Corrosion behaviour of molybdenum-implanted stainless steel

Abstract A low-molybdenum austenitic stainless steel (UNS S30100) has been surface implanted with molybdenum ions, using various doses of 50 keV and 140 keV ions at room temperature. It is found that in aqueous sulphate/chloride solutions similar to the constitution of sea-waters the implantation does not affect the potentiostatically-determined critical pitting potential, but does change the density and morphology of corrosion pits. Pitting initiation after the addition of chloride at a fixed potential indicates little change in the time for measurable current increase, but the rate of increase of the current is much lower for implanted material. Detailed examination using optical microscopy, scanning electron microscopy, transmission electron microscopy and selected area diffraction suggests that the pits produced in implanted material are hemispherical with smooth covers of unattacked alloy. The use of half-implanted samples demonstrates that molybdenum implantation causes the formation of smooth-covered pits rather than “lacy” attack characteristic of stainless steel attack in chloride solutions. The energy of implantation also affects the density of pit nucleation, suggesting that passive films formed after implantation at low energies are not able to completely protect the steel surfaces.