Electroless deposition and some properties of Ni-Cu-P and Ni-Sn-P coatings

In this review, a summary of our published results, completed with new unpublished data are considered together with some of other authors' publications, making an attempt to reveal the mechanism of the third element co-deposition in electroless Ni-P plating and to define its influence on the ternary coatings' properties. A model explaining the triple role of Cu added to the solution for electroless (EL) Ni-P plating is described: as a stabilizer [Cu(I)]; as an accelerator; and as a stability-affecting agent, forming a dispersed solid phase in the solution. The disproportionation reaction of Cu(I) has been taken into account. A planned experiment was performed using full-effect factorial design with two levels of three process variables, and the response surfaces were constructed. The interaction between the factors was revealed. The results are in harmony with the aforementioned model. In low-tin Ni-Sn-P coatings, the alloy components are uniformly distributed both on the surface and through the thickness. In contrast, high-tin coatings exhibit three-dimensional areas enriched in Sn and impoverished in Ni and P. As the reason for their formation, the disproportionation reaction of Sn(II) is suggested, supposed to be locally predominant over the hypophosphite oxidation. EL deposition of pure Sn onto Ni-Sn-P substrates is shown. The inclusion of Cu or Sn in EL Ni-P increases the thermal stability of amorphous state, ensures the preservation of a paramagnetic behavior and improves the corrosion resistance.