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Elucidation of the Sulfide Corrosion Mechanism in Piston Pin Bushings
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on April 14, 2020 by SAE International in United States
Today, downsizing is realizing lighter and more compact engines, but at the same time, the use of turbochargers and other supercharging devices in order to supplement power and torque is increasing their power density, resulting in higher thermal and mechanical loads. In such environment, corrosion of the copper alloy bushes (piston pin bushes) that are press-fitted into the small ends of the conrods is becoming an issue. It is known that automotive bearing materials such as bushes suffer sulfidation corrosion as a result of reacting with an extreme-pressure additive (Zn-DTP) in the lubricating oil, but the reaction paths remain unclear. The research discussed in this paper therefore tried to elucidate the reaction paths in the reaction between Zn-DTP and copper in actual vehicle environments. Unit corrosion tests were conducted in order to identify the effect of the state of degradation of the oil and its temperature and copper content on corrosion. The results of these tests suggested that the direct reaction between copper and Zn-DTP was not the main factor in the corrosion under study, but that intermediate reaction products were the major contributors to corrosion. Complementary use of synchrotron radiation XRD and XAFS at the Aichi Synchrotron Radiation Center made it possible to precisely identify the structures of the intermediate reaction products. The reaction paths for the corrosion reaction were elucidated by comprehensive evaluation of the energy generated in each reaction path using the identified structures of the intermediate reaction products and first principles calculations.