Action of Carbon in Stress Corrosion Cracking of Mild Steel in Nitrate Solutions

Abstract

Abstract The effect of total carbon content on susceptibility of iron to stress corrosion cracking (SCC), on the depth of intergranular corrosion without stresses, anodic polarization curves, and the general corrosion rate was examined on decarburized Armco iron and its ferritic or martensitic alloys with carbon up to 1.4% in 5N NH4NO3 and 5N Ca(NO3)2, pH 5.0, at 75 and 100 C (167 and 212 F). The susceptibility to SCC and intergranular corrosion increased with the carbon content increasing up to about 0.009% or above, and it diminished with further increase in the carbon content. Carbon decreased the ability of iron to passivate (depassivating action), but it also promoted deposition of magnetite and enhanced its protective properties (impeding action). It is suggested that the observed effect of carbon on SCC and intergranular corrosion results from the predominance of the depassivating action at low carbon contents, and the predominance of the impeding action at higher carbon contents.