Abstract: Black pits appeared on the surface of a marine diesel engine crankshaft after final finishing, which resulted in the failure of the product. By means of macroscopic analysis, macro examination, chemical composition analysis, SEM microscopic analysis, metallographic analysis, energy spectrum analysis and so on, the causes of the black pits on the crankshaft surface were analyzed. The results show that the black pits on the crankshaft surface were pitting corrosion pits which resulted from the corrosion of the crankshaft because it was exposed to the corrosive medium during the machining process. The final finishing allowance was small and the corrosion affected layer could not be completely removed, so black pits formed on the crankshaft surface.