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, E_c. Above E_c 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 E_c 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 E_c. 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 E_c; whereas, for aq. FeCl₃ the limiting potential is 0 mv (sce), fully 430 mv below E_c. 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.