Corrosion mechanism of nanocrystalline Zn–Ni alloys obtained from a new DMH-based bath as a replacement for Zn and Cd coatings

Abstract

Nanocrystalline Zn–Ni alloys obtained from a newly developed 5,5′-dimethylhydantoin (DMH)-based bath are proposed as a replacement for Zn and Cd coatings due to their excellent corrosion resistance and environmentally friendly properties. However, the mechanism of the superior corrosion performance of the Zn–Ni samples compared with the Zn and Cd coatings has not been greatly investigated. In the present study, the corrosion mechanisms of Zn–Ni alloys and Zn and Cd coatings have been studied. The results show that the corrosion resistance of the deposits is directly dependent on the composition of the products formed during corrosion. The excellent corrosion resistance of the Zn–Ni samples is due to the appearance of simonkolleite in the corrosion products. With increasing current density, the amount of simonkolleite decreases; thus, the corrosion resistance of the Zn–Ni alloys decreases with increasing current density. The XPS studies indicate a higher amount of simonkolleite and an additional protective Ni-rich layer on the Zn–Ni alloys compared to the Zn coating. Moreover, the superior corrosion resistance of the Zn–Ni alloys compared to the Cd coating is associated with the loose structure of CdCl₂·H₂O formed on the Cd coating. Compared with the Zn and Cd coatings, nanocrystalline Zn–Ni alloys display excellent corrosion resistance due to the rapid rate at which zinc is lost from their hydrophilic surfaces at the initial immersion. After a long immersion time, the higher amount of simonkolleite in the corrosion products with hydrophobic surfaces further increases their polarization resistance. The results of Tafel curves and electrochemical impedance spectra (EIS) confirm the above facts that Zn–Ni alloys have better corrosion resistance than Zn and Cd coatings and represent the best alternative to Zn and Cd coatings.

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1. Introduction

Cd coatings can be used as sacrificial deposits in industrial applications to protect steel and reduce the induced embrittlement of high strength steel. However, due to stringent environmental regulations and its toxic nature, the application of Cd is restricted.^1,2 Zn coatings have been proposed as a replacement for Cd to provide sacrificial protection for steel because of the low standard electrode potential of Zn (E₀ = −1.07 V vs. SCE).³ In sacrificial coatings, the corrosion potential of the coating should be more negative than that of the steel. The dissolution of Zn is rapid in corrosion environments owing to the large driving force between Zn and the steel, which reduces the lifetime of Zn coatings. This can be resolved by alloying Zn with Ni to increase the corrosion potential of the coatings. In recent years, great interest has been focused on Zn–Ni alloys due to their better mechanical characteristics, higher thermal stability and superior corrosion resistance compared to Zn and other Zn alloys.^4,5 It is reported that the corrosion resistance of Zn–Ni deposits with Ni content in the range of 8 wt% to 14 wt% is five times greater than that of pure zinc.⁶ Some researchers have also found that the single γ phase structure of Zn–Ni alloys has better corrosion resistance.^7,8 It is clear that Zn–Ni alloys can be considered to be the best alternative for Cd and Zn coatings.

The baths used to electrodeposit Zn–Ni alloys can be divided into two types: acid and alkaline. In general, the deposits obtained from acid baths contain a mixture of γ phase and δ phase and often suffer from poor alloy distribution.^9,10 Contrastingly, alkaline baths have relatively high throwing power, and only γ phase can be detected in the coatings. A number of complexing agents, such as sodium acetate,^11,12 triethanolamine,¹³ amine,¹⁴ ethylenediamine,¹⁵ tartrate,¹⁶ glycinate,^17,18 citrate and urea, are employed to stabilize Zn²⁺ and Ni²⁺ in alkaline baths. However, these alkaline baths have lower current efficiency compared with acid baths, and their uses are limited. For this reason, a new alkaline bath with high current efficiency using 5,5′-dimethylhydantoin (DMH) as a complexing agent has been proposed for commercial applications.¹⁹ DMH has been successfully used in Au²⁰ electrodeposition. However, to the best of our knowledge, no detailed literature reports or studies have been proposed to study the corrosion resistance of Zn–Ni alloys using DMH as a complexing agent in Zn–Ni alloy deposition. Therefore, the application of DMH in Zn–Ni alloys is meaningful and novel.

Various techniques have been used to characterize the corrosion resistance of deposits, such as the measurement of open circuit potentials (OCPs), Tafel curves and electrochemical impedance spectroscopy (EIS).^21,22 The main corrosion mechanism of Zn–Ni alloys involves dealloying²³ and dezincification²⁴ of Zn. The formation of corrosion products is considered to be the main reason for the improved corrosion resistance of the deposits. This is due to the fact that the corrosion products can act as barriers against diffusion, reducing the rate of corrosion. Zinc oxide (ZnO), zinc hydroxide (Zn(OH)₂), smithsonite (ZnCO₃), simonkolleite (Zn₅(OH)₈Cl₂·H₂O) and hydrozincite (Zn₅(CO₃)₂(OH)₆) are the common corrosion products of Zn and Zn alloys.²⁵ Among these, simonkolleite has been identified as the most effective barrier, owing to its low solubility.²⁴ However, the prospective role of simonkolleite in nanocrystalline Zn–Ni coatings has rarely been analyzed compared with that in Zn and Cd coatings, and it is essential to investigate the formation of corrosion products during corrosion.

Surface wettability has a significant effect on the corrosion resistance of deposits.²⁶ A hydrophobic surface is the desired result to prevent corrosion, as it limits the contact between the corrosion environment and the coatings;²⁷ therefore, hydrophobic surfaces have been used in the corrosion inhibition of many coatings.²⁸ A corroded sample with a hydrophobic surface can prolong the life of the coating after corrosion. It is well known that the hydrophobicity of the surface is determined by the surface roughness of the deposits. However, to date, few detailed studies have focused on the effects of surface wettability on the corrosion resistance of Zn–Ni alloys.

In this study, a new DMH-based alkaline bath was proposed to deposit nanocrystalline Zn–Ni alloys. The corrosion resistance of deposits is associated with their surface morphology and the chemical state of the corrosion products on the deposits. For this purpose, SEM and EDS tests were carried out to analyze the surfaces of the corroded deposits. OCPs, potentiodynamic polarization and EIS measurements with various immersion times were used to investigate the effect of current density on the corrosion resistance of Zn–Ni alloys. The corrosion resistance of the Zn–Ni coatings was also compared with that of Zn and Cd coatings, and the main parameters which affect the corrosion process were evaluated. Specifically, the mechanism of the superior corrosion resistance of Zn–Ni coatings compared with Zn and Cd coatings was analyzed by XPS and EIS in detail.

2. Experimental

2.1 Electrodeposition of Zn–Ni alloys, Zn and Cd

Three Zn–Ni alloys and Zn and Cd coatings were electrodeposited with thicknesses of about 20 μm. The parameters are as follows.

The Zn–Ni alloys were electrodeposited from a DMH-based bath with the following composition: DMH 140 g L⁻¹, Na₄P₂O₇·10H₂O 40 g L⁻¹, ZnSO₄·7H₂O 70 g L⁻¹, NiSO₄·6H₂O 30 g L⁻¹, K₂CO₃ 95 g L⁻¹ and additives 40 mg L⁻¹. The additives consisted of two aromatic compounds and a wetting agent. The pH of the bath was adjusted to 9 to 10, the bath temperature was 50 °C and the agitation speed was 1000 rpm. The current densities used to electrodeposit the Zn–Ni alloys were 3 A dm⁻², 6 A dm⁻² and 9 A dm⁻², and the corresponding coatings are marked as Zn–Ni (A), Zn–Ni (B) and Zn–Ni (C), respectively.

The Zn coatings were deposited from the above bath without the addition of Ni. The bath temperature was 30 °C and the agitation speed was 1000 rpm. A current density of 2 A dm⁻² was used to obtain Zn deposits with the best corrosion resistance.

The Cd coatings were deposited from a cyanide-based bath with CdO 25 g L⁻¹, NaCN 120 g L⁻¹, Na₂CO₃ 10–60 g L⁻¹ and NaOH 10–30 g L⁻¹. The pH of the bath was about 13 and the bath temperature was 25 °C. The applied current density was 3.5 A dm⁻² over 15 min to obtain a coating thickness of 20 μm.

Carbon steel plates with dimensions of 4 cm × 5 cm were used as the cathodic substrates. Before electrodeposition, the plates were degreased with 30 wt% sodium hydroxide solution at 50 °C for 5 minutes and the surfaces were activated with 50% hydrochloric acid for a few seconds. After these steps, the plates were washed with distilled water and placed into the bath immediately to avoid the formation of an oxide layer. After electrodeposition, the Zn–Ni alloy coatings were washed with distilled water, then dried with cold air.

2.2 Characterization of the coatings and corrosion products

Scanning electron microscopy (SEM, Helios Nanolab 600i) with energy dispersive X-ray spectroscopy (EDS) was used to analyze the surface morphology and composition of the deposits. The surface roughness of the deposits was determined by atomic force microscopy (AFM, Bruker) in contact mode. X-ray diffraction (XRD, D/max-γβ) was employed to analyze the phase structure and average grain size of the deposits. The measurements were performed in the 2θ range from 20° to 100° with Cu Kα radiation at 40 kV and 40 mA (λ = 0.1546 nm) at a scan rate of 0.02° per second. The grain size was calculated by Scherrer's formula:

D = kλ/β cos θ (1)

where D is the average grain size of the deposits (nm), k is a constant (0.89), β is the full width at half maximum (FWHM) and θ is the reflectance angle.

Anodic linear sweep voltammetry (ALSV) was employed to analyze the effect of Ni on the phase structure of the deposits. The coatings were deposited on a RDE Pt electrode. The deposits were dissolved in the deposition electrolyte with a sweep rate of 1 mV s⁻¹ at 25 °C.

The surface morphology and phase structure of the corrosion product layer were examined by SEM and XRD after the coatings were immersed in 3.5% NaCl for 24 h. The samples immersed in 3.5% NaCl for 24 h were used to interpret the composition of the corrosion product film. The surface analysis of the corrosion products was carried out using X-ray photoelectron spectroscopy (XPS). A PHI 5700 ESCA System (Physic Electronics, USA) was used to measure the XPS. The excitation source was Al Kα radiation (1486.6 eV). The binding energies of all elements have been calibrated against the carbon contamination at 284.6 eV. The corrosion product layer was sputtered with 3.0 keV argon ions at a current of 0.5 μA for 4 min. The sputtering area was 4 mm × 4 mm and the sputtering rate was 2 nm min⁻¹.

The wettability of the surface of the deposits and corrosion products was characterized by contact angle (CA) using a contact angle measure meter (JC 2000D5, Shanghai Zhongchen Digital Technology Apparatus Co., Ltd.) at room temperature. 3 μL droplets of ultrapure water were dropped onto the samples, and the average of at least three points was measured at different positions for each sample.

2.3 Corrosion studies by electrochemical measurements

Three types of electrochemical methods were used to characterize the corrosion resistance of Zn–Ni alloys, Zn and Cd coatings: Open Circuit Potentials (OCPs), Tafel curves and electrochemical impedance spectra (EIS). These measurements were performed in a three-electrode cell using a platinum foil as the counter electrode (CE) and saturated calomel electrode (SCE) as the reference electrode with a CHI750D electrochemical workstation at 25 °C.

The OCPs were recorded for 24 h and 240 h in 3.5% NaCl solution. Tafel curves were used to determine the corrosion potential and corrosion current of the deposits. The coatings were immersed in 3.5% NaCl solution for 0.5 h to stabilize the OCP value. Then the Tafel curves were recorded in the range of −0.25 to 0.25 V with respect to the OCP. The coatings after 24 h immersion were also measured by the Tafel technique to characterize the protective corrosion products. EIS was performed with different immersion times. The impedance spectra were measured at the OCP in the frequency range between 10⁵ Hz and 10⁻² Hz with an amplitude of 5 mV. ZSimpWin software was used to fit the EIS data.

3. Results and discussion

3.1 Characteristics of the coatings

3.1.1 Surface morphologies and phase structure of the coatings. Fig. 1a–c shows the surface morphologies of Zn–Ni (A), Zn–Ni (B) and Zn–Ni (C). Regardless of the current density, the deposited Zn–Ni alloys are homogenous and compact and cover the substrates completely. A rectangular pyramid structure can be observed in the deposits, and the grain size is about 15 to 25 nm. With increasing current density, the overpotential increases, resulting in an increase of the nucleation rate and a decrease of the grain size. This result is in agreement with the investigation of El-Sherik et al.²⁹ From Fig. 1c, it can be seen that some grains cluster together and form colonies; thus, clearer colony boundaries appear. The clusters formed in Zn–Ni (C) can be attributed to the fast growth of grains and clusters on the nuclei compared with the nucleation rate,^30,31 which further increases the roughness of the Zn–Ni deposits (Fig. 2c). When the current density increases from 3 to 9 A dm⁻², the Ni content in the deposits slightly increases from 13.71 wt% to 14.83 wt%. Although the changes in the composition of the deposits are slight, the effect of current density on the surface morphologies of the deposits is remarkable. Colonies are more readily formed at larger current densities.^32,33 According to a previous study,²¹ colonies on the microscale are observed at larger current densities. However, these colonies are still in the nanoscale range (200 to 500 nm) in the present research, which is indicative of better corrosion resistance compared to the microcrystalline Zn–Ni alloys.²¹ It is observed that Zn and Cd coatings are also displayed in Fig. 1d and e for comparison. The incorporation of Ni into the Zn matrices slightly alters the morphology of the Zn coatings. The Zn coating is composed of hemispherical structures, a common characteristic of Zn deposits.³⁴ Furthermore, a different morphology can be seen in the Cd coatings. Instead of the platelet morphology, hexagonal crystals with some nodules are observed, which is a typical structure of Cd deposits.³⁵

Fig. 1 SEM images of (a) Zn–Ni (A), (b) Zn–Ni (B), (c) Zn–Ni (C), (d) Zn and (e) Cd coatings.

Fig. 2 AFM images of (a) Zn–Ni (A), (b) Zn–Ni (B), (c) Zn–Ni (C), (d) Zn and (e) Cd coatings.

The typical three dimensional (3D) surface morphologies, representing Zn–Ni (A), Zn–Ni (B), Zn–Ni (C), Zn and Cd coatings, are shown in Fig. 2. The scanned area is 2.0 μm × 2.0 μm on the micrometer level. It can be clearly seen that Zn–Ni (A) has a more compact and homogeneous structure (more uniform grain size). However, colonies with larger grain sizes can be easily observed in Zn–Ni (C). Zn–Ni (C) has more cavities about 60 nm deep, in which the deposit is significantly thinner. The increase of these cavities is due to the generation of hydrogen. When the deposition current density is higher, a drastic evolution of hydrogen bubbles is observed from the cathode. Thus, the Zn–Ni alloy deposition is disturbed, which results in clearer colony boundaries.³⁶ These results are in accordance with the SEM analysis. Compared with the Zn–Ni alloys, the Zn coating displayed in Fig. 2d has a relatively rough and heterogeneous surface. Furthermore, hexagonal crystals can also be observed in Fig. 2e in the Cd coating. On the other hand, the surface roughness of the Zn–Ni alloys and the Zn and Cd coatings is shown in Table 1. The roughness does not change significantly as the current density increases from 3 to 6 A dm⁻². In contrast, a significant increase of the surface roughness can be observed in Zn–Ni (C), which is related to the appearance of cavities. The roughness of the Zn–Ni deposits is the same or smaller compared to the Cd coating. This can be associated with the excellent leveling capability of the composite additives in the investigated bath;¹⁹ meanwhile, a much greater surface roughness is observed in the Zn coating.

Table 1 The surface roughnesses of the Zn–Ni alloys and the Zn and Cd coatings

Sample	Rq (nm)	Ra (nm)	Rmax (nm)
Zn–Ni (A)	21.5	17.4	120
Zn–Ni (B)	22.1	18.1	127
Zn–Ni (C)	30.8	25.3	194
Zn	51.3	41.7	276
Cd	27.5	22.3	157

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The XRD patterns of the Zn–Ni alloys and the Zn and Cd coatings are shown in Fig. 3. All the coatings exhibit a crystalline structure in the diffractograms. The Zn–Ni alloys are γ-Ni₅Zn₂₁ phase, which is the typical phase observed in Zn-13% to 15%-Ni alloys. Regardless of the current density, the preferred orientation is the (411) plane. Also, γ phase with a body centered cubic structure is the desired Zn–Ni alloy phase for maximum corrosion protection in a chloride environment in relation to η phase.³⁷ Thus, the Zn–Ni coatings may have excellent corrosion resistance in the investigated range of current densities. Both the Zn and Cd coatings show typical diffraction patterns of Zn and Cd deposits, as shown in Fig. 3d and e, respectively. Ni atoms can be incorporated into the Zn lattice, which causes a significant distortion of the structure of Zn, resulting in the difference between Zn and the Zn–Ni alloys.³⁸ The average grain size was calculated based on the Scherrer equation, which is suited to detect grain sizes less than 100 nm. With increasing current density, the grain size of the Zn–Ni alloys decreases from 24.4 nm to 17.2 nm, confirming the analysis of the SEM results. The grain sizes of the Zn and Cd coatings are 37.6 nm and 51.3 nm, respectively.

Fig. 3 XRD patterns of the Zn–Ni alloys and the Zn and Cd coatings.

3.1.2 ALSV curves of the coatings. The ALSV method was used to determine the phase structure of the Zn–Ni alloys. As shown in Fig. 4, one can clearly see that the pure Zn coating starts to dissolve at about −1.4 V and has only one anodic peak at about −1.33 V. For the Zn–Ni alloys, two distinct anodic peaks are registered. However, only γ phase is detected in the Zn–Ni alloys based on the XRD analysis. Therefore, the first peak at about −1.31 V corresponds to the dissolution of Zn from the γ phase. This peak overlaps the dissolution peak of pure Zn. The second peak at −1.0 V corresponds to the dissolution of Ni from the remainder of the γ phase. According to the literature,³⁹ the height of the peaks gives a prediction about the quantity of the related phase in the deposits. It is notable that the height of the first anodic peak for all the Zn–Ni coatings is clearly smaller than the first anodic peak for the pure Zn coating, indicating a decrease of the current efficiency in the Zn–Ni alloys owing to the codeposition of Ni. A similar result was obtained with the codeposition of Mo in Zn–Co alloy.⁴⁰ The Zn–Ni (C) and Zn–Ni (A) coatings have the lowest and highest amounts of Zn content in their deposits, respectively. This analysis is also supported by the EDS results. In a typical ALSV curve, a higher dissolution potential corresponds to better corrosion resistance.⁴⁰ The potential of the first peak for the Zn–Ni alloys is more positive than that of the Zn coatings, and it shifts to a more negative potential by increasing the current density, indicating better corrosion resistance of the Zn–Ni (A) deposit. As for the Cd coating, it starts to dissolve at about −1.08 V and has one anodic peak at about −0.89 V. Furthermore, the height of the first peak for Zn–Ni alloys is significantly larger compared to the Cd coatings. It can be therefore concluded that the codeposition of Ni with Zn should cause a significant increase in the corrosion resistance of the deposits.

Fig. 4 ALSV voltammograms of the Zn–Ni alloys and the Zn and Cd coatings deposited on a Pt electrode at a scan rate of 1 mV s⁻¹ at 50 °C.

3.2 Corrosion resistance

3.2.1 Open circuit potentials (OCPs) and contact angle (CA). To investigate the sacrificial behavior of the deposits, open circuit potential measurements (OCPs) of the Zn–Ni alloys and the Zn and Cd coatings measured for 24 h and 240 h were studied, respectively. As shown in Fig. 5a, the OCPs of Zn–Ni (A) and Zn–Ni (B) are much closer to that of Cd, while the OCP of Zn–Ni (C) lies between those of Zn and Cd. The OCP of steel is −0.64 V. Hence, the differences in OCPs between Zn–Ni (A) or Zn–Ni (B) and steel are much smaller compared to those of Zn–Ni (C) and Zn, indicating a much lower driving force of the Zn–Ni (A) and Zn–Ni (B) coatings during corrosion. The OCPs of all the Zn–Ni alloys start to increase and then stabilize to a nearly constant value owing to dezincification. It can be clearly seen that the zinc loss rate of Zn–Ni (A) is much faster than that of Zn–Ni (B) and Zn–Ni (C). This may be due to the wettability of the surface of the deposits. It is well known that the contact angle (CA) of a coating is related to its microstructure, surface morphology and chemical composition.⁴¹ As can be seen in Fig. 6, the CA increases from 70.1° to 118.2° when the current density increases from 3 A dm⁻² to 9 A dm⁻², indicating that the nature of the Zn–Ni coatings changes from hydrophilic to hydrophobic; that is, the formation of clusters in Zn–Ni (C) leads to an increase of the CA. Thus, the contact between the solution and the deposit is inhibited by air, reducing the chance of the dezincification reaction and resulting in a relatively slower zinc loss rate for the Zn–Ni (C) coating. Therefore, the Zn–Ni (A) coating with a hydrophilic surface accelerates the formation of a stable corrosion product layer, which can significantly improve the corrosion resistance of Zn–Ni (A) at the initial corrosion. As shown in Fig. 5b, the value of OCPs in Zn–Ni (A) is almost constant during 240 h immersion, suggesting that the protective layer on Zn–Ni (A) deposits has not been damaged after 240 h and that Zn–Ni (A) can withstand a long immersion time. Fig. 7 displays the CA of the corroded surface of the Zn–Ni alloys and the Zn and Cd coatings after 24 h immersion. The CAs of Zn–Ni (A) and Zn–Ni (B) increase from 70.1° and 79.3° to 140.9° and 108.7°, respectively. However, the CA of Zn–Ni (C) decreases from 118.2° to 94.4° after 24 h immersion, indicating the hydrophobic nature of the corrosion products formed on the surface of Zn–Ni (A). The stable and hydrophobic corrosion product layer on Zn–Ni (A) after corrosion can increase the corrosion resistance of the Zn–Ni deposits. It can be seen that the hydrophobic corrosion products after corrosion result in much better corrosion resistance of Zn–Ni (A) compared to Zn–Ni (B) and Zn–Ni (C). On the other hand, the CAs of the corrosion products in the Zn and Cd coatings also show decreasing trends after the corrosion tests. Hence, Zn–Ni (A) alloy has a better protective effect for steel than the Zn and Cd coatings during a long period of immersion.

Fig. 5 The OCPs of the Zn–Ni alloys and the Zn and Cd coatings for (a) 24 h, (b) 240 h.

Fig. 6 CA measurements on: (a) Zn–Ni (A), (b) Zn–Ni (B), (c) Zn–Ni (C), (d) Zn and (e) Cd coatings before immersion.

Fig. 7 CA measurements on: (a) Zn–Ni (A), (b) Zn–Ni (B), (c) Zn–Ni (C), (d) Zn and (e) Cd coatings after 24 h immersion.

3.2.2 Characterization of the corrosion products. Fig. 8 shows the XRD patterns of the Zn–Ni alloys and the Zn and Cd deposits after 24 h immersion. Simonkolleite (Zn₅(OH)₈Cl₂·H₂O), hydrozincite (Zn₅(CO₃)₂(OH)₆) and zinc oxide (ZnO) are confirmed as the main corrosion products for the Zn–Ni alloys and the Zn coating during immersion. These corrosion products are the typical products for Zn alloys and Zn during immersion tests.⁴² As shown in Fig. 8a, the intensities of δ-Ni₃Zn₂₂ and γ-Ni₅Zn₂₁ are much greater than those of the corrosion products, indicating that the Zn–Ni alloys have not dissolved completely during immersion. It has been reported that simonkolleite has a hexagonal lattice with very low solubility,⁴³ which can offer effective protection to deposits. Therefore, simonkolleite in corrosion products has a significant effect on the corrosion performance of Zn–Ni alloys and Zn coatings. Moreover, the remaining Ni-rich layer of Zn–Ni alloys after corrosion offers additional protection to the deposits compared to the Zn deposits. As for the Cd coating, the formation of a mixture of corrosion products, CdO, Cd(OH)₂ and CdCl₂·H₂O, is observed. The corrosion products formed on Zn–Ni coatings can offer effective protection to the underlying coating compared to the corrosion products formed on the Cd coating due to the low solubility and compactness of simonkolleite.²⁴ Hence, real passivation of Zn–Ni alloys can be achieved.

Fig. 8 XRD patterns of (a) Zn–Ni alloys, (b) Zn and Cd coatings obtained after 24 h of immersion in 3.5% NaCl solution.

It is worth noting that the peaks of simonkolleite, hydrozincite and zinc oxide often overlap in XRD patterns. Also, similar corrosion products are observed on the surface of Zn–Ni alloys and Zn coatings. Therefore, XPS studies were carried out to further understand the composition of the corroded Zn–Ni alloys and Zn coatings. The composition of Zn, Ni, C, O and Cl in the corroded surface of the Zn–Ni alloys and Zn coatings was analyzed by EDS and XPS tests. The results are shown in Tables 2 and 3, respectively. It is clear that the surface composition measured by EDS is significantly different from the result obtained by XPS after 24 h immersion. This is predictable, because the composition of corrosion products is heterogeneous with regard to thickness. The sampling depth of EDS is at the micron level, whereas the sampling depth of XPS is no more than 5 to 6 nm. Although the Ni content increases with increasing current density according to XPS, it decreases according to the EDS results. This behavior is due to the faster dezincification rate at the initial stage of corrosion of Zn–Ni (A). The enrichment of Zn in the outer layer of corrosion products indicates the segregation of Zn from the bulk to the surface. This is evidenced by the low Ni content (0.13% to 0.32%) in the surface of the corrosion products of the Zn–Ni alloys by XPS. The composition of C according to EDS is much higher than that measured by XPS. The excess of C likely originates from contamination of the corroded samples. The amount of O and Cl decreases with increasing current density according to the EDS analysis. The same trend is also observed in the XPS spectra.

Table 2 EDS analysis of the composition of the corrosion products of the Zn–Ni alloys and the Zn coating after 24 h exposure

Samples	Composition (at%)
	C	O	Zn	Cl	Ni
Zn–Ni (A)	16.40 (2.4)	51.12 (1.8)	24.64 (1.2)	5.21 (0.59)	2.63 (0.81)
Zn–Ni (B)	17.82 (1.3)	48.25 (3.9)	27.56 (0.73)	4.72 (0.15)	1.65 (0.46)
Zn–Ni (C)	19.69 (1.7)	46.37 (2.1)	28.82 (1.6)	4.19 (0.47)	0.93 (0.54)
Zn	21.66 (2.1)	48.45 (2.5)	25.47 (2.4)	4.42 (0.74)	—

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Table 3 XPS analysis of the composition of the corrosion products of the Zn–Ni alloys and the Zn coating after 24 h exposure

Samples	Composition (at%)					The relative share of O (at%)
	C(asCO3)	O	Zn	Cl	Ni	ZnO	Zn5(OH)8Cl2·H2O	Zn5(OH)6(CO3)2	H2O
Zn–Ni (A)	1.57	55.14	36.67	6.49	0.13	16.51	29.18	9.45	—
Zn–Ni (B)	1.64	54.68	37.94	5.51	0.23	20.07	24.77	9.84	—
Zn–Ni (C)	1.71	54.45	38.58	4.94	0.32	21.95	22.23	10.27	—
Zn	1.87	58.51	34.81	4.81	—	18.10	21.65	11.23	7.53

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Fig. 9 shows the decomposition of the C 1s and O 1s spectra, respectively. As seen in Fig. 9a, the C 1s spectra can be decomposed by three contributions centered at 284.6 eV, 286.3 eV and 289.4 eV. The first two carbon peaks at 284.6 eV and 286.3 eV are related to the adventitious carbon state C–C and organic C–O, respectively. According to ref. 44 and 45, hydrozincite (Zn₅(CO₃)₂(OH)₆) and zinc carbonate (ZnCO₃) are observed at 289.1 eV and 289.6 eV, respectively. However, XRD analysis only shows the appearance of hydrozincite. Therefore, the inorganic carbonate species at 289.4 eV indicates the presence of the common corrosion product hydrozincite. Fig. 9b displays the O 1s spectra. Three contributions of the peaks are observed for the Zn–Ni alloys; these are located at about 531.2 eV, 532.0 eV and 530.0 eV, respectively. The dominant peak at 531.2 eV is assigned to simonkolleite.⁴⁶ The other peaks at 532.0 eV and 530.0 eV are associated with hydrozincite and zinc oxide, respectively. Meanwhile, for the Zn coating, in addition to the above three peaks, the fourth peak at 539.8 eV is related to H₂O. The Zn 2p and Zn LMM spectra are employed to determine the chemical state of zinc. As shown in Fig. 10, the Zn Auger spectrum is employed because the binding energy between possible Zn²⁺ and Zn is very small in the Zn 2p spectrum. The peak observed at 499.4 eV in Fig. 10b is a typical characteristic of simonkolleite and hydrozincite.⁴⁶ To identify the zinc bonds, the modified Auger parameter (α′) is also useful. The value of the modified Auger parameter (α′) for zinc is 2009.3 eV. According to ref. 47 and 48, the reference Wagner data for simonkolleite and hydrozincite are 2009.5 eV and 2009.6 eV, respectively. These values are close to our calculated data, suggesting the co-existence of simonkolleite and hydrozincite in the corrosion products of the Zn–Ni alloys and the Zn coating.

Fig. 9 XPS spectra of (a) C 1s and (b) O 1s on the corroded surfaces of the Zn–Ni alloys and Zn after 24 h immersion.

Fig. 10 XPS spectra of (a) Zn 2p and (b) Zn LMM on the corroded surfaces of the Zn–Ni alloys and Zn after 24 h immersion.

Cl 2p and Ni 2p are also recorded in Fig. 11. Fig. 11a displays the Cl 2p spectra for Zn–Ni alloys and Zn deposits. The peak of Cl 2p at 198.6 eV confirms the appearance of simonkolleite. The results are in line with the Zn 2p and O 1s spectra and the XRD results. A shoulder appears in the Cl 2p spectra, especially for Zn–Ni (A). This is related to the spin–orbital splitting.⁴⁴ The Ni peak centered at 852.8 eV indicates the appearance of metallic nickel.^49,50 As a result, a Ni-rich layer forms during corrosion, which offers additional protection to the Zn–Ni alloys. This confirms that the corrosion products of the Zn–Ni alloys consist of zinc compounds and a Ni-rich layer.

Fig. 11 XPS spectra of (a) Cl 2p and (b) Ni 2p on the corroded surfaces of the Zn–Ni alloys and Zn after 24 h immersion.

The composition of corrosion products on the surface of the Zn–Ni alloys varies with increasing current density in accordance with the analysis displayed in Table 3. As seen in Fig. 9, when the applied current density is 3 A dm⁻², the shares of zinc oxide and simonkolleite relative to the total amount of O are 29.94% and 52.92%, respectively. The remainder, in an amount of 17.14%, is hydrozincite. When the current density is increased to 9 A dm⁻², in relation to the total O, the contribution of simonkolleite decreases to 40.83%, while the share of zinc oxide increases to 40.31%. However, the share of hydrozincite changes only slightly and is estimated at 18.86%. Based on ref. 51, (hydroxy) carbonates have a buffering effect and can provide additional protection for Zn–Ni alloys and Zn against corrosion. It can be clearly seen that the share of simonkolleite is over 50%; thus, this can be considered to be the main corrosion product at 3 A dm⁻². However, the main corrosion products become simonkolleite and zinc oxide at 9 A dm⁻² due to the almost identical contributions of these two components. Considering the corrosion products of the Zn coating, the shares of simonkolleite, hydrozincite and zinc oxide in the total O are 37.01%, 19.19% and 30.93%, respectively. The remaining 12.87% share of O is related to H₂O.

To summarize, the surface analysis results are in accordance with the XRD assignments of simonkolleite, hydrozincite and zinc oxide and additionally show that the Ni-rich layer is also part of the corrosion products of the Zn–Ni alloys.

During corrosion of the Zn–Ni alloys and Zn, the dissolution of zinc is balanced by the reduction of oxygen at the cathodic areas; subsequently, Zn(OH)₂ is formed. Zn(OH)₂ can also dehydrate to form zinc oxide (eqn (2)). In the presence of NaCl solution, Na⁺ can move towards the cathode, while Cl⁻ can migrate towards the anodic sites filled with dissolved zinc. The active Cl⁻ and pH values are beneficial to the precipitation of simonkolleite (eqn (3)).

Zn + ½O₂ + H₂O → Zn(OH)₂ → ZnO + H₂O (2)

4ZnO + Zn²⁺ + 2Cl⁻ + 5H₂O → Zn₅(OH)₈Cl₂·H₂O (3)

Generally, the hydrolysis of Zn²⁺ is in excess of that of Cl⁻, and the transformation from zinc oxide into simonkolleite is incomplete. Therefore, the corrosion products of Zn–Ni alloys and Zn include zinc oxide and simonkolleite. Furthermore, the atmospheric carbon dioxide dissolves in the corrosive media and forms carbonate ions. Thus, zinc oxide can also react with carbonate ions to form hydrozincite (eqn (4)).

5ZnO + 2H⁺ + 2CO₃²⁻ + 3H₂O → Zn₅(CO₃)₂(OH)₆ + 2OH⁻ (4)

The surfaces of the Zn–Ni alloys and Zn coatings become dark gray after 24 h immersion. The predominance of simonkolleite in the corrosion products may be attributed to its low solubility compared with hydrozincite and zinc oxide. It has been reported that simonkolleite has excellent insulating performance⁵² and can be considered to be a barrier, limiting the oxygen reduction at the cathode and thus quenching the general activity. However, zinc oxide can be used as a catalyst and is favourable to oxygen diffusion due to its semiconducting nature.^53,54 Therefore, Zn–Ni alloys have better corrosion resistance compared to Zn coatings owing to their higher amount of simonkolleite. The decrease of corrosion resistance with increasing current density is also related to the decrease of simonkolleite and the increase of zinc oxide in the corrosion products.

According to the analysis of Odnevall et al.,⁵⁵ the layered structure of simonkolleite and hydrozincite facilitates conversion from one phase to another under appropriate conditions. The relatively higher amount of simonkolleite in the corrosion products may be due to different reasons. First, simonkolleite is formed prior to the precipitation of hydrozincite owing to its low solubility. Second, the precipitation of simonkolleite is directly dependent on a high concentration of available Cl⁻. This condition is more readily met than the requirements to form hydrozincite. Furthermore, the formed hydrozincite may be instable in a Cl⁻ ion-rich environment and can decompose into simonkolleite according to eqn (5) after a long period of immersion in NaCl solution.

Zn₅(CO₃)₂(OH)₆ + 2Cl⁻ + 2OH⁻ → Zn₅(OH)₈Cl₂ + 2CO₃²⁻ (5)

According to the combined XRD, EDS and XPS results, the better corrosion performance of Zn–Ni (A) is related to two aspects. At the initial stage of corrosion, the corrosion product film on the surface of Zn–Ni (A) can be formed at a much quicker rate due to its hydrophilic nature. It has been mentioned that the formed simonkolleite in the corrosion products is insoluble and is unfavourable to the diffusion of oxygen. Hence, the higher composition of simonkolleite in the corrosion products of Zn–Ni (A) offers more effective protection for the underlying coating against corrosion compared to other Zn–Ni alloys and the Zn coating after long periods of immersion. Moreover, the hydrophilic nature of the corroded surface of Zn–Ni (A) also contributes to the enhanced corrosion performance.

The surface morphologies of the Zn–Ni alloys and the Zn and Cd coatings after 24 h immersion are shown in Fig. 12. It can be clearly seen that the corrosion products formed on Zn–Ni (A) have nano-scale thicknesses in the vertical direction and micro-scale thicknesses in the other directions. The nano-microstructure surface of the corrosion products on Zn–Ni (A) results in an increase of the CA after 24 h immersion. Localized corrosion occurs on the Zn–Ni (B) surface, and web-like cracks appear in Zn–Ni (C). The release of tensile and compressive stress may be the cause of the formation of cracks in Zn–Ni (C) after a long exposure time.²⁶ The CA of Zn–Ni (C) decreases with the formation of cracks, which is related to the easier penetration of the corrosive media into the corrosion product layer through the cracks. Thus, the protective effect of the corrosion product on Zn–Ni (C) is decreased compared to that of Zn–Ni (A). On the other hand, the corroded Zn surface has a uniform surface with flake-like or lamellae structures. The lamellae structure is micro-scaled and is unfavorable to an increase of the CA. The surface of the Cd coating is filled with irregular corrosion products after immersion. However, the corrosion products of the Zn and Cd coatings are loose, and some microholes can be observed. The slight decrease in CA for the Zn and Cd coatings can be associated with the holes existing in the loose structures of the corrosion products. This is also the reason why a superior corrosion resistance of Zn–Ni alloys can be obtained compared to the Zn and Cd coatings during corrosion.

Fig. 12 SEM images of the corroded surfaces of the Zn–Ni alloys and the Zn and Cd coatings after 24 h immersion in 3.5% NaCl solution.

3.2.3 Tafel curves. The Tafel polarization curves of the Zn–Ni alloys and the Zn and Cd coatings were investigated after exposure of the samples to 3.5% NaCl for 0.5 h (stabilizing the OCP) and 24 h, respectively. Table 4 shows the corrosion parameters of the samples obtained from the polarization curves. There is no passivation in all deposits. The E_corr values for all coatings are more negative than that of steel (E_corr = −0.64 V_(SCE)), indicating that all the coatings can act as sacrificial deposits for the steel. No major constraints of the anodic dissolution of zinc can be observed in Fig. 13a and b. It has been mentioned that the main cathodic reduction for Zn alloys and Zn is oxygen reduction, which is the rate controlling step of the corrosion.⁵⁶ Furthermore, the superior corrosion resistance of the deposits corresponds to a more positive E_corr or a lower j_corr.⁵⁷ Fig. 13a shows the polarization curves after stabilizing the open circuit potential for 0.5 h. The E_corr of the Zn–Ni alloys negatively increases and the j_corr increases rapidly with increasing deposition current density. This behavior is due to the faster dezincification rate of Zn–Ni (A) compared to Zn–Ni (B) and Zn–Ni (C), which is in turn related to the hydrophilic surface of Zn–Ni (A). The hydrophobic properties of Zn–Ni (C) can limit the corrosion reaction between the deposits and the corrosive media, resulting in the slower dezincification rate. Thus, the formation of the protective Zn corrosion product and the Ni-rich layer is inhibited and decreases the corrosion resistance of Zn–Ni (C). The more negative E_corr and the larger value of j_corr also indicate the poor corrosion resistance of the Zn deposits. The E_corr of Zn–Ni (A) is similar to that of the Cd coating; however, the j_corr of Zn–Ni (A) is smaller than that of the Cd deposits.

Table 4 The corrosion potentials and the corrosion current of the Zn–Ni alloys and the Zn and Cd coatings after 0.5 h and 24 h immersion

t (h)	Samples	Ecorr (V vs. SCE)	jcorr (μA cm^−2)
0.5	Zn–Ni (A)	−0.86	22.01 (1.13)
	Zn–Ni (B)	−0.89	25.77 (1.52)
	Zn–Ni (C)	−0.92	30.81 (1.37)
	Zn	−1.10	40.07 (1.81)
	Cd	−0.84	35.10 (1.48)
24	Zn–Ni (A)	−0.77	31.07 (1.25)
	Zn–Ni (B)	−0.80	46.81 (2.04)
	Zn–Ni (C)	−0.87	47.80 (1.85)
	Zn	−1.16	56.94 (1.62)
	Cd	−0.79	54.69 (2.39)

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Fig. 13 Polarization curves of the Zn–Ni alloys and the Zn and Cd coatings after immersion in 3.5% NaCl solution for (a) 0.5 h and (b) 24 h.

Fig. 13b shows the potentiodynamic polarization of the deposits after 24 h immersion in 3.5% NaCl solution. Compared with the data shown in Fig. 13a, the j_corr values of the corroded Zn–Ni alloys increase with increasing current density after 24 h immersion, indicating a decrease of the corrosion resistance. The j_corr of Zn–Ni (A) is smaller compared to that of the Cd coating, suggesting better corrosion resistance of the corroded Zn–Ni (A). The dominant amount of simonkolleite and the additional Ni-rich layer in Zn–Ni (A) offered better corrosion production for the underlying coating compared with the corrosion products formed on Cd (CdCl₂·H₂O). The increase in the CA of the corroded surface of the deposits can also increase the corrosion resistance of the deposits. The CA of Zn–Ni (A) increases to 140.9° after 24 h immersion. However, the CA of Cd decreases to 109.3° after 24 h immersion. The larger value of CA for Zn–Ni (A) (Fig. 7) can hinder the infiltration of the electrolyte through the porous corrosion product layer. Thus, the diffusion of oxygen becomes slower and the corrosion reaction can also be limited.

3.2.4 Electrochemical impedance spectra. The corrosion mechanism of the Zn–Ni samples can potentially be estimated by EIS. Fig. 14 shows the Nyquist impedance plots of Zn–Ni (A), Zn–Ni (B) and Zn–Ni (C) as a function of exposure time in 3.5% NaCl solution. The corrosion reaction occurs at the interface of the coatings and the corrosion products through the pores of the corrosion product film. It is noted that three well-defined capacitance arcs are observed in the Zn–Ni alloys and Zn. Meanwhile, two capacitance arcs exist in Cd. A protective oxide film is established on the surface of the Zn–Ni alloys and Zn deposits due to exposure to air.¹⁹ Hence, a very small arc at high frequencies corresponds to an unstable passive film. Generally, the middle frequency loops are recorded as the corrosion products and the low frequency loops are related to the electrical double layer.⁵⁸ Owing to the discontinuity (porous corrosion products) or inhomogeneities of the surface of the deposits, a constant phase element (defined as Q) is used to replace the pure capacitor. The equivalent circuit in Fig. 15a models three time constants, in which a pair of elements in parallel, namely, R_s, corresponds to the resistance of 3.5% NaCl solution. R₁ and CPE₁ correspond to the resistance and capacity of the oxide films formed in air, respectively. R₂ indicates the resistance of the electrolyte which permeates the pores of the corrosion product layer. CPE₂ represents the dielectric character capacitance of the porous corrosion product layer. R₃ and CPE₃ indicate the charge transfer resistance and the double layer capacitance formed on the interface of the electrolyte/metal coating through the pores, respectively. The Warburg diffusion impedance, W, is introduced in Fig. 15b to indicate the diffusion process. R₁ and CPE₁ describe the protective corrosion product film. R₂ and CPE₂ represent the electric double layer. The circuits exhibit a good fit to the experimental data. The standard deviations are on the order of 10⁻⁴, and the relative error for each parameter is less than 9%. Also, the calculated data show acceptable values. The measured data are denoted by scattered symbols and the fitted data are denoted by lines (Fig. 16).

Fig. 14 Nyquist plots for (a) Zn–Ni (A), (b) Zn–Ni (B) and (c) Zn–Ni (C) after 0.5 h, 6 h, 12 h, 24 h and 48 h of immersion in 3.5% NaCl solution.

Fig. 15 Nyquist plots for (a) Zn and (b) Cd after 0.5 h, 6 h, 12 h, 24 h and 48 h of immersion in 3.5% NaCl solution.

Fig. 16 Equivalent circuit models used for fitting the Nyquist data: (a) for the Zn–Ni alloys and the Zn coating, and (b) for the Cd coating.

The XRD and XPS results show that simonkolleite is the main corrosion product on the surface of the Zn–Ni alloys and Zn. It is noted that the simonkolleite cannot directly inhibit the anodic dissolution of Zn,⁵⁹ and the inhibition effect can be related to the restriction of oxygen diffusion through the porous corrosion product layer. Thus, the corrosion process may be controlled by this process. This has also been evidenced by the Tafel plots of the Zn–Ni alloys (Fig. 12). The presence of Warburg impedance observed at the Nyquist plots for the Cd coating reflects the anodic diffusion of Cd²⁺ from the Cd surface to the bulk solution and the cathodic diffusion of oxygen from the bulk solution to the Cd surface through the porous corrosion product layer. A relaxation of the diffusion line (less than 45° to the real axis) is due to the fact that the diffusion layer is restricted to a finite length. Furthermore, finite length diffusion is a good model to interpret the diffusion of oxygen though the porous corrosion product layer.⁶⁰

The calculated parameters of the Zn–Ni alloys and Zn are shown in Table 5. The C₁ and R₁ values exhibit little change over the whole immersion time. R₂ decreases significantly over 24 h immersion for the three Zn–Ni alloys and Zn, indicating that the corrosion rate decreases drastically. This behavior is due to the increase in the porosity of the corrosion product layers; that is, the corrosive media can more easily penetrate through the corrosion products during immersion. This is evidenced by the increase of C₂. Some irregular values of C₂ may be indicative of the slow dissolution of corrosion products or a decrease in porosity. The chloride ions can adsorb on the corrosion product film and damage it. The slight increase in the value of R₂ after 24 h exposure indicates the established chemical equilibrium of corrosion processes within 24 h immersion. The value of R₃ decreases for all Zn–Ni alloys and Zn during 24 h immersion. After that, it remains nearly unchanged from 24 to 48 h. The changes in the values of the parameters (R₃ and C₃) indicate that the deposits do not lose their protective properties. This can be evidenced by the stable OCP values, as shown in Fig. 5.

Table 5 Optimum fit parameters of Zn–Ni alloys and Zn deposits with various immersion times

Samples	t (h)	Rs (Ω cm^2)	R1 (Ω cm^2)	n1	C1 (μF cm^−2)	R2 (Ω cm^2)	n2	C2 (μF cm^−2)	R3 (Ω cm^2)	n3	C3 (mF cm^−2)
Zn–Ni (A)	0.5	3.38	10.89	1	0.19	591.6	0.88	14.24	1163	0.34	9.73
	6	3.10	11.13	1	0.19	498.6	0.90	14.76	1013	0.42	11.32
	12	5.09	9.50	1	0.40	401.8	0.77	32.30	862.8	0.45	14.32
	24	2.89	10.15	1	0.17	234.7	0.83	35.24	765	0.58	12.28
	48	4.28	12.92	1	0.46	207.9	0.77	30.40	703.6	0.41	10.13
Zn–Ni (B)	0.5	2.53	11.26	1	0.21	553.2	0.82	37.35	808	0.53	19.53
	6	5.23	9.92	1	0.20	471.2	0.88	11.77	737.4	0.31	7.11
	12	3.15	11.31	1	0.31	344.6	0.86	16.66	683.6	0.56	12.73
	24	4.92	8.31	1	0.23	168	0.82	41.93	501.8	0.66	13.59
	48	3.39	15.04	0.83	1.35	153	0.74	270.4	422.1	0.56	5.98
Zn–Ni (C)	0.5	3.15	11.31	1	0.21	412	0.87	16.74	698.4	0.35	6.96
	6	2.84	13.86	0.94	0.53	322.2	0.81	28.09	560.9	0.42	7.58
	12	3.23	10.05	1	0.15	183.2	0.83	41	486.4	0.33	13.53
	24	3.51	11.09	0.98	0.16	231.3	0.72	100.3	423.4	0.71	4.18
	48	2.96	16.85	1	0.38	236.7	0.89	328	404.9	0.66	12.22
Zn	0.5	2.91	10.97	1	0.21	296.5	0.81	59.45	443.7	0.43	15.08
	6	2.64	16.25	1	0.36	246.1	0.73	58.42	387.8	0.44	8.80
	12	2.87	14.93	0.91	0.56	176.1	0.77	465.9	439.5	0.52	24.02
	24	2.79	21.05	0.97	0.46	191.7	0.62	314.9	325.9	0.70	12.03
	48	4.97	13.31	1	0.10	219.2	0.65	783	243.8	0.75	8.32

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Fig. 15 shows the Nyquist plots of the Cd coatings. The results are also shown in Table 6. R₁ decreases and C₁ increases over 48 h immersion, indicative of the increase of the porosity of the corrosion product layer. The R₂ also decreases, which indicates that the active surface increases. This demonstrates the decrease of the corrosion resistance of the Cd coatings over 48 h immersion. However, the change in C₂ does not support the above analysis.

Table 6 Optimum fit parameters of Cd deposits with various immersion times

Samples	t (h)	Rs (Ω cm^2)	R1 (Ω cm^2)	n1	C1 (μF cm^−2)	R2 (Ω cm^2)	n2	C2 (μF cm^−2)	W (Ω cm^2 s^−0.5)
Cd	0.5	3.89	47.76	1	1.99	835.3	0.81	25.8	0.02612
	6	3.57	39.38	1	2.04	711.9	0.82	18.8	0.03131
	12	3.15	29.66	0.79	20.1	627.9	0.89	42.6	0.01781
	24	3.72	25.41	0.67	47.5	527.6	0.81	28.7	0.02322
	48	3.95	29.05	0.77	50.3	474.4	0.86	26.3	0.0289

---

Fig. 17 shows the contrast of the polarization resistance R_p among Zn–Ni alloys, Zn and Cd coatings, respectively. The sum of the three resistances R₁, R₂ and R₃ can be characterized as the total polarization resistance for the Zn–Ni alloys and Zn, whereas the total polarization resistance of Cd can be assumed to be the sum of R₁ and R₂. It is noted that the R_p values of the Zn–Ni alloys are higher than these of Zn and Cd. Also, these values decrease with increasing current density. It is well-known that higher R_p values lead to better corrosion resistance. Therefore, the corrosion resistance of Zn–Ni alloys is superior to that of the Zn and Cd coatings. Furthermore, the corrosion resistance of Zn–Ni alloys decreases by increasing current density. The above results confirm the analysis of Tafel tests.

Fig. 17 Time dependence of R_p for the Zn–Ni alloys and the Zn and Cd coatings during 48 h of immersion in 3.5% NaCl solution.

4. Conclusions

The corrosion mechanism of Zn–Ni alloys and Zn and Cd coatings has been studied in this work. According to the above analysis, the following conclusions can be made.

Nanocrystalline Zn–Ni alloys are obtained in a novel alkaline bath with DMH as the complexing agent. The qualitative compositions of the corrosion products of the Zn–Ni alloys and Zn are similar, while their quantitative compositions are different to some extent. The XRD patterns indicate that simonkolleite, hydrozincite and zinc oxide are the main corrosion products in the Zn–Ni alloys and Zn. The XPS analysis further confirms that the corrosion products formed on the Zn–Ni alloys have higher amounts of simonkolleite than that formed on Zn; also, Zn–Ni (A) is enriched with simonkolleite compared to Zn–Ni (B) and Zn–Ni (C). According to the Tafel and EIS results, the corrosion resistance of Zn–Ni alloys decreases with the increase of current density. Zn–Ni alloys exhibit lower j_corr values and higher polarization resistance than the Zn and Cd coatings. The mechanism of the superior corrosion resistance of the Zn–Ni alloys compared to the Zn and Cd coatings can be divided into two stages. At the initial stage of immersion, the better corrosion resistance of Zn–Ni (A) is due to its higher dezincification rate compared to the other deposits. This is in turn related to the variety of surface morphologies and wettabilities of the different Zn–Ni alloys. After a long immersion time, the excellent corrosion performance of Zn–Ni (A) is associated with the composition of its corrosion products. The corrosion products formed on Zn–Ni (A) have a higher amount of simonkolleite and a lower amount of zinc oxide. As a result, the rate of corrosion is suppressed. The additional Ni-rich layer in Zn–Ni alloys also contributes to better corrosion performance compared to Zn. Moreover, the formed corrosion product layers on Zn–Ni alloys offer much better protection than the CdCl₂·H₂O layer on the Cd coating due to its lower porosity and compact structure. Based on the above results, it is concluded that Zn–Ni alloys with better corrosion resistance can be obtained at a lower current density in the investigated bath, and Zn–Ni alloys can provide excellent protection compared to Zn and Cd coatings.

Acknowledgements

The authors are grateful for financial support from the State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (2015DX09) and National Water Pollution Control and Management Technology Major Projects (2013ZX07201007-003-02).

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