Chemical conversion coatings sometimes referred to as chromate conversion, chem film, Iridite or Alodine refer to a process of applying a thin barrier coating on the substrate to passivate aluminum alloys to slow the formation of corrosion. It is gelatinous film that has self-healing properties and improves paint adhesion on aluminum alloys and die castings. It can also be applied to zinc die castings.
Materials qualified under Mil-DTL-5541F produce coatings that range in color from clear to iridescent yellow or brown.
They are all electrically conductive and impart some measure of corrosion protection on the substrate.
Visual inspection may be difficult since breaks in the coating are not easily seen.
Type I is Hexavalent Chromate
Type II is ROHS compliant, hex free chromate. This meets the ROHS and EELV requirements.
Class IA coatings are intended to provide corrosion protection as a stand alone coating and improve adhesion of subsequent operations such as paint on aluminum substrates.
Class 3 chemical conversion coatings are used as a corrosion preventative film where electrical contact is still required. Many times it is applied under masking operations prior to anodic coatings in accordance with MIL-A-8625-F.
The primary difference between Class IA coatings and Class three coatings is thickness.
Chromate conversion coatings are commonly used on parts in the: