The scientific community is yet to reach consensus regarding the influence of tungsten (W) additions on the localized corrosion resistance of super duplex stainless steels (SDSS). In this work, three SDSS varying mainly in their W composition were studied to elucidate the effects of W as a solid solution element on crevice corrosion resistance. The localized corrosion resistance of these materials was studied using the potentiodynamic-galvanostatic-potentiodynamic (PD-GS-PD) technique instead of the broadly used cyclic potentiodynamic polarization (CPP) technique described in the ASTM G61 standard due to the high corrosion resistance of the materials. Additionally, the long-term crevice corrosion resistance of the W-free and high-W SDSS was investigated in natural seawater to validate the results obtained with the PD-GS-PD technique.
Figure 1. Crevice repassivation potential (ER,Crev) or transpassive potential (ETrans) obtained for all the materials.
The objective of this investigation was to quantify the crevice corrosion resistance of commercial SDSS as a function of W content. Therefore, three SA materials differing mainly in their W content were compared. Firstly, PD-GS-PD tests were conducted at incremental temperatures to determine changes in TR,Crev as a function of W content. The results were, then, validated using long-term potentiostatic testing in natural seawater.
Figure 2. Natural seawater long-term tests of extruded pipe materials (a) UNS S32750 at OCP, (b) UNS S39274 at OCP, (c) UNS S32750 polarised to +600 mVAg/AgCl, and (d) UNS S39274 polarised to +600 mVAg/AgCl, the picture inserts show a specimen after exposure to illustrate the attack.
Why is it important?
This study showed W had a beneficial effect when added up to 2.1 wt.% in solid solution. In this case, the data did not suggest an optimal W concentration range, in contrast with the observations made on isothermally treated SDSS. Additionally, the long-term natural seawater data validated the crevice corrosion trends observed with the short-term PD-GS-PD technique.
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C. Torres, R. Johnsen, M. Iannuzzi, "Crevice Corrosion of Solution Annealed 25Cr Duplex Stainless Steels: Effect of W on Critical Temperatures," Corros Sci 178 (2021): p. 109053. https://doi.org/10.1016/j.corsci.2020.109053