It can be seen from many tests that the pre-cleaning of the metal has a great influence on the life of the coating film. The rust and scale on the iron surface need to be removed because if there is rust and scale on the metal surface, it can promote the generation of local primary galvanic couples, and the primary galvanic couples are the source of rust contact. Metals should be cleaned not only of rust and scale but also of dirt and oil before painting with paint samples, because dirt and oil will reduce the adhesion of the paint film, which will also lead to the most rapid damage to the paint film. As already pointed out, there is a relationship between the adhesion of the paint film and its corrosion resistance.
Dirt and oil on the metal can be removed with lye or solvents (gasoline, kerosene for paint). Rust and scale on the metal can basically be removed by two methods: chemical methods and mechanical methods. The chemical cleaning method (chemical contact removal method) is to dissolve the oxide by immersing the metal in a bath containing acid (sulfuric acid, hydrochloric acid, etc.). (Related instruments: PH meter)
The speed of the chemical etching process depends on the acid used, the concentration of the acid and the temperature.
Mechanical cleaning methods for metals should include manual surface cleaning with steel wire brushes, emery or pumice, sand cleaning with sandblasting machines, mechanical treatment with emery wheels or other abrasive materials, and fire cleaning with special acetylene lamps. When cleaning metal, it is often carried out first by contact method and then by mechanical cleaning method.
Metals can also be cleaned by etching with the addition of "addition agents". The addition of additives in the acid, that is, the addition of surfactants, will more or less reduce the dissolution rate of iron. Since the metal cleaning method has a great influence on the stability of the paint film, all the samples to be painted need to go through the same cleaning method when doing the comparative test.
Yakubovich and Iskra 5° studied the effect of various methods of cleaning metal on the stability of paint films. It is well established that chemical cleaning of metals can promote corrosion. In this case, corrosion begins with the formation of individual small spots on the surface that can attack the paint; the result is favorable conditions for the rapid growth of rust groups. In addition, as a result of the chemical cleaning of the iron, sometimes a texture can be produced very quickly on the coating film. It seems that mechanical methods, especially the metal (steel) brush cleaning method, are good ways to clean metal. The author arranges all metal cleaning methods in the following order: metal brush, sandblasting machine, pumice stone, sandpaper (by hand), contact removal in sulfuric acid with additive, contact removal in hydrochloric acid with additive, Etching method in hydrochloric acid without addition agent.
The paint film on the metal surface cleaned with metal brush and sandblasting machine is the most stable; the paint film on the surface cleaned with hydrochloric acid is the least stable.
Therefore, when carrying out the atmospheric test of the coating film on the iron surface, it is better to only use mechanical methods to clean the surface.
