Surface Dacromet Rust Prevention Treatment of Powder Metallurgical Parts

Dacromet, named Zinc Chromium Coating domestically, is a new type of anti-corrosion coating mainly composed of zinc powder, aluminum powder, chromic acid, and deionized water.

In recent years, Dacromet surface spraying technology has emerged for powder metallurgy parts. Dacromet surface spraying originated in European and American countries in the 1950s and initially belonged to military classified technology. Later, it gradually became popular worldwide and domestically. There are various types of Dacromet spraying fluids, including chromium containing and chromium free according to environmental requirements.

There are various types of Dacromet coatings, but the basic composition of the coatings can be summarized as follows:

1. Metal: Composed of substances such as zinc and aluminum, mainly consisting of ultrafine flake zinc and ultrafine flake aluminum.

2. Solvent: It is an inert organic solvent, such as ethylene glycol.

3. Inorganic acid components: such as chromic acid.

4. Special organic matter: It is a thickening and dispersing component of the coating liquid, mainly composed of cellulose like white powder.

The protective effect of Dacromet film on powder metallurgy parts and steel substrates can be summarized as follows:

1. Barrier effect: Due to the overlapping layers of sheet zinc and aluminum, it hinders the process of corrosive media such as water and oxygen reaching the substrate, which can serve as an isolated shielding effect.

2. Passivation effect: During the treatment process of Dacromet, chromic acid reacts with zinc, aluminum powder, and base metal to form a dense passivation film, which has good corrosion resistance.

3. Cathodic protection effect: The main protective effect of zinc aluminum chromium coating is the same as that of zinc coating, which provides cathodic protection to the substrate.

Advantages and disadvantages of Dacromet:

Advantage

Dacromet is a new type of surface treatment technology, which is a "green electroplating" compared to traditional electroplating processes. Its advantages include the following:

1. Super strong corrosion resistance: The thickness of the Dacromet film is only 4-8 μm. But its rust prevention effect is more than 7-10 times that of traditional electroplating, hot-dip galvanizing, or coating methods. The standard parts and pipe fittings treated with Dacromet process did not show red rust after salt spray resistance test for more than 1200h.

2. No hydrogen embrittlement: The treatment process of Dacromet determines that there is no hydrogen embrittlement phenomenon in Dacromet, so Dacromet is very suitable for coating load-bearing parts.

3. High heat resistance: Dacromet can withstand high temperature corrosion, with a heat resistance temperature of over 300 ℃. However, the traditional galvanizing process has already peeled and scrapped when the temperature reaches 100 ℃.

4. Good adhesion and recoating performance: The Dacromet coating has good adhesion to the metal substrate and strong adhesion to other additional coatings. The treated parts are easy to spray and stain, and the adhesion to the organic coating even exceeds that of the phosphating film.

5. Good permeability: Due to the electrostatic shielding effect, deep holes, narrow seams, and inner walls of pipe fittings are difficult to electroplate with zinc, so the above-mentioned parts of the workpiece cannot be protected by electroplating. Dacromet can enter these parts of the workpiece to form a Dacromet coating.

6. pollution-free and pollution-free: During the entire process of production, processing, and workpiece coating, Dacromet does not generate wastewater or exhaust gas that pollutes the environment, and does not require the treatment of three wastes, reducing treatment costs.

Shortcoming

Although Dacromet has many advantages, it also has some shortcomings, mainly reflected in:

1. Some Dacromet contains chromium ions that are harmful to the environment and human health, especially hexavalent chromium ions that have carcinogenic effects. (Under current environmental policies, the current Dacromet no longer contains hexavalent chromium ions)

2. The sintering temperature of Dacromet is high, the time is long, and the energy consumption is high.

3. The surface hardness and wear resistance of Dacromet are not high, and the products coated with Dacromet are not suitable for contact and connection with copper, magnesium, nickel, and stainless steel components, as they can cause contact corrosion, affecting the surface quality and anti-corrosion performance of the products.

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

5. The conductivity of Dacromet coating is not very good, so it is not suitable for conductive connection parts, such as grounding bolts of electrical appliances.

The process flow and precautions of Dacromet:

Organic solvent degreasing——Mechanical polishing (powder metallurgy sandblasting)——Spraying——Baking——Secondary spraying - Baking ——Drying

Precautions:

1. Dacromet will rapidly age when exposed to light, so the coating process of Dacromet should be carried out indoors.

2. If the baking temperature of Dacromet is too low or too high, it will lose its anti-corrosion ability. Dacromet should be baked within an appropriate temperature range.

3. The usage cycle of Dacromet is very short, so it should be used up as soon as possible.

Dacromet has poor wear resistance, so it should be topcoated with Dacromet before applying other wear-resistant coatings.