Fundamental Studies of Corrosion on Soft Metal/Lubricant Interfaces Using X-ray Spectroscopy

Automotive engine corrosion presents a significant economic and engineering challenge, with estimated global costs reaching billions.1 The main research focus has been on the steel components of engines, but copper has traditionally been used in critical engine parts due to its high heat conductivity and there is an increasing use of Cu-based alloys in the main engine components.2,3 Cu is known to be susceptible to corrosion by common lubricant additives such as zinc dialkyl dithiophosphates (ZDDP) and molybdenum dithiocarbamate (MoDTC). The mechanistic details of Cu corrosion by these additives are essentially unknown. The research reported in this thesis therefore sought to obtain information on the mechanisms of Cu corrosion by MoDTC and ZDDP in typical lubricant phases at elevated temperature. To achieve practical relevance, studies of Cu corrosion were performed under the conditions of the high-temperature corrosion bench test (HTCBT), which is widely used in commercial lubricant R&D for the qualitative assessment of metal corrosion by lubricant formulations. Advanced surface characterization techniques, including X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS) were used to determine the chemical species and phases formed on Cu metal surfaces. The strong corrosive effect of MoDTC on Cu was immediately apparent, revealing the chemical nature of dark tarnish layers developed within a short time interval, and tracing its slow chemical transformations to substantial Cu2S corrosion layers. For ZDDP, results revealed varying tarnish compositions as a function of its concentration in the lubricant. As for MoDTC the corrosion ultimately forms Cu2S as the final product. In addition to characterizing the corrosion of Cu surfaces by MoDTC and ZDDPs, developments of surface-sensitive operando and in situ techniques for XAS analysis by total electron yield (TEY) detection were carried out. An in situ XAS cell was designed to subject metal samples to HTCBT conditions, allowing for multi-modal XAS characterization with sensitivity to bulk and surface speciation, as well as the detection of impurity species. Integration of a TEY detector for an existing research tribometer for real-time surface characterization during tribological testing was demonstrated.