Chromate conversion coatings on aluminum are also known as yellow Bonderite, Iridite, and Alodine, or Chem Film. They are used for everyday aluminum components, hardware, automotive wheels, heat sinks and aerospace applications (aircraft hulls, side and torsion struts, shock absorbers, flight control systems, and landing gear). Chromate conversion coating is applied by dipping, spraying, or brushing, and the coating thickness doesn’t change the dimensions of the coated part. If applied to a surface before the primer, it can improve the adhesion of both primer and paint.
What is Chromate Conversion Coating?
Chromate Conversion Coating is a process that converts the aluminum surface properties, unlike plating, which applies a coating onto the aluminum’s surface. It is typically used to passivate magnesium, copper, silver, tin, zinc, aluminum, and steel alloys. It can also be used as a decorative finish, primer, corrosion inhibitor, or to retain electrical conductivity. Chromate results in no measurable buildup on the parts and provides excellent corrosion resistance. Chromate coating minimizes surface oxidation, and it is used as an undercoat for adhesive applications or paint.
The process is named after hexavalent chromium, the chromate found in chromic acid, which is highly regulated because it is toxic. Non-hexavalent chromium-based processes are now becoming more available at a commercial level. Chromate conversion coatings are usually recognized by their distinctively greenish-yellow, iridescent color.
Everything Depends on the Specific Alloy Chromated
In today’s market, there are a few tremendous commercial conversion coatings, while there are also some bogus ones (that don’t even contain chromium salts). For example, coatings that leave a yellow color are based on potassium permanganate. If left bare, those processes don’t provide any corrosion protection.
The appearance of a chromate conversion coating depends on the alloy being chromated. Five different alloys processed with the same pretreatment and chromate application can produce five different hues of chemical film. For example, the widest inconsistencies and variations are typically seen in cast aluminum alloys. Color and uniformity will vary from one alloy and another as well as from an etched surface to a polished surface.
Next, what’s also dependent on the specific alloy being chromated is corrosion resistance. The purer the aluminum alloy, the better the product will perform in a corrosion test. The same chromate conversion coating applied to a wrought aluminum alloy can exceed 200-hrs in neutral salt testing per ASTM B117. This lies in stark contrast when compared to a cast aluminum alloy with silicon content exceeding 1 percent, which will last less than 24-hrs in the same conditions.
As the pH goes up, the coating reaction slows, which can cause the coating to become very light. If the pH goes down, the reaction gets faster, which may result in a powdery coating that wipes off.
Aluminum chromate conversion coatings improve the appearance of the metal while serving as an excellent base for adhesives, paint, and other adhesion-requiring applications.
If you are interested in aluminum chromate conversion coating or have questions about the processes we use, feel free to contact us.
