THE ELECTROCHEMICAL OXIDATION OF GOLD IN 0.6 M NACL AND 0.3 M NA2SO4 SOLUTIONS

In order to clarify aspects of dealloying and stress corrosion crackin g of copper-gold alloys, the electrochemical oxidation behavior of pur e polycrystalline gold in 0.6 M NaCl and in 0.3 M Na2SO4 solutions was studied using low-scan rate cyclic potentiodynamic polarization, pote ntial stepping, and Scratching techniques. During cyclic polarization (0.2 mV/s); a limiting current density (anodic current plateau) was ob served over the potential region from 200 to 600 mV/SCE, independent o f the oxygen concentration, the stirring of the solution, or the prese nce of complexing or noncomplexing ions. Consistent with previous find ings in the literature, the anodic current plateau was assumed to be a ssociated with the formation of a two-dimensional, submonolayer, hydro us Au-O surface film, eventually transforming into a thicker oxide fil m. The measured limiting current density (0.1 mA/cm(2)) was approximat ely one tenth of that previously observed for Cu-25 atom percent (a/o) Au between 300 and 430 mV/SCE, the domain of observed stress-corrosio n cracking for this alloy, in which case the current is mainly due to selective dissolution of copper. The current transients produced by bo th potential stepping and scratching of gold in the potential region o f the anodic plateau were compared with transients observed for Cu-25 a/o Au. Arguments were made for attributing these to the faradaic refo rmation of the hydrous Au-O film and the faradaic selective dissolutio n of copper to form gold of submonolayer coverage, respectively. It wa s found that the current transients produced on scratching gold decay exponentially over the first similar to 50 ms followed by an inverse p ower-law decay up to the end of the measuring period of 400 ms. The si gnificance of these results to transgranular stress-corrosion cracking of copper-gold alloys is discussed.