Corrosion Behaviour of High-Mn TWIP Steels with Electroless Ni-P Coating

An attempt has been taken to improve the corrosion resistance of twinning-induced plasticity (TWIP) steels (Fe-25Mn and Fe-25Mn-3Al) by electroless Ni-P coating. Corrosion resistance in 3.5% NaCl and 0.1M H2SO4 solutions was characterized by the potentiodynamic polarization curves and Tafel method calculations. It was found that Ni-P coating improved the corrosion resistance by the factor of 4 and 40 in 3.5%NaCl and 0.1M H2SO4, respectively. Post-heat treatments, commonly used to increase hardness and wear resistance of Ni-P deposits, were found to reduce the corrosion resistance, even though the 1 h annealing at 700°C less than at 350°C. Also, the adhesion of the coating on the steels was impaired in annealing. The lowered corrosion resistance in annealing can be suggested to result from the crystallization of the amorphous coating and the formation of Ni3P particles, but at 700°C, the formation MnO layer on the top of the coating promoted the resistance by enhancing the passivation in the 3.5% NaCl solution.