Fabrication of Zn-Ni-Mn alloy by electrodeposition and its characterization

3.1 Cyclic voltammograms

Electrodeposition technique was used to prepare Zn, Ni and Mn individual metals and Zn-Ni, Ni-Mn and Zn-Mn binary alloys as well as Zn-Ni-Mn ternary alloy deposited on steel rod substrates. Sulfate bath solution at 25.0°C was first considered by cyclic voltammetry to adjust the potential ranges during the various redox processes. Figure 1A (a, b) shows a voltammogram of Zn and Ni single-handedly in sulfate media. It was observed that only one reduction peak was detected for each element in the potential scan through the negative path, and each peak was linked with the reduction of both Zn(II) and Ni(II) ions. Deposition of Zn and Ni started at −1.11 V and −0.885 V, separately. Also it was observed that in all voltammograms, a cathodic peak appeared at −0.54 V, ascribed to the codeposition of sulfur released during the reduction of sulfate groups at the cathode in the presence of H₂SO₄, as reported in previous work (Abou-Krisha et al., 2008). When the direction of the potential scan was changed to the positive direction at −1.3 V, an anodic peak was detected at −0.873 V for Zn and at −0.146 V for Ni. Each peak corresponds to oxidation of deposited Zn and Ni, respectively.

Figure 1:

(A and B): i – E curves for steel in (a) Zn, (b) Ni, (c) Mn, (d) Ni-Mn, (e) Zn-Mn, (f) Zn-Ni and (g) Zn-Ni-Mn baths and a scan rate of 5 mV/s at 25.0°C.

In the cyclic voltammogram of the individual Mn metal, the anodic peak that appeared at −0.113 V was ascribed to the dissolution of deposited sulfur on the cathode surface. The height of this peak is decreased in the existence of Ni and Zn in the bath. It was observed from the same voltammogram that a cathodic peak for Mn deposition cannot be detected, indicating that Mn does not deposit under these conditions.

It is noteworthy that little work has been performed to determine the electrodeposition mechanism of pure metallic manganese. This lack in earlier research is probably due to the difficulty of reviewing the electrodeposition of Mn, specifically attributable to the very low potential value of the Mn²⁺/Mn⁽⁰⁾ duo in aqueous solution (EMn2+/Mn=−1.18 V vs. SHE). Moreover, reduction of Mn²⁺ to Mn is always accompanied by hydrogen gas evolution, which makes it hard to obviously detect the manganese reduction procedure. The study is also difficult because Mn is easily oxidized, and the surfaces of the manganese coatings obtained are converted to oxides, forming a dark oxide film on the surface when they are exposed to air (Díaz-Arista et al., 2006).

Sulfur-containing additives (e.g. ammonium sulfate, ammonium thiocyanate and sulfur dioxide) are commonly added to solutions from which manganese is electrodeposited. It has been reported (Lewis et al., 1976) that the addition of such compounds to the bath leads to deposition of manganese in the brittle α phase. In addition, the use of ammonium salts as additives has been reported to increase the efficiency of Mn(II) reduction.

To obtain a cathodic peak for Mn, electronegative potential greater than −1.3 V must be applied to the cathode. Figure 2 shows a typical voltammogram for Mn that was obtained in the absence and in the presence of ammonium thiocyanate (NH₄SCN) in the potential range from −0.4 V to −1.700 V. In the voltammogram recorded in the absence of NH₄SCN, the cathodic current density rapidly increased when the potential was close to −1.4 V. This increase was associated with two parallel reduction reactions: the reduction of H₂O (hydrogen evolution) and the reduction of Mn(II) ions. An oxidation peak was noticeably not observed through the potential scan in the positive direction. A similar behavior was reported by Sylla et al. (2003). They ascribed the lack of this peak to the instability of the manganese deposits, which simply dissolve in acidic media. It was reported in a previous study (Díaz-Arista et al., 2006) that an electrochemical quartz crystal microbalance that was used to deposit manganese under these conditions was dissolved via a non-faradaic process (i.e. mass changes that were not associated with the charge transfer processes) during a potential scan from −1.2 V to −0.7 V in the positive direction.

Figure 2:

i – E curves for steel in Mn bath [(A) without and (B) with 0.01 m NH₄SCN] and a scan rate of 5 mV/s at 25.0°C.

The addition of NH₄SCN to the bath solution had a strong effect on the voltammogram of Mn. In the course of the potential scan in the cathodic direction, the increase in cathodic current density, which is associated with the reduction of H₂O and Mn(II) ions, occurred at more negative potentials than in the absence of NH₄SCN. Therefore, the overpotential for the reduction of Mn(II) ions and hydrogen evolution reactions increased in the presence of the additive. This may be attributed to the presence of the additive slowing down the reaction rate of Mn(II) reduction. It is noteworthy that there is an anodic peak (I_a) for Mn dissolution in the presence of NH₄SCN. This result explains that the manganese phase made in the potential scan in the cathodic direction in the existence of NH₄SCN was established and easily achieves electrochemical oxidation. The sulfur that remained in the form of sulfide had occluded the coating due to the reduction of SCN⁻ ions during the electrodeposition procedure.

In the cyclic voltammogram of the Zn-Ni system (as shown in Figure 1B f), the deposition reaction started at approximately −1.075 V. Previous studies have shown that zinc reduction processes are controlled by mass transport (Díaz-Arista et al., 2005). Detection of several peaks through the electrochemical oxidation of Zn-Ni has been ascribed to the successive oxidation of different phases (Trejo et al., 2003). Accordingly, the anodic sweep as shown in the voltammogram in Figure 1B f includes four dissolution current peaks at −0.822, −0.550, −0.294 and −0.160 V. The first dissolution peak was related to the dissolution of Zn from the pure Zn phase, the second peak was related to the dissolution of Zn from the γ-Ni₅Zn₂₁ and/or δ-Ni₃Zn₂₂ phases, the third peak was ascribed to the dissolution of Ni from the γ-Ni₅Zn₂₁ and/or δ-Ni₃Zn₂₂ phases, while the fourth dissolution peak can be attributed to the dissolution of the porous Ni matrix remaining after the preferential dissolution of Zn from the Zn-enriched phases.

For the Zn-Mn system (as shown in Figure 1B e), the deposition process started at approximately −1.060 V, while an anodic peak was observed at −0.865 V. It was observed in the voltammogram that only deposition of Zn was detected, while the reduction of Mn(II) ions did not appear. Also, this voltammogram was analogous to the voltammogram of the individual Zn metal, except that the deposition potential and the anodic peak shifted to more positive values, from −1.11 V to −1.06 V and from −0.873 V to −0.865 V, respectively. This indicated that Mn did not deposit with Zn under these conditions to form the Zn-Mn alloy.

The Ni-Mn system (as shown in Figure 1B d) was more interesting because the voltammogram was more modified than the voltammogram for the individual Ni metal. The differences were attributed to the incorporation of Mn. On the negative-going potential scan, four deposition processes were observed. The first deposition peak started at −0.585 V owing to the codeposition of sulfur. The second deposition process started at −0.906 V as a result of the deposition of Ni. The third deposition process started at −1.008 V, which was ascribed to the deposition of Mn. The fourth deposition process started at −1.169 V, which was attributed to the hydrogen evolution side reaction combined with the manganese deposition reaction. By switching the direction of the potential scan toward the positive direction at −1.3 V, an anodic peak was observed at −0.132 V, which was attributed to dissolution of the deposited Ni. In the Ni-Mn system, as shown in Figure 1A and B, the dissolution peak of pure Ni (Figure 1A b) was clearly lower than the dissolution peak of Ni (Figure 1B d), which means that Ni was deposited with a higher content when alloyed with Mn, in agreement with the Ni amount in the deposit, as listed in Table 1.

Interestingly, no Mn dissolution peak was observed when the Mn(II) and Ni(II) solutions were mixed. This was attributed to the large difference between the deposition potentials of the two cations. A displacement reaction (1) can occur during deposition:

Beyond dissolution of Mn at the anodic current, Mn will also dissolve, according to reaction (2):

The above two reactions describe the quick dissolution of Mn before the anodic potential is applied, which can explain the absence of a dissolution peak for Mn in the Mn(II) and Ni(II) bath. Wu et al. (2008) reported similar results for the electrodeposition of a Mn-Co alloy from a sulfate bath. In a previous study, a similar result was reported by Sylla et al. (2003) using a KCl+H₃BO₃ matrix, who also ascribed the lack of a Mn dissolution peak to instability of the manganese deposits, which are easily dissolved in acidic media. However, after a detailed study using a quartz crystal microbalance and SEM and EDX, Díaz-Arista et al. (2006) proposed that the lack of a Mn dissolution peak was due to Mn oxidation to Mn(OH)₂.

In the cyclic voltammogram of the Zn-Ni-Mn system (as shown in Figure 1A g), the deposition procedure started at approximately −1.115 V (peak c₁), which was analogous to the Zn deposition (c), while hydrogen evolution side reaction was indicated by c₂ (Figure 1A g). Thus, it seems that the deposition potential was mainly determined by Zn. In the anodic sweep, the four peaks of the anodic current −0.965, −0.835, −0.550, and −0.294 V show the differences between the Zn-Ni-Mn and Zn-Ni systems. The peak at −0.965 V was attributed to dissolution of Mn from the Mn_0.27Zn_0.73 phase, the peak at −0.835 V was attributed to dissolution of Zn from the pure zinc phase, the peak at −0.55 V was ascribed to dissolution of Zn from the γ-Ni₅Zn₂₁ and/or δ-Ni₃Zn₂₂ phases, and the peak at −0.294 V was correlated with dissolution of Ni from the nickel phases. The addition of Mn to the Zn-Ni system clearly resulted in the disappearance of the fourth anodic peak that was observed in the Zn-Ni system. Moreover, the height of the first peak in the Zn-Ni system at −0.822 V increased, and the peak slightly shifted to a more negative value from −0.822 V to −0.835 V. The low height of the first anodic peak in the Zn-Ni-Mn system was due to the low content of the Mn_0.27Zn_0.73 phase, which therefore indicated the low content of Mn in the deposited alloy.

3.2 XRD and EDXF measurements

Figure 3 exhibits the EDXRF peaks for the Zn-Ni-Mn coating electrodeposited on a steel substrate at 10 mA cm⁻² for 10 min. The peaks corresponding to the iron observed in the spectrum are due to the steel substrate. Because the EDXRF electron beam depth is higher than the deposit thickness obtained. The peaks corresponding to Zn clearly had the largest intensities, indicating that Zn was the most readily deposited metal, while Mn had the lowest intensity, indicating that Mn had the lowest content in Zn-Ni-Mn. The incorporated sulfur in the deposit was observed, as discussed previously.

Figure 3:

EDXRF patterns for the Zn-Ni-Mn alloy electrodeposited on a steel substrate of area 2 cm² at 10 mA cm⁻² and 25.0°C for 10 min from a Zn-Ni-Mn bath.

Figure 4 shows the chemical composition and the phase analysis by XRD of the obtained Zn-Ni-Mn coating. The results revealed two high-intensity peaks and seven low-intensity peaks appeared that correspond to the pure hcp-Zn phase. Also, peaks of the hcp-Mn_0.27Zn_0.73 phase, cubic ε-ZnMn phase and cubic γ-Ni₅Zn₂₁ and/or tetragonal δ-Ni₃Zn₂₂ phases appeared. The peaks of iron corresponded to the steel substrate, i.e. steel sheets. This probably appeared because the X-ray electron beam penetrated the thinner deposited layer of the Zn-Ni-Mn alloy. Several intensive peaks in the XRD pattern showed the highly crystalline structure of the obtained Zn-Ni-Mn coatings. It is important to note that the peaks of the different phases observed in our study for the Zn-Ni-Mn alloy are very close, and some peaks were superimposed on one each other. The diffraction lines of the Mn_0.27Zn_0.73 and ε-ZnMn phases are very close to each other; therefore, the Mn_0.27Zn_0.73 phase or the ε-ZnMn phase may be present.

Figure 4:

XRD patterns for the Zn-Ni-Mn alloy electrodeposited on a steel substrate of area 2 cm² at 10 mA cm⁻² and 25.0°C for 10 min from a Zn-Ni-Mn bath.

3.3 Galvanostatic technique

The potential-time dependence curves for the deposition of Zn, Ni and Mn single-handedly, and Zn-Ni, Ni-Mn and Zn-Ni-Mn alloys on steel rods substrate at 10 mA cm⁻² for 10 min are shown in Figure 5. In the case of pure Ni, the deposition obviously required low overpotential to produce the first nucleus, and the deposit rate increased at lower cathodic potentials, whereas the deposition of pure Zn occurred with higher nucleation overpotential, and the deposit rate increased at higher cathodic potentials. However, the deposition of Mn required more electronegative potential than Zn or Ni to generate the first nucleus, and the rate of deposit increased at the highest electronegative potentials. The Zn-Ni, Ni-Mn and Zn-Ni-Mn alloys were codeposited at moderate overpotential values positioned between the Ni overpotential and the Zn or Mn overpotentials. The deposition potential of the Zn-Ni system was closer to the deposition potential of Zn than that of Ni, indicating that the amount of Zn in the deposit was greater than that of Ni. This is because the deposition of nickel is intensely inhibited by the presence of zinc, while the deposition of zinc is encouraged by the presence of nickel (Zech et al., 1999). This means that deposition of the Zn-Ni system was an ACD.

Figure 5:

E – t curves for steel at 10 mA cm⁻² and 25.0°C for 10 min in (A) Zn, (B) Ni, (C) Mn, (D) Ni-Mn, (E) Zn-Ni and (F) Zn-Ni-Mn baths.

Additionally, the deposition potential of the Ni-Mn system was much closer to the deposition potential of Ni than that of Mn, indicating that the main deposited metal in the Ni-Mn system was Ni. This means that the codeposition of the Ni-Mn system is a normal codeposition. On the other hand, the Zn-Ni-Mn alloy was codeposited at moderate overpotentials between those of Ni-Mn and Zn-Ni, which were deposited at more anodic and more cathodic overpotentials, respectively. The galvanostatic curve of the Zn-Ni-Mn system was located in between the galvanostatic curves of Ni or Ni-Mn and that of Zn or Zn-Ni, but was very far from the galvanostatic curve of Mn. This means that the main two components of the Zn-Ni-Mn alloy were Zn and Ni, but a very small amount of Mn was also deposited in the alloy. These results, in accordance with the results obtained from the AAS and EDXRF, showed that Zn was deposited in the highest amount, while Mn was deposited in the lowest measurable amount.

3.4 Anodic linear sweep voltammograms

The various phases and chemical forms present in the alloy will produce different current peaks in ALSV (Despić et al., 1988). Therefore, the peak structure obtained is characteristic of the components in the alloy and the phase structures of the deposit (Zećević et al., 1998).

The phase structures were investigated in a Na₂SO₄ solution containing complex forming ions, where Zn-Ni-Mn alloys completely dissolve. Pure Zn is well known to dissolve in a Na₂SO₄ solution; on the other hand, zinc alloys did not dissolve without a minor quantity of a complex forming agent, such as EDTA, present, and then together Zn and its alloys will dissolve. Similar results for dissolution of Zn-Ni alloys were obtained by Bajat et al. (2002).

Figure 6 shows the ALSV attained for individual Zn and Ni metals, and for Zn-Ni, Ni-Mn and Zn-Ni-Mn alloys through the dissolution of the deposits acquired galvanostatically at 10 mA cm⁻² for 10 min at a scan rate of 5 mV s⁻¹. It was observed from the results that both pure Zn and pure Ni had only a single anodic dissolution peak at −0.708 V and −0.089 V, respectively. Moreover, the dissolution peak of Zn was very high, but the Ni dissolution peak was very low, indicating that pure Zn was deposited with higher content. Pure Ni was deposited with lower content, which may be due to the surface effect. It is very important to note that pure Ni was deposited with lower content when deposited alone and with higher content when deposited alloyed with Mn. For the Zn-Ni alloy, there were three anodic dissolution peaks at −0.743 V, −0.394 V and −0.166 V. The first peak was attributed to dissolution of Zn from the pure zinc phase, the second peak was ascribed to dissolution of zinc from the γ-Ni₅Zn₂₁ and/or δ-Ni₃Zn₂₂ phases and the third peak was attributed to dissolution of Ni from its phases.

Figure 6:

ALSVs for Zn, Ni, Ni-Mn, Zn-Ni and Zn-Ni-Mn deposits obtained galvanostatically at 10 mA cm⁻² and 25.0°C for 10 min on a steel rod from (A) Zn, (B) Ni, (C) Ni-Mn, (D) Zn-Ni and (E) Zn-Ni-Mn baths, in 0.5 m Na₂SO₄+0.05 m EDTA at 5 mV/s and 25.0°C.

For the Zn-Ni-Mn alloy, as shown in Figure 6, there were also three anodic dissolution peaks at −0.660 V, −0.448 V and −0.177 V. The first anodic peak was attributed to the dissolution of zinc from the pure Zn phase, while the second peak was related to the dissolution of Zn from the γ-Ni₅Zn₂₁ and/or δ-Ni₃Zn₂₂ phases, which was verified by XRD as shown in Figure 4. The third anodic peak, which appeared at a more noble potential of −0.177 V, was ascribed to the dissolution of nickel from its phases. It was observed from the result that a difference between the heights of the dissolution peaks of the Zn-Ni-Mn and Zn-Ni alloys is very clear, which may be ascribed to the codeposition of manganese. Additionally, the first dissolution peak clearly shifted in the more anodic direction from −0.743 V in the Zn-Ni alloy to −0.660 V in the Zn-Ni-Mn alloy. In the second anodic peak, a clear shift toward the negative direction from −0.394 V in the Zn-Ni alloy to −0.448 V in the Zn-Ni-Mn alloy was observed. For the third dissolution peak, there was a small shift in the cathodic direction from −0.166 V in the Zn-Ni alloy to −0.177 V in the Zn-Ni-Mn alloy. It is interesting to note that the height of the anodic Zn dissolution peak when it was deposited as individual metal was higher than the peaks of the Zn-Ni or Zn-Ni-Mn alloy. This means that the amount of the pure Zn phase in these alloys was less than that in the single metal deposit.

3.5 Potentiostatic current density-time curves

Figure 7 exhibits the potentiostatic current density-time curves of pure Zn, Ni and Mn alone, and of the Ni-Mn and Zn-Ni-Mn alloys electrodeposited on steel rods at −1.300 V for 10 min. As shown from the potentiostatic curves, pure Mn required more anodic current density to form the first nucleus, and the deposit increased at very low cathodic current densities, while pure Ni needed more cathodic current density to form the first nucleus, and the deposition rate increased at very high cathodic current densities. Pure Zn needed moderate cathodic current density to create the first nucleus, and the deposit increased at moderate cathodic current densities that were between those of Mn and Ni. The potentiostatic curve of the Zn-Ni-Mn system was positioned between the potentiostatic curves of Ni and Zn, but it was very far from the potentiostatic curve of Mn, indicating that a small amount of Mn deposited in the alloy. The current density of the Zn-Ni-Mn alloy shifted to a more cathodic value as the time increased with low-frequency and magnitude fluctuations. This decrease in the cathodic current density during the first few minutes was attributed to the covering of the steel substrate by the Zn-Ni-Mn alloy, which may favor hydrogen evolution. However, the persistence of this decrease for larger times was due to a dendritic growth of the deposit that increases the active area (Sylla et al., 2005b). The fluctuations may have been caused by the formation of the hydrogen evolution side reaction via H₂ bubbles (Sylla et al., 2003).

Figure 7:

i – t curves for steel at −1300 mV and 25.0°C for 10 min in (A) Zn, (B) Ni, (C) Mn, (D) Ni-Mn, (E) Zn-Ni and (F) Zn-Ni-Mn baths.

3.6 Potentiodynamic polarization curves

The corrosion resistance of the deposits was evaluated from the potentiodynamic polarization curves as shown in Figure 8. It was obtained for pure Zn, Ni and Mn and for Zn-Ni, Ni-Mn and Zn-Ni-Mn alloys, which was electrodeposited potentiostatically at −1.300 V for 10 min from standard bath and then put in 0.05 m HCl as corrosive agent. The corrosion potential (E_corr.), corrosion current density (i_corr.) and polarization resistance (R_p) values were determined from the curves as listed in Table 1. It can observe from the results that the pure Zn had the lowest corrosion resistance when immersed in 0.05 m HCl because it had the most negative E_corr. and the highest i_corr. values, while Ni had the highest corrosion resistance because it had the lowest negative E_corr. and the lowest i_corr. values. It was also clear from the results that the Zn-Ni-Mn coating had a more positive E_corr. value than the Zn-Ni coating, indicating that the ternary alloy showed higher corrosion resistance than the binary one. The addition of Mn to the bath containing Ni shifted the corrosion potential of Ni to a more negative value from −0.284 V to −0.409 V. It is interesting to note that the corrosion potential of Mn is not similar to the corrosion potential of Mn but represents the corrosion potential of the steel rod substrate used in the electrochemical measurements because Mn does not deposit under these conditions, and if it was deposited, the value of E_corr. must be more negative than −0.212 V as Mn is more active than Zn and Ni.

Figure 8:

Log i – E curves for Zn, Ni, Mn, Ni-Mn, Zn-Ni and Zn-Ni-Mn deposits obtained potentioststically at −1300 mV and 25.0°C for 10 min on a steel rode from (A) Zn, (B) Ni, (C) Mn, (D) Ni-Mn, (E) Zn-Ni and (F) Zn-Ni-Mn baths, then put in 0.05 m HCl as corrosive agent at 2 mV/s and 25.0°C.

It has been reported (Baldwin & Smith, 1996) that the potential difference of the covering and the substrate acts as a powerful force for corrosion of the sacrificial coating and defense of steel under rusting conditions. Because of the relatively large difference in the electronegativities of Zn and steel, rapid dissolution of Zn occurs under corroding conditions and reduces the coating life of pure Zn on steel, damaging the coating. This problem of rapid dissolution is mitigated by alloying Zn with Ni and Mn, which bring the E_o of the coating closer to that of the steel substrate and reduce the driving force for dissolution, enhancing corrosion resistance for a longer period and increasing the durability of the coating.

Several models have been proposed to explain the ACD of Zn alloys (Fabri-Miranda et al., 1996; Yan et al., 1996; Chen & Sun, 2001; Bahrololoom et al., 2003; Huang & Sun, 2004; Lodhi et al., 2007; Ortiz-Aparicio et al., 2007; Lichušina et al., 2008). In the display study, the competitive adsorption between Zn²⁺ and Ni²⁺ (or its monovalent intermediate) for the occupation of the active sites prompted the preferential deposition of Zn and led to ACD (Abou-Krisha et al., 2008). A few researchers (Ohba et al., 2006) have inspected the anomalous behavior and stated that it may have been due to the slower deposition kinetics of iron group metals at various concentrations of electrolytes.

3.7 The chemical composition of deposit alloys

In the absence of Mn²⁺, as shown in Table 1, the percentages of nickel and zinc in the Zn-Ni alloy were 11.8% and 88.2%, respectively, but after adding Mn²⁺ to the bath, the nickel content in the deposit decreased to 8.3%, while the zinc content slightly increased to 89.3%. This decreased nickel content may have been replaced by manganese in the deposit. The addition of manganese to the bath containing nickel and zinc first led to an increase in the alloy current efficiency from 78.9% in the absence of Mn²⁺ to 86.4% after codeposition of manganese (i.e. for the Zn-Ni-Mn alloy) as shown in Table 1. Secondly, an increase in the thickness of the deposit from 2.15 μm for the Zn-Ni alloy to 2.38 μm for the Zn-Ni-Mn alloy was also obtained. Thirdly, a shift toward the positive direction in the E_corr. from −0.865 V for the Zn-Ni alloy to −0.756 V for the Zn-Ni-Mn alloy and a decrease in the i_corr. from 41.4 μA/cm² for the Zn-Ni alloy to 25.8 μA/cm² for the Zn-Ni-Mn alloy were achieved. Finally, the R_p increased from 3.76 kΩ for the Zn-Ni alloy to 5.93 kΩ for the Zn-Ni-Mn alloy. Based on these results, the ternary Zn-Ni-Mn coating showed higher corrosion resistance properties in comparison with the Zn-Ni coating.

In Table 1, Mn codeposition was found to be normal because Mn showed the most negative electrochemical potential, and thus, it was the least present in the deposit. It is interesting that Mn can be codeposited with Ni or with Ni and Zn under the studied conditions, where it does not deposit in its pure form. Similarly, to induce codeposition, where a given element can be codeposited to form an alloy but cannot be codeposited in its pure form, interactions between the components in the deposit may shift the deposition potential of the less noble metal to more positive values (Chassaing et al., 1989).

3.8 A dark brown deposit formed on the anode (electrodeposition on the anode)

During electrodeposition of pure Mn or its alloys at the cathode from the baths containing Mn²⁺ ions, a dark brown deposit formed on the counter platinum electrode surface. The dark brown deposit as an anodic electrodeposition can be designated by the following equations (Ghaemi & Binder, 2002; Walanda et al., 2005).

At the cathode:

At the anode:

The reduction of H⁺ ions at the cathode leads to an increase in pH, while the anodic reaction produces a decline in pH.

As the above four reactions are taking place, other reactions also occur at the electrodes, with the most prominent being (Ghaemi & Binder, 2002; Walanda et al., 2005):

Because the multivalent ions represent a significant fraction of the Mn deposited at the cathode, reactions (7) and (8) cannot be neglected. Most of the Mn³⁺ and Mn⁴⁺ ions stay in the solution deposit at the anode as hydroxide/oxide precipitates. The overall of reactions (4) and (8) is:

The mechanism of the electrodeposition of manganese dioxide at the anode is (Ghaemi & Binder, 2002):

3.9 Surface morphology

Figure 9A and B show the SEM micrographs for the Zn-Ni and Zn-Ni-Mn alloys obtained on steel substrates at 10 mA/cm² and 25.0°C for 10 min. The Zn-Ni binary alloy exhibits a semi-cauliflower morphology, while the Zn-Ni-Mn ternary alloy exhibits a granular morphology. The Zn-Ni alloy had a rough, non-homogeneous and non-uniform morphology, while the morphology of the Zn-Ni-Mn alloy was smoother and more homogeneous. This indicates that the addition of Mn to the Zn-Ni alloy modified the crystal growth during the electrocrystallization of the coating. Moreover, the Zn-Ni-Mn alloy possessed finer grain size than the Zn-Ni alloy, resulting in increased compactness, hardness and adherence of the coating in the presence of Mn. Thus, the addition of Mn to the Zn-Ni alloy improved the structural properties and surface morphology of the produced alloy. As seen in Figure 9A and B, the two coatings completely covered the substrate without cracks.

Figure 9:

SEM micrographs (at magnification of 8000×) for (A) Zn-Ni (B) Zn-Ni-Mn alloys deposited on a steel substrate of area 2 cm² at 10 mA/cm² and 25.0°C for 10 min from (A) Zn-Ni and (B) Zn-Ni-Mn baths.