Recently, the demand for corrosion resistance in automobiles has steadily increased, prompting car manufacturers to continuously look for corrosion resistance improvement measures. Additionally, the widespread distribution of chloride compounds for road de-icing during winter results in the corrosion of the vehicle body surface. Furthermore, environmental concerns associated with the post-treatment process of hexavalent chromium (Cr⁶⁺) used to enhance corrosion resistance in Zn plating have become a pressing issue, especially with the global prohibition of Cr⁶⁺ usage by the European Union. Thus, enhancing the corrosion resistance of automotive parts through plating and post treatment with Cr is essential. Until recently, Zn plating and trivalent chromium (Cr³⁺) post-treatment were predominantly used, with efforts focused on improving the corrosion resistance of these processes. However, pure Zn plating requires an increased thickness to enhance the corrosion resistance, leading to challenges related to the peeling of the Zn plating layer. Moreover, the corrosion resistance of commercially available Cr³⁺ post-treatment is lower than that of Cr⁶⁺, which poses a challenge. Consequently, Zn alloy plating, particularly Zn–Ni alloy plating, has been adopted as a plating technology that addresses the drawbacks of pure Zn plating while increasing the corrosion resistance [1,2].

Zn–Fe alloy plating is a frequently employed method among various zinc alloy plating techniques. However, the easy oxidation of Fe ions makes it challenging to achieve a homogeneous Fe alloy content within the plating layer, affecting the lifespan and manageability of the plating bath. Conversely, Zn–Co alloy plating is recognized for its excellent corrosion resistance among Zn alloy plating methods, but it has the drawback of higher maintenance costs than other non-iron alternatives. Zn–Ni alloy plating offers superior uniformity in the plating layer and exceptional adhesion, minimizing delamination. Owing to these benefits, Zn–Ni alloy plating is widely used for commercial applications in Japan and Europe. Although Zn–Ni alloy plating techniques have been established and applied in acidic and chloride baths, using them in alkaline baths often involves hazardous substances, such as cyanides. The corrosion resistance of Zn–Ni alloy plating varies with the Ni content, typically exhibiting the best performance in the range of 11–18 wt% Ni. Zn–Ni alloy plating in alkaline baths has superior corrosion resistance to those in acidic baths [3]. This enhanced corrosion resistance is attributed to the formation of the r phase, which occurs solely within a specified range of Ni content.

The conditions for achieving this phase are vital for enhancing the corrosion resistance of the Zn–Ni alloy plating. Selecting these conditions depends on the Ni vacancy content in the plating layer and factors, such as the molar ratio of Ni ions to Zn ions, pH, temperature, current density, and flow speed of the plating solution. Thus, adjusting each variable appropriately during the manufacturing process is necessary to achieve the desired phase. Zn–Ni alloy plating methods can be categorized based on whether the electrolyte atmosphere is acidic or alkaline. In acidic atmospheres, the metal content in the plating solution is notably higher than that in alkaline atmospheres, and the Ni content within the plating layer is higher than that under alkaline conditions. The hardness of the plating obtained under acidic atmospheres tends to be higher, and the current efficiency approaches 100%. However, the uniformity of the plating is slightly lower than that under alkaline conditions, and the manufacturing cost is generally higher. The plating processes primarily involve electrochemical methods. Domestic research has primarily focused on mechanical properties, such as current efficiency, glossiness, and corrosion resistance under various plating conditions in acidic atmospheres. However, research on the reaction characteristics essential for continuous plating is limited [4]. Despite the lower equilibrium potential of Zn than Ni in Zn–Ni alloy plating, anomalous co-deposition occurs, resulting in a larger deposition of Zn than expected based on the Zn–Ni concentration ratio in the plating solution. Additionally, the potential shifts by a few hundred millivolts toward the anode, leading to underpotential deposition, which facilitates plating. This behavior is attributed to a decrease in the free energy of the Zn–Ni alloy deposited on the cathode surface, promoting a potential shift. However, different perspectives exist regarding anomalous co-deposition. Brenner [5] postulated that at high current densities, a substance interfering with the Ni deposition forms on the electrode surface; however, the identity of this substance remains unclear. Dahms [6] studied the anomalous co-deposition phenomenon during Ni–Fe alloy plating using a mercury electrode and found that this phenomenon occurred in oxygensaturated solutions but not in oxygen-free solutions. Dahms and Croll [7] indicated that Ni deposition is hindered when the hydrogen evolution rate surpasses the limiting current density, and they attributed the anomalous co-deposition phenomenon to the preferential adsorption of iron hydroxide on the cathode surface, obstructing Ni deposition. Higashi et al. [8] explained that anomalous co-deposition in Zn–Co alloy plating can be attributed to the high pH on the electrode surface during plating in alkaline baths, which favors hydroxide ion formation. Therefore, Ni deposition was hindered by the formation of hydroxide compounds in the alkaline plating baths. For instance, if an electrically conductive Zn plate corrodes, it forms ZnCl₂·4Zn(OH)₂ and zinc oxide (ZnO). Among these products, ZnCl₂·4Zn(OH)₂ is known to exhibit significant resistance to corrosion [9]. Satoh et al. [10] observed the presence of more ZnCl₂·4Zn(OH)₂ in the corrosion products of plated Zn–Ni and Zn plates, consistent with the results of Okada [9]. The adhesion of corrosion products to the substrate was stronger in the Zn–Ni plating than in the Zn plating, contributing to the superior corrosion resistance of the Zn–Ni alloy plating. The corrosion resistance of the Zn–Ni alloy plating was dependent on the Ni content. Generally, the best corrosion resistance is achieved with 11–13 % Ni [11–13]. This superior corrosion resistance is due to the r phase formation within the specified Ni content range. In the products generated during the plating process, it has been observed that Ni is incorporated into the hexagonal η phase at concentrations up to an average of 4 %. At concentrations exceeding 9.5 % Ni, the plating layer manifests a body-centered cubic structure, characteristic of the r phase (Ni₅Zn₂₁). Furthermore, at Ni contents exceeding 70 %, the plating layer comprises an FCC α phase, incorporating Ni and Zn. The r phase is essential for enhancing corrosion resistance, but it is vulnerable to external stress and hardly undergoes deformation. Lambert and Hart [11] demonstrated a 35 % reduction in the plating layer thickness for 0 wt% Ni and 15 % for Zn-5 wt% Ni during tensile tests. However, for Ni content of 9 % or higher (r phase), deformation was minimal but resulted in numerous cracks. These results underscore the r phase’s resistance to deformation and external stress. This study evaluated the corrosion resistance of Zn–Ni alloy plating in an alkaline bath without the use of CN, aiming to achieve a high Ni alloy vacancy rate similar to that of an acidic bath and the r phase (Ni₅Zn₂₁) generation. Additionally, a post-treatment involving tribasic sodium phosphate and sulfuric acid was applied to assess the corrosion resistance of the Zn–Ni alloy plating using electrochemical behavior analysis.

The basic composition and process conditions of the plating solutions used in this study are listed in Table I. The plating solution was prepared using extra pure grade reagents dissolved in deionized water in precise quantities. ZnO and NiCl₂ were used as the sources of Zn and Ni, respectively. Complexing agents aminoacetic acid (AAA) and triethylamine (TEA) were employed [14], and sodium hydroxide was used to create an alkaline environment for plating. The electrochemical test specimens of Zn–Ni alloy plating were produced with a uniform thickness of approximately 20 μm using the same charge quantity. For preparing the Cr³⁺ post-treatment solution, chromium trioxide was used as the source of Cr³⁺. Small amounts of trisodium phosphate and sulfuric acid were added as reducing agents, and ethanol was added to facilitate rapid drying and substitution. A trivalent chromate solution was then prepared and used for post-treatment at a pH of 1.8 to 2.0.

Table I. Composition of the plating solution and plating process conditions..

Composition of Zn-Ni alloy plating solution	Concentration (mol/L)	Composition of trivalent chromate solution	Concentration (mol/L)
ZnO	0.08–0.16	Cr2O3	0.05
NiCl2	0.00–0.08	Na3PO4	0.65
AAA	0.60	H2SO4	0.04
TEA	0.12	C2H5OH	0.25
NaOH	2.00	-	-
Plating process conditions
Current density		0.5–3.0 A/dm2
Temperature		25 °C
pH		≥ 14

3.1. Analysis of Ni content in the plating layer based on the plating solution composition and plating process conditions

Maintaining the concentration of NiCl₂ at 0.02 M, Zn–Ni alloy plating tests were conducted by varying the concentration of ZnO. The results of the Ni content analysis of the plating layers are summarized in Fig. 1.

Figure 1. Analysis of the Ni plating layer content according to ZnO concentration.

The Ni content increased when the ZnO concentration varied from 0.08 to 0.10 M. However, at concentrations above this range, the Ni content tended to decrease. The plating efficiency also remained relatively stable within the range of ZnO concentrations from 0.08 to 0.12 M, at approximately 57 %. However, at ZnO concentrations of 0.14 M and above, the plating efficiency sharply decreased to around 21 %, indicating poor plating results under these conditions.

The results in Fig. 2 show that the Ni content increases with the NiCl₂ concentration. At NiCl₂ concentrations of 0.06 and 0.08 M, the Ni content increased to 24.1 and 24.5 %, respectively, suggesting a potential improvement in reducing vacancies compared to conventional CN alkaline baths. Moreover, the plating efficiency increased with the Ni ion concentration. At a concentration of 0.015 M, the plating efficiency was below 30 %. However, at concentrations of 0.02 M and higher, it rapidly rose to over 50 %. Beyond 0.04 M, the plating efficiency reached approximately 60 %, indicating no significant increase beyond this point.

Figure 2. Analysis of Ni content change according to NiCl₂ concentration in the Zn−Ni layer.

The results of the analysis of Ni content in the plating layer with increasing plating solution temperature are presented in Fig. 3. The plating efficiency remained at approximately 58 %, with a high ZnO concentration of 0.12 M and a NiCl₂ concentration of 0.02 M, when tested at temperatures of 25, 40, and 50 °C. The increase in temperature led to only a slight increase of approximately 1 wt% in the Ni content, and there was no significant impact on the content variation. In general, in various alloy electroplating solutions containing Ni, as the solution temperature increases, the Ni void fraction expands, leading to a change in the alloy ratio. Consequently, temperature becomes a crucial variable influencing the alloy composition. However, under the zincate bath conditions, even with an increase in the plating solution temperature, it is deemed a critical indicator with significant implications. This is because temperature variation demonstrates minimal impact on the alloy composition, causing an increase of only approximately 1 wt%.

Figure 3. Analysis of the Ni plating layer content according to temperature.

The results of analyzing the Ni content in the plating layer by increasing the current density with the same charge quantity are presented in Fig. 4. At a current density of 0.5 A/dm², the Ni content was approximately 2 wt%, higher than normal, and it decreased at higher current densities. Additionally, this trend affected the plating efficiency. The efficiency decreased as the Ni and Zn contents decreased and increased, respectively. However, Li et al. [14] reported that the Ni content was not significantly affected by the current density in alkaline baths. This discrepancy was likely due to the different complexing agents and additives used in this study. The results of this study indicate that as the current density increases, the powdery precipitates easily detach from the plating layer surface. This phenomenon was observed at current densities above 2 A/dm², suggesting that nucleation caused by complexing agents and additives significantly affected the alloy crystal growth and plating layer growth rate.

Figure 4. Analysis of the Ni plating layer content according to current density.

3.2. Analysis of corrosion resistance and crystal structure of Zn–Ni alloy plating layer

The results of measuring the corrosion rate of the Zn–Ni alloy plating layer according to the Ni content are presented in Fig. 5, expressed in mils penetration per year (mpy). An increase in the Ni content of the plating layer demonstrated a decreasing tendency in corrosion rate. The corrosion rate reached its minimum value at the Ni content of 17.2 %; the corrosion rate increased again beyond this point. Specifically, at an Ni content of 8.7 wt%, the corrosion rate was 14.1 mpy. As the Ni content increased, the corrosion rate decreased, reaching its lowest value of 4.1 mpy at 17.2 % Ni content. With a further increase in Ni content, the corrosion rate increased again to 8.2 mpy at 25.4 % Ni content.

Figure 5. Corrosion rate due to the changes in the Ni content among the plating layers.

Figure 6 illustrates the corrosion rates of the alloy plating layer with respect to the processing time of the trivalent chromate treatment. The results indicated that up to a treatment time of 30 s, the corrosion rate decreased as the treatment time increased, demonstrating a significant improvement in the corrosion resistance. Within the tested range, the highest corrosion resistance was observed at 30 s. Moreover, when the treatment time exceeded 40 s, the corrosion rate tended to increase, indicating that longer treatment times increased the corrosion rate.

Figure 6. Corrosion rate according to the changes in the chromate processing time.

Shibuya et al. [12] investigated the variation in the nickel composition of Zn–Ni alloy plating in sulfuric acid baths and studied the changes in corrosion resistance based on the composition under different electroplating conditions. They reported that the alloy plating layer exhibited the best corrosion resistance when its Ni composition fell within the range of 10–16 % and was predominantly composed of the γ phase. Figure 7 shows the crystal structure of the Zn–Ni alloy plating layer obtained via X-ray diffraction (XRD) analysis. The crystal structures before and after the trivalent chromate treatment were compared. The analysis revealed the presence of alloy structures in the γ phase, including γ-Ni₂Zn₁₁, γ-Ni₅Zn₂₁, and γ-Ni₃Zn₂₁ [15]. When subjected to the trivalent chromate treatment, the effect of the CrO passivation film was observed as a peak in the amorphous phase, even though its intensity was relatively low.

Figure 7. Crystal structure analysis using XRD before (a) and after (b) trivalent Cr treatment.

Figure 8 illustrates the scanning electron microscopy (SEM) images of the Zn–Ni alloy electrodeposition layer surface morphology with varying NiCl₂ concentrations. It is evident that the surface structure changed as the Ni concentration increased. Up to 0.01 M, the morphology exhibited growth in the form of rounded circular crystals rather than distinct crystalline structures. Starting from 0.02 M, a pattern of rounded half-moons covering the entire surface becomes more prominent. Slight cracking was observed at concentrations of NiCl₂ exceeding 0.03 M. This is believed to be influenced by Cl−, affecting electrode stress.

Figure 8. SEM surface images according to the NiCl₂ concentrations of (a) 0.005, (b) 0.010, (c) 0.020, and (d) 0.030 M, respectively.

Figure 9 illustrates the surface based on the treatment time with Cr³⁺ after Zn–Ni alloy plating. This depicts the process of CrO film formation through substitution reactions owing to selective Zn leaching depending on the treatment time. It changed into a shape similar to that of NiCl₂ 0.01 M before trivalent chromate treatment [Fig. 8(b)]. Additionally, a slightly viscous sol-gel-like solution flowing down the surface of the plated layer due to the reaction with Cr³⁺ was observed. A water-washing and drying process is essential to achieve a faster reaction to increase corrosion resistance. During trivalent chromate processing, the electrons released from the zinc dissolution undergo a substitution reaction with chromium. The CrO film significantly affected the crystal structure.

Figure 9. SEM surface images of trivalent chromate treatment for (a) 20, (b) 30, (c) 40, and (d) 50 s, respectively.

The corrosion resistance of the Zn–Ni alloy plating material was found to be significantly dependent on the Zn and Ni contents within the plating layer. Consequently, the plating process conditions that allowed control of the Ni void content in the Zn–Ni alloy plating layer within the range of 8.7–25.4 % were identified. The optimal corrosion resistance was obtained at a Ni content of 17.2 % in the Zn–Ni alloy plating layer. The corrosion rate increased when the Ni content exceeded this value. The significant improvement in corrosion resistance can be inferred from the increase in corrosion potential (Ecorr) due to the post-treatment with Cr³⁺. The best corrosion resistance was achieved with a treatment time of 30 s. Considering a pH of 1.8–2.0, which is strongly acidic, it is crucial to form a film under conditions similar to those observed in the SEM analysis of the relationship between treatment time and corrosion resistance.

For Zn–Ni alloy plating, the Ecorr displayed values less than −1.0 V. After treatment with Cr³⁺, a potential increase of approximately 200 mV was observed, and the corrosion current density potential also indicated a decrease in the current density ranging from log10⁻² to log10⁻³ A/cm². These observations suggest a significant enhancement in corrosion resistance post-treatment with Cr³⁺.

The XRD analysis of the crystal structure of the Zn–Ni alloy plating layer confirmed the formation of γ phase alloy structures known for their excellent corrosion resistance, such as γ-Ni₂Zn₁₁, γ-Ni₅Zn₂₁, and γ-Ni₃Zn₂₁, in the plating layer.

The treatment with Cr³⁺ led to the selective leaching of Zn, increasing the Ni content by approximately 1.5–3.0 wt% within the Zn–Ni alloy plating layer. Considering the optimal Ni void content, it was determined that the corrosion resistance was best when the Ni content in the Zn–Ni alloy plating layer was approximately 15.5 % before treatment with Cr³⁺ and approximately 17.2 % after treatment.

This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2021R1F1A 1046135) and Semiconductor major track (Materials, Components, Equipment) project supported by the Ministry of Education and the Ministry of Trade, Industry and Energy (No. P0022196).

The authors declare no conflicts of interest.