Selective corrosion of duplex stainless steel bars in acid Part 1: Effect of the composition, microstructure and anodic polarizations

The selective corrosion of the phases comprised in four duplex stainless steels (DSS) is studied in acid media (H2SO4+HCl solutions). The work considers traditional UNS S32205 grade as well as other economical-advantageous grades (UNS S32001 and UNS S32304). The samples of the lean DSS come from bars formed by hot working (HW), while those of the S32205 come both from HW bars and bars obtained by cold working. The amount of strain-induced martensite in the austenitic phase of DSS that the processing has induced is highly dependent on the DSS composition. The change in the relative corrosion resistance of the different phases comprised in the microstructure with the potential is considered. The influence of the composition of the DSS is discussed. It has been checked that, at potentials close to the corrosion potential (E-corr), the corrosion of the ferritic phase is more favored than that of the austenite. Increasing potentials in the anodic activation region tends to favor the selective corrosion of the austenite. The trend to selective dissolution is less marked for the least alloyed DSS. The strain-induced martensite in the austenitic phase exhibits a corrosion resistance clearly higher than that of the ferrite at E-corr or potentials close to E-corr. The possible influence of TiN precipitates in the corrosion development is also analyzed.