Dacromet of surface Treatment

What is Dacromet?

Dacromet is a new type of anti-corrosion coating with zinc powder, aluminum powder, Ming acid and deionized water as the main components. The substrate material using Dacromet is mainly steel and non-ferrous metals. After the coating liquid reacts with the metal substrate, a layer of silver-gray anti-corrosion film will be formed on its surface, which can isolate the substrate from the air and play a role in protecting the workpiece.

The protective effect of dacromet film layer on steel matrix can be summarized as follows:

Anti-corrosion mechanism

• Barrier effect: Due to the overlapping of sheet zinc and aluminum layers, it impedes the process of water, oxygen and other corrosive media to reach the matrix, and can play an isolating shielding role.

• Passivation: In the treatment process of dacromet, chromic acid reacts chemically with zinc, aluminum powder and matrix metal to generate a dense passivation film, which has good corrosion resistance.

Advantages

• Super corrosion resistance: the thickness of Dacrosol film is only 4-8um, but its anti-rust effect is more than 7-10 times that of traditional electric galvanizing, hot dip galvanizing or coating method. The standard parts and pipe joints treated by Dacromet process did not show red rust after salt spray resistance test for more than 1200h.

• No hydrogen brittleness: The processing process of Dacromet determines that Dacromet does not have hydrogen embrittlement, so Dacromet is very suitable for the coating of stressed parts.

• High heat resistance: Dacromet can resist high temperature corrosion, and the heat resistance temperature can reach 300℃C or more. The traditional galvanizing process has been scrapped when the temperature reaches 100 ° C.

• Good bonding force and recoating performance: Dacromet coating has a good bonding force with the metal matrix, and has a strong adhesion with other additional coatings, and the treated parts are easy to spray and color, and the bonding force with the organic coating even exceeds the phosphating film.

• Good permeability: Due to the electrostatic shielding effect, the deep hole, slit, inner wall of the workpiece and other parts are difficult to electroplate zinc, so the above parts of the workpiece can not be protected by electroplating method. Dacromet can enter these parts of the workpiece to form a dacromet coating.

• Low friction: Adding polymer materials in Dacromet coating, the friction coefficient of the properly treated workpiece is 0.06~0.12, and the general treatment is 0.12~0.18, which can meet the needs of industry.

• Good weather resistance: Dacromet can withstand the erosion of sulfur dioxide, acid rain, smoke and dust under a certain thickness. It’s used in municipal projects. After testing, its SO2 resistance test can be up to 3 weeks, in addition, it also has a certain corrosion resistance to chemicals, and has good corrosion resistance in gasoline and oil.

• The coating thickness is easy to control: the thickness of the general Dacromet coating, two coating two drying can be controlled between 6~8um, for the surface anti-corrosion treatment of fasteners, its matching accuracy can meet the accuracy of 69/6H, and there will be no phenomenon of hot-dip galvanized fasteners easy to destroy the coating in operation.

• Low coating hardness: the hardness of Dacromet coating itself is only 1~2H, coupled with its thin, Dacromet coating is not suitable for moving parts or under high wear resistance conditions.

Disadvantages

• Some Dacromet contains chromium ions harmful to the environment and human body, especially hexavalent chromium ions have carcinogenic effects.

• The sintering temperature of dacromet is higher, the sintering time is longer, and the energy consumption is large.

• The surface hardness of Dacromet is not high, the wear resistance is not good, and Dacromet coated products are not suitable for contact and connection with copper, magnesium, nickel and stainless steel parts, because they will produce contact corrosion, affecting the surface quality and corrosion resistance of products.

• The surface color of Dacromet coating is single, only silver white and silver gray, which is not suitable for the personalized needs of automobile development. However, different colors can be obtained through post-treatment or composite coating to improve the decorative and matching of truck parts.

• The electrical conductivity of Dacromet coating is not very good, so it is not suitable for electrically connected parts, such as electrical grounding bolts.

Process flow

Organic solvent degreasing — mechanical polishing — spraying — baking — secondary spraying — baking — drying.

Matters needing attention

• Dacromet will age rapidly in the light, so the coating process of Dacromet should be carried out indoors

• Too low or too high a baking temperature will make dacrococcus lose its anti-corrosion ability, and Dacrococcus should be baked in a suitable temperature range

• Dacromet has a short life cycle, so it should be used as soon as possible.

• Dacromet has poor wear resistance, so it should be top coated with Dacromet, and then coated with other coatings with wear resistance.

The anti-rust mechanism of dacromet coating is generally considered to be the following

• The controlled self-sacrifice protection of zinc powder

• Chromic acid in the treatment of the workpiece surface to form a dense oxide film that is not easy to be corroded;

• A coating composed of dozens of layers of zinc and aluminum sheets superimposed together forms a shielding effect and increases the path through which the intruder reaches the surface of the workpiece. Electrogalvanizing is to directly cover the surface of the steel with a layer of zinc, corrosion current is easy to flow between the layers, especially in the salt spray environment, greatly reduce the protection current to make zinc easy.

• Consumption produces white or red rust in the early stages of treatment. The dacromet treatment is composed of a sheet of zinc covered with chromic acid compounds, moderate conductivity, so it has excellent corrosion resistance. The layers of zinc sheets are superimposed on each other to form a shield, and the rate of zinc precipitation is controlled even in the salt spray test.

• Moreover, because the chromic acid compound in dacrosol dry film does not contain crystal water, its high temperature resistance and corrosion resistance after heating are also very good.