Oxidation and reduction reactions responsible for galvanic corrosion of ferrous and reactive metals in the presence of a perfluoropolyether lubricant: Fomblin Z-DOL

We recently reported unambiguous evidence for galvanic corrosion involving a perfluoropolyether (PFPE) lubricant, Fomblin Z-DOL, and a commercial Al a lloy. Here, we extend these measurements to a series of high-purity metals: Al, Ti, Cr, Fe, Co, and Ni where freshly abraded metal electrodes are pair ed with thermally oxidized electrodes. By comparing the electrochemical pot entials of the various electrode combinations, we can assign the oxidizing half reactions in cells with freshly abraded Fe, Co, and Ni to the oxidatio n of the metal to its highest normal oxidation state. The electrochemical p otentials for cells containing Al, Cr, and Ti are consistent with the oxida tion of partially oxidized species (Ti2+ --> Ti3+ + e(-) in the case of Ti) at the anode. The continuous oxide films that form on the reactive metals (Al, Cr, and Ti) apparently prevent the oxidation of the underlying metal. Nevertheless, partially oxidized species are available for further oxidatio n. In contrast, the less reactive metals (Fe, Co, and Ni) do not form conti nuous oxide layers, leaving bare metal exposed to the electrolyte and thus available for oxidation. The reduction potentials measured with thermally o xidized Al cathodes are consistent with the electrolytic release of F- ions from the surface. Similar reactions are expected at the other oxide cathod es. Implications with respect to hard drive lubrication with Fomblin Z-DOL and possible corrosion/degradation mechanisms are discussed. (C) 2001 Elsev ier Science B.V. All rights reserved.