Abstract
Thermal films from zinc dialkyl dithiophosphate (ZDDP), an ashless shortchain dithiophosphate (DDP-1) and a longchain ashless dithiophosphate (DDP-2) were generated on steel substrates. XANES data was used to calculate phosphates chain lengths, P/S ratios and sulfate/sufide ratios at different time intervals and mechanistic models were proposed. The results indicate that presence of zinc metal cation in ZDDP allows thermal films to grow for extened period of time with the formation of ZnSO4, ZnS and polyphosphates of Zn. For DDP-1, the lower flash point results in quicker saturation of growth. DDP-2 exhibits a continous growth rate for extended period of time because of its thermal stability. DDP-1 and DDP-2 thermal films are composed of phosphates, sulfates and sulfides of iron.
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