There are two top dogs in the game of protecting aluminum. Both anodizing and chromate prevent corrosion and protect the surface, but each does it in its own way.
When oxygen in the air touches aluminum, corrosion happens. Unlike steel and iron, which produce iron oxide or rust, when aluminum is exposed to oxygen a thin hard oxide layer forms. Rust is flaky and brittle, but you can’t easily remove the powdery white or dull gray, aluminum oxide. Some types of aluminum corrode more readily than others. But 5052-H32, for example, is very durable and corrosion resistant.
Chromate and anodizing are conversion coatings. That means instead of adding a protective layer on top of the material, the process chemically converts the surface layer of the substrate, then builds from that base. But the two processes differ in how this conversion happens.
MIL-A-8625: Anodic Coating of Aluminum (Anodizing)
Industries that deal heavily with aluminum parts tend to use Type II and Type III anodizing. They are electrolytic processes in which the part you’re plating acts as an anode, meaning it gets a negative charge. Here’s how it works: A charge flows through the part while it is dipped in an acidic solution. Electrolysis converts the surface of the material into protective aluminum oxide. It’s a process that forms a hard, porous, and robust surface. From there, a variety of colored dyes can be used to create a cosmetic part, black being the most common.
Type III or hardcoat uses a process similar to Type II, in that it uses sulfuric acid during the process. Stricter controls on temperature, solution, and duration create a much harder surface than Type II anodizing. This improves wear resistance, as well as lubricating and dielectric properties. The effect of the extended plating time causes “clear” anodizing to come out gray, although the surface can be dyed another color. It’s important to note that hardcoat is not a good surface if you want to apply powder coating or paint.
The chromate conversion process results in an iridescent to slightly golden iridescent film. Chromate offers corrosion resistance while adding the advantage of being conductive. Also called chemical film, this finish is added by submerging the aluminum in a bath containing chromic acid. In the past, the process used hexavalent chromium, but these days it’s falling out of favor due to its high toxicity, with trivalent chromium being the most common replacement.
Chromate conversion coatings come in two thicknesses: Class 1A and Class 3. Class 1A is the thicker of the two and is used primarily for corrosion protection, although you do lose some electrical conductivity. Thinner Class 3, is often used under an anodized layer and powder coat where masked areas require conductivity. Class 3 is better suited as the substrate for paint and powder coat, too.
Chromate is not as hard or wear-resistant as anodizing, but it is a self-sacrificing coating. When you scratch or damage the surface, soluble chromate from areas adjacent to the damage will slowly cover the exposed metal.
Bottom Line: Anodizing vs. Chromate Finishes
Anodizing and chromate are the two most common ways to increase corrosion resistance on your aluminum parts. Putting it all together, this chart provides some overall guidance to help you make an informed decision for your parts.
|Characteristic||Anodize Type II||Anodize Type III||Chromate|
|Increase in surface hardness||√||√√|
|Substrate for paint/powder||√||√|
|Builds up on surface||√√||√√||√|