The effects of alloying elements, such as nickel and chromium, on the corrosion rate of linepipe steels have been investigated using electrochemical techniques; suitable alloy designs are proposed for both weld metal and base metal. Anodic-polarization measurements revealed that the addition of nickel to the weld metal depolarizes the anodic-polarization curves while the addition of chromium to the weld metal polarizes the cathodic-polarization curves. The differences in corrosion-current densities between the base metal and the weld metal obtained by the Stern-Geary method were independent of nickel content; on the other hand, the corrosion-current density became smaller by the addition of chromium to the weld metal. The role of nickel and chromium on the corrosion rate was mechanistically considered from the viewpoint of corrosion-film characteristics using the a.c. impedance technique. It was elucidated that addition of chromium results in a very tight corrosion film, with higher corrosion resistance arising from formation of Cr2O3 on the surface of the weld metal.
