Determinant parameters of surface morphology to corrosion behaviour of cold-rolled auto sheet steel

To elucidate the effect of surface morphology on corrosion behaviour of cold-rolled steel, three-dimensional (3D) surface profiles, average corrosion rate and potential differences (PD) for local areas of cold-rolled steel were investigated using a 3D white-light interference surface profilometer, polarization curves and atomic force microscope. Results showed that the linear relationship between surface roughness Sa and corrosion rate, achieved based on polished steels, was not valid for the as-received interstitial-free steels. An innovative surface parameter, the proportion of active area (Pa), was proposed and verified to have a distinct positive correlation with the corrosion rate of as-received cold-rolled steels. Moreover, it was found that the specimen with different surface textures were involved in diverse corrosion stages, and thus, the corrosion rate was determined by different surface parameters. As a result, a three-stage corrosion evolution, including initial dissolution (stage I), peak levelling (stage II) and general thinning (stage III), was deduced, in which Pa and local PD played predominant roles to stage I and stage II, respectively.