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 (0.290V). Nevertheless, chromium phosphide (CrP4) shifted the anodic corrosion curve toward a nobler region and produced a passive region that was more stable against corrosion.