The Role of Tungsten on the Phase Transformation Kinetics and its Correlation with the Localized Corrosion Resistance of 25Cr Super Duplex Stainless Steels

Abstract

Super Duplex Stainless Steels (SDSS) have excellent corrosion resistance due to their high concentration of alloying elements like Cr, Mo, and N. There is still, however, disagreement on the role of tungsten in the corrosion resistance of stainless steels. In this regard, the influence of tungsten on tertiary phase precipitation kinetics remains a chief source of controversy. In this study, three different SDSS with different tungsten contents have been investigated, namely, UNS S32750 (W-free), S32760 (0.6 wt% W), and S39274 (2.1 wt% W). Different isothermal aging conditions were studied, followed by microstructure characterization using scanning electron microscopy, energy dispersive X-ray spectroscopy, electron backscatter diffraction, and transmission electron microscopy to quantify the type and volume fraction of tertiary phases and intermetallic compounds. Time-Temperature-Transformation-Corrosion maps were constructed by quantifying the changes in pitting corrosion resistance caused by the precipitation of incremental amounts of deleterious phases. Results showed that 2.1 wt% W additions retarded the precipitation kinetics of all tertiary phases—including σ-phase—favoring the formation of χ-phase. Both χ- and σ-phase affected corrosion resistance, reducing the critical pitting temperature by 10 °C–20 °C at concentrations well below 1 vol%.

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

“…NaOH with an applied potential of 1.5 V for 10 s. Figure 1 shows the microstructure of each material after the heat treatments. The γ and α ratio in these materials was measured by electron backscatter diffraction (EBSD) in a previous work [5], obtaining a 57.2 and 47.6 vol.% α for UNS S32750 and UNS S39274, respectively. These values are within the acceptable 40 to 60 vol.% range for each phase in SDSS.…”

“…On the other hand, there were significant differences regarding the dissolution ratio of W. In 1 M HCl, d' W was close to 1; however, in 7 M LiCl, the results indicated a possible surface enrichment of W on the surface, as d' W < 1. Figure 9 illustrates the E corr * and η obtained from the anodic potentiodynamic polarization curves used for Galvele's critical acidification model, together with the estimated E crit calculated using Equation (5). Even though both materials differed in the E corr * and η, the resulting E crit was similar for both materials in 1 M HCl.…”

“…Contrarily, in 7 M LiCl, the dissolved concentration of metal cations released by UNS S39274 was larger than that of UNS S32750, apart from the tests performed at 60 • C, where UNS S39274 experienced a much lower dissolution than UNS S32750. Figure 9 illustrates the Ecorr * and η obtained from the anodic potentiodynamic polarization curves used for Galvele's critical acidification model, together with the estimated Ecrit calculated using Equation (5). Even though both materials differed in the Ecorr * and η, the resulting Ecrit was similar for both materials in 1 M HCl.…”

“…Tungsten (W) is another element that has been shown to improve corrosion resistance. However, the effect of W on localized corrosion resistance is still disputed, as discussed in detail elsewhere [5,6].…”

“…A recent publication reviewed and studied the influence of W additions to SDSS on the precipitation kinetics of tertiary phases. In this regard, tertiary phases such as σand χ-phase are well known to be deleterious to both mechanical properties and corrosion resistance [5]. The authors showed that about 2 wt.% W reduced the total concentration of deleterious phases, and retarded the precipitation of σ-phase, favoring the formation of χ-phase at grain and phase boundaries.…”