Use of the Critical Acidification Model to Estimate the Influence of W in the Localized Corrosion Resistance of 25Cr Super Duplex Stainless Steels

Abstract

Two super duplex stainless steels (SDSS) with different W content, namely UNS S32750 (W-free) and UNS S39274 (2.1 wt.% W), were tested in simulated crevice corrosion environments to determine the influence of W on their corrosion resistance. Anodic potentiodynamic polarization experiments were performed in two different crevice-like-solutions: 1 M HCl (as reference for a pH = 0 environment), and 7 M LiCl, adjusted to the same pH value. Galvele’s critical acidification model was used to estimate the theoretical critical potential (Ecrit) and comparatively evaluate the corrosion resistance of the two SDSS. The anodic potentiodynamic polarization results showed a statistically significant difference between the two materials in only one test condition, i.e., 7 M Cl− at 60 °C. Additionally, the quantification of chemical dissolution of the metal cations after the tests suggested a surface enrichment in W only in the 7 M chloride solution. Scanning electron microscope (SEM) analysis indicated a uniform dissolution experienced by UNS S32750 in this environment, whereas UNS S39274 suffered selective corrosion of the ferrite-phase. These observations were reflected in a slight increase in the Ecrit values of UNS S32974 estimated with Galvele’s model.