Surface preparation

The main precondition for the successful application of powder coating material is an uncontaminated surface of the substrate. A contaminated surface can cause defects in the appearance of the coating (tubercles, pits, etc.) and may cause poor adhesion between the coating and the substrate. The most common impurities are oils, grease, surface corrosion, scales, in general, particles lying free and sticking. Only a “clean surface” in many cases is not sufficient to achieve the required corrosion protection. In cases where anti-corrosion protection is an advantage, it is necessary to resort to a preliminary chemical treatment.

Cold-rolled steel sheets shown on the photos were degreased with a solvent (trichlorethylene) before applying the powder coating material (A) and (B) treated with zinc phosphate. One powder coating was applied under the same conditions on the plates, and in both cases they were exposed to 1000 hours in a salt chamber (5% salt solution at 35 ° C).

Often, “solvent degreasing” is used as the only pre-treatment before applying the powder coating and varnish. To know what can happen when such a system is exposed to corrosive effects, we can apply the tests in a chamber with salt spraying. We compare it with the purification system recommended as pretreatment for a higher corrosion class.

Consequently, a number of factors influence the choice of the pre-treatment method:

• Type and quality of metal;
• The state of the surface, i.e. degree and type of pollution;
• Finished products, their scope of application and protection requirements;
• Economic and environmental requirements.

Selecting the cleaning process, it is necessary to proceed from the type of impurities that must be eliminated, and, simultaneously, from the type of the treated substrate. Solvents remove only “greasy” impurities, such as oils, fats, greases, etc. Degreasing is often used with acidic, neutral or alkaline chemicals. Such reagents can eliminate corrosive products, scales and other oxides.

Mechanical cleaning

Mechanical processes are used both to eliminate such impurities as slag from welding, scale, etc., and to improve the adhesion of the subsequent coating. If sandblasting is used (the abrasive should be chosen in accordance with the type of substrate and the required roughness), it is important to know that greasy sewage must be eliminated in advance.

Phosphating. Phosphoric iron

The treatment with phosphoric iron (often called thin-layer phosphatizing) gives very good adhesivity and does not adversely affect the mechanical properties of the paint layer. Phosphoric iron provides good corrosion protection in low and medium corrosion classes, although it can not compete with zinc phosphate. Iron phosphate can be sprayed or immersed. The number of degrees of the process is in the range from 2 to 7 and depends on the base metal and on the required protection. In comparison with zinc phosphating, the ferriferous is cheaper and easier to implement. Typically, the thickness of the phosphate layer is from 0.3 to 1.0 g / m2.

Phosphoric acid zinc

Preliminary treatment with phosphoric acid zinc achieves a thicker layer than when ferrous phosphatizing, and this layer is more firmly attached to the base. Phosphoric acid zinc also has very advantageous adhesion properties, although in some cases it can reduce the mechanical integrity (elasticity) of the system. Phosphoric acid zinc provides remarkable anti-corrosion resistance and is recommended for pre-treatment of steel and galvanized steel in a medium of high corrosion classes. Phosphoric acid zinc can be used in devices for spraying and immersion. The number of process degrees is from 4 to 8. Zinc phosphating is more expensive than ferrous phosphating, due to higher costs for both equipment and the process itself.

Chromatization

In the group  of chromatic process, there are a number of different systems. The choice of the system depends on the type of metal or alloy, the kind of object (the method of its manufacture: casting, pressing) and, of course, the quality requirements.

Chromatization can further be divided into:

• Thin layer chromatography;
• Green chromating;
• Yellow chromating.

The last of these processes is most often used before the application of powder materials. The number of degrees of the process can differ depending on how intensively one needs to process the product for chromating, for example, by etching, neutralization, etc., and how much subsequent washing is required.

Ecology

In particularly demanding applications of aluminum, special anodization processes and chromating are considered equally suitable as its pretreatment. To obtain good results, the thickness and porosity of the anodizing layer and the type of aluminum alloy are important.

Traditional chemical pretreatment means the use of chemicals subject to strict standards, and the cleaning and laying of waste is expensive. As an alternative, so-called non-rinse processes were developed in which they do not work with heavy metals, hence, purification is less difficult, and waste is not classified as hazardous. Alternative pretreatment processes are still under development, but are now available for steel and aluminum in corrosion classes 0.1 and 2.

The choice of the method suitable for pretreatment depends on a number of factors. In addition to the requirements for the final use of the product, attention should be paid to the combination of powder coating and metal, the planned volume of production and the capabilities of the existing premises. In order to find a solution, special knowledge is needed.

We recommend contacting the leading chemical suppliers for pre-treatment advice.

Corrosive class
0	Interior with relative humidity below 60%.
	Very low risk of corrosion.
1	Unheated, well ventilated rooms.
	Low risk of corrosion.
2	Exterior with a continental climate, far from the sea and industrial plants.
	Average degree of corrosion risk.
3	Premises with variable temperature and humidity.
	In densely populated areas or near industrial plants.
	Near the water areas and near the coast.
	High risk of corrosion.
4	Constant, high humidity.
	Near industrial plants for the production or processing of chemicals.
	Very high risk of corrosion.

Caution: Aluzinc and galvanized materials are often supplied with a protective chromatographic layer. This chromate can cause problems in subsequent surface treatment. We recommend to consult with the supplier whether the chromatographic layer is suitable for the application of powder coating materials.