Electroless Deposition of Ni–Sn–P and Ni–Sn–Cu–P Coatings

Abstract

The surface morphology and the elemental distribution of low- and high-tin Ni–Sn–P coatings have been investigated. It is shown that in low-tin Ni–Sn–P coatings there is a uniform distribution of the alloy components, both on the surface and through the thickness. The main mechanism of electroless alloy deposition in this case is based on the well-known hypophosphite oxidation as a source of electrons for the metals (Ni and Sn) and phosphorus reduction. In high-tin coatings, a nonuniform distribution of the components is observed, both on the surface and through the coating thickness. Three-dimensional areas enriched in tin and impoverished in Ni and especially in P have been observed using scanning electron microscopy with energy-dispersive X-ray spectroscopy, scanning Auger electron spectroscopy, and Auger electron spectroscopy. The disproportionation reaction of Sn(II) is suggested as being predominant over the hypophosphite oxidation in these three-dimensional areas and is responsible for their formation. The introduction of copper in the solution is giving an additional opportunity to reveal the role of the oxidation of Sn(II) into Sn(IV) as a source of electrons.