hence a camera constant D X d was established Subsequently, the diameters 'D' of the rings from the corrosion f i l m were measured and the distance 'd' between c r y s t a l planes of the corrosion f i l m were established. The 'd' spacing and r e l a t i v e i n t e n s i t i e s were matched with those l i s t e d i n the ASTM X-ray powder d i f f r a c t i o n catalog and were confirmed by comparing with t h e o r e t i c a l estimates of e l e c t r o n d i f f r a c t i o n i n t e n s i t i e s c a l c u l a t e d by Crowe & 83 Tromans .The specimen surfaces were then chemically analysed using an ETEC Autoscan scanning electron microscope equipped with energy d i s p e r s i v e X - r a y s p e c t r o s c o p y ( E D S ) s y s t e m . 2.4 Corrosion P o t e n t i a l Measurement To monitor f r e e l y corroding p o t e n t i a l s , s o l u t i o n preparation was s i m i l a r to that for p o l a r i s a t i o n t e s t s . A c e l l and a specimen very s i m i l a r to the one used f o r anodic p o l a r i s a t i o n tests was used. Table-3 gives the composition of solutions f o r which corrosion p o t e n t i a l s were monitored. A l l the measurements were done 0 at a temperature of 92 C and with nitrogen purging. Corrosion potentials were monitored for specimens with; 1. an i n i t i a l a i r formed f i l m a f t e r mechanical p o l i s h i n g and 2. a f t e r a i r formed f i l m was removed c a t h o d i c a l l y at -1.30 VSCE 32 Table - 3 Solution Compositions used f o r Corrosion P o t e n t i a l Measurements S l . No. NaOH A1 20 3.3H 20 Na 2C0 3 Na 2S0^ Organics 1 2m 2 3m - - - -3 3m 0.5m -4 4m - - - -5 4m 1.0m - - -6 2m - 0.45m 7 2m - 2.2 g/1 Acetic 8 2m - 4.4 g/1 Formic 9 2m 0.5m 0.45m 2.7 g/1 10 I n d u s t r i a l bayer s o l u t i o n . 33 and 3. a f t e r introduction of oxygen by aeration (medical grade a i r ) . 2.4.1. Procedure A f t e r the so l u t i o n had warmed up to the required temperature, the specimen was introduced and the p o t e n t i a l was monitored with respect to a room temperature standard calomel electrode. A f t e r the s t a b i l i s a t i o n of p o t e n t i a l , the specimen was ca t h o d i c a l l y polarised to -1.30 Vg^g f o r 15 minutes and then allowed to d r i f t . Once the p o t e n t i a l had s t a b i l i s e d , nitrogen purging was stopped and the so l u t i o n continuously aerated. The p o t e n t i a l was recorded a f t e r i t s t a b i l i s e d . 2.5 Fracture Mechanics Experiments Double Cantilever Beam (DCB) specimens, Fig-7, were used f o r fract u r e mechanics experiments. Both constant load and constant d e f l e c t i o n tests were used. The K c a l i b r a t i o n for the DCB specimen 62 i s given by Brown and Srawley f o r a/W upto 0.6 and W/H 5, = P.a (3.45 + 2.415 (H/a))/ B.H 3 / 2 (3) where P i s the load. The specimens were made from 25.4mm thick ASME SA 516 grade 70 s t e e l of composition given i n Table-1. Although c a l c u l a t i o n of V H A V H A a = 25.4 mm II = 11.6 mm B = 25.4 mm W = 66.8 mm F i g - 7 DCB specimen geometry. (a) 2. Z tin U : 8 (b) 1. DCB specimens 2. M i l d s t e e l anode 3. M i l d s t e e l counter electrodes A. r e f l u x condenser 5 • Temperature probe 6. Nitrogen purge 7. Luggin c a p i l l a r y 8. Keating tape. F i g - 8 Schematic of f r a c t u r e mechanics test c e l l . 36 stress i n t e n s i t y at the crack t i p by loading the specimen i s straightforward i n constant load t e s t s , they are not so i n constant d e f l e c t i o n t e s t s . The method of a p p l i c a t i o n of load i n constant d e f l e c t i o n DCB specimens i s given i n Appendix-2. fi 3 ASTM E399-78a s p e c i f i e s that the thickness of the specimen 2 'B must be g r e a t e r than 2.5 (K^/o^) i n order f o r plane s t r a i n conditions to predominate.For the specimen geometry and material used, t h i s condition i s attained when stress i n t e n s i t y l e v e l s were less than 42 MPa/m. The specimens were i n i t i a l l y polished before being mounted onto a Sonntag f a t i g u e machine. They were fatigue cracked at a K^ . l e v e l of 13 ± 11 MPa/m. Once the fatigue crack had extended 2 to 3 mm, the l e v e l was reduced to 10 ± 9 MPa/m and the crack was further extended by a millimeter. P r i o r to mounting i n the c e l l , the specimens were marked f o r i d e n t i f i c a t i o n . . Then, they were wrapped with Teflon tape, except f o r a s m a l l area near the crack t i p , to reduce c u r r e n t requirements. In case of constant load t e s t s , the specimen and the c e l l were assembled and mounted i n a tensometer frame. In case of cons t a n t d e f l e c t i o n t e s t s , 4 specimens with d i f f e r e n t l e v e l s were used at a time. A f t e r the specimens were bolt loaded (see Appendix-2 f o r procedure), n i c k e l wires were spot welded to specimens and the others ends of the wires were connected i n p a r a l l e l to a c e n t r a l mild s t e e l rod of 6mm diameter which acts as a common conductor, Fig-8. Because of th i s a l l the 4 specimens 37 were maintained at the same p o t e n t i a l . Mild s t e e l rods of 6mm diameter were used as counter electrodes. A standard calomel electrode with s a l t bridge and Luggin c a p i l l a r y was used as a reference electrode. Fracture mechanics tests were done i n 3 d i f f e r e n t s o l u t i o n types; (1) Increasing concentrations of sodium hydroxide, (2) Bayer solutions supplied by Arvida lab-Alcan, (3) 4m NaOH + lm A1 20 3 s o l u t i o n . The p o t e n t i a l s that were applied were based on r e s u l t s obtained from slow s t r a i n rate experiments. Various models of potentiostats were used to maintain the p o t e n t i a l of the specimen. They include ECO model 549 and Wenking model 70 TS1 potentiostats. 0 A l l the tests were done at a temperature of 92 C and with nitrogen purging. In constant d e f l e c t i o n t e s t s , potentials of the specimen varied by 10 to 15 m i l l i v o l t s from the co n t r o l l e d p o t e n t i a l . No e f f o r t was made to analyse the p o s s i b i l i t i e s of any corrosion product wedging stresses i n constant d e f l e c t i o n t e s t s . 2.5.1 Procedure In the constant load t e s t s , once the so l u t i o n had warmed up to the required temperature, a load precalculated to give the desired stress i n t e n s i t y was applied on the specimen. The specimen, which was the working electrode, was c a t h o d i c a l l y p o l a r i s e d for 30 minutes to reduce any oxide f i l m on the surface. Then, the p o t e n t i a l of the specimen was raised i n the anodic (+ve) d i r e c t i o n 38 to the desired p o t e n t i a l . The test was allowed to run for 15 to 20 days. D i s t i l l e d water was added everyday to make up for evaporation losses. At the end of the tes t , the specimen was removed, washed with water and alcohol and dri e d . In the constant d e f l e c t i o n t e s t s , the specimens were bolt-loaded before the s t a r t of the t e s t . The experimental procedure was very s i m i l a r to that of constant load t e s t s . The specimens were l a t e r cooled i n l i q u i d nitrogen and overloaded to f r a c t u r e . The broken specimens were washed with alcohol and dr i e d . Crack extensions from the fatigue crack were measured at the center of the crack front and at l e a s t two points between the centre and end of the crack front on each side with an o p t i c a l microscope. Very small crack extensions were measured using SEM. Both average and maximum crack length was found. Knowing the time of the t e s t , both average and maximum crack v e l o c i t i e s were computed. The frac t u r e surfaces were cleaned by immersing the specimen i n i n h i b i t e d acid s o l u t i o n composed of 4 mis of 35% 2butyne-l,4-diol + 3 mis of HC1 + 50 mis of d i s t i l l e d water and suspending i t i n u l t r a s o n i c cleaner for 15 to 20 minutes. The specimen was subsequently cleaned with water and alcohol and dr i e d . The fr a c t u r e surface was examined with an ETEC autoscan scanning electron microscope (SEM). 2.6 C y c l i c Voltammetry C y c l i c voltammetry was performed to chart the electrochemical 39 spectrum f or d i f f e r e n t s o l u t i o n compositions, including sodium hydroxide solutions of d i f f e r e n t concentrations, and to f i n d the di f f e r e n c e , i f any, i n the voltammograms. The s t e e l specimen and the c e l l were s i m i l a r to those used for the p o l a r i s a t i o n t e s t s . A l l 0 the t e s t s were done at a temperature of 92 C and with nitrogen purging. A PAR model 173 potentiostat, with model 175 u n i v e r s a l programmer, was used to measure and program the l i m i t i n g voltages and scan rate. A Houston Instrument Omnigraphic recorder was used to plot the voltammograms. Table-4 gives the compositions of the solutions f o r which voltammetry was done. 2.6.1 Procedure The e x p e r i m e n t a l procedure was very s i m i l a r to the electrochemical tests previously explained. Instead of mild s t e e l counter electrodes, two graphite electrodes were used. Instead of the t r a d i t i o n a l s a l t bridge-luggin c a p i l l a r y set up for measuring the electrode p o t e n t i a l , a 3mm diameter Teflon tube closed at one end with a z i r c o n i a plug and f i l l e d with saturated KC1 was used. The other end of the tube f i t t e d into a small p l a s t i c container which contained the calomel electrode i n saturated KC1. A l l tests were done at a scan rate of 50 mv/sec between two l i m i t i n g p o t e n t i a l s . 40 Table-4 Solution Compositions Used f o r C y c l i c Voltammetry S l . No. NaOH A1 20 3.3H 20 Na 2C0 3 Na 2S0^ Organics 1 2m 2 3m - - -3 3m 0.5m - -4 4m - - -5 4m 1.0m - -6 2m - 0.45m -7 2m - - 2.2 g/1 Acetic 8 2m - - 4.4 g/1 Formic 9 I n d u s t r i a l bayer solut i o n , (see Table-2) 41 3. RESULTS 3.1 P o l a r i s a t i o n Behaviour A n o d i c p o l a r i s a t i o n c u r v e s of t h e s t e e l i n d i f f e r e n t concentrations of sodium hydroxide are given i n Fig.9. Increases i n c a u s t i c concentration of sodium hydroxide resulted i n an 2 i n c r e a s e d peak c u r r e n t d e n s i t y from approximately 54 A/m to 88 2 A/m , whereas, the p o t e n t i a l ranges for the act i v e region, a c t i v e -passive region and passive region remained almost constant. Fig-10 demonstrates the e f f e c t of alumina tri-hydrate addition to NaOH s o l u t i o n having a constant, free, concentration of 2m NaOH based on the stoichiometric formation of alumina according to equation (4) ; 2NaOH + A1 20 3 N a 2 A l 2 0 4 + H 20 (4); Fig-11 shows the e f f e c t of alumina additions to caustic solutions having a constant i n i t i a l concentration of 5m NaOH and va r i a b l e free concentration. In both cases, increases i n the concentration of alumina reduced the peak current density i e . , increased the ease of passivation. Fig.12 confirmed that alumina decreased the peak current density i n solutions of lower caustic concentration representative of spent Bayer c a u s t i c i t y ( I B S ) . Addition of sodium carbonate to 2m NaoH s o l u t i o n increased the 42 F i g - 9 Anodic p o l a r i s a t i o n curves i n NaOH s o l u t i o n s . 43 C U R R E N T DENSITY, A/m2 F i g - 10 Anodic polarisation curves i n NaOH + A l ^ solutions. (Constant free NaOH concentrations) 44 0.01 0.1 1 10 100 1000 CURRENT DENSITY, A/m2 F i g - 11 Anodic p o l a r i s a t i o n curves i n NaOH + A l 2 ° 3 s o l u t i o n s . (Variable 'free NaOH concentrations') 45 a m 0.1 1 10 100 1000 C U R R E N T DENSITY, A/m 2 F i g - 12 Anodic p o l a r i s a t i o n curves i n NaOH + A^O^ s o l u t i o n s . ( C a u s t i c i t y corresponding to IBS) 46 anodic peak current density and also broadened the peak. The active- p a s s i v e t r a n s i t i o n region moved to more noble p o t e n t i a l s . However, addi t i o n of alumina overwhelmed the e f f e c t of carbonate by reducing the peak current density as shown i n Fig.13. Low molecular weight organic acids of the type found i n spent Bayer l i q u o r (IBS) were added i n d i v i d u a l l y to 2m NaOH solu t i o n s , the e f f e c t s being shown i n Fig.14. Only formic acid and a c e t i c acid produced any s i g n i f i c a n t changes. Although both e x h i b i t a second anodic peak, the one i n formic acid was more pronounced. Fig-15 compares the anodic p o l a r i s a t i o n diagram obtained for the IBS s o l u t i o n supplied by Arvida lab-Alcan with simulated bayer s o l u t i o n (SB) prepared i n the laboratory. The e f f e c t of additions of low molecular weight organic acids i s also shown. A l l the p o l a r i s a t i o n diagrams except SB+formic appeared s i m i l a r . 3.2 SSRT Studies The percentage reduction i n area, number of secondary cracks and apparent crack v e l o c i t i e s were p l o t t e d against p o t e n t i a l i n 2m sodium hydroxide (Fig-16), 4m sodium hydroxide+lm alumina (Fig-17) and IBS solutions (Fig-18). Table-5 gives the corresponding data. In a l l the 3 solutions, f a s t e s t apparent crack v e l o c i t i e s were observed i n the cathodic region at -1.20 V g (, E, with IBS v e l o c i t i e s ( 9 . 0 6 0 X 1 0 - 1 0 /sec) being the highe s t and aluminate v e l o c i t i e s (2.55X10 m/ sec) the lowest. Also, lowest reductions In area were 47 001 0.1 1 10 100 1000 CURRENT DENSITY, A/m2 F i g - 13 Anodic p o l a r i s a t i o n curves i n NaOH + Na„CO_ s o l u t i o n s . 48 CURRENT DENSITY*, A/m2 F i g - 16 Anodic p o l a r i s a t i o n curves i n NaOH + organic a c i d s . 49 001 0.1 1 10 100 1000 C U R R E N T DENSITY, A/m 2 F i g - 15 Anodic p o l a r i s a t i o n curves i n IBS & simulated Bayer (SB) . 50 + 0-34 £ -0-10 5-0-54 -0-98 10 20 30 40 2 4 6 8 10 40 60 i 10 100 VxlO No Of Crocks • No cracking. App Crock Velocity %RedninAreo Current m/sec DensiIyA/rt' Fig - 16 % Reduction in area, No. of cracks(N) and Apparent crack velocity (V ) i n 2.0 mols/Kg NaOH a P P + 0-34 01 0 — 054 -0 98 _ l _ _ i _ _l_ 10 20 30 40 2 4 6 8 10 40 60 I 10 OO Vn I0 ' ° No. of crocks App. Crack Velocity %Redrwn Area Current m/sec Density A*n No Cracking Fig - 17 % Reduction in area, No. of Cracks (N) and Apparent crack velocity (V_ ) in 4.0 mols/KG NaOH +1.0 mol/KG A1_0„ app „+ 0-34 010 o. -054 -0-98 10 20 30 40 2 4 6 8 10 V , io'° 40 60 I 10 100 App Crock Velocity %RedninAreo Current m/sec DensityA/m No of crocks « No Cracking Fig - 18 % Reduction in area, No. of cracks(N) and Apparent crack velocity ( v a p p ) i n I B S * 51 exhibited at t h i s p o t e n t i a l . In the anodic region, i n a l l the three solutions, there were no s i g n i f i c a n t changes i n percentage reduction i n area, over the po t e n t i a l range tested; see Table-5. The largest number of secondary cracks and maximum apparent crack v e l o c i t i e s were noticed at p o t e n t i a l s i n the active-passive t r a n s i t i o n zone. In the case of aluminate solutions only, the number of cracks and crack v e l o c i t i e s at p a s s i v e p o t e n t i a l s were comparable to those at a c t i v e p o t e n t i a l s . Compared to the anodic region, a smaller number of crack were observed at cathodic p o t e n t i a l s i n a l l the three s o l u t i o n s . M e t a l l o g r a p h i c examination r e v e a l e d t h a t the crack propagation was intergranular. A t y p i c a l example i s shown i n F i g - 1 9 . Fig-20 shows a f r a c t u r e d slow s t r a i n r a t e t e s t specimen, tested i n IBS s o l u t i o n , with a large number of secondary cracks. This specimen was tested at a p o t e n t i a l i n the a c t i v e -passive t r a n s t i o n zone. Fig-21 shows a fractured t e n s i l e specimen t e s t e d i n IBS s o l u t i o n at a p o t e n t i a l of - 1.2Vg^ E ( c a t h o d i c region). Specimens tested at cathodic potentials i n the other two solutions revealed a s i m i l a r f r a c t u r e appearance with very l i t t l e secondary cracking. Unlike corrosion films observed i n caustic and IBS solutions, which were thick and black, aluminate solutions produced a th i n , l i g h t grey f i l m . 52 Table-5 Results of SSRT Composition Potential No.of Cracks Apparent % Redn.in Area Crack Velocity mv/SCE m/sec -0.200 0 0 60.05 -0.760 0 0 63.03 -0.920 4 2.166X10"11 56.96 -0.930 1 8.703X10-*11 60.56 2m NaOH -0.940 6 9.907X10"11 62.05 -0.980 20 3.611X10"10 63.50 -1.200 6 7.518X10"10 39.34 -1.250 1 1.064X10"10 36.63 -1.300 1 7.796X10"10 39.78 -0.430 3 1.018X10~10 47.00 -0.950 0 0 52.11 4m NaOH + -0.970 2 1.574X10"10 55.37 lm alumina -1.040 0 0 47.29 -1.100 0 0 44.94 -1.200 2 2.550X10"10 33.41 -0.430 3,3 2.950X10"10 51.55 -0.760 1,0 4.037X10"10 51.55 -0.931 28,9 2.259X10"10 46.12 -0.954 20,10 3.620X10"10 45.83 Bayer -O.980 10,10 5.185X10"10 49.30 Solution -1.010 40,40 7.000X10"10 58.52 -1.060 32,40 9.028X10"10 42.54 -1.200 7,4 9.060X10"10 38.84 f 53 F i g - 19 T y p i c a l metallograph of sectioned and etched SSRT specimen showing intergranular cracking 200 X Fractured SSRT specimen i n I n d u s t r i a l Bayer sol u t i o n (IBS) E = - 1 .20 V o r f f 10 X 55 3.3 Corrosion P o t e n t i a l s Table-6 shows the s t a b i l i s e d corrosion potentials of the s t e e l specimens i n solutions of d i f f e r e n t compositions. In the simple sodium hydroxide solutions, the p o t e n t i a l of the specimen was either i n the a c t i v e or passive region. In a l l other cases, the p o t e n t i a l was i n the passive region. Once the a i r formed f i l m was removed c a t h o d i c a l l y , the p o t e n t i a l of the specimen was always i n the active or active-passive region, with the s i n g l e exception of 2m sodium hydroxide+formic a c i d . 3.4 E l e c t r o n D i f f r a c t i o n E l e c t r o n d i f f r a c t i o n data obtained from the corrosion f i l m on s t e e l specimens immersed i n 4m sodium hydroxide and 4m sodium hydroxide+alumina s o l u t i o n i s given i n Table-7 and Table-8 r e s p e c t i v e l y . The patterns were obtained from fractured t e n s i l e test specimens which had been po l a r i s e d to potentials just noble to the a c t i v e peak. Both patterns exhibited Fe and Fe^O^ r e f l e c t i o n s showing t h a t the f i l m was p r i m a r i l y Fe^O^ s p i n e l . Alumina, which influences the p o l a r i s a t i o n curve dramatically, did not influence the e l e c t r o n d i f f r a c t i o n pattern. X-ray energy analysis of the specimen surface a f t e r t e s t i n g i n aluminate solutions showed aluminum was present i n the surface f i l m at these p o t e n t i a l s probably as a mixed Fe,0, - F e 9 A l 9 0 , s p i n e l . Table-6 Corrosion P o t e n t i a l s 56 * P o t e n t i a l of * P o t e n t i a l a f t e r * P o t e n t i a l a f t e r the specimen the a i r formed aeration of Composition with an a i r f i l m was removed so l u t i o n by med-formed f i l m c a t h o d i c a l l y c a l a i r 2.0m NaOH -0.860 (P) -1.080 (a) -1.070 (a) 3.0m NaOH -1.136 (a) -1.150 (a-P) -1.136 (a) 4.0m NaOH -0.830 (P) -1.120 (a) -3.0m NaOH + 0.5m alumina -0.472 (P) -1.052 (a) -0.344 (P) 4.0m NaOH + 1.0m alumina -0.496 (P) -1.100 (a) -0.652 (P) 2.0m NaOH + 0.42m Na 2C0 3 -0.695 (P) -1.156 (a) -0.376 (P) 2.0m NaOH + 4.4g/l Acetic -0.350 (P) -1.048 (a-P) -0.378 (P) 2.0m NaOH + 2.2g/1 Formic -0.406 (P) -0.366 (P) -Bayer -0.740 (P) -1.136 (a-P) -0.570 (P) * A l l potentials measured with respect to standard calomel electrode. a : ac t i v e ; P : Passive; a-P : active-passive t r a n s i t i o n ; 57 Table-7 E l e c t r o n D i f f r a c t i o n Data i n 4m NaOH Solution Ring No. Intensity Diameter D inches Observed 'd' 0 A F e 3 ° 4 0 (hkl) A a-Fe (hkl) 0 A 1. V.weak 0.44 2.967 (220) 2.966 - -2. weak 0.52 2.510 (311) 2.530 - -3. S.broad 0.64 2.039 (400) 2.096 (110) 2.0268 4. W.diffuse 0.82 1.590 (333)(511) 1.614 - -5. weak 0.890 1.460 (440) 1.483 - -0 Camera constant Dd = 1.3054 in.A (ave); Based on Gold c a l i b r a t i o n Table-8 E l e c t r o n D i f f r a c t i o n Data i n 4m NaOH + lm Alumina Solution Ring No Intensity Diameter 'D' inches Observed 0 A »d» F e 3 0 o 4 (hkl) A a-Fe (hkl) 0 A 1. weak 1.360 2.553 (311) 2.53 2. s.broad 1.7150 2.024 (400) 2.096 (110) 2.0268 3. spotty arc 1.790 1.940 (331) 1.926 - -4. d i f f u s e 2.120 1.638 (333)(511) 1.614 - -5. weak broad 2.370 1.465 (440) 1.483 (200) 1.43 6. spotty arc 2.460 1.411 (531) 1.419 - -7. strong 2.965 1.171 (444) 1.2112 (211) 1.1702 8. spotty arc 3.00 1.157 (551)(711) 1.176 - -9. di f f u s e arc 3.08 1.127 (642) 1.1214 -0 Camera constant Dd = 3.472 in.A (ave); Based on i r o n r i n g s . 58 3.5 C y c l i c Voltammetry Fi g s . 22,23 and 24 show t y p i c a l voltammograms obtained for the s t e e l i n sodium hydroxide solutions of 2m,3m and 4m concentrations r e s p e c t i v e l y . The potentials were cycled between -1.40 V to 0.4 64 ^SCE* ^ o n e °^ t h e ^ figures show current peaks I but show peak II and peak I I I . The usual electrochemical mechanism assigned to peak 64 I i s adsorption of hydrogen or formation of hydroxyl ions With increase i n concentration of NaOH, peak I I , which i s e s s e n t i a l l y a shoulder, exhibits a s l i g h t l y higher current density, and moves towards a lower p o t e n t i a l . Peak III also exhibits a higher current density with increase i n concentration and t h i s increase i s quite s i g n i f i c a n t compared to peak I I . Various electrochemical mechanisms have been ascribed to peak II and peak 64 I I I . The products of these electrochemical mechanisms have been 64 v a r i o u s l y d e s c r i b e d as F e ( 0 H ) 2 , F e 3 ° 4 o r F e 2 ° 3 * T h e P e a k s a r e generally sharp and well defined. No further peaks appear u n t i l l the p o t e n t i a l c o r r e s p o n d i n g to oxygen evo l u t i o n (+ 0.340 Vg^g), although an i n e x p l i c a b l e 'bump' appears just a f t e r peak I I I . On reversing the p o t e n t i a l , peak IV, appears as a poorly defined s h o u l d e r . The c u r r e n t d e n s i t y c o r r e s p o n d i n g to peak IV, i P ^ , increases with increase i n scan rate. Peak IV was thought to be a 64 c o n j u g a t e of peak I I I . The r e d u c t i o n p r o c e s s e s t h a t are manifested i n peak IV always occured at a p o t e n t i a l more negative than peak I I I . Peak V, the l a s t reduction peak before the a p p e a r e n c e of h y d r o g e n e v o l u t i o n , i s t h o u g h t to be 500 375 250 125 0 125 250 375 500 (+) (-) 2 Current Density (A/m ) Fig - 22 Cyclic Voltammogram in 2m NaOH -0.2 0.0 u in -0.4 G V o -0.6 -0.8 -1.0 -1.2 III 4.0m NaOH f i r s t scan s t a b i l i s e d scan 500 375 (+) 250 125 125 250 375 500 (-) ON Current Density (A/m ) F i g - 24 C y c l i c Voltammogram i n 4m NaOH 62 a conjugate of peak II and always appeared at potentials negative to peak I I . There was an i n i t i a l increase i n current density with increase i n concentration and the magnitude of th i s increase, decreased at higher concentration of NaOH. In aluminate solutions, Figs.25 and 26, a l l the above peaks were observed except t h a t the c u r r e n t d e n s i t y of peak I I I , i P -j, was s i g n i f i c a n t l y reduced. I t i s noteworthy that peak II current was the same i n both sodium hydroxide and aluminate sol u t i o n s . I t i s clear from t h i s that the mechanism by which a l u m i n a r e d u c e s i P ^ o c c u r s i n t h e p o t e n t i a l r e g i o n between p o t e n t i a l s c o r r e s p o n d i n g to peak II and I I I . On comparing sodium hydroxide and aluminate solutions, the slope dE/dl i n the region between peak II and peak I II was greater for aluminate s o l u t i o n s . I f peak I I i s a s c r i b e d to f o r m a t i o n of HFe02» i t i s clear that alumina becomes incorporated into the f i l m only a f t e r the formation of HFe02 and t h i s i n c o r p o r a t i o n i n c r e a s e s the resistance of the f i l m . The voltammograms obtained i n 2m NaOH + 0.42 Na2C0 3 s o l u t i o n are shown i n Fig.27. Compared to sodium hydroxide solutions, t h i s f i g u r e shows an ad d i t i o n a l peak (hereafter c a l l e d peak I l i a ) at pot e n t i a l s p o s i t i v e to peak I I I but well before oxygen evolution. Although peak I II and peak I l i a look l i k e s p l i t wave, i n d i c a t i n g strong adsorption of reactant (sodium carbonate), t h i s can be F i g - 25 C y c l i c Voltammogram i n 3.0m NaOH + 0.5m A1„0 -o.2r o.o -0 .2 w -0 .4 u GO *' -0 .6 •H 4-1 C 0) 4J o -0 .8 -1 .0 -1 .2 -1 .4 500 (+) 375 4.0m NaOH + 1.0m A 1 2 0 3 . 3 H 2 0 f i r s t scan s t a b i l i s e d scan III 250 125 500 (-) F i g -Current Dens i ty (A/m ) 26 C y c l i c Voltammogram i n 4.0m NaOH + 1.0m A 1 2 0 3 66 discounted because conjugate adsorption waves are symmetrical about iP and no conjugate wave i s seen i n the reduction c y c l e . Figs.28 and 29 shows the voltammograms obtained when organic acids, normally present i n IBS so l u t i o n , are added to 2m sodium hydroxide s o l u t i o n . Both show a small, a d d i t i o n a l peak I l i a , s i m i l a r to one found i n sodium hydroxide+sodium carbonate so l u t i o n s . Both occured at approximately the same p o t e n t i a l as that found i n sodium hydroxide+sodium carbonate solutions and both showed no conjugate peaks. Contrary to r e s u l t s obtained i n p o l a r i s a t i o n t e s t s , where the anodic peak increases with addition of carbonate or organic acids, c y c l i c voltammetry showed an in e x p l i c a b l e decrease i n peak I I I . The voltammogram obtained i n i n d u s t r i a l Bayer s o l u t i o n (IBS) i s shown i n Fig-30. This showed no a d d i t i o n a l peaks but displayed a very large peak I I I . Peak V was not v i s i b l e i n the p o t e n t i a l range tested. 3.6 Fracture Mechanics A l l the v e l o c i t i e s g i v e n i n t h i s s e c t i o n and the following sections are the maximum crack v e l o c i t i e s that were observed i n plane s t r a i n conditions. Since most of the crackfronts were bowed, t h e c r a c k l e n g t h s a t t h e c e n t e r of t h e specimen were longer than that measured at sides by a small Fig - 28 Cyclic Voltammogram in 2.0m NaOH + 2.3 g/1 Formic acid.' Fig - 29 Cyclic Voltammogram in 2,0m NaOH + 4.4 g/1 Acetic ac +0.2 0.0 -0 .2 w w-0.4 -0.6 0) o -0 .8 -1 .0 -1 .2 h H I I n d u s t r i a l Bayer S o l u t i o n f i r s t scan s t a b i l i s e d scan 500 (+) 375 250 125 125 250 375 Current Density ( A/m 2 ) F i g - 30 C y c l i c Voltammogram i n IBS. 500 (-) 70 amount.(0.5mm). As previously mentioned i n 'experimental' section, both an average and maximum crack growth increments were measured and corresponding crack v e l o c i t i e s computed. The e f f e c t of stress i n t e n s i t y on crack v e l o c i t y i n 2m NaOH at 0 92 C i s shown i n Fig-31. Table-9 gives stress i n t e n s i t i e s at which cracking was observed and duration of t e s t s . No cracking was observed at stress i n t e n i t i e s below 29 MPa/m, despite the test being conducted f o r ~740 Hrs. The K-j-g^ Q of the s t e e l i n 2m NaOH was around 29 MPa/m. The p o t e n t i a l that was applied was i n the active-passive region. Fig-32 showed that upto 35 MPa/m the crack v e l o c i t y was i n region I. The V-K plots obtained for 4m and 8m sodium hydroxide solutions are also shown i n Fig-31. The p o t e n t i a l s at which t e s t i n g was done were -1.070 V c „ , and -1.030 Vg^g r e s p e c t i v e l y . Table-10 and Table-11 give the corresponding data. The Region II v e l o c i t y was almost the same i n both the s o l u t i o n s a f t e r 30 MPa/m. I t i s clear from the figure that K^-g^ moves towards lower K values with increasing sodium hydroxide concentrations. The e f f e c t of stress i n t e n s i t y on crack v e l o c i t y i n 4m NaOH+lm alumina s o l u t i o n i s summarised i n Fig-32 and Table-12 . A l l the crack v e l o c i t i e s obtained were believed to be Region I v e l o c i t i e s based on the data. The V-K plot of 4m NaOH i s also plotted i n the same figu r e f o r purposes of comparison. I t i s clear that aluminate present i n the so l u t i o n was responsible f o r the predomination of Region II stress i n t e n s i t i e s below 22 MPa/m. At stress i n t e n s i t i e s 71 • • 2m NaOH G © 4m NaOH • 0 8mNaOH Stress Intensity, MPaffi Fig - 31 Effect of stress intensity on crack velocity in NaOH solutions of different concentrations. 72 Table-9 E f f e c t of Stress Intensity on CracksVelocity i n 2m NaOH at 92 C and at -0.980 V O O T, Stress Intensity Duration Crack V e l o c i t y (MPa/m) (Hrs) (m/sec) * 29 744 0 32 744 4.00X10" 1 1 35 744 1.94X10" 1 0 No crack propagation observed at stress i n t e n s i t i e s below 29 MPa/m. Table-10 E f f e c t of Stress Intensity on Crack V e l o c i t y i n 4m NaOH 0 ;  at 92 C and at -1.070 V 0„„ Stress Intensity Duration Crack V e l o c i t y (MPa/m) (Hrs) (m/sec) 21.4 480 1.273X10" 1 1 22.0 480 l .oooxio - 1 1 25.5 - 26.4 720 5.090X10" 1 0 30.0 - 30.3 504 9.149X10" 1 0 35.0 - 35.2 480 9.317X10" 1 0 40.0 - 42.0 720 6.940X10" 1 0 73 Table-11 E f f e c t pf Stress Intensity on Crack V e l o c i t y i n 8M NaOH 0 at 92 C at -1.030 V„„„ Stress Intensity Duration Crack V e l o c i t y (MPa/m) (Hrs) (m/sec) 22.00 - 24.40 672 1.120X10 - 9 26.00 - 26.90 600 6.064X10 1 0 30.00 - 30.02 720 8.526X10 1 0 33.37 - 34.00 720 7.639X10 1 0 38.00 - 38.81 720 7.300X10 1 0 40.13 - 42.00 720 8.290X10" 1 0 Table-12 E f f e c t of Stress Intensity on Crack V e l o c i t y i n 4m NaOH + lm alumina Solution at"92 C and at -0.960 V^ -,^  Stress Intensity Duration Crack V e l o c i t y (MPa/m) (Hrs) (m/sec) 21.9 - 24.0 528 1.350X10"9 24.7 - 30.0 528 1.350X10"9 30.2 - 38.0 528 1.540X10~9 34.8 - 42.0 528 2.390X10 - 9 74 -O 4m NaOH O o 4 m Na 0H+1 m A^O^ LTD Q—Q 18 22 26 30 34 38 42 Stress Intensity, MPa/m Fig - 32 Effect of stress intensity on crack velocity i 4 mol/Kg NaOH + 1 mol/Kg A l ^ . 75 above 38 MPa/m, the increase i n crack v e l o c i t y may be either due to the approach of Region III or experimental s c a t t e r . The V-K plot i n the IBS so l u t i o n i s given i n Fig-33. Table-13 gives the corresponding data. The crack v e l o c i t i e s seem to f a l l i n Region II which lay between that of 4m NaOH and 4m NaOH+lm alumina so l u t i o n s . Table-14 summarises the Region II crack v e l o c i t i e s f o r the d i f f e r e n t s o l u t i o n s . 3.7 FraCtOgraphyOf SCC A l l the fractographs shown i n the following section show crack propagation d i r e c t i o n from top to bottom of f i g u r e . Detailed fractography was conducted on the fracture mechanics, DCB, specimens. A representative fractograph of the specimen tested i n 2m NaOH i s shown i n Fig.34 f o r a l e v e l of 32 MPa/m. The surface was heavily corroded and i t was very d i f f i c u l t to i d e n t i f y the fracture path. Specimens cracked at higher stress i n t e n s i t y displayed s i m i l a r features. In 4m NaOH, the fracture mode was intergranular. Fractographs obtained from specimens cracked at 30 MPa/m and 40 MPa/m are shown i n Fig-35. Some of the f l a t features look l i k e intergranular fa c e t s . In some regions the grains appear deformed. 76 i o " T r o U U o > g 10 id* IBS _0Q _ <S>_ i © „ _ 18 22 26 30 34 38 42 Stress Intensity MPoM Fig - 33 Effect of stress intensity on crack velocity i n IBS. 77 Table-13 E f f e c t of Stress Intensity"on Crack V e l o c i t y i n Bayer 0 Solutions (Supplied"by Aryida Lab;, Alcan) at 92 C and at -1.020 V S C E Stress Intensity Duration Crack V e l o c i t y (MPa/m) (Hrs) (m/sec) 22.00 - 22.38 504 1.196X10"9 25.00 - 25.43 504 1.201X109 28.00 - 28.65 504 1.300X109 29.80 - 30.00 480 9.780X10 1 0 30.00 - 30.60 504 8.760X10 1 0 33.40 - 34.00 480 1.279X10 - 9 42.00 - 42.24 480 9.780X10" 1 0 Table 14 Region II Crack V e l o c i t i e s Composition Crack V e l o c i t y m/ sec 2m NaOH 4m NaOH 8m NaOH 4m NaOH + lm alumina IBS -not known-9.0 X 10~ 1 0 7.5 X 10" 1 0 1.2 X 10~ 9 9.0 X 10~ 1 0 78 79 At the highest caustic concentration tested (8m NaOH), the fracture surface was severely corroded making i t d i f f i c u l t to i d e n t i f y the i n d i v i d u a l grains;eg., Fig-36. But secondary cracks which penetrate deep into the surface can be seen. Fractographs obtained from specimens that cracked i n aluminate solution,Fig-37, were very s i m i l a r to that of 8m NaOH. However, there was a noticeable lack of secondary cracks, or, i f present, they were not very deep. Crack surfaces from specimens obtained by SCC i n IBS solutions showed some unique c h a r a c t e r i s t i c s ; eg., Fig-38. The surface was heavily corroded at lower stress i n t e n s i t i e s (22 MPa/m),- but at higher stress i n t e n s i t i e s , 32 MPa/m there were some evidences of t r a n s g r a n u l a r c r a c k i n g . Some n o t i c e a b l e f e a t u r e s a r e t r a n s c r y s t a l l i n e cracking and feather l i k e structures. These are d i f f e r e n t from the small, randomly oriented lamellae which may be p e a r l i t e colonies. In g e n e r a l , t h e r e was s u f f i c i e n t d i s s o l u t i o n to make i n t r e p r e t a t i o n of crack paths d i f f i c u l t f o r most micrographs. F i g - 35 Fracture surfaces a f t e r testing i n 4.0 moles/Kg NaOH, -1 .070 V „ r F , (a) 30 MPa/m, (b) 40 MPa/m (b) 800 X Fig - 36 Fracture surfaces a f t e r testing i n 8.0 moles/Kg NaOH, -1.030 V , (a) 26 MPa/m, (b) 38 MPa/m (c) 800 X F i g - 38 Fracture surfaces a f t e r testing i n IBS -1.020 V g C E , (a) 22MPa/m (b) 22 MPa/m, (c) 38 MPa/m 85 DISCUSSION 4.1 Polarisation Behaviour 4.1.1 Mechanism of Alumina Inhibition The polarisation diagrams showed that the peak current density for iron dissolution increased with increasing NaOH concentration, whereas, increasing additions of alumina to cau s t i c s o l u t i o n s decreased the peak current density. The inhibiting effect of alumina additions on the peak current density i s readily explained by the deposition of an iron-aluminate film on the electrode surface. Aqueous E-pH equilibrium diagram show that in caustic solutions iron dissolves i n the active range of potentials as HFe02 ions*'"'* according to equation (5) Fe + 2H20 • HFe0~ + 3H++ 2e~ (5) and alumina forms A102 ions. A1 20 3 + H20 -• 2 A102 + 2H + (6) At the electrode surface, due to kinetic effects, the concentration of HFeOj ions may reach concentrations which are su f f i c i e n t l y high to exceed the solubility product of iron aluminate, which i s then deposited on the electrode surface to hinder further dissolution, as shown i n equation (7). H20 + HFe0~ + 2A10~ - F e A l ^ + 30H_ (7) 86 Consequently, the equilibrium constant, K, i s given by K = (OH -) 3 / (HFeO~) ( A10~) 2 (8) assuming (H^O) and ( F e A l ^ ) = 1 This shows that for a constant sodium hydroxide concentration, the increasing additions of alumina require smaller concentration of (HFeO^) i o n s i n order to exceed the s o l u b i l i t y product; i e . , greater i n h i b i t i n g e f f e c t i s obtained. Consistent with above explanations, comparison of c y c l i c voltammograms i n NaOH & NaOH + A1 2 0 3 solutions (Figs. 25 & 27) show that there was no change i n peak II current density and the i n h i b i t i n g e f f e c t started to occur between peaks II & I I I , i n d i c a t i n g t h a t f o r m a t i o n o f H F e 0 2 i o n s was nec e s s a r y f o r i n h i b i t i o n . Also X-ray energy dispersion analysis of s t e e l surfaces, which had been subjected to active-passive p o t e n t i a l s , revealed the presence of aluminum i n the surface f i l m . At h i g h e r p o t e n t i a l s , i n the passive region, A10 2 ions may be i n c o r p o r a t e d into the ir o n oxide passivating films to form a mixed s p i n e l , b e c a u s e Fe^O^, F e 2 0 3 and F e A l 2 0 ^ a l l have s p i n e l structures. EDS analyses of the surfaces of electrodes subjected to passivating potentials confirmed the presence of aluminum. 0 A l a t t i c e parameter of 8.3545 A was calculated from electron 87 d i f f r a c t i o n data f o r the f i l m o b t a i n e d i n 4m NaOH + lm A^O^ s o l u t i o n (see Table - 8); Calculated l a t t i c e parameter L a t t i c e parameter of Fe^O^ i s L a t t i c e parameter of ^2^3 * s L a t t i c e parameter of FeA^O^ i s 8.3545; 8.3963; 8.350; 8.113; The observed l a t t i c e parameter were far too removed to account for the presence of FeA^O^ f i l m . There could be l i t t l e FeA^O^ i n the Fe^O^ f i l m , forming a mixed s p i n e l because aluminum was detected by EDS. The reason f o r the absence of r i n g corresponding to the presence of F e A ^ O ^ was a strong i n d i c a t i o n that the c r y s t a l l i t e s were extremely s m a l l , i n d i s t i n g u i s h a b l e from an amorphous structure. The l i t e r a t u r e suggests that amorphous films are 69 more p r o t e c t i v e , have a lower l i m i t i n g thickness, lower passive current d e n s i t i e s , less susceptible to breakdown, reform at higher rates and are more d u c t i l e than c r y s t a l l i n e f i l m s . The p o s s i b i l i t y that alumina additions provided an i n h i b i t i n g action v i a an adsorption e f f e c t was dismissed because the c y c l i c voltammograms showed no adsorption-desorption peaks i n the range tested, whereas, the deposition mechanism appears consistent with a l l observations. I t i s i n t e r e s t i n g to note that other w o r k e r s ^ have observed an i n h i b i t i o n of i r o n d i s s o l u t i o n i n caustic solutions i n the presence 3- 3- 2-of a d d i t i o n s of GaOg, GeO^, CrO^ & MoO^ i o n s , which they a t t r i b u t e d to a deposition process. 88 4.1.2 E f f e c t of Carbonate and Aluminate Additions to NaOH The increase i n peak current density (Fig-14) when carbonate was added to NaOH so l u t i o n suggests that the i r o n oxide f i l m which formed at the active-passive nose was porous (non-protective). However, additions of alumina overwhelmed the e f f e c t of carbonate by reducing the peak current density. This i n h i b i t i n g action may be a t t r i b u t e d to deposition of i r o n aluminate, as postulated i n the previous section or due to formation of a heterogeneous f i l m c o n s i s t i n g of both carbonate and aluminate. Heterogeneous films 71 72 are known to be more protective than homogeneous films ' . Awad 73 and Mansour , i n a study on the e f f e c t of sodium aluminate as a p i t t i n g corrosion i n h i b i t o r of mild s t e e l i n a l k a l i n e solutions, observed that an heterogeneous f i l m consisting of i r o n oxide with aluminate and carbonate ions was more protective than films obtained i n sodium carbonate or a l k a l i n e solutions alone. 4.1.3 E f f e c t of Organic Acid Additions Addition of low molecular weight organic acids to NaOH solutions showed an a d d i t i o n a l peak (Fig-15) i n p o l a r i s a t i o n diagrams which was also seen i n the c y c l i c voltammograms (peak I l i a ) . The presence of t h i s peak may be ascribed to oxidation of an 74 adsorbed group; eg.COOH according to equation (9) & (10). 89 (Adsorption ) HCOOH ± e Fe-H , + Fe-COOH , (9) r surface ads ads (Oxidation ) F e ~ H a d s H + + e ~ Fe-COOH « + H + + CO„ + e~ (10). ads 2 The absence of Fe-COOH a d s o r p t i o n - d e s o r p t i o n peaks on the voltammograms may be a t t r i b u t e d to the fact that the pot e n t i a l s at which these processes occur were lower than the p o t e n t i a l range tested. The i n d u s t r i a l Bayer soluton (IBS), which contains both formic and a c e t i c acid, i n addition to other high molecular weight organic acids, did not show the oxidation peak (peak I l i a ) . The reason for i t s absence i s unclear. I t may be due to the formation of metal organic complexes which are not oxidised i n the p o t e n t i a l range tested. 4.2 Slow S t r a i n Rate Tests 4.2.1 Cracking at Cathodic P o t e n t i a l s The cracking observed at cathodic potentials i n the three solutions tested (pg. 50) may be due to a hydrogen embrittlement 18 mechanism. S i n g b e i l and Tromans have calculated the r e v e r s i b l e hydrogen electrode p o t e n t i a l from equation (11) which was based on 80 studies by Biernat & Robins E H +/ H„ = 0.052 -0.072 pH - 0.036 log (a H„), V 0„_ (11) 90 0 (LA 1 Q At 92 C , pH = 12.75 and a H 2 = 0.25 atm . E H +/ H 2 = -0.845 V S R E ; (-1.085 v g C E ) . This shows that the potentials at which the largest apparent crack v e l o c i t i e s and g r e a t e s t r e d u c t i o n i n areas were observed, correspond to potentials at which i t was thermodynamically f e a s i b l e f o r hydrogen to be produced. I t i s probable that cracking occured by propagation through a crack t i p region which was embrittled by the adsorption or absorption of the ca t h o d i c a l l y 37 generated atomic hydrogen Specimens tested i n a l l three solutions showed, only a small number of secondary cracks. This indicated crack i n i t i a t i o n was not an easy process; i e . , a c r i t i c a l combination of microstructure and impurity elements may be necessary. Among the three solutions, specimens tested i n IBS showed the l o n g e s t c r a c k s , w h i l e those t e s t e d i n NaOH + A 1 2 0 3 s o l u t i o n s displayed smaller crack lengths. The presence of a large number of impurities, both organic and inorganic, i n the IBS may have alt e r e d one or more of the parameters that a f f e c t hydrogen cracking; eg., overpotential f o r hydrogen evolution, permeation rate of hydrogen in t o the metal l a t t i c e and hydrogen recombination reaction on the metal s u r f a c e . Smialowski^ quotes from the work of Bogotskia and Frumkin that many organic and inorganic compounds added to the e l e c t r o l y t e a f f e c t both the overpotential of hydrogen evolution 91 reaction and permeation rate. 4.2.2 Cracking At Anodic P o t e n t i a l s In the anodic region, the largest number of cracks (N) and maximum crack v e l o c i t y (Vapp) were seen at pot e n t i a l s i n the active-passive t r a n s i t i o n region. Under these conditions, largest 'N' and Vapp. were observed i n IBS. The maximum SCC s u s c e p t i b i l i t y at active-passive t r a n s i t i o n potentials can be r e a d i l y explained i n terms of the f i l m rupture-dissolution model ( i e . , crack advance occurs by alternate cycles of f i l m rupture, d i s s o l u t i o n and repa s s i v a t i o n ) . I t has been reported previously i n the l i t e r a t u r e 77 78 t h a t u n s t a b l e f i l m s e x i s t at these p o t e n t i a l s ' . Hence, the crack w i l l spend more time i n the d i s s o l u t i o n stage. Film i n s t a b i l i t y and modification of the repassivation rate by the presence of impurities i n the environment, both organic and inorganic, are expected to influence both i n i t i a t i o n and the o v e r a l l crack propagation rate. This could account f o r the fact that the IBS showed large N and Vapp. I t i s known that various 79 s o l u t i o n additives decrease the s t a b i l i t y of passive films Another c h a r a c t e r i s t i c of the f i l m rupture model relevant to SCC i s the mechanical behaviour of the f i l m , eg. n o n - c r y s t a l l i n e f i l m s are more d u c t i l e ^ t h a n c r y s t a l l i n e f i l m s . Hence, i n a slow s t r a i n rate t e s t , a greater s t r a i n w i l l be required to break a d u c t i l e f i l m . In th i s s i t u a t i o n , crack i n i t i a t i o n w i l l be delayed 92 u n t i l l a t e r stages of the test and the frequency of f i l m rupture events w i l l be decreased during crack propagation. Both of these events could account for lower Vapp i n NaOH + alumina solutions ( i e . , amorphous d u c t i l e i r o n aluminate film) compared to NaOH solutions ( c r y s t a l l i n e i r o n oxide f i l m ) . Consistent with the anticipated e f f e c t of f i l m c h a r a c t e r i s t i c s on SCC, the c y c l i c voltammograms, p o l a r i s a t i o n diagrams, electron d i f f r a c t i o n and EDS of s t e e l surfaces indicated that the nature of the f i l m at the active-passive t r a n s i t i o n p o t entials was dependent on the environment. 4.3 Fracture Mechanics Tests 1 * ----- r 4.3.1 E f f e c t of Sodium Hydroxide Concentration on Crack V e l o c i t y Fig-32 showed that increasing concentrations of NaOH moved region I towards lower stress i n t e n s i t y values while, region II v e l o c i t i e s for both 4m NaOH and 8m NaOH solutions remained more or less unchanged. Movement of region I can be explained by changes i n r e p a s s i v a t i o n k i n e t i c s . D i e g l e & Vermilyea*^ i n t h e i r studies on 0 transient corrosion of s t e e l i n caustic solutions at 85 C, observed t h a t r e p a s s i v a t i o n i s slower i n h i g h e r c o n c e n t r a t i o n NaOH sol u t i o n s . Therefore, i n accordence with the f i l m rupture-d i s s o l u t i o n model, the frequency of repassivation events would decrease with increasing caustic concentration, allowing the crack to spend a greater time i n the d i s s o l u t i o n stage. Repassivation 93 rates are not expected to influence region II because the crack t i p i s considered to be e s s e n t i a l l y f i l m f r e e . 4.3.2 Comparison of 'Crack ' V e l o c i t i e s from Slow "Strain 'Tests 'and  Fracture Mechanics Tests Table-15 shows apparent crack v e l o c i t i e s obtained from SSRT and region II v e l o c i t i e s from the fracture mechanics experiment. In the case of the 4m NaOH + lm A^O^ s o l u t i o n , an order of magnitude differe n c e was seen between the two v e l o c i t i e s , while i n IBS the two crack v e l o c i t i e s are comparable. The better c o r r e l a t i o n i n IBS can be explained by the presence of mechanically weak passive films which allow cracks to i n i t i a t e early i n the t e s t . Consequently, under the r i s i n g l o a d (K^) conditions i n the SSRT experiment, the p r o b a b i l i t y of the crack reaching region II v e l o c i t y i s increased. Consistent with the postulated weaker films i n IBS, numerous secondary cracks should also be present i n the specimens, as observed. I n the 4m NaOH + lm A^O-j s o l u t i o n , the s m a l l e r number of secondary cracks suggest that crack i n i t i a t i o n was d i f f i c u l t , consistent with the presence of an amorphous and presumably d u c t i l e passive f i l m on the specimen. Hence the cracks i n the SSRT specimens are u n l i k e l y to reach region II v e l o c i t y and w i l l spend more time i n region I. A l t e r n a t i v e l y , the crack may spend most of i t s time i n region II but the Vapp has been underestimated because 94 Table-15 Comparison of Apparent"Crack V e l o c i t i e s From SSRT  With Region II Crack V e l o c i t i e s . Composition P o t e n t i a l VSCE No. of Cracks Crack V e l o c i t y (SSRT) Crack V e l o c i t y (DCB) 2m NaOH 4m NaOH + lm alumina Bayer _ -0.980 -0.960 -1.060 15 3 40 3.611X10 - 1 0 1.574X10" 1 0 . 9.027X10" 1 0 -not known-1.35X10 - 9 1.20X10"9 T a b l e - 1 6 C a l c u l a t e d and Observed Crack. V e l o c i t i e s i n Solutions Composition Calculated region II crack v e l o c i t i e s , (m/sec) Observed region II crack v e l o c i t i e s , (m/sec) 4m NaOH IBS 4m NaOH + ... lm alumina 2.46 X 10"J 7.35 X 10~ 9 1.28 X 10~ 9 a 9.0 X 10-1JJ =• 9.0 X 10~ 1 0 ? 1.2 X l O - 9 Table-17 Calculated Overpotentials Based on Estimated Values of i c o r r U si n8-Faraday' s Law 6 (mm) C0HP , 3 (moles/m ) Overvoltage ( v o l t s ) 1.0 93.90 0.180 0.5 46.95 0.169 0.1 9.39 0.143 0.05 4.69 0.132 0.01 0.93 0.107 0.005 0.46 0.096 95 i t might only have been propagating during the l a t e r stages of the test (Vapp was based on t o t a l test time). 4.4 Estimation of 'Concentration Overvoltage Cracking at anodic p o t e n t i a l s appears to be consistent with the general p r i n c i p l e s of the f i l m rupture-dissolution model. In region I I , where the crack t i p i s expected to be contineously bare, Faraday's law may be employed to predict the crack v e l o c i t y . V = i W / F p (12) £L where i i s the anodic current density at the crack t i p , W i s the equivalent weight of i r o n , F i s the Faraday's constant and p i s the density of i r o n . I t i s possible to cal c u l a t e from equation (12), an upper l i m i t i n g crack v e l o c i t y near the active-passive t r a n s i t i o n p o t entials using the peak current density obtained from the p o l a r i s a t i o n diagrams. Table-16 gives the calculated (predicted) crack v e l o c i t i e s f or 4m NaOH, 4m NaOH + lm A^O^ and IBS sol u t i o n s . There was a large difference between observed and predicted crack v e l o c i t i e s except i n 4m NaOH + lm A^O-j s o l u t i o n . This may be explained by the fac t that the d i s s o l u t i o n processes i n the r e s t r i c t e d confines of the crack t i p region were under mixed a c t i v a t i o n - d i f f u s i o n control rather than pure a c t i v a t i o n c o n t r o l . The following analysis represents an attempt to estimate the region II concentration ( d i f f u s i o n ) overvoltage. 96 The crack progresses by d i s s o l u t i o n of i r o n according to equation (13); Fe + 2H 20 HFeO~ + 3H + + 2e~ (13) Due to k i n e t i c s of d i f f u s i o n , the concentration of HFe0 2 ions w i l l be higher near the surface of the metal than at some distance removed from the surface. Under steady state conditions, the c o n c e n t r a t i o n at the outer Helmholtz plane (OHP) w i l l be CL.™ , f a l l i n g t o an e q u i l i b r i u m b u l k c o n c e n t r a t i o n C^mi^ a t a at a d i s t a n c e '6' from t h e OHP. The c u r r e n t d e n s i t y ' i ^ ' corresponding to transport of ions across the d i f f u s i o n layer ' <5' .82 i s given by equation 14; i n = nFD (C_„_ - C, ,. ) D v OHP bulk /•< i s (14) From (12) and (14) and equating i = i n , we get <COHP " Cbulk> = V 6 p / n D W . (15) Hence, may be calculated from (15) for various values of 6, as shown i n Table - 17, by using the following values f o r the relevant parameters; W = 27.9 X 10~ 3 Kg p = 7.86 X 10 3 Kg/m3 97 n = 2 —9 2 D » 1.5 X 10 m/s, based on representative values for aqueous ions. —3 3 C, ,, =10 moles/m , the i n i t i a l concentration of bulk dissolved i r o n species i n a l l so l u t i o n s . -9 V = 1.0 X 10 m/sec, the average r e g i o n I I v a l u e s , F = 96,500 A.S. eq u i v a l e n t " 1 . Subsequently, the d i f f u s i o n (concentration) overvoltage may be c a l c u l a t e d f r o m e q u a t i o n (16) which i s based on the Nernst equation n D = 0.0362 log ( C 0 R p / C b u l k ) (16) Values of rP are l i s t e d i n Table-17. The analysis showed that even with a small d i f f u s i o n layer thickness (0.005 mm) concentration ( d i f f u s i o n ) overvoltages of the order of 80-90 mv can occur. A two hundred times change i n d i f f u s i o n layer thickness ( from 0.005 to 1 mm ) resulted i n a change i n the overvoltage by a factor of approximately 2. Using the p o l a r i s a t i o n diagram, i t i s possible to estimate rP by an alternate method, not requiring the values of 6 and D. F i r s t , the current density required to account for the observed crack , o v e l o c i t y i s determined from equation (12) and the p o t e n t i a l on the p o l a r i s a t i o n diagram corresponding to t h i s current density i s 98 determined. This value i s then subtracted from the p o t e n t i a l a p p l i e d d u r i n g the s t r e s s c o r r o s i o n c r a c k i n g tests to y i e l d n ° . Using t h i s procedure, 4m NaOH solutions showed a concentration o v e r v o l t a g e of HOmv whi l e 4m NaOH + lm A^O^ and IBS showed an overvoltage of 80-90 mv. Comparison with Table-17 shows that these correspond to a 1 <S' of 0.05 and 0.005 mm resp e c t i v e l y , which are 82 quite reasonable when compared with values i n the l i t e r a t u r e 4.5 Coalescence of Secondary Cracks The SSRT studies have shown that the crack(frequency) nucleation i s dependent on the environment conditions. Large numbers of secondary cracks, even i f short, may be more serious than a sing l e large crack. These i n d i v i d u a l small cracks, while i n e f f e c t i v e i n t h e i r early stages of growth, may coalesce and lead to a very large change i n stress i n t e n s i t y at the crack t i p and cause catastrophic f a i l u r e . Therefore, during inspection of s t e e l reactor vessels, l i f e t i m e predictions based on the propagation of the largest detectable crack may not give a conservative estimate. A t t e n t i o n must be d i r e c t e d to i n c o r p o r a t i n g t h i s v a r i a b l e (secondary cracks) i n the l i f e t i m e p r e d i c t i o n of a reactor v e s s e l . 5. CONCLUSIONS 99 The corrosion and SCC behaviour of i r o n i n hot caustic and caustic aluminate solutions was consistent with the following conclusions: 1. Addition of alumina to NaOH solutions i n h i b i t s corrosion of s t e e l , p o s s i b l y by forming mixed i r o n oxide-iron aluminate f i l m or an amorphous iron-aluminate f i l m . 2 . At anodic p o t e n t i a l s , slow s t r a i n rate tests (SSRT) showed that crack s u s c e p t i b i l i t y was more pronounced i n the active-passive t r a n s i t i o n region. 3. F r a c t u r e mechanics t e s t s at a c t i v e - p a s s i v e t r a n s i t i o n potentials e x hibit t y p i c a l crack dependent (region I) and crack independent (region II) behaviour, with the k i n e t i c s being consistent with f i l m r upture-dissolution models of cracking. Region II v e l o c i t i e s were r e l a t i v e l y unchanged by composition v a r i a t i o n i n the environments studied, whereas, region I and, hence behaviour were affected by caustic concentrations. A. A c o r r e l a t i o n of apparent crack v e l o c i t i e s from the SSRT studies with region II v e l o c i t i e s from fracture mechanics tests can be obtained, provided the SSRT specimens show large numbers of secondary cracks. 5. Concentration ( d i f f u s i o n ) overvoltages with minimum values of 100 ~ 80-85 mv w i l l always occur i n the closed confines of the crack t i p region, which may reach a maximum of around 150 mv. 101 BIBLIOGRAPHY 1. 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Privat e Communication, Alcan International Limited. 62. Brown & Srawley, Plane S t r a i n Crack Toughness Testing, STP410, ASTM, Philadelphia (1966). 63. ANSI/ASTM Standard E399-78a (1978). 64. M.C.H.Mckubre & D.D.Macdonald, Electrochemical k i n e t i c behaviour of Fe,Ni & Zn i n concentrated sodium hydroxide 104 solutions. Report to D.O.E., Contract no.EM-78-C-01-5159 (1979). 65. M.Pourbaix, Atlas of Electrochemical E q u i l i b r i a i n Aqueous solutions, NACE, P.171, (1974). 66. Dragutin M.Drazic, Chen Shen Hao, Corros. Science,23,7,pp.683-686 (1983). 67. B.G.Ateya & H.W.Pickering, Proc. of IV I n t n l . Symp. on P a s s i v i t y , ' P a s s i v i t y of metal 1, Ed.Robert F. Frankenthal & Jerome Kruger,PP.350-368, The Electrochemical Society Inc. (1978). 68. C.L.Foley, J.Kruger & C.J.Bechtoldt, J.Electrochem S o c : Electrochemical Science, 114,10,pp.994-1001. 69. A.G.Revesz & I.Kruger, as i n Ref. 67, PP.137-155. 70. R.D.Terns & R.N.Parkins, Corrosion I n h i b i t i o r s , F i f t h , Vol.3, PP.857-871 (1980). 71. R.A.Legault & W.J.Betlin, Mater. Prot. Performance, (9),35,(1970). 72. J.B.Lumsden & Szklarska-smialowska, Corrosion.34,pp.169-176 (1978). 73. G.H. Awad & S.A. Mansour, as i n Ref.70, pp.873-883. 74. Ines Fonseca, Jiang Lin-Cai & Derek Fletcher, J.Electrochem Soc: Electrochemical and Technology,130,11,pp.2187-2192 (1983). 75. D.Singbeil & D.Tromans, J.Electrochem. S o c , 129,pp.2669-73 76. M.Smialowski,as i n Ref.10, pp.405-422 (1973). 77. J.S.Lewelyn Leach, as i n Ref.10,pp.16-20. 78. R.W. Staehele, as i n Ref.10, pp.180-207. 79. J.R. Galvele, as i n Ref.67. 80. R.J. Biernat and R.G. Robins, Electrochimica Acta,14, pp. 819-820 (1969). 81. Handbook of chemistry and physics, 63rd E d i t i o n , R.C. Weast,ed.,CRC press Inc., West palm beach, F l . , 1982-83, pp.D-175,F-51. 82. J.0. Bockris & A.K.N. Reddy: Modern aspects of Electrochemistry Plenum press, New York, NY, Vol.2, pp. 1055-1059, (1970). 83. D.C.Crowe and D.Tromans, Symp. on Metallography and Corrosion, Calgary , (1983). 106 APPENDIX- 1 Ca l c u l a t i o n of S t r a i n Rate Speed of the motor used = 12 RPH Reduction r a t i o of Gear = 25/162 Speed of shaft = (25/162)/12 RPH « 30X10"3 RPM 19 turns of shaft to move cross head 1/16" 19X16 turns of shaft to move cross head 1" 19X16 RPM of shaft = l"/min of cross head speed .'. 30X10 - 3 RPM of shaft = 9.868X10 - 5 in/min. of cross head speed = 1.644X10 - 6 in/sec For a gage length of 1": St r a i n rate = 1.644X10~ 6/sec - 2.0X10"6 /sec 107 APPENDIX^2 Method Of C a l c u l a t i n g Stress "Intensity i n Constant "Deflection Tests To c a l c u l a t e the load and consequently the stress i n t e n s i t y at the crack t i p , load Vs. d e f l e c t i o n curves were plotted f o r d i f f e r e n t crack lengths. A DCB specimen was prepared and cracked incrementally. For each crack length, the d e f l e c t i o n of the specimen with increase i n load was plotted with an Instron t e s t i n g machine. D e f l e c t i o n was measured by f i x i n g two knife edges 5mms. apart, using a r a l d i t e r e s i n , on top of the specimen. An Instron C0D(Crack Opening Displacement) gage was used to measure d e f l e c t i o n . F i g . A l gives the load d e f l e c t i o n plots for various crack lengths, given as the dimensionless number a/W, where a i s the crack length and W i s the width of the specimen. Compliance, which i s the r a t i o of d e f l e c t i o n to load, was found by measuring the slope of the curves. Table A - l gives the compliances obtained for d i f f e r e n t crack lengths. A non-linear c u r v e - f i t t i n g routine UBC *0LSF (supported by UBC computing center) was used to f i t a 5th degree polynomial to the compliance data. F i g -A2 shows the f i t t e d curve, along with measured points. Table A-2 gives the calculated compliance for d i f f e r e n t crack lengths based on the polynomial. To obtain the desired stress i n t e n s i t y at the crack t i p the specimen was i n i t i a l l y fatigue pre-cracked. A compliance value was obtained for the r e s u l t i n g crack length using the polynomial 108 0 1 2 3 4 5 6 7 Lood ( KN) Fig - Al Load-Deflection curves. 110 Table A - l Compliance of the Specimen as a Function of Crack Length Sl.No. a a/W Compliance 1. 25.4 0.380 5.20 2. 28.4 0.425 5.80 3. 31.4 0.470 6.80 4. 34.4 0.5157 8.80 5. 37.4 0.5607 12.20 6. 40.4 0.605 14.60 7. 46.4 0.695 17.4o SUM OF SQUARES WAS= 0.764617E-01 a/W COMPLIANCE FITTED COMP RESIDUALS DEGREE OF CHOSEN POLYNOMIAL WAS A B SIGMA O.3800 0.4250 0.4700 O.5157 O.5607 0.6070 0.6950 0.520000E+01 0.5800O0E+01 0.68OOO0E+01 0.880000E+01 0. 122000E+02 0.146000E+02 0.174000E+02 0.519978E+01 0.586254E+01 0.665199E+01 0.898485E+01 0.120769E+02 0.146364E+02 0.173981E+02 0.215713E-03 -0.625357E-01 0.148011E+00 -O.184845E+O0 0.123072E+00 -O.363767E-01 0.188877E-02 0.397739E+01 0.611394E+01 0.580422E+01 0.533241E+01 0.514234E+01 0.0 0.347007E+00 0.572510E+00 0.491893E+00 0.351532E+00 0.468248E+02 0.147843E+01 0.110944E+01 0.148270E+00 O.138307E+00 0.581429E+01 -0.238247E+04 0.375198E+01 0.236897E+04 0.515324E+00 -0.926487E+03 -0.854195E+00 0.178300E+03 -0.295690E+00 -0.168703E+02 0.628532E+00 0.628532E+00 NEW - a/W NEW-COMPL 0.380 0.519978E+01 0.385 0.538052E+01 0.390 0.551723E+01 0.395 0.561832E+01 0.40O 0.569148E+01 0.405 0.574375E+01 0.410 0.578152E+01 0.415 0.581056E+01 0.420 0.583603E+01 c 0.425 0.586254E+01 0.430 0.589412E+01 0 0.435 0.593430E+01 0.440 0.598609E+01 0.445 0.605203E+01 0.450 0.613419E+01 1ST DERIV. 411445E+01 314532E+01 235117E+01 171792E+01 123196E+01 880155E+00 649846E+00 528838E+00 505413E+00 0.568321E+00 706784E+00 910496E+00 116962E+01 147480E+01 181714E+01 0.460 0.465 0.470 0.475 0.480 0.485 0.490 0.495 0.500 0.505 0.510 0.515 0.520 0.525 0.530 0.535 0.540 0.545 0.550 0. 555 0.560 0.565 0.570 0.575 0.580 0.585 0.590 0.595 0.600 0.605 0.610 0.615 0.620 0.625 0.630 0.635 0.640 0.645 0.650 0.655 0.660 0.665 0.670 0.675 0.680 0.685 0.690 0.635336E+01 0.649245E+01 0.665199E+01 0.683212E+01 0.703268E+01 0.725322E+01 0.749300E+01 0.775106E+01 0.802622E+01 0.831707E+01 0.862208E+01 0.893951E+01 0.926755E+01 0.960425E+01 0.994758E+01 0.102955E+02 0.106459E+02 0.109966E+02 0.113455E+02 0.116907E+02 0.12O3O0E+02 0.123616E+02 0.126837E+02 0.129945E+02 0.132927E+02 0.135768E+02 0.138457E+02 0.140985E+02 0.143346E+02 0.145536E+02 0.147554E+02 0.149402E+02 0.151086E+02 0.152617E+02 0.1540O6E+02 0.155272E+02 0.156438E+02 0.157528E+02 0.158576E+02 0.159617E+02 0.160694E+02 0.161854E+02 0.163151E+02 0.164643E+02 0.166397E+02 0.168484E+02 0.170983E+02 0.258007E+01 0.298525E+O1 0.339673E+01 0.380798E+01 0.421293E+01 0.460600E+01 0.498207E+01 0.533648E+01 0.566505E+01 0.596408E+01 0.623034E+01 0.646105E+01 0.665394E-K)1 0.680717E+01 0.691940E+01 0.698975E+01 0.701781E+01 0.700364E+01 0.694779E+01 0.685126E+01 0.671553E+01 0.654254E+01 0.633473E+01 0.609498E+01 0.582666E+01 0.553360E+01 0.522012E+01 0.489098E+01 0.455144E+01 0.420723E+01 0.386452E+01 0.353000E+01 0.321079E+01 0.291450E+01 0.264920E+01 0.242346E+01 0.224628E+01 0.212717E+01 0.207608E+01 0.210346E+01 0.222020E+01 0.243769E+01 0.276778E+01 0.322278E+01 0.381549E+01 0.455918E+01 0.546757E+01 113 function. The load necessary to obtain the desired stress i n t e n s i t y was calculated by the equation; Kj = (P.a /BH J /^). (3.45 + 2.415 (H/a)). (3) Since both load and compliance were known, i t was possible to f i n d out d e f l e c t i o n by m u l t i p l y i n g the load (KN) with compliance. Two opposing b o l t s , rounded at the ends, were threaded through the two beams of the specimen and tightened to d e f l e c t the beams apart by the calculated amount of d e f l e c t i o n using either Instron or MTS COD gauges. Once the specimen was placed i n the corrosive environment, there was an increase i n crack length due to SCC. Since the d e f l e c t i o n remains constant, the load decreases. There was an i n i t i a l increase i n stress i n t e n s i t y , due to increase i n crack length although the load decreases, but l a t e r the drop i n load predominated so that e f f e c t i v e l y there was an o v e r a l l drop i n stress i n t e n s i t y . At the end of the test the f i n a l crack length was measured and the new f i n a l compliance was found from Table-A2. The e f f e c t i v e load at the end of the test was then recalculated by using the f i n a l compliance and the d e f l e c t i o n value, which remained constant throughout the t e s t . The f i n a l (new) K-j. was calculated by i n s e r t i n g the load into the above equation (3). 

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