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Electrochemistry of Stress Corrosion Cracking of Brass M Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1986 Electrochemistry of stress corrosion cracking of brass M. Moustafa Said Seleet Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Physical Chemistry Commons Recommended Citation Seleet, M. Moustafa Said, "Electrochemistry of stress corrosion cracking of brass " (1986). Retrospective Theses and Dissertations. 8038. https://lib.dr.iastate.edu/rtd/8038 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. INFORMATION TO USERS This reproduction was made from a copy of a manuscript sent to us for publication and microfilming. While the most advanced technology has been used to pho­ tograph and reproduce this manuscript, the quality of the reproduction is heavily dependent upon the quality of the material submitted. Pages in any manuscript may have indistinct print. In all cases the best available copy has been filmed. The following explanation of techniques is provided to help clarify notations which may appear on this reproduction. 1. Manuscripts may not always be complete. When it is not possible to obtain missing pages, a note appears to indicate this. 2. When copyrighted materials are removed from the manuscript, a note ap­ pears to indicate this. 3. Oversize materials (maps, drawings, and charts) are photographed by sec­ tioning the original, begiiming at the upper left hand comer and continu­ ing from left to right in equal sections with small overlaps. Each oversize page is also filmed as one exposure and is available, for an additional charge, as a standard 35mm slide or in black and white paper format. * 4. Most photographs reproduce acceptably on positive microfilm or micro­ fiche but lack clarity on xerographic copies made from the microfilm. For an additional charge, all photographs are available in black and white standard 35mm slide format.* *For more information about black and white slides or enlarged paper reproductions, please contact the Dissertations Customer Services Department. IMwrsiiy Microfilins LstasiîaéksïaS 8615086 Select, M. Moustafa Said ELECTROCHEMISTRY OF STRESS CORROSION CRACKING OF BRASS Iowa State University PH.D. 1986 University Microfilms In ter n Eltionsil 300 N. zeeb Road, Ann Arbor. Ml 48106 Electrochemistry of stress corrosion cracking of brass by M. Moustafa Said Seleet A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY Department: Chemistry Major; Physical Chemistry Approved; Signature was redacted for privacy. In Charge of Major Work Signature was redacted for privacy. Signature was redacted for privacy. For the Gradate College lowa State University Ames, Iowa 1986 ii TABLE OF CONTENTS Page INTRODUCTION 1 Stress Corrosion Cracking of Brass in Ammoniacal Solutions 1 In tarnishing solutions 10 In nontamishing solutions 12 Stress Corrosion Cracking of Brass in Nonammoniacal Solutions 15 Stress Corrosion Cracking of Pure Copper 21 Summary of Stress Corrosion Cracking Studies: The Unsolved Problems 23 EXPERIMENTAL 27 Material 27 Stress corrosion cracking samples 27 Electrochemical tests samples 27 Solutions 28 Measurement Techniques 29 Stress corrosion cracking tests 29 Electrochemical tests 29 Surface analysis tests 32 RESULTS AND DISCUSSION 33 Brass in Ammoniacal and Ammonium Salts Aqueous Solutions 33 Stress corrosion cracking tests 33 Electrochemical tests 36 Surface analysis tests 86 Brass in Nonammoniacal Aqueous Solutions 107 Stress corrosion cracking tests 107 Electrochemical tests 107 Surface analysis tests 134 ill Page Pure Copper in Aqueous Solutions 137 Stress corrosion cracking tests 137 Electrochemical tests 138 Stress Corrosion Cracking Mechanisms 141 Dissolution mechanism 141 Brittle mechanical failure by adsorption model 145 SUMMARY 147 CONCLUSIONS 149 LITERATURE CITED 151 ACKNOWLEDGMENTS 158 APPENDIX A. THEORETICAL BASICS OF POTENTIAL/pH DIAGRAM CONSTRUCTION 159 APPENDIX B. CHEMICAL REACTIONS OF Cu-NH -H 0 AND Zn-NH^-H^O SYSTEMS ^ 161 APPENDIX C. STABILITY CONSTANTS OF Cu"*", Cu"^, AND Zn"^ IONS IN VARIOUS COMPLEXING SOLUTIONS 163 APPENDIX D. MATHEMATICAL SOLUTION OF THE DIFFERENTIAL EQUATIONS OF Cu+- AND Cu++-COMPLEXES GROWTH RATES 164 APPENDIX E. BRIEF DISCUSSION OF ENERGY DISPERSIVE X-RAY ANALYSIS TECHNIQUE FUNDAMENTALS 168 1 INTRODUCTION Stress Corrosion Cracking of Brass In Ammoniacal Solutions The brass Industry has been troubled since Its beginning with the stress corrosion cracking (SCC) of various forms of brass. SCC was called "season cracking" because of Its resemblance In appearance to the cracking of wood during seasoning; however, this term Is misleading since SCC is a result of internal strains localized in effect by cor­ rosive action [1]. It was known that brasses stored in a dry, and clean atmosphere, although carrying high internal strains, cracked only after a long time, while a damp atmosphere was especially active in producing such cracking, particularly if it carried ammonia vapor from decaying organic matter [2], Past experience indicated also that cracking was more frequent in brasses carrying a higher percent of zinc, Zn. The effect of Zn-content on the number and kind of phases in brass alloy is shown in the phase diagram of brass. Figure 1. Al­ though it has been generally accepted that corrosion is an inevitable factor, the few articles that appeared in the beginning of this cen­ tury were all devoted to studying the effect of internal stresses and how to release them [3,4]. Thompson and Tracy [5] observed that the addition of zinc, phosphorous, arsenic, antimony, silicon, nickel or aluminum to copper results in alloys that are subject to SCC in a moist ammoniacal atmosphere. They identified two modes of cracking, Intercrystalllne and transcrystalline. However, no failure mechanisms were discussed. 2 Extensive systematic study of SCC of brass was initiated by Matt- sson's work, published in 1961 [6]. By that time, the potential/pH diagrams were introduced by Pourbaix [7]. The potential/pH diagram of a certain metal, in some systems, defines the stability regions of different possible forms of this metal produced by its possible chemi­ cal and electrochemical reactions with the system at different values of potential and pH. The theoretical basics of the potential/pH dia­ gram construction are discussed in Appendix A. The potential/pH dia­ grams for most of the elements were constructed by Pourbaix; however. It Is Important to note that they are restricted to binary systems of pure elements with water, e.g., Cu-H^O, Zn-H^O. So, where additional species (e.g., NH^, CN~) are present, the potential/pH diagram must be modified to Include the possible reactions of these species with the element under consideration. Mattsson [6] constructed the potentlal/pH diagrams for the ternary systems: Cu-NHg-HgO and Zn-NHg-H^O (Figures 2a,b). He used these dia­ grams to explain his observations of the relationship between the time- to-crack of brass samples in ammoniacal copper sulphate solutions and the pH of the solution. The chemical reactions used by Mattsson to con­ struct the potential/pH diagrams are shown in Appendix B. To initiate the brass cracking, stress was created in the brass samples by shaping them into loops, where only the outer surface of the loop was under tensile stress. The disadvantage of the loop method, used also by Thompson [8], is the Inability to determine or vary the applied stresses. 3 •C Atomic Percentage Zinc •/? 20 30 40 30 60 70 $0 90 1100 I0S3' 1000 900 900 10. 1400 73 700 —^aa 600 roi '•.3 1000 soo •454\ Ml 400 c- 600 300 200, 400 Cu 20 30 40 50 60 70 80 90 Zn Weight Percentage Zinc Figure 1. Phase diagram of copper-zinc alloys o« OS OS > 05 S 0 2 01 •0 5 Cu pH a a. Cu-NH^-H^O system b. Zn-NH^-H^O system Figure 2. Potential/pH diagram of ternary systems containing 1 g mole NHg and 0.05 M CuSO^ at 25°C [6] 4 The brass (63 Cu-37 Zn) loops were immersed in solutions of different pH and observed visually for crack appearance. All the solutions had the same composition of 1 g-mole of NH^ [added as NHg solution or (NH^)2S0^, in proper ratio to adjust the required pH] +0.05 g-atom/L of Cu, added as CuS0^"5 HgO. Mattsson identified two kinds of solu­ tions , tarnishing and nontarnishing, according to the appearance of the sample after the test. In tarnishing solutions, 6.3<pH<7.3, a black surface coating of Cu^O was observed. Time-to-cracking, t^, was found to decrease with increasing pH, passing through a minimum at pH= 7.3; however, no cracking was identified at pH=2. The cracking mode was intergranular in tarnishing solutions, and transgranular in non­ tarnishing solutions [3.9<pH<5.5 and 7.7<pH<11.2]. The anodic reaction was assumed to be zinc dissolution at all pH values, in addition to cop­ per dissolution at 7.8^pH^1.2. The cathodic reaction was supposed to be the reduction of Cu 2+-complexes at pH values where these complexes are stable, and the reduction of Cu 2+-ions elsewhere. Hoar and Scully [9] observed the importance of yielding of the metal (upon stress) for the appearance of a large increase in dissolution current, and postu­ lated an electrochemical mechanism for the SCC propagation. Takano and Shimodaira [10] observed the SCC of a-brass (70/30) in Mattsson's solu­ tions at pH=7.4, but not at pH=2.0 or 10.0. They suggested that the cracking observed by Mattsson at pH=10.0 was due to mechanical failure caused by the thinning of the sectional area of the specimen by severe corrosion.
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