Effect of plating time for electroless nickel coating on rebar surface: An option for application in concrete structure

The ternary Ni–P–Fe alloy coating was developed by an electroless process using a citrate bath. During the steel sample dip in bath, Fe will first get dissolved from the rebar surface and would subsequently get introduced in the bath as Fe2+. This, in turn, would activate the surface and Ni, P and Fe are co-deposited on the activated rebar surface. The first deposition layer would act as catalyst for subsequent metal deposition. The coating was characterised using SEM, EDS and XRD techniques. The bath pH was found to play a strong role on process kinetics as well as composition of the coating but not on coating structure. The weight percentage of Ni, P and Fe in the coating was found to have a relationship with the bath pH. The Tafel and salt spray test were conducted to find out corrosion resistance performance of the coatings. Electrochemical behavior of the coated rebars in simulated concrete environment was influenced by the pH of the concrete pore solution and the ‘P’ content in the coating. Corrosion potential and the corrosion rate of the coating increased with increase in ‘P’ content in the coating and pH of the pore solution, whereas the resistance against chloride attack of the coating increased with increase in ‘P’ content. The marginal drop in the bond strength of the coated rebar with the concrete structure may be due to the higher ‘P’ content which is expected to provide improved surface smoothness.