Comparison of mechanical, corrosion, and hydrogen permeation properties of electroless Ni-Zn-P alloys with electrolytic Zn-Ni and Cd coatings

Novel Ni-Zn-P alloys containing 74 wt% Ni was prepared from a sulfate electrolyte using an electroless process. The mechanical, corrosion, and hydrogen permeation resistance characteristics of the developed Ni-Zn-P alloys were studied and compared with those of electrodeposited Cd and Zn-Ni coatings. Comparison of the mechanical properties shows that the developed alloy possesses engineering attributes comparable to those of Cd coatings. Corrosion studies in a 0.5-M sodium sulfate (Na₂SO₄) + 0.5-M boric acid (H₃BO₃) solution at pH 7.0 reveals that the dissolution rate of Ni-Zn-P alloys is five times lower than that of Zn-Ni and Cd coatings. In addition to the corrosion and engineering properties, comparison of the hydrogen permeation characteristics of the developed coating was made with that of Cd and Zn-Ni coatings. The exchange current density, i_o: thickness dependent adsorption-absorption rate constant, k″: recombination rate constant, k₃: and the surface hydrogen coverage, θ_s, that characterize the hydrogen evolution kinetics of the coatings were obtained by applying a mathematical model to experimental results. From the resulting data analysis, Ni-Zn-P alloy was seen not only to provide longer life under corroding media, but also reduces greatly the risk of the substrate being exposed to hydrogen embrittlement when compared to electrodeposited Cd and Zn-Ni coatings.