Introduction

Owing to their decent cathodic and barrier protective natures and moderate processing costs, electrodeposited zinc and zinc alloys are being employed as metallic coatings for numerous applications, ranging from automotive to electronic parts. Zinc alloys that comprise metals of the iron group, particularly zinc-nickel (Zn-Ni) with 8-14 wt.% Ni, have received high interest as they exhibit enhanced corrosion resistance over that of pure Zn, while their electrochemical potentials remain anodic to that of steels (Ref 1-4). This thus allows coatings to be used in harsher environments, improves the coatings’ service life, and enables the production of thinner coatings that offer better formability and weldability.

In the past decade, various electrodeposition techniques were developed to improve the fabrication of Zn-Ni alloy coatings. This includes a range of direct current electrodeposition processes, conducted with either acidic or alkaline plating baths, which have been explored to control Zn and Ni contents and corresponding phase structures in the coatings (Ref 5-9). Pulse electrodeposition later received increasing interest as the technique provides homogeneous coatings with fine grains and limited amount of defects (Ref 10-12). Recently, reverse-pulse electrodeposition, whereby a current with cathodic and anodic pulse waveforms is periodically applied to the plating system, was investigated as an alternative route to produce Zn-Ni alloy coatings (Ref 13). The use of single-bath chemistry coupled with a series of pulsating parameters for tailoring a good range of compositions (11-100%Ni), phase structures, and morphology has proved the technique useful for controlling the alloys’ characteristics.

Although the fabrication processes of Zn-Ni alloy coatings have been intensively examined, systematic studies that investigate the influence of the alloys’ characteristics on their corrosion properties remain limited and sometimes unclear. Pech-Canul et al. (Ref 3) and Ramanauskas et al. (Ref 4) successively show that Zn-12%Ni withstands red rust formation almost two times better than pure Zn under a salt spray environment and exhibits six times lower corrosion current density in a chloride solution. The work by Beltowska-Lehman et al. (Ref 5) shows that the corrosion resistance of Zn-Ni alloys is generally improved with increasing Ni contents (2-17%). However, the authors also note that a comparable level of corrosion resistance is evidenced in the coatings of distinct Ni contents obtained from different electrolyte formulations. They suggest that not only the Ni content but also phase compositions influence the alloys’ corrosion properties. These arguments and observations are also echoed in the studies by Byk et al. (Ref 7) and Gavrila et al. (Ref 14). On the contrary, Hammami et al. (Ref 15) show that the polarization resistance of Zn-Ni coatings reduces with an increase of Ni content from 10 to 14%. As for pulsed coating deposits, Pagotto et al. (Ref 10) explore the corrosion resistance of Zn-Ni coatings prepared by pulsed electrodeposition. The authors find that the deposits exhibit fine-grained morphology and show relatively low corrosion resistance, compared to those prepared with direct currents.

In the present study, we establish the relationship of the reverse-pulse electrodeposited Zn-Ni alloy coatings’ characteristics to their corrosion behaviors in a saline environment. This is achieved through polarization analysis and electrochemical impedance spectroscopy, coupled with analysis of the deposits’ microstructure. In addition to the properties of the coatings fabricated by the reverse-pulse electrodeposition method, the study also addresses some mechanistic views of the effects of composition and microstructure on corrosion properties of Zn-Ni alloys.

Experimental

Zn-Ni alloy specimens were prepared by the reverse-pulse electrodeposition technique using a plating bath with aqueous bath chemistry: 100 g/L ZnCl2, 200 g/L NiCl2·6H2O, 40 g/L H3BO3, with pH of 3.5, and the temperature maintained at 35 °C. A copper substrate (1.0 × 2.4 cm2), polished to a mirror-like finish and activated by 10% H2SO4, was a cathode, and platinum was used as an anode. Periodic pulse current, consisting of forward peak current density (I f) of 0.08 A/cm2 and reverse peak current density (I r) of 0, 0.04, 0.06, 0.08, or 0.1 A/cm2, was applied to the electrodes using a pulse rectifier (Dynatronix, DUPR 10-3-6). The forward- and reverse-pulse durations (t f and t r) were controlled at 20 and 3 ms, respectively. The specimens are termed A, B, C, D, and E, as specified in Table 1. Phase composition, alloy contents, and surface morphology of the electrodeposits were characterized by x-ray diffractometry (XRD), energy-dispersive spectroscopy (EDS), and scanning electron microscopy (SEM), respectively. For nanocrystalline deposits, their grain sizes were determined using the x-ray integral breadth method. The linear intercept method was employed to estimate the grain sizes of microcrystalline deposits.

Table 1 The characteristics of the Zn-Ni alloy deposits processed with various reverse-pulse current densities (I r)
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The corrosion studies were executed using a μAutolab/FRA2 potentiostat in aerated 3.5 wt.% NaCl solutions. A saturated calomel electrode (SCE) and platinum were used as reference and counter electrodes, respectively. The potentiodynamic polarization tests were conducted from the open circuit potentials with a scan rate of 1 mV/s. All samples were immersed in the saline solution for 1 h prior to potentiodynamic testing for stabilization. The Tafel regions ±50 mV with respect to the tip of the polarization curves were selected for the evaluation of the corrosion potential (E corr) and the corrosion current density (i corr) by Tafel extrapolation. Alternating current impedance measurements were performed on the specimens that were immersed in the saline solution for 1 and 24 h, using the frequency range of 0.01-100 KHz with 10 mV peak amplitude above the corrosion potentials. The equivalent circuit simulation program NOVA 1.7 was employed for EIS data analysis.

Results and Discussion

Alloys’ Characteristics

The five sets of Zn-Ni coatings (A, B, C, D, and E) were successfully fabricated with uniform morphology. The specimens are distinct by the nickel content, phase structure, and grain size, as summarized in Table 1. Figure 1a-e illustrates surface morphology of these coatings.

Fig. 1
figure 1

(a)-(e) Surface morphology of the electrodeposited Zn-Ni alloy coatings of sets A-E, processed with I f = 0.08 A/cm2 and I r = (a) 0, (b) 0.04, (c) 0.06, (d) 0.08, and (e) 0.1 A/cm2, respectively

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The coating A which was prepared with non-reversing pulsation exhibits micron-sized grains with 11 wt.% Ni and a γ phase. Through the introduction of the anodic waveforms, the coatings B, C, D, and E are characterized by nanosized grains with Ni content in a 13-17 % range. The smaller grain sizes of the coatings B, C, D, and E may be attributed to the re-nucleation of the deposits, which is promoted under the reverse-pulse electrodeposition mode (Ref 16). In these coatings, the γ phase remains the sole phase. The high Ni specimens (i.e., 17% in set C), however, show a distinct preference for a pyramidal texture, whereas the other sets exhibit a typical nodular surface morphology having a preferential (330) crystallographic orientation. While the structure of most deposits is fairly compact, the coating E appears somewhat porous, owing to a high level of zinc and nickel dealloying at a high anodic current in the deposition process. The deposits’ compositions and corresponding phase structures observed here are also controlled mainly by dezincification and nickel dissolution upon reverse pulsing, as discussed in detail in Ref 13.

Potentiodynamic Polarization Study

Figure 2(a) presents the polarization curves of the coatings in various sets. For the most part, all coatings exhibit a similar shape of polarization curves, and no obvious passivation is detected. Table 2 summarizes the corrosion potential, corrosion current density, and corresponding corrosion rate of the deposits as deduced from the polarization tests. It can be observed that the corrosion potentials generally increase with Ni content. The coating E, however, shows relatively high corrosion potential in spite of its low Ni content. In a separate study, we performed a polarization test on a copper substrate and determined its relatively high corrosion potential of −0.2 V versus SCE. The results therefore suggest the presence of interconnected porosity in coating E which may allow the electrolyte to permeate to the copper substrate. As shown in Table 2, the corrosion rates, as determined from the corrosion current densities, are on the order of 0.1 mm/year which appear comparable among the different groups, with the set B specimen showing relatively high corrosion resistance.

Fig. 2
figure 2

(a)-(b) Potentiodynamic curves of the Zn-Ni alloy coatings of sets A-E in (a) the equilibrium corrosion potential vicinity and in (b) the highly anodic region

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Table 2 The corrosion potentials (E corr), corrosion current densities (i corr), and corrosion rates (CR) as determined from the potentiodynamic polarization study
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Figure 2(b) shows a close-up picture of the polarization curves in the highly anodic region above −0.2 V vs. SCE. It is evident that the curves’ profiles exhibit peaks and valleys in this high potential region. The development of these features may be rationalized by the preferential zinc dissolution from the alloys, which ultimately leads to the presence of a top layer enriched with Ni that acts as a barrier for further corrosion attack (Ref 10, 15). Furthermore, it has been noted that in a 3.5% NaCl solution, zinc hydroxide chloride can be formed which serves as a pseudo-passive layer that provides an inhibition effect for the deposits (Ref 3, 17, 18). From Fig. 2(b), it can be observed that the inhibition effect is relatively more pronounced in coatings B and C, whereas at the other end, low polarization peaks are observed as expected in the porous set E coating.

To further understand the mechanistic view of the corrosion behaviors of the deposits and examine the evolution of the corrosion behaviors over time, non-destructive EIS measurements were thus performed as discussed in the subsequent section. Owing to porosity, coating E was omitted from the EIS measurement.

Electrochemical Impedance Measurements

Figure 3(a) and (b) presents Bode plots of the coatings measured at the 1st and 24th h, respectively, showing the magnitude of the impedance with respect to frequency. A similar shape of Bode plots is observed in all cases, with impedance monotonically decreasing with an increase of frequency. The absolute impedance values of coatings A and B (ZA and ZB) appear relatively high, especially over the low and medium ranges of frequency. After 24 h of immersion, the EIS measurements give Bode plots with shapes conformed to those found at the 1st hour. However, the magnitude of the impedance decreases, suggesting a decrease of corrosion resistance of the coatings over immersion time. ZA in particular decreases markedly and becomes the lowest, whereas, ZB remains relatively high. On the other hand, the impedance magnitude of coating C (ZC) is somewhat stable over time.

Fig. 3
figure 3

(a)-(b) Bode plots of the Zn-Ni alloy coatings of sets A-D after being immersed in 3.5% NaCl solutions for 1 and 24 h

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The corresponding Nyquist plots are shown in Fig. 4(a) and (b). These sets of data are analyzed for the equivalent circuit, which is found to be composed of a capacitance (C) connected in parallel to a charge transfer resistance (R c) and a diffusion-control Warburg element (W). Additionally, the whole unit is connected in series with a solution resistance (R s). The schematic view of this equivalent circuit is shown in the inset of Fig. 4. The determined circuit parameters of the four sets of the coating specimens are presented in Table 3.

Fig. 4
figure 4

(a)-(b) Nyquist responses of the Zn-Ni alloy coatings of sets A-D after being immersed in 3.5% NaCl solutions for 1 and 24 h

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Table 3 The values of EIS equivalent circuit parameters of coatings A-D after being immersed in 3.5% NaCl solutions for 1 and 24 h
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At the 1st h, the R s values of all specimens are comparable and appear smaller than R c, signifying the conductive nature of the saline solution and that the solution resistance does not control the corrosion. Instead, the charge transfer resistance and diffusion-control Warburg impedance dominate at high and low ranges of frequency, respectively. Relatively low values of R c and relatively large magnitude of admittance of the Warburg element (Y o) in coatings C and D contribute to their low impedance values at all ranges of frequency. Conversely, high R c and low Y o in coatings A and B contribute to their comparatively high absolute impedance values. The capacitance, which represents an electrical double layer on the interface between an electrode and its surrounding electrolyte, generally appears to be similar among the different groups of specimens at ~20 μF. The coating C, however, shows about three times higher capacitance value than the others, suggesting its relatively small impedance magnitude.

Following 24 h, the R s’s remain approximately equivalent to that at the 1st h, suggesting that no significant change has occurred to the solution. The double-layer capacitance values of all specimens reduce considerably, resulting in a more significant contribution to corrosion resistance by this circuit element. This suggests that over time upon immersion, the specimens experience changes in their double-layer characteristics, possibly attributed to a development of a pseudo-passive layer. Nevertheless, since the absolute impedance magnitudes of the specimens are evidently decreased, the increase of the double-layer impedance appears not as important compared to the modulations of R c and Y o, which for the most part are reduced and increased, respectively. This explains the increase of the corrosion rates over time as observed in coatings A, B, and D. On the other hand, despite an increase of Y o, the large decrement of capacitance and a balanced increase of R c lead to a minimal change of the impedance magnitude of set C specimen over time. Compared to all sets of the specimens, with its lowest R c and highest Y o, coating A thus shows the highest decrement of impedance and, correspondingly, relatively poor corrosion resistance.

Effects of Deposits’ Characteristics on Corrosion

From the polarization and EIS analysis coupled with the specimens’ characterization, we may elucidate the corrosion behaviors of the reverse-pulse electrodeposited specimens as follows.

Comparing coatings A and B, we can deduce that the corrosion resistance is enhanced with the increase of Ni content (e.g., from 11 to 14%) and reduction of grain size, or predominantly by one of these parameters. The improved corrosion resistance is achieved through enhancement of the charge transfer resistance and retardation of diffusion of corrosion products away from the cathode’s surfaces. As the results from the polarization study have suggested, the specimens with refined grain size facilitate the formation of the pseudo-passive films, which in turn could help enhance the corrosion resistance of the deposits.

Despite the high Ni content (17%), in the initial stage of corrosion, the γ phase with a pyramidal texture preference found in coating C yields relatively low corrosion resistance, compared to a nodular (330) texture. This could be owed to the relatively low binding energy of atoms aligned in such non-closed pack crystallographic configuration. At prolonged time, however, a significant improvement of the charge transfer resistance is evidenced, rendering the corrosion resistance of the pyramidal deposits fairly stable over time. The rougher surface morphology and relatively high effective surface area of these deposits may promote pseudo-passive film formation and retard the migration of corrosion products and, correspondingly, help sustain their corrosion-resistant performances.

The characteristics of the deposits from sets B and D are very similar in terms of nickel content, phase composition, and grain size. Nevertheless, the set B deposits exhibit better corrosion resistance than the set D ones. This could be attributed to some distinction in the distributions of crystallographic orientations of their surface grains. Furthermore, since the set D deposits were processed with relatively high anodic currents that preferentially oxidize zinc, and subsequently nickel, from the deposits, the presence of submicron voids and defects, which could serve as local sites that promote corrosion reactions, can be expected on their surfaces.

Conclusions

The relationship between the reverse-pulse electrodeposited Zn-Ni alloy coatings’ characteristics and their corrosion behaviors in a saline environment has been established in the study. Deposits with refined nodular grains of γ phase and increased nickel contents are obtained from the reverse-pulse processing. Such materials’ characteristics affect the charge transfer resistance and facilitate a formation of the pseudo-passive films, leading to the enhancement of the corrosion resistance of the alloys. Further increase of anodic current density upon electrodeposition introduces higher nickel contents and pyramidal grains, which give rise to relatively low corrosion resistance with a higher level of stability of the deposits. Ultimately, an excessively high anodic current density leads to the development of a porous structure that does not effectively provide barrier protection to the underlined substrates.