Evolution of texture and phase constitution in Ni-P coatings with phosphorous addition and its effect on the coating corrosion behaviour

Abstract

ABSTRACT Ni-P coatings with varying amounts of phosphorous in lower (<1wt%) composition were electrodeposited over mild steel substrate. Microstructure analysis was performed using scanning electron microscopy, x-ray diffraction, and electron backscatter diffraction. Increasing the P content in the Ni-P coatings gradually transformed the coatings from crystalline to nanocrystalline. Minor additions of P in the Ni coating matrix (0.04 wt.% P) shifted the growth texture from (110) to (111). The corrosion behaviour of the coatings was highly sensitive to the phosphorous content along with its correlation with the transformation from crystalline to nanocrystalline. For the low concentration of P in the Ni-P coating matrix, a monotonous increase in strain within the crystalline phase and galvanic coupling due to lower volume fraction of the anodic nanocrystalline phase was observed, which led to the deterioration of the corrosion protection properties. High P content led to an increase in the nanocrystalline region and thus nullified the intense galvanic coupling and subsequently increased the corrosion resistance again. This study revealed that the texture and volume fraction of the crystalline phase can determine the corrosion behaviour of the Ni-P coatings, and at an optimum phosphorous concentration, highest corrosion resistance behaviour can be achieved.