Chemical surface preparation

In the initial stage of any painting process, the surface is pre-treated. This is the most time-consuming and time consuming process that is often not given due attention, but is a prerequisite for quality coverage. Surface preparation determines the quality, durability, elasticity and durability of the coating, contributes to the optimal adhesion of the powder paint to the painted surface and to improve its anti-corrosion properties.

When removing contaminants from the surface, it is important to choose the most appropriate treatment method and composition used for this purpose. Their choice depends on the material of the surface being treated, the type, the degree of contamination, as well as the requirements for the conditions and terms of operation.

For pre-treatment of the surface before painting are used methods of degreasing, removal of oxide films (abrasive cleaning, etching) and the application of the conversion layer (phosphating, chromatization). Of these, only the first method is required and the rest are applied depending on the specific conditions.

The surface preparation process involves several steps:

Surface cleaning and degreasing;
phosphating (phosphates of iron or zinc);
rinsing and fastening;
drying coating.
In the first stage, the surface is degreased and cleaned. It can be carried out mechanically or chemically. Mechanical cleaning uses steel brushes or grinding wheels, and depending on the size of the surface may be rubbed with a clean cloth dipped in solvent. Chemical cleaning is carried out using alkaline, acidic or neutral substances, as well as solvents that are used depending on the type and degree of contamination, type, material and size of the surface to be treated, etc.

During treatment with the chemical composition of the parts can be immersed in a bath with a solution or subjected to jet treatment (the solution is supplied under pressure through special openings). In the latter case, the efficiency of the treatment is significantly increased, since the surface is also subjected to mechanical action, in addition, a continuous flow of pure solution to the surface.

The application of the conversion sublayer prevents moisture and contaminants from causing the peeling and subsequent destruction of the coating.

Phosphating and chromatic treatment of the treated surface with the application of a thin layer of inorganic paint helps to improve the adhesion (“adhesion”) of the surface with the paint and protects it from rust, increasing its anti-corrosion properties. Typically, the surface is treated with iron phosphate (for steel surfaces), zinc (for galvanic elements), chromium (for aluminum materials) or manganese, and chromium anhydride. Chromium plating or anodizing methods are often used for aluminum and its alloys. Zinc phosphate treatment provides the best protection against corrosion, however, this process is more complicated than others. Phosphating can increase the adhesion of paint to the surface 2-3 times.

To remove oxides (these include scale, rust and oxide films), abrasive cleaning, (shot blasting, shot blasting, mechanical) and chemical cleaning (etching) are used.

Abrasive cleaning is carried out with the help of abrasive particles (sand, shot), steel or cast iron pellets, as well as nuts that are fed to the surface at high speed by compressed air or by centrifugal force. Abrasive particles hit the surface, breaking off pieces of metal with rust or scale and other contaminants. Such cleaning increases the adhesion of the coating.

It should be remembered that abrasive cleaning can only be applied to materials over 3 mm thick. The right choice of material plays a big role, as too much of a fraction can lead to a large surface roughness, and the coating will lie unevenly.

Etching is the removal of contaminants, oxides and rust by the use of etching solutions based on sulfuric, hydrochloric, phosphoric, nitric acid or caustic soda. The solutions contain inhibitors that slow down the dissolution of already cleaned areas of the surface.

Chemical cleaning is more productive and easier to use than abrasive, but after that it is necessary to wash the surface of the solutions, which necessitates the use of additional treatment facilities.

In the final stage of surface preparation is used passivation of the surface, ie its treatment with compounds of chromium and sodium nitrate. Passing-on prevents secondary corrosion. It can be used both after degreasing the surface and after phosphating or chromating the surface.

After rinsing and drying, the surface is ready for powder coating.