Dacromet surface treatment
Dacromet is the translation and abbreviation of Dacromet, which is called Dacromet, dacrust and dikelon. Zinc chromium coating is a new anticorrosive coating named zinc powder, aluminum powder, chromic acid and deionized water.
Dacromet coating liquid has a wide variety of types, but the basic composition of the coating solution can be summarized as follows:
1. metal: it is composed of zinc, aluminum and other substances, mainly including small scale zinc and fine scale aluminum.
2. solvent: inert organic solvent, such as glycol, etc.
3. inorganic acid components: such as chromic acid.
4. special organic matter: it is the viscosity increasing dispersing component of coating solution, and its main component is cellulose white powder.
Anti corrosion mechanism
The protective effect of Dacromet film on steel matrix can be summarized as follows:
1. barrier effect: because of the overlapping of sheet zinc and aluminum layers, it hinders the process of water and oxygen corrosion media to the substrate, and can play an isolated shielding role.
2. passivation: during Dacromet treatment, chromic acid reacts with zinc, aluminum powder and base metal to form a dense passivation film, which has good corrosion resistance.
3. cathodic protection: the main protective effect of zinc aluminum chromium coating is the same as that of galvanized coating, which is to protect the substrate.
Advantages and disadvantages
Dacromet is a new surface treatment technology. Compared with the traditional electroplating process, Dacromet is a kind of "green plating". The advantages are as follows:
1. Strong corrosion resistance: Dacromet film is only 4-8 thick μ m. But its rust prevention effect is 7-10 times of that of traditional galvanizing, hot galvanizing or coating. The standard parts and pipe joints treated by Dacromet process have not red rust after salt spray resistance test for more than 1200h.
2. No hydrogen brittleness: Dacromet treatment process determines that Dacromet has no hydrogen embrittlement, so Dacromet is very suitable for the coating of stressed parts.
3. High heat resistance: Dacromet can resist high temperature corrosion, and the heat resistance temperature can reach more than 300 ℃. The traditional galvanizing process, when the temperature reaches 100 ℃, it has become skin scrapped.
4. Good bonding force and repainting performance: Dacromet coating has good binding force with metal matrix, and has strong adhesion with other additional coatings. The treated parts are easy to paint and coloring, and the binding force with organic coating even exceeds phosphating film.
5. Good permeability: due to electrostatic shielding effect, the deep hole, slit, inner wall of pipe fittings and other parts of the workpiece are difficult to be plated with zinc, so the above parts of the workpiece cannot be protected by electroplating. Dacromet can enter these parts of the workpiece to form Dacromet coating.
6. No pollution and pollution: Dacromet will not produce waste water and waste gas which pollutes the environment during the whole process of production, processing and workpiece coating, and it does not need three wastes treatment, which reduces the treatment cost
Chromium free Dacromet
Although Dacromet has many advantages, it also has some shortcomings, mainly reflected in:
1. Some Dacromet contain chromium ions harmful to environment and human body, especially hexavalent chromium ions, which have carcinogenic effect（ Under the environmental protection policy, Dacromet has no hexavalent chromium ions)
2. Dacromet has high sintering temperature, long time and high energy consumption.
3. Dacromet has low surface hardness and poor wear resistance, and Dacromet coated products are not suitable for contacting and connecting with copper, magnesium, nickel and stainless steel parts, because they will produce contact corrosion, which will affect the surface quality and corrosion resistance of the products.
4. Dacromet coating has a single surface color, only silver white and silver gray, which is not suitable for the personalized needs of automobile development. However, different colors can be obtained by post-treatment or composite coating to improve the decorative and matching of truck parts.
5. Dacromet coating is not very conductive, so it is not suitable for conductive parts such as grounding bolts of electrical appliances.
Organic solvent degreasing - mechanical polishing - spraying - baking - secondary spraying - baking - drying.
matters needing attention
1. Dacromet is aging rapidly in light, so the coating process of Dacromet should be carried out indoors.
2. Dacromet can lose its anticorrosive ability because of its low and high baking temperature. Dacromet should be baked in the proper temperature range.
3. Dacromet has a short life cycle, so it should be used up as soon as possible.
4. Dacromet has poor wear resistance, so Dacromet should be coated on top and then other coatings with wear resistance.
1. Corrosion protection of various high-strength chassis small and medium supports, connectors, exposed parts and fasteners on trucks, such as various special-shaped steel supports, bolts (including horse riding bolts, wheel bolts, etc.), nuts, etc. It has been explicitly suggested that Dacromet coating should be used for fasteners with grade 10.9 on the automobile, because Dacromet coating has high corrosion resistance and can ensure no hydrogen embrittlement. High strength steel has the risk of hydrogen embrittlement in pickling and electroplating. Although hydrogen can be removed by heat treatment, it is difficult to clean it. Dacromet coating process does not need pickling and activation, nor leads to electrochemical reaction of hydrogen evolution, which avoids hydrogen embrittlement.
2. Corrosion protection of metal parts in high heat environment such as heat insulation plate, exhaust pipe, radiator, engine head and other parts. The passivation film on the surface of traditional zinc coating will be destroyed at about 70 ℃, and the corrosion resistance will decrease sharply. The curing temperature of Dacromet coating is about 300 ℃, the chromate polymer in the coating does not contain crystalline water, and the coating is not easy to be destroyed at high temperature, and it shows excellent anticorrosive property. The parts treated by Dacromet process are used for a long time at 250 ℃, and their appearance is almost unchanged and the corrosion resistance is very strong.
3. Corrosion protection of various elastic parts on trucks, such as hoops, semicircle clamps, springs, leaf springs, etc. The strength and hardness of these parts are required to be high, and hydrogen embrittlement will be produced during electroplating. For example, if the hydrogen removal is not good, the dynamic load is easy to tear or corrode fatigue for a long time, which will bring safety hazards to the vehicle driving, especially for trucks, the working environment is relatively bad, so the surface treatment requirements for these parts are relatively high, Dacromet coating has high corrosion resistance and high weather resistance, which is very suitable for surface treatment of such automobile parts.
4. Corrosion protection of various pipe and cavity parts with complex shapes on trucks, such as exhaust pipe and muffler of engine. If the parts are electroplated, the coating formed is uneven due to the complex shape of such parts and the limitation of electroplating process. Even the leakage plating may occur, which will reduce the corrosion resistance of the parts, which directly affects the performance and service life of the automobile parts. Dacromet process adopts impregnation and other methods, which does not have the problem of poor coating ability and depth ability in electroplating process, and Dacromet treatment solution has excellent permeability, and can form a good protective film in the tight contact gap, especially for the anticorrosion of automobile parts with complicated pipe and cavity, Some assembly parts are also suitable for Dacromet treatment after assembly. In addition, Dacromet coating is not subject to the corrosion of various oils, antifreeze, cleaning agents and other chemicals, and the chemical resistance and weather resistance are also very good, so Dacromet is very suitable for the corrosion protection of truck chassis and external parts
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