Applications of ion implantation for the improvement of localized corrosion resistance of M50 bearing steel

Pitting corrosion of M50 alloy steel bearings used in turbojet engines has been found to be a severe problem. The difficulty arises when salt-spray condensates accumulate in the engine lubricants of aircraft not in regular use. Ion implantation was applied to this problem because in the early stages of this work it was shown to be able to maintain both the dimensional stability and the contact fatigue lifetime of the M50 bearings. Qualitative tests, which simulated the geometry and thermal cycle conditions leading to pitting of the M50 bearing surface, were performed using oil containing 3 ppm NaCl. Initially it was found that chromium surface alloys containing 20-25% chromium substantially reduce the level of attack. Prior to further corrosion simulation tests, potentio-kinetic studies were carried out on M50 implanted with chromium, molybdenum and titanium in order to screen both the passivating tendency of the surface alloys formed and their resistance to localized forms of corrosion. Singular additions of chromium, molybdenum and titanium were found to increase the resistance of M50 to localized breakdown significantly. The highest resistance to localized breakdown was found for a multi-implantation of chromium and molybdenum.