Electrochemical Corrosion Behavior of Nanocrystalline Metal Thin Film Prepared By Magnetron Sputtering -- a Recent Work Report

The small grain size and the high volume fraction of grain boundaries result in significantly different corrosion behavior between nanocrystalline materials and corresponding coarse crystalline materials. The results by several electrochemical techniques indicate that in active dissolution, nanocrystallization accelerates the corrosion reactions; in passivation, nanocrystallization increases the passive ability. Further researches indicate that for passive materials such as stainless steel, Al alloys and Ni alloys, nanocrystallization changes the composition of the passive film, which improves the enrichment of passive elements such as Cr. In local corrosion, nanocrystallization increases the unstable points on the surface of the materials, which increases the possibility of local corrosion. However, the excellent ability of element diffusion helps heal the local corrosion point, which inhibits the growth of the local corrosion.

In recently, we use the first principles calculation to study the energy change in the passive film and on the interface. We found that NC thin film promotes the formation of more Cr2O3 to increase the growth rate of passive film: more Cr₂O₃ increases the formation energy E_f of the passive film. Cr₂O₃ is apt to form at the interface to make the system most stable, which possesses lowest E_R. The diffusion rate of defects and others across the passive film is promoted on NC thin film, which results in the formation of a Cr atom enriched layer at the oxides scale/metal interface. All of those reflect the super passive ability of nanocrystallization structure.

And our group also found a thin membrane passive film over the growing pit on sputtering nanocrystalline stainless steel thin film for the first time, which has two layer structures and possesses higher elastic modulus and hardness. This thin membrane passive film will influence the corrosion behavior of material, especially pitting corrosion.