Black spots on carbon steel after contact to lubricating oil with extreme pressure additives: an XPS study

Unrevealed synergistic effects of certain additive composition of lubricating oil cause the formation of black spots on guiding planes of cast iron. We performed an X-ray photoemission study comparing samples with black spots to samples without. Sputter depth profiles of the elements Fe, O, C, Si, Ca, Mn, S, P and N were recorded. While the shapes of the iron and oxygen spectra do not differ markedly, the thickness of the iron oxide is much larger on black spots. While Si and Mn were found in detectable concentrations in the oxide layer of samples with and without black spots, Ca, S and P were detected on black spot samples only. Since the detected iron oxides are of dark colour and Mn is known to form dark compounds with O, S and P, we conclude that a thick iron oxide layer containing Mn compounds is responsible for the colouring of the black spots.