Effect of Mo content on the corrosion resistance of (CoCrFeNi)_1-xMo_x thin films in sulfuric acid

(CoCrFeNi)(1-x)Mo-x thin films with various Mo content (0-10 at.%) were grown by magnetron sputtering on a stainless steel substrate. The films with 0-2 at.% presented two crystal structures: one FCC phase and one sigma phase, while films with higher Mo content only had the FCC structure. All films have a (111) texture and follow the topography of the substrate. The corrosion resistance of the films was evaluated in 0.05 M H2SO4 at room temperature and at 80 degrees C. It was observed that the corrosion current densities considerably decreased for Mo > 2 at%, and that the current densities were higher at the elevated temperature. Scanning Kelvin Probe Force Microscopy showed a large potential difference between the main FCC phase and sigma phase for the Mo0-2 films. This would suggest that preferential dissolution of the FCC phase occurs over the sigma and reduces the corrosion resistance. Such preferential dissolution does not occur for the higher Mo content films with only the FCC phase. The high corrosion resistance was also attributed to the inhibition of Fe and Cr dissolution by Mo and the stabilisation of the Cr enriched oxide by incorporating Mo oxides into the passive film, identified by X-ray photoelectron spectroscopy. The low corrosion current densities (below 1 mu A/cm(2)) make these thin films possible candidates for protective coatings of bipolar plates in PEM fuel cells.