Research on zincate zinc plating and development and application of additives

In the electroplating industry, galvanizing accounts for 60% of production, and in the electromechanical industry, galvanizing accounts for about 70%. Eliminating highly toxic cyanide from galvanizing is the key to achieving cyanide free plating. In recent years, cyanide free zinc plating has been used in production, with ammonium salt zinc plating being the most widely used. However, due to the severe corrosion of ammonium salt zinc plating electrolyte on equipment and factories, it is not easy to achieve automated production. When the process control is poor, the passivation film severely discolors during storage, transportation, and use, reducing the appearance quality of the product. The above shortcomings still need to be further overcome. Therefore, in recent years, many units have been researching cyanide free alkaline zinc plating, some of which are used to produce actual alkaline triethanolamine zinc plating. Due to the poor chelating ability of triethanolamine to zinc, poor dispersion ability of the plating solution, low production efficiency, and unable to meet production needs, new progress has been made in the research of alkaline zincate zinc plating.
The purpose of studying cyanide free alkaline zincate zinc plating was initially to gradually start from the process and achieve high production efficiency, but the road is difficult. For example, zincate solutions composed of zinc oxide and sodium hydroxide can only produce sponge like deposits. Therefore, it is necessary to study some additives with excellent cyanide properties to improve the electrochemical characteristics of the electrolyte, and efforts are still being made to this day.
There are several types of zinc salt zinc plating additives to choose from:
The development of cyanide-free electroplating in China is relatively fast, with zincate zinc plating accounting for the main part of cyanide-free zinc plating. The key component additives mainly include DPE and DE series.
DPE is the most widely used variety and is an ideal organic additive to replace cyanide zinc plating compared to traditional cyanide zinc plating processes. The solution using DPE process has good stability, simple maintenance, and convenient operation. The varieties produced include DPE - Ⅰ and DPE - Ⅱ (DPE - Ⅱ is rarely used).
DPE - I is a water-soluble polymer formed by the condensation of dimethylaminopropylamine and epoxy propane, suitable for 15-20 μ Coating of m; DPE - III is a water-soluble polymer condensed with dimethylaminopropylamine, ethylenediamine, and propylene oxide, suitable for use in the range of 20-25 μ The coating of m. Both are of actual non-toxic grade.
DE zincate zinc plating additive is widely used in China. It is a water-soluble polymer formed by the condensation of dimethylamine and epichlorohydrin, and its process specifications are also close to those of cyanide zinc plating.
It is crucial to master the dosage and characteristics of additives in on-site production. Due to the lack of effective analytical methods to check the actual additive content in the tank liquid, it is important to have a reasonable understanding of the dosage, and neither too little nor too much is sufficient. Excessive inclusion of additives can easily cause brittle faults in the coating. Additives, as the name suggests, can only be added frequently and rarely, even under normal circumstances. For the zincate zinc plating process with DPE as an additive, a simple Hull cell test can usually be used to judge. When there are too many additives, the low current density area of the test piece is not bright, and on the contrary, the low current density area is bright and the high current density area is not bright.
The use of DPE type additives alone is acceptable under general conditions, but the brightness is not sufficient. In order to obtain a satisfactory zincate zinc plating layer, it is necessary to add a brightener and use it in conjunction with additives. Brighteners include triethanolamine, aromatic aldehydes, or nitrogen heterocyclic compounds. Domestic market available brighteners include WA (W906), WB, ZB-80, ZBD-81, and other varieties, which have similar properties and can effectively improve the appearance quality of the coating and refine the grain size, but they are also being improved.
When converting cyanide zinc plating bath solution, attention should be paid to the fact that the composition of cyanide zinc plating is most similar to that of the DPE additive zinc plating system. In order to use cyanide free electroplating and eliminate cyanide pollution, the most commonly used method is natural filtration. This can maintain normal production and gradually adjust the composition to the specifications of cyanide free electroplating.
Using this method for filtration, stop adding cyanide first, reduce the concentration of the main salt, switch to using iron anodes, increase the content of caustic soda proportionally, and control NaCN: Zn=(2-3:1). Due to the decrease of NaCN, the complexation effect weakens, and the influence of impurities is manifested. When ZnO drops to 20g/L and NaOH rises to 100g/L, Na2S or Zn powder can be added to treat the tank liquid, followed by low current electrolysis treatment. When analyzing NaCN below 10g/L, ZnO should be adjusted to 12g/L, and NaOH should be adjusted to 120g/L. At this time, the tank solution should be treated again and DPE - III (4mL/L) additive should be added. When NaCN is further reduced, brightener WB or other types of brighteners can be added.
Galvanizing is a widely used variety in electroplating. In order to improve the quality of galvanizing, both domestically and internationally, in addition to the composition of the plating solution, the transformation and improvement of electroplating equipment, and the maintenance and management of the plating solution, it also focuses on researching effective additives and conducting research work on the following aspects:
(1) Reduce the brittleness of the coating. From the analysis and comparison of cyanide free zincate zinc plating layer and cyanide plated zinc plating layer, the former has a high content of hydrocarbons in the coating, indicating the presence of organic inclusions, which leads to the brittleness of the coating. Therefore, it is an important issue to require an appropriate molecular weight and structural organic additive.
(2) Improve the current efficiency of alkaline zinc plating system. Due to the development of modern industry, more and more attention is being paid to economic benefits. Improving the current efficiency of the system can not only improve productivity, but also reduce hydrogen embrittlement.
(3) Prevent discoloration issues. This issue is not necessarily determined by the nature of the additive, but is closely related to the interference of impurities in the plating solution, post plating treatment, and improvement of passivation process. However, the antioxidant ability of the additive and the degree of inclusion in the coating are also factors that cause discoloration of the coating. Obviously, the above mentioned issues are topics that need to be studied and improved.