ABSTRACT
Super duplex stainless steels are ferritic-austenitic stainless steels with 25 wt% Cr and a pitting resistance equivalent (PRE) = 40. Even though other elements, such as Cr and Mo, have been studied in much more detail than W, research has shown that an optimal composition exists, in which W in solid solution improves localized corrosion resistance. Outside this range, W is ineffective or even detrimental. However, the mechanisms by which W improves localized corrosion resistance in stainless steels are unclear. For example, debate still exists as to whether W enhances passivity or facilitates repassivation.
In this work, two super duplex stainless steels were investigated: a W-free (UNS S32750) and a 2.1 wt% W-containing (UNS S39274) grade. The first goal was to study differences in the overall localized corrosion resistance; while the second objective was to gain an insight into the mechanisms by which W affects the localized corrosion performance of super duplex stainless steels. The work was divided into two parts. Firstly, anodic potentiodynamic polarization curves were conducted in different simulated pit environments at various temperatures. Two simulated pit solutions were tested: 1 M HCl and 7 M LiCl adjusted to a pH of 0. In the second part, crevice repassivation potentials were measured using the potentiodynamic-galvanostatic-potentiodynamic technique in 3.5 wt% NaCl pH = 6.5 solution. This investigation will provide evidence regarding the influence of W in passive film stability and repassivation kinetics
INTRODUCTION
Super duplex stainless steels (SDSS) are a type of highly alloyed stainless steels, with at least 25 wt% Cr. Additionally, SDSS have a dual austenitic and ferritic microstructure, with an ideal ratio between phases as close to one as possible.1 Because of their high Cr, Mo, and N content, SDSS are widely used in corrosive environments where high corrosion resistance is required. However, SDSS can suffer localized corrosion in seawater and are limited to a maximum service temperature of 20°C according to NORSOK M-0012 and ISO 214573.
