Influence of some anions on the stress corrosion of low alloy steels in aqueous solutions

Abstract

Polarization and stress corrosion behaviour of two types of low alloy steels, AISI 4130X and AISI 4145, were studied in agitated 0.05 M sodium acetate and 0.05 M trisodium phosphate solutions saturated with natural gas at room temperature. The magnitude of currents, shown in potentiokinetic polarization curves in the range of —0.8 V(SCE) to +0.8 V(SCE) in these media, is very small compared with earlier results obtained in sulphate and chloride solutions. Stress corrosion experiments conducted using a slow strain rate technique under cathodic conditions indicate the onset of hydrogen embrittlement on both types of steel in sulphate, chloride, acetate and phosphate solutions. Under anodic conditions, the results in 0.05 M sodium acetate solutions indicate strong susceptibility to stress corrosion compared with no susceptibility in 0.05 M trisodium phosphate solutions if there is no onset of pitting. These results are in contrast with results showing strong anodic dissolution and consequent absence of stress corrosion under anodic conditions in sulphate and chloride media. Results of Auger and IR analyses of anodic surface films formed in acetate and phosphate solutions are presented and discussed in relation to their observed mechanical behaviour.