Pulse reverse electrodeposition and characterization of nanocrystalline zinc coatings

Abstract

Nanocrystalline zinc coatings are produced through pulse reverse electrodeposition in an acid sulfate electrolyte with polyacrylamide (PAM) as the only additive and are characterized by field-emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The influence of the additive and electrodeposition parameters on the surface morphology, grain size and crystallographic preferred orientation of the coatings are investigated. The mechanical, wear and corrosion resistance properties of the nanocrystalline and coarse-grained zinc coatings are also evaluated by nanoindentation, a ball-on-disc tribometer and an immersion test, respectively. The results show that changing the electrodeposition parameters cannot produce the nanocrystalline zinc coating in the absence of the additive, and the PAM plays a crucial role in reducing the grain size of the coating from the micro to nano scale. The grain size of the coating decreases asymptotically with an increased forward pulse current density in the presence of PAM, and the pulse reverse current makes the coating smoother. The hardness of the nanocrystalline zinc coating (31 nm) is almost three times than that of the coarse-grained counterpart. The wear and corrosion resistance properties of the nanocrystalline zinc coating are superior to its coarse-grained counterpart.

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Introduction

Zinc is an anodic coating relative to carbon steel, iron and low alloy steel, which can play a role in electrochemical protection when it forms a micro-corrosion battery with steel based metal and the damaged area of the coating surface is not too big. Moreover, the zinc coating is stable in air and it forms a layer of dense alkaline zinc carbonate film, which plays a role in the physical protection of the substrate in a humid environment. Moreover, the low cost and easy application of zinc also makes the electrodeposition of a zinc coating a major surface protection technique and a widespread application for the protection of steel against corrosion. As abovementioned, the success of using zinc as a steel coating can be attributed to its sacrificial nature, but the zinc coating does not alter the mechanical properties of the underlying base metal, and thereby the coating must be thick enough to endure attack of a corrosive environment. Thus, the poor mechanical properties of the conventional coarse-grained zinc coating have hindered its application in many fields. This provides the need to develop thinner electrodeposited coatings with improved properties such as hardness, corrosion, friction and wear, etc.^1–3

Nanocrystalline electrodeposits have exhibited many unusual mechanical, physical, chemical and electrochemical properties due to its grain size below 100 nm and high-volume fraction of the grain boundary.⁴ Further research have demonstrated that the nanocrystalline coatings possess excellent wear resistance,^5,6 corrosion resistance,^7,8 ductility,^9,10 hardness^11,12 and electrochemical properties¹³ compared with conventional coarse-grained zinc coatings. Therefore, it is of great significance to improve the properties of conventional zinc coatings through the nano-electrodeposition technology.

So far, several nanocrystalline zinc coatings were obtained from various galvanizing baths, such as an alkali,¹⁴ chloride,¹⁵ sulfate,¹⁶ acetate¹⁷ or citrate¹⁸ system. Among these baths, the sulfate bath has a high current efficiency, less energy consumption, solution stability, simple maintenance characteristics, which can be used in high speed plating. Yan et al.¹⁹ produced the nanocrystalline zinc coatings with a laminated structure through direct current electrodeposition from a sulfate bath, but it only achieved a nanoscale in the thickness direction not in the three dimensional grain size. Li M. C. et al.²⁰ obtained the nanocrystalline zinc coatings from a sulfate bath containing mixtures of thiourea and benzalacetone by pulse electrodeposition, with average grain sizes in the range of 60 to 77 nm. The results show that it was not effective enough to induce the formation of nanocrystalline zinc in a single addition or additive-free system. Gomes²¹ also investigated the electrochemistry behavior of the electrodeposition of nanocrystalline zinc coatings in a sulfate bath with cetyl trimethyl ammonium bromide (CTAB), sodium dodecyl sulphate (SDS) and octylphenolpoly(ethyleneglycolether)_n, n = 10, (Triton X-100) as surfactants and obtained a needle shape zinc coating with average grain sizes in the range of 20 to 40 nm. It is also proven that the additive plays a crucial role in the process of grain refinement.

The current modes also have a significant effect on the nanocrystallized surfaces of coatings in the presence of additives. The most commonly used current modes in the electrodeposition of zinc coatings include a direct current, pulse current and pulse reverse current. Among these techniques, the pulse electrodeposition (or single pulse electrodeposition) had significant advantages in the control of the grain size, surface morphology and preferred orientation of the coating than the direct current electrodeposition.²² Compared with the pulse electrodeposition, the pulse reverse electrodeposition (or double pulse electrodeposition)^23–26 can significantly improve the thickness distribution, eliminate hydrogen embrittlement and leaves the surface of the coating in an activated state, so as to prepare smoother, denser, lower porosity and better adhesion coatings. Moreover, the anodic dissolution effect of the pulse reverse current was beneficial to reduce the actual thickness of the diffusion layer and improve the cathodic current efficiency, and further speed up the deposition rate of coating as well. Therefore, the zinc coatings produced by pulse reverse electrodeposition had better corrosion resistance than the coatings prepared through pulse electrodeposition,²⁷ which fabricated similar coatings in the bath without additives to the bright coatings prepared by direct current electrodeposition in the bath with additives.

So far, most research works have focused on the pulse electrodeposition of nanocrystalline zinc coatings and the electrochemical corrosion behaviour of the coatings.^2,3,14,18 There are rare studies on the pulse reverse electrodeposition of nanocrystalline zinc coatings and the mechanical properties of the coatings. In this study, we produce nanocrystalline zinc coatings by combining the advantages of an additive and the pulse reverse current, and investigate their effect on the surface topography, crystallographic preferred orientation and grain size of the coatings. Specially, interest is focused on the different hardness, corrosion and tribological behavior between the nanocrystalline zinc coatings and conventional coarse-grained zinc coatings.

Experimental

Electrodeposition of nanocrystalline zinc coatings

Zinc electrodeposition was carried out in a two-electrode cell containing a 200 mL sulfate bath. The substrates used were copper sheets of 4 × 4 cm² area, which were pretreated by immersion in a solution of 10% (volume ratio) hydrochloric acid at room temperature for 10 s, and rinsed with distilled water. A zinc plate of 99.99% purity was used as the anode. The sulfate bath was prepared by dissolving 100 g L⁻¹ of ZnSO₄·7H₂O and 20 g L⁻¹ of H₃BO₃ in deionized water. Once the solids were dissolved, the pH value of the solution was adjusted to 1–2 by the addition of dilute sulphuric acid, followed by the addition of 1 g L⁻¹ of the nonionic type polyacrylamide (PAM MW = 2 000 000) as the grain refiner. The bath was heated under stirring until the PAM was completely dissolved and the electrolyte became transparent to complete the configuration process of bath. All solutions were prepared with AR grade chemicals and deionized water.

The pulse reverse electrodeposition or double pulse electrodeposition (The periodic reverse pulse current with pulses in both the positive and negative direction.) was conducted by varying the pulse forward current density (J_f) from 1 to 4 A dm⁻² and the pulse reverse current density (J_r) from 0 to 0.5 A dm⁻². In order to study the influence of the reverse current density on the surface topography, crystallographic preferred orientation and grain size of the zinc coatings, the current-on time (T_on) and current-off time (T_off) were set to fixed values of 0.2 ms and 0.8 ms, respectively, and the electrodeposition time was 60 min in a magnetically stirred system at room temperature, for all experiments.

The efficiency of the current electrodeposition (η) was calculated by Faraday's law according to eqn (1). The amount of mass deposited in theory was compared with that of the actual zinc mass deposited at the same current density and time.

where m₁ is the measured weight before electrodeposition (in g), m₂ is the weight after electrodeposition (in g), I is the current that flows through the cathode (in A), t is the time that the current flows (in h) and K is the electrochemical equivalent of Zinc (in 1.22 g (A⁻¹ h⁻¹)).

Grain size characterization of coatings

A field-emission scanning electron microscope (FESEM, Helios Nanolab 600i) with energy dispersive X-ray spectroscopy (EDS) was used to examine the surface and cross-section morphology, grain size and the components of the coatings. The grain size distribution of the coatings was investigated by the Nano Measure software, which was determined by measuring more than 100 points in the FESEM image randomly to avoid feature point close clustering. Then the average grain size of the coatings was obtained through Gaussian fitting of the measured results. The surface roughness was characterized by an atomic force microscope (AFM, Bruker Multimode 8), working in the intermittent contact mode. X-ray diffraction (XRD, Rigaku Corporation Dmax-3B) was carried out using Cu Ka radiation in order to determine the crystallographic texture, crystallographic preferred orientation and approximate average grain size of the coatings. The grain size of the coatings was calculated using Scherrer's formula. Moreover, considering the sensitivity limitation of FESEM and the calculation error of XRD in the grain size of the coatings, transmission electron microscopy (TEM, JEM-2100) was performed to determine the grain size of the coating obtained from the optimum bath. Samples of nanocrystalline zinc coating were removed from the substrate using a sapphire knife without destroying the substrate, and then dispersed in acetone. A drop of the solution was placed on a carbon-coated grid and inserted into the TEM. In any testing program, three time replicates should be included to certify the reproducibility.

Properties evaluation of the coatings

Mechanical performance. The hardness values of the specimens were evaluated based on the load-depth curves obtained by the nanoindentation tests (nanoindenter XP, MTS Systems Corporation). All measurements were made at a 1 μm penetration depth with a Berkovich diamond indenter. Typically, 5 indents were obtained for individual specimens, from which the average values were calculated.

Friction and wear property. The friction coefficients of the coatings were tested on a ball-on-disk tribometer (Center for Tribology, HIT, China) with a 52 100 steel ball of 10 mm diameter as the friction partner. All experiments were performed under a load of 1 N and a sliding speed of 0.188 m s⁻¹ without lubrication at room temperature with a relative humidity of approximately 40%. The friction coefficient curves were recorded continuously during the testing process.

Corrosion behaviour. The corrosion resistance property of the samples was evaluated by an immersion test, and a 3.5% NaCl solution (room temperature and closed environment) was used as the corrosive medium. After the corrosion tests, the surface morphology and element composition of the corrosion products were analysed by FESEM and EDS.

Results and discussion

Effect of pulse current density on grain size

In order to determine whether the change of J_f reflects the surface morphology or not, a SEM study was performed, and the images are shown in Fig. 1. These SEM micrographs were obtained at J_f of 1, 3, 5 and 7 A dm⁻². The SEM images show that the surface morphology and grain size are markedly affected by the pulse current density. In the basic zinc sulphate bath (additive-free sulphate bath), the coatings display the hexagonal zinc plates aligned parallel to the substrate, which indicates that the hexagonal structure of zinc is preserved in the zinc electrodeposition. By comparing the coatings at different values of J_f from the additive-free bath, it is found that the grain size increased gradually with an increase in J_f. This result indicates that changing the electrodeposition parameters cannot produce nanocrystalline zinc, and the nucleation rate is lower than the growth rate of the coatings in the absence of additives. The smallest grain size was produced at J_f = 1 A dm⁻².

Fig. 1 Effect of J_f on the surface morphology of zinc electrodeposited from a sulphate bath without additives: (a) 1 A dm⁻², (b) 3 A dm⁻², (c) 5 A dm⁻², and (d) 7 A dm⁻².

Effect of the additive on grain size

Fig. 2 shows the influence of additives on the surface morphology of the coatings. The additive is chosen according to Youssef's research,^2,28 in which nanocrystalline zinc coatings (56 nm) are electrodeposited by a pulse current from zinc chloride electrolytes (ZnCl₂, NH₄Cl and H₃BO₃; pH = 4.7) with polyacrylamide and thiourea as the additives. The result shows that the PAM leads to a progressive reduction in the grain size from a micro to a nano-scale (<100 nm), when compared with the coatings prepared from the additive-free solution (Fig. 1). The results indicate that the PAM plays a primary role in grain refinement in this optimized sulfate bath. According to Youssef's research, the reduction of the grain size by organic additives is mainly related to the combined effects of the increase in cathode overpotential, the retardation of continuous grain growth, the increase in the nucleation of continuous grain growth and the increase in the nucleation rate. The mechanism of the PAM is similar to the conventional zinc plating brightener, which suppresses the propagation of reagent particles and decreases the concentration of reactive ions, thereby increasing the overpotential of the cathode by organic additive absorption on the surface of the electrode.¹⁵ A relatively smoother and more uniform distribution of grain size was produced when the concentration of the PAM was 1 g L⁻¹, as shown in Fig. 2b. As the PAM concentration continues to increase, the coatings could relatively coarsen (as shown in Fig. 2c and d) and the PAM precipitates out.

Fig. 2 FESEM morphology of zinc electrodeposited at J_f of 1 A dm⁻² from a sulphate bath with different concentrations of the additive: (a) 0.5 g L⁻¹, (b) 1 g L⁻¹, (c) 1.5 g L⁻¹, and (d) 2 g L⁻¹.

The influence of different J_f on the microstructure characteristics of the coatings was examined in the range of 1 to 4 A dm⁻², as shown in Fig. 3. The coatings, prepared from bath with PAM (1 g L⁻¹), are considerably homogeneous, dense and fine with approximately spherical-shaped particles. The grain size of coatings decreases gradually with increase of J_f from 1 to 3 A dm⁻². This trend can be explained by Sherik's theory,²⁹ which states that increasing pulse current density can increase the overpotential, which increases the free energy to form new nuclei and results in a higher nucleation rate and smaller grain size. Saber et al.¹⁵ observed similar effects for the nanocrystalline zinc electrodeposition in chloride electrolyte systems. They explained this trend and attributed the reduction of the grain size to the high overpotential associated with the high pulse current density. At J_f = 4 A dm⁻², the grain relatively coarsens, therefore the J_f should be controlled under 4 A dm⁻². The smallest grain size of the coatings was obtained at a J_f of 3 A cm⁻², as shown in the image (Fig. 3c). The FESEM studies also indicate that pulse current has an important action on grain size reduction in the presence of an additive.

Fig. 3 FESEM morphology of zinc electrodeposited at different J_f from a sulphate bath with an additive (1 g L⁻¹): (a) 1 A dm⁻², (b) 2 A dm⁻², (c) 3 A dm⁻², and (d) 4 A dm⁻².

Effect of pulse reverse current on grain size

The influence of the pulse reverse currents on the surface morphology of the coatings, are shown in Fig. 4 and 5. These FESEM micrographs of the coatings are obtained from J_r of 0.2 A dm⁻² to 0.5 A dm⁻². As shown in Fig. 4, the pulse reverse current does not have an obvious impact on the grain size of the coatings with the increase in the J_r from 0.2 to 0.4 A dm⁻², when compared with the coatings produced at a J_f of 3 A dm⁻² (Fig. 3c). Due to the addition of the pulse reverse current, the morphology of the coatings was more uniform and denser. Sequentially, increasing the J_r from 0.4 to 0.5 A dm⁻² results in a progressive increase in the grain size of the coating and the coating surface is relatively coarse and loose.

Fig. 4 FESEM images of zinc electrodeposited in the presence of a pulse reverse current of (a) J_f = 2 A dm⁻², J_r = 0.2 A dm⁻², (b) J_f = 3 A dm⁻², J_r = 0.3 A dm⁻², (c) J_f = 4 A dm⁻², J_r = 0.4 A dm⁻², and (d) J_f = 5 A dm⁻², J_r = 0.5 A dm⁻² from a sulfate-based electrolyte with an additive (1 g l⁻¹).

Fig. 5 AFM images of the nanocrystalline zinc coatings obtained at (a) J_f = 3 A dm⁻² and (b) J_f = 3 A dm⁻², J_r = 0.3 A dm⁻² from a sulfate bath with an additive (1 g l⁻¹).

The surface morphology of the coatings electrodeposited at different pulse reverse current densities was also studied by AFM measurements, as shown in Fig. 5. Through comparison, it is found that the surface of coatings obtained at a J_r = 0.3 A dm⁻² is smoother. Combination of the SEM and AFM results suggests that the pulse reverse current does not have an obvious effect on the grain sizes, but achieves the effect of levelling off the coatings while keeping the J_f constant. The reason might be that the reverse current dissolved the particles with a larger grain size on the surface of the coatings, which made the coatings smoother. An increase in the J_f has the effect of decreasing the crystallite size, which has already been suggested in the preceding section. Other recent studies investigated the viability of the pulse current (The periodic pulse current with a pulse only in the positive direction.) in electrodeposition for better control of the structure and properties of nanocrystalline zinc coatings and similar results were obtained.^30,31 The pulse reverse electrodeposition not only kept the advantages of the pulse electrodeposition but also improved the performance of levelling off the coatings, so the pulse reverse electrodeposition has a dual role in both the smoothing and grain refinement of the coatings compared to direct current electrodeposition. Moreover, the anodic dissolution of the reverse current makes the metal ion concentration of the cathode surface rise rapidly, which is favourable for using a higher pulse current density in the later cathode cycle. And the high pulse current density heightens the nucleation rate much more than the growth rate of the coating, and thus produces a coating with a finer grain size.

The effects of increasing J_r on the preferred orientation and the grain size of coatings are shown in Fig. 6 and 7, respectively. Fig. 6 shows that the crystallographic orientation of the coatings includes the five obvious diffraction peaks of Zn (100), (101), (110), (200) and (201) associated with a hexagonal structure. The diffraction peak intensity corresponding to the (110) plane is larger than the other peaks, indicating that the deposits have a well preferred orientation along the (110) direction when the J_r increases from 0 to 0.3 A dm⁻². However, sequentially increasing the J_r from 0.4 to 0.5 A dm⁻² alters the preferred orientation to the (100) crystal plane. The XRD analysis results indicated that the reverse current has a significant influence on the crystallographic orientation. This is attributed to the pulse current, which had a considerable effect on overpotential, thereby influencing the preferred orientation of coatings when the additive was present in the bath, as reported in previous studies.^22,28

Fig. 6 XRD spectra of zinc electrodeposited at different J_r from a sulfate-based electrolyte with an additive (1 g L⁻¹).

Fig. 7 Crystalline size of zinc electrodeposited at different J_r from a sulfate-based electrolyte with an additive (1 g L⁻¹).

The average grain size of the coatings calculated by Scherrer's formula,³² at different J_r is shown in Fig. 7. It can be inferred that the reverse current is not effective enough to reduce the grain size, but when the J_r is increased from 0.4 to 0.5 A dm⁻², the largest grain size is produced. This trend is in good agreement with the surface morphology analysis results, shown in Fig. 4.

Fig. 8 gives the TEM photomicrographs of the coatings which are electrodeposited at J_f of 3 A dm⁻² and J_r of 0.3 A dm⁻² from a basic sulfate bath containing a PAM concentration of 1 g L⁻¹. Microstructure characterization by means of the TEM observations indicates that the nanocrystalline zinc coating consists of ultrafine crystallites in sizes ranging from 26 nm to about 42 nm, as shown in Fig. 8a. In presence of the additive (PAM), the coating shows an approximately spherical-shaped structure and the average grain size is about 31 nm, which is in reasonable agreement with the observed results from the FESEM images and the average grain sizes obtained by the XRD technique. The corresponding selected-area electron diffraction (SAED) pattern can be indexed by the (100), (101), and (110) crystal planes of zinc, as shown in Fig. 8b. The nanocrystalline zinc coating also was characterized by EDS (Fig. 8c), which shows that it is mainly composed of zinc. The detected oxygen is merely slight and its presence may be from the bath and the adsorption of oxygen in the air. The EDS results also indicate that the nanocrystalline coating is pure zinc and not affected by oxidation after the electrodeposition and before insertion into the TEM.

Fig. 8 TEM images and corresponding EDS spectra of zinc electrodeposition with J_f of 3 A dm⁻² and J_r of 0.3 A dm⁻² from a sulfate-based electrolyte with an additive (1 g L⁻¹): (a) TEM image of zinc electrodeposition, (b) Diffraction rings of zinc electrodeposited, and (c) EDS pattern for zinc electrodeposition.

Current efficiency of the nanocrystalline zinc coating

The current efficiencies of the coarse-grained (from the basic bath) and nanocrystalline (from the optimum bath) zinc coatings are measured by weight measurements, whose quantities are about 76% and 72%, respectively. The difference between the coarse-grained zinc and nanocrystalline zinc indicates that the additive decreases the current efficiency while it refines the grain size of zinc coating. This result can be attributed to fact that the absorption of the additive suppresses the propagation of the reagent particles and decreases the concentration of the reactive ions, thereby increasing the overpotential of the cathode (as stated above). The values of the current efficiency are adequate to meet the requirements of the nanocrystalline zinc coatings production.

Mechanical performance of the nanocrystalline zinc coating

Fig. 9 shows a cross-sectional morphology of the nanocrystalline zinc coating and hardness distribution along the thickness direction. Fig. 9a gives a cross-sectional observation of the nanocrystalline zinc coating, which is obtained at a J_f of 3 A dm⁻² and J_r of 0.3 A dm⁻² for 60 min from the optimum bath. It can be demonstrated that the nanocrystalline zinc coating with an average thickness of 24 μm has firm adherence to the substrate and almost no defects in the deposited regions, and the nanocrystalline zinc coating with a smooth surface has a uniform thickness distribution. The hardness-displacement curves through the thickness of the coarse-grained and nanocrystalline zinc coatings, are shown in Fig. 9b. It can be demonstrated that the hardness of the coarse-grained and nanocrystalline zinc coatings are 0.54 and 1.53 GPa, while the penetration depth is 1000 nm. The hardness of the nanocrystalline zinc coating is almost three times as that of the coarse-grained zinc coating. This proves that the surface nanocrystallization of the coating could apparently enhance the hardness of the coarse-grained zinc coating.

Fig. 9 Cross-section morphology (a) and hardness distribution (b) of the nanocrystalline zinc coating.

Tribological property of nanocrystalline zinc coating

The grain size distribution of the nanocrystalline zinc coating was determined from the FESEM image by measuring more than 100 grains, as shown in Fig. 10a. The grains of the coating are highly dispersed with values ranging from 25 nm to 40 nm and the average grain size is 31 nm. Fig. 10b shows that the variation of the friction coefficients with the sliding duration for the coarse-grained and the nanocrystalline zinc coatings at an applied load of 1 N and a sliding speed of 0.188 m s⁻¹. It can be demonstrated that the friction coefficients in both samples increase evidently in the initial period of sliding and tend to achieve steady-state values. For the coarse-grained zinc coating, the friction coefficient varies in the range of 0.5–0.7 and the values are very unstable. Compared with the coarse-grained zinc coating, the friction coefficient of the nanocrystalline zinc sample is between 0.2 and 0.3. The steady-state friction coefficients for the nanocrystalline zinc coating are evidently lower than that of the coarse-grained zinc coating. The differences between the coarse-grained and nanocrystalline zinc coatings prove that the surface nanocrystallization can decrease the friction coefficients of the deposit evidently. In general, under a certain load, the lower the friction coefficients, the better the wear resistance ability is.^33–35 Hence, it can be concluded that the nanocrystalline zinc coating exhibited excellent friction-reduction behavior and better wear resistance performance when compared with the coarse-grained zinc under the same wear conditions.

Fig. 10 Grain size distribution (a) and friction coefficients (b) of the nanocrystalline zinc coating.

Corrosion resistance evaluation of the nanocrystalline zinc coating

After 100 h of immersion in a 3.5% NaCl solution, the FESEM observation was carried out for the coarse-grained and nanocrystalline zinc coatings, as shown in Fig. 11. The images display that both surfaces are covered with corrosion product layers. In the case of the coarse-grained zinc coating, the corrosion sites are discrete and local sites are corroded badly. It looks like that the coating is melted down. As for nanocrystalline zinc coating, the surface has been completely covered with a fine and uniform corrosion product layer, and has no abscission of the coating because of the serious corrosion. The EDS spectrum of the elemental composition of the coating surface shows that the deposits predominantly contain Zn and O. It is noteworthy that for both coatings in the same immersion times, the atomic oxygen content in the coarse-grained zinc coating is less than that of the nanocrystalline zinc coating. It can be deduced that the nanocrystalline zinc coating is more easily oxidized and forms a dense and complete corrosion product layer during the immersion initial stage. According to the literature,^3,36 the corrosion product layers on both zinc coating surfaces are mainly composed of ZnO, Zn(OH)₂ and Zn₅(OH)₈Cl₂·H₂O (simonkolleite). In comparison with the coarse-grained zinc coating, the enhanced corrosion resistance of the nanocrystalline zinc coating is mainly due to the better protection of the corrosion product layer. The nanocrystalline structure enhances both the kinetics of passivation and the stability of the passive film formed.² This can be explained by the fact that the nanocrystalline materials are characterized by a high-volume fraction of the grain boundary, so metal atoms that are at the grain boundaries possess a higher activity and are prone to corrosion. The volume fraction of the grain boundary is increased with the surface nanocrystallization of the coating, thus the number of the surface activity sites is increased, which makes the surface of the nanocrystalline zinc coating rapidly form a protective corrosion product film compared to the coarse-grained zinc coating.

Fig. 11 FESEM morphologies and element compositions for the corroded surfaces of the coarse-grained (a) and nanocrystalline (b) zinc deposits after 100 h of immersion.

Conclusions

Application of the pulse reverse current for nanocrystalline zinc electrodeposition from a sulfate bath containing PAM as the only additive resulted in a more homogeneous, finer and denser distribution of grains, in comparison to the single pulse electrodeposition samples. The following main findings resulted from the present investigation:

In additive-free electrolytes, no nanocrystalline zinc coating is obtained just by changing the pulse current density. The PAM can effectively reduce the grain size from a micro to nano scale mainly due to the increases in the overpotential of the cathode by absorption on the electrode surface. The grain size of the nanocrystalline zinc coatings decreases gradually with an increased pulse current density. The investigations also show that the pulse reverse current alters the preferred orientation and levelling of the coatings.

A pure nanocrystalline zinc coating with an average grain size of 31 nm and the (110) crystallographic preferred orientation is produced in this system under optimal conditions, and the thickness and surface grain size distribution are highly uniform. The hardness of the nanocrystalline zinc coating is almost three times as that of the coarse-grained zinc. The friction coefficient of the nanocrystalline zinc coating is much lower and more stable than that of the coarse-grained zinc coating, which indicates that the nanocrystalline zinc coating exhibited an excellent friction-reduction behaviour and better anti-wear performance.

The results of the immersion test indicate that the nanocrystalline zinc coating shows a much higher corrosion resistance than the coarse-grained zinc coating. The corrosion resistance property of the nanocrystalline zinc coating is mainly due to the nanocrystalline structure which enhances both the kinetics of passivation and the stability of the formed passive film. Moreover, the current efficiency of the nanocrystalline zinc coating is 72%, which is adequate to meet the requirements of the production. Therefore, the pulse reverse electrodeposition of the nanocrystalline zinc coating is quite an effective method to improve the properties of coarse-grained zinc coatings.

Notes and references

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