Pretreatment

The surface of the component to be coated must be thoroughly cleaned of all grease and dust particles by means of physical or chemical processes. Improvements to corrosion resistance and the later surface coating bond are also part of the pretreatment process. Typical pretreatments include:

- for plain carbon steel: degreasing, pickling, phosphating, passivation

- for aluminum: degreasing, deoxidation, chromodizing or the application of a non-chromated conversion coating based on zirconium, molybdenum, titanium, or silicate

The surfaces of aluminum components generally receive a chromate conversion treatment. Following degreasing and an acid wash the components are placed in a chromate bath. The bath is strongly acidic (pH value 1 to 2) and has as its most important ingredients chromic acid and complex fluorides. As a protection against corrosion, zinc surfaces are also chro-mated in order to prevent the formation of white rust. In some cases, mechanical processing (for example the sanding of damaged paint areas on a motor vehicle) can also be part of the preparation process. Often an undercoat or primer is applied and serves as a bond between the component material and the surface coating. The undercoating also provides protection against corrosion and consists of chromates as well as zinc and lead compounds.

0 0

Post a comment