Microstructural and corrosive interactions in phosphorus ion implanted 304L stainless steel—II. Alterations in corrosion resistance by implantation

The optimum fluences for homogeneous amorphous phase production were determined in Part I of this work for accelerating potentials of 50, 100, 150, 175 and 200 kV. Polarization measurements of these amorphous specimens in room temperature 1.0 N H2SO4 at a scan rate of 0.1 mV s−1 in the present work show a decrease in the critical anodic and passive current densities by factors between 2 and 10 when compared to corresponding values for unimplanted, electropolished specimens. These decreases become less pronounced with increasing implantation energy. The presence of crystalline phases, especially b.c.c. phase, within the surface region of the amorphous phase degrades the observed amorphous phase improvements. Recrystallization of the amorphous structure removes all corrosion resistance improvements obtained from the P+ implantation. As discussed herein, the cause of the electrochemical improvements is a combination of a microstructural effect imparted by the amorphous phase and a chemical effect derived from the implanted phosphorus when present in quantities of ⋍40% within ⋍25 Å of the surface. Chromium depletion at the specimen surface during implantation, as well as during annealing, also degrades the electrochemical properties and acts in competition with improvements caused by the added phosphorus.