Recent developments in the surface-mobility stress-corrosion-cracking mechanism

Abstract The surface-mobility stress corrosion cracking (SCC) mechanism has been successful in explaining numerous cases of SCC where deleterious films are formed on the metal surface. Recent work has shown two important new facts: (1) Surface mobility could be acting in systems where the presence of specific metal cations in the solution is essential. In such cases SCC is strongly related to the exchange current density, i 0 , between the metal and the solution. An equation for the calculation of surface diffusivity, based on i 0 has been developed and successfully applied to SCC of Ag–Cd and Cu–Zn alloys. (2) When studying the effect of the strain rate (SR) on the crack propagation rate (CPR), it was found that when increasing the SR a monotonic increase on the CPR occurs. When plotting the log CPR versus log SR, a significant difference is found between intergranular stress corrosion cracking (IGSCC) and transgranular stress corrosion cracking (TGSCC). The slopes for IGSCC ( α =0.5–0.7) are considerably higher than those for TGSCC ( α =0.2–0.3). This difference in behaviour is found even when single crystals and polycrystals of the same alloy are compared under the same experimental conditions. This observation could explain why some authors reported a sort of ‘intergranular cleavage’ and had difficulty in finding an equivalent ‘transgranular cleavage’.