I. Comparison of advantages and disadvantages of the two processes
The process characteristics of acidic zinc-nickel alloy electroplating and alkaline zinc-nickel alloy electroplating are similar to those of acidic zinc plating and alkaline zinc plating. The main advantages and disadvantages of these two processes are compared below.
Advantages of alkaline zinc-nickel alloy electroplating / Disadvantages of acidic zinc-nickel alloy electroplating
1. Good throwing power (dispersion ability), with relatively uniform coating thickness distribution.
1. Relatively low throwing power (dispersion ability), with relatively non-uniform coating thickness distribution.
2. The difference in nickel content in the coating between high and low current density areas of the workpiece is small.
2. The difference in nickel content in the coating between high and low current density areas of the workpiece is large.
Disadvantages of alkaline zinc-nickel alloy electroplating / Advantages of acidic zinc-nickel alloy electroplating
1. Low current efficiency, generally around 50%, with slow coating deposition rate of about 0.25 μm/min.
1. High current efficiency, generally around 90%, with fast coating deposition rate, up to over 1.0 μm/min.
2. High production operating costs.
2. Low production operating costs, about 50% lower than the alkaline process.
3. Relatively difficult to maintain normal operation of the plating bath.
3. Relatively easy to maintain normal operation of the plating bath.
4. Cannot plate cast iron, carburized steel, or high-carbon steel.
4. Can plate cast iron, carburized steel, or high-carbon steel.
II. Selection between the two processes
From the comparison of the characteristics and advantages/disadvantages of the two processes above, it can be concluded that the choice of process depends on the material and geometric shape of the workpiece to be plated. Except for materials such as cast iron and carburized steel, the choice of process for workpieces made of other metal materials (mainly carbon steel) will depend on the geometric shape of the workpiece. The selection principle is as follows: if there is a large difference between the current density at the high current density area and that at the low current density area of the workpiece during electroplating, the alkaline process should be used; otherwise, the acidic process should be used. For example, for workpieces with blind holes, or workpieces where the center area is far from the edge area (such as impellers), the current density difference between the high and low current density areas is very large. In this case, if the acidic process is used, the coating thickness and nickel content differences between low and high current density areas will be much greater than those obtained with the alkaline process. When the workpiece is simple in shape, such as various cylindrical rollers, pistons, fasteners, etc., the current density on the workpiece surface is relatively consistent. In this case, the acidic process should be selected, because the two major advantages of the alkaline process are no longer significant, while its own disadvantages are quite prominent.
III. A perspective on the current status of zinc-nickel alloy electroplating in China
China's research on zinc-nickel alloy began in the 1980s, and all of it focused on acidic chloride electroplating processes. In this research, the author successfully solved the key technical problem of acidic chloride zinc-nickel alloy electroplating by identifying and determining the optimal combination of additives to effectively inhibit nickel deposition in the low cathode current density region. With this breakthrough, the process could be used for rack plating of steel parts with relatively complex shapes, as well as for barrel plating. Since 1993, several enterprises have successively used this process for industrial mass production. One of the main processed products was cable trays, with a total processing volume of 10,000 to 20,000 tons over nearly ten years. However, as far as the author knows, although acidic zinc-nickel alloy electroplating began to be applied in industrial production in the 1990s, it was still in the initial stage in terms of both product variety and quantity. As for alkaline zinc-nickel alloy electroplating, no reports of industrial production use were seen in the 1990s.
The rapid development and popularization of zinc-nickel alloy electroplating in China has occurred only in recent years, but only the alkaline process has developed; the acidic process has not developed correspondingly. As mentioned earlier, the process characteristics of acidic and alkaline zinc-nickel alloy electroplating are similar to those of acidic and alkaline zinc plating. The process selection depends on the shape of the workpiece (except for certain steel types): some workpieces are suitable for the alkaline process, and some for the acidic process. Therefore, logically, the usage ratio of the two zinc-nickel alloy electroplating processes should be roughly close to the ratio between acidic zinc plating and alkaline zinc plating, rather than showing a dramatic disparity.
It is known that acidic zinc plating accounts for more than 50% of the total electroplated zinc in China, while alkaline zinc plating accounts for about 40%. Although there are no national statistics for acidic zinc-nickel alloy electroplating, the author estimates that it does not exceed 10%. The reason for such a low proportion is that a large number of workpieces that should have adopted the acidic zinc-nickel alloy process are currently generally plated with the alkaline zinc-nickel alloy process. Taking coal mine single hydraulic props as an example: the national usage exceeds one million units, and numerous plating shops dedicated to this processing generally use the alkaline zinc-nickel alloy process rather than the acidic process. Based on the author's R&D and production practice with both processes, for such workpieces, under the same coating nickel content, same coating thickness, and similar corrosion resistance, the acidic zinc-nickel alloy process can reduce production costs by about 50% and increase output by more than two times. The author does not intend to discuss the reasons for the abnormal development pattern of the two zinc-nickel alloy electroplating processes in China in recent years, but only predicts that this situation will change in a few years, and the acidic zinc-nickel alloy process will gradually rise to the proportion it should occupy.