Initiation of borohydride-reduced nickel boron plating on steels with various carbon and nickel content.

Electroless nickel-boron coatings are widely used in industrial on various substrates: ferrous and non-ferrous alloys mainly but also in some cases non-metallic materials. However, their growth process is still not fully understood and the influence of the nature of the substrate on this process is completely unknown.
The formation of electroless nickel-boron was observed on 5 ferrous alloys: a mild steel, a high carbon unalloyed steel, a cryogenic steel (that contains 9 wt.% nickel), an austenitic stainless steel and an austeno-ferritic (duplex) stainless steel.
Nickel-boron films were prepared by electroless deposition, using sodium borohydride as a reducing agent. Samples were immersed in a plating bath for times ranging from 5 s to 60 min. The influence of the nature of the substrate on the initial deposition of the coatings was investigated in detail: the initiation mechanism was identified for all substrates and it was found to be related to catalytic oxidation of the reducing agent rather than to a displacement process.
The delay before initiation was influenced by the nickel content of the coating and by a high number of grain boundaries.
In all cases, the plating rate varied with plating time, with a slower period during the first 10 minutes that corresponds to morphological modification of the coating.