Surface treatment before industrial paint coating

Iron and steel metal components are often exposed to the atmosphere. Due to the role of oxygen and water in the atmosphere and the presence of SO2, CO2 and H2S, they are prone to corrosion, leading to the performance degradation and even damage of the components. Therefore, it is necessary to eliminate the adverse factors that affect the quality of the coating and improve the protective performance of the coating in the pre coating treatment.

The surface treatment of steel before painting mainly includes degreasing, derusting, phosphating, oxidation, passivation and sealing, etc.

1. Degreasing

Use solvents, such as gasoline 120 # solvent oil, alcohol, diluent, cleaning agent, emulsifier, electrolysis and other methods to remove mineral, vegetable grease and other dirt on the surface of steel components, so as to ensure the adhesion of the coating.

2. Derusting

Use manual and power tools, chemical pickling and blasting (sand blasting and shot blasting) methods to remove the oxide skin and rust scale on the surface of steel components, ensure the cleanness, flatness and roughness of the coated substrate, and strengthen the adhesion of the coating.

Manual rust removal: use a shovel, sandpaper, and wire brush to remove loose, raised oxide skin, loose rust and dirt on the steel surface.

Power derusting: use power wire brush, dynamic elastic grinding wheel or grinding wheel to remove most of the oxide skin, rust chips and dirt on the surface of steel components.

Pickling; Dilute solution of hydrochloric acid or sulfuric acid (10~25% Hcl, 10~30% H2S O4) is used to soak and spray the oxygen on the steel surface

Scale, rust scale and dirt shall be removed by oxidation (after removing dirt and rust, the acid radical remaining on the steel surface shall be neutralized with 10% NaoH alkali and rinsed with water to ensure the steel surface is neutral to avoid secondary pollution).

Spray rust removal; Use the machine to strike and rub the steel surface with quartz sand or steel shot through high-speed movement, so as to remove the oxide skin, rust and oil stain on the steel component surface. Restore the true color of steel, which is the best rust removal method. Some methods are used for large-scale machinery and equipment, long-term protection and anti-corrosion projects. Because of the high professional level and high cost of construction, this method is rarely used for most projects.

Rust and rust removal standards: The international rust removal standard is IS08501-1:1988, and the national rust removal standard is GB-8923-88. The two standards are equivalent.

Rust is divided into four grades: A refers to the presence of oxide skin without rust, B refers to the occurrence of rust, with some oxide skin peeling off, C refers to the occurrence of oxide skin peeling off, with rust, the oxide skin can be scraped off, with a small amount of pitting, D refers to serious rust, with common pitting, with all oxide skin peeling off.

Rust removal methods are divided into two categories: Sa refers to manual tools and power tools.

There are four rust removal grades by shot blasting: Sal: slight rust removal, no visible grease, dirt, loose oxide skin and rust on the steel surface; Sa2: Thoroughly remove the rust. The steel surface is free of visible grease, scale, oxide skin and rust. Sa/22.5: Thoroughly remove the rust. The steel surface is free of grease, dirt. The oxide skin and rust scale are completely removed, showing a slight metallic color. Sa3 is the most thorough rust removal. The steel surface is clean, free of any grease, dirt, rust scale and oxide skin traces. The uniform metallic luster can be seen.

Rust removal with manual or power tools can be divided into two levels, St2 and St3, and St2 are relatively complete. The steel surface shall be free of visible grease, dirt, uneven oxide scale and rust. St3 shall be completely derusted. The steel surface shall be free of grease, dirt, and most oxide scales shall be removed without rust.

Rust removal shall reach St3 or Sa2.5 level to ensure firm adhesion and long-term protection of the coating.