The base character of aluminium means it reacts quickly with oxygen in the atmosphere to form a compact, resistant layer, which also contains other impurities from the surrounding environment. This layer is not suitable as a protective layer due to its varying thickness and also for decorative reasons. The components to be coated must therefore undergo pretreatment tailored specifically to the subsequent finishing process in order to satisfy the requirements for the relevant coating with regard to purity, surface texture and activation:
For powder coating and wet painting, a chemical pretreatment is carried out that finishes with the application of the conversion film. This conversion film is generally based on chromate.
Alternatively, some finishing partners are able to use a chromate-free conversion film, but it must be specified with the order.
Pre-anodisation is possible as an alternative pretreatment for powder coating and wet painting with a guarantee against filiform corrosion (creep corrosion).
When choosing chromate-free and pre-anodisation pretreatments, the finishing partner will be selected and contracted based on this criterion.
The anodising process places extremely high demands on the surface. Coarser residues from manufacturing are removed during preliminary purification, whilst the fine surface texture is produced during the mechanical pretreatment. The surface textures available from the pretreatment are summarised in DIN 17611:
Possible surface textures from the anodising pretreatment
- E0 = degreasing and deoxidation
- E1 = grinding
- E2 = brushing
- E3 = polishing
- E4 = grinding and brushing
- E5 = grinding and polishing
- E6 = pickling