Specific Effect of Metal Cations on Stress Corrosion Cracking
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henomenon of season cracking of brass was discovered at the beginning of the past century,[1] but it was reproduced in the laboratory only when it was found that the presence of Cu2+ ions was necessary to induce cracking, and this discovery led to the development of the Mattson solution.[2] Since then, many authors have investigated the phenomenon,[3,4,5] but no explanation of why the presence of Cu2+ is necessary to induce cracking was given, excepting the fact that it modifies the electrode potential. Surprisingly, the specific effect of metal ions on stress corrosion cracking (SCC), although occasionally reported, has never been seriously considered from a mechanistic point of view. The specific effect of certain cations on SCC has been observed in various systems.[6–13] The SCC has been found for Cu alloys,[6–9] Ag alloys,[10,11,12] and Au alloys[13] when ions of the more noble component of the alloy (Cu, Ag, and Au ions, respectively) are present in the solution. In particular, Ag-Cd alloys, with compositions ranging from Ag-10Cd (at. pct) up to Ag-40Cd (at. pct), are known to be susceptible to SCC in silver nitrate aqueous solutions, at the equilibrium potential of Ag.[11] The aim of the present work was to determine whether this is a more general phenomenon. To that purpose, Ag-20Zn (at. pct) samples were tested in solutions containing Ag+ ions at the equilibrium potential of Ag.
S.B. FARINA, Researcher, and G.S. DUFFO´, Researcher, are with the Comisio´n Nacional de Energı´ a Ato´mica and the Consejo Nacional de Investigaciones Cientı´ ficas y Te´cnicas, (1650) San Martı´ n, Buenos Aires, Argentina. J.R. GALVELE, Professor, is with the Instituto Prof. J. Sabato, Univ. Nac. Gral. San Martı´ n, (1650) San Martı´ n, Buenos Aires, Argentina. Contact e-mail: [email protected] Manuscript submitted October 2, 2006. Article published online July 17, 2007. 1872—VOLUME 38A, AUGUST 2007
The samples used were 0.08-cm-diameter wires of Ag-20Zn (at. pct). The chemical composition of the alloy was (wt pct) Zn13.2, Al < 0.005, Pb < 0.002, Si < 0.002, Fe < 0.002, Mg < 0.0002, Cu 0.2 ± 0.05, and Ag balance. The yield strength, after heat treatment, was 88 ± 1 MPa. The specimens were degreased with acetone, annealed for 1 hour in argon at 600 C, and air cooled. Prior to the measurements, the surface of the samples was again degreased with acetone and dried with hot air. Tests were performed at constant potential with a Hounsfield tensometer that was modified in the laboratory to obtain an initial strain rate of 4.3 · 10)6 s)1. The cell used in the wire straining tests is described in a previous publication.[14] The solutions used were 1 M AgNO3, 0.5 M AgNO3 + 0.5 M NaNO3, 0.1 M AgNO3 + 0.9 M NaNO3, 0.05 M AgNO3 + 0.95 M NaNO3, 0.01 M AgNO3 + 0.99 M NaNO3, and 1 M NaNO3. A constant nitrate concentration of 1 M was maintained. The solutions were prepared with analytical grade reagents and deionized water (resistivity = 18 MWÆcm). The tests were performed at the equilibrium potential for the Ag+ + e– = Ag reaction. The electrode poten
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