Study of the galvanic coupling between the phases of duplex stainless steel

The interest of the industry in duplex stainless steels is steadily increasing, due to their very good mechanical properties and higher resistance to localized corrosion, compared to the classical austenitic stainless steel. Indeed, the additional presence of the ferritic phase in the duplex improves the corrosion resistance of the alloy. Nevertheless, besides these very good properties, this family of alloys is still sensitive to corrosion in very aggressive environments, such as high chloride concentration and/or very alkaline media. Moreover, the corrosion mechanism of these alloys is still not very well understood. As an example, the 2205 and 2304 duplex stainless steels show two different corrosion patterns. In the first case, the pits are formed preferentially in the austenitic phase while in the second one on both phases. It is suggested that this difference is related to the galvanic coupling between the two phases. Two parallel investigations are being performed in order to better understand the process. The first approach consists in selectively dissolving each phase of the 2205 and 2304 alloys and then investigating their corrosion properties through electrochemical impedance spectroscopy (EIS). In the second study, pure austenitic and ferritic stainless steel coupons are tightly coupled to reproduce an electrochemical model sample of the duplex stainless steel. The galvanic coupling process of this model is also investigated through local electrochemical impedance spectroscopy.