Effect of heat-treatment temperature and borohydride concentration on corrosion behaviour of ENB coating

Abstract

Electroless Ni-B (ENB) alloy coatings are extensively used due to their good tribological, physical, electrical and mechanical properties. The behaviour of coatings generally depends on the concentration of bath parameters, heat-treatment temperature, as well as heat-treatment duration. The current study was carried out to deposit coatings over steel specimens with various NaBH4 concentrations and heat-treated at various temperatures to study the significance of NaBH4 concentration as well as heat-treatment temperature on corrosion resistance of the Ni-B coatings. Ni-B coatings were heat-treated for an hour at different temperatures for example, 350°C, 450°C and 550°C. Electrochemical impedance spectroscopy (EIS) and Potentiodynamic polarization (PDP) test methods were employed to analyse the corrosion behaviour of the as-deposited as well as heat-treated coatings. The tests were conducted against a corrosive environment consisting of 3.5% NaCl solution. It was observed that the as-deposited coatings with low boron content exhibit a mixture of amorphous and nanocrystalline structures. The same gradually becomes amorphous with the rise in sodium borohydride concentration. The increase in boron content with sodium borohydride concentration in the coating bath led to the transformation of phase structure. This amorphous phase structure of as-deposited coatings further transforms into a crystalline structure upon heat treatment. This crystallinity increases with heat-treatment temperature. The as-deposited coatings show cauliflower-like surface morphology at all concentrations of NaBH4 but the same becomes a coarse-grained structure with clustered aggregates leading to a rough surface. The as-deposited coatings show better corrosion resistance behaviour compared to heat-treated ones, especially at the higher temperature.