Understanding the low corrosion potential and high corrosion resistance of nano-zinc electrodeposit based on electron work function and interfacial potential difference

Abstract

Nano-electrodeposition has been demonstrated to be more effective in protecting materials from corrosion, compared to conventional electrodeposition. However, debate still exists regarding why nanocrystalline electrodeposits show more negative corrosion potentials but lower corrosion rates than those of coarse-grained electrodeposits. In this work, we investigated corrosion behaviors of electrodeposited nanocrystalline and microcrystalline zinc coatings in a 3.5 wt% NaCl solution, and related them to the electron work functions of the coatings in order to understand the addressed issue based on the electron stability. It is demonstrated that the corrosion potential (ϕ_m) of a deposit depends on its electron work function (φ_m) and the contact potential difference at the electrodeposit/solution interface (Δ^m_sψ). φ_m reflects the stability of electrons in the deposit, while Δ^m_sψ reflects the environmental influence on the chemical stability of the deposit, which affects the actual corrosion rate. The present study shows that electrons in the nanocrystalline zinc coating are less confined due to its high-density grain boundaries, corresponding to lowered φ_m. However, Δ^m_sψ of the nanocrystalline coating is more positive than that of coarse-grained one, leading to a lower corrosion rate. This study helps end the debate and provide relevant information for optimize nanocrystalline electrodeposits.

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Introduction

Electrodeposited zinc coatings have been extensively used in the manufacturing, automotive, and construction sectors to protect steel components from atmospheric corrosion with low costs and merit of easy fabrication. Zinc may serve as a sacrificial coating to prevent corrosion of the steel beneath even if the zinc coating is scratched or damaged. Many efforts have been made to improve the performance of zinc coating with prolonged service lifetime.^1–4 Deposition of nanocrystalline zinc coating is an effective approach due to the facts that nanocrystalline zinc coating does not only provide a higher corrosion resistance but also improved mechanical properties, higher wear resistance and brightness without post-processing treatment, compared with those made using other methods.^5–9 Many studies have been reported in the literature, which were conducted to investigate variations in the corrosion behavior of zinc coating with respect to the grain size from micro to nano-scale in different environments and corresponding mechanisms. For instance, Youssef et al.¹⁰ investigated corrosion of nanocrystalline zinc coating in comparison with coarse-grained zinc coating in a 0.5 M NaOH solution and observed that the corrosion rate of nanocrystalline zinc coating was about 60% lower than that of electrogalvanized steel. M. C. Li and Naik et al.^11–13 showed that the corrosion rate of nanocrystalline zinc coating was less than 1/2 of that of coarse-grained zinc coating in a 3.5 wt% NaCl solution. Ramanauskas et al.¹⁴ also demonstrated that the corrosion resistance of nanocrystalline zinc coating was greater than that of its coarse-grained counterpart in a 0.6 M NaCl + 0.2 M NaHCO₃ solution. However, regardless of its lower corrosion rate, nanocrystalline zinc coating exhibits a more negative corrosion potential than the coarse-grained one.^10,12,13 Nanocrystalline nickel, iron and cobalt coatings also show similar trend.^15–17 A material with a more negative corrosion potential is more prone to corrosion and usually possesses a lower resistance to corrosion. Thus, it is of importance to understand this contradictory phenomenon in order to optimize nanocrystalline coatings.

In this study, we decipher this contradiction based on the electron stability and environmental influence on the surface stability, reflected by the electron work function and corrosion potential, respectively. The corrosion potential (ϕ_m) is determined by not only the electron work function of the electrodeposit (φ_m) but also the potential difference at the electrodeposit/solution interface (Δ^m_sψ). Thus, although ϕ_m shifts to more negative levels as the grain size decreases from micro to nano-scale, Δ^m_sψ may rise due to the enhanced activity of nano-coatings ascribed to its lowered work function or electron stability. As a result, the corrosion rate can be reduced. Such relationships and mechanisms for variations in ϕ_m, φ_m and Δ^m_sψ with a decrease in grain size are studied and discussed in details in this article.

Experimental

Coarse-grained and nanocrystalline zinc coatings were electrodeposited on carbon steel substrates (2 × 2 cm²) using a sulfate bath (ZnSO₄·7H₂O 100 g L⁻¹ and H₃BO₃ 20 g L⁻¹) in absence and presence of polyacrylamide (1 g L⁻¹) as the grain refiner, respectively. A detailed description of the bath configuration and processing conditions have been reported in previous publications.^7,18 Performances of zinc coatings in different baths were examined with cyclic voltammetry tests, which were performed at the room temperature (25 ± 1 °C) in a conventional three electrode cell controlled with a CHI 750D electrochemical workstation having platinum plate, saturated calomel electrode (SCE) and glassy carbon electrode as the auxiliary, reference and working electrode, respectively. Prior to each cyclic voltammetry analysis, the bath was deoxygenated for 15 min with ultrapure nitrogen and a weak flux of nitrogen was maintained during the test, and Na₂SO₄ (200 g L⁻¹) was needed as the conductive salt. In addition, the absorption spectra of different baths were recorded using a UV-vis spectrophotometer (UV-vis DRS, PG, TU-1900/1901) to study the coordination environment of zinc ions in bath.

After electrodeposition, zinc coatings were rinsed immediately with deionized water, then dried before experiments were performed for property evaluation and microstructure characterization. Surface morphology, grain size distribution and electron work function of zinc coatings were characterized using a multimode atomic force microscope with a Kelvin Probe (Brucker, US). X-ray diffraction (XRD, Rigaku Corporation Dmax-3B) with Cu Kα radiation was carried out to determine the crystalline texture or preferred crystallographic orientation and approximate average grain size of the zinc coatings. The orientation with maximum texture coefficient value (TC) value was the preferred orientation of the coatings, which was calculated using the following eqn (1):¹⁹

where I_hkl is the peak intensity of (hkl) crystallographic plane of the zinc electrodeposits, I_0,hkl is the peak intensity of (hkl) crystallographic plane according to the JCPDS card no. 00-004-0831, and n is the total number of crystallographic planes being considered. The grain size was estimated using the following Scherrer's formula²⁰where D is the grain size (in nm), K is a constant (0.89), λ is the X-ray wavelength (0.154 nm), β is the full width at half maxima in 2θ degrees, and θ is the diffraction angle.

The electrochemical corrosion behaviours of coarse-grained and nanocrystalline zinc coatings in a 3.5 wt% NaCl solution were evaluated with corrosion potential and potentiodynamic polarization tests. The measurements were also carried out in the conventional three electrode cell controlled by a CHI 750D electrochemical workstation. The zinc coating was the working electrode with a geometrical working area of 1 × 1 cm². The corrosion potential of zinc coatings was measured repeatedly over a period of 1 h. Potentiodynamic polarization curves were obtained by changing the electrode potential in the range of ±500 mV around the open-circuit potential against SCE at a scan rate of 1.0 mV s⁻¹.

Results and discussion

Experimental phenomena

AFM two-dimensional (2D) and three-dimensional (3D) morphologies of coarse-grained and nanocrystalline zinc coatings are illustrated in Fig. 1. The coarse-grained zinc coating made using the sulfate bath has an average grain size around 5 μm. The zinc coating electrodeposited using the sulfate bath containing polyacrylamide has much smaller grains in the range of 30–50 nm (average: 41 nm) with a more uniform and finer surface structure. XRD patterns (Fig. 2) of both the coatings indicate that the coarse-grained zinc coating has a (002) preferred orientation, while the nanocrystalline zinc coating has a well preferred orientation along (110) direction and the average grain size is 31 nm, calculated using Scherrer's formula. This grain size value is consistent with that (41 nm) determined from the AFM images. Moreover, through comparing the 3D AFM morphology of coarse-grained zinc coating (Fig. 1b) with that of nanocrystalline zinc coating (Fig. 1d), one may see that the nanocrystalline zinc coating is smoother. Both the 2D and 3D AFM results confirm that the zinc electrodeposition electrolyte possesses excellent grain refinement and levelling capabilities in the presence of polyacrylamide. The formation of nanocrystalline zinc coating is attributed to the fact that the polyacrylamide increases the overpotential of the cathode by absorption on the surface of the electrode during the electrodeposition process (Fig. 3). As shown in Fig. 3, with the addition of polyacrylamide into the sulfate bath, the reduction peak (P_CG) shows obvious shifted to more negative potential (P_NC) indicating that the cathode polarization occurred. The UV-vis spectrums of sulfate bath in the absence or presence of polyacrylamide display only one absorption peak at around 200 nm, suggesting that the coordination environment of Zn²⁺ does not change with the addition of polyacrylamide and the negative shift in cathodic peak potentials must be ascribed to the adsorption of polyacrylamide on active sites of cathode surface (as shown by the inset in Fig. 3).²¹

Fig. 1 AFM images of coarse-grained (a, b) and nanocrystalline (c, d) zinc coatings. The inset in image (c) shows the grain size distribution of nanocrystalline zinc coating.

Fig. 2 XRD patterns of coarse-grained and nanocrystalline zinc coatings.

Fig. 3 Cyclic voltammograms for sulfate baths in absence (for coarse-grained zinc) and presence (for nanocrystalline zinc) of polyacrylamide at a scan rate of 100 mV s⁻¹. The arrows indicate scan directions of voltages. The inset shows UV-vis spectra of the two baths.

Marine environment is a common corrosive medium for zinc coatings used in industry. Therefore, potentiodynamic polarization technique is used to evaluate corrosion behaviours of coarse-grained and nanocrystalline zinc coatings in the 3.5 wt% NaCl solution (simulated seawater¹¹). Typical polarization curves of coarse-grained and nanocrystalline zinc coatings are displayed in Fig. 4a and b, respectively. A summary of electrochemical parameters determined based on the polarization curves is given in Table 1. As shown, the nanocrystalline zinc coating has a more negative ϕ_m (−1.109 V) than coarse-grained zinc coating (−1.055 V), indicating that the nanocrystalline zinc coating is more prone to corrosion. However, the nanocrystalline zinc coating appears to be more protective than the coarse-grained one against corrosion, evidenced by its lower i_corr. The corrosion rate of the nanocrystalline zinc coating (18.6 μA cm⁻²) is less than 1/2 of that of the coarse-grained one (48.1 μA cm⁻²). The result is consistent with those of studies by other researchers.^12,13

Fig. 4 Polarization curves of coarse-grained (a) and nanocrystalline (b) zinc coatings in 3.5 wt% NaCl solution at 25 ± 1 °C, and those of (a′ – coarse-grained and b′ – nanocrystalline) zinc coatings/solution interface i.e., (Δ^m_sψ), determined using eqn (3).

Table 1 Electrochemical parameters of coarse-grained (a) and nanocrystalline (b) zinc coatings in a 3.5 wt% NaCl solution at 25 ± 1 °C, including experimental data obtained in the present study and those from references^a

Samples	ϕm (V)	φm (eV)	Δ^msψ (V)	Rp (Ω cm^2)	icorr (μA cm^−2)	References
a The EWFs (φm) of coarse-grained and nanocrystalline zinc coatings are values relative to that of the silicon probe as a reference.
a	−1.055	0.092	−1.147	450.4	48.1	Present study
	−1.040	0.092	−1.132		35.4	12
	−1.100	0.092	−1.192		28.1	13
b	−1.109	−0.241	−0.868	918.7	18.6	Present study
	−1.020	−0.241	−0.779		17.7	13
	−1.200	−0.241	−0.959		15.8	13

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Although the lower corrosion potential of nanocrystalline coating seems contradictory to its higher corrosion resistance, this is explainable if when intrinsic electrochemical behaviour and external factors are taken into consideration. The intrinsic electrochemical behaviour of a material is largely reflected by its electron work function (EWF), which is the minimum energy required to extract electron from inside a solid to its surface without kinetic energy.^22,23 EWF is a measure of intrinsic proneness of a material to corrosion. A lower EWF corresponds to a higher tendency of electrons for participating in corrosion reactions. Previous studies have demonstrated that the electron work function of materials is related to their corrosion resistance.^24,25 However, corrosion is also dependent on external factors i.e. the environment. Thus, one needs to look at both the intrinsic behaviour and the environment in order to explain and predict the performance of materials during corrosion processes.

Intrinsic electron behavior and environmental factors on corrosion

As indicated earlier, corrosion is dependent on both intrinsic behaviour and environmental influence. Or in other words, the corrosion potential (ϕ_m) depends on electron work function (φ_m) of the material and the contact potential difference at the material/solution interface (Δ^m_sψ), expressed as^26–28where e is the elementary charge. Thus, the corrosion potential is only a measure of the apparent nobility of a material in a specific environment, depending on the intrinsic nobility or electron stability characterized by EWF and the environment-dependent Δ^m_sψ. This is the reason why the ranking of materials in galvanic series changes with the corrosive environment, since the apparent nobility of materials varies with the environment.²⁸ Owing to the fact that EWF is influenced by microstructure constituents such as grain boundaries, ϕ_m varies with microstructure. This may explain why some materials show completely revered behavior when corroded respectively in alkaline and acidic solutions as their grain size decreases from micro-scale to nano-scale.^16,29

In order to understand the difference in performance between nanocrystalline and coarse-grained zinc coatings based on their electron behavior, a multimode atomic force microscope with a nano-Kelvin probe was used to map variations in local electron work function for coarse-grained and nanocrystalline zinc coatings within a scan area of 10 μm × 10 μm. Fig. 5a and b show the surface morphologies of scan areas on coarse-grained and nanocrystalline zinc coatings. Corresponding surface electron work function maps are illustrated in Fig. 5a′ and b′, respectively. In the EWF maps, darker regions have lower work functions. One may see that the coarse-grained zinc coating shows higher work function than the nanocrystalline one. This has been more clearly illustrated by corresponding line profiles of EWF of the coarse-grained and nanocrystalline zinc coatings. As shown, EWF of the zinc coating is lowered as the grain size decreases from micro-scale to nano-scale. The large fluctuation in EWF of the coarse-grained coating is ascribed to the fact that EWF in the grain boundary regions is lower than that of grains. The average φ_m of the coarse-grained zinc coating is 0.33 eV higher than that of the nanocrystalline coating.

Fig. 5 AFM morphologies (a, b) and corresponding in situ EWF (a′, b′) maps of coarse-grained (a, a′) and nanocrystalline (b, b′) zinc coatings; and EWF profiles of coarse-grained and nanocrystalline zinc coatings relative to EWF of the silicon probe as a reference (c), measured along dashed lines in (a′, b′).

Substituting the average φ_m values of coarse-grained and nanocrystalline zinc coatings, determined from the EWF maps (Fig. 5) into eqn (3), values of Δ^m_sψ for different coatings are calculated and presented in Table 1. Accordingly, the polarization curves of coarse-grained and nanocrystalline zinc coatings at the electrodeposit/solution interface are illustrated in Fig. 4 (curves a′ and b′). As illustrated, Δ^m_sψ of the coarse-grained zinc coating (−1.147 V) is actually more negative than that of the nanocrystalline zinc coating (−0.868 V), although ϕ_m of coarse-grained zinc coating is more positive than that of the nanocrystalline one. As indicated by eqn (3), the corrosion rate of zinc coatings depends on the reaction in the interfacial region between the electrodeposit and the solution. As a result, the nanocrystalline zinc coating with higher Δ^m_sψ shows a lower i_corr than the coarse-grained zinc coating, which has a more negative Δ^m_sψ (as shown by curves a′ and b′ in Fig. 4 and Table 1). This explains that the contradiction between more negative corrosion potential and smaller corrosion rate of nanocrystalline zinc coating in contrast with the coarse-grained zinc coating in the NaCl solution (as shown in Fig. 4 and Table 1) as well as in a NaOH solution as reported in ref. 10 (data are given in Table 2).

Table 2 Electrochemical parameters of coarse-grained (a) and nanocrystalline (b) zinc coatings in NaOH solutions. The ϕ_m and i_corr data of zinc coatings are borrowed from ref. 10^a

Samples	ϕm (V)	φm (eV)	Δ^msψ (V)	icorr (μA cm^−2)
a The EWFs (φm) of coarse-grained and nanocrystalline zinc coatings are values relative to that of the silicon probe as a reference.
a	−1.455	0.092	−1.547	229
b	−1.470	−0.241	−1.229	90

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The positive shift of Δ^m_sψ of zinc coatings with the reduction of grain size from micro- to nano-scale is attributed to the decrease in work function. Electrons in coarse-grained zinc coating are more confined than those in nanocrystalline zinc coating, which renders the nanocrystalline zinc coating more active than the coarse-grained one. Consequently, electrons at surface of the nanocrystalline coating are more active and easier enter into the NaCl solution (Fig. 6a) and participate in the electrode reaction:^30,31

Zn → Zn²⁺ + 2e⁻ (4)

Fig. 6 A model for the corrosion reaction of nanocrystalline zinc coating in a NaCl solution.

This reaction increases the concentration of Zn²⁺ near the nanocrystalline zinc coating and electrons dissociated from the reaction interact with dissolved oxygen and water molecules in the solution, forming OH⁻ (Fig. 6b) described as:³²

The OH⁻ ions further react with Zn²⁺, generating a layer of corrosion product, mainly Zn(OH)₂, on surface of the zinc coating (Fig. 6c). There may be other corrosion products such as Zn₅(OH)₈Cl₂·H₂O and ZnO, produced by from the following chemical reactions^33–37

5Zn²⁺ + 8OH⁻ + 2Cl⁻ + H₂O → Zn₅(OH)₈Cl₂·H₂O (6)

Zn₅(OH)₈Cl₂ + 2CO₃²⁻ → Zn(CO₃)₂²⁻ + 2Cl⁻ + 4H₂O + 4ZnO (7)

Zn(CO₃)₂²⁻ + 2OH⁻ → ZnO + 2CO₃²⁻ + H₂O (8)

The formed insoluble corrosion product layer on surface of the nanocrystalline zinc coating inhibits further corrosion (Fig. 6d),^10–13 leading to a decrease in the corrosion rate (Fig. 4 and Table 1). A corrosion product layer also forms on surface of the coarse-grained zinc coating; however, its protection ability is lower than that of the corrosion product layer on nanocrystalline zinc coating,^5,11 evidenced by higher stability of the layer with elevated Δ^m_sψ. As shown in Table 1, the corresponding polarization resistance (R_p) of the nanocrystalline zinc coating (918.7 Ω cm²) is two times as large as that of the coarse-grained one (450.4 Ω cm²).

To further verify the relationship between corrosion resistance and corrosion product layer of the zinc coatings, we synthesized corrosion product layers on surfaces of the coarse-grained and nanocrystalline zinc coatings by immersing the coatings in a 3.5 wt% NaCl solution for 100 hours. Fig. 7 illustrates variations in the corrosion potential of the coarse-grained and nanocrystalline zinc coatings with corrosion product layers in the NaCl solution. As shown, the corrosion potential of the corrosion product layer on nanocrystalline zinc coating is more positive than that of the corrosion product layer on the coarse-grained zinc coating, indicating that the corrosion product layer on the nanocrystalline coating has higher stability than that on the coarse-grained coating.

Fig. 7 Variation of corrosion potential with time for coarse-grained and nanocrystalline zinc coatings in 3.5 wt% solutions after 100 h of immersion.

Conclusions

We investigated corrosion behaviors of coarse-grained and nanocrystalline zinc coatings and correlated the corrosion potential with the electron stability. The objective of the study is to clarify the mechanism for the lower corrosion potential but smaller corrosion rate of the nanocrystalline zinc coating, in comparison with those of coarse-grained zinc coating based on the electron work function theory. According to the theory, the corrosion potential (ϕ_m) is determined by electron work function of the material (φ_m) and the contact potential difference (Δ^m_sψ) at the electrodeposit/solution interface: ϕ_m = φ_m/e + Δ^m_sψ. The former is a measure of the electron stability of the electrodeposit and the latter reflects the chemical stability of corrosion product, e.g., an oxide scale or an adsorption layer. φ_m of zinc electrodeposit decreases with a decrease in the grain size from micro-scale to nano-scale, rendering surface electrons of the nanocrystalline zinc coating more active to participate in corrosion reactions than those of the coarse-grained zinc coating. This accelerates the formation of a more protective corrosion product layer, leading to a positive shift of Δ^m_sψ and consequently enhanced corrosion resistance, though the nanocrystalline coating has a lower EWF. Δ^m_sψ is dependent on both the material and the environment, which influences the performance of materials in different solutions and their ranking in terms of corrosion resistance.

Acknowledgements

The authors are grateful for financial support from the Natural Science and Engineering Research Council of Canada, Suncor Energy Inc., GIW Industries Inc., Shell Canada Ltd., Magna International Inc. and Volant Products Inc.

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