Stress-Corrosion Cracking of Cu-25Au Single Crystals in Aqueous Chloride Media

Abstract

Transgranular stress-corrosion cracking (T-SCC) in disordered copper-25 atomic percent gold single crystals was studied in two chloride media: 2% (0.13M) FeCl3 and oxygen-free 0.6M NaCl. Potentiostatic polarization, constant deflection tests, and high-resolution SEM examination were used to relate the electrochemical behavior with susceptibility to T-SCC in this system. Selective electro-dissolution of copper occurs over the investigated potential domain, and polarization is characterized by a region of potential in which the current is very low ( ~ 2–4 microamps/cm2), increasing only very slowly with increasing potential up to a critical potential, Ec. Above Ec the current increases by a factor of 103 in a very narrow domain of rising potential ( ~ 50 mv). In both chloride media the critical potential was found to be ~ +430 mv (sce). At and above Ec a porous gold-rich “sponge” is readily formed on the surface, and in aq. FeCl3 metallographic evidence for sponge formation was found in the low-current region of potential, well below Ec. In both media, fracture surfaces display the characteristic cleavage-like features seen in other T-SCC systems. The lower limit of potential for susceptibility in aq. NaCl occurs at 400 mv (sce), slightly below Ec; whereas, for aq. FeCl3 the limiting potential is 0 mv (sce), fully 430 mv below Ec. For both media, the upper limit of potential, if it exists, is greater than 600 mv (sce). These results are analyzed in order to distinguish between proposed mechanisms for T-SCC.