Corrosion resistance of electroless Ni–low B coatings

Abstract

The corrosion resistance of electroless (EL) Ni–low B coatings, obtained using an alkaline borohydride-reduced electroless plating bath, with varying concentrations of NaBH4 (0?2–1?0 g L21), in 3?5%NaCl, was evaluated. The rate of deposition, boron content and the size of the nodules of the EL Ni–low B coatings were increased while the crystallinity of the coating was decreased with increasing concentration of NaBH4. The change in chemical composition and decrease in crystallinity did not seem to have any influence on the corrosion resistance of the EL Ni–low B coatings of the present study, as opposed to the nodular growth with a columnar structure which had a profound effect. The results of polarisation and electrochemical impedance spectroscopy (EIS) studies confirm penetration of the corrosive medium through the columnar nature of the coating and ascertain its dominating influence on the corrosion resistance of EL Ni– low B coatings over other factors. The results of the present study again confirm the poor corrosion protective ability of EL Ni–B coatings and justify the selection of EL Ni–P coatings for applications that warrant high corrosion resistance.