Effect of Inclusions on Acid Corrosion Resistance of Free-cutting Steel Crankshaft

The effect of MnS inclusions on the acid corrosion resistance of an induction-hardened free-cutting steel crankshaft was investigated by optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that the MnS inclusions exposed on the crankshaft surface could introduce the pitting corrosion in microscale initially at the interfaces of MnS inclusion/metal matrix, which acted as nucleation sites of pitting corrosion in the medium of 10% nitric acid in water. The products of MnS anodic oxidation could accelerate further oxidative corrosion strongly in a surrounding micro zone and cause permanent damages in the form of corrosion pits to the crankshaft subsurface. The statistical analysis shows that the maximum width of the corrosion pits was concentrated at 30-55 μm, and the maximum depth at 10-25 μm.