Nucleation and Growth of Stress Corrosion Cracks in Austenitic Steels with Varying Ni and Mo Contents

The growth rates and densities of cracks formed by stress corrosion in MgCl2 solution at 153 C have been compared in several austenitic steels. These include two commercial steels, Type 316 and 310, and experimental melts with 17% Cr and varying Ni and Mo contents. The familiar {110} slot growths occurred below slip traces in steels of Type 316 and 310 and in Mo-containing alloys but were not observed in the Mo-free steels (although stress corrosion cracks formed in such steels when the Ni content was low). Crack nucleation at nonmetallic inclusions was observed in the Type 310 steel as previously observed in Type 316 steels and crack growth rates were similar in these two types of steel. In the Mo-free, low Ni content steels, nucleation occurred at both pits and grain boundaries. The crack growth rates in such steels were much lower, and the critical stresses for cracking higher, than in Mo-containing steels. Similar changes in the electrochemical behavior were observed on either increasing the Ni content or adding Mo and yet these additions give opposite effects on SCC susceptibility. An increased passivation rate and stable oxide formation can explain the detrimental effect of Mo. The beneficial effect of Ni additions to the 17% Cr steels is not considered to be simply that of increasing the stacking fault energy, γ, since Type 310 steel of similar γ is more susceptible. Chemical factors such as the slow passivation of etched regions are probably of equal importance. A slightly modified slip dissolution theory is discussed.