Transgranular stress-corrosion cracking of disordered Cu-25Au in aqueous chloride and sulfate media

Abstract

Transgranular stress-corrosion cracking (T-SCC) in disordered copper-25 at. pct gold single crystals was studied in two chloride media: 2 pct (0.13 M) FeCl3 and oxygen-free 0.6 M NaCl. A limited number of tests were also performed on polycrystals in the chloride media and also in acidified 1 M Na2SO4. Potentiostatic polarization, constant deflection tests, and high-resolution SEM examination were used to relate the electrochemical and fractographic features of T-SCC in these systems. Exclusive selective electro-dissolution of copper occurs over the potential domain investigated, and the polarization behavior is characterized at low potentials by a “passive≓ domain in which the current is very low (2 to 4 microamps/cm2), increasing only very slowly with increasing potential up to a “transpassive” potential,E c. AboveE c the current increases by a factor of 103 in a very narrow domain of rising potential ( ≈ 50 mv). In both chloride media Ec 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 also found in the low-current region of potential, well below Ec. In all three media, fracture surfaces display the characteristic cleavage-like features scen in other T-SCC systems. For the constant deflection loading employed in the present investigation, the lower limit of potential for susceptibility to cracking in aq. NaCl occurs at +400 mv (sce), which is slightly below Ec; whereas, for aq. FeCl3 the limiting potential is 0 mv (sce), considerably belowE c. For both chloride media, the upper limit, if it exists, is greater than +600 mv (sce). These results are analyzed in order to distinguish between proposed mechanisms for T-SCC.