Tailoring surface sensitivity towards nitrate reduction for enhanced electrolytic zinc phosphate deposition

Electrolytic phosphating is a wet surface treatment involving local alkalinization of the electrolyte mediated by H+-consuming reactions, such as hydrogen evolution (HER) and nitrate reduction (NO3RR). In this study, the role of NO3RR during electrolytic zinc phosphate coating deposition was investigated on a variety of substrates. Through the use of in-situ differential electrochemical spectroscopy (DEMS), we demonstrate that substrates with higher selectivity towards NO3RR lead to the formation of zinc phosphate coatings with better coverage and finer crystal size. Additionally, we present a strategy to improve selectivity towards NO3RR through the co-deposition of metallic copper clusters. The addition of Cu2+ in the electrolyte with a concentration as low as 0.01 M can lead to improvements of up to 33 % in the coating weight of Zn3(PO4)2 coatings on structural steel. These results could open enable the extension of phosphating treatments to substrates that are traditionally more challenging to coat, with potential applications in the fasteners industry and in wire drawing lines.