Anomalous co-deposition mechanism

There are three kinds of co-deposition mechanisms mentioned in literature review [29-44]：(1) Hydroxide suppression mechanism (HSM) theory (2) Underpotential deposition (UPD) theory (3) Exchange current density theory. The Underpotential deposition theory (UPD) is apparently not suitable in this study, where multi-layer coating will form instead of continuous deposition for the termination of UPD once the monolayer was deposited and the ions in solution should detect only the last layer deposited on the surface. In Hedge’s study of Exchange current density theory, five current densities (1, 2, 3, 4 and 5 A/dm²) were selected, and potentiodynamic polarization scans were used to determine the limiting current density iL. With the comparison of the ratio of partial current density and limiting current density, it was found that Zn deposition was heavily influenced by mass-transport limitation at high applied current densities, while the rates of Ni and Fe deposition were not. However, the electrodeposition process was conducting with bath stirring, which reduced the effect of mass-transport limitation. Therefore, this research is aimed at examining the accuracy of Hydroxide suppression mechanism (HSM) theory. At the beginning of electrodeposition process, the pH value near cathode rises rapidly due to the evolution of hydrogen at ambient high current density.As the critical pH value for the precipitation of iron-group metal hydroxides is significantly higher than that for the precipitation of zinc hydroxide, the later will form and inhibit the deposition of iron-group metal. HSM theory indicates that anomalous co-deposition (ACD) occurs when continuous zinc hydroxide film covered fully on the cathode surface. Chassaing [54] proposed a reaction model in

chloride electrolyte at various temperatures and polarizations. The deposition of nickel-rich alloys was attributed to a mixed intermediate (ZnNi⁺ads) at low polarizations and high temperatures. While at high cathodic polarizations and low temperatures, which anomalous co-deposition often occurs, zinc preferential discharge is attributed to the intermediate Zn⁺_ads, a catalyst for the deposition of zinc rich deposits.

5.1.1 Variations of nickel contents in the Zn-Ni coating

Based on HSM theory, the deposition of nickel was strongly inhibited by the presence of zinc hydroxide. In this study, the nickel contents in the coating increase as deposition temperature rises. Moreover, the increasing type of nickel contents changes from linear growth to parabolic growth at three different bath contents ratio. With 40%

Ni²⁺ ions in the bath, it is found 11.52wt% nickel in the coating at 30℃, and then gradually increases to 30.02wt% when deposition temperature reaches 60℃. All nickel contents are below the CRL, which means the main electrodeposition process is anomalous co-deposition. As the Ni²⁺ ions elevated to 45%, the increment of nickel contents is divided into three regions. In first region where deposition temperature ranges from 30℃ to 45℃, nickel contents in the coating rises slowly from 12.09wt% to 17.45wt% with increasing temperature. Subsequently, in the second region between 45 and 55℃, there is a distinguished increment of nickel contents from 17.45wt% to 30.68wt% compared with the region one. As the temperature exceeds 55℃, it comes into the third region where nickel contents rises dramatically to 62.45wt%. In the region three, the nickel contents exceed the CRL, which indicates that normal co-deposition dominates the electrodeposition process instead of anomalous co-deposition. When Ni²⁺

ions in the bath reach 50%, nickel contents in the coating increase 15.78wt% to

co-deposition occurs when the deposition temperature is above 55℃. It can be assumed that the reduction of nickel ions is more sensitive to deposition temperature than zinc ions in HSM theory, for the need of nickel ions diffusing through the zinc hydroxide to form the precursor ZnNi⁺ads, and then reduces to both nickel and zinc metals on the cathode while there is another reduction path for zinc directly from zinc hydroxide. In our study the normal co-deposition tended to appeared in the condition of high temperatures and high Ni²⁺ ion percentages in the bath. The results support HSM theory in two respect：(1) There must be enough Ni²⁺ ions in the bath to cause normal co-deposition (2) The deposition of nickel strongly depends on the deposition temperature.

5.1.2 Effects on current efficiency

The current efficiency reduces as deposition temperature increases, which might due to the fierce hydrogen evolution at high temperatures. According to the literature, the process of hydrogen evolution involves a loosely absorbed intermediate ZnH⁺_ads.

With 40% Ni²⁺ ions in the bath, the current efficiency is moderately decreased from 99.7% to 93.2%. As Ni²⁺ ions increases to 45%, similar to the increment of nickel contents inside the coating, the declinement of current efficiency with deposition temperature is separated into three parts. In the range of 30℃ to 45℃, the current efficiency slowly decreases from 99.3% to 95.1%. Subsequently, the reduction of current efficiency becomes larger, changes from 95.1% to 90.8% in the range of 45℃ to 55℃. When the deposition temperature keeps rising, the current density sharply declines to 77.8% at 60℃. At high temperatures protons were thermal activated, which led to the acceleration of proton diffusing through the zinc hydroxide film to form the

intermediate ZnH⁺ads. ZnH⁺ads was then reduced on the cathode surface to generate hydrogen. The results reveals that the current efficiency of the Zn-Ni coatings are relatively low at the conditions where normal co-deposition occur：90.8% (45% Ni²⁺

ions, 60℃) and 77.8% (50% Ni²⁺ ions, 60℃). It can be found in this study that both increase the Ni²⁺ ions percentage and deposition temperature will reduce the current efficiency while deposition temperature makes a bigger effect. From the respect of HSM theory, the increment of Ni²⁺ ions percentage will inhibit the formation of a continuous and compact zinc hydroxide film and deposition temperature will accelerate the diffusion of nickel ions and proton, and both of them will express the decrease of current efficiency. Hence, the results of current efficiency do support the HSM theory.

5.1.3 Differences in surface morphologies and microstructures

The grain shape transited from small granules to pyramid like and then big granules with the rise of deposition temperatures. At high temperatures and high Ni²⁺ ions percentage in the bath (45%Ni²⁺, 60℃; 50% Ni²⁺ ions, 55℃; 50% Ni²⁺ ions, 60℃), the generation of internal stress results in the formation of cracks. This result corresponds to the condition where normal co-deposition might occur, and the cracks will reduce both the corrosion resistance and mechanical properties. According to XRD patterns, it shows that Ni5Zn21(γ) was the main phase in most cases, and NiZn(β1) appeared at the conditions where cracks were produced.