Effect of phosphorus doping on some properties of electroplated Zn–Ni alloy coatings

Zn–Ni alloys were electrodeposited on carbon steel plates and their morphology, composition and structure were evaluated as a function of the amount of Na2H2PO2 added in the plating bath. The presence of phosphorus in Zn–Ni alloys, even in very small amounts, showed effectively a remarkable performance in comparison with the corresponding P-free alloy coating. In fact, Na2H2PO2 added in the plating bath offered a way of optimizing the Ni content in the deposit. Meanwhile, the microhardness was observed to increase. Conversely, these coatings showed smaller crystallite size. Data of XRD had shown that the quantity of η-phase decreased. An inhibitory effect of phosphorus co-deposition was also observed in 3% NaCl solution. Corrosion properties of Zn–Ni alloy coatings were evaluated by means of potentiodynamic polarization and electrochemical impedance spectroscopy. The doped coatings exhibited considerably enhanced corrosion resistance and barrier properties.