Surface Finishing Equipment and Systems

Chem film, also called chemical film, is a chemical conversion coating. It typically refers to a chromate conversion coating applied to aluminum and its alloys. The chemfilm coating provides a layer of corrosion protection and a base for paint and primer. When applied as a thin coat, chem film can allow for electrical conductivity.

Chem film coatings are marketed under brand names such as Alodine^®, Iridite^® and Chromicoat^®.

The chemfilm coating process is often governed by the commonly used MIL-DTL-5541 standard, which specifies different types and classes of chem film coatings.

A chromate conversion coating is a type of chemical conversion coating that is typically applied to aluminum alloys. Other types of chemical conversion coatings include iron phosphate and zinc phosphate coatings, typically applied to steel.

A chromate conversion coating protects the underlying aluminum against corrosion, serves as a base for paint adhesion, and can allow for electrical conductivity when applied as a thin coat.

Chromate conversion coatings for aluminum are widely used in the aerospace industry, as well as with everyday hardware such as nuts and bolts.

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.

Coating Thickness

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.

Surface Roughness

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.

Applications

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.

The primary difference between Alodine (chem film) and anodizing is that anodizing is an electrolytic process and Alodine (chem film) is not.

Alodine (chem film) and anodizing are both processes used in aluminum finishing to improve corrosion resistance. Both transform the outer layer of the surface of the metal.

But Alodine (chem film) and anodizing are NOT the same.

Anodizing is an electrolytic finishing process that applies electrical current to the metal while it is immersed in an electrolyte solution. In addition to increased corrosion resistance, aluminum anodizing is used for improved lubrication or to allow dyeing (coloring). It is generally more expensive to anodize than to apply chem film and requires greater technical expertise.

In contrast, Alodine (chem film) coatings do NOT apply electrical current to the aluminum. The metal is immersed in a tank, often at room temperature. Compared to anodizing, chem film is less expensive and requires less technical expertise. Unlike anodizing, a chem film coating can allow aluminum to retain electrical or thermal conductivity. This makes chem film the preferred choice when aluminum parts need to be electrically grounded.

Best Technology offers equipment for Alodine (chem film) and titanium anodizing, but we do not currently offer equipment for aluminum anodizing.

A chemical conversion coating is a surface treatment applied to metals in which the metal undergoes a chemical reaction at the surface. This chemical reaction transforms the metal surface into a thin protective layer.

When applied to aluminum, a chemical conversion coating is sometimes called an aluminum conversion coating.

Chemical conversion coatings provide increased corrosion resistance and a uniform, inert surface for powder coating or paint adhesion.

Chemical conversion coatings are typically applied via immersion in a chemical tank or bath, or by brushing or spraying.

Types of chemical conversion coatings vary according to the metal substrate:

• Iron phosphate coatings – Used with steel
• Zinc phosphate coatings – Used with galvanized steel
• Chromate conversion coatings – Used with aluminum alloys

Best Technology offers phosphating tank lines for iron phosphate coatings and zinc phosphate coatings, and Alodine tank lines for chromate conversion coatings.