Investigation of electroless plating of Ni–W–P alloy films

Abstract

The electroless deposition of Ni–W–P alloy coatings onto metal substrates using H₂PO₂ ⁻ as reducing agent from solutions containing nickel sulfate, sodium tungstate, sodium citrate, ammonium sulfate and other additives was studied. At most temperatures (60–80 °C) and pHs (7–11) investigated, bright and coherent coatings uniform in appearance were produced. Phosphorous and tungsten contents ranging from 3.5 to 8 wt % and 0.5 to 6 wt %, respectively, were obtained depending upon solution temperature and pH. Trends such as the effects of pH and temperature on average metal deposition rate and the P content in the alloy are similar to that reported previously for the Ni–P system. Correlation of open-circuit potentials with events occurring at the electrode surface in different solutions and polarization curves provide strong evidence that Ni²⁺ ions participate in W and P deposition, H₂ evolution and H₂PO₂ ⁻ oxidation and that H₂PO₂ ⁻ ions participate in cathodic reduction. This indicates that the partial reactions for the Ni–W–P system do not occur independently of one another.