Abstract
The oil and gas industry frequently uses type 25Cr super duplex stainless steels (SDSS) both for seawater applications and for pipes and components exposed to hydrocarbon environments topside and subsea. During the last years, the effect of tungsten on pitting and crevice corrosion resistance has been debated among corrosion experts. In particular, some authors claim that W has a strong synergistic effect with molybdenum when added above a certain threshold value. In the solution annealed condition, tungsten-containing SDSS have been claimed to offer corrosion resistance on a par with higher-grade steels such as type 6Mo (UNS S31254) super austenitic stainless steel grades.
The objective of this investigation was to examine the effect of W on localized corrosion of two SDSS: a high-W (UNS S39274) and a low-W (mod. UNS S32750) grade. Both plain samples (pitting corrosion) and samples with artificial crevices (crevice corrosion) were exposed. Tests were conducted in a 3.50-wt% NaCl solution or natural seawater with temperature ranging from 30°C to 90°C. Cyclic Potentiodynamic polarization scans according to ASTM G-61, critical pitting temperature tests according to ASTM G-150, and long-term seawater open circuit potential exposures of creviced samples were conducted.
The outcome of the testing showed that W addition has a positive effect on both pitting and crevice corrosion resistance. We propose a new parametric definition of Pitting Resistant Equivalent.
Introduction
Duplex and super duplex stainless steels (DSS and SDSS, respectively) are steels composed of a two-phase ferritic-austenitic microstructure, the components of which are both stainless, i.e. they contain more than 13-wt% chromium (Cr) (1) . Although the ferrite content of DSS and SDSS can vary between 35 and 55%, manufacturers balance the steels close to the ideal 50-50% ferrite/austenite ratio (1,2) . DSS are ferritic/austenitic stainless steels with 22-wt% Cr and have a corrosion resistance on par with austenitic grades of similar Cr content (1-4) . Examples of DSS include UNS S32205 and S31803. In contrast, SDSS are defined based not only on their chromium content but also on the alloy's Pitting Resistant Equivalent (PRE) (3,4) . In this regard, the PRE is an empirical formula that attempts to correlate the complex beneficial effect of the main alloying elements using a simple compositionally derived "pitting index" (5) . While NORSOK M-001(3) defines PRE based on Cr, molybdenum (Mo), and nitrogen (N) (Eq. 1) (3,6) , ISO2 21457 (4) includes tungsten (W) into the PRE expression (Eq. 2) (4) .
