What Is Involved in Zinc Plating?

Galvanizing refers to the surface treatment technology of plating a layer of zinc on the surface of metals, alloys or other materials to play a role of beauty and rust prevention. The main method used is hot-dip galvanizing.

With the development of galvanizing process, high performance

In the plating tank containing the galvanizing bath, the parts to be plated after cleaning and special pretreatment are used as

In the actual production process, common factors affecting the speed and quality of galvanizing are:

(1) The pretreatment is incomplete. There is an oxide film on the surface of the workpiece, which affects the normal deposition of zinc.

(2) Poor electrical conductivity. The current is consumed on the wire, and the current distributed to the surface of the workpiece is too small.

(3) The carbon content of the workpiece is high.

There are two types of galvanizing processes: hot galvanizing and cold galvanizing. Cold galvanized is also called electrogalvanizing . Here we mainly discuss electrogalvanizing. There are many types of electro-galvanization. However, according to the PH value of the galvanizing bath, there are two main types: alkaline galvanizing and acid galvanizing.

Galvanized alkaline galvanized

The alkaline galvanizing process refers to that the pH of the plating solution is alkaline. But due to different complexing agents, they are divided into two types: cyanide zinc plating and zincate zinc plating. Cyanide galvanization is a very old type of plating. There are three main components in the bath: the main salt zinc oxide, the complexing agent sodium cyanide, and the conductive salt sodium hydroxide (commonly known as fire alkali). There were no brighteners in early cyanide plating solutions. With the improvement of people's aesthetic requirements, a brightening agent has been added to the cyanide zinc plating solution. The cyanide zinc plating process is stable and the coating is fine. The plating solution has good dispersion ability. According to the content of sodium cyanide, it is divided into three types: high cyanide, medium cyanide and low cyanide zinc plating. The biggest disadvantage of cyanide galvanizing is that it is too toxic and serious to the environment.

Zinc salt galvanizing is a rapid galvanizing process that has developed rapidly in the past 30 years. Its main component is the main salt zinc oxide, complexing agent and conductive salt sodium hydroxide (commonly known as fire base). In order to obtain a fine bright coating with good dispersion ability, a brightener must also be added. The main development period of domestic zincate galvanization was the era of cyanide-free galvanization in the 1970s. The famous DPE galvanizing process and DE galvanizing process have been used until now. Although this process is not as stable and detailed as the cyanide galvanizing process. But its biggest advantage is cyanide-free. The harm to the environment is much smaller. New developments have been made in zincate galvanizing, defects such as blistering and brittleness have been overcome, and the dispersing ability has been greatly improved.

Galvanized acid galvanized

The pH value of the plating solution is 4-6. The second achievement of cyanide-free zinc plating in the 1970s was cyanide-free ammonium chloride zinc plating. It goes hand in hand with zincate alkaline galvanizing. It uses zinc chloride as the main salt, ammonium chloride as the complexing agent and conductive salt. Citric acid and aminotriacetic acid are auxiliary complexing agents. Polyethylene glycol and thiourea are used as coating refiners. The plating layer obtained from this plating solution is fine and has good dispersing ability. The plating solution has low toxicity. However, the main disadvantage is that the plating solution is unstable, and the ammonium chloride gas emitted is very corrosive to the plating equipment . Current efficiency is also low. Strong sensitivity to impurities. Narrow operating temperature range. Has been basically eliminated by the potassium chloride zinc plating process.

Galvanized potassium chloride (or sodium chloride) galvanizing process

The potassium chloride (or sodium chloride) galvanizing process is a new cyanide-free galvanizing process developed in the 1980s. Its main components are: the main salt of zinc chloride, the general content is 70-90g / L. Potassium chloride is used as a conductive salt, and the content ranges from 140-280g / L, which can be arbitrarily adjusted according to different needs. Boric acid is used as a pH buffer to stabilize the pH between 4.6-5.6. Since this plating solution works at room temperature, the solubility of boric acid is not high, and the general content is controlled at 25-30g / L.

These three components alone cannot produce qualified zinc coatings. Add some additives to get a bright, detailed galvanized layer. The advantages of potassium chloride zinc plating are stable plating solution, bright and detailed coating, low cost, high current efficiency, and non-toxicity. The disadvantage is that the dispersing ability of the plating solution is slightly worse than that of alkaline zinc plating, and the brittleness of the coating is also large. Nevertheless, the introduction of potassium chloride zinc plating was immediately popular. Development is very rapid, has occupied half of the galvanized mountains. From the market point of view, the current galvanizing process is mainly two plating types: alkaline zincate and acid potassium chloride zinc plating. Other galvanizing processes have dwarfed and slowly withdrew from the historical stage.

There is also a sulfate zinc plating process in the acid zinc plating process. Its main ingredient is zinc sulfate, the main salt. The content is between 250-500g / L. Alum or aluminum sulfate is used as the conductive salt. The content is between 30-50g / L. Conductive salts are also added with sodium sulfate or sodium chloride. In addition to the above, some additives are added to make the coating fine. The earliest use of dextrin or gum, later invented some special brighteners to make the coating brighter and more detailed. The biggest advantage of sulfate zinc plating is that it can use a larger current density (1-5A / dm2) and fast plating speed. However, the disadvantage is that the dispersion ability of the plating solution is very poor, and it is not suitable for plating more complex workpieces.

Due to its particularity, sulfate zinc plating is mainly used in industries with simple shapes and continuous productivity, such as iron wire, steel strip, and steel plate. In addition to the above-mentioned several commonly used galvanizing processes, there are also pyrophosphate galvanizing, fluoroborate galvanizing, and hydroxy acid galvanizing, but they are not widely used and will not be introduced.