Effect of pH on anomalous co-deposition and current efficiency during electrodeposition of Ni-Zn-P alloys

Abstract

Electrodeposition of Ni-Zn-P alloy coatings was studied from an electrolyte containing hypophosphorous acid (H3PO2) as a source of phosphorus. In this study, the effect of electrolyte pH on deposit composition, current efficiency and anomalous codeposition was investigated in detail. With increase in pH of the electrolyte, the Zn and P-content of the NiZnP alloy coating decreased with commensurate increase in Ni-content. The faradic efficiency of deposition was found to increase with increase in pH and ultimately saturated beyond pH=10. At lower pH, the anomalous effect of Zn2+ was prominent which turned into non-anomalous at pH≥9.0. Evidence of simultaneous electroless deposition was noticed during electrodeposition and the corresponding deposition rate was nearly unaffected due to electrolyte pH variation. X-ray diffraction investigation confirmed the deposition of amorphous NiZnP alloys throughout the composition range studied here. Scanning electron microscope (SEM) examination of as-deposited surface showed smooth to nodular growth pattern with multiple cracks depending upon the pH values. The cross-section examination of deposited Ni-Zn-P alloy microstructure revealed growth of multiple layers with non-uniform thickness ranging from 100nm to 1μm along the growth direction. Corrosion experiments were performed in static as well as in potentiodynamic conditions in an electrolyte containing Na2SO4, H3BO3 under neutral condition. Coatings with Zn content in the range 18–22wt% were found to display open circuit potential (OCP) of −0.630 to −0.600V vs. SCE which are more active but close to steel substrate OCP and hence can be used as a sacrificial coating for the protection of steel for a longer duration.