Phosphating is a common pretreatment technique used in principle to phosphate the surface of steel. It can also be applied to non-ferrous metals (such as aluminum, zinc, etc.). Parts.
Phosphating is a process that forms a phosphate conversion film through chemical and electrochemical reactions, and the formed phosphate conversion film is called a phosphate film.
The main purpose of phosphating is to protect the base metal, prevent metal corrosion to a certain extent, and can be used as a primer before painting to improve the adhesion and corrosion resistance of the coating layer. Friction, lubrication effect in cold working of metals.
Classification of phosphates
⒈ Classification by phosphating temperature
⑴ High temperature type
The treatment time at 80-90℃ is 10-20 minutes, and the thickness of the phosphate film formed is 10-30g/m2. The ratio of the free acid content to the total acid content of the solution is 1: (7-8).
Advantages: The film has strong corrosion resistance and good adhesion.
Disadvantages: Long heating time, large amount of solution volatilization, large energy consumption, excessive phosphate precipitation, unstable free acidity, uneven crystal thickness, rarely used.
⑵Medium temperature type
50-75℃, treatment time 5-15 minutes, phosphating film thickness 1-7g/m2, solution free acidity to total acidity ratio 1: (10-15)
Advantages: Stable free acidity, easy to control, short phosphating time, high production efficiency, corrosion resistance is basically the same as high temperature phosphating film, has many applications.
⑶Low temperature type
30-50℃ Energy saving and easy to use.
⑷Normal temperature type
10-40℃ normal temperature (low temperature) phosphating (in addition to adding oxidizing agent, also adding accelerator), time 10-40 minutes, solution free acidity to total acidity ratio is 1: (20-30), film thickness is 0.2 - 7g/m2.
Advantages: No heating required, low chemical consumption, stable solution.
Disadvantages: Long processing time and complicated solution preparation.
⒉ Classification by composition of phosphating solution
⑴ Zinc phosphate
⑵ Zinc calcium phosphate
⑶ Iron-based phosphating
⑷ Manganese phosphating
⑸ Complex phosphating Phosphating solution is composed of elements such as zinc, iron, calcium, nickel, and manganese.
⒊ Classification by phosphating method
⑴ Chemical phosphating
The workpiece is immersed in the phosphating solution and phosphating is carried out using a chemical reaction. It is widely used.
⑵ Electrochemical phosphating
In the phosphating solution, the workpiece is connected to the positive electrode and the steel material is connected to the negative electrode to perform phosphating.
⒋ Classification by quality of phosphate coating
⑴ Thick (thick film phosphating) The film weight is 7.5g/m2 or more.
⑵Semi-thick (medium-thick phosphating) The film weight is 4.6-7.5g/m2.
⑶Lightweight (thin film phosphating) The film weight is 1.1-4.5g/m2.
⑷Semi-lightweight (special thin film phosphating) The film weight is 0.2-1.0g/m2.
⒌Classification by process
⑴Immersion phosphating
Suitable for high, medium and low temperature phosphating. Features: The equipment is simple, only a heating tank and a corresponding heating device are required. It is best to use a stainless steel or rubber-lined tank. Side of the tank.
⑵Spray phosphating
Application process
1. Pre-degreasing → Degreasing → Rust removal → Water washing → (surface adjustment) → Phosphating → Water washing → Post-phosphating treatment (electrophoresis, powder coating, etc.).
2. Major aluminum and zinc parts
3. Phosphate blackening solution
When used at room temperature, phosphating protection, also known as steel coloring agent, is formed in one step. Dilute 1:4-5 before use, soak at room temperature for about 30 minutes, and finally seal for protection.
4. Treatment process: Removal of oil and rust - Immersion in anti-rust water - Phosphating and blackening - Drying - Sealing protection [1].
Medium-low temperature phosphating can be used to treat large-area workpieces such as automobiles, refrigerators, and washing machine housings. Features: Short processing time, fast film formation reaction rate, high production efficiency, and the phosphate film obtained by this method is dense and uniform crystallization, thin film with good corrosion resistance.
