Effect of carbon content and microstructure on the corrosion resistance of low alloy steels

The corrosion resistance and corrosion behavior of low alloy steels with different carbon contents and different microstructures were investigated and compared with a commercial weathering steel 09CuPCrNi. Optical microscopy and SEM were employed to examine the microstructures of these steels. It was revealed that the dominant microstructure in the steel cooled in water after rolling is lath-like bainite while the microstructure in the steel cooled in air after rolling is the mixture of acicular ferrite, granular bainite, M/A islands and little cementite (pearlite). Cyclic wet-dry test was carried out to evaluate the corrosion resistance of these steels. The results indicate that the corrosion resistance of the low carbon steel with 0.03 96 C or 0.196 C, which was cooled in water after rolling, excels that of 09CuPCrNi steel. Provided with low carbon content, the influence of on the corrosion resistance of the tested steel is not obvious. With a higher carbon content, the bainitic steel with single phase exhibits a higher corrosion resistance than multi-phase steel constituted by ferrite and cementite (pearlite). No marked distinction can be detected in the corro-sion resistance of the steels with different carbon contents when they were cooled in water after rolling. When cooled in air after rolling, the steel with low carbon content exhibits a higher corrosion resistance than the steel with a higher carbon content. Through observations on rust layers with SEM, it can be found that a compact inner rust layer forms in the specimen with excellent corrosion resistance after long corrosion.