Effect of Retained Austenite on Corrosion Behavior of Ultrafine Bainitic Steel in Marine Environment

In this study, the impact of retained austenite on corrosion initiation, propagation, and resistance of ultrafine bainitic steels in marine environments based on the one-step and two-step bainitic isothermal transformation was investigated using a combination of theoretical calculations and experimental methods. According to the results, the microstructure of ultrafine bainitic steels was composed of parallel arranged bainite ferrite (BF) and retained austenite (RA). The fraction of the block RA was significantly reduced, whereas the amount of the film RA increased through the two-step bainitic transformation. In the initial stage of corrosion, micro-galvanic effects occurred between the multiphases due to the difference in nobility, leading to the selective dissolution of BF sheaves, which is adjacent to the block RA in the one-step bainitic steel. However, uniformly distributed film RA in the two-step bainitic steel acted as a barrier against corrosion propagation. The electrochemical measurements and neutral salt spray tests revealed a relatively lower corrosion rate for the two-step bainitic steel, indicating a higher corrosion resistance than the one-step bainitic steel. The corrosion products layer mainly consisted of α-FeOOH, γ-FeOOH, and Fe 3 O 4 , which were stable and favorable for the formation of a protective rust layer.