The stress-corrosion cracking of mild steel in nitrate solutions

Abstract

The stress-corrosion cracking of 0.08% C and 0.30% C steels in boiling nitrate solution has been investigated. The principal aim has been to assess the role of stress in stress-corrosion cracking. Slow strain rate tensile tests in boiling nitrate solution have enabled the stress required to initiate stress-corrosion cracks to be measured and compared with data from tensile tests in the absence of nitrate solution. Constant strain stress-corrosion tests in a hard-beam tensometer have been used in addition to conventional static stress-corrosion tests. The dependence of stress-corrosion propensity on grain size, strain rate and pre-strain has been studied. Thin film electron microscopy has been used to compare the dislocation arrangements in a steel susceptible to stress-corrosion cracking with those in a more resistant steel. Initial stages in the corrosion of mild steel in nitrate solutions have been studied by examination of thin foils in the electron microscope, both before and after their immersion in nitrate solution. The investigation has revealed no evidence for purely mechanical stages in stress-corrosion cracking and previous evidence for such stages has been shown to be capable of alternative interpretation. A cracking mechanism is proposed, by which failure occurs as a result of continuous electrochemical corrosion. The role of stress is attributed to that of opening up corrosion fissures, so as to make the corrodent available at the fissure tip.