Elucidation of the Sulfide Corrosion Mechanism in Piston Pin Bushings 2020-01-1079

Recent trends to downsize engines have resulted in lighter weight and greater compactness. At the same time, however, power density has increased due to the addition of turbocharger and other such means to supplement engine power and torque, and this has increased the thermal and mechanical load. In this kind of environment, corrosion of the copper alloy bushing (piston pin bushing) that is press-fitted in the small end of the connecting rod becomes an issue. The material used in automobile bearings, of which the bushing is a typical example, is known to undergo sulfidation corrosion through reaction with an extreme-pressure additive Zinc Dialkyldithiophosphate (ZnDTP) in the lubricating oil. However, that reaction path has not been clarified. The purpose of the present research, therefore, is to clarify the reaction path of ZnDTP and copper in an actual engine environment. In order to ascertain the effects of the heat, copper content, and state of deterioration of the oil on corrosion, component corrosion tests were conducted. The results of the component corrosion tests suggested that the main factor in the corrosion mechanism was not the direct reaction between ZnDTP and copper, but rather that intermediate products contribute greatly. The structure of the intermediate products was identified by making complementary use of X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) analysis at the Aichi Synchrotron Radiation Center. The corrosion reaction path was then clarified by making use of the structure of the products obtained in this way together with first principles calculation to comprehensively evaluate the energy generated in each reaction path.