Zinc-nickel alloy coatings electrodeposited by pulse current and their corrosion behavior.

Steel in subtropical monsoon regions encounters severe atmospheric corrosion. In an attempt to provide better corrosion protection, zinc-nickel alloys were electrodeposited on steel by pulse current. Some electric variables (average current density and pulse cycle) and bath temperature on chemical compositions and grain size of coatings were studied. Next, zinc-nickel alloys electrodeposited on steel were prepared for atmospheric corrosion tests. In addition, the determination of the chemical compositions and the grain size of the Zn-Ni alloy coatings were conducted by scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDS) and scanning probe atomic force microscope (AFM). Furthermore, Tafel plots were employed to evaluate the alloy corrosion behavior.

Keywords: Pulse plating, Zn-[N.sub.2] alloy coatings, atmospheric corrosion testing, SEM, EDS, XRD, electrodeposition, Zn, [N.sub.2], iron, cast iron, steel

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In general, zinc and zinc alloys electrodeposited on steel are of interest due to improved corrosion protection. (1) However, Zn-Ni and Zn-Co coatings have received more attention than zinc coatings because of their higher degree of corrosion resistance to sodium chloride in the atmospheric environment. (2-5) Furthermore, Baldwin, et al., Felloni, et al., Fratesi, et al., and Zaki, et al. also noted that Zn-Ni alloy electrodeposits had better corrosion resistance to sodium chloride in the atmospheric environment. (6-9) In addition, Ramanauskas, et al. (10) found that one-year weight losses for pure Zn coatings and Zn-Ni coatings were 9 g/[m.sup.2] and 5 g/[m.sup.2], respectively, whereas in another test site, one-year weight losses for pure Zn coatings and Zn-Ni coatings were 12 g/[m.sup.2] and 6 g/[m.sup.2], respectively. Although Hsu and Kao, et al. found that electrodeposited Zn-Cd alloy coatings had the best anticorrosion behavior, their electrodeposition waste was not easily treated. (11,12) Moreover, Kalantary also showed that the corrosion resistance of the Zn-Ni alloy electrodeposits was better than that of other zinc alloys of equal thickness with respect to cost considerations. (13) Therefore, electrodeposited Zn-Ni alloys provide a better alternative for corrosion protection.

Electrodeposited Zn-Ni alloy coatings are mainly utilized to improve the corrosion resistance of automobile steel bodies. (14) However, they also have been used in other applications such as electrocatalytic water electrolysis and electronic industries. In addition, due to their better mechanical properties, they are being considered for plating aircraft components. (14)

Natorski mentioned that current efficiencies increase when highly conductive chemicals such as KCl are added to chloride baths. (15) In addition, Alfantazi, et al. mentioned that in order to obtain a higher current efficiency, the pH value in a chloride bath of electrodeposited Zn-Ni alloy coatings should be adjusted to 3.5 ~ 4.5. (16) Furthermore, Ashassi-Sorkhabi, et al. found that the ratios of [Ni.sup.2+]/[Zn.sup.2+] (0.5 ~ 1.5) in a chloride bath had little effect on the chemical compositions of deposits. (2) Thus, in this study, [Ni.sup.2+]/[Zn.sup.2+] = 0.5 was chosen and [H.sub.3]B[O.sub.3] (60 g/l) was used to control pH = 4.4 of the chloride bath to which KCl (160 g/l) was added, according to the literature. (17)

Pulse plating has advantages over direct current plating because it produces smooth, fine-grained, and more compact coatings. (18-20) Therefore, in this work, pulse plating was chosen to electrodeposit thin films of the Zn-Ni alloys from an acid chloride bath. The effect of operating variables such as bath temperature, average current density, and pulse cycle on the compositions and grain size of electrodeposited Zn-Ni alloys was measured. According to the literature, (7) the compositions, structure, and porosity of electrodeposited Zn-Ni alloys also influence their corrosion behavior. Outdoor exposure tests are necessary in order to build electrodeposited Zn-Ni alloy atmospheric corrosion databases, so such tests were conducted. In addition, the compositions, grain size, and crystalline structure of the Zn-Ni alloy coatings were examined by scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDS), scanning probe atomic force microscopy (AFM), and X-ray diffraction (XRD), respectively. Furthermore, Tafel plots were employed to evaluate the corrosion behavior of the Zn-Ni alloys. In addition, the morphology of the cross-sections of specimens exposed outdoors was examined by SEM. Moreover, rust layers of corrosion specimens were analyzed by Fourier transform infrared spectroscopy (FTIR).

EXPERIMENTAL

Carbon steel [SS400; chemical composition (wt%): C (0.13), Si (0.02), Mn (0.77), P (0.021), S (0.007), Cu (0.01), Cr (0.02), Ni (0.01), V (0.002), and Al (0.032)] coupons sized 2 cm x 7 cm were chosen as cathodes. The anodes were made of zinc and measured 2 cm x 7 cm. Before electrodeposition, SS400 was etched in 50% (volume/volume) aqueous HC1 and then rinsed with distilled water. It was abraded with coarse (mesh #100) SiC paper, and then with fine (mesh #600) SiC paper. Next, it was rinsed with distilled water and acetone and then ultrasonically with acetone for five minutes. It was then rinsed with distilled water alone. Next, it was neutralized in 10% (volume/volume) aqueous NaOH for two minutes and then rinsed with distilled water alone. Finally, active sites were produced by putting coupons into 5% (volume/volume) aqueous HCl for 30 min and then rinsing with distilled water alone. The bath compositions and deposition conditions for Zn-Ni deposits are given in Table 1. The solution was prepared using distilled water and reagent grade chemicals. In addition, the deposition equipment includes pulse/DC power supplier (precision of time: 1 ms, E: 0 ~ 30 V and I: 0 ~ 20 A), deposition cell, and temperature control system. Furthermore, [T.sub.off]/[T.sub.on] refers to the ratio of the time that the pulsed current is off versus the time that it is on. The setting of [T.sub.on] (1 ms) was fixed and [T.sub.off] (1, 2, 3, 4, and 5 ms) was varied.

[FIGURE 1 OMITTED]

After deposition, the deposits were immediately rinsed with distilled water and then dried by heating in air at 353 K for 30 min. Next, the determination of the chemical compositions of the Zn-Ni alloy coatings was conducted by SEM (Hitachi S3000N, Japan) combined with EDS (Horiba, England). The SEM was operated in the range 20 ~ 25 keV. Additional information on the grain size of the Zn-Ni alloy coatings was obtained by AFM (Digital Instrument, Nanoscope III, USA) operating in the tapping mode. In addition, the XRD (MAC Science, Japan) was used to characterize the crystalline structure of the Zn-Ni alloy coatings.

Atmospheric corrosion tests were carried out at a station on the roof of a four-story building, open to the air and located 2500 to 3000 meters away from the seashore. The two sides (front and rear) of different Ni percentage coatings were exposed to the atmosphere on open racks at an angle of 45[degrees] facing south. Insulators (plastic material) were inserted between the test specimens (different percentages of zinc in Zn-Ni alloys) and racks. After different exposure durations were determined by corrosion rates (the higher the corrosion rate, the shorter the exposure period), the test specimens were immersed in a rinsing solution [100 g of [(N[H.sub.4]).sub.2][S.sub.2][O.sub.8] dissolved in deionized water and total volume of 1000 ml of an aqueous solution] until the complete elimination of corrosion products was achieved. The specimens were then rinsed with distilled water, dried, and weighed to four decimals, so that weight losses due to atmospheric corrosion could be obtained. Atmospheric corrosion rates are dependent on [Cl.sup.-] deposition fluxes, S[O.sub.2] deposition fluxes, temperature, the material under test, and the time of wetness. Thus, [Cl.sup.-] deposition fluxes were measured by the wet candle method, according to ISO 9225, and S[O.sub.2] deposition fluxes were to be made in accordance with ISO 9225 at the same station of atmospheric corrosion tests and at different periods determined by exposure duration of tests on the specimens. In addition, the time of wetness could then be determined from RH/temperature measured in the meteorological station near the atmospheric corrosion station.

Tafel plots were obtained through an EG & G Model 273A corrosion measurement system. The electrolyte, a solution of 3.5% NaCl, was prepared from reagent grade chemicals and distilled water. A three-electrode corrosion cell consisting of Ag/AgCl was used as the reference electrode, Pt as the counter electrode, and our test specimens (different percentages of zinc in Zn-Ni alloys) cleaned of corrosion products were used as the working electrode. All the corrosion experiments were conducted at scan voltages (-250 mV ~ -250 mV), scan rate (1.66E-3 mV/sec), and room temperature. The potentials reported were against the Ag/AgCl reference electrode except where stated. In addition, examination of the surface morphology of the cross-sections of specimens exposed to outdoor conditions was conducted by SEM (Hitachi S3000N, Japan), operated in the range 20 ~ 25 keV. Furthermore, the rust layers of the corrosion specimens were analyzed by FTIR (Perkin Elmer LEE-59).

[FIGURE 2 OMITTED]

RESULTS AND DISCUSSION

The effect of the plating bath temperature on the Ni percentages in the deposits was studied in the range of 303 ~ 353 K. These experiments were carried out with pulse current at the bath compositions and deposition conditions (see Table 1) whose average current density (150 mA/[cm.sup.2]) and [T.sub.off]/[T.sub.on] ratio (4) were chosen according to the literature of Felloni, et al. (7), Alfantazi, et al. (16), and Pagotto, et al. (17) In addition, the better deposition conditions (average current density = 150 mA/[cm.sup.2] and [T.sub.off]/[T.sub.on] = 4) for plating zinc-nickel in the deposits were also confirmed by carrying out intersection experiments among three parameters (plating bath temperature, average current density, and pulse cycle) and examination of the surface morphology of the Zn-Ni alloy coatings by optical microscopy (OM). Figure 1 shows the variety in Ni compositions of deposits as a function of the plating bath temperature. The higher the plating temperature of the bath solution was, the higher the Ni compositions of deposits. This behavior is confirmed in the literature. (14) The reason behind this behavior may be that [Zn.sup.2+] activity (lower activation energy) is higher than [Ni.sup.2+] activity (higher activation energy). Thus Zn(OH)[.sub.2] is more easily formed than Ni(OH)[.sub.2], and then more Zn(OH)[.sub.2] is reduced to form Zn. However, Ni with higher activation energy is very temperature-sensitive, according to Arrhenius' law. Therefore, Ni(OH)[.sub.2] is more easily formed and then more Ni(OH)[.sub.2] is reduced to form Ni at higher temperature. In addition, Figure 2 shows how grain size varied with plating bath temperature. At the temperature 313 K, the grain size of deposits reached a minimum size of 1.754 [micro]m and the Ni composition was 11.89 wt%. As the maximum corrosion resistance for Zn-Ni alloys was reported in the range of 12 ~ 14 wt% nickel, (14) whose value approached 11.89 wt% nickel plated at the temperature 313 K, so this temperature (313 K) was chosen as a better temperature for other parts of this study. It may be that the higher the temperature, the higher the growing rate of crystals and the lower the formative rate of nuclei. In addition, the finer grain size provides greater corrosion resistance according to atmospheric corrosion tests (Figure 8) and Tafel extrapolation tests (Figure 9). The reason behind this behavior may be that the finer the grain size was, the more compact the structure of the Zn-Ni coatings became, which is not easily attacked by [Cl.sup.-],... etc. Thus, the finer grain size of the Zn-Ni coatings could improve corrosion resistance. Furthermore, from a comparison of curves in Figure 3, it shows that [eta] phase, which is the hexagonal structure of rich zinc, is only found at T = 303 K (9.65 wt% Ni); [gamma] phase, which is the body-centered cubic structure of [Ni.sub.5][Zn.sub.21], gradually disperses when the temperature of the bath solution increases; and [alpha] phase, which is the face-centered cubic structure of rich nickel, is only found at T = 353 K (59.21 wt% Ni). XRD patterns also reflect that the Ni compositions of deposits increase when the temperature of the bath solution increases.

[FIGURE 3 OMITTED]

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[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

The effect of the average current densities on the Ni percentages in the deposits was studied in the range of 50 ~ 200 mA/[cm.sup.2]. The temperature of the bath solution was chosen at 313 K, according to the previously mentioned better temperature on the percentage of nickel in the deposits, and [T.sub.off]/[T.sub.on] = 4 was also chosen. Figure 4 shows that average current densities had little effect on the Ni compositions (10 ~ 14 wt%) of deposits. This result is similar to that found in the literature. (16) Moreover, Figure 5 shows how grain size varied with average current densities. At the average current density 150 mA/[cm.sup.2], the grain size of deposits reached a minimum whose grain diameter was 1.754 [micro]m and the Ni composition was 11.89 wt%. So this average current density (150 mA/[cm.sup.2]) was chosen as a better plating current density for other parts of this research.

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

The effect of [T.sub.off]/[T.sub.on] ratios on the percentages of nickel in deposits was studied in the range of 1 ~ 4. The temperature of the bath solution and the average current density were chosen at 313 K and 150 mA/[cm.sup.2], respectively, according to the chosen better temperature and average current density on the percentage of nickel in the deposits. Figure 6 shows that pulse cycle had little effect on the Ni compositions of deposits. In addition, Figure 7 shows how grain size varied with [T.sub.off]/[T.sub.on] ratios. At [T.sub.off]/[T.sub.on] = 4, the grain size of deposits reached a minimum whose grain diameter was 1.754 [micro]m and the Ni composition was 11.89 wt%. So [T.sub.off]/[T.sub.on] = 4 was chosen as a better pulse cycle for other parts of this study.

The results of atmospheric corrosion tests for the three test specimens (sample A: 11.89 wt% Ni and grain diameter = 1.754 [micro]m; sample B: 22.4 wt% Ni and grain diameter = 4.102 [micro]m; and sample C: 59.21 wt% Ni and grain diameter = 7.265 [micro]m) are illustrated in Figure 8. The weight loss for sample A was less than that for both samples B and C. The reason behind this behavior may be that the finer the grain size, the more the compact structure of the Zn-Ni coatings, which is not easily attacked by [Cl.sup.-],... etc. Thus, the finer grain size (1.754 mm) of the Zn-Ni coatings (sample A) could improve corrosion resistance. In addition, it may be the greater wt% of Zn in sample A and that those protective layers [Zn(OH)[.sub.2] and Zn[Cl.sub.2]] of corrosion built up more easily on sample A than on samples B and C. In addition, after 150 days, the curves for all samples began to plateau due to dense enough protective layers being formed. Furthermore, the gradual formation of corrosion layers was almost certainly responsible for the decrease in corrosion rates (slopes of Figure 8) on all the samples with increasing time. Moreover, the annual average [Cl.sup.-] deposition flux, annual average S[O.sub.2] deposition flux, and the time of wetness was 40.76 mg/day-[m.sup.2], 3.24 mg/day-[m.sup.2], and 0.512 year, respectively, in the exposure site.

In addition, corrosion data obtained from Tafel curves (see Figure 9) were [i.sub.corr] = 8.075E-7 A/[cm.sup.2] ([E.sub.corr] = -0.458 V) for sample A, [i.sub.corr] = 1.128E-5 A/[cm.sup.2] ([E.sub.corr] = -0.599 V) for sample B, and [i.sub.corr] = 2.748E-5 A/[cm.sup.2] ([E.sub.corr] = -0.823 V) for sample C. Thus, sample A had the best corrosion resistance due to the lowest corrosion current density and sample C had the worst anticorrosion because of the highest corrosion current density. For tendencies of anticorrosive characteristics, the results of Tafel extrapolation tests were similar to those seen in the atmospheric corrosion tests (Figure 8) and the literature. (8,21)

Furthermore, at the end of the exposure period, cross-sections were prepared for SEM evaluation. Figures 10, 11, and 12 show SEM images of samples A, B, and C after 229 days of outdoor exposure, and illustrate less corrosion pitting due to better corrosion resistance of the Zn-Ni coatings and exposure to the lower [Cl.sup.-] deposition flux (42.4 mg/day-[m.sup.2]) and S[O.sub.2] deposition flux (3.29 mg/day-[m.sup.2]) in the exposure environment.

Moreover, according to the literature of Shastey and Townsend, (22) the corrosion mechanisms for the Zn-Ni alloy coatings are:

[Zn.sup.2+] + 2 O[H.sup.-] [[k.sub.1].[right arrow]] Zn(OH)[.sub.2] (1)

[Zn.sup.2+] + 2 [Cl.sup.-] [[k.sub.2].[right arrow]] Zn[Cl.sub.2] (2)

Rust layers of atmospheric corrosion tests for samples A, B, and C were analyzed by FTIR. Figure 13 illustrates that the compositions of all the rust layers were Zn(OH)[.sub.2] and Zn[Cl.sub.2], which confirmed the products of corrosion mechanisms.

CONCLUSIONS

The plating bath temperature was the main factor affecting the percentages of nickel in the deposits. However, the plating current densities and pulse cycle had little effect on the Ni compositions of deposits. Better deposition conditions for plating zinc-nickel in the deposits were 313 K, 150 mA/[cm.sup.2], and [T.sub.off]/[T.sub.on] = 4, where grain size in the deposit reached a minimum. For atmospheric corrosion tests, the curves for all samples began to plateau after 150 days due to dense enough protective layers being formed. The weight loss for sample A was smaller because of its finer grain size in the deposits and the greater wt% of Zn. The results of Tafel extrapolation tests were similar to those of atmospheric corrosion tests. In addition, the rust layers analyzed by FTIR were composed of Zn(OH)[.sub.2] and Zn[Cl.sub.2], which confirmed the products of corrosion mechanisms. Organic coatings applied over the Ni-Zn coating treatment will be considered as future work.

References

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(2) Ashassi-Sorkhabi, H., Hagrah, A., Parvini-Ahmadi, N., and Manzoori, J., "Zinc-Nickel Alloy Coatings Electrodeposited from a Chloride Bath Using Direct and Pulse Current," Surf. Coat. Technol., 140, 278 (2001).

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(8) Fratesi, R., Roventi, G., Brang C., and Simoncini, S., "Corrosion Resistance of Zn-Co Alloy Coatings," Surf. Coat. Technol., 63, 97 (1994).

(9) Zaki, N., "Zinc-Nickel Alloy Plating," Met. Finish., 6, 57 (1989).

(10) Ramanauskas, R., Muleshkova, L., Maldonado, L., and Dobrovolskis, P., "Characterization of the Corrosion Behaviour of Zn and Zn Alloy Electrodeposits: Atmospheric and Accelerated Tests," Corros. Sci., 40, No. 2/3, 401 (1998).

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(14) Ashassi-Sorkhabi, H., Hagrah, A., Parvini-Ahmadi, N., and Manzoori, J., "Zinc-Nickel Alloy Coatings Electrodeposited from a Chloride Bath Using Direct and Pulse Current," Surf. Coat. Technol., 140, 278 (2001).

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(16) Alfantazi, A.M., Page, J., and Erb, U., "Pulse Plating of Zn-Ni Alloy Coatings," J. Appl. Electrochem., 26, 1225 (1996).

(17) Pagotto, S.O., de Alvarenga Freire, C.M., and Ballester, M., "Zn-Ni Alloy Deposits Obtained by Continuous and Pulsed Electrodeposition Processes," Surf. Coat. Technol., 122, 10 (1999).

(18) Ibl, N., "Some Theoretical Aspects of Pulse Electrolysis," Surf. Coat. Technol., 10, 81 (1980).

(19) Maksimovic, M.D. and Dimitrijevic, Z.S., "Fundamental Aspects of Pulsating Current Metal Electrodeposition V: The Determination of the Optimal Frequency Range," Surf. Coat. Technol., 17, 3 (1982).

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(21) Fratesi, R. and Roventi, G., "Electrodeposition of Zinc-Nickel Alloy Coatings from a Chloride Bath Containing N[H.sub.4]Cl," J. Appl. Chem., 22, 657 (1992).

(22) Shastey, C.R. and Townsend, H.E., "Mechanisms of Cosmetic Corrosion in Painted Zinc and Zinc-Alloy Coated Sheet Steels," Corrosion, 45, No. 2, 103 (1989).

Chuen-Chang Lin ([dagger]) and Chi-Ming Huang -- National Yunlin University of Science and Technology*

* Department of Chemical Engineering, 123 University Road, Sec. 3, Yunlin, Taiwan, R.O.C.

([dagger]) Author to whom correspondence should be addressed. Ph: 886-5-534-2601 x4619; fax: [phone]; [e-mail].

Table 1 -- Bath Compositions and Deposition Conditions

Bath Compositions and
Deposition Conditions       Quantity

Zn[Cl.sub.2]                90 g/l
Ni[Cl.sub.2] * 6[H.sub.2]O  80 g/l
[H.sub.3]B[O.sub.3] (pH)    60 g/l (4.4)
KCl                         160 g/l
average current density     50, 100, 150, 200 mA/[cm.sup.2]
temperature                 303, 313, 323, 333, 343, 353 K
[T.sub.off]/[T.sub.on] (a)  1, 2, 3, 4, 5

(a) The ratio of the time that the pulsed current is off versus the time
that it is on.