Corrosion Behavior of Newly Developed High-Strength Bainitic Railway Wheel Steels

High-strength bainitic wheel steels based on a new steel composition, namely MS2 [0.47% C, 0.87% Mn, 0.51% Si, 0.02% Mo, 0.03%Cu, 0.031% S, 0.033% P and rest Fe (weight percent)], were prepared by austempering treatment at 400 °C for 10, 30, 60 and 120 min. Corrosion behavior of the bainitic MS2 steels and normalized ferritic–pearlitic MS2 steel was studied and compared with that of the conventional ferritic–pearlitic wheel steel. Immersion test, electrochemical polarization and salt fog tests were carried out in 3.5% NaCl. Bainitic steels prepared with higher holding duration during austempering exhibited improved corrosion resistance on salt fog exposure. In contrast, bainitic steels prepared with lower holding duration during austempering resulted in better corrosion resistance on electrochemical polarization test. Salt fog test revealed gradual improvement in corrosion resistance for the bainitic steels with the successive holding. Improved corrosion behavior for the bainitic steel was related to the tiny and uniform spreading of iron carbide in the ferritic matrix resulting in the uniform coverage of the protective oxide layer on the steel surface. The enrichment of Si on the corroded surface of bainitic steel favored the formation of supermagnetic α-FeOOH of smaller particle size, improving the protective character of the rust against corrosion. Corrosion mechanisms for the ferritic–pearlitic and bainitic wheel steels were put forward.