Improvement of corrosion resistance of Ni−Mo alloy coatings: Effect of heat treatment

Graphical abstract: - Highlights: • Conjunction between SEM, EIS, and Tafel measurements to obtain a coat with dense morphology and without crack. • An inverse Hall-Petch effect is observed after annealing the coatings, i.e. the coatings get harder as the grain size is increased by increasing annealing temperature up to 600 ^oC. • Heat treatment can improve the mechanical and corrosion properties of coatings. - Abstract: In this paper, Ni−Mo alloy coatings were deposited from bath containing sodium citrate, nickel sulphate, and sodium molybdate. Essentially, this work is divided into two mains parts: (i) the optimization on the coatings deposition parameters and (ii) the effect of the heat treatment. Polarization curves and electrochemical impedance spectroscopy were acquired using potentiostat/galvanostat and a frequency response analyzer, respectively. Morphology and chemical composition of the coatings were investigated by scanning electron microscopy and energy dispersive spectroscopy, respectively. Polarization curves at different condition revealed that electroplating at temperature 40 ^oC, pH 9 provides a dense coating with high efficiency. Following the optimization of the deposition parameters, the coatings were annealed at 200, 400, and 600 ^oC for 25 min. The results showed that the coatings obtained at temperature 40 ^oC, pH 9, and annealing at 600 ^oC has the highest corrosion resistance and microhardness.