Effect of coating bath composition on the properties of electroless nickel-boron films

The effect of borohydride, thallium acetate, ethylenediamine and sodium hydroxide concentrations, and the coating bath temperature on both the coating rate and boron content of the electroless Ni-B films was investigated systematically. The Ni-B coating rate increased with the increase in thallium acetate and sodium hydroxide concentrations, but it was not very sensitive to the borohydride concentration. Below 90 g L− 1 ethylenediamine concentration the coating efficiency was significantly low and above this value as the ethylenediamine concentration increased the coating rate decreased slightly. Below 85 °C the coating rate was very low and above this temperature it was insensitive to the bath temperature. The boron content of Ni-B film increased with the increase in the borohydride concentration and the bath temperature, and decreased with the increase in thallium acetate and ethylenediamine concentrations. Up to 50 g L− 1 sodium hydroxide concentration, the boron content of the film increased and above this concentration it was insensitive to the sodium hydroxide concentration. As the boron content of Ni-B film increased, both the corrosion resistance and microhardness of Ni-B film increased. Heat treatment brought significant improvement in the microhardness but the corrosion resistance of Ni-B film was observed to decrease due to the disappearance of the amorphous characteristics of the as-deposited Ni-B film and the formation of the Ni-B compound phases.