Effect of microstructure on the corrosion resistance of duplex stainless steels: Materials performance maps

Abstract

Although super duplex stainless steels (25Cr SDSS) are extensively used in oil and gas production due to their excellent combination of mechanical properties and corrosion resistance they are susceptible to the precipitation of deleterious phases during heat treatment and manufacturing operations including welding. Deleterious phases in turn affect both localized corrosion resistance and mechanical properties. Despite the fact that current international standards such as ISO 21457 NORSOK M-001 and NORSOK M-630 treat most 25Cr SDSS grades as equivalent debate still exists as to whether alloying elements such as tungsten (W) accelerate or retard the formation of detrimental phases. Understanding the effect of alloying elements on phase transformation kinetics can help streamline fabrication by optimizing for example welding procedures.In this work the effect of W on the precipitation kinetics of two 25Cr SDSS grades namely UNS S32750 (W-free) and UNS S39274 (High-W) was quantified by constructing Time-Temperature-Transformation (TTT) diagrams. The effect of tertiary phases on localized corrosion resistance was investigated as a function of volume fraction and type of precipitate with a focus on Sigma and Chi phase formation. The critical pitting and crevice temperatures (CPT and CCT respectively) of the various metallurgical stages were determined using both cyclic potentiodynamic polarization (CPP) and ferric chloride (FeCl3) immersion testing.Materials performance maps (MPM) were developed by combining TTT diagrams with CPT and CCT inferred from repassivation potentials (CCT|ERP). Given that MPM provide information about phase transformation kinetics and corrosion resistance simultaneously TTT-CPT(CCT) diagrams could become an indispensable materials selection tool that could be incorporated as parts of international materials selection standards.