# Concentration Influence of Complexing Agent on Electrodeposited Zn-Ni Alloy
## 1 Introduction  
Steels are the most common materials used in diverse industries, such as construction, automotive, energy, and aerospace. To meet application requirements, they are tough and durable with high yield and ultimate tensile strengths. While operating, steels are subjected to corrosive environments, such as an aqueous solution, leading to an initial stress corrosion crack. Unfortunately, the corrosion-resistant property of steel is too low. Therefore, a sacrificial protective coating is essential to prolong the life expectancy of steel. Zinc (Zn) electroplating, also known as galvanizing, is the most widely used metal coating. Electroplated binary Zn-X alloys, in which X is an iron group metallic element (Fe, Co, or Ni), possess enhanced properties, especially corrosion resistance, compared to pure Zn [1; 2; 3; 4; 5]. Zn alloys have been used as a sacrificial coating to protect stainless steel, as their reduction potential is lower than that of stainless steel. Due to the low reduction potential, these alloys corrode more preferentially than steels, thereby increasing the lifetime of materials. Owing to its excellent corrosion-resistant properties, Zn-Ni alloy coating has generated considerable interest recently [6; 7; 8; 9; 10]. Electrodeposition is an electrochemical method highly preferable for preparing coating due to its unique ease of controlling the thickness and chemical composition of the coatings via the deposition conditions, such as current density, pH, and composition bath. In this technique, the deposition of coating is carried out onto a cathodic substrate by the reduction of metallic cations. Conventional electrodeposition of Zn-Ni alloy is conducted in aqueous solutions, and Zn-Ni alloy coatings can be obtained from sulfate, chloride, acid, weak acid, and alkaline bath solutions [11; 12; 13; 14]. Weak acid baths usually operate at high current densities, yielding Zn-Ni alloy coatings with a relatively low Ni content and non-uniform distribution of Ni element throughout the deposit. Conversely, alkaline electrolytes operateat low current density and provide uniform Ni distribution with a higher content than that produced by acid baths. Alkaline solutions, including cyanide and zincate, yield higher throwing power than their counterparts [15; 16; 17]. While cyanide solution shows high toxicity in the environment, zincate is an eco-friendly candidate. Moreover, zincate solution has some advantages such as low hydrogen embrittlement and uniform plating layers. Additionally, the bath solution is less corrosive to equipment, affordable, and easy to control. The typical composition of zincate solution consists of Zn and Ni ion compounds, such as ZnO, and Ni\({}_{2}\)SO\({}_{4}\cdot\)6H\({}_{2}\)O, sodium hydroxide (NaOH), and organic additives [18]. The addition of additives in electroplating is essential as they impact the growth and structure of the deposits, thereby enhancing the appearance and properties of coatings or the performance of the bath. Hydrogen generation evolution reaction (HER) during Zn and Zn alloy electrodeposition may cause hydrogen embrittlement. The existence of hydrogen traps may yield residual stresses, resulting in delayed failures of the electroplated coatings. Hydrogen generation can be specified according to the reduction that takes place at the cathode as follows. First, Ni\({}^{+2}\) and Zn\({}^{+2}\) ions are dissolved into adsorbed monovalent ions [19]. \[Ni^{2+}+e^{-}\to Ni_{ad}^{+}\]  
\[Zn^{2+}+e^{-}\to Zn_{ad}^{+}\]  
Subsequently, these adsorbed ions are reduced to metallic ions, which combines with hydrogen ion (H\({}^{+}\)) to form \(NiH_{ad}^{+}\), and \(ZnH_{ad}^{+}\). \[Ni_{ad}^{+}+e^{-}\to Ni\]  
\[Zn_{ad}^{+}+e^{-}\to Zn\]  
\[Ni+H^{+}\to NiH_{ad}^{+}\]  
\[Zn+H^{+}\to ZnH_{ad}^{+}\]  
Finally, these species are reduced to form metallic Zn and Ni, and to release H\({}_{2}\). \[NiH_{ad}^{+}+H^{+}+2e^{-}\to Ni+H_{2}\]  
\[ZnH_{ad}^{+}+H^{+}+2e^{-}\to Zn+H_{2}\]  
Electroplated Zn-Ni alloys exist in three phases \(\alpha\), \(\gamma\), and \(\eta\)[20].