What is the Dacromet coating process like?

1, Zinc-based micro-coating metal anti-corrosion coating liquid (Dacromet) coating process

Dacromet coating process: the raw materials are prepared into water-soluble paint, and then coated on the surface of the pre-treated workpiece, after pre-baking and sintering, a layer of inorganic film is formed. The basic process is as follows: workpiece degreasing → degreasing Rust (shot blasting) → dipping (or spraying) → drying → pre-baking → sintering → cooling → inspection → packaging.

① Degreasing and degreasing are divided into two types: organic solvent or alkaline solution. From the perspective of environmental protection, alkaline degreasing should be used. After degreasing, the surface of the workpiece is required to be infiltrated by water. Alkaline degreasing is based on The cleanliness of the surface determines that the workpiece is soaked at room temperature for 10-20 minutes.

Alkaline degreasing formula (mass fraction W percent): OP-100.5 percent; 70 percent AES2.0 percent; Na2SiO31.0 percent; H2O balance.

Put the workpiece in a closed container, bring in the cleaning agent under high pressure, and spray it for cleaning. Since the surface of the workpiece is mineral anti-rust oil, a compound surfactant with good emulsification, dispersion and solubility is selected.

② Shot blasting In order to avoid hydrogen embrittlement and environmental pollution, pickling is not used for rust removal, but shot blasting is used. The diameter of the steel shot used in the shot blasting machine is 0.1-0.6mm, and the dust is removed by compressed air. After being collected by a special powder collector, it is collected and treated intensively. The degreasing and rust removal must be thorough, otherwise the adhesion and corrosion resistance of the coating will be reduced.

③ Dip coating The processed workpiece is immersed in the pre-prepared dacromet coating solution. The workpiece is generally dipped for 2-3 minutes under slight vibration, and then dried. If the workpiece is large, spray coating. Dip coating or spray coating Finally, after inspection, if there is unevenness or missing coating, it can be repainted by brush.

④Pre-baking, curing the coated workpiece, placed on the stainless steel mesh belt, requiring the workpiece not to stick to each other, entering the sintering furnace for 10-30min; curing for 15-30min. Pre-baking, curing temperature and time are mainly determined by the thickness of the coating It is determined by the size of the workpiece and different coating liquids, and is controlled by the conveying speed of the stainless steel mesh belt conveyor.

⑤ After post-treatment, if the coating on the surface of the fastener appears rough in appearance after curing, use a hard brush to clean the surface of the fastener.

2, Dacromet coating quality problems

① The quality of the coating solution is unstable

Due to the high density of zinc powder, it is easy to precipitate when preparing the coating, which makes the coating liquid layered. Therefore, a high-efficiency thickener must be selected. Sodium polyacrylate is used to thicken the coating liquid. When the amount added is too large, the coating liquid is easy to condense glue, so as to shorten the service life. It can be dissolved in water first, and the effect of adding it in the form of aqueous solution is better.

② Coating surface color

Generally, the surface color of the treated fastener is silver-gray. In the production process, it is found that the ratio of zinc powder and aluminum powder has the greatest influence on the color. Increasing the amount of aluminum powder can increase the silver-white color of the surface of the fastener. The amount of aluminum powder After the increase, it will have a certain impact on the corrosion resistance of the fastener, because zinc is a sacrificial anode to protect the metal substrate, while aluminum forms an oxide film to protect the metal substrate, but there is a certain proportional relationship between the two, which is not only from From the perspective of corrosion resistance, it is also considered from the perspective of production cost.

③The influence of pH value of chromic acid

In the Dacromet coating solution, there is another component that must be considered. In the actual preparation process, if the pH value of chromic acid is too high, the reaction between chromic acid and the slurry will be very violent, and a large amount of heat will be released, and finally the entire solution will be solidified. If it is diluted with water and then coated, the surface adhesion of the workpiece will decrease and it will easily peel off. The test found that the pH value of chromic acid is between 1 and 4. Within this range, the pH value Low, chromic acid reacts with the slurry, slightly exothermic, but the adhesion decreases with the increase of PH value.

④ Effect of stirring time on the quality of zinc-chromium film

After the solution material is prepared, it must be stirred to make the slurry, chromic acid and other components mix evenly and react and mature. Stir for 2-4 hours to make the coating mix evenly and mature. During the use, it should be stirred continuously. However, the prepared coating solution has a service life, and it can be used for 3 days if it is stored in an airtight container and kept stirring.

⑤Influence of wetting agent

Dacromet-coated workpieces are all metal parts. The coating liquid wets on the surface of the workpiece and spreads well, which directly affects the quality of the coating. If it is wet and spreads poorly, the surface of the fastener will be leaked, and it takes more Only one coating can cover the surface of the workpiece, resulting in uneven coating and reduced corrosion resistance. The combination of low-foaming non-ionic surface active wetting agent and reducing agent ethylene glycol, diacetone alcohol, etc. can achieve better results.

⑥Influence of reducing agent

However, when the drying temperature is about 80 degrees Celsius, the water in the coating solution volatilizes more, while other components are less oxidized. When sintered at about 300 degrees Celsius, the oxidation and volatilization occurs rapidly, resulting in a large number of pinholes in the film layer, which affects the coating quality. Therefore, a composite reducing agent should be used, so that when the transition is from about 80 degrees Celsius to about 300 degrees Celsius, the reducing agent components can be oxidized into small molecules and volatilized more uniformly, and at the same time, the reducing agent components that have not participated in the reaction Can evaporate relatively uniformly and slowly.

⑦Relationship between coating thickness and viscosity

The viscosity of the coating liquid has a great influence on the thickness of the film layer. If the viscosity is too small, the film layer will be thin, and the corrosion resistance will not meet the requirements. It will take multiple coatings to complete, so that the efficiency cannot be improved. If the viscosity is too high, then one coating If the film layer is too thick, it is easy to peel off during the sintering process, which will affect the surface quality and corrosion resistance.