LOCALIZED CORROSION OF NICKEL-MOLYBDENUM ALLOYS

Transcription

1 I 1 LOCALIZED CORROSION OF NICKEL AND NICKEL-MOLYBDENUM ALLOYS I I' I

2 LOCALIZED CORROSION OF NICKEL AND NICKEL-MOLYBDENUM ALLOYS BY MOHAMMED ZAMIN, M.Eng. A Thesis Submitted to the School of Graduate Studies in Partial Fulfilment o the Requirements for the Degree Doctor of Philosophy McMaster University January 1976 M,OHAMMEO ZAM1'N 1977 I ---~ ~,..{,ie:..,,~k... ~ [J~

3 , DOCTOR OF PHILOSOPHY (1976Y (Metallurgy and Materials Science) McMASTER UNIVERSITY Hamilton, Ontario TITLE: AUTHOR: Localized Corrosion of Nickel and Nickel-Molybdenum Alloys Mohammea Zamin, B.Tech. (Hans) (Indian Institute 'of Technology, Kharagpur). D.I.I.T. (Indian Institute of Technology, Kharaqpur). M. Eng. (McMaster University). SUPERVISOR: Professor M. B. Ives NUMBER OF PAGES: xix,287 \ \

4 ABSTRACT This investigation is concerned with the study of the localized corrosion of nickel and the effect of the addition o molybdenum on its corrosion properties. To this end, alloys containing up to 30 wt % Mo have been studied in ln H 2 S0 4 and chloride containing environments by potentiostatic and galvanostatic techniques and 'in-situ' microscopic observations. The anodic oxidation of nickel has been studied by the galvanostatic technique. The experimental. res~lts have been used to develop the kinetic equation describing the film growth on nickel. It has been shown that an inverse, logarith- "- mic behaviour is obtained in that the film,growth current density is inversely proportional to the charge stored in the film. The resistivity of the film'has been calculated and its value suggests that the film is pure and almost stoichiometric NiO. The significance of the Flade potential of nickel in ln H 2 S0 4 solu~ion is discussed in view of the results obtained and those existing in li~erature.. The implications of the critical potential for determining the susceptibility to pitting corrosion of metals and alloys has been investigated for Ni and Ni-Mo alloys. It is concluded th~t the critical poten- tial does not adequat:ei.y describe the electrochemical conditions! I required. for the p'itting corrosl.qn oflni and Ni- Mo alloys in sulfuric acid solutions containing chloric;}e iqns. iii

5 The passive electrode has been treated as a metalsemiconductor system and justification has been provided for the assumption of a 100% effic~ency for the current density during cathodic reduction. The charge required to reduce the film formed on Ni and Ni-Mo alloys in IN H 2 S0 4 and in the presence of chloride ions is reported. Some information is provided on the nature of the film on the Ni-Mo alloys in chloride containing solutions. Studies are reported on the polarization behaviour of the Ni-Mo alloys. in synthetic sea. water and on the corrosion morphology I obtained after 30 day immersion in Isynthetic sea water and an acidic chloride containing solution. It is proposed that the improved resistance to corrosion of Ni~Mo alloys in naturally corroding systems is due to a sluggish anodic reaction imparted by a slow hydration of the metallic ions, while under anodic polarization in the presence of chloride ions it is due to the presence of a diffusion barrier film of M00 3 The results obtained on the ~i-mo alloys have been explained on the basis of a defect model for the pass{ve film on the alloys. iv

6 ACKNOWLEDGEMENTS My sincerest thanks and gratitude to my supervisor, Dr. M. B. Ives, for his guidance, encouragement and counsel throughout the duration of my graduate work. His sensitiveness to the problems that beset one, both academically and personal- ~,,;J ly, can only be paralleled by his warm friendship. I shall always cherish my association with him. My thanks also to Dr. W.W. Smeltzer and Dr. O. E. Hileman who served as members of my Ph.D. Supervisory Committee. I deeply appreciate their keen interest, advice and guidance throughout my Ph.D. research. I thank Dr. R. Kelly for pringing to my attention the solid-state implications of simple electrode processes, for his candid comment$ and enlightening discussions. I am grateful ~o Dr. G.R. purdy for his help towards the x-ray diff~action part of this work. His eager involvement and insight were invaluable. lowe my thanks to the technical staff'of both the Department of Metallurgy and Materials Science and the. " Ins~itute for Mat.erial.s Research - in particular to Mr.. Frank Gibbs, Mr. Fred Smith, Mr. Fred Pearson and Mr. Martin Van~ Oosten. I also gratefully acknowledge the expertise of Mr. Rudy Pal~e, the university glass blower. I appreciate ~he help ~ffered by Mr. Henry Behmann, Department of Chemical i Engi1eering in obtain~ng for ~he \ nickel in solution. the atomic absorption analysis v

7 The manuscript was typed with skill and remarkable patience by Mrs. Helen Kennelly, to whom I give special thanks. Tne figures were drawn with. precision by Mr. David Hodgson, whose general expertise and constant help I gratefully acknowledge. My heartfelt thanks to two of my special friends who saw me through the ups and downs of graduate study. To Mrs. Angela Manolescu and Mr. Satinder Vig my gratitude for their understanding, friendship ~nd moral support. I thank the following for providing the financial assistance - McMaster university, in the form of a graduate assistantship, and the Defense Research ~oard and the National Research Council, in the form of research grants to Dr. Ives. Last and by no means least, my thanks to a very special person. In the short time that I have known he~, her friendship and counsel, kindness and sincerity, and above all, her love and understanding have shown me an entirely new aspect of life and made the last "and most difficult days of graduate work more enjoyable. To Catherine Jean Good, my deepest gratitude. Finally, there are many others who, either directly or indirectly, were involved at various stages of this work. ~ They are too numerous to mention by name, yet have been too kind to be completely forgotten. To all of them, I remain deeply indebted. vi

8 TABLE OF CONENTS CHAPTER I INTRODUCTION 1.1 Significance of Localized Corrosion Development of Nickel-Base Alloys A - Status of the Hastelloys 1.3 Aims of the Present Investigation An Outline of the Presentation which Follows 9 CHAPTER I I REVIEW OF LITERATURE THE ELECTROCHEMISTRY OF CORROSION 2.1 The Concept of Anodic Polarization The Phenomenon of passivity Theories of- Passivity 2.2.l(a) Generalized Film Theory 2.2.l(bt Electron Configuration Theory The Phenomenon of Localized Corrosion Theories of Localized Corrosion Theories of Pitting Corrosion l.l(a) Penetration Theory l.l(b) Displacement ~heory 25 ~ Theory of Intergranularo 26 Corrosion Comments on the Theories of Localized Corrosion 29 METALLURGY OF THE NICKEL-MOLYBDENUM SYSTE~ 2.4 The Binary' Phase Diagram 2.5 "hases 'and Structures of th~. System The a-phase vii

9 2.5.2 The B-phase The y-phase The 6-phase CORROSION CHARACTERISTICS OF NICKEL-MOLYBDENUM ALLOYS NICKEL AND 2.6 Corrosion of Nickel Passivation Behaviour Localized Corrosion Behaviour 2.7 Corrosion of Nickel-Molybdenum Alloys CHAPTER III EXPERIMENTAL TECHNIQUES AND PROCEDURE 3.1 Materials and Specimen Preparation 3.2 Description of Apparatus Corrosion Cell Assembly Potential Measuring Device Purification of Nitrogen Electrical Circuit 3.3 Experimental Techniques Potentiostatic Polarization Potentiostatic Activation Film Formation Study Cathodic Reduction Studies Repassivation Studies Anodic-Cathodic Polarization I Weight Loss Measurements Genera~ Techniques Metallography viii

10 Page Replication Electron Microscopy X-ray Diffraction Atomic Absorption Spectroscopy 3.5 Reproducibility of Results CHAPTER IV RESULTS: THE CORROSION BEHAVIOUR OF NICKEL 4.1 Introduction 4.2 Passive Film Thickness and Cathodic Reduction Behaviour 4.3 Galvanostatic Film Growth 4.4 The Critical Potential 4.5 Sununary CHAPTER V: RESULTS: CHARACTERIZATION AND CORROSION BEHAVIOUR OF NICKEL-MOLYBDENUM ALLOYS 5.'1 Characterization of the Alloys The Corrosion Behaviour of Nickel Molybdenum Alloys Sulfuric Acid Environment Anodic Polariza~ion Characteristics Film Thickness Measurements Anodic~Cathodic Polarization 1~ Chloride Containing Environments Anodio Polarization Characteristics Film Thickness Measurements Potentiostatic Activation Repassivation Characteristics Weight Loss Measurements 190 ix

11 Effect of Metallurgical structure The Polarization Characteristics of Molybdenum Polarization Behaviour in Sy~thetic Sea Water Immersion Test 5.3 Summary ~ Page ,....~ CHAPTER VI DISCUSSION 6.1 Introduction Kinetics and Mechanism of the Passive Film 210 Formation on Nickel 6.3 Significance of the Flade Potential of 222 Nickel.6.4 The Concept of the Critical Potential Validity of Film Thickness Measurements ~ General Discussion of the Results Comparison of the Results with Those 243 in Literature Anodic-Cathodic Polarization Corrosion of the Alloys 6.7 Mechanism of Corrosion of the Alloys 6.B Effect of Molybdenu~ Addition on the Corrosion Properties of Nickel CHAPTER VII CONCLUSIONS AND REMARKS APPENDIX A 7.1 Conclusions 7.2 Remarks INDEXING OF X-RAY DIFFRACTION DATA APPENDIX B DERIVATION OF THE KINETIC EQUATION FOR INITIAL FILM THICKNESS x

12 Page APPENDIX C MECHANICAL PROPERTIES OF Ni-Mo ALLOYS REFERENCES xi

13 LIST OF ILLUSTRATIONS Figure Caption Schematic representation of an anodic polarization curve. Various symbols used in the text are also shown. The nickel-rich end of the Ni-Mo phase diagram at lower temperatures 44., The variation of lattice parameter of the a-solid solution (a Q ) as a function of molybdenum content for N~-Mo alloys44. The potential-ph diagram for the nickel-water system at 25 C. 66 Schematic diagram of the corrosion cell. The window for 'in-situ' microscopic observations is at the bottom. Schematic. diagram of the cell for the determination of corrosion rates by weight loss measurements33. Schematic circuit diagram of the polarization currentloo. Catho~ic reduction curves for nickel at 2 A i ~A/cm in IN H2S04 for various times of anodic oxidation at +600 mv. Electrode potential-total charge consumed (E-QT) curves for nickel in IN H2S04 at various growth current densities. The curves are uncorrected for dissolution. Electrod~ potential-charge stored in f~lm (E-Qf) curves for nickel in IN H2S04 at various growth current densities. The curves are corrected for dissolution..anodic polarization curves for nickel of varying grain size in 0.02 M Cl- solution (ph=o. 4) Nature of attack on polycrystalline nickel with varying grain size in 0.02 M Clsolution after polarization to mv xii

14 Figure Caption 4-6 Anodic polarization curve for nickel in 0.02MCl- solution (ph==0.4). (a), (b), (g) refer to stages at which optical micrographs were taken to record the development of corrosion morphology shown in Figure 'In-situ' optical micrographs showing the development of surface morphology on a nickel specimen polarized in 0.02 M Clsolution (ph=o.4). The potentiostatic polarization curve is shown in Figure 4-6 and the micrographs refer to stages shown on the curve. 600x 102 and Potential-current density curves for nickel of varying grain size after being held at +400 mv for 28 mins in(~n H2S04 followed by the addition of Cl--ions to yield 0.02 M CI- solution Optical photomicrographs of the surface of the alloys: 109 and The variation of lattice parameter of the a-solid solution (a o ) as a function of molybdenum content for Ni-Mo alloys X-ray diffractometer trace for Ni-22Mo alloy showing the low angle extra peaks associated with the alloy Schematic representation of the effect of lattice strain on the Debye-line width and position X-ray diffractometer trace for Ni-15Mo (single. phase). and Ni-22Mo alloy (two phase) showing high angle p~ak broadening, possibly suggesting coherent precipitation of B(Ni4Mo) in the a-matrix. 118 I 5-6 X-ray diffraztometer trace for Ni-30Mo alloy in various heat treated conditions. 121 I,i 'I I I ) SEM fractographs. Anodic polarization curves for Ni and Ni-Mo alloys in IN H 2 S t I xiii

15 Figure 5-9 Caption l Determination of reducibility of the film on Ni-15Mo alloy. Page Cathodic red~ction curves for Ni-5Mo alloys at 2.7 ua/cm in IN H2S04 after various times of anodic oxidation at +6bo mv Cathodic reduction curves for Ni-5Mo, Ni-lOMo and Ni-15Mo alloys at 2.7 ua/cm 2 in IN H2S04 after 60 mins of anodic oxidation at +600 mv Transmission electron micrographs of two stage gold shadowed carbon replicas of surface of Ni-lSMo,alloy after various electrochemical treatments (a) Anodic-Cathodic polarization curves for nickel and Ni-Mo alloys in IN H2S04 (Overvoltage vs Current Density) (b) Anodic-Cathodic polarization curve for a Ni-22Mo alloy in IN H2S04 (Electrode potential vs Current Density) Anodic polarization curves for Ni and Ni-Mo alloys in 0.02 M Cl- solution (ph=0.4) \ 5-18 I 5-19 t Anodic polarization curves for Ni and Ni-Mo 142 alloys in M Cl- solution (ph=0.4). Anodic polarization curves for Ni and Ni-Mo 143 alloys in M Cl- solution (ph=0.4). Anodic polarization curves for Ni and Ni-Mo 144 alloys in 0.09 M Cl- solution (ph=0.4). Anodic polarization curves for Ni.and Ni-Mo 145 alloys in 0.33 M Cl- solution (ph=0.4). Anodic polarization curves for Ni and Ni-Mo 146 alloys in 0.50 M Cl- solution (ph=0.4) Surface morphology of the specimens after potentiostatic polarization in 0.02 M Clsolution (ph=0.4). Surface morphology of the specimen after potentiostatic polarization in 0.02 M Clsolution (ph=o.4). Jtiv

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