How does zinc phosphate prevent corrosion?

Zinc phosphate is an inorganic compound with the chemical formula of Zn3(PO4)2. It is a white crystalline powder, soluble in inorganic acid, ammonia water and ammonium salt solution, insoluble in ethanol and almost insoluble in water. It is mainly used in medicine and dentistry. Adhesives, also used in anti-rust paints, phosphors, etc.

Zinc phosphate basic information

Chinese name: 磷酸锌

Foreign name: Zinc phosphate

Chemical formula: Zn3(PO4)2

Molecular weight: 386.170

CAS accession number: 7779-90-0

EINECS accession number 231-944-3

Melting point: 900 ℃

Water solubility: almost insoluble

Density: 3.99 g/cm³

Appearance: white crystalline powder

Zinc 2 Phosphate Properties

Colorless orthorhombic crystal or white microcrystalline powder. Apparent density 0.8 ~ lg/cm3. Soluble in mineral acid, ammonia water, ammonium salt solution; insoluble in ethanol; almost insoluble in water, and its solubility decreases with increasing temperature. When heated to 100 ℃, it loses 2 crystal waters and becomes anhydrous. Deliquescent and corrosive.

Zn Phosphate Production Method

1. Zinc oxide method: add diluted 15% phosphoric acid solution into the reactor, slowly add zinc oxide slurry with a concentration of about 20% under stirring to react to generate zinc phosphate, keep the temperature below 30 ℃, add zinc phosphate dihydrate The seed crystal is heated to 80°C under the condition of pH 3, filtered, washed with hot water, pulverized, and dried at 90°C to obtain the finished product of zinc phosphate dihydrate.

2. Metathesis method: add trisodium phosphate solution into the reactor, slowly add zinc sulfate solution under stirring, carry out metathesis reaction at 70-80°C, filter and rinse with water to remove sulfate ions after complete reaction, and then pulverize and drying at 120°C to obtain the finished product of zinc phosphate dihydrate.

Zinc Phosphate Rust Prevention Mechanism

There are various theories about the rust prevention mechanism of zinc phosphate. Meyer proposed in 1963 that zinc phosphate forms a complex with carboxylic acid in the coating system. It is believed that zinc phosphate slowly dissociates into phosphate ions inside the coating, and the condensed phosphate ions react with the metal surface to form complex and adhesive Me-Zn -P2O5 compounds cover the film, passivate the metal, or form a complex complex between the metal surface and the paint. In addition, zinc ions also form insoluble complexes, which play a role in cathodic protection. Especially in the initial corrosion process, due to the presence of crystal water, it reacts with -OH and -COOH in the coating, and may produce a gel. This reaction is carried out by the mixing of zinc phosphate in the multi-layer primer, when the -OH, -COOH groups in the primer paint react with the phosphate, the inner layer adhesion will be obtained.

Harrison et al. proposed in 1965 that the corrosion of steel due to contamination by ammonium salts is very intense. Zinc phosphate can inhibit ammonium ions, and through the penetration and diffusion of the paint film, the metal surface is slowly "phosphated" and maintained in a passive state. The presence of phosphate radicals will greatly reduce the harm of ammonium ions and control the corrosion of metals. Zinc ions may form cathodic protection on the steel surface, preventing further corrosion of the metal. Soluble phosphate reacts with ferrous ions generated in the cathodic region of the metal surface to form a cathodic inhibitor of insoluble precipitation coating. However, zinc phosphate is almost insoluble, so its role has been suspected. Regarding this issue, C1ay et al. conducted a constant current polarization test of iron on the extract of zinc phosphate, which confirmed its anti-corrosion effect. Forms a tough paint film that stabilizes rust.

Zinc Phosphate Applications

Zinc phosphate is a white non-toxic anti-rust pigment. It is a new generation of non-toxic and pollution-free anti-rust pigment with excellent anti-rust and corrosion effect.

Zinc phosphate is mainly used as the base material of various coatings such as aldol, phenolic, epoxy resin, etc., to prepare various water-resistant, acid-resistant and anti-corrosion coatings, such as: phenolic paint, epoxy paint, acrylic paint, high-build paint and water-soluble paint Resin paint, etc. It is widely used in dry ships, automobiles, industrial machinery, light metals, household appliances and metal containers for food, etc. Flame retardant for rubber and synthetic polymer materials. It can also be used for phosphating treatment of steel, medical industry, etc.