# Concentration Influence of Complexing Agent on Electrodeposited Zn-Ni Alloy
## 1 Introduction
Specifically, phase \(\alpha\) is defined as a solid solution of Zn in Ni with the Zn amount smaller than 30%, possessing a face-centered cubic (FCC) structure. Phase \(\eta\) is a solid solution of Ni in Zn with a Ni amount smaller than 1%, possessing a hexagonal crystal structure. Phase \(\gamma\) is an intermetallic compound with a body-centered cubic (BCC) structure, containing the Ni amount from 10 to 30%. The phase \(\gamma\) of Zn-Ni alloy yields high corrosion resistance and good mechanical properties. Hence, electroplated phase \(\gamma\) is commonly applied as a corrosion protection layer for steels and metals [21]. Previous studies reported that the Zn-Ni alloy containing 12-18 wt.% of Ni content yielded 3-8-fold higher corrosion resistance than pure Zn coating [22]. This excellent property was attributable to the existence of the \(\gamma\) phase in the microstructure. The Zn-Ni alloy moreover exhibited a uniform and smooth surface without the degradation of strength, and ductility of steel. In addition, the phase composition can be determined by changing the variables of electrodeposition, such as the Ni/Zn ion ratio in bath solution, the current density, temperature, and stirring speed. One previous study proved that when the equilibrium potential of Zn was \(-1.00\), \(-0.94\), and \(-0.67\) V versus a saturated calomel electrode (SCE), the \(\eta\), \(\gamma\), and \(\alpha\) phases, respectively were deposited from a chloride electrolyte with Ni/Zn ion ratio of 5.0 [23]. To improve the corrosion resistance of Zn-Ni alloy coatings, an extrinsic factor can be added to the film. Notably, a chromate conversion coating is widely adopted as a corrosion barrier. Different from Cr\({}_{2}\)O\({}_{7}\)\({}^{2-}\) or CrO\({}_{4}\)\({}^{2-}\) in hexavalent passivation, which preferentially tends to oxidize, the Cr\({}^{3+}\) ion in trivalent passivation is stable with the lack of oxidation. The oxidizing incapability of Cr\({}^{3+}\) creates a thinner passivation film than that of Cr\({}^{6+}\). The variables of chromate conversion coating, such as duration, temperature, and pH, directly affect the growth, bonding strength, and corrosion resistance of the passivation film. Our group reported adding a chromate coating as a post-treatment to increase the corrosion resistance of Zn-Ni coatings [24]. The combination of NaF additive concentration and eco-friendly trivalent chromate treatment time was investigated. When both the NaF concentration and chromate treatment time increased, the cracks on the chromate film reduced, suggesting that NaF accelerated the formation of chromate film. Meanwhile, the corrosion current decreased when the NaF concentration increased. Due to the presence of the chromate film, passivation was formed, suppressing the corrosion current, and improving the corrosion resistance. In addition, high-corrosion-resistant Zn-Ni alloys also require intrinsic factors, i.e., a fine microstructure with either a high Zn content or a high Ni content. Particularly, a high Zn content, which is less noble than iron, should be obtained to use as a sacrificial film, or a high Ni content, which is nobler than iron, should be obtained to use as a barrier film. Previous research has mostly focused on factors that increase the Ni content since a higher amount of Ni leads to better corrosion resistance of coatings [25; 26]. One effective approach is the addition of complexing agents in the plating solution [27; 28]. The interaction between metallic ions and complexing agents that contain ions or functional groups can result in the formation of a complex. Hence, the complexing agent is expected to bind with Ni ions during electroplating, thereby reducing Ni hydroxide precipitation and improving Ni deposition efficiency. [PERSON] et al. reported that the usage of complexing agent 5,5\({}^{\prime}\)-dimethylhydantoin induced strong adsorption on electrodes [29]. The deposition of Zn-Ni alloy was changed from normal to anomalous co-deposition with decreasing the applied deposition potential. [PERSON] et al.