Corrosion study of standard duplex stainless steel UNS S31803 and lean duplex stainless UNS S32304 in liquor from paper and pulp mill

Abstract

Duplex stainless steel (DSS) was employed to distinguish between two phases: austenite and ferrite. The microstructure was analyzed using magnetic force microscope (MFM) and scanning Kelvin probe microscopy (SKPFM). MFM was used to distinguish the phases austenite and ferrite, due to magnetic features (ferrite is ferromagnetic and austenite is paramagnetic). SKPFM was applied to evaluate the variation of Volta potential between austenite () and ferrite (). Also, MFM/SKPFM technique showed to be a powerful tool in studying the phases in duplex stainless steels without surface etching necessarily. In this study, X-Ray Diffraction (XRD) and Rietveld Refinement were performed to identify and quantify the ferrite and austenite phase of cold and hot rolled duplex stainless steels (UNS S31803) and lean duplex stainless steels (UNS S32304). Electrochemical impedance spectroscopy (EIS) was applied to evaluate the chemical behavior of duplex and lean duplex stainless steels in white, green, and black liquors of paper and pulp industry. Rietveld analysis results showed a higher austenite content than the standard limit for duplex steels in the hot rolled condition. The hot rolling condition plays a major role in improving corrosion resistance in white liquor mainly for the lean duplex steel. The cyclic potentiodynamic polarization behavior of duplex stainless steel (DSS) and lean duplex stainless steel (LDSS) was studied in white and green liquors from a pulp and paper processing plant. The corrosion behavior in industrial and also synthetic liquors was compared. The polarization curves of the duplex steels in synthetic white liquor were shifted to lower potentials and higher current densities in relation to the steel in industrial white liquor, which proved to be less aggressive to the duplex steel. The duplex steels also showed the highest values of transpassive potential in industrial white liquor. Cold and hot rolled duplex and lean duplex steels in green liquor showed the lowest values of transpassive potential. Specimens of duplex stainless steel (DSS) were exposed at high temperature (200 ºC) in industrial white liquor from a Brazilian kraft mill using an autoclave to simulate the same conditions as a digester processing. Tafel extrapolation method and weight loss were used to evaluate corrosion behavior of steels. The results showed that the corrosion rates of DSSs increase with temperature increases. As well as selective dissolution of the ferrite phase