This study examines the effect of cobalt addition on the corrosion resistance of low alloy steel in a 10wt.% sulfuric acid solution at room temperature. All specimens show passive behaviors over the potential range, 0.5–1.55V_SCE, and the passive current density decreased with increasing Co addition. The Co-containing steels also show higher polarization resistance and lower corrosion rate. The beneficial effect of Co is attributed to the formation of a uniform and continuous rust layer due to an interaction between Co and other elements such as Fe, O and S.