Effect of phosphorus content on the mechanical, microstructure and corrosion properties of supermartensitic stainless steel

In this study, the effect of high and low phosphorus contents on the mechanical and corrosion properties of two supermartensitic stainless was evaluated as a function of tempering temperature (570, 620, 670 and 690 °C). The best heat treatment condition (tempered at 620 °C) was determined by conducting potentiodynamic polarization tests in natural seawater. At all temperatures, the two tempered steels presented nearly the same values of hardness, yield strength (0.2%), ultimate tensile strength and elongation. A high phosphorus content promoted the formation of nanoprecipitates of CrP4 between the retained austenite (3%) and martensitic grain boundaries. This structure significantly improved the impact toughness. The steels also presented similar pit potentials (0.290 V). Nevertheless, chromium phosphide (CrP4) shifted the anodic corrosion curve toward a nobler region and produced a passive region that was more stable against corrosion.