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 alloy. 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 paired with thermally oxidized electrodes. By comparing the electrochemical potentials of the various electrode combinations, we can assign the oxidizing half reactions in cells with freshly abraded Fe, Co, and Ni to the oxidation of the metal to its highest normal oxidation state. The electrochemical potentials for cells containing Al, Cr, and Ti are consistent with the oxidation 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 oxidation. In contrast, the less reactive metals (Fe, Co, and Ni) do not form continuous oxide layers, leaving bare metal exposed to the electrolyte and thus available for oxidation. The reduction potentials measured with thermally oxidized Al cathodes are consistent with the electrolytic release of F− ions from the surface. Similar reactions are expected at the other oxide cathodes. Implications with respect to hard drive lubrication with Fomblin Z-DOL and possible corrosion/degradation mechanisms are discussed.