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Crevice corrosion behaviour of nickel based alloys in neutral chloride solutions Mulford, Stephen John 1985

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CREVICE CORROSION BEHAVIOUR OF NICKEL BASED ALLOYS IN NEUTRAL CHLORIDE SOLUTIONS by STEPHEN J . MULFORD B.A.Sc, (Metallurgical Engineering), The University of B r i t i s h Columbia, 1982 A THESIS SUBMITTED IN PARTIAL FULFILMENT 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 this thesis as conforming to the required standard THE UNIVERSITY OF/ BRITISH COLUMBIA June 1985 © Stephen J . Mulford, 1985 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 for 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 available for reference and study. I further agree that permission f o r extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by hi s or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of [Y\c<*Jl Btn°jiA<-tVi/\ The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 D A T E T^/^ /o . h<YS ABSTRACT Crevice corrosion experiments have been conducted on Inconel 600 and Inconel 625 exposed to two p r i n c i p l e test solutions of 1 M NaCl and 1 M N a C l + 0.01 M N a 2 S 2 0 3 (Sodium T h i o s u l p h a t e ) a t t h r e e temperatures, 22°C, 55 °C and 80°C. The crevice corrosion tests were performed i n a c o r r o s i o n c e l l which was c o n s t r u c t e d from PTFE (Polytetrafluoroethylene, Teflon) and Pyrex g l a s s . Features of the c e l l included the u t i l i z a t i o n of an a r t i f i c i a l Teflon-metal cr e v i c e and provisions to monitor crevice corrosion current, a c t i v e c r e v i c e c o r r o s i o n p o t e n t i a l and active crevice pH. A d d i t i o n a l experiments i n c l u d e d p o t e n t i o d y n a m i c a n o d i c p o l a r i z a t i o n tests on pure N i , A l l o y 600, and A l l o y 625 i n bulk s o l u t i o n environments and i n simulated crevice s o l u t i o n s . Crevice corrosion morphology and compositional analysis of the corrosion products was studied using a scanning electron microscope equipped with an X-ray energy d i s p e r s i v e spectroscopy (EDS) system. Results show that c r e v i c e corrosion rates increase with increasing temperature for A l l o y 600 i n both p r i n c i p l e test s o l u t i o n s . X-ray EDS a n a l y s i s i n d i c a t e d t h a t an i n s o l u b l e n i c k e l s u l p h i d e corrosion product formed on A l l o y 600 i n a so l u t i o n of 1 M NaCl + 0.01 M Na 0S 90~. For the A l l o y 600, i n a s o l u t i o n of 1 M NaCl + 0.01 M Na 9S„0., i i i n i t i a t i o n times were s i g n i f i c a n t l y reduced and crevice c o r r o s i o n propagation rates enhanced, as compared to A l l o y 600 i n 1 M NaCl. The decrease i n i n i t i a t i o n times has been a t t r i b u t e d to the -2 d e s t a b i l i z i n g nature of the S^O^ species on the passive oxide f i l m . Enhanced propagation rates have been a t t r i b u t e d to the presence of r^S i n the c r e v i c e s o l u t i o n and the f o r m a t i o n of an adsorbed s p e c i e s Ni(H„S) ., which enhances the anodic d i s s o l u t i o n r e a c t i o n . The H~S i n 2 ads 2 the a c t i v e c r e v i c e s o l u t i o n o r i g i n a t e d from the thermodynamically -2 f a v o u r e d electrochemical reduction of the S20^ species i n the a c t i v e c r e v i c e s o l u t i o n . Experiments on A l l o y 625, which i s alloyed with molybdenum, (Mo), show that i t was v i r t u a l l y immune to crevice corrosion as compared to A l l o y 600 which i s not alloyed with Mo. The resistance of A l l o y 625 to crevice corrosion i n i t i a t i o n has been a t t r i b u t e d to the s t a b i l i z i n g nature of MoC^ i n the passive oxide f i l m . For an a c t i v e l y -2 corroding system, the formation of the molybdate species MoO^ may act as an anodic i n h i b i t o r and e f f e c t i v e l y enhance the repassivation of the passive f i l m . i i i TABLE OF CONTENTS Page Abstract i i Table of Contents i v L i s t of Tables v i L i s t of Figures v i i L i s t of Symbols and Abbreviations x Acknowledgement x i 1. INTRODUCTION 1 1.1. Temperature 3 1.2. Bulk Solution 3 1.3. Crevice Solution 4 1.4. Influence of Sulphur Species 5 1.5. Alloy Composition 6 1.6. Crevice Geometry 6 1.7 Crevice Corrosion Test Methods 7 1.8 Objectives 8 2. EXPERIMENTAL 2.1. Materials 10 2.2. Specimen 10 2.3. Solution 12 2.4. Corrosion C e l l 12 2.5. Test Set-Up 16 i v Page 2.6. pH Experiments 16 2.7. Pol a r i z a t i o n Experiments 17 2.8 Microscopy 21 3. RESULTS 23 3.1. Introduction 23 3.2. Electrochemical Measurements ; 23 3.2.1. Potential Measurements 27 3.2.2. Corrosion Rates 28 3.2.3. I n i t i a t i o n Time 30 3.3. Other Crevice Corrosion Tests 31 3.4. pH Measurements 35 3.5. Pol a r i z a t i o n Tests 39 3.6. EDS 50 3.7. Crevice Corrosion Morphology 55 3.7.1. Crevice Location 55 3.7.2. Morphological Features of Crevice Corrosion .... 55 4. DISCUSSION 63 4.1. Role of Thiosulphate Ion 63 4.2. Role of Molybdenum 71 5. SUMMARY 74 BIBLIOGRAPHY 75 APPENDIX 78 v LIST OF TABLES Page TABLE I Nominal Composition of Ni-Alloys 11 I I Crevice Corrosion Data Summary for Alloy 600 and Alloy 625 24 I I I Calculated Reduction Potentials for Eg^ n^-2/ g a n d ES/H 2S 6 6 v i LIST OF FIGURES Figure Page 1 Crevice Corrosion C e l l (crevice open). a) N2 purge, b) thermistor probe, c) thermometer, d) Luggin c a p i l l a r y , e) vent to condensor, f) KC1 s a l t bridge (containing a cottom thread), g) c e l l walls (glass), h) heating tape, i ) specimen, j) telfon disc, k) copper contact, 1) zirconia plug 13 2 Details of c e l l assembly when crevice i s created (closed), k) copper contact, 1) Luggin c a p i l l a r y , m) 0-ring gaskets, n) specimen 15 3 Modified Teflon-metal crevice assembly for pH experiments 18 4 Test c e l l for polarization studies. a) N2 bubbler, b) thermistor probe, c) Luggin c a p i l l a r y , d) working electrode, e) graphite counter electrode, f) heating pad 20 5 Crevice corrosion current and potential vs time for Alloy 600 i n 1 M NaCl, (80°C). Crevice closed at Time = 0. (Test #4) 25 6 Effect of cooling the solution from 80°C to room temperature (20°C) on the crevice current and potential vs time behaviour for Alloy 600 i n 1 M NaCl. (Test #24). Arrows i n the Figure denote the time that the heater was turned o f f . Active crevice corrosion ceased (current 0) at Time = 71 hours and a solution temperature of = 30°C 33 7 Effect of cooling the solution from 80°C to room temperature (20°C) on the crevice current and potential vs time behaviour for Alloy 600 In 1 M NaCl + 0.01 M Na 2S 20 3. (Test #25). Arrows i n the Figure denote the time that the heater was turned o f f . The solution had cooled to 20 °C at Time = 35 hours 34 8 SEM photograph of oxidation products formed on Alloy 625 i n 1 M NaCl + 0.01 Na 2S 20 3, (95°C) 36 9 X-Ray EDS spectra of the oxidation product formed on Alloy 625 i n 1 M NaCl + 0.01 M Na 2S 20 3, (95°C), and of the matrix 37 10 Active crevice pH vs time for Alloy 600 i n 1 M NaCl + 0.01 M Na 2S 20 3, 22°C. (Test #23) 38 v i i Figure * Page 11 Anodic polarization curves of Alloy 600, i n 1 M NaCl, 1 M NaCl + 0.01 M Na 2S 20 3, and for Pt wire i n 1 M NaCl + 0.01 M Na 2S 20 3, (20°C) AO 12 Corroded surface of Alloy 600 polarization sample exposed to 1 M NaCl + 0.01 M Na 2S 20 3 at 20°C. Refer to Figure 11 42 13 Anodic polarization curves of pure Ni in a N i C l 2 solution, (pH = 2.2). Effect of increasing concentrations of Na2S. (80°C) 44 14 Anodic polarization curves of pure Ni i n 0.5 N HC1, pH = 0.6, and i n 0.5 N HC1 + 0.01 M Na2S. (20°C) 45 15 Anodic polarization curves of Alloy 600 i n 0.5 N HC1. Effect of the addition of 0.01 M Na2S. Temp. = 20°C 46 16 Anodic polarization curves of Alloy 625 i n 1 M NaCl and 1 M NaCl + 0.01 M Na 2S 20 3. Temp. = 20°C 48 17 Anodic po l a r i z a t i o n curves of Alloy 625 i n 1 M NaCl and in 1 M NaCl + 0.01 M Na 2S 20 3. (80°C) 49 18 X-Ray EDS spectra of corrosion products formed on Alloy 600 i n 1 M NaCl solution (80°C) compared with the Alloy 600 matrix. (Test #4) 51 19 SEM photograh of Ti precipitates found on the surface of the corrosion products formed on Alloy 600 exposed to 1 M NaCl solution (80°C). (Test #4) 52 20 X-Ray EDS spectra of Ti precipitates as shown i n Figure 19 53 21 X-Ray EDS spectra of black corrosion products formed on All o y 600 i n 1 M NaCl + 0.01 M Na 2S 20 3 (55°C) 54 22 General appearance of active crevice corrison sites formed on Alloy 600 i n a) 1 M NaCl (80°C), b) 1 M NaCl + 0.01 M Na 2S 20 3. (55°C) 56 23 SEM photographs i l l u s t r a t i n g the development of crevice corrosion morphology of Alloy 600 i n a) 1 M NaCl (55°C), b) 1 M NaCl (55°C) advanced stages, c) 1 M NaCl (80°C), well developed crevice, d) 1 M NaCl + 0.01 M Na 2S 20 3 (20°C) early stages 58 v i i i Figure Page 24 SEM photographs of corrosion products formed within crevice corrosions s i t e s i n a) Al l o y 600, 1 M NaCl (80°C), b) Alloy 600, 1 M NaCl + 0.01 M Na 2S 20 3 (55°C) 60 25 SEM photograph of Voluminous black corrosion products formed within crevice corrosion s i t e s , Alloy 600, 1 M NaCl + 0.01 M Na 2S 20 3 (55°C) 61 26 SEM photographs of crevice i n t e r i o r s following the removal of corrosion products with an inhi b i t e d acid solution a) Alloy 600 i n 1 M NaCl (80°C), b) Alloy 600 i n 1 M NaCl + 0.01 M Na 7S 90o (55°C) 62 i x LIST OF SYMBOLS AND ABBREVIATIONS Symbols A EmV E v F i """final R ave R max P V (SCE) W uA Abbreviations E corr EDS ESCA nA PTFE SCE SEM measured crevice corrosion area, electrochemical p o t e n t i a l i n m i l l i - v o l t s . electrochemical p o t e n t i a l i n v o l t s . Faraday average current density. f i n a l current at termination of crev i c e corrosion t e s t , average crevice corrosion rate, maximum crevice corrosion rate, density. v o l t s with respect to the saturated calomel electrode, equivalent weight, micro Amperes. corrosion p o t e n t i a l . X-Ray Energy Dispersive Spectroscopy. Electron Spectroscopy for Chemical A n a l y s i s . nano-Amperes. Poly t e t r a f l u o r o e t h y l e n e . Saturated Calomel Electrode. Scanning Electron Microscope. AKNOWLEDGEMENT I wish to express my s i n c e r e thanks to my res e a r c h supervisor, Dr. Desmond Tromans for his support and encouragement throughout the project. I also wish to thank the students, f a c u l t y and sta f f of the Metallurgical Engineering Department for making my stay a most memorable experience. Special thanks goes to my wife, Nosrat, for her u n f a i l i n g patience, and understanding from the very beginning. F i n a n c i a l a s s i s t a n c e provided by the Department of Me t a l l u r g i c a l Engineering and by the Cy and Emerald Keyes Foundation i s greatly appreciated. x i 1. INTRODUCTION Crevice corrosion i s one form of lo c a l i z e d corrosion which i s characterized by the l o c a l breakdown of a protective passive f i l m and corrosion i n shielded s i t e s wherever a crevice i s formed. Examples of crevice s i t e s Include lap j o i n t s , threaded or riveted connections, gaskets, porous welds, and marine or debris deposits [1,2]. Common engineering alloys susceptible to crevice corrosion include Stainless Steels, Titanium a l l o y s , Aluminum alloys and, the subject of th i s t h e s i s , Nickel-based a l l o y s . The mechanism of crevice corrosion has been studied i n some d e t a i l by many researchers [3,4,5,6]. O l d f i e l d and Sutton developed a mathematical model to show the influence of several parameters on the i n i t i a t i o n and propagation of crevice corrosion [7]. Their model i s based on the generally accepted mechanism of crevice corrosion, which includes four stages. Stage I i s the depletion of oxygen i n the crevice solution. Oxygen transport from the bulk solution to the crevice i n t e r i o r f a l l s under d i f f u s i o n control and the cathodic reduction of oxygen, which maintains the passive layer, rapidly consumes the dissolved oxygen i n the crevice. Transport of oxygen by convection and hydrodynamic processes play no role within the r e s t r i c t e d confines of the crevice. 2 During Stage I I , metal cations pass through the passive f i l m into the crevice e l e c t r o l y t e . To maintain e l e c t r o n e u t r a l i t y , anions (e.g. C l ions) migrate into the crevice solution and with time increase i n concentration. At the same time, hydrolysis reactions involving metal cations increase the a c i d i t y within the crevice. These hydrolysis reactions take the form of [8]: M n + + H 20 + M(OH) ( n - 1 ) + H + (1) During Stage I I I , the crevice solution becomes s u f f i c i e n t l y aggressive due to the increase i n a c i d i t y and chloride ion concentration that i t breaks down the passive oxide f i l m . Active d i s s o l u t i o n of the metal begins at the end of th i s stage. The time period preceeding active dissolution of the metal i s known as the i n i t i a t i o n time. I t i s a measure of a metals resistance to crevice corrosion. During Stage IV, propagation of the active crevice proceeds. Studies of crevice corrosion have shown that several important factors [6] influence crevice corrosion behaviour. These factors i n c l u d e bulk solution environment and composition (e.g. C l content, content, pH, temperature), crevice solution (electrochemical reactions), a l l o y composition, crevice type and geometry. In e f f e c t , these factors are m e t a l l u r g i c a l , environmental, and geometrical, and are generally i n t e r r e l a t e d . 3 1.1. Temperature Raising the temperature has been shown to increase the severity of crevice corrosion i n several a l l o y systems. These include stainless steels i n chloride solutions [17], the Ni-based a l l o y s Hastelloy G and Inconel 625 i n neutral and acid chloride environments [10], and several Ni-based alloys i n 4.5 N H 2S0 4 and 4.5 N HC1 [11]. 1.2. Bulk Solution Nickel based alloys (Ni-Cr) are completely r e s i s t a n t to c r e v i c e c o r r o s i o n i n neutral aereated water. With the addition of Cl ions, Ni-based alloys become susceptible to p i t t i n g and crevice corrosion [15]. Jackson and Rooyen [14] showed that i n a p o l a r i z a t i o n test for an experimental Ni-based a l l o y i n 1 N HC1 at 40°C, an increase i n the chloride ion concentration caused an increase i n the anodic current density. A mathematical model developed by Kain [8] predicted that by increasing the chloride concentration for a given crevice gap, the time to breakdown should decrease. Subsequent multiple crevice tests on st a i n l e s s steels i n d i l u t e seawater showed that generally fewer s i t e s i n i t i a t e d i n d i l u t e seawater as compared to f u l l strength seawater te s t s . In addition, depths of penetration i n d i l u t e seawater were an order of magnitude less than those measured i n f u l l strength seawater. 4 1.3. Crevice Solution The crevice solution composition plays a major role i n the l o c a l i z e d anodic d i s s o l u t i o n of an a l l o y i n an a c t i v e l y corroding crevice. An extensive review on the subject has been given by Turnbull [16]. The factors which influence the crevice solution are the a l l o y composition, bulk solution composition, crevice geometry, time and p o t e n t i a l . I t i s not the intention to review each factor i n d e t a i l . However, i t i s clear that once a crevice has formed, the solution chemistry inside the crevice changes with time. O l d f i e l d and Sutton [7] i n t h e i r mathematical model, show that i t i s metal hydrolysis, and the subsequent f a l l i n pH, which has a major influence on the crevice s o l u t i o n . Permanent breakdown i n p a s s i v i t y and active d i s s o l u t i o n of the a l l o y occurs only when the crevice solution attains a c r i t i c a l composition. This crevice solution i s referred to as the c r i t i c a l crevice s o l u t i o n , defined i n terms of pH and C l ion concentration. Hydrolysis e q u i l i b r i a data [3,4,7,16] suggest that for stainless s t e e l s , i t i s the hydrolysis of Cr which has the most dramatic effect on lowering the crevice solution pH, and to a lesser extent Ni and Fe hydrolysis. Calculations by Bernhardsson [4] have shown that for stainless steels i n chloride environments Cr, and especially Mo, cause a decreased pH i n the crevice due to hydrolysis effects and that Ni has no appreciable e f f e c t . 5 1.4. Influence of Sulphur Species The addition of thiosulphate has been investigated by several researchers. Tromans and Frederick [17] have investigated the e f f e c t of thiosulphate on crevice corrosion of a u s t e n i t i c s t a i n l e s s s t e e l s i n a n e u t r a l chloride environment. Analysis of corrosion products, together with thermodynamic and k i n e t i c considerations indicated that increased crevice corrosion propagation rates were due to the formation of s u l p h i d e ( I ^ S ) i n the a c t i v e c r e v i c e which c a t a l y z e d the anodic d i s s o l u t i o n of i r o n . Sury [18] s t u d i e d the e f f e c t of sulphide ( r ^ S ) adsorption on the corrosion behaviour of i r o n , cobalt and n i c k e l i n a c i d s o l u t i o n s . P o l a r i z a t i o n curves showed a pronounced d e p o l a r i z a t i o n e f f e c t of r^S on the anodic d i s s o l u t i o n of Ni i n aereated H-SO, s o l u t i o n at 25°C. The 2 4 r e s u l t s showed that s u l p h i d e (HS species) adsorption i s a predominant fac t o r i n determining the corrosion rates of pure i r o n and n i c k e l , and t h e i r a l l o y s . Newman and co-workers [20] investigated Stress Corrosion C r a c k i n g (SCC) behaviour of s e n s i t i z e d 304 S t a i n l e s s S t e e l i n thiosulphate s o l u t i o n s . They showed that hydrogen sulphide produced at the crack t i p could i n p r i n c i p l e account for SCC behaviour, the source o f t h e H^S b e i n g t h a t p r o d u c e d c h e m i c a l l y as a r e s u l t o f t h e a c i d i f i c a t i o n of the thiosulphate s o l u t i o n . 6 1.5. A l l o y Composition S e v e r a l r e s e a r c h e r s have s t u d i e d the e f f e c t of a l l o y composition on crev i c e corrosion behaviour. The b e n e f i c i a l e f f e c t s of Mo and Cr i n s t a i n l e s s s t e e l s have been known for a long time [12,13,38]. This has also been shown to be the case for nickel-based a l l o y s . For example, T r a i s n e l et a l [11] showed that Mo and Cu s h i f t the corrosion p o t e n t i a l towards p o s i t i v e values and s i g n i f i c a n t l y reduce hydrogen evolution i n a c i d i c environments. Mo was found to be about 3 times as e f f e c t i v e as Cr for the passivation e f f e c t . Asphahani [10] showed that the high Mo content of Hastelloy C-276 nickel-based a l l o y contributed to exce l l e n t resistance to c r e v i c e corrosion, as compared to A l l o y 625 of lower Mo content, when tested i n an acid c h l o r i d e environment. 1.6. Crevice Geometry O l d f i e l d and Sutton [7] i n t h e i r mathematical model have shown that both c r e v i c e depth and gap are important g e o m e t r i c a l considerations. Jackson and Rooyen [14] studied the crevice corrosion behaviour of Ni-Cr-Mo-Fe a l l o y s and showed that c r e v i c e attack increased i n s e v e r i t y as the crevice became t i g h t e r ( i . e . more narrow). S i m i l a r i l y , Rosenfeld [3] showed that crevice corrosion rates increase as the width of the crev i c e decreases for a 2Crl3 s t a i n l e s s s t e e l i n 0.5 M NaCl. 7 1.7. Crevice Corrosion Test Methods A thorough review on electrochemical test methods i n crevice corrosion i s given by Jsseling [6]. A variety of methods may be u t i l i z e d for quantifying crevice corrosion behaviour. These include s t a t i s t i c a l studies u t i l i z i n g multiple crevice assemblies [2,21], p o l a r i z a t i o n techniques [6,14], simulated crevice experiments u t i l i z i n g metal to non-metal a r t i f i c i a l crevices, and monitoring either crevice potential or crevice current or both [22,23,24]. Each of these methods have thei r advantages and disadvantages. Multiple crevice assemblies are advantageous since they may be placed i n the actual operating environment. Their disadvantage l i e s i n that an experiment may take several weeks to complete, especially when the corrosion rate i s low. Also, no d i s t i n c t i o n between i n i t i a t i o n and propagation stages of crevice corrosion may be made. Pol a r i z a t i o n tests are probably the most common testing method used i n the laboratory. Using simulated crevice solutions, the anodic d i s s o l u t i o n behaviour of a metal may be obtained over a range of potentials (potentiodynamic). These polarization tests are fast (usually one hour or less) and the chemistry or environmental conditions may be changed to investigate the influence of d i f f e r e n t operating parameters on corrosion behaviour. The main disadvantage of these tests i s that i t i s unknown just how much of an effect polarizing a metal w i l l have on i t s true corrosion behaviour. Also, i f simulated crevice 8 solutions are used, certain assumptions have to be made regarding the composition and pH of the crevice solution. Crevice corrosion experiments using a r t i f i c i a l crevices have been employed with good r e s u l t s . The advantage of t h i s method i s that i n i t i a t i o n and propagation behaviour may be obtained by monitoring both the corrosion potential and corrosion current. Provisions can be made to monitor crevice solution pH. Another advantage of t h i s method i s that the tests are r e l a t i v e l y short (less than one week), experimental conditions can be reproduceably controlled and quantitative data may be obtained. 1.8. Objectives I t i s the objective of t h i s study to quantify the crevice c o r r o s i o n behaviour of Inconel 600 and Inconel 625, using electrochemical techniques, i n neutral chloride solutions i n the presence and absence of thiosulphate species (sodium thiosulphate). The electrochemical techniques include a r t i f i c i a l crevice corrosion experiments^and p o l a r i z a t i o n experiments. A second objective i s to determine the role that thiosulphate -2 i o n (^O^ ) has on the i n i t i a t i o n and propagation of crevice corrosion by combining the electrochemical studies with corrosion product analysis and thermodynamic calculations. 9 I t i s hoped that the results may be used i n industry, e s p e c i a l l y the pulp and paper industry, where chloride environments containing various sulphur species, including thiosulphate, p r e v a i l . Currently used Fe-base a l l o y s , mainly stainless s t e e l s , are p a r t i c u l a r l y prone to l o c a l i z e d corrosion i n acid chloride environments leading to considerable equipment losses [25]. The presence of thiosulphate has been implicated i n aggravating the corrosion problem on paper machines. It i s a p o s s i b i l i t y that nickel-based alloys may be a viable cost e f f e c t i v e a l t e r n a t i v e to replace the stainless steels by reducing the frequency of corrosion related f a i l u r e s . 10 2. EXPERIMENTAL 2.1. M a t e r i a l s Crevice c o r r o s i o n experiments were conducted on as-received * c o m m e r c i a l grade, c o l d r o l l e d and annealed I n c o n e l A l l o y 600 and Inconel A l l o y 625. These a l l o y s were obtained i n sheet form of thicknesses 1/8" (3.2 mm) and 1/16" (inches) (1.6 mm) r e s p e c t i v e l y . Nominal compositions are shown i n Table 1. 2.2. Specimen Square specimens (63.5 x 63.5 mm) were sectioned from the sheet and wet p o l i s h e d to 600 g r i t on SiC p o l i s h i n g d i s c s . Subsequently, they were washed, degreased in acetone and then rinsed i n ethanol. The polished surface was then p a r t l y coated with a protective c r e v i c e r e s i s t a n t alkyd enamel paint (Glyptal G-1201, Canadian General 2 E l e c t r i c Company) leaving an exposed polished surface area of 23.76 cm (55 mm d i a . ) . The coated sample was baked for 24 hours at 120°C and then stored i n a dessicator p r i o r to use. This heat-cured r e s i n minimized microscopic crevices [26] between the coating and metal. INCO Trademark TABLE 1 Nominal Composition of Ni-Alloys Wt.% Alloy Ni Cr Fe Mn Mo Cu Si Other 600 75.5 15.5 8.0 .5 - .25 .25 (C + S) < .1 625 60 21.5 4.0 .2 9 .25 .25 (Al + Ti) .8 max (Cb + Ta) 4.0 12 2.3. Solutions Two p r i n c i p l e test solutions were used for the crevice corrosion experiments. They were aqueous solutions of neutral 1 M NaCl and 1 M NaCl + 0.01 M Na2S20^, prepared from reagent grade chemicals and d i s t i l l e d water. Additional tests using solutions of 0.01 M and 1.0 M sodium thiosulphate i n the absence of chloride ion were also used. 2.4. Corrosion C e l l A schematic of the corrosion c e l l i s given i n Figure 1. The c e l l base and l i d were constructed of polytetrafluoroethylene (PTFE, or Teflon). The c e l l wall was a 4 mm thick pyrex glass tube with an inside diameter of 14.4 cm. These materials were chosen for their good chemical inertness to neutral and acid chloride solutions. The test specimen, located i n the base of the c e l l , was secured into position by a plexiglass flange pressing against the back face of the sample and bolted into place. A rubber 0-ring gasket placed between the unpolished back face of the sample and the plexiglass ensured a leak tight seal. A copper b o l t , screwed into the plexiglass flange, made e l e c t r i c a l contact at the unpolished specimen surface and was e l e c t r i c a l l y connected to a supplementary cathode chamber. Here, a platinum mesh counter electrode acted as the cathode upon which the reduction of dissolved oxygen occurred. The cathode chamber was f i l l e d with 1 M KC1 and e l e c t r i c a l l y connected to the c e l l solution v i a a s a l t bridge. The s a l t bridge consisted of 3 mm inside 13 | 1 Teflon Plexiglass Pyrex glass Air Figure 1. Crevice Corrosion C e l l (crevice open). a) N 2 purge, b) thermistor probe, c) thermometer, d) Luggin c a p i l l a r y , e) vent to condenser, f ) KC1 s a l t bridge ( c o n t a i n i n g a cotton t h r e a d ) , g) c e l l w a l l s ( g l a s s ) , h) heating tape, i ) specimen, j ) Telfon d i s c , k) copper contact, 1) porous zi r c o n i a plug. 14 diameter Teflon tubing f i l l e d with 1 M KC1 and plugged with a porous z i r c o n i a plug where i t terminated i n the test c e l l . A cotton thread was placed i n s i d e the KC1 s a l t bridge to prevent e l e c t r i c a l disconnection due to the formation of bubbles. The leak rate was measured to be approximately 5 ml per 24 hour period at 80°C. A i r was bubbled into the cathode chamber during the experiment to provide a continuous supply of fresh oxygen for the reduction reaction. Incorporated into the design of the c e l l were provisions to monitor the crevice corrosion current and p o t e n t i a l . The corrosion potential was monitored with an externally placed saturated calomel r e f e r e n c e e l e c t r o d e (E /.C(-,I,N = E /c,„^s - .2416 v o l t s at 25°C) V^OIVEJJ v^bntij connected to the c e l l v i a a saturated KC1 s a l t bridge. External placement of the calomel reference electrode at room temperature ensured a constant reference potential at a l l testing temperatures. The corrosion current was monitored by placing a zero impedance ammeter into the c i r c u i t as shown i n Figure 1. Tests were conducted i n 1.25 1 of solution at 22°C, 55 °C and 80°C. For the elevated temperature te s t s , the c e l l was heated externally using a Thermolyne heating tape wrapped around the c e l l and the solution controlled to ± 1°C by a thermistor probe (Teflon coated) and a temperature c o n t r o l l e r . An a r t i f i c i a l crevice was formed by placing a 46 mm diameter Teflon disc onto the polished metal surface as shown i n Figure 2 and applying a constant load by means of a torque wrench. 15 SSS Tef,on I I P lex ig lass G Figure 2. D e t a i l s of c e l l assembly when c r e v i c e i s created (closed). k) copper contact, 1) Luggin c a p i l l a r y , m) 0-ring gaskets, n) specimen. 16 2.5. Test Set-up A specimen was placed i n the base of the c e l l and a l l e l e c t r i c a l connections were made. With a i r bubbling through the cathode chamber, N 2 purging of the c e l l and a l l monitors running, the test solution was added. Following a period of approximately three hours, the a r t i f i c i a l Teflon-metal crevice was closed by lowering the Teflon disc onto the specimen (Figure 2). A consistent closure force was obtained by screwing a threaded plexiglass cylinder onto the t e f l o n disc using a torque wrench set to 10 i n . l b s . (1.13 N»m). During the experiment, both corrosion current and potential were continuously monitored with respect to time. The corrosion current was measured using a Keithley Model 177 d i g i t a l meter coupled to a Fischer Recordall Series 5000 chart recorder. The potential was measured using a Corning Model 125 pH meter coupled to a Sargent SRG chart recorder. The tests were allowed to run u n t i l the corrosion current and potential attained a reasonably steady value. This usually took approximately 24 hours, although longer and shorter tests were also conducted. At the termination of an experiment, the c e l l was emptied, the sample removed, rinsed f i r s t with d i s t i l l e d water and then ethanol, dried and stored i n a dessicator. 2.6. pH Experiments A set of experiments were conducted i n order to determine the approximate pH i n an a c t i v e l y corroding crevice. To do t h i s , a modified 17 Teflon disk was designed to accommodate a pH electrode (Figure 3). The Teflon disc was tapered to form an annulus with an inside diameter of 13 mm and an outside diameter of 27 mm. Specimens for the pH experiments had a 12 mm diameter hole machined to a depth of 1 mm and c e n t r a l l y located so that the sensing bulb of a pH electrode would f i t into i t when the a r t i f i c i a l crevice was closed. The entire surface was coated with Glyptal paint, except for a small area with a diameter of ~ 3 mm located d i r e c t l y beneath the pH electrode. The pH was monitored continuously throughout the te s t . Following the experiment, the specimen was checked to see that corrosion had occurred close to the t i p of the pH electode. The general test procedure was i d e n t i c a l to the crevice corrosion experiments. I n i t i a l l y , pH measurements were made at elevated temperatures. However, unreliable data were obtained, due to i n s t a b i l i t i e s i n the pH readings and absence of corrosion. The most meaningful result was obtained at room temperature (22°C) i n a 1 M NaCl + 0.01 M Sodium Thiosulphate solution. 2.7. P o l a r i z a t i o n Tests P o l a r i z a t i o n tests were conducted using an EG & G Princeton Applied Research Model 350 A Corrosion Measurement System. Tests were conducted on Alloy 600, Alloy 625 and pure n i c k e l at 22°C and 80°C. The solutions used were 1 M NaCl, 1 M NaCl + 0.01 M Na 2S 20 3 > 0.5 N HC1, 0.5 N HC1 + 0.01 M Na_S (Sodium Sulphide) and a s o l u t i o n of N i C l , Figure 3. M o d i f i e d T e f l o n - m e t a l c r e v i c e assembly f o r pH experiments. 19 adjusted to pH ~ 2.2 by adding d i s t i l l e d water to a saturated solution of n i c k e l c h l o r i d e ( N i C l 2 ) . One p o l a r i z a t i o n test was conducted i n 1 M NaCl + 0.01 M N a 2 S 2 0 3 using platinum ( P t ) wire as the working electrode. The po l a r i z a t i o n (working) electrodes were prepared by spot 2 welding Ni wire (.020 inches diameter) to approximately 1 cm pieces of each a l l o y , i n s u l a t i n g the Ni wire with glass tubing, mounting the specimen i n cold cure (Quick Mount) epoxy r e s i n , followed by mechanical polishing to 600 g r i t . A schematic of the p o l a r i z a t i o n c e l l i s shown i n Figure 4. The c e l l was a 3-necked Pyrex f l a s k . The positions of the working electrode and graphite electrode are shown i n the figure. Nitrogen was bubbled into the c e l l prior to the test to lower the oxygen concentration. A Teflon coated thermistor probe was used for maintaining the solution temperature, and heating was provided by using a heating pad controlled by a Yellow Springs Inc. Model 71 temperature c o n t r o l l e r coupled to a variable transformer. The potential was measured by an externally placed saturated calomel reference electrode, bridged to the c e l l v i a a Teflon tube containing saturated KC1 solution. A cotton thread was strung through the entire length of the Teflon tube to avoid e l e c t r i c a l disconnection due to the formation of bubbles i n the tube. The end of the Teflon tubing (plugged with cotton thread) was placed ~ 1 mm from the working 2 0 Figure 4. Test c e l l for p o l a r i z a t i o n studies. a) N bubbler, b) thermistor probe, c) Luggin c a p i l l a r y , 2 d) working electrode, e) graphite counter electrode, f ) heating pad. 21 e l e c t r o d e s u r f a c e . No p o t e n t i a l c o r r e c t i o n s were made f o r concentration or thermal gradients. The sample was placed i n the c e l l and a l l the appropriate connections were made. The free corrosion p o t e n t i a l was allowed to s t a b i l i z e for at le a s t 5 minutes p r i o r to the potentiodynamic scan. The p o t e n t i a l was then scanned i n the p o s i t i v e d i r e c t i o n at 0.5 mV/S, s t a r t i n g at a p o t e n t i a l —250 mV below the free c o r r o s i o n p o t e n t i a l , to produce a p l o t of p o t e n t i a l (V „ ) vs. log 2 current density i n nA/cm . 2.8. Microscopy Following the crev i c e corrosion t e s t s , the Gl y p t a l paint was removed by soaking i n acetone f o r one half hour. A v i s u a l examination of the sample was made to note the p o s i t i o n of the creviced area with respect to the Luggin c a p i l l a r y . O p t i c a l photographs were taken of the creviced area using a Zeiss stereoscopic low magnification m e t a l l u r g i c a l microscope. Crevice areas were c a l c u l a t e d from measurements taken from these photographs. Various magnifications were used ranging from 9X to 50X, depending on the si z e of the corroded area (creviced area). The crevice area was used to compute the average current density and from t h i s value the average penetration rate could be cal c u l a t e d using Faraday's laws of e l e c t r o l y s i s . 22 Morphological features of crevice corrosion were examined with an ETEC Autoscan scanning electron microscope (SEM) at 20 KeV e x c i t a t i o n . This instrument was equipped with an X-ray energy dispersive spectroscopy (EDS) system for q u a l i t a t i v e chemical analysis of the c o r r o s i o n products. For the 1 M NaCl + 0.01 M Na 2S 20 3 t e s t s , the corrosion products were scraped from the crevice using a wooden pick, to avoid matrix effects during the chemical analysis. SEM photographs of the corrosion product inside the crevice were taken (1 M NaCl and 1 M NaCl + 0.01 M N a 2 S 2 0 3 t e s t s ) and EDS of these photographed regions were conducted. Following the SEM analysis the corrosion product was removed by soaking the sample for a few minutes i n an i n h i b i t e d acid solution which contained 3 ml concentrated HC1 + 4 ml of 2-butyne 1, 4 d i o l (35 pet aqueous solution) + 50 ml of d i s t i l l e d water. This inh i b i t e d acid solution was used to minimize chemical etching of the underlying matrix. Following removal of corrosion product, the crevice depth was measured using a Reichert Universal MeF metallurgical l i g h t microscope. This instrument employed a micrometer controlled focussing stage accurate to ± 1 micron along the o p t i c a l axis. By focussing on the polishing marks adjacent to the crevice, then focussing down into the deepest part of the crevice, the maximum depth of penetration could be accurately determined by the difference i n micrometer readings. From thi s value, the maximum penetration rate could be determined. 23 3. RESULTS 3.1. Introduction Table 2 summarizes the experimental data for Alloy 600 and Al l o y 625. Included i n the data are the solutions used, the test temperature i n °C, the crevice potential E m V(SCE), the test period i n hours, the i n i t i a t i o n time (Active crevice time = Test period -I n i t i a t i o n period), the average current density i n Amperes per meter 2 square (A/m ), the average c r e v i c e r a t e i n mm/yr and the maximum crevice rate i n mm/yr. 3.2. Electrochemical Measurements General behaviour of the c e l l for a closed crevice i s shown for A l l o y 600 at 80°C i n Figure 5. Two curves are shown. The upper curve Is a plot of corrosion current i n uA vs. time i n hours. The lower curve i s measured p o t e n t i a l i n mV plotted on the same time scale. For a l l of the tests conducted, where corrosion had occurred beneath the Teflon-metal crevice, an increase In current and a simultaneous decrease i n potential to more active values ( i . e . more negative p o t e n t i a l s ) , was always observed. Other researchers [3,5,6,9,22,23] have observed similar potential-current vs time behaviour i n electrochemical testing, and have shown that the onset of active d i s s o l u t i o n of the a l l o y i s characterized by an increase i n current and a simultaneous f a l l i n the measured potential to more active values. TABLE 2 Crevice Corrosion Data Summary for Alloy 600 and Alloy 625 Test t Solution (M) T CO Crevice E mV Test Period (hr.) Initiation Period (hr.) Ave i A/m2 Raye mm/yr max mm/yr 1 1 NaCl 80 -290 42.5 2.5 16.4 17.1 34.0 2 80 -267 44.5 0 12.2 12.7 30.0 3 80 -245 74.5 3.5 9.6 10.0 20.0 4 80 -205 26 0 8.7 9.1 16.0 5 80 -181 29.6 1.7 5.1 5.3 10.4 6 80 -264 . 14.6 0 8.9 9.3 30.0 7 55 -128 26.4 .4 1.74 1.8 3.4 8 55 - 92 25.8 .5 1.15 1.2 3.5 9 20 - 37 75 - No -10 20 + 10 28.2 - Corrosion -11 1 NaCl + .01 80 -305 68.3 0 16.2 17.0 15.4** 12 Na9S,0, 80 -250 3.5 .75 - - 317++ 13 80 -250 3.25 0 100 105.0 296** 14 80 -300 8.5 0.1 31 33.0 135 15 55 -300 14.33 0 4.48 4.7 34.0 16 55 -325 22.0 0 2.53 2.6 24.0 17 20 -216 27.71 0.1 6.16 6.4 49.1 18 20 -323 23.2 0 .45 .47 4.9 19 20 -300 24.9 .7 .5 .52 1.81 20 20 -315 27.5 .9 .42 .44 3.6 21 .01 Na 2S 20 3 80 + 32 72 - - - -22 1.0 Na 2S 20 3 80 - 35 45.5 - - - -23 1 NaCl + 22 -250 pH Experii lent .01 Na 2S 20 3 24 1 NaCl 80 -180 85.6 12.6 2.61 2.70 3.17+ 25 1 NaCl + 80 -316 46.5 0.9 14.6 15.3 24 + .01 Na,S,0, Alloy 625 26 1 NaCl + .01 80 -130 115 No Corrosion 27 Na 2S 20 3 95 -290 61.5 4.5 1 670 701 * * Oxidation Product on Surface. Apparent active behaviour for a period of ~ 25 minutes at -290 mV V Used 5 in-lb (.565 N-m) torque. + Heater turned off, system allowed to cool to room temperature. ++ Two deep pits on polished surface, crevice not closed. ** Creviced under glyptal coating. 25 0 10 20 30 40 Time (hours) Figure 5. Crevice corrosion current and potential vs time for Al l o y 600 i n 1 M NaCl, (80°C). Crevice closed at Time = 0. (Test #4). 26 For the Alloy 600 corrosion experiments i n thiosulphate solutions i n the absense of chloride ions (Tests #21 and 22), no sudden increase i n current- or f a l l i n potential to more active values were observed, neither was there any physical evidence on the sample surface to indicate active d i s s o l u t i o n of the a l l o y . In general, upon adding solution to the c e l l and with a l l monitors running and the crevice open, a transient positive current r i s e was observed, which f e l l to less than 1 uA within a few minutes. For the elevated temperature t e s t s , the current rose continuously u n t i l the test temperature (either 55°C or 80°C) was reached, followed by a decrease to a very low value (< 1 uA). Following a period of approximately three hours, giving time for the potential and current to a t t a i n reasonably steady values, the Teflon-metal crevice was closed (Time = 0 hours i n Figure 5). A b r i e f transient response was observed for both potential and current immediately following crevice closure, after which both the potential and current s t a b i l i z e d . The current assumed a value < 1 uA and the s t a b i l i z e d p o t e n t i a l values ranged from -50 i V . ^ to +30 mV,,-^ , for but JKJCJ most of the test s . The elapsed time between crevice closure and the onset of crevice corrosion (active di s s o l u t i o n of the a l l o y ) was recorded as the i n i t i a t i o n time. Sometimes no i n i t i a t i o n time was observed meaning 27 that active crevicing was observed immediately following crevice closure. The active crevicing period was the time following the i n i t i a t i o n of active behaviour to the termination of the t e s t , and th i s was the time upon which the maximum corrosion rate was calculated. The curves shown i n Figure 5 represent t y p i c a l experimental data common to the tests i n which active crevice corrosion was observed. 3.2.1. Po t e n t i a l Measurements The active crevice potential varied somewhat for the various tests as can be seen i n Table 2. Generally, active potentials for the 1 M NaCl s o l u t i o n t e s t s f e l l i n the range -200 mVcr,„ to -300 mVor,_,. The a c t i v e crevice potentials for the 1 M NaCl + 0.01 M Na 2S 20 3 t e s t s , subsequently referred to as thiosulphate t e s t s , were i n the -300 m Vg^£ range for the majority of the tests. For the 1 M NaCl tests , the crevice potentials became increasingly more noble, as the temperature decreased. For the thiosulphate t e s t s , the potentials at a l l temperatures were i n the -300 mV,,.,, range. For the th i o s u l p h a t e t e s t s i n the absence of chloride SCE ions, Tests #21 and #22, no corrosion was observed and the potentials s t a b i l i z e d at comparatively noble potentials. At room temperature, 20°C, the Alloy 600 did not corrode. The a l l o y assumed a stable noble potential for the duration of the test with no signs of transient active behaviour, such as fluctuations 28 i n the potential for br i e f periods i n the negative or more active d i r e c t i o n . For the Inconel Alloy 625, no corrosion occurred i n any of the solutions, except i n one test at 95°C i n which transient active behaviour was observed for a short period. The active potential assumed a minimum value of -290 mV f o r ~ 25 minutes and then rose to more noble values as repassivation occurred. In general, for the 1 M NaCl tests, the corrosion potential was dependent on temperature. The lower the temperature, the more noble the p o t e n t i a l . This i s expected because at the higher temperatures the dis s o l u t i o n k i n e t i c s are faster, r e s u l t i n g i n higher exchange current densities and a corresponding f a l l i n p o t e n t i a l . Under these circumstances, the more active potentials should result i n higher corrosion rates, which was i n fact the case as w i l l be shown i n the next section. 3.2.2. Corrosion Rates Referring to Table 2, two corrosion rate parameters are compared, R and R , both expressed i n mm/yr. r ave max R i s the average crevice corrosion rate at the end of the ave te s t . R was calculated from Faraday's Laws using the formula ave 29 iW R = =—, where W = equivalent weight (Kg), i = average current Fp 2 density (A/m ) i = I f i n a l ( t o t a l area creviced) 3 3 F = Faraday (96,500 A«sec/equiv.) and p = density = 8.7 x 10 Kg/m for A l l o y 600. A weighted value for W was based on the a l l o y ^ I 3*^" 2*^  3™^* composition and charges on the ions of Ni , Cr , Fe , Mo , Cu , 4+ S i . These values are c o n s i s t e n t w i t h the E-pH e q u i l i b r i a for the range of potentials encountered i n th i s work [27]. The value of i used to calculate the average crevice c o r r o s i o n r a t e R & v e » I s the average current density prevailing at the end of the crevice corrosion test. I t i s not the average value prevailing throughout the test because the v a r i a t i o n of crevice corrosion area with time i s unknown. I f the active crevice current ( I) remains f a i r l y constant, then i t i s probable that the active crevice a r e a (A) remains f a i r l y constant and R i s the average c r e v i c e ' ave b corrosion rate throughout the test. Because of the uncertainty i n the accuracy of measuring the average current density, and therefore the average penetration rate, a second parameter, the maximum crevice rate was a l s o calculated. This maximum rate, R , was calculated from the max maximum crevice depth measured and the active crevice time. Since both the crevice depth and the active crevice time could be accurately measured, this value i s believed to be a more r e l i a b l e means to compare propagation rates among the various t e s t s . Also, for p r a c t i c a l 30 reasons, the maximum rate i s the most relevant parameter to consider for penetration of containers and tanks. The general results for both the 1 M NaCl tests and the thiosulphate tests indicate that with increasing temperature, the crevice corrosion rate increases. At 20°C, A l l o y 600 w i l l not corrode i n a solution of 1 M NaCl. At a temperature of 55°C Alloy 600 does corrode, and increasing the temperature further to 80°C results i n an even higher crevice corrosion rate. For the Al l o y 600 thiosulphate t e s t s , i t can be seen that the corrosion rate increases with increasing temperature. Comparing the results shows that at a l l test temperatures the corrosion rates are s i g n i f i c a n t l y higher when the thiosulphate i s present (up to a factor of 10). Both average and maximum crevice rates showed t h i s trend. For the thiosulphate tests at 80°C, i t was observed that active crevice corrosion i n i t i a t e d under the Glyptal paint with very high corrosion rates (eg. Test #13). In one test (Test #12), two deep p i t s were observed on the free surface with a maximum corrosion rate exceeding 300 mm/yr before the crevice was closed. 3.2.3. I n i t i a t i o n Time As i s shown i n Table 2, the addition of thiosulphate ion reduces the i n i t i a t i o n time dramatically. Immediately upon crevice closure, corrosion would begin for test temperatures of 55°C and 8.0°C. Tests #9 and #10 at 20°C showed that i n the absence of thiosulphate 31 ion, A l l o y 600 did not corrode following crevice closure, but with the addition of thiosulphate ion to the inactive system (crevice closed) the samples began to corrode within one hour (Tests #19 and #20). These r e s u l t s suggest that the thiosulphate ion has an effect on both the i n i t i a t i o n and propagation of crevice corrosion. This i s i n contrast to results on stainless s t e e l [17] under sim i l a r experimental conditions i n which i t was concluded that the thiosulphate ion had a dramatic effect on propagation rates but l i t t l e or no effect on the i n i t i a t i o n time. 3.3. Other Crevice Corrosion Tests Two tests #21 and #22 using solutions of 0.01 M thiosulphate and 1 M thiosulphate i n the absence of chloride ion at 80°C exhibited no corrosion. I t was concluded that chloride ion had to be present for the thiosulphate ion to have any effect and that the thiosulphate ion alone was incapable of i n i t i a t i n g crevice corrosion. I t i s l i k e l y that while chloride ion has the effect of influencing the i n t e g r i t y of the passive layer, the thiosulphate ion helps to promote and accelerate the anodic d i s s o l u t i o n of the metal. Two tests #24 and #25 were conducted to show what effect reducing the temperature would have on an already a c t i v e l y corroding crevice for both the 1 M NaCl and the 1 M NaCl + 0.01 M thiosulphate solutions. These tests involved heating the solution up to 80°C, closing the crevice, then after ensuring active crevice corrosion had 32 occurred, turning the heater off and allowing the system to cool. Consistent with the previous crevice corrosion data, the sample tested i n 1 M NaCl (Test #24, Figure 6) ceased corroding when the temperature had f a l l e n to ~ 30°C. Both the current and the p o t e n t i a l f e l l continuously during cooling and upon cessation of active crevice corrosion, there was a sharp decrease i n current and a sudden increase i n potential to a more noble value. However, for the sample exposed to 1 M NaCl + 0.01 M thiosulphate, the active crevice continued to corrode right down to room temperature (Test #25, Figure 7). No repassivation occurred for this sample. Upon cooling, the potential d r i f t e d slowly i n the noble d i r e c t i o n increasing from -255 mV_,-_ to -249 m V p . , , and the current decreased from a value of approximately 16 to a steady value of 10.4 |iA over a 24 hour period. The system however, remained a c t i v e , that i s the metal continued to corrode. These results were consistent with the previous experiments (Tests #9, and #10) which showed that the Inconel 600 w i l l not corrode at room temperature i n 1 M NaCl i n the absence of thiosulphate ions. In the presence of thiosulphate ions, Inconel 600 w i l l corrode at room temperature and upon cooling from 80°C an a c t i v e l y corroding crevice w i l l not repassivate. Two tests, #26 and #27 were conducted on the A l l o y 625 at 80°C and 95°C i n 1 M NaCl + 0.01 M N a ^ O j . No corrosion was observed for the test at 80°C. For the test at 95°C, an anodic current was 33 > E c a> o < A. 3 c o 'in o o 0) o -100 -200 l—I—I—i—r J Heater off 1 1 1 1 1 1 ' V 30 50 Time (hours) 70 Figure 6. Effect of cooling the solution from 80°C to room temperature (20°C) on the crevice current and potential vs time behaviour for Alloy 600 i n 1 M NaCl. (Test #24). Arrows i n the Figure denote the time that the heater was turned o f f . Active crevice corrosion ceased (current -> 0) at Time = 71 hours and a solution temperature of - 30°C. 34 < c CO 1_ 3 o a> Q tn > E o D_ 40 32 24 16 8 0 -100 -200 -300 -400 - J - i Heater off 1 _L Heater off J l _ 10 20 30 40 Time (hours) 50 Figure 7. Effect of cooling the solution from 80°C to room temperature (22°C) on the crevice current and potential vs time behaviour for Alloy 600 i n 1 M NaCl + .01 M Na 2S 20 3. (Test #25). Arrows i n the Figure denote the time that the heater was turned o f f . The temperature had cooled to 22°C at Time = 35 hours. 35 observed for a period of approximately 25 minutes after which the sample repassivated. The average corrosion rate datum i s not considered to be r e l i a b l e since no penetration was observed. However, subsequent SEM analysis revealed an oxidation product on the surface, as shown i n Figure 8. An apparent enrichment of Fe and depletion of molybdenum, chromium and n i c k e l , as shown i n Figure 9, was revealed by EDS analysis. This indicates that 95°C may be a borderline temperature for crevice corrosion under these conditions. 3.4. pH Measurements PH measurements were conducted i n the crevice corrosion c e l l using a modified crevice assembly, (Figure 3), as described e a r l i e r . A bulb type s i l v e r - s i l v e r chloride/pH electrode was used and the pH meter (Corning Model 125) was calibrated using a standard buffer solution with a pH = 4.01 at 25°C. The most successful test was conducted at room temperature i n 1 M NaCl + 0.01 M Na 2S 20 3. The c e l l was f i l l e d with 1.25 X of 1 M NaCl solution and then the crevice assembly containing the pH electrode was lowered into place leaving the t i p of the pH electrode ~ 2 mm above the metal surface. F o l l o w i n g t h i s , 3.1 g of N a 2 S 2 0 3 was dissolved i n 10 ml of d i s t i l l e d water and then added to the c e l l . Referring to Figure 10, upon addition of the thiosulphate (T = 0), a b r i e f period of ~ 3 minutes elapsed, followed by a sudden f a l l i n pH. The i n i t i a l pH was 7 and within 2 hours, f e l l to pH = 4. The lowest pH recorded was 3.58. 36 1300X Figure 8. SEM photograph of oxidation products formed on Al l o y 625 i n 1 M NaCl + 0.01 Na 2S 20 3, (95°C). 37 10 I 0 V „ 10 3 o o o> o 10 g 1 0 10^ i o 2 k 10 S k a MoL, Matrix Corrosion product 1 Cr Cr Fe Fe Ni Ni ka k£k a kyg k a k£ 1 0 X-ray energy , keV Figure 9. X-Ray EDS spectra of the oxidation product formed on Alloy 625 i n 1 M NaCl + 0.01 M Na 2S 203, (95°C), and of the matrix. 1—I—I—I—I—I—I—I—I—I—I—I—I—f 0 2 4 6 8 10 12 Time (hours) Figure 10. Active crevice pH vs time for Alloy 600 in 1 M NaCl + 0.01 M NaoS„0„, 22°C. (Test #23). 2 2 3 LO CO 39 After several hours the pH s t a b i l i z e d at a value of 4. A check of the po t e n t i a l indicated a value of -250 mV SCE. Following the t e s t , the sample was checked to confirm that corrosion had occurred In the v i c i n i t y of the sensing bulb of the pH electrode. Since crevice corrosion did occur near the pH electrode, the measured pH i s believed to be a reasonable estimate of the upper pH range encountered i n an active crevice for t h i s system. The actual value may be lower because i t i s d i f f i c u l t to monitor the pH at the exact surface location where crevice corrosion occurs. I t i s encouraging to observe that a f a l l i n pH did occur, however, the f i n a l value obtained should be taken as an approximate value only. The intention of t h i s experiment was to v e r i f y that a lowering of pH does occur during crevice corrosion i n t h i s system. 3.5. P o l a r i z a t i o n Tests A series of p o l a r i z a t i o n tests were conducted on pure N i , A l l o y 600 and Alloy 625. The purpose of these tests was to determine the anodic disso l u t i o n behaviour of these materials i n solutions i d e n t i c a l to the crevice corrosion experiments and In solutions simulating the expected acid chloride conditions inside the a c t i v e l y corroding crevice. This allowed a comparison to be made between the two alloys and pure N i , and to correlate the p o l a r i z a t i o n behaviour to the actual crevice experiments. 40 Figure 11. Anodic polarization curves of Alloy 600, i n 1 M NaCl, 1 M NaCl + 0.01 M Na 2S 20 3, and for Pt wire i n 1 M NaCl + 0.01 M Na 2S 20 3, (20°C). 41 Figure 11 shows two anodic polarization curves for Alloy 600 i n 1 M NaCl and 1 M NaCl + 0.01 M Na^Og at 20°C and, i n addition, an anodic curve for Pt wire i n a solution of 1 M NaCl + 0.01 M Na^O.^ i s also plotted. As can be seen, the addition of thiosulphate has a dramatic effect on the polarization behaviour of the Alloy 600. The curve i s shifted to the right (as compared to Alloy 600 i n 1 M NaCl) and very high anodic current densities are observed above the corrosion potential (E ). v x corr With the addition of thiosulphate, severe p i t t i n g and c r e v i c i n g was observed on the metal surface at a potential only 100 mV above E as shown i n Figure 12. A v i s u a l examination during the cor r e test revealed that when a p i t formed on the surface, a green viscous f l u i d flowed from i t . This f l u i d would run down the surface of the v e r t i c a l l y mounted specimen and severely etch the surface. The effect was so pronounced that deep crevicing was observed whenever this f l u i d contacted the Alloy 600 surface. In contrast to t h i s , Alloy 600 i n 1 M NaCl exhibited no macroscopic p i t t i n g and no apparent crevicing along the epoxy-metal interface, up to a potential of 0.4 V o l t s ^ ^ , ^ . I t was suspected that the large increase i n current density may have been due to oxidation of the thiosulphate to higher oxidation states. The Pt wire anode was placed i n the thiosulphate solution to determine the magnitude of th i s contribution. As can be seen (Figure 11), the oxidation of the thiosulphate contributes very l i t t l e to the 42 Figure 12. Corroded surface of Alloy 600 pol a r i z a t i o n sample exposed to 1 M NaCl + 0.01 M Na 2S 20 3 at 20°C. Refer to Figure 11. 43 o v e r a l l current density. For example, comparing the Pt and Alloy 600 i n 1 M NaCl + 0.01 M Na2S20.j, there are several orders of magnitude difference i n anodic current density at a potential of zero v o l t s . Figure 13 shows the p o l a r i z a t i o n behaviour of pure n i c k e l with the addition of increasing concentrations of sulphide (Na2S) to an a c i d i c N i C l 2 s o l u t i o n adjusted to pH = 2.2. This solution roughly simulates the expected acid chloride conditions within an a c t i v e l y corroding crevice. The p o l a r i z a t i o n curves i n Figure 13 show that increasing the molar concentration of sulphide increases the anodic d i s s o l u t i o n of Ni reflected by the higher anodic current d e n s i t i e s . The potential at which the sulphide becomes unstable at pH = 2.2 and 80°C i s -147 mV„nv, (Refer to Table 3, Equation ( 2 ) i i i , page 66). Below t h i s p o t e n t i a l , the sulphide i s stable and enhances the. anodic d i s s o l u t i o n of the Ni. Figure 14 i s a p o l a r i z a t i o n diagram of pure Ni i n 0.5 N HC1 pH = 0.6 at 22°C. In these highly a c i d i c conditions, Ni exhibits a predominantly active behaviour. With the addition of sulphide (0.01 M Na2S), the curve i s s h i f t e d to more a c t i v e p o t e n t i a l s and higher anodic current densities are observed. Both Figures 13 and 14 show that the active disso l u t i o n af pure Ni i s enhanced i n the presence of Sulphide ions ( i . e . H_S , a r i s i n g from the reaction: Na_S + 2H + •* 2 Na + + H 2S). 44 03 T T T T Ni in N iC i 2 ,PH • 2.20 + 0.05 ( a ) No sulphide a d d e d (b) QOOIM Sulphide ( N a 2 S ) (c) 0.01 M Sulphide ( d ) 0.1 M Sulphide o. in o > -O.I ( a ) (b)> -0.3 0.5 1 1 1 10 10" 10° Current density (nA/cm 2) 10 8 Figure 13. Anodic p o l a r i z a t i o n curves of pure Ni i n a N i C l 2 s o l u t i o n , (pH = 2.2). E f f e c t of increasing concentrations of Na 2S. (80°C). 45 Figure 14. Anodic p o l a r i z a t i o n curves of pure Ni i n 0.5 N HC1, pH = 0.6, and i n 0.5 N HC1 + 0.01 M Na2S. (20°C). 46 Figure 15. Anodic po l a r i z a t i o n curves of Alloy 600 i n 0.5 N HC1. Effect of the addition of 0.01 M Na2S. Temp. = 20°C. 47 Figure 15 shows the effect of the addition of sulphide (0.01 M Na2S) on the anodic po l a r i z a t i o n behaviour of Al l o y 600 i n a solution of 0.5 N HC1 at 20°C. With the addition of sulphide, a peculiar behaviour j u s t above E c o r r Is observed. At higher potent i a l s , around zero v o l t s SCE, an increase i n current density i s observed i n the presence of sulphide ions. P o l a r i z a t i o n tests conducted on Alloy 625 i n solutions of 1 M NaCl and 1 M NaCl + 0.01 M Na^O-j at 20°C and 80°C are shown i n Figures 16 and 17 respectively. Figure 16 shows that the addition of thiosulphate has no appreciable effect on the anodic d i s s o l u t i o n of A l l o y 62 5. Only a s l i g h t lowering i n E c Q r r and a very small increase i n the anodic dissol u t i o n current i s observed. At 80°C (Figure 17), a more active corrosion potential i s observed with the addition of thiosulphate and the curve i s shifted to the right to higher current de n s i t i e s . However, a v i s u a l examination following the p o l a r i z a t i o n tests showed that no macroscopic p i t t i n g or crevicing had occurred up to 1.0 volts SCE for any of the tests conducted on Alloy 625. The results of the po l a r i z a t i o n tests show that the addition of sulphide w i l l enhance the anodic d i s s o l u t i o n of pure Ni and Alloy 600 i n acid chloride solutions. I t has also been shown that the presence of thiosulphate Ion i n a neutral chloride solution lowers the p i t t i n g potential and enhances the anodic d i s s o l u t i o n of Alloy 600 by several orders of magnitude. I t has been shown that Alloy 625 i s not susceptible to enhanced corrosion i n the presence of thiosulphate i n a 48 Figure 16. Anodic p o l a r i z a t i o n curves of Alloy 625 i n 1 M NaCl and 1 M NaCl + 0.01 M NaoSoOo. Temp. = 20°C. 49 i—i—i—i—i—i—r—r Alloy 625, IM NaCl Alloy 625,1 M NaCI + QOlM Na 2S 20 3 Figure 17. Anodic p o l a r i z a t i o n curves of All o y 625 i n 1 M NaCl and i n 1 M NaCl + 0.01 M N a 2 S 2 0 v (80°C). 50 neutral chloride solution. The Alloy 625 exhibited superior resistance as compared to Alloy 600 i n a l l tests conducted. 3.6 EDS Composition analyses were conducted by X-ray energy dispersive spectroscopy on the corrosion products produced i n the 1 M NaCl tests and the thiosulphate tests. Figure 18 shows an EDS spectra of one of the 1 M NaCl tests at 80°C. The figure represents log counts vs X-ray Energy. EDS of the corrosion product and of the matrix (~ 1 cm from the crevice s i t e on the polished surface) are compared. The plots are e s s e n t i a l l y i d e n t i c a l except that the corrosion product i s enriched i n Ti probably originating from Ti p r e c i p i t a t e s . Figure 19 i s an SEM photograph showing these p r e c i p i t a t e s . Figure 20 compares the composition of the T i precipitates with that of the matrix. A prominent T i peak i s present for the p r e c i p i t a t e , but i s absent for the matrix. Other EDS spectra confirmed the presence of Ti enrichment i n the corrosion product. I t i s unknown whether these precipitates are carbides, or n i t r i d e s [40], or oxides. EDS analysis on the black corrosion product obtained from the Alloy 600 thiosulphate tests indicates a prominent S peak as well as a prominent Ni peak, as shown i n Figure 21. The Cr peak i s suppressed, suggesting that the corrosion product i s predominantly a sulphide of N i . 51 I 0 4 1 1 Ti Cr Cr Fe ka kg kp ka i i ' i • Ni Ni fa »<£ i I 0 3 Matrix 1 I I I 1 . o 2 — — ID coun 10 — 1 1 i cr o . o 4 Corrosion — product tens c i o 3 1 • 2 10 in ' 1 • / / - ; • .: * * I * *. *» i X - ray energy, keV Figure 18. X-Ray EDS spectra of corrosion products formed on A l l o y 600 i n 1 M NaCl solution (80°C) compared with the Alloy 600 matrix. (Test #4). 52 1000X Figure 19. SEM photograph of Ti precipitates found on the surface of the corrosion products formed on Alloy 600 exposed to 1 M NaCl solution (80°C). (Test #4). 53 4 0 0 0 h -I 5 X-ray energy, keV Figure 20. X-Ray EDS spectra of Ti precipitates as shown i n Figure 19. 54 4000 | 3000 3 O u fc-o I 2000 c ® lOOOh-X-ray energy, keV Figure 21. X-Ray EDS spectra of black corrosion products formed on All o y 600 i n 1 M NaCl + 0.01 M Na 2S 20o (55°C). 55 3.7. Crevice Corrosion Morphology 3.7.1. Crevice Location In the majority of the 1 M NaCl t e s t s , active crevice corrosion s i t e s tended to be located at the edge of the Teflon-metal crevice. The general appearance of an active crevice s i t e i s shown i n Figure 22a. The crevice corrosion was highly l o c a l i z e d with l i t t l e tendency for a second crevice s i t e to i n i t i a t e . For the thiosulphates t e s t s , corrosion occurred most often at the edge of the Teflon-metal crevice, i n i t i a t i n g at more than one lo c a t i o n , and covered a larger area, as shown i n Figure 22b. In addition corrosion at the i n t e r i o r of the Teflon-metal crevice was observed, which was not the case i n the 1 M NaCl t e s t s . For some of the thiosulphate tests at 80°C, crevice corrosion occurred beneath the Glyptal coating, and this tended to be a problem for these te s t s . 3.7.2. Morphological Features of Crevice Corrosion The morphological development of crevice corrosion for a l l o y 600 i n 1 M NaCl i s shown i n Figures 23a, 23b and 23c. These SEM photographs were taken near the edge of t y p i c a l active crevices and are assumed to represent the morphological changes accompanying i n i t i a l 56 Figure 22. General appearance of active crevice c o r r o s i o n s i t e s formed on A l l o y 600 i n a) 1 M NaCl (80°C), b) 1 M NaCl + 0.01 M Na 2S 20 3. (55°C). 57 active crevice growth. In the early stages of crevice corrosion a p i t t i n g behaviour was observed. Newly formed p i t s tended to a l i g n i n the d i r e c t i o n of the polishing marks as shown i n Figure 23a. For aggressive active conditions which i s a c h a r a c t e r i s t i c of the higher temperatures (55°C and 80°C), these p i t s grew rapidly and eventually coalesced to form a crevice, as shown i n Figures 23b and 23c. For the less aggressive conditions, for example at lower temperatures (20°C), these p i t s developed slowly and their coalescence led to a grain etching effect as shown i n Figure 23d for Alloy 600 i n 1 M NaCl + 0.01 M N a 2 S 2 0 3 at 20°C. For these low temperature samples a broad etched area formed on the metal surface but a well developed c r e v i c e d i d not form. For the t h i o s u l p h a t e t e s t s at higher temperatures (55°C and 80°C), the morphological development of crevice corrosion was observed to be similar to the 1 M NaCl tests. An examination of the corrosion products formed on Alloy 600 i n 1 M NaCl revealed es s e n t i a l l y a dried mud appearance, as shown by Figure 24a. This corrosion product was present i n a l l of the corroded samples including the thiosulphate te s t s . Although the corrosion products for the thiosulphate tests were found to be of a d i f f e r e n t chemical composition from the 1 M NaCl experiments, the appearance of the corrosion product was similar as shown i n Figure 24b. The major difference between the crevice corrosion tests from a macroscopic point of view was that i n the thiosulphate test s , the crevices were f i l l e d 58 Figure 23. SEM photographs i l l u s t r a t i n g the development of crevice corrosion morphology of Alloy 600 i n a) 1 M NaCl (55°C), b) 1 M NaCl (55°C) advanced stages, c) 1 M NaCl (80°C), well developed crevice, d) 1 M NaCl + 0.01 M Na 2S 20 3 (20°C) early stages. 59 with a loo s e l y adherent black deposit, probably sulphide (NiS, N i . ^ ) , as has been shown by the EDS r e s u l t s . An SEM photograph of the loose black corrosion product i s shown i n Figure 25. Following the removal of the corrosion products with an inh i b i t e d acid s o l u t i o n , SEM photographs revealed an e s s e n t i a l l y electro-polished surface with preferential grain boundary etching as shown i n Figure 26a for Alloy 600 tested i n 1 M NaCl. Removal of the corrosion product from the thiosulphate tests revealed s i m i l a r morphological features, as shown i n Figure 26b. 60 b 100OX Figure 24. SEM photographs of corrosion products formed within c r e v i c e corrosions s i t e s i n a) A l l o y 600, 1 M NaCl (80°C), b) A l l o y 600, 1 M NaCl + 0.01 M NaoSoOo (55°C). 61 Fig u r e 25. SEM photograph of Voluminous b l a c k c o r r o s i o n products formed w i t h i n c r e v i c e c o r r o s i o n s i t e s , A l l o y 600, 1 M NaCl + 0.01 M N a 2 S 2 0 3 (55°C). 62 b 400X Figure 26. SEM photographs of c r e v i c e i n t e r i o r s f o l l o w i n g the removal of c o r r o s i o n products w i t h an i n h i b i t e d a c i d s o l u t i o n a) A l l o y 600 i n 1 M NaCl (80°C), b) A l l o y 600 i n 1 M NaCl + 0.01 M N a 2 S 2 0 3 (55°C). 63 4. DISCUSSION 4.1. Role of Thiosulphate Ion The corrosion rate data for a l l o y 600 shows a remarkable difference between the 1 M NaCl tests and the thiosulphate t e s t s . In a d d i t i o n to i t s a c c e l e r a t i n g e f f e c t on c o r r o s i o n r a t e s , the thiosulphate ion also had a pronounced influence on the i n i t i a t i o n times. However, chloride ions had to be present, otherwise corrosion would not occur. To understand the influence that thiosulphate ion has on the i n i t i a t i o n and propagation of crevice corrosion, i t i s necessary to know the probable stable sulphur species i n the crevice s o l u t i o n . Recent studies by Tromans and Frederick [17] on crevice corrosion of AISI 304, 316L and UHB 904L SS i n 1 M NaCl have shown that the pH and potentials within active crevices l i e i n the region where the reduction — 2 of ^O^ to H 2S i s thermodynamically favoured. They a t t r i b u t e d enhanced crevice corrosion rates to the c a t a l y t i c effect of R^S on the anodic d i s s o l u t i o n of Fe. Extending this to the Ni-based a l l o y s , the potentials and pH measured i n this study also favour the formation of H2S by the electrochemical reduction of the thiosulphate species. The following sequence of reactions show a possible reaction path by which thiosulphate ion may be reduced to H2S: 64 -2 + 6H + 4e •*• 2S + 3H„0 (2) S + 2H + 2e •»• H„S (3) 2"(aq) These equations are consistent with E-pH thermodynamics for the sulphur-water system [28]. The above reactions show that formation of elemental sulphur i s a possible intermediate reaction product, and i t may Influence the sulphur may indeed affect the d i s s o l u t i o n and passivation of N i . Marcus and Oudar [29], studied the anodic d i s s o l u t i o n behaviour of Ni doped with S, i n 0.1 N l^SO^ using electrochemical and radiochemical (S radiotracer) techniques, Auger spectroscopy and ESCA. They concluded that adsorbed sulphur i s a catalyst for the anodic d i s s o l u t i o n reaction and that the presence of a surface monolayer i s responsible for the i n h i b i t i o n of passivation. s u l p h i d e s (H^S) i n stainless s t e e l crevices [17] or p i t s [33] exposed to neutral chloride solutions containing thiosulphate ions may be due to the r e d u c t i o n of the thiosulphate species to H^S. S i m i l a r i l y , the chemical d i s s o l u t i o n of sulphide Inclusions i n crevices [30] of sta i n l e s s steels exposed to neutral chloride environments may also generate ^ S . The corrosion products formed are insoluble sulphides of N i , Cu and Mo [17,30] and the more soluble Fe sulphides. d i s s o l u t i o n behaviour. Experimental evidence has indicated that However, other researchers argue that the presence of 65 The experimental results i n th i s study have indicated that an a c t i v e p o t e n t i a l (~ -300 mV ) and a c i d (pH ~ 3.6-4.0) conditions SOL p r e v a i l i n the a c t i v e l y corroding thiosulphate te s t s . Table 3 shows the calculated reduction potentials for reactions (2) and (3) i n the presence of 0.01 M dissolved sulphur species for the three test temperatures used i n the crevice corrosion experiments. Comparison with crevice corrosion potentials (Table 2) show that both reactions are thermodynamically possible at pH = 4 or l e s s . Physical evidence i n the form of a voluminous black corrosion product within the crevice, which was found by X-ray EDS analysis to contain Ni and S peaks, confirmed that a sulphide of n i c k e l formed. These observations are consistent with E-pH diagrams for the Ni-S-H^O system [20,34,35,36] which show that the potential-pH conditions observed i n th i s study l i e i n the s t a b i l i t y region for the insoluble n i c k e l sulphide species (NiS or Ni^S2)' S i m i l a r i l y , Tromans and Frederick [17] observed a ni c k e l sulphide corrosion product that formed on 304 S.S. i n a Teflon-metal crevice experiment at 22°C i n 1 M NaCl + 0.01 M Na2S20.j. By comparing X-ray EDS spec t r a of the c o r r o s i o n product to that of NiS ( m i l l e r i t e ) they suggested that N i . ^ was the l i k e l y compound formed. The absence of FeS and the presence of small amounts of CuS and MoS was consistent 1 with t h e i r r e l a t i v e l y high s o l u b i l i t i e s i n the crevice solution. The enhanced anodic d i s s o l u t i o n of Fe and i t s alloys i n the presence of aqueous H~S at low potentials and pH's has been attributed 66 TABLE 3 Calculated Reduction Potentials for E g 0 - 2 / s a n d ES/H S E -2 ( 1 ) E s 2 o 3 2 / s E ( 2 )  hS/H 2S Temp °C pH m VSHE m VSCE m VSHE m VSCE 22 22 22 22 0 2 4 6 433 256 81 - 95 199 15 -161 -337 201 83 - 33 -150 - 41 -158 -275 -392 55 55 55 55 0 2 4 6 457 261 65 -131 215 19 -177 -373 228 98 - 32 -162 - 14 -144 -274 -404 80 80 80 80 0 2 4 6 474 264 54 -156 232 22 -188 -398 249 109 - 31 -171 7 -133 -272 -413 *> E S 2 0 3 2 / S E s 2 o - 2 / s (2) i ) E S/H2S 1 1 5 ES/H 2S 1 1 4 > ES/H 2S 0.462 - .088 pH + .0146 log [ S 2 0 ~ 2 ] , V$m at 22°C 0.490 - .098 pH + .0163 log [ S 2 0 ~ 2 ] , V g H E at 55°C 0.509 - .105 pH + .0175 log [ S 2 0 ~ 2 ] , V g H E at 80°C .142 - .0585 pH - .0293 log [H 2S], .163 - .065 pH - .0325 log [H 2S], .179 - .070 pH - .0351 log [H 2S], VSHE 3 t 2 2 ° C VSHE a t 5 5 ° C VSHE a t 8 0 C 67 to the ads o r p t i o n of the HS species and the formation of the surface species Fe(SH) [37]. S i m i l a r l y t h i s has been extended to explain the same phenomenon for active nic k e l d i s s o l u t i o n i n sulphuric acid [18,31]. The following set of reactions i l l u s t r a t e s the possible mechanism by which r^S may c a t a l y s e anodic d i s s o l u t i o n of Fe or Ni [31,37], M + H.S, . = MHS~ + H + (4) 2 (aq) ads MHS~ -»• MHS+ + 2e (5) ads MHS+ + H + •*• M 2 + + H.S, . (6) 2 (aq) where M = Fe,Ni As can be seen by the above reactions, H^ S i s regenerated and can be considered as a true c a t a l y s t . For the thiosulphate experiments, the presence of a ni c k e l sulphide corrosion product Indicated that the sulphur species i s consumed during the anodic oxidation of the a l l o y and a mechanism to account for the observed enhanced crevice corrosion propagation rates must be considered which does not involve the regeneration of H_S. 68 The following set of equations outline a possible sequence of reactions Involving aqueous r^S i n which the sulphide i s consumed; N i + H 2 S ( a q ) * N i ( H2 S>ads ( 7 ) intermediate steps N i ( H 2 S ) a d s * N i ( H 2 S ) a d s + e" ( 8 ) N i ( H 2 S ) ^ d g * N i ( H 2 S ) 2 + + e" (9) NiS + 2H + (NiS precipitates i n crevice and deposits on surface) or, Ni(H_S) +, ->• NiS + 2H + + e~ (10) ' 2 ads (NiS adheres on surface) Overall Ni + H2S •* NiS + 2H + + 2e~ (11) Comparing the two possible reaction paths, (9) and (10), either the adsorbed species may pass into solution and precipitate as a bulk corrosion product, or the adsorbed species may decompose on the surface r e s u l t i n g i n a corrosion f i l m . For the thiosulphate test s , there was a loose black corrosion product which f i l l e d the crevice. This observation suggests that a nic k e l sulphide precipitated from solution and deposited on the surface, consistent with the right hand reaction path of equation (9). This reaction, while not a true c a t a l y t i c reaction because sulphur species are consumed, may increase the k i n e t i c s of the disso l u t i o n reaction, thereby increasing crevice corrosion rates. 69 Since sulphur species are consumed i n the crevice solution there must be s u f f i c i e n t thiosulphate present i n order to maintain the proposed anodic d i s s o l u t i o n reaction. By cal c u l a t i n g the t o t a l area under the current-time curve obtained from a* crevice corrosion experiment, the t o t a l number of coulombs passed during active crevice corrosion may be determined. From this value, the t o t a l number of equivalents of sulphur consumed during active crevice corrosion may be compared to the t o t a l number of equivalents available i n the bulk s o l u t i o n . This c a l c u l a t i o n was performed for Test #14 and d e t a i l s are shown i n Appendix 1. The calculated number of sulphur equivalents consumed are several orders of magnitude less than what are available i n the bulk solution. Hence, there i s adequate thiosulphate present i n the solution to account for the t o t a l anodic charge passed during active crevice corrosion. I t should be noted that i f d i f f u s i o n i s r e s t r i c t e d from the bulk solution into the crevice solution then the reaction rate may come under d i f f u s i o n control, which could l i m i t the o v e r a l l d i s s o l u t i o n reaction r e s u l t i n g i n a continuously decreasing propagation rate as time progresses. Po l a r i z a t i o n results for Ni and Alloy 600 (Figures 13,14,15) showed that the presence of sulphide as ^ S i n acid chloride solutions, enhanced the anodic d i s s o l u t i o n reaction. For the Alloy 600 i n neutral 1 M NaCl, the addition of thiosulphate considerably reduced the passive po t e n t i a l range (Figure 11), and greatly increased the passive anodic current density. The crevice corrosion results also showed that Alloy 70 600 was not only susceptible to enhanced propagation rates i n the presence of thiosulphate, but that i n i t i a t i o n times were reduced. This would suggest that the thiosulphate ion species may, by i t s e l f , promote breakdown of the passive f i l m and enhance crevice corrosion. However, i n the absence of chloride ion, thiosulphate ion had no apparent effect and did not i n i t i a t e crevice corrosion, as shown by Tests #21 and #22. Thus, there must be a synergistic effect between chloride and thiosulphate. The present study indicates that the presence of chloride ion i s essential for the i n i t i a t i o n of crevice corrosion i n Inconel 600. I t affects not only the i n t e g r i t y of the passive f i l m , but also effects changes i n the pH of the crevice e l e c t r o l y t e . According to the adsorbed ion displacement model for p a s s i v i t y breakdown [38], the role of C l i n the i n i t i a t i o n s t a g e i s to compete wit h OH ions f o r adsorption s i t e s on the metal l a t t i c e . Once chloride i s adsorbed onto the metal l a t t i c e , i t leaves the metal atom "open" to further attack. Sometimes i t i s possible for the oxide layer to repair i t s e l f , which may occur, for example, i f the crevice conditions are not severe enough to promote active behaviour. I t i s a modern concept that the most important process c o n t r o l l i n g the resistance of a material to p i t t i n g may be the repair of the passive oxide f i l m , rather than i t s a b i l i t y to r e s i s t breakdown [33]. When breakdown occurs, the repassivation rate can play an important role i n determining whether passivity can be restored or whether l o c a l i z e d attack such as crevice corrosion can be i n i t i a t e d [42]. 71 The role of thiosulphate on the i n i t i a t i o n of crevice corrosion may be to impair the i n t e g r i t y of the passive oxide f i l m (or retard the repassivation rate i n the dynamic f i l m breakdown-repair -2 p r o c e s s ) so t h a t i n the presence of S^O^ and C l i o n , c r e v i c e c o r r o s i o n may occur at a lower temperature than i n the presence of Cl ion alone. This i s supported by the p o l a r i z a t i o n r e s u l t s as shown by Fig u r e 11 f o r A l l o y 600 i n 1 M NaCl + 0.01 M N a ^ O j . The p i t t i n g p o t e n t i a l i s reduced considerably and the passive anodic current density i s increased as compared to Alloy 600 i n 1 M NaCl. S i m i l a r l y , the crevice corrosion results for Alloy 600 i n 1 M NaCl thiosulphate ion show that thiosulphate must be present i n order for crevice corrosion to i n i t i a t e at room temperature. In the absence of thiosulphate, crevice corrosion did not i n i t i a t e . In addition, the presence of thiosulphate reduces i n i t i a t i o n times at the higher temperatures as compared to Alloy 600 i n 1 M NaCl. Thus, the experimental evidence shows that the effect of the thiosulphate ion i n the presence of C l ion, i s to impart a high degree of i n s t a b i l i t y to the passive oxide f i l m , r e s u l t i n g i n shorter i n i t i a t i o n times for a given temperature, and lowering the temperature at which crevice corrosion w i l l i n i t i a t e . 4.2. Role of Molybdenum The preceeding discussion has shown that the s t a b i l i t y of the oxide f i l m i s an important factor i n determining whether crevice corrosion w i l l i n i t i a t e or not. Therefore, i t i s reasonable to assume 72 that any a l l o y i n g elements which may improve the i n t e g r i t y of the passive oxide f i l m , (eg. by increasing resistance to breakdown or by improving i t s a b i l i t y to repair i t s e l f ) , may negate the detrimental e f f e c t of C l ions and thiosulphate ions on the i n i t i a t i o n of crevice corrosion. Evidence for this i s reflected i n the studies on Alloy 625. The p o l a r i z a t i o n results (Figures 16 and 17) and the crevice corrosion r e s u l t s showed that A l l o y 625 was immune i n 1 M Sodium Chloride so l u t i o n , and the addition of thiosulphate ion had no appreciable ef f e c t on p o l a r i z a t i o n behaviour or crevice corrosion behaviour. The major differences between Alloy 600 and Alloy 625 are the molybdenum and chromium contents (Table 1). Alloy 600 i s not alloyed with molybdenum as i s Alloy 625. Alloy 625 also has a higher Cr content as compared to Alloy 600. Several studies have been conducted on the passivation behaviour of stainless steels alloyed with molybdenum In neutral and acid chloride environments using a v a r i e t y of electrochemical and surface analysis techniques [39,43,44]. These studies have shown that Mo has no effect on the passive f i l m composition or thickness. Instead, i t has been proposed that molybdenum dissolves from the metal -2 su b s t r a t e and i s oxidized to molybdate ions (MoO^ ) i n solution. The molybdate ion then adsorbs on the metal surface and acts as an anodic i n h i b i t o r . Consequently, molybdenum promotes the formation of a stable passive f i l m at defects, thereby accelerating repassivation. 73 For the Alloy 625 corrosion experiment i n thiosulphate at 95°C, Test #27, the active potential assumed a transient value of -290 mV c„„ f o r ~ 25 minutes. The low potential conditions inside an active SCE -2 crevice do not favour solely the formation of the MoO^ ion, according to E-pH equilibrium diagrams for the Mo-H20 system at 25°C [27]. There i s also an Mo02 region of s t a b i l i t y which i s passive and a region where 3+ Mo i s s o l u b l e In solu t i o n , depending on the pH. Unfortunately, the E-pH e q u i l i b r i u m i s unknown at 95°C f o r the Mo-H20 system i n the presence of chloride ions, so that a direct comparison to the crevice corrosion experiments can not be made. In addition, i t i s also unknown what pH may exist under these conditions. However, the crevice corrosion experiment did show active behaviour for approximately 25 minutes. I t i s possible that i t took some time for the Mo to dissolve -2 i n t o s o l u t i o n , o x i d i z e to MoO^ , and then act as an i n h i b i t o r r e s u l t i n g i n repassivation. However, i t can not be ruled out that Mo02 may have played a role i n the repassivation process. Since only l i m i t e d experimental data was obtained a d e f i n i t e statement regarding the r o l e of Mo i n passivation behaviour can not be made for t h i s system and requires further investigation. 74 5. SUMMARY Crevice corrosion rates for Alloy 600 In solutions of 1 M NaCl and 1 M NaCl + 0.01 M ^282 0 3 increase with increasing temperature. Crevice corrosion i n i t i a t i o n times are reduced and crevice corrosion propagation rates increase with the addition of 0.01 M N a2 S2°3 t o 1 M N a C 1 f o r AHoy 600 as compared to Alloy 600 i n 1 M NaCl solutions. A l l o y 600 did not c r e v i c e corrode i n solutions of 0.01 M Na2S203 and 1 M ^2820^. For crevice corrosion to occur, chloride ions had to be present. For A l l o y 600 i n 1 M NaCl + 0.01 M N a 2 S 2 0 3 , c r e v i c e c o r r o s i o n results i n a black corrosion product, i d e n t i f i e d as a sulphide of n i c k e l . The crevice corrosion behaviour of Alloy 625 i s superior to Alloy 600 i n 1 M NaCl and 1 M NaCl + 0.01 M Na 2S 20 3 solutions. C r e v i c e corrosion rates for Alloy 600 i n 1 M NaCl + 0.01 M Na 2S 20 3 are enhanced due to the formation of a reduced sulphur species, probably H^S, i n the active crevice s o l u t i o n . HL^ S adsorbs onto the metal surface and enhances the d i s s o l u t i o n k i n e t i c s . The superior resistance to crevice corrosion for Alloy 625 i n 1 M NaCl + 0.01 M Na 2S 20 3 i s probably due to the s t a b i l i z i n g effect of alloyed molybdenum on passive f i l m formation and, i n the active state, the a b i l i t y of Mo to promote the repassivation of an ac t i v e l y corroding surface. 75 BIBLIOGRAPHY 1. W.D. France J r . , ASTM STP 516, 1972, p. 164-200. 2. D.B. Anderson, ASTM STP 576, 1976, p. 231-242. 3. I.L. Rosenfield, Localized Corrosion, NACE-3, 1974, p. 373-398. 4. S. Bernhardsson et a l . , ICMC (8th) 1981, Vol. 1, p. 193-198. 5. I. Matsushima et a l . , 7th ICMC (Rio de Janeiro) B r a s i l , 1978, p. 506-516. 6. F.P. U s s e l i n g , Br. Corrosion J . , V o l . 15, (2), 1980, p. 51-69. 7. J.W. O l d f i e l d and W.H. Sutton, Br. Corros. J . , Vol. 13, (1), 1978, p. 13-122. 8. R.M. Kain, Corrosion, Vol. 40, (6), 1984, p. 313-321. 9. D. Tromans and L. Frederick, Corrosion, Vol. 39, (8), 1983, p. 305-312. 10. A.I. Asphahani, Materials Performance, 1980, (8), p. 9-21. 11. M. Traisnel et a l . , 8th ICMC, 1981, Vol. 1, p. 355-360. 12. E.A. L i z l o v s , ASTM STP 516, 1972, p. 201-209. 13. M.B. Rockel, Corrosion, Vol. 29, (10), 1973, p. 393-396. 14. R.P. Jackson and D. van Rooyen, ASTM STP 516, 1972, p. 210-221. 15. H.H. Uhlig, Corrosion and Corrosion Control, 2nd Ed., 1971, p. 75. 16. A. Turnball, Corrosion Science, V o l . 23, (8), p. 833-870. 17. D. Tromans and L. Frederick, Corrosion, Vol. 40, (12), 1984, p. 633-639. 18. P. Sury, Corrosion Science, Vol. 16, 1976, p. 879-900. 19. R. Bandy et a l . , Corrosion Science, Vol. 23, No. 9, p. 995. 76 20. R.C. Newman et a l . , Met. Trans. A., Vol. 13A, (11), 1982, p. 2015-2026. 21. J.D. Rushton and L.L. Edwards, Corrosion Problems of Alloys to Bleach Plant Environment, 1980, p. 55-73. 22. D.A. Jones and N.D. Greene, Corrosion, Vol. 25, (9), 1969, p. 367-370. 23. T.S. Lee, ASTM STP 727, 1981, p. 43-68. 24. S. Henrickson and M. Asberg, 7th Scandinavian Corrosion Congress Trondheim, 1975, p. 200. 25. L.H. Lali b e r t e and W.A. Mueller, Proc. Symp. Electrochemical Techniques for Corrosion, (NACE), 1976, p. 11-17. 26. N.D. Greene et a l . , Technical Note, Corrosion, Vol. 21, (9), 1965, p. 275-276. 27. M. Pourbaix, Atlas of Electrochemical E q u i l i b r i a i n Aqueous Solutions, NACE, Houston, Texas, 1974. 28. R.J. Biernat and R.G Robins, Electrochimica Acta, 1969, Vol. 4, p. 809-820. 29. P. Marcus and J . Oudar, 8th ICMC, Vol. 1, 1981, p. 106-107. 30. S. ZaKipour, C. Leygraf, 8th ICMC, Vol. 1, 1981, p. 181-186. 31. M. Kesten and H.G. F e l l e r , Electrochimica Acta, 1971, Vol. 16, p. 763-778. 32. J . Horvath and M. Novak, Corrosion Science, 1964, V o l . 4, p. 159-178. 33. R.C. Newman et a l . , Corrosion, Vol. 38, No. 5, 1982, p. 261-265. 34. E. Peters, Met. Trans. B, Vol. 713, December 1976, p. 505-517. 35. J . Horrath and M. Novak, Corrosion Science, V o l . 4, 1964, p. 159-178. 77 36. D.D. MacDonald and B.C. Syrett, Corrosion, V o l . 35, No. 10, 1979, p. 471-475. 37. Z.A. Iofa et a l . , Electrochimica Acta, V o l . 9, 1964, p. 1645-1653. 38. Jerome Kruger, Passivity and i t s Breakdown on Iron and Iron Base A l l o y s , USA-JAPAN Seminar, NACE, 1976, p. 91-98. 39. H. Ogawa et a l . , Corrosion, Vol. 34, No. 2, Feb., 1978, p. 52-60. 40. William F. Smith, Structure and Properties of Engineering A l l o y s , McGraw-Hill Series i n Materials Science and Engineering, 1981, p. 469. 41. Akitsugu Okuwaki et a l . , Met. Trans. B, Vol. 15B, (12), 1984, p. 609-615. 42. A.G. Revesz and J . Kruger, Proc. of the 7th Int. Cong, on Met. Corrosion, Rio de Janeiro, B r a s i l , 1978, Vol. 1, p. 331-340. 43. K. Sugimoto and Y. Sawada, Corrosion, Vol. 32, No. 9, 1976, p. 347-352. 44. T. Jossic and J . Talbot, 8th ICMC, Vol. 1, 1981 p. 455-459. 45. W. Yang and A. Pourabaix, 8th ICMC, Vol. 1, 1981, p. 172-178. 78 APPENDIX 1 Calculation to determine the number of equivalents of sulphide consumed during the anodic d i s s o l u t i o n of Al l o y 600. Considering Ni d i s s o l u t i o n only. For experiment #14, the t o t a l number of coulombs passed «= uA-hr x 3600 s/hr. x 10~ 6 A/uA = (42)(5) x 3600 x 10~ 6 = 0.756 coulombs = Area under the current-time curve Current (Amperes) Time (sec) Thus, number of equivalents of Ni consumed = n, PlZ^ coulombs— ' ^ 96,500 coulombs/equiv. = 7.8 x 10~ 6 equiv. One equivalent of Ni w i l l consume one equivalent of H2S (equation 11). 79 Number of equivalents r^S consumed = 7.8 x 10 ^ 2-Number of equivalents of S available v ia reduction of $2®3 t o H 2 ^ .01 moles n o c o 2 equiv. _ 0 c / m ~ 2 \ = x 1.25 I x ~S = 2.5(10 ) equiv. S. mole n The t o t a l number of sulphide equivalents available far exceed the t o t a l number of equivalents consumed during the anodic d i s s o l u t i o n reaction. 

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