Effects of secondary phases on localized corrosion of duplex stainless steel, and micro-electrochemical characteristics of polycrystalline zinc and duplex stainless steel

Abstract

Effects of Secondary Phases on Localized corrosion of Duplex Stainless Steel Mechanical and corrosion properties of austenitic-ferritic (α + γ) duplex stainless steels (DSS) are significantly degraded with precipitation of secondary phases such as sigma (s), chi (c) and alpha prime (α´) phase when exposed to temperatures of 250℃ ~ 950℃ during improper heat treatment or welding. In view of corrosion, these secondary phases act as preferential sites for the initiation of metastable pitting corrosion, which can develop into stable pitting corrosion if the degree of occlusion of pit satisfies the critical condition for the growth into stable pit. Metastable pitting corrosion is very important in industrial aspect, because it can develop into stable pitting corrosion after a long period of time even in less corrosive condition than expected from the conventional potentiodynamic test or critical pitting temperature measurements. An electrochemical noise measurement (ENM), through which electrochemical potential or current noises associated with the initiation and repassivation of metastable pitting can be quantitatively analyzed, is considered to be most suitable for investigating effects of secondary phases on the metastable pitting corrosion of DSS. Effects of the secondary phases on the initiation and propagation of localized corrosion of 25Cr duplex stainless steel (DSS) were examined by electrochemical noise measurements in a ferric chloride solution, and the results were discussed with microstructural variations with aging using a X-ray diffraction (XRD) and a scanning electron microscope (SEM). Power spectral density (PSD) and corrosion admittance $(A_c)$ spectrum, as a measure of the severity of localized corrosion, were used to analyze quantitatively the electrochemical noises of the alloy with aging. With an aging at 850℃, passive film of Fe-25Cr-7Ni-3Mo-0.25N alloy became significantly unstable due to the initiation of metastable pitting corrosion ar...