PASSIVITY BREAKDOWN DUE TO DISCONTINUOUS PRECIPITATION DURING AGING OF 21CR-1OMN-5NI STAINLESS-STEEL

Discontinuous precipitation in a 21Cr-1OMn-5Ni steel was found to resu lt in breakdown of passivity when tested in a 0.5 M H2SO4 + 0.01 M KSC N solution. The electrochemical response was similar to that due to se nsitization of conventional stainless steels. The ageing time and temp erature affected the resistance of the passive film, with lower ageing temperatures being more severe in terms of passivity breakdown. The e ffect of interlamellar spacing on the chromium depletion, the effect o f volume diffusion on the healing of the matrix, and the solubility pr oduct reaction for precipitation, were considered to explain the obser ved different strengths of passive films formed on specimens aged at d ifferent temperatures and times. The observance of good corrosion prot ection in spite of the presence of profuse precipitates was in contras t to the effect of grain-boundary carbide proliferation on the corrosi on resistance of conventional stainless steels.