Electrodeposition of Copper Clusters during Electrolytic Zinc Phosphating to Increase Nitrate Reduction Selectivity
Authors: Lissandrello Federico , Lukatskaya Maria R. , Magagnin Luca 
Published: December 2024

Despite being one of the most widespread and well-established surface treatments, conversion phosphating is often scrutinized because of its negative impact on the environment. Electrolytic phosphate coatings are perhaps the primary candidates to lead the phasing out of the old conversion technology, while also offering similar material performance. [1] In this embodiment of the process, the formation of phosphate crystals is promoted by the application of a cathodic potential to the substrate, triggering hydrogen evolution (HER) and nitrate reduction (NO3R) at the interface. These two reactions consume H+ ions, leading to a local alkalinization of the electrolyte and eventually to phosphate precipitation. So far, very little attention has been placed on the effects of the two reactions on the coatings’ properties. On one hand, HER is driven by direct reduction of protons and will always take place as long as the electrolyte is acidic; on the other hand, the formation of H2 bubbles at the interface can interfere with the coating formation itself. Conversely, NO3R does not lead to formation of gas byproducts, but its contribution to the total cathodic current depends strongly on the activity of the substrate.

In this work, we established a correlation between the properties of zinc phosphate coatings obtained by the electrolytic method and the NO3R activity of the substrate they are deposited on. For the purposes of the study, differential electrochemical mass spectrometry (DEMS) was used to evaluate the HER partial current density in-operando, under various NO3
- concentrations. In this regard, Au and Cu substrates were selected because of their widely different selectivity towards NO3R. [2] The results highlighted drastically improved properties for the zinc phosphate coatings obtained on copper, suggesting a positive correlation between substrate NO3R selectivity and zinc phosphate coating quality.

The importance of these results is then discussed in relation to the treatment of low-alloy steel, which has the greatest relevance in industrial applications. In this regard, the coating weight of zinc phosphate coatings obtained under various NO3
- concentrations is evaluated and compared to copper. The findings are also corroborated with SEM and XRD measurements, to determine the coating morphology and composition.

Finally, the addition of Cu2+ to the phosphating formulation is proposed as a mean to increase the selectivity towards NO3R during the deposition. Under the cathodic polarization maintained during the coating process, Cu2+ ions can be directly reduced on the steel surface, leading to the formation of metallic Cu clusters, which can then act as catalysts towards NO3R. Additions of Cu2+ as low as 5 mM in the solution led to improvements in the coating weight and homogeneity, indicating a better utilization of the NO3
- ions in solution.

In conclusion, NO3R is found to have beneficial effects during the formation of phosphate coatings via the electrolytic deposition technology. Direct electrodeposition of metallic Cu clusters with minimal modification of the base electrolyte was found to be a viable method to improve the activity of the substrate towards NO3R. The findings of this study could help the development of novel electrolytes for electrolytic phosphating, with particular attention to other strategic alloys, such as those based on Mg and Al, that could greatly benefit from this technology.

References

[1] F. Lissandrello, N. Lecis, L. Magagnin, Investigating the effect of current density in ultra-fast electrolytic zinc phosphate deposition, Electrochim Acta 478 (2024) 143840. https://doi.org/10.1016/J.ELECTACTA.2024.143840.

[2] G.E. Dima, A.C.A. De Vooys, M.T.M. Koper, Electrocatalytic reduction of nitrate at low concentration on coinage and transition-metal electrodes in acid solutions, Journal of Electroanalytical Chemistry 554–555 (2003) 15–23. https://doi.org/10.1016/S0022-0728(02)01443-2.