Influence of iron element on the structure and corrosion resistance of Ni-P coatings in different corrosive environments

Ni-P coatings and Ni-Fe-P coatings with the same thickness and the same phosphorus content were prepared by electroless deposition. The effects of iron on the structure, morphology and corrosion resistance of Ni-P coatings were studied. The results show that the iron can improve the flatness of the Ni-P coatings and increase its crystallite size to make it appear crystalline state. The corrosion behavior of the two coatings was investigated in 3.5 wt% NaCl solution, saturated CO2 solution and 0.1 mol.L- 1 H2SO4 solution. During the corrosion process, iron can promote the conversion of Ni(OH)2 to NiO in the passive film. In 3.5 wt% NaCl solution, the Ni-Fe-P coatings exhibited better corrosion resistance, which was due to the fact that NiO can effectively inhibit the ion exchange during the corrosion process. In saturated CO2 solution, the corrosion resistance of Ni-P coatings was more excellent. This is because the corrosion reaction mainly occurs between the coating and the substrate, and the smaller crystallite size of the Ni-P coatings can effectively prevent the intrusion of corrosion ions. In 0.1 mol.L- 1 H2SO4 solution, the Ni-Fe-P coatings was seriously damaged, which was not suitable for metal protection in strong acid environment.