Corrosion characteristics of duplex surface-treated spheroidal graphite cast iron

Spheroidal graphite cast iron was treated by a duplex surface treatment process, which was combined plasma nitriding and oxidation. The surface and cross-sectional microstructures and corrosion characteristics were analyzed. Potentiodynamic polarization curves were obtained at room temperature from samples immersed in 3% NaCl solution. In the case of oxidation after nitriding, an oxide layer (double layers of Fe2O3 and Fe3O4) was found to be formed on top of the compound layer (γ′-Fe4N). The value for the average roughness (Ra) of the plasma nitrided sample was higher than that of the plasma oxidation-treated sample. The untreated sample showed typical graphitic corrosion, and the plasma treatment was found to increase the corrosion resistance.