What is the electrical conductivity of chromate conversion coatings, also called Alodine® or chem film, when applied to aluminum alloys?
Unlike anodizing, Alodine coatings offer electrical conductivity for aluminum parts, when applied according to Class 3 of the MIL-DTL-5541 standard. Class 3 chemical conversion coatings protect against corrosion where low electrical resistance is required.
Because Class 3 coatings are thinner, they provide less corrosion resistance than Class 1A, but the trade-off is better electrical conductivity.
Technically the coating itself is not conductive, but rather the Class 3 coating is thin enough to allow the aluminum to retain its electrical conductivity.
The other factor that affects Alodine conductivity is surface roughness. A rougher surface finish allows greater conductivity due to breaks in the coating. In contrast, highly polished surfaces have lower conductivity due to lack of breaks in the coating.
To meet the Class 3 MIL-DTL-5541 standard, parts must have electrical resistance of no more than 0.005 – 0.010 ohms per square inch under nominal electrode pressure of 200 psi with a flat contact.
Common applications for electrically conductive Alodine coatings include parts that need to be electrically grounded.
In addition to electrical conductivity, Alodine coatings offer thermal conductivity and are used in applications such as heat sinks.
Chromate conversion coatings such as Alodine are an effective solution for aluminum alloys when both corrosion resistance and conductivity are important.