Accelerated Testing to Investigate Corrosion Mechanisms of Carburized and Carbonitrided Martensitic Stainless Steel for Aerospace Bearings in Harsh Environments

Carburizable martensitic stainless steels (MSSs) are attractive candidates for bearings due to their high corrosion resistance, high hardness, and high temperature performance. Wear performance in tribocorrosion applications is strongly influenced by the surrounding environment. Electrochemical testing was used to evaluate three different surface treatments on AMS 5930 steel developed for advanced gas turbine engine bearing applications: low temperature (LTT), high temperature (HTT), and carbonitrided (CN). HTT had a higher corrosion rate that increased with time, whereas LTT and CN had lower corrosion rates that were stable over time. Accelerated testing revealed that surface treatment significantly influenced how corrosion propagated: HTT was more uniform; conversely, LTT and CN showed localized attack. Degradation mechanisms developed from electrochemical methods provide rapid insight into long-term wear behavior

Corrosion Initiation and Propagation on Carburized Martensitic Stainless Steel Surfaces Studied via Advanced Scanning Probe Microscopy

Historically, high carbon steels have been used in mechanical applications because their high surface hardness contributes to excellent wear performance. However, in aggressive environments, current bearing steels exhibit insufficient corrosion resistance. Martensitic stainless steels are attractive for bearing applications due to their high corrosion resistance and ability to be surface hardened via carburizing heat treatments. Here three different carburizing heat treatments were applied to UNS S42670: a high-temperature temper (HTT), a low-temperature temper (LTT), and carbo-nitriding (CN). Magnetic force microscopy showed differences in magnetic domains between the matrix and carbides, while scanning Kelvin probe force microscopy (SKPFM) revealed a 90–200 mV Volta potential difference between the two phases. Corrosion progression was monitored on the nanoscale via SKPFM and in situ atomic force microscopy (AFM), revealing different corrosion modes among heat treatments that predicted bulk corrosion behavior in electrochemical testing. HTT outperforms LTT and CN in wear testing and thus is recommended for non-corrosive aerospace applications, whereas CN is recommended for corrosion-prone applications as it exhibits exceptional corrosion resistance. The results reported here support the use of scanning probe microscopy for predicting bulk corrosion behavior by measuring nanoscale surface differences in properties between carbides and the surrounding matrix