Aqueous passivation of an Fe-based alloy containing two refractory metals

There are a few examples of alloys with two or more passivating components where the specific role of each component in forming the passive film is understood. We examined the electrochemical passivation behavior of Fe 1-x Cr x V y alloys in 0.1 M H 2 SO 4 in terms of the Cr + V content. Our results showed that for y \(≤\) 0.04 we obtained an excellent fit to the percolation passivation theory, but for y > 0.04, the theory underestimated the number of monolayers dissolved in order for passivation to occur by about a factor of two. The chemical short-range order parameters for the alloys were characterized using neutron scattering; however, the differences among the alloys for y \(≤\) 0.04 and y > 0.04 were minimal. Energy and wavelength dispersive spectroscopy revealed that all the alloys contained nanometer-scale V-oxygen clusters that served as sites for the initiation of pitting. Pit densities, pit diameters and pit depths were characterized using focused ion-beam machining, scanning electron microscopy and digital image analysis. These results allowed us to develop an analysis based on Faraday’s law for the difference in the number of monolayers dissolved for y = 0.04 and y > 0.04, and we found excellent agreement between our experimental results and the analysis. Our results provide a path forward for evaluating passivation behaviors of alloys that exhibit pitting.