Effects of sigma phase on the initiation and propagation of pitting corrosion of duplex stainless steel

Abstract

Effects of a phase on the initiation and propagation of pitting corrosion of duplex stainless steel were examined using critical pitting temperature (CPT) and electrochemical noise measurements in a ferric chloride solution, and the results were discussed with microstructural variations with aging time. Power spectral density (PSD) and corrosion admittance (A(o)) spectrum as measures of the severity of localized corrosion were used to analyze the electrochemical noise quantitatively. With aging at 850degreesC, the passive film of the alloy becomes significantly unstable due to the initiation of metastable pitting corrosion, which is represented by the increase in PSD. In the admittance spectrum, the value of negative A, (the state for pitting corrosion) was found to increase with aging. This may be attributed to the precipitation of a phase, which depletes Cr and Mo around the alpha/sigma and/or gamma/sigma phase boundaries.