Electrochemical noise analysis of localized corrosion of duplex stainless steel aged at 475 °C

Effects of precipitation of alpha prime (α′) phase on the localized corrosion of Fe-25Cr-7Ni-3Mo-0.25N duplex stainless steel (DSS) were examined by electrochemical noise measurements in ferric chloride solution. Power spectral density (PSD) and corrosion admittance (Ac) spectrum as a measure of the severity of localized corrosion were used to analyze quantitatively the electrochemical noise of the alloy with aging. Electrochemical potential and current noises of the alloy in passive state were found to increase with aging at 475 °C, as represented by the increase in PSD. This suggests that passive film was degraded due to the formation of Cr-depleted region around Cr-rich α′ precipitates that was formed on a nanometer scale within ferrite grains during the aging. In the corrosion admittance spectrum, the deviation from zero Ac value increased with aging. In addition, the number of positive Ac corresponding to a state of uniform corrosion was higher than that of negative Ac for the over-aged alloy. Numerous micro-galvanic cells formed on a nanometer scale between Cr-rich α′ precipitate and neighboring Fe-rich phase in ferrite grains led to a selective-uniform dissolution of ferrite in the aged alloy accompanied with pitting corrosion.