Alodine (Chromate Conversion Coating)
At a Glance
About the Process
Chromate conversion coating, more commonly known as chem film or by the brand name of Alodine, is a chemical coating that passivates and protects aluminum from corrosion. It is also used as a base layer before priming and painting parts. The standard most commonly specified in engineering applications is MIL-DTL-5541F, which refers specifically to coating of aluminum alloys. Other metals may be chromated but in these cases the process is not called Alodine.
This protective layer is much thinner than a layer of anodize, and while both are created by immersion of parts into a bath, Alodine is a simple chemical coating and the process does not use electrical current. In fact, Alodine can even be applied by simply painting the chemical onto the surface of the part.
For the reasons above, Alodine is a cheaper process than anodizing, but also more prone to scratches, wear, and physical harm. The most common color of the coating is an iridescent greenish-gold, and thus it may also be used for cosmetic purposes. However, this color comes from hexavalent chromium, which is toxic, so for RoHS compliance needs there are clear versions of the coating available as well.
- Plugging threaded/reamed holes not required unless tolerances are very tight, as thickness change is minimal.
- Alodine coatings offer a much greater electrical conductivity than bare aluminum.