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Caustic cracking susceptibility of SAE A 516 Gr. 70 steel in alkaline sulphide solutions Ramakrishna, Shankar 1984

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CAUSTIC CRACKING SUSCEPTIBILITY OF SAE A 516 Gr. 70 STEEL IH ALKALINE SULPHIDE SOLUTIONS by @ SHANKAR RAMAKRISHNA B. Tech., Banaras Hindu University, 1981 A THESIS IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF METALLURGICAL ENGINEERING We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA January 1984 ® Shankar Ramakrishna, 1984 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and study. I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the head o f my department o r by h i s o r her r e p r e s e n t a t i v e s . I t i s understood t h a t 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 g a i n s h a l l not be allowed without my w r i t t e n p e r m i s s i o n . Department o f / V y l ^^M^r^cs>J C^^-^^s^^^y The U n i v e r s i t y o f B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date / ° - ( - t^&g. A B S T R A C T The stress corrosion cracking (SCC) of A516 Gr. 70 steel was investigated in three solution composed of 3.35 m NaOH, 2.5 m NaOH + 0.42 m Na2S, and 3.35 m NaOH + 0.02 m Na2S. The electrochemical potential for maximum susceptibility was assessed by the slow strain rate testing technique (SSRT), and was found to reside in the active passive transition zone in each solution (-1 Vggg in the 3.35 m NaOH solution, and -0.88 Vg^g in the solution containing sulphide ions). Some secondary cracking was visible at potentials corresponding to the passive zone in the 3.35 m NaOH + 0.42 m Na2S solution, indicating that the material was mildly susceptible to stress corrosion cracking at anodic protection potentials. Since most industrial failures have occurred, in the vici n i t y of welds, a series of tests with a weld incorporated in SSRT specimen was conducted to ascertain whether or not changes in microstructure affected stress corrosion susceptibility. The fusion zone of a single pass weld was found to be most susceptible to cracking in alkaline sulphide solutions, at potentials corresponding to the active-passive transition. The fracture mechanics technique, u t i l i z i n g fatigue precracked to study the effects of stress intensity, electrochemical potentials, microstructure, and heat treatment, on crack propogation rates in the 3.35 m NaOH + 0.42 m Na2S solutions. Both stress intensity dependent (regions I and III) and stress intensity independent (region II) - i i i -cracking propogation behavior was observed. Region II crack velocities of the order of 4 x 10~ 1 0 m/sec were observed at -0.88 VgcE» a n a" 2 x 10~*° m/sec at -0.75 VgQg. No significant change in region II crack velocity was observed when the base material was subjected to a simulated stress r e l i e f anneal (650°C for 1 hr.) and tested at -0.88 VgQE. The region II crack velocity through a material with a dendritic microstructure (fusion zone of a weld) was found to be approximately 1 x 10~*° m/sec. A mechanism for failure due to coalescence of cracks and not due to the penetration of a single crack through the wall, has been suggested. The applicability of anodic protection in prolonging the service l i f e of digesters has been examined. Although no experiments were conducted to determine the mechanism of crack propogation, hydrogen embrittlement has been ruled out as a possible mechanism contributing to failure. The results obtained are expected to find applications in the pulp and paper industry. - i v -TABLE OF CONTENTS Page ABSTRACT i i TABLE OF CONTENTS i v LIST OF TABLES v i LIST OF FIGURES v i i LIST OF SYMBOLS AND ABBREVIATIONS x ACKNOWLEDGEMENT x i i CHAPTER 1 - INTRODUCTION 1 1.1 Stress Corrosion Cracking .. 1 1.2 O r i g i n of the Present Work 2 1.3 Theories of Stress Corrosion Cracking 5 1.4 Structure and Properties of Welds 10 1.5 E f f e c t of Heat Treatment on SCC S u s c e p t i b i l i t y . . . . 14 1.6 SCC Testing Techniques 14 CHAPTER 2 - EXPERIMENTAL 21 2.1 Scope of the present work 21 2.2 Specimen 23 2.3 Equipment and Apparatus 26 2.4. Experimental Procedures.... 29 2.4.1 Show S t r a i n Rate Test Experiments 29 2.4.2 Fracture Mechanics Experiments 33 2.4.3 Arodic P o l o r i z a i o n curves 35 CHAPTER 3 - RESULTS 37 3.1 Anodic P o l a r i z a t i o n Tests 37 3.2 Slow S t r a i n Rate Testing Technique 41 3.2.1. 2.5m NaOH + 0.42 m NallS. 41 3.2.2 3.35m NaOH 43 3.2.3 3.35 m NaOH + 0.42 m Na 2S 43 3.2.3.1. Base M a t e r i a l 43 3.2.3.2. Welded Samples 47 - v -Page 3.3 Fracture Mechanics Results 55 3.3.1 General comments 55 3.3.2 Effect of Stress Intensity 56 3.3.3 Effect of Potential 56 3.3.4 Effect of Microstructure 59 3.3.5 Effect of Stress-Relief Anneal 60 3.4 Fractography 60 3.4.1 General Comments 60 3.4.2 Effect of Stress Intensity and Potential... 62 3.4.3 Effect of Stress Relief Anneal 74 3.4.4 Effect of Microstructure 74 CHAPTER 4 - DISCUSSION 79 4.1 Slow Strain Rate Testing 79 4.2 Cracking in Digesters 80 4.3 Analysis of Partial Surface Cracks 81 4.3.1 General Coments 81 4.3.2 Leak before Catostrophic failure 82 4.3.3 Catastrophic failure before leak 84 4.4 Effect of residual stresses 86 4.5 Anodic Protection of digesters 87 CHAPTER 5 - CONCLUSIONS 89 REFERENCES 91 - v i -LIST OF TABLES page Table 1. Yield Strength and Chemical Composition of steels 22 2. SSRT Test Results in 3.35m NaOH 44 3. SSRT Test results in semulated White Liquor 48 4. Secondary Cracking in the Base Material in the 3.35m NaOH + .42m Na£S environment 49 5. Secondary Cracking in the Welded Specimen in the 3.35m NaOH + 0.42M Na2S environment 54 6. C r i t i c a l coalsced crack data 85 - v i i -LIST OF FIGURES Page Figure 1 A typical continuous Kraft digester 3 Figure 2 Location of different welds in a continuous digester 11 Figure 3 Stress-strain curves for specimen tested in SCC susceptible and inert environments using SSRT, where A Q and A g c c are the areas under the respective curves 17 Figure 4 Schematic of a typical log V - Kj curve, showing regions I, II and III 20 Figure 5 SSRT specimen geometry 24 Figure 6 DCB specimen geometry 25 Figure 7 DCB specimen incorporating the fusion zone of a weld in the crack plane 27 Figure 8 SSRT test c e l l 29 Figure 9 Calibration curve for DCB specimen 31 Figure 10 Fracture mechanics test c e l l 34 Figure 11 Anodic polarization test c e l l . . . . . 36 Figure 12 Anodic polarization curve of A516 Gr 70 steel in 2.5 m NaOH + 0.42 m Na2S at 92°C 38 Figure 13 Anodic polarization curves of A516 Gr 70 steel in 3.35 m NaOH and 3.35 m NaOH +0.42 m Na2S solutions at 92°C 39 Figure 14 Anodic polarization curves of A516 Gr 70 steel in 3.35 m NaOH at 92°C with varying Fe"1-1" ion concentrations 40 Figure 15 Effect of potential upon reduction in area in 2.5 m NaOH + 0.42 m Na2S superimposed upon the anodic polarization curve obtained in the same solution 42 Figure 16 Effect of potential upon reduction in area for 3.35 m NaOH superimposed upon the anodic polarization curve obtained in the same solution 45 - v i i i -Page Figure 17 Secondary cracking at -0.98 Vg^g in a 3.35mNaOH environment at 92°C 46 Figure 18 Effect of potential upon reduction in area in 3.35 m NaOH + 0.42 m Na2S superimposed upon the anodic polarization curve obtained in the same solution 50 Figure 19 Secondary cracking at -0.88 V S C E in a 3.35m NaOH + 0.42 m Na2S environment 51 Figure 20 Secondary cracking at -0.750 VgQg in a 3.35m NaOH + 0.42 m Na2S environment 52 Figure 21 Oxide scale on a SSRT specimen at -0.88 Vg C E in a 3.35 m NaOH + 0.42 m Na2S environment 53 Figure 22 Effect of stress intensity on crack growth in a 3.35 m NaOH + 0.42 m Na2S environment at - ° ' 8 8 V S C E 5 7 Figure 23 Effect of potential on crack growth in a 3.35 m NaOH + 0.42 m Na2S environment 58 Figure 24 Effect of microstructure and heat-treatment on crack propagation in a 3.35 m NaOH + 0.42m Na2S environment 61 Figure 25 Fatigue surface, on the DCB specimen, showing evidence of etching 63 Figure 26 B r i t t l e overload failure zone, showing a transgranular mode of failure 64 Figure 27 Variation of fractography with stress intensity in a 3.35 m NaOH + 0.42 m Na2S environment at -0.750 VgQE 65 Figure 28 Stereographic photographs of crack front at 40 MPa/m, -0.75 V.„_ 67 Figure 29 Transgranular secondary cracks at 40 MPa/m, " ° - 7 5 V S C E 6 8 Figure 30 Variation of fractography with stress intensity in a 3.35 m NaOH + 0.42 m Na2S environment at -0.88 Vg C E 69 - i x -Page Figure 31 Intergranular crack path at 40 MPa/m, -0.88 Figure 32 Stereographic photographs of crack front at 40 MPa/m, -0.88 V___, 72 Figure 33 Intergranular secondary cracks at 40 MPa/a, - ° ' 8 8 V S C E 7 3 Figure 34 SCC crack front i n the heat treated specimen at 40 MPa/m, -0.88 V„„_, 75 ' SCE Figure 35 Intergranular secondary cracks i n heattreated specimen at 40 MPa/m, -0.88 V___ 76 SCE Figure 36 Fractograph of DCB specimen incorporating a weld, i n d i c a t i n g the crack front and the overload region 77 Figure 37 SCC crack front i n the DCB specimen incorporating a weld 78 Figure 38 Proposed mode of propogation of flaws i n the digester w a l l 83 - x -LIST OF SYMBOLS AND ABBREVIATIONS SYMBOLS a Crack length B Thickness of fracture mechanics specimen E P o t e n t i a l H Fracture Mechanics specimen height Kj Stress i n t e n s i t y factor for mode I opening Kj. C r i t i c a l stress i n t e n s i t y factor KISCC Treshold SCC stress i n t e n s i t y f a c t o r P Load V Crack v e l o c i t y ^SCE P o t e n t i a l with respect to a standard saturated calomal electrode W Fracture mechanics specimen length Applied stress Y i e l d stress ys ABBREVIATIONS COD Crack opening displacement gauge DCD Double contilever Beam frac t u r e mechanics specimens FCP Free Corrosion p o t e n t i a l HAZ Heat aff e c t e d zone. LEFM Linear E l a s t i c Fracture Mechanics - x i -MIG Metal Inert gas welding PTFE Poly Tetra f l u o ethylene (PTFE) SAW Submerged Are Welding SCC Stress Corrosion Cracking SEM Scanning metal are welding SMAW Shielded metal are welding SSRT Slow s t r a i n rate t e s t i n g technique - x i i ACKNOWLEDGEMENT I would l i k e to express my gratitude to my supervisors, Dr. D. Tromans, and Dr. E.B. Howbolt, f o r t h e i r guidance and counsel throughout the course of t h i s project. Special thanks go to D. Crowe and R. Sriram f o r the help they gave during the course of my experiments. I appreciate the work done for me by H. Tump, M. McLeod, E. Klassen, M. Mager and many others of t h i s department. I would also l i k e to thank a l l my fellow graduate students f o r making my stay here both enjoyable and worthwhile. Assistance provided by B.C. Hydro i n radiographing the welds i s g r a t e f u l l y acknowledged. F i n a n c i a l assistance from the National Science and Engineering Research Council, and Macmillan Bloedel Ltd. i s g r a t e f u l l y appreciated. - x i i i -It may not seem, therefore, that we may a n t i c i p a t e f a i l u r e of t h i s type occurring i n unsuspected places, so the I n d u s t r i a l implications of t h i s phenomenon do not loom large. F a i l u r e s are rare and w i l l become more so as understanding spreads...." 1 Discussion of Paper by Hodge, J.C. and M i l l e r , J.L., Trans of ASM, 28 (1940). - 1 -CHAPTER 1 INTRODUCTION 1.1 Stress Corrosion Cracking Stress corrosion cracking (SCC) could be defined as the penetration and cracking of a metal under the conjoint action of a t e n s i l e stress and a corrosive environment, at a rate i n excess of that produced by e i t h e r factor a c t i n g s i n g l y . One wishes that Brooks' forecast appearing i n the frontpiece of th i s thesis had been cor r e c t , for i n d u s t r i a l surveys show that SCC i s reponsible f o r approximately 33% of the f a i l u r e s i n the chemical industry,-* and continues to occur at unsuspected l o c a t i o n s , a fact that was brought home by the recent catastrophic f a i l u r e of a pulp and paper m i l l digester at Pine H i l l , Alabama,^ which managed to shake the industry out of i t ' s present state of complacence. SCC i s p a r t i c u l a r l y dangerous because i t i s not r e a d i l y detectable, and the f i r s t i n k l i n g that a stress corrosion problem e x i s t s i s often the catastrophic f a i l u r e of a piece of equipment. In addition to the p r o h i b i t i v e cost of repair and/or replacement of a valuable piece of equipment, and the attendant loss of production, the r i s k of i n j u r y to the operating personnel, makes the phenomenon p a r t i c u l a r l y d i s t a s t e f u l . SCC was once associated with s p e c i f i c metal-environment systems l i k e brass i n ammoniacal sol u t i o n s , a u s t e n i t i c s t a i n l e s s s t e e l i n brine, high strength s t e e l i n moist H2S environments, and environments as - 2 -innocuous as d i s t i l l e d water. It is now recognised that the phenomenon is more widespread and no single theory can account for a l l occurrences of SCC. A large number of variables could interact synergistically to affect the stress-corrosion reaction. In addition to specific metal-environment combinations, the microstructure, the metallurgical history of the metal in solution, presence of damaging ions in solution, nature of the oxide film formed on the metal, nature of the kinetics of the competing electrode reactions, and the presence of residual stresses on the metal, could affect the stress corrosion behaviour. Furthermore, the stress, and the environment at the tip of a stress corrosion crack could differ significantly from the bulk conditions frequently used to characterize SCC. 1.2 Origin of the Present Work Several industries deal with highly alkaline solutions on a regular basis. One of these i s the pulp and paper industry. In the Kraft process, which i s the most widely used pulping process, and where SCC has become a serious problem, a solution primarily containing 1-4 m NaOH + 0.2 - 0.6 M Na2S, termed as white liquor, i s used to dissolve lignin from the wood chips, leaving behind cellulose fibres, or pulp. The chips and white liquor came into contact with each other in large pressure vessels termed digesters (currently made of A516 Gr. 70 steel plates welded together, and illustrated in Fig. 1), where they encounter temperatures between 100 - 165°C, and pressures of 687 - 1,200 kPa. - 3 CHIP LINE IMPREGNATION ZONE HEATING ZONE DIGESTION ZT)NE WASHING ZONE BLOW LINE Figure 1 - A t y p i c a l continuous Kraft digester. - 4 -When dige s t i o n i s complete, the spent l i q u o r containing l i g n i n and other soluble residue from the wood chips and oxidized sulphur species, i s separated from the pulp. The spent l i q u o r , or 'black l i q u o r ' , as i t i s now c a l l e d , i s subjected to a serie s of rejuvenating operations to restore i t s o r i g i n a l NaOH and S ion concentrations, and remove impurities from sol u t i o n . Besides NaOH and Na2S, white l i q u o r also contains impurities l i k e Na2S203, Na2S0it, Na2C03, NaCl and other organic impurities which cannot be removed by the rejuvenating process.^. Most SCC problems i n the process were encountered i n the digester. A digester i s e s s e n t i a l l y a c y l i n d r i c a l pressure v e s s e l , with an i n t e r n a l cone at the top. During September 1980, i n an accident i n v o l v i n g a digester at Pine H i l l , Alabama, the en t i r e 18 f t base diameter upper cone, welded to the c y l i n d r i c a l base was blown away. The Q f a i l u r e was at t r i b u t e d to intergranular Stress Corrosion Cracking. The problem encountered i n the Pine H i l l , Alabama, digester was not unique, but served to focus attention on the problem. S i g n i f i c a n t s h e l l corrosion was experienced i n a continuous digester at a m i l l i n Vancouver Island, as far back as 1967. In 1977, cracking of the t r a n s i t i o n zone welds of a continous digester i n Mackenzie, B.C. was 9 a c c i d e n t a l l y discovered when the l e v e l i n d i c a t o r was being i n s t a l l e d . Severe cracking was encountered when a soda process ( u t i l i z i n g only NaOH for pulping) continuous digester at Pennsylvania, i n 1978. This was subsequently controlled by i n s t a l l i n g at anodic protection s y s t e m . 9 ' 1 0 . In 1980, the f i l l e t welds attaching the centre-pipe support plates to the s h e l l In a digester at Grande P r a r i e , A l b e r t a , were found to be cracked. - 5 -Continuous neglect of this problem culminated in the accident at Pine H i l l . Subsequent careful investigations conducted by the TAPPI continuous Digester cracking task force found that 65% of digesters and impregnation vessels contained some cracking.^ Several a u t h o r s h a v e suggested that the active-passive range of potentials, as defined by the anodic polarization curve, was the region where the metal was most prone to SCC i n , alkaline environments. The present work was undertaken to determine whether or not (i) welded structures are more prone to SCC; and ( i i ) anodic protection would be a useful tool in controlling SCC in continuous digesters. 1.3 Theories of Stress Corrosion Cracking No unified theory of SCC exists that can explain a l l occurrances of cracking. A number of 'simplistic' models have been advanced to explain specific situations. However, there are a number of features common to a l l SCC problems.^ (i ) The existance of a tensile stress, either applied, or present as a residual stress. (Ii) The alloy usually shows good resistance to general corrosive attack by the environment causing cracking, ( i i i ) The microscopic appearance of stress corrosion fracture in b r i t t l e , although the metal i t s e l f may exhibit significant d u c t i l i t y in a mechanical test. - 6 -( i v ) In order for SCC to occur, a threshold stress must be exceeded. (v) There e x i s t s a p o t e n t i a l , or range of p o t e n t i a l s where the metal i s most prone to SCC. Three d i s t i n c t schools of thought e x i s t among researchers attempting to model SCC. The electrochemically based models require that the sides of a crack become passivated, whereas the material at the crack t i p a c t i v e l y d i s s o l v e s . Vermilyea and D i e g l e ^ * ^ and S t a c h l e ^ consider that d i s s o l u t i o n at the crack t i p occurs because of the l o c a l i z e d rupture of the passive f i l m . Thus, the development of a crack i s c o n t r o l l e d by a balance between the rate of passivation, and the rate of d i s s o l u t i o n at the crack t i p . In contrast to t h i s view, | O I Q O f ) 91 S c u l l y , Parkins, " and Hoar and Jones''1 consider the material at the crack t i p to be constantly a c t i v e . The absence of passivation can be explained either by assuming the existance of regions that are d i f f i c u l t to passivate or by supposing that a material under p l a s t i c deformation i s more d i f f i c u l t to passivate than the surrounding non-deforming material. In a l l cases, the s p e c i f i c e f f e c t of agressive ions i s explained on the basis of t h e i r influence on the rate of repassivation. 9?—OL In the SCC theory proposed by Uhl i g , * ^ the formation of cracks i s considered to be a phenomenon of mechanical f r a c t u r e , rather than a corrosive problem. The theory assumes cracking to be caused by the adsorption of the SCC provoking anions onto those regions which are i n a state of s u b - c r i t i c a l tension. By t h i s adsorption, the surface - 7 -energy i s lowered so g r e a t l y that the material w i l l f racture at a stress l e v e l below the nominal y i e l d l i m i t . This theory i s presumably applicable only i n a small number of SCC systems. However, Scully 1®* 1 9 has pointed out that adsorption of anions can be regarded as important i n SCC by v i r t u e of t h e i r influence on the rate of electrochemical d i s s o l u t i o n . The Hydrogen-embrittlement mechanisms, the t h i r d group of model used to explain SCC, has been considered by some workers to be a d i f f e r e n t phenomena from SCC. However, since cathodic reactions (which could produce hydrogen under the proper conditions) are as much a part of the cracking processes as anodic reactions, i t would be inappropriate to exclude hydrogen embrittlement from a discussion of SCC. The hydrogen embrittlement model postulates that fracture r e s u l t s from the production of a b r i t t l e region at the crack t i p because of the in t r o d u c t i o n of hydrogen i n t o the a l l o y , v i a cathodic processes. The production of b r i t t l e regions has been at t r i b u t e d to decohesion,^ new phase f o r m a t i o n , ^ a n d pinning of d i s l o c a t i o n s . ^ The environment can a f f e c t the process by i n f l u e n c i n g the rate of production of hydrogen ions, and by enhancing or i n h i b i t i n g the rate of entry of hydrogen into the metal, by the i n t e r p o s i t i o n of adsorbed, or re a c t i o n layer f i l m s . In aqueous systems, the po t e n t i a l and pH at the crack t i p determine whether or not i t i s thermodynamically f e a s i b l e to evolve hydrogen. 28 The observation of Benedicts that hydrogen gas was evolved when i r o n f i l i n g s were immersed i n white l i q u o r , caused workers to speculate as to whether hydrogen embrittlement could, i n f a c t , be - 8 -responsible for b r i t t l e f a i l u r e i n the pulp and paper industry. However, t h i s observation has not been independently confirmed. It has been observed that hydrogen enters s t e e l more r e a d i l y from sulphide s o l u t i o n s , when the pH i s below 5. This may be a consequence of increased hydrogen ion a c t i v i t y , or may be a r e s u l t of the change In stable species as the pH decreases. Biernat & Robins report that dissolved H 2S i s stable at a low pH, but i s transformed i n t o the HS~ species when the pH i s about 5 (H2S/HS has a pk of 6.7). At s t i l l higher pH values, the stable species i s the S ion. I t appears that the sulphur species must be present as H 2S i n an acid medium for s i g n i f i c a n t hydrogen embrittlement to occur. The digesters are usually i n contact with highly a l k a l i n e solutions (pH = 12-13) for most of t h e i r l i f e t i m e . The only time a s o l u t i o n of a lower pH could be i n contact with the digester wall i s during acid cleaning of the screens within the d i g e s t e r . The impregnation zone, where most of the stress corrosion O f ) cracks are observed u i s not immersed i n the a c i d , but some condensation of the vapours could occur on the walls of the impregnation zone. I t i s highly u n l i k e l y that the pH could be lowered i n t o the region where H 2S i s the stable species, due to condensation. If the moisture that condensed on the walls to be a c i d i f i e d , most cracking would be expected i n the region around the screen. However, no SCC cracks are observed i n t h i s r e g i o n . 7 The aforementioned f a c t s estabish that Hydrogen Embrittlement could be ruled out as a cause of SCC occurs i n Kraft digesters. Other authors have attempted to model s p e c i f i c aspects of SCC of - 9 -m i l d s t e e l i n a l k a l i n e solutions, based on anodic d i s s o l u t i o n theories. B i g n o l d , 3 1 Doig & F l e w i t t , 3 2 and M e l v i l l e , 3 3 have a l l examined the e f f e c t s of a change i n p o t e n t i a l down the crack length on SCC, and Mogensen et a l 3 ^ have investigated the d i s s o l u t i o n and repassivation k i n e t i c s of oxide formation at various p o t e n t i a l s . It must be noted that none of the mechanisms i n any of the three groups of models specify, or imply, an i n t e r g r a n u l a r or transgranular crack path. Examination of SCC cracks by t r a d i t i o n a l methods have shown that SCC of mild s t e e l follows a predominantly integranular path In a l k a l i n e s o l u t i o n s , ^  »3-* although transgranular cracking has been reported i n some instances. The c o r r e l a t i o n of SCC with passive f i l m behavior has led to several i n v e s t i g a t i o n s of the properties of these f i l m s . C y c l i c voltammetry experiments, have shown that i n i t i a l f i l m formation occurs i n the a c t i v e region as an adsorbed layer of Fe(OH) 2. At higher p o t e n t i a l s , further oxidation of i r o n occurs and the passive f i l m oxidation to e i t h e r FeOOH or FesOi^. This occurs at the active-passive t r a n s i t i o n . At s t i l l higher p o t e n t i a l s , the FeOOH or Fe30i+ oxidizes to form Y-Fe 203. 3^ I t has been shown that the films formed at the SCC susceptible p o t e n t i a l s are more b r i t t l e than the Y-Fe203 films formed at higher p o t e n t i a l s , and therefore more l i k e l y to rupture under the a p p l i c a t i o n of a t e n s i l e s t r e s s . 3 ^ Other s t u d i e s ^ ' 7 - ! »38 have shown that f i l m rupture at SCC susceptible potentials produces large current transients before repassivation can occur. At other p o t e n t i a l s , rupture - 10 -of the passive f i l m causes l i t t l e , or no increase, i n the anodic current density. This behaviour has been r e l a t e d to the thermodynamic s t a b i l i t y of the soluble i r o n species, HFeO^ ( d i h y p o f e r r i t e ) , i n the p o t e n t i a l range where these t r a n s i e n t s are produced, and the rate of r e p a s s i v a t i o n of the exposed metal. The passivation process involves the following 36 two-step sequence. HFe02~ > Fe(0H>2 •* passive f i l m In the presence of CO3 or S ions, the d i h y p o f e r r i t e ions may combine with the carbonate (or sulphide) ions to form ferrous carbonate (or ferrous sulphide) which i s l e s s soluble, and more stable than ferrous hydroxide. The intermediate step i n the passivation process becomes r e l a t i v e l y s t a b i l i z e d , and a complete layer of y-FeOOH becomes more d i f f i c u l t to form. The range of conditions for possible SCC i s therefore extended. 104 Structure and Properties of Welds Since a l l the cracking problems i n the digester occur i n the v i c i n i t y of welds, i t would be prudent to examine the structure and properties of various region i n the weld. F i g . 2 indicates the f r o n t view of a digester, and the l o c a t i o n s of various fusion welding processes used i n i t s construction. The weld can be divided i n t o the fusion zone and the Heat - 11 -— S M A W S A W 1 1 —- S M A W — S M A W — S M A W S A W " * SUBMERGED ARC WELDING S M A W — SHIELDED METAL ARC WELDING Figure 2 - Location of different welds i n a continuous digester. - 1 2 -A f f e c t e d Zone (HAZ). The fusion zone i s the r e s u l t of s o l i d i f i c a t i o n of the molten metal pool. The heat from the molten metal would a f f e c t the base material adjacent to the deposited molten metal. This region comprises the HAZ. The process of s o l d i f i c a t i o n of the molten weld pool i s fundamentally si m i l a r to ingot s o l i d i f i c a t i o n . Conditions which promote c o n s t i t u t i o n a l supercooling, I.e. fast growth rates, high a l l o y contents, and extreme values of the solute d i s t r i b u t i o n c o e f f i c i e n t , e x i s t i n the weld pool. This promotes breakdown of the fusion boundary i n t e r f a c e , ^ 0 which r e s u l t s i n the development of a d e n d r i t i c s t r u c t u r e . Dendritic growth i s associated with segregation, either as coring, or accumulation of solute i n the i n t e r d e n t r i t i c spaces. The o r i g i n a l columnar structure of the weld may be refined i n grain s i z e , and becomes equiaxed by heating to above the A3 temperature, or between the A^ and A 3 temperatures. Such refinement takes place when multiple passes are deposited. The heat of each succeeding bead reheats the previously deposited beads, r e f i n i n g the grains of those portions of the weld layers that are reheated to the c r i t i c a l temperature range. The HAZ i s comprised of a narrow band of material adjacent to the fusion l i n e , which has been heated to peak temperatures ranging from the i n i t i a l plate temperature, to the melting point of the metal, at the fusion boundary. Depending upon the maximum temperature attained at any point i n the HAZ, and the cooling rate at that point, a wide variety of microstructures ranging from coarse p e a r l i t e , b a i n i t e , to even martensite could be produced. In multipass welds, the structure of a - 1 3 -given region can be a l t e r e d s u b s t a n t i a l l y by the l o c a l i z e d heating caused by subsequent weld passes. The l o c a l heating and cooling accompanying weld cycles, causes l o c a l s t r a i n s , which i n turn produces re s i d u a l stresses. Thus, i n terms of the SCC behaviour of a welded j o i n t , we are confronted with two p o s s i b i l i t i e s : ( i ) regions with high r e s i d u a l t e n s i l e stress which serves to i n i t i a t e SCC; or ( i i ) m i c r o s t r u c t u r a l changes which play a more dominant role than r e s i d u a l stresses i n the phenomenon of SCC. I f the f i r s t scenario were true, digestors which have been subjected to a f u l l stress r e l i e f anneal would be immune to SCC. However, i n p r a c t i c e , cracking problems have been encountered even i n digesters which have been subjected to a f u l l stress r e l i e f anneal.^ Furthermore, dense networks of stress corrosion cracks have been observed i n the fusion zones of welds in dige s t e r s , whereas fewer stress corrosion cracks were observed i n ei t h e r the HAZ or the base m a t e r i a l . ^ Hurst and Cowen have also reported cracking i n the fusion zone of welds exposed to strongly a l k a l i n e environments. Also no c o r r e l a t i o n has been obtained between the SCC s u s c e p t i b i l i t y and welding procedure.^ A l l the above suggests that r e s i d u a l stresses do not play a dominant r o l e i n SCC. This prompted the author to follow t e s t i n g techniques which completely eliminate the e f f e c t of r e s i d u a l stresses, and allowed examination of the s u s c e p t i b i l i t y of various microstructures to SCC. - 14 -1.5 E f f e c t of Heat Treatment on SCC S u s c e p t i b i l i t y During the weld c y c l e s , the base material adjacent to the fusion zone could be heated to a temperature which approaches i t s melting point. In addi t i o n the microstructure of the parent material i n the HAZ would be a l t e r e d . Furthermore, a f t e r welding, the parent material surrounding the weld i s subjected to a stress r e l i e f anneal at approximately 650°C f o r 1 hr. I t i s possible that d i f f e r e n t microstructures would e x h i b i t s u s c e p t i b i l i t y to SCC to varying degrees. However, very l i t t l e work has been done on the e f f e c t of heat treatment and/or thermal h i s t o r y on the stress corrosion s u s c e p t i b i l i t y i n a l k a l i n e environments. Bohnenkamp^0 reported that dead loaded specimen quenched and tempered at 400 - 700°C f a i l e d f a s t e r , by a f a c t o r of 2 - 4, than specimen quenched and tempered at 300 - 400°C. Bohnankamp also suggested that the d i s t r i b u t i o n of carbon, rather than the absolute carbon content, plays a more important r o l e i n determining the SCC resistance of a material. He reported a higher rate of d i s s o l u t i o n of carbides i n NaOH solutions, which accounts for the intergranular attack which has been observed i n low carbon s t e e l s . Furthermore, observations that s t e e l s with s u f f i c i e n t l y low carbon and nitrogen contents are immune to intergranular attack"* 1 provide a d d i t i o n a l support to t h i s . 1.6 SCC T e s t i n g Techniques Due to the large number of variables which a f f e c t SCC, many - 1 5 -d i f f e r e n t techniques have been used to study the phenomenon. They range from r e l a t i v e l y simple tests which monitor the l i f e t i m e of a stressed sample exposed to corrosive environments, the s o p h i s t i c a t e d t r i b o - e l l i p s o m e t r i c experiments, where passive f i l m growth k i n e t i c s and current density are measured with time. Since SCC can be a r e l a t i v e l y slow process, by laboratory standards, many SCC tests are performed by increasing the r e l a t i v e aggressiveness of the environment, v i s a v i s , the t e s t specimen. This could be done by increasing the concentration of the aggressive species i n the environment and/or inc r e a s i n g the temperature, or introducing a notch or precrack i n the test specimen. This serves to ( i ) decrease the time required f o r i n i t i a t i o n of a stress corrosion crack; and ( i i ) increase the rate of propogation of the s t r e s s corrosion crack; so that r e s u l t s could be obtained within a reasonable period of time. The two r e l a t i v e l y new methods of p a r t i c u l a r i n t e r e s t to t h i s study are the Slow S t r a i n Rate Technique (SSRT) and the f r a c t u r e mechanics approach. These compare favourable with the t r a d i t i o n a l accelerated SCC t e s t s , and y i e l d information without having to increase the aggressiveness of the environment. The primary advantages of the SSRT are (a) i t provides a rapid laboratory method for evaluating SCC s u s c e p t i b i l i t y i n s o l u t i o n of laboratory or i n d u s t r i a l - 16 -i n t e r e s t . Usually a test never l a s t s f o r more than 48 hrs, compared to the 60+ days for constant load, or constant s t r a i n t e s t s ; and (b) the r e s u l t s are p o s i t i v e , i . e . , a f a i l u r e always occurs, e i t h e r i n the d u c t i l e (overload) manner, or prematurely, due to SCC, i n the b r i t t l e mode. The SSRT tests demonstrates the fa c t that SCC i s dependent on the l o c a l s t r a i n rate produced at the crack t i p , rather than the t e n s i l e s t r e s s per se. P a r k i n s ^ postulated that creep or s t r a i n rate i s the c o n t r o l l i n g parameter i n SCC. He suggested that cracks w i l l propagate only i f electrochemical conditions which cause cracking were established before creep i n the p l a s t i c zone ahead of the crack t i p was exhausted. Studies have shown that there i s an upper and lower l i m i t of s t r a i n rate that w i l l produce SCC. Both above and below t h i s range, the material w i l l f a i l by d u c t i l e , microvoid coalescence. The upper l i m i t occurs as a r e s u l t of i n s u f f i c i e n t crack advance, with respect to the amount of s t r a i n occurring, while the lower l i m i t i s thought to be due to a p r o t e c t i v e f i l m which establishes i t s e l f over the crack t i p f a s t e r than i t can be ruptured by the s t r a i n . The optimum s t r a i n rate for producing SCC varies with the metal, environment, electrochemical p o t e n t i a l , and temperature, and must be experimentally determined f o r 8 44 ea.ch system. • . A t y p i c a l SSRT test consists of p u l l i n g apart, to f a i l u r e , a c y l i d r i c a l t e n s i l e specimen exposed to the appropriate environment, at a low, constant, extension rate. Since the onset of SCC w i l l a f f e c t the - 17 -Figure 3 - Stress-strain curves for specimen tested i n SCC susceptible and inert environments using SSRT, where AQ and Agcc are the areas under the respective curves. - 18 -relative d u c t i l i t y of the test specimen, as Illustrated in Fig. 3, SCC susceptibility can be characterized by mechanical parameters which include % elongation, % reduction in area, or even time to failure. Visual confirmation of secondary cracks on the specimen surface could be used to reinforce the conclusion obtained from this test. The SSRT has been found to be a more severe test of SCC than other traditional tests, and has produced SCC for metal-environment combinations where SCC was d i f f i c u l t to intitiate. 4 3» 4 5 Semi-quantitative data, in the form of apparent crack growth rates has also been obtained from the slow strain rate t e s t s . ^ The fracture mechanics approach to the study of SCC is an extension of Linear Elastic Fracture Mechanics (LEFM) which is used to A O quantify fracture in materials of limited d u c t i l i t y . This technique follows the growth of a crack as a function of the stress intensity at the crack t i p , which is characterized by Kj for a mode I crack opening. The stress intensity is directly proportional to the applied load, and the square root of the crack length, a l l other factors being equal. At sufficiently high values, the stress intensity approaches Kjrj, when spontaneous unstable crack growth occurs. In most fracture mechanics experiments, the crack growth rate, v, is measured as a function of the stress intensity. A plot of log v VS stress intensity usually shows three distinct regions. In region I, at low Kj values the crack propogation rate is stress-intensity dependent. Region II, where the rate of crack propagation is independent of stress-intensity, and f i n a l l y region III where crack - 19 -propagation becomes Kj dependant at high stress intensities. At very high stress intensities, when Kj = K I C, unstable crack propagation occurs. On the other end of the Kj spectrum, for values of Kj below a particular threshold stress intensity denoted by KjgQfj, no crack propagation is detectable. A schematic log V-Kj plot i s provided in Fig. 4 . The complex interaction of stress intensity and environment variables in regions I and II, makes i t d i f f i c u l t to treat data mechanistically. However, the presence of the relatively stress independent region II is useful in determining the effects of temperature, solution composition, viscosity etc., on crack propagation, which eventually help elucidate the rate controlling mechanism of crack propagation. Moreover, fracture mechanics allows separation of the i n i t i a t i o n from the crack propagation stages, which makes i t a useful tool for AO kinetic studies. 7 - 20 -F i g u r e 4 - Schematic of a t y p i c a l l o g V - K j c u r v e , showing reg ions I , I I and I I I . - 21 -CHAPTER 2 EXPERIMENTAL 2.1 Scope of the Present Work During the course of this work, two test methods, namely the Slow Strain Rate Technique (SSRT), and the Fracture Mechanics technique, were conducted with three different alkaline solutions. The SSRT experiments were employed to determine the conditions where the test material was most prone to SCC, in solutions composed of 3.35 moles/kg NaOH, 2.5 moles/kg NaOH + 0.42 moles/kg Na2S, and 3.35 moles/kg NaOH + 0.42 moles/kg Na2S. This technique has been used successfully by other workers to study SCC i n alkaline solutions.* 1> 3 8»^ The material used was a A516 Gr. 70 steel, obtained as hot-worked plates 1/2" (1.27cm) and 1" (2.54cm) thick, whose composition is indicated in Table 1. Since hot-worked plates are used directly in the construction of digesters in pulp and paper plants, tests were conducted in the as-received condition. Based on the SSRT results, specific potentials were chosen for the fracture mechanics study, to investigate the effect of stress-intensity and potential on v, the crack propagation rate. Detailed fractography was conducted on the fracture surfaces produced by both test techniques. The solutions used were prepared under nitrogen purged conditions, using singly d i s t i l l e d water and reagent grade chemicals (NaOH pellets, and Na2S • 9H20 crystals). Table 1 - Yield Strengths and Chemical Composition of Steels Yield Strength MPa Tensile Strength MPa Hardness No. Chemical Composition (%) Material VHN C Si Mn P S A516 Gr. 70 0.5 in plate 308 525 171 0.18 0.22 1.12 0.021 0.007 A501 Gr. 70 1 in. plate 343 498 - 0.19 0.23 1.16 0.021 0.008 E70S Welding electrode 572 189 - - - - -- 23 -2.2 Specimen T e n s i l e SSRT specimen, were machined from the 1/2" p l a t e , with the t e n s i l e axis p a r a l l e l to the R o l l i n g D i r e c t i o n . The co n f i g u r a t i o n and dimensions of these specimen are indicated i n F i g . 5. In a d d i t i o n , t e s t s were conducted i n t e n s i l e SSRT specimen incorporating both the fusion zone, and the HAZ of a weld, i n the gauge section, as i l l u s t r a t e d i n F i g . 5(b). This ensured that a wide v a r i e t y of microstructures were simultaneously i n contact with the environment. Welds were made by machining 3/8" (9.5mm) deep groves Into the p l a t e , perpendicular to the r o l l i n g d i r e c t i o n , and depositing weld metal Into the groove with an automated MIG welding machine, and a E70S wire electrode. P r i o r to use, these specimen were polished with 300 g r i t paper, degreased with chlorothane, and dried with ethanol. Double Cantilever Beam (DCB) specimen, machined from the 1" (25.4mm) plate whose geometry i s shown i n F i g . 6, were used f o r the Fracture Mechanics experiments. The Beam height (H) f o r these specimen was 11.6 mm, specimen length from the loading l i n e (W) was 66.7 mm, and the specimen thickness (B) was 25.4 mm. The length of the i n i t i a l machined notch from the loading l i n e was 25.4 mm, and the beams were threaded to receive grips for the a p p l i c a t i o n of a load. The faces perpendicular to the crack plane were polished to a 0.3 um f i n i s h , washed with water, then dried with ethanol, and stored i n a dessicator u n t i l further use. Brown and Srawley^ have shown that the c a l i b r a t i o n f o r the DCB geometry i s given by - 24 -FUSION ZONE BASE MATERIAL —i 9-5 * -(a) BASE METAL ( b ) WELD INCORPORATED ALL DIMENSIONS IN mm Figure 5 - SSRT specimen geometry. - 25 -V H A V H A Figure 6 - DCB specimen geometry. The above calibration equation i s valid only when a/w j< 0.6 and W/H > 5. In order to compare the crack propagation rates of the as-cast dendritic structure (as in the fusion zone of a weld) vis a vis the hot-worked microstructure DCB specimen with a machined notch incorporated in the fusion zone of a weld were u t i l i z e d . This was done by machining a 3/8" deep groove parallel to the rolling direction, on the 1/2" thick plate, and depositing molten metal into the groove with a MIG 1" welding machine. The above is illustrated in Fig. 7. Subsequently -5-o (3.2mm) of the base metal was ground way, suitable sections were cut out, and notches were incorporated within the fusion zone. As in the previous case, the beam height was 11.6 mm, the specimen length from the loading line was 66.7 mm, but the specimen thickness was 9.53 mm. 2.3 Equipment and Apparatus SSRT experiments were carried out on a modified, vertic a l l y mounted hounsfield tensometer, capable of achieving a cross-head speed of .00015 in /min, corresponding to a strain rate of 2.5 x 10~6/sec. The cross-heads were pulled apart by a motor rotating at 12 rev./hour, whose drive shaft is coupled to a reducing gear system. A schematic diagram of the c e l l used i s provided in Fig. 8. The c e l l was - 27 -BASE METAL—J FUSION ZONE D C B SPECIMEN Figure 7 - DCB specimen incorporating the fusion zone of a weld i n the crack plane. - 28 -constructed of FEP or PTFE fluocarbon polymers, and the s o l u t i o n within the c e l l was heated by an e x t e r n a l l y wrapped, resistance heated tape. Temperature c o n t r o l of ±1°C was maintained by connecting the heating tape, through a Variac power transformer, to a YSI model 71 temperature c o n t r o l l e r . A FEP coated thermistor probe (YSI model 400) was inserted i n t o the c e l l . Two spectrographic grade graphite counter electrodes were positioned at opposite ends of the c e l l , as were a Nitrogen purge l i n e , and a r e f l u x condenser. An external, saturated calomel electrode, maintained at room tempeature, was connected to the c e l l v i a a luggin c a p i l l a r y , which was positioned approximately 1 mm from the gauge se c t i o n of the specimen. A cotton thread was run through the luggin c a p i l l a r y to ensure stable p o t e n t i a l c o n t r o l . P o t e n t i a l control was maintained by a Wenking Model 68 Potentiostat. A few t e s t s were conducted with 0.5 - 1 mm of the surface of the gauge section removed electrochemically In a s o l u t i o n comprising of 25 gms. chromium t r l o x i d e , 135 ml a c e t i c acid and 7 ml water. These t e s t s were performed to ascertain the influence of r e s i d u a l stresses on the surface, on secondary cracking. DCB specimen were fatigue precracked by a Sonntag SF-l-U fatigue t e s t i n g machine. Fatigue grips were constructed of high speed t o o l s t e e l . A u n i v e r s a l grip was used to ensure loading along a single plane. The maximum value during fatigue was maintained below the Kj value f o r subsequent SCC t e s t s . The DCB specimen were self-loaded by opposing b o l t s . A c a l i b r a t i o n curve of d e f l e c t i o n of the beams VS load acting along the loading l i n e for the DCB specimen was generated, - 29 -LUGGfo CAPILLARY— N s PURGE P T F E C A P -FEP BEAKER SPECIMEN COUNTER ELECTRODE J J-SOLUTION '-COUNTER ELECTRODE PTFE SEAL Figure 8 - SSRT test c e l l - 30 -and i s illustrated in Fig. 9. Deflection of the beam was measured by a MTS model 632 COD gauge. Fracture mechanics tests were conducted in a Teflon basin, heated from the outside by a resistance heated tape. A nitrogen purge line, FEP coated thermistor probe, luggin capillary, and a FEP coated thermometer were inserted into the Teflon basin. Temperature was controlled in the same manner as in the SSRT experiments. Low carbon steel wires were used as counter electrodes. The text specimen were connected in parallel to ensure that a l l the specimen were maintained at the same potential. Polarization curves were generated by a EG & G Model 135 potentiostat using a scan rate of 1 mv/sec. Fracture surfaces were examined with an ETEC Autoscan Scanning Electron Microscope (SEM), using a secondary electron imaging mode and 20 keV excitation. 2.4 Experimental Procedures 2.4.1 Slow Strain Rate Test Experiments The test c e l l was assembled as illustrated in Fig. 8, and mounted on the modified hounsfield tensometer. About 500 ml of test solution was poured into the c e l l . A small tensile load was applied as the specimen was brought up to a temperature of 92°C. When the test temperature was attained, a potential of -1.3 VgcE w a s applied to the - 31 -Figure 9 - C a l i b r a t i o n curve for DCB specimen - 32 -specimen for a period of 10 minutes, i n order to reduce a l l the oxides present on the surface of the gauge section. A preselected p o t e n t i a l was subsequently applied to the specimen, a f t e r which the sample was pulled apart at a s t r a i n rate of 2.5 x 10~ 6. Temperature and p o t e n t i a l were maintained constant throughout the t e s t , and nitrogen was c o n t i n u a l l y bubbled through the c e l l i n order to provide a s t i r r i n g a c t i o n and minimize oxidation of S ions. In order to reduce the surface area of the metal exposed, and consequently, the amount of current supplied by the potentiostat, and changes i n the s o l u t i o n concentration due to d i s s o l u t i o n of i r o n , the en t i r e specimen, with the exception of the gauge section, was covered with Teflon tape. At the conclusion of each t e s t , the two halves of the t e s t specimen were cleaned, f i r s t with d i s t i l l e d water, and then with ethanol, and d r i e d . The reduction i n c r o s s - s e c t i o n a l area of the f r a c t u r e region was measured with a t r a v e l l i n g microscope. The oxide scale present on the f a i l e d halves were removed by suspending them i n an i n h i b i t e d acid s o l u t i o n composed of 4ml 35% 2 butyne-1,4 d i o l + 3 ml HC1 + 50 ml H2O, i n an u l t r a s o n i c cleaner for 2-3 minutes. The f a i l e d ends were subsequently examined under the SEM. Secondary cracking was observed at some p o t e n t i a l s . The frequency of these cracks was measured by l o n g i t u d i n a l l y sectioning the f a i l e d halves through the centre, p o l i s h i n g to a 0.06 um f i n i s h , etching with 2 v o l . % N i t a l , and observing the sample under an o p t i c a l microscope. The regions where the welded samples f a i l e d , were also examined i n a s i m i l a r manner. - 33 -2.4.2 Fracture Mechanics Experiments DCB samples utilized for fracture mechanics experiments on the base metal, both in the as-received condition and after a 650°C anneal to simulate Industrial stress r e l i e f . Most specimen were precracked at a maximum stress intensity of 20 MPa Vm, but test specimen which were loaded to a low Kj_ level were recracked at a maximum Kj level of 15 MPa/m, unti l the machined notch was extended by 3-4 mm. Test specimen in the as-received conditon were loaded with opposing bolts. The deflection required to maintain a particular Kj level was extrapolated from the calibration curve, fig.9, and the specimen was bolt-loaded t i l l the desired deflection was obtained. Nickel wires were spotwelded onto the sample prior to loading. The spotwelded joint was coated with an epoxy resin to minimize degradation of the joint within the corrosion c e l l . Several specimen were connected in parallel to one Potentiostat. The schematic diagram of the fracture mechanics corrosion c e l l is provided in Fig. 10. DCB specimen in the heat treated condition (annealed for 1 hr at 650°C), and the specimen incorporating the fusion zone of a weld in the crack plane, were loaded in a hounsfield tensiometer. Iron wires were used as counter electrodes, as both graphite and platinum electrodes were observed to erode during the course of the test, which lasted for 3 weeks. Potential and temperature control were effected in a manner explained previously. At the conclusion of each test, the samples were broken open in liquid nitrogen. The corrosion deposits in the crack were removed with - 34 -r LUGGIN CAPILLARY r—IRON COUNTER-ELECTRODE D C B SPECIMEN P T F E BASIN Figure 10 - Fracture mechanics test c e l l - 35 -an i n h i b i t e d a c i d s o l u t i o n , and crack extension was measured with a t r a v e l l i n g microscope. A l l measurements were made with respect to a f i d u c i a l / r e f e r e n c e l i n e . Due to the small crack extensions, the crack growth increment was v e r i f i e d when the sample was viewed i n the SEM. The path of the secondary cracks ( i . e . , cracks branching from the microcrack) was determined by sectioning the sample l o n g i t u d i n a l l y , p o l i s h i n g to a 0.06 um f i n i s h , etching with a 2 v o l . % N i t a l s o l u t i o n , and f i n a l l y viewing the polished face i n a SEM. 2.4.3. Anodic Polarization Curves A l l p o l a r i z a t i o n curves were generated potentiodynamically. Small square sections were cut from the pl a t e s , and Ni wire was spotwelded to one of the ends. The sample was mounted i n an epoxy mould. Before the commencement of each t e s t , the specimen was ground to a 300 g r i t f i n i s h . A l l p o l a r i z a t i o n tests were conducted i n a Teflon c e l l , provided with graphite counter electrodes, as i l l u s t r a t e d i n F i g . 11. Temperature control at 92°C was maintained i n a manner explained i n section 2-3, and N 2 was continually bubbled through the c e l l s . Just before the commencement of each t e s t , the specimen was po l a r i z e d to, and held at -I.2V5CE f or 15 mintues, a f t e r which an anodic p o l r a i z a t i o n scan was i n i t i a t e d . No attempt was made to correct for any small p o t e n t i a l differences due to thermal and concentration gradients i n the s a l t bridge. - 36 -LUGGIN CAPILLARY PURGE PTFE LID F E P BEAKER I 2 Z Z THERMISTOR PROBE I COUNTER ELECTRODE EPOXY RESIN WORKING ELECTRODE Figure 1 1 - Anodic p o l a r i z a t i o n test c e l l - 37 -CHAPTER III RESULTS 3.1 Anodic Polarization Tests The anodic p o l a r i z a t i o n curves generated for t h i s work were 12 13 51 s i m i l a r to those presented by other authors. • •** J X. Figures 12 and 13 show the anodic behaviour for the as-received material i n 2.5 m NaOH + 0.42 m Na2S; and 3.35 m NaOH and 3.35 m NaOH + 0.42 m Na 2S r e s p e c t i v e l y . A l l the curves show a d i s t i n c t and well defined active peak, and a t r a n s i t i o n from an active to a passive behaviour. The important d i f f e r e n c e s between the anodic behavi our of the metal i n NaOH, and the NaOH + Na2S solutions were: ( i ) a s h i f t i n the onset of passive behaviour from -1 Vg^g i n the NaOH s o l u t i o n , to -900 mV S C E i n the s o l u t i o n containing S ions, ( i i ) a much higher current density attained by the sulphide s o l u t i o n (simulated white l i q u o r ) , both at the act i v e peak, and i n the passive zone, ( i i i ) an increase i n current density i n the presence of S ions at potentials above -725 mVgQg, due to the oxidation of i o S ions to higher oxidation states. The Free Corrosion P o t e n t i a l (FCP) was found to reside i n the ac t i v e zone at approximately -1.1 V S C E i n the deaerated simulated white l i q u o r s o l u t i o n . However, the FCP was raised when the l i q u o r was - 38 -Figure 12 - Anodic polarization curve of A516 Gr 70 steel in 2.5 m NaOH + 0.42 m Na2S at 92°C - 39 -3.35 m NaOH I 1 1 1 1 I I -2 - 1 0 I 2 3 4 10 10 10 10 10 2 10 10 CURRENT DENSITY (A/M ) Figure 13 - Anodic polarization curves of A516 Gr 70 steel in 3.35 m NaOH and 3.35 m NaOH + 0.42 m Na2S solutions at 92°C - 40 -Figure 14 - Anodic polarization curves of A516 Gr 70 steel in 3.35 m NaOH at 92°C with varying Fe"*-*" ion concentrations - 41 -aerated. Thus, the FCP i n digesters u t i l i z i n g improperly prepared and stored white l i q u o r would be greater than -1.1 Vgfjg, and could conceivably reside i n the active-passive t r a n s i t i o n zone. ++ —6 —•+ Increases i n the Fe ion concentration from 10 M to 10 M did not a f f e c t the p o t e n t i a l for the onset of the active-passive t r a n s i t i o n , as i l l u s t r a t e d i n F i g . 14. 3.2 Slow Strain Rate Testing Technique 3.2.1 2.5 m NaOH + 0.42 m Na2S F i g . 15 shows the r e s u l t of the SSRT experiment superimposed on the anodic p o l a r i z a t i o n curve of the s t e e l i n the same caustic sulphide environment. The r e s u l t s are plotted as % Reduction i n area VS P o t e n t i a l , as i l l u s t r a t e d i n F i g . 15. The minimum reduction i n area occurs i n the region of t r a n s i t i o n from the active to the passive behaviour. The Active-Passive T r a n s i t i o n (APT) has been reported to be 13 the region most prone to SCC. J However, the % reduction i n area of the f a i l e d samples i n the APT zone d i f f e r e d from that i n other regions of the anodic p o l a r i z a t i o n curve by only 13%. This, combined with the fa c t that no secondary cracking was observed i n the f a i l e d specimen, appeared to suggest that the s t e e l was not very prone to SCC i n the white l i q u o r with a caustic concentration of 2.5 m NaOH. SSRT tes t s conducted a f t e r removal of the outer 0.5 - 1 mm by e l e c t r o p o l i s h i n g yielded s i m i l a r r e s u l t s . This estabished that the - 42 -Figure 15 - Effect of potential upon reduction i n area in 2.5 m NaOH + 0.42 m Na2S superimposed upon the anodic polarization curve obtained In the same solution. - A3 -formation of secondary cracks were not impeded by residual stresses introduced by machining. 3.2.2 3.35 m NaOH In order to determine i f the OH- ion concentration had any influence on the SCC behaviour, a series of SSRT tests were conducted in the 3.35 m NaOH solution, the minimum reduction in area occurred in the active-passive transition zone, as illustrated in Fig. 16 and tabulated in Table 2. Once again, the % reduction in area of the failed samples at the potentials corresponding to the Active-Passive transition of the anodic polarization curve was not very marked. However, the presence of secondary cracks penetrating the surface of the specimen near the region where failure occurred, confirmed SCC as the cause of premature failure, as il l u s t r a t e d in Fig. 17. When the failed halves were viewed by the SEM, an annulus of material affected by the environment was observed. No secondary cracking was observed in samples tested at potentials corresponding to either the active, or the passive regions. 3.2.3 3.35 m NaOH + 0.42 m Na2S 3.2.3.1 Base Material Since i t was apparent that the stress corrosion susceptibility of the steel was affected by the OH- ion concentration, a series of SSRT tests were conducted in 3.35 m NaOH + 0.42 m Na2S solutions. Minimum - 44 -Table 2 - SSRT Test Results i n 3.35 m NaOH Po t e n t i a l (Vgfjg) ^ Reduction i n Area 0 47.7 -0.500 46.8 -0.750 47.4 -0.950 34.4 -0.980 34.1 -1.05 41.8 -1.2 40.8 - 4 5 -Figure 16 - Effect of potential upon reduction i n area for 3.35 tn NaOH superimposed upon the anodic po l a r i z a t i o n curve obtained i n the same solution. - 46 -, I mm, Figure 17 - Secondary cracking at -0.98 V S C E i n a 3.35m NaOH environment at 92°C. - 47 -reduction in area was obtained at potentials corresponding to the Active-Passive transition, as illustrated in Fig. 18. Also, extensive secondary cracking was observed at these potentials, as illustrated in Fig. 19. Some secondary cracking was observed at potentials corresponding to the passive zone, as illustrated in Fig. 20, and at less noble potentials where hydrogen is expected to be evolved. The apparent crack velocities, i.e., the ratio of the largest crack length, to the duration of the test was found to be 1.85 x 10" m/sec in the active passive transition zone, 8.1 x 10~ 1 0 m/sec is in passive zone, and 1.31 x IO - 9 m/sec at -1.8 Vg E fj, as illustrated in Tables 3 and 4. Thus the presence of S ions enhanced SCC at potentials, other than the active-passive transition region. 3.2.3.2 Welded Samples SSRT tests were conducted on tensile samples with a weld incorporated in the gauge section. Failure occurred in the parent material at potentials residing in the Active and Passive zones. However, in samples which were tested at potentials corresponding to the Active-Passive transition zone, failure was observed to occur in the fusion zone of the weld. Extensive secondary cracking was observed at these potentials, with most of the cracks occurring in the fusion zone. The apparent crack velocity of the secondary cracks in the base metal was found to be 1.88 x 10" m/sec which is comparable to that obtained in the SSRT tests conducted on samples fabricated from the as-received material, under the same conditions (Table 3). The apparent crack - 48 -Table 3 - SSRT Test results in simulated White Liquor % Reduction in Area Potential V „ r F 2.5 M NaOH + Na2S 3.35 M NaOH + Na2S 0 47.7 47.5 -0.5 47.9 --0.6 - 47.0 -0.7 - 47.5 -0.75 47.4 46.5 -0.88 35.5 35.5 -0.85 34.1 35.7 -0.906 43.9 --1.0 - 45.5 -1.05 41.8 --1.20 - 44.2 -2.10 — 18.2 - 49 -•Table 4 - Secondary Cracking i n the base material i n the 3.35 m NaOH + 0.42 m Na 2S environment. P o t e n t i a l Frequency of Apparent Crack V e l o c i t y V „ r F Secondary Cracks* m/sec -0.50 8 9.2 X IO" 1 0 -0.65 6 8.5 X I O " 1 0 -0.70 7 8.1 X IO" 1 0 -0.75 6 8.3 X IO" 1 0 -0.85 18 1.85 X IO" 9 -0.88 19 1.9 x IO" 9 -1.05 3 3.9 X IO" 1 0 -2.1 14 1.3 X IO" 9 * T o t a l number of secondary cracks on sectioned sample. - 50 -% REDUCTION IN AREA 10 30 50 70 90 ~I I I 1 1 1 1 1 T 3.35 M NQOH +' 0.42 M N0*S BASE MATERIAL „ -2 - 1 0 I 2 3 4 10 10 10 10 10 10 10 CURRENT DENSITY (AITlp./M ) Figure 18 - Effect of potential upon reduction in area in 3.35 m NaOH + 0.42 m Na2S superimposed upon the anodic polarization curve obtained in the same solution. - 51 -, 08mm t Figure 19 - Secondary cracking at -0.88 VgQg i n a 3.35m NaOH + 0.42 m Na2S environment - 52 -Figure 20 - Secondary cracking at -0.750 V$cE i n a 3.35m NaOH + 0.42 m Na2S environment - 53 Table 5 - Secondary Cracking in the welded specimen in the 3.35 m NaOH + 0.42 m Na2S environment. Potential BASE MATERIAL HAZ FUSION ZONE Ve.„ Freq. of Cracks V Freq. of Cracks V Freq. of Cracks V oOfi I I I m/sec m/sec m/sec -0.650 4 6.5 x 10 - 1 0 -0.750 6 6.2 x 10" 1 0 -0.850* 4 1.88 x IO"9 12 7.4 x 10 - 1 0 36 1.38 x IO - 9 0.880* 5 1.85 x IO - 9 8 7.1 x IO - 1 0 32 1.24 x IO - 9 -1.3 1 6.94 x 10~ 1 0 - 7.1 x 10~ 1 0 32 1.24 x IO"9 •Failure in the fusion zone. - 55 -velocity in the HAZ and fusion zone was 7.4 x 1 0 - 1 0 m/sec and 1.38 x 10 _ m/sec respectively. However, the apparent velocities of most of the cracks in the fusion zone was 6.4 x IO -* 0 m/sec, i,e., numerous long cracks were obtained. The apparent crack velocity, and the frequency of cracking results for both types of specimen, i.e. those made of the as-received material, and those incorporating the weld, are tabulated in Tables 4 and 5 respectively. 3.3 Fracture Mechanics Results 3.3.1 General Comments Fracture mechanics tests were conducted in a 3.35 m NaOH + 0.42 m Na2S environment, since i t was found to interact more severely with the base material. When the DCB specimen were sp l i t open in liquid nitrogen after exposure to simulated white liquor for a period of three weeks, i t was possible to visually distinguish the boundaries betwen the fatigue precrack, stress corrosion crack, and the b r i t t l e overload regions. Both, the fatigue precrack, and the stress corrosion cracks were observed to t r a i l at the edges of the specimen. This was possibly due to the presence of residual compressive stresses at the surface layer, after the rolling operations, due to changes in stress state, or subtle differences in the crack liquid composition at the free surface. The average crack velocity was the mean increment of the stress corrosion crack growth, measured at a minimum of 15 locations. Since - 56 -very small crack increments were obtained during each t e s t , there was l i t t l e change i n the Kj value during t e s t i n g . Hence, f o r p r a c t i c a l considerations, the fracture mechanics tests could be considered to be under constant Kj conditions. 3.3.2 Effect of Stress Intensity The log of the crack v e l o c i t y i s pl o t t e d against the s t r e s s i n t e n s i t y i n F i g . 22. the log v V/S K x p l o t e x h i b i t s three d i s t i n c t regions. The stress independent crack v e l o c i t y , i . e . , the stage II crack propagation rate was found to be approximately 4 x 1 0 - 1 0 m/sec at -0.88 VgQg (corresponding to the active-passive t r a n s i t i o n region of the anodic p o l a r i z a t i o n curve). The threshold stress i n t e n s i t y f or SCC was approximately 20 MPa/m. An increase i n the stress i n t e n s i t y above 30 MPa/m indi c a t e s the onset of region II crack propagation, as i l l u s t r a t e d i n F i g . 22. 3.3.3 Effect of Potential DCB specimens were tested at -0.88 V S C E and -0.75 V g C E , i . e . , p o t e n t i a l s corresponding to the active-passive t r a n s i t i o n zone, and the passive zone of the anodic p o l a r i z a t i o n curve r e s p e c t i v e l y . The threshold stress i n t e n s i t i e s for SCC, at both the above mentioned p o t e n t i a l s were found to be approximately 20 MPa/m. The stage II crack propagation rate was found to be approximately 4 x 10~ 1 0 m/sec at -0.88 - 57 -10 K l $ c c 30 40 50 60 65 STRESS INTENSITY ( K,) MPo^fa Figure 22 - E f f e c t of stress i n t e n s i t y on crack growth i n a 3.35 m NaOH + 0.42 m Na2S environment at -0.88 V S C E - 58 -Figure 23 - Effect of potential on crack growth in a 3.35m NaOH + 0.42 m Na2S environment. - 59 -VgCE a n c* x 10~ 1 0 m/sec at -0.75 Vgfjg, as illustrated in Fig. 23. Thus, the ratio of the stage II velocities at -0.88 Vgfjg and -0.75 Vgfjg was found to be 2.6, which compared favourably with the ratio of the apparent crack velocities at the aforementioned potentials, obtained from the SSRT tests, which was found to be 2.35. Also, the threshold stress intensity for crack propagation was not significantly affected by a change in potential. These results suggest that digester cracking rates could be reduced by at least a factor of 2, by imposing an anodic protection potential of -0.75 VgQg, where the current requirement i s not excessive. 3.3.4 Effect of Microstructure Failure was observed to occur preferentially in the fusion zone of the weld, in SSRT tests conducted at potentials corresponding to the APT zone. Hence DCB crack velocity measurements were made with the crack plane situated in the fusion zone of the weld, at a potentials corresponding to the active-passive transition zone, to help explain this phenomenon. The stage II crack velocity in the fusion zone was found to be approximately 1 x 10~ 1 0 m/sec. Thus the preferential failure in the fusion zone during SSRT cannot be attributed to higher crack rates. - 60 -3.3.5 Effect of Stress-Relief Anneal In order to r e l i e v e stresses generated during f a b r i c a t i o n , the regions around the welded j o i n t s are subjected to a stress r e l i e f anneal at temperatures as high as 650°C for 1 hour. The heat treatment does not cause any s i g n i f i c a n t spheroidization of carbides, or grain growth. The stage II crack v e l o c i t y of the heat treated material, at -0.88 Vgfjg, did not d i f f e r s i g n i f i c a n t l y from the stage I I v e l o c i t y of the base mat e r i a l , as i l l u s t r a t e d i n F i g . 24. 3.4 Fractogxaphy 3.4.1 General Comments A l l fractography of the stress corrosion cracks were taken as close to the leading edge as possible, i n order to minimize the e f f e c t of general corrosion on topography. The test specimen were covered with a black layer of corrosion product when they were removed from the test s o l u t i o n , at the conclusion of each experiment. The corrosion product was thicker on the machined notch, and fatigue precrack, as compared to the str e s s corrosion crack. The surface films were removed with an i n h i b i t e d acid s o l u t i o n , which produced no p i t t i n g , nor introduced any a r t e f a c t s onto the fracture surface. The fatigue zone and the zone of overload f a i l u r e were e a s i l y - 61 -STRESS INTENSITY ( K, ) MPo/ivl Figure 24 - E f f e c t of microstructure and heat-treatment on crack propagation i n a 3.35m NaOH + 0.42m Na 2S environment. - 62 -discernable from the SCC crack. The fatigue surface was featureless, and had retained i t s original banded structure. Evidence of etching of the surface by the environment was visible, as illustrated in Figure 25. In contrast, the zone of overload failure showed a b r i t t l e , transgranular mode of failure, as illustrated In Fig. 26. A l l photographs have been mounted, such that cracks propogate from l e f t to right. 3.4.2 Effect of Stress Intensity and Potential The crack surfaces of a l l the specimen tested at -750 mVg^ g exhibited, the roughened surface characteristics of the general dissolution phenomenon, obliterating most of the fine fractographic features. Hence, the crack path could not be readily identified, as illustrated in Fig. 27 (a-d). Examination of stereographic photographs (Fig. 28a-b) did not reveal any intergranularly faceted regions. Hence i t was concluded that the crack path was transgranular. Deep secondary cracks were observed. Longitudinal sectioning of the sample revealed that these secondary cracks followed a transgranular path, as illustrated In Fig. 29. The amount of dissolution occurring at the secondary cracks appeared to increase with increasing stress intensity. This suggests that dissolution is a dominant mechanism for crack propagation. The crack front of the specimen tested at -880°C also exhibited dissolution effects, as illustrated in Fig. 30a-c. However the crack path was found to be intergranular by metallograph!c examination of the - 63 -Figure 25 - Fatigue surface, on the DCB specimen, showing evidence of etching - 64 -Figure 26 - B r i t t l e overload f a i l u r e zone, showing a transgranular mode of f a i l u r e - 65 -- 66 -Figure 27c - Kj - 50 MPa/m Figure 27 - Va r i a t i o n of fractography with s t r e s s i n t e n s i t y i n a 3.35 m NaOH + 0.42 m Na2S environment at -0.750 VgQg Figure 28 - Stereographic photographs of crack front at 40 MPa/m, -0.75 V S C E - 68 -Figure 29 - Transgranular secondary cracks at 40 MPa/m, -0.75 V S C E Figure 30a - K x - 25 MPa/m Figure 30b - K x = 35 MPa/m - 70 -Figure 30c - Kj « 50 MPa/m Figure 30 - Variation of fractography with stress intensity in a 3.35 m NaOH + 0.42 m Na2S environment at -0.88 Vgfjg - 71 -Figure 31 - Intergranular crack path at 40 MPa/m, -0.88 VSCE - 72 -Figure 32 - Stereographic photographs of crack front at 40 MPa/m, -0.88 V S C E - 73 -Figure 33 - Intergranular secondary cracks at 40 MPa/a, -0.88 V S C E - 74 -primary and secondary crack perpendicular to the crack plane, before the sample was fractured i n a l i q u i d Nitrogen environment, as i l l u s t r a t e d i n F i g . 31. Examination of stereo photographs of the stress corrosion crack front revealed a mildly faceted topography, as i l l u s t r a t e d i n F i g . 32a-b. secondary cracks, which were intergranular i n nature were v i s i b l e , as i l l u s t r a t e d i n F i g . 33. 3.4.2 Effect of Stress Relief Anneal Samples which had been annealed were exposed to the test s o l u t i o n for only 2 weeks at a p o t e n t i a l of -880 mVSfjg. Since, a l l f i n e features had not been o b l i t e r a t e d to a very great extent, by general d i s s o l u t i o n , i t was possible to discern that the crack had propagated i n an intergranular manner, as i l l u s t r a t e d i n F i g . 34. However, the p o s s i b i l i t y that the heat treatment could have promoted a more pronounced Intergranular f a i l u r e cannot be ruled out. 3.4.3 Effect of Microstructure The stress corrosion crack appeared to propagate along the i n t e r d e n d r i t i c spaces, i n the specimen where the crack was incorporated i n the fusion zone of the weld, as i l l u s t r a t e d i n F i g . 36-37. I n t e r d e n d r i t i c spaces are regions of solute segregation. Hence, t h i s r e s u l t was not unexpected. - 75 -Figure 34 - SCC crack front i n the heat treated specimen at 40 MPa/m, -0.88 V S C E - 76 -Figure 35 - Intergranular secondary cracks i n heat treated specimen at 40 MPa/m, -0.88 V__w - 77 -Figure 36 - Fractograph of DCB specimen incorporating a weld, i n d i c a t i n g the crack front and the overload region. - 78 -Figure 37 - SCC crack front i n the DCB specimen incorporating a weld - 79 -CHAPTER IV DISCUSSION 4.1 Slow Strain Rate Testing The SSRT has proved to be a remarkably v e r s a t i l e technique i n assessing the SCC s u s c e p t i b i l i t y of the s t e e l , under any set of environmental conditions. I t was a r e l a t i v e l y rapid t e s t , which y i e l d e d a great deal of information. This technique, however, exhibited some drawbacks. An annulus of metal at the surface was affected during the SSRT t e s t s , where cracks formed and grew r a d i a l l y inwards. This annulus e f f e c t i v e l y reduces the load carrying c a p a b i l i t y of the specimen. In metal-environment conditions where the rate of crack growth was low, the annulus was of a small depth, and consequently, the load carrying c a p a b i l i t y of the sample was aff e c t e d to a very small degree. When the percentage reduction i n area i s the only parameter used to gauge SCC s u s c e p t i b i l i t y , misleading, or at best, inconclusive r e s u l t s were obtained. In such s i t u a t i o n s , r e s u l t s were better interpreted by combining data on the frequency of secondary cracking with that of the percentage reduction i n area. Specimen which were thought to be immune to SCC based on area reduction (as i n the passive region i n simulated white l i q u o r environmnts) were found to exhibit a small d e f i n i t e , degree of s u s c e p t i b i l i t y to SCC when r e s u l t s were interpreted i n terms of these other parameters. Since the secondary cracks i n f a i l e d SSRT specimens were - 80 -d i s t r i b u t e d uniformly, the frequency of secondary cracking on the specimen surface, was related to the frequency of cracks along the i n t e r s e c t i o n of any l o n g i t u d i n a l section with the surface (drawing an analogy with the p r i n c i p l e s of quantitative o p t i c a l microscopy). i . e . N Q = K N p ( 1 ) N Q = frequency of secondary cracks/unit area of the surface of the specimen. Np = frequency of secondary cracks/unit length on a l i n e a r intercept of the l o n g i t u d i n a l section with the surface. K = geometric constant Merely measuring the % reduction i n area, as a gauge of SCC s u s c e p t i b i l i t y , tended to overestimate the immunity of the base material from s u s c e p t i b i l i t y to SCC at anodic protection p o t e n t i a l s . The presence of a few secondary cracks at potentials corresponding to the passive zone, indicated a small degree of s u s c e p t i b i l i t y to SCC at these p o t e n t i a l s . Use of a combination of secondary cracking data, and data on the % reduction i n area, to i n t e r p r e t SSRT test r e s u l t s has not been reported i n the l i t e r a t u r e . The r a t i o of the apparent crack v e l o c i t i e s obtained from the SSRT specimen, tested under d i f f e r e n t environments, was approximately equal to the r a t i o of the stage II crack v e l o c i t i e s . Hence, for the s t e e l examined i n t h i s study the SSRT tests were a rapid test method for the comparison of the the r e l a t i v e region II crack v e l o c i t i e s under d i f f e r e n t conditions. Such a s i t u a t i o n could apply to other metal/environment systems. 4»2 Cracking i n Digesters Cracking has been observed i n the fusion zone of welds i n Kraft - 81 -di g e s t e r s . Since the t e n s i l e strength of the fusion zone i s usually greater than that of the base material, f a i l u r e i n t h i s region i s c l e a r l y due to environmental i n t e r a c t i o n . F a i l u r e occured i n the fusion zone, during SSRT t e s t s , only at p o t e n t i a l s corresponding to the active- p a s s i v e t r a n s i t i o n zone. The free corrosion p o t e n t i a l (FCP) was found to reside i n the active zone. However, aeration of i n d u s t r i a l C O white l i q u o r due to improper preparation, and/or storage, would lead to oxidation of the S ions, according to equation 2. 2x S 2 _ + (x - 1) 0 2 + 2(x - 1) H 20 -»• 2 S x 2 ~ + 4(x - 1) OH" (2) 2 _ In p r a c t i c e , an equilibrium which controls the S x concentration Is 2— 2— 2— established between the S x , S and S 203 ions i n s o l u t i o n , according to equation 3. 4 S x 2 _ + 6(x - 1) OH" * 2(x - 1) S 2~ + (x - 1) S 2 0 3 2 " + 3(x - 1) H 20 (3) 2_ The e f f e c t of adding S x to white l i q u o r has been shown to raise the corrosion p o t e n t i a l . Therefore, during commercial pulping, the FCP could be raised to the active-passive t r a n s i t i o n zone where cracking problems i n the weld fusion zone could be encountered. 4.3 Analysis of Pa r t i a l Surface Cracks 4.3.1 General Comments F a i l u r e of digesters could r e s u l t from the growth of stress corrosion cracks, r a d i a t i n g r a d i a l l y outwards, from the Inside of the digester w a l l . - 82 -The stress i n t e n s i t y , Kj, for a p a r t i a l surface crack of C O e l l i p t i c a l shape subjected to a t e n s i l e stress i s given by Kj - Y oVa/Q (4) Y = T/% (5) Q = f(a/2c) = [$ 2 - 0.212 (oVoys) 2] (6a) 2 2 • - J Q / 2 / U - C 2 a ] 8 i n 2 6 d e ( 6 b ) c where 'a' i s the crack depth, and '2c' is the crack length at the surface. 4.3.2 Leak Before Catastrophic Failure Consider the scenario where a single partial surface crack nucleates at a region of local breakdown of passivity, and propagates as an approximately semi-circular flaw, as illustrated in Fig. 38a. Under these conditions, Kj i s given by Kj. = | OV I M ( 7 ) 5 5 The driving force for the propagation of this flaw would be provided by a tensile, hoop stress, - 83 -Figure 38 - Proposed mode of propagation of flaws in the digester wall - 84 -°HOOP = I t C 8 ) (For vessels with a large diameter to thickness ratio) P = internal pressure D <= internal diameter of vessel t = wall thickness of vessel The Hoop stress was found to be 245 MPa and 123 MPa for the 2.54 cm and 5.08 cm thick vessels respectively, u t i l i z i n g a typical internal pressure of 1130 KPa (165 psi), and wall thickness of 2.54 cm (1") and 5.08 cm (2"). The Kj value of the flaw, at penetration of the semi-circular crack through the vessel with a 2.54 cm wall thickness was found to be 44.05 MPa /m. For a 5.08 cm thick vessel, the Kj value of the flaw was found to be 31.28 MPa/m, at the point of penetration through the wall. The above values of stress intensity correspond to the stage II of the log v - Kj plot. Such flaws could cause the digesters to leak, but would not cause catastrophic failure of the type experienced at Pine H i l l , Alabama. 4.3.3 Catastrophic Failure Before Leak Consider the situation where 'n' equally spaced partial surface cracks nucleate, and propagate as semi-circular cracks, coalesce to give an approximately e l l i p t i c a l crack, as illustrated in Fig. 38b. Applying the condition = KQ, equation (4) may be employed to determine the total crack length (2c) of coalesced crack, at which catastrophic failure would occur for specific crack penetration values, 'a', that are Table 6 - C r i t i c a l Coalsced Crack Data a 12 C 2C (cm.) n Frequency Kj of Penetration •a' cm. of Flaws** m~ Single Flaw MPa/m No Residual Stress With Residual Stress* No Residual Stress* With Residual Stress No Residual Stress* With Residual Stress 1.71 - 0.005 - 343.0 - 199 59 36.1 1.78 - 0.03 - 60.0 - 32 56 36.8 1.9 - 0.1 - 19.0 - 9 53 38.1 2.03 - 0.125 - 16.25 - 7 49 39.4 2.16 - 0.16 - 13.5 - 5 46 40.6 2.29 0.01 0.18 228.6 12.7 99 5 44 41.8 2.35 0.02 0.2 117.5 11.74 49 4 42 42.3 2.41 0.04 0.21 60.3 11.5 24 4 41 42.9 •Residual Stress = 35 MPa. **Frequency of Flaws = 1/a. - 86 -less than the wall thickness of 2.54 cm. The Hoop stress i s considered to be the only driving force for crack propagation. The results of the above caculation are li s t e d in Table 5. Since a valid Kjfj value could not be determined from the 1" (2.54 cm) plate, a typical KQ value of 70 MPa/m for the material has been assumed. Also, from the model, 'n', the minimum number of flaws which would have to coalesce to produce a flow of c r i t i c a l dimensions, could be determined from the condition: (n + 1) = 2c/a (9) SSRT test results indicate that the phenomenon of i n i t i a t i o n of cracks i s potential dependant, and is most pronounced in the fusion zone of the weld, at potentials corresponding to the APT. The frequency of cracks in the failed SSRT specimen incorporating a weld, tested at the APT was found to be 72/cm. Although the SSRT test results cannot be directly applied to a practical situation, i t illu s t r a t e s the fact that an extremely high frequency of nucleation points per unit length could be obtained in the fusion zone. This could cause catastrophic failure after a penetration considerably smaller than the wall thickness. 4 04 Effect of Residual Stresses Residual stresses are introduced in the vessel due to the welding operations. These stresses provide an increased driving force for crack propagation in longitudinal welds. Utilizing the analysis which has ben enunciated in Section 4.3, and applying a residual stress of 35 MPa, a coalesced flaw was found to attain c r i t i c a l dimension, after a - 87 -penet ra t ion of 1.71 cm, i . e . , the minimum depth of penetrat ion f o r ca tas t roph ic f rac ture was l ess than that ca l cu la ted i n the absence of a r e s i d u a l s t r e s s . Thus, i f crack nuc lea t ion i s s u f f i c i e n t l y frequent fo r coalescence to occur , the presence of r e s idua l s t resses could s i g n i f i c a n t l y reduce the l i f e t i m e of a s t ruc tu re . However, i t must be recognized, that i n add i t ion to the hoop s t r e s s , the flaw i s a l so acted upon by a t e n s i l e , l o n g i t u d i n a l s t ress (normal to the hoop s t ress ) which could a f f ec t the crack propagat ion . The e f f e c t of the l o n g i t u d i n a l s t ress has been neglected i n the ca l cu l a t i ons conducted above. Thus, the r esu l t s d isp layed i n Table 6 are a l l very conservat ive i n nature, and could be used only as a gu ide l i n e s . Consistent with the coalescence hypothes is , numerous cracks have been observed i n d i g e s t e r s . For e . g . a discontinuous network, 1/8" deep has been observed to run approximately 360° around the circumference of the d iges te r . A l s o , severa l small cracks have been observed transverse to the weld, 1/8" long and 1/16" deep, with 2" long cracks i n the v i c i n i t y , suggesting a coalescence of smaller c r a c k s . 5 6 4.5 Anodic Protection of Digesters The l i f e t ime of d iges ters can be increased by e i t h e r : ( i ) decreas ing the rate of penetrat ion of the f law; or ( i i ) reducing the frequency of nuc leat ion of c racks . A slower rate of penetrat ion could enable the operator to detect the presence of a f law, before i t a t t a ins c r i t i c a l dimensions, during the pe r iod i c inspec t ions which a d iges te r i s subjected t o . A reduct ion - 88 -of the frequency of nucleation of flaws would serve the same purpose. It would necessitate an increased depth of penetration before the crack can attain a c r i t i c a l length. This increases the chances of detection of the flaw before i t attains c r i t i c a l dimensions. Polarization of the A516 Gr.70 steel to potentials in the passive zone brings about a two fold reduction in the crack propogation rate, and a marked reduction in the frequency of nucleation of cracks (lower frequency of cracking in SSRT tests). As a consequence, a flaw could be more easily detected before i t could attain c r i t i c a l dimensions. Although anodic protection might not completely eliminate the problem of cracking in digesters, i t could control the problem, and greatly reduce the attendent risks of catastrophic failure. - 89 -CHAPTER V CONCLUSIONS The results of the present work on M1G welded A516 Gr. 70 steel in a 3.35 m NaOH, 2.5 m NaOH + 0.42 m Na2S, and 3.35 m NaOH + 0.42 m Na2S solutions support the following conclusions: (i) The base material is mildly susceptible to SCC at higher caustic concentrations. The potentials of maximum susceptibility resided in the active-passive transition zone of the anodic polarization curve. ( i i ) The fusion zone of the weld in the steel plate exhibited a preferential susceptibility to SCC at potentials corresponding to the active-passive transition zone. At a l l other potentials, the weld (both the fusion zone and the HAZ) exhibited no preferential susceptibility to SCC. ( i i i ) Anodic polarization was found to decrease the frequency of cracking in simulated white liquor solutions. (iv) The steel exhibited both stress intensity dependent, and potential dependent cracking. The region II crack velocity at -0.75 VgcE (corresponding to the passive zone) was lower than the region II cracking velocity at -0.88 V S c E (corresponding to the active passive transition) by a factor of 2. (v) The region II crack velocity in the fusion zone (dendritic microstructure) was found to be significantly lower than that in the as-received, hot rolled material. Annealing the base material for 1 hr. at 650°C (simulating a thermal stress r e l i e f anneal) however did not - 90 -result in any change in the stage II velocity, as compared to the as-received material. (vi) A hypothesis that crack growth occurs by coalescence stress corrosion cracks has been advanced. - 91 -REFERENCES 1. Hodge J.C., Miller J.L.; Trans ASM; 28; 1940; 25. 2. Scully J.C., Fundamentals of Corrosion; Permagon Press (New York); 1975 3. Spidel, M.O., Fourt P.M.; Stress Corrosion Cracking and Hydrogen  Embittlement of Iron based alloys; ed. R.W.Stachle, J.Hochman, R.D.McBright, and J.E.Slater; NACE; Houston, (1977); PPST-60. 4. Smith K; Pulp and Paper; (10); 1981; pp.66-69 5. Bennett, D.C; TAPPI; V64; (9); 1981; pp.75-77. 6. Hertzberg, R.W.: Deformation and Fracture Mechanics of Engineering  Materials; J. Wiley & Sons; New York; 1976. 7. Yeske R; SCC of continuous Digestors - Report to DCRC: Insititute of Paper Chemistry; 1983. 8. Boulton L. H. and Miller, N.A.; APPITA; V36; (2): pp.126-129 9. Bennett, D.C; Paper presented at the Fourth International symposium on corrosion in the Pulp & Paper Industry; Stockholm, Sweden, May 30-June 2, 1983. 10. Bennett, D.C; Pulp & Paper Industry Corrosion Problems - V.3 -third International symposium or corrosion in the Pulp & Paper Industry, 1980; NACE: Houston; 1982. 11. Humphries, M.J.; and Parks, R.N.; Corrosion Science; 7; 747; (1967). 12. Tromans, D; J. Electrochemical Society; 127; 1253; 1980 13. Singbeil, D.L.; M.A.Sc. Thesis; University of British Columbia; 1981 14. Kruger, J; Stress Corrosion Cracking; ed. J.Yahalom, A.Adadjem; Freud Publishing House (Israel); (1980); pp 6-37 15. Vermilyea, D.A.; & Diegle, R.B.: Corrosion; 1976, 32, 26 16. Diegle, R.B.; Vermilyea, D.A.; J. Electrochemical Soc; 1975, V122, 180. 17. Stachle, R.W.; Theory of Stress Corrosion Cracking in Alloys; ed. J.C. Scully; NATO, Brussels; 1971; p.223 18. Scully, J.C; Corrosion Science; 1967; V7; p.197 - 92 -19. Scully, J.C; Corrosion Science; 1968; V8; P513 20. Parkins, R.N.; Theory of Stress Cracking in Alloys; ed., J.C. Scully, NATO, Brussels; 1971; p.167 21. Hoar, T.P. & Jones, R.W.; Corrosion Science; 1973, V13, p.725 22. Uhlig; H.H.; Fundamental Aspects of Stress Corrosion Cracking; Ed. R.W. Stachle, A.J. Forty; Houston, (1969) p.85 23. Uhlig, H.H.; Perusal, K.E. & Talerman, M.; Corrosion; V30; 1974; p.229 24. Uhlig, H.H.; Hixon, D; Corrosion; V32; 1976; p.56 25. Troiano, A.R.; Trans. A.S.M.; V62; 1960; P.54 26. Tromans, D.; Birlay, S.S.; Corrosion; V27, 1971, p.63 27. Stachle, R.W.; Latanision, R.M.: Scripta Met; V.2, 1968, p.667 28. Haegland, B.: Roald, B.; Norsk Skogindustrie; V9, (10); 1955, p.351-364 29. Biernat, R.J.; Robins, R.G.; Electrochemica Acta; V17, 1972, p.1261-1283 30. Olson, N.J.; Kilgone, R.H.; Danielson, M.J.: Pool, K.H.; Paper presented at the Fourth International Symposium on Corrosion in the  Pulp & Paper Industry, Stockholm, Sweden, May 30-June 2, 1983 31. Bignold, G.J.; Corrosion; V28, (8), 1972, p.307 32. Doig, P.; Flewitt, P.E.J.; Met Trans A, V98, 1978, p.357 33. Melville, P.H.; British Corrosion Journal; V14, (1); 1979, p.15 34. Mogensen, M.: Maahn, E.: Bech-Neilson, G.; Bri t i s h Corrosion  Journal; V l l , 1976, 181 35. Schmidt, H.W.; Gagner, P.J.: Heinemann, G.; Pogacar, C.F., and Wyche, F.H.; Corrosion; 7. 295 , (1951) 36. Rionochl, J.E.; Berry, W.E.: Corrosion; V28; 1972; 151 37. Mazille, H.: & Uhlig, H.H.: Corrosion; V28; p.427; 1972 38. Humphries, M.J. , Parkins, R.N.; Fundamental Aspects of Stress  Corrosion Cracking; ed. R.W. Stachle, A.J. Forty, D.Van Rooyen; NACE; Houston, (1969), p.384 39. Diegle, R.B.; Vermilyea, D.A.; Corrosion, V32, 411, 1976 - 93 -40. Davies, G.J.; Garland, J.G.; International Metallurgical Reviews; V20; 1975 41. Private Communication with Prof. D. Tromans; University of British Columbia 42. Ambrose, J.R.; Kruger, J.: Corrosion; V28; p.30, 1972 43. Parkins, R.N.; Stress Corrosion Cracking; The Slow Stain Rate  Testing Technique STP 665; ed. G.M. Ugiansty & J. H. Payer, ASTM, Philadelphia, (1979), p.5-25. 44. Payer, T.H.; Berry, W.E.; and Boyd, W.K.; St ress Corrision Cracking: The Slow Strain Rate Technique, STP 665; ed. G.M.Ugiansty & J. H. Payer; ASTM, Philadelphia; (1979); p.61 45. Mom, A.J.; Decchar, R.T.; V.D.Wekken, C.J.; Schntre, W.A.; Stress  Corrosion Cracking: The Slow Strain Rate Technique, STP 665, ed. G.M. Ugiansty and J.H. Payer; ASTM, Philadelphia, (1979); p.305 46. Scully J.C; Stress Corrosion Cracking: The Slow Strain Rate  Technique STP 665; ed. G. M. Ugiansty and J.H. Payer; ASTM, Philadelphia, (1979); p.237 48. Brown, B.F.; Met. Rev.; V13, 171, 1968. 49. Wei, R.P.; Fundamental Aspects of Stress Corrosion Cracking; ed; R. W. Stachle, A. J. Forty and D. Van Rooyen; NACE; Houston; (1969) p.104 50. Bohnenkamp K.; Fundamental Aspects of Stress Corrosion Cracking; ed. Stachle R.W., Forty, A.J.; Van Rooyen D; NACE; Houston (1969) 51. Parkins R.N.; J. Iron and Steel Institute; 172; (1952); p.149. 52. Brown, B.F., Srawley, J.E.; Plane Strain Crack Toughness Testing STP. 410; ASTM, Philadelphia, (1966) 53. Singbeil D; Tromans D; Pulp and Paper Industry Corrosion Problems Vol 3; NACE; Houston, Texas; (1982); p.40 54. Wensley, D.A., Charlton, R.S.; Corrosion; V36, 385; (1980) 55. Hertzberg R.W.; Deformation and Fracture Mechanics of Engineering  Materials; John Wiley & Sons; (1976) 56. Private Communication of Prof. D. Tromans with Wensley D.A.; Macmillan Bloedel Research, Vancouver 

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