Combined electrochemical and surface analysis measurements revealed that 304L stainless steel, implanted with P, oxidizes at near-transpassive anodic potential in 1 N H2SO4 to give corrosion products that affect the corrosion rate differently depending on the amount of P at the surface of the metal. At low concentrations, P assisted corrosion by facilitating dissolution of the oxidation products; at high concentrations, the products were insoluble and precipitated to form a thick, electrically resistive film. The resultant voltage drop contributed to lower currents as long as the thick film was stable. The dependence of corrosion rate on P concentration is believed to contribute to the two-peak current response observed for the material in potential-step measurements.