Corrosion of Sensitized Austenitic Stainless Steel in Hot Aqueous Solutions Under Natural and Electrochemical Control

The corrosion of sensitized austenitic stainless steel in dilute acid at 289 C (552 F) was investigated with electrochemical and corrosion techniques. Grain boundary crevices form initially at the corrosion potential, but penetration is not significant as the crevices fill with corrosion product. Severe intergranular attack occurs if the sensitized alloy is polarized to a passive potential in a pH 3 H₂SO₄ electrolyte. The general corrosion morphology changes significantly with potential. A thick layered oxide forms if the alloy is polarized at the peak of the active region while anodic protection is achieved in the passive region. Fe-Ni-Cr alloys, where the chromium content was <18%, were used in the polarization and corrosion studies. The chromium composition of these alloys were similar to the local grain boundary compositions that might exist after a particular sensitizing treatment. The anodic polarization kinetics of these alloys depends on solution pH and alloy chromium content. At pH 3 or less, alloys containing <12.5% chromium do not passivate. At small anodic polarizing potentials these alloys exhibit extremely high corrosion currents. Exposure under open circuit conditions showed that corrosion increases with decreasing pH and chromium content. The corrosion rate of the alloys containing <12.5% chromium appeared to be limited by the diffusion of hydrogen ions to the corroding surface. The corrosion rate-limiting step of alloys containing >12.5% Cr appears to be a chemical reaction.