Influence of Fe2+ oxidation and its antioxidant ascorbic acid as additive in Zn-Ni-Fe electrodeposition process on a low carbon steel

Zn-Ni-Fe co-deposits were obtained on a low carbon steel with and without ascorbic acid (AA) in solution and compared to Zn-Ni by electrochemical experiments. After the spontaneous oxidation of Fe2+ to Fe3+ and subsequent Fe(OH)(3) formation, the tests were performed to verify the effects of this transformation on the Zn-Ni-Fe co-deposition process. The other solution containing AA was prepared to avoid Fe2+ oxidation in the solution to obtain Zn-Ni-Fe, however, this organic compound also influenced the electrodeposition due to complexation with the metal cations and adsorption on the cathodic surface. The characterization of the coatings was performed after obtaining the co-deposits by galvanostatic electrodeposition. It was verified that Fe2+ did not significantly influence the Zn-Ni-Fe electrodeposition process using the parameters of this experiment, but only when it was oxidized to Fe3+ and formed Fe(OH)(3). The Fe(OH)(3) and AA acted as inhibitors to the electrodeposition, through different mechanisms. Fe(OH)3 resulted in higher gamma phase (Zn21Ni5) content than eta phase (Zn89Ni1) or pure Zn in the Zn-Ni-Fe co-deposit compared to Zn-Ni, besides promoting more compact coating. AA resulted in higher eta phase or pure Zn content than gamma phase in both Zn-Ni and Zn-Ni-Fe compared to the co-deposits obtained without the organic compound in solution. Moreover, the presence of AA promoted the formation of Zn-Ni and Zn-Ni-Fe coating less rough than these co-deposits obtained without AA due to its effect as additive.