Effect of ε-carbide (Fe2.4C) on Corrosion and Hydrogen Diffusion Behaviors of Automotive Ultrahigh-Strength Steel Sheet

Effects of epsilon-carbide (Fe2.4C) on corrosion and hydrogen diffusion behaviors of ultra-strong steel sheets for automotive application were investigated using a number of experimental and analytical methods. Results of this study showed that the type of iron carbide precipitated during tempering treatments conducted at below A(1) temperatures had a significant influence on corrosion kinetics. Compared to a steel sample with cementite (Fe3C), a steel sample with epsilon-carbide (Fe2.4C) showed higher corrosion resistance during a long-term exposure to a neutral aqueous solution. In addition, the diffusion kinetics of hydrogen atoms formed by electrochemical corrosion reactions in the steel matrix with e-carbide were slower than the steel matrix with cementite because of a comparatively higher binding energy of hydrogen with epsilon-carbide. These results suggest that designing steels with fine epsilon-carbide distributed uniformly throughout the matrix can be an effective technical strategy to ensure high resistance to hydrogen embrittlement induced by aqueous corrosion.