Anodizing is a “Conversion Coating”, how to define coating thickness

Anodizing is a “Conversion Coating”, how to define coating thickness

Anodizing is a “Conversion Coating”, and is very different than paints, plating and other common coatings on metal. While paints and plating sit on top of the surface of the aluminum, anodizing converts the outer layer of aluminum to aluminum oxide, so the coating is fully integrated with the aluminum substrate. This is why anodizing doesn’t chip or flake like paint- is completely integral with the underlying metal.

Etching into the substrate

Anodizing is a Conversion Coating because the surface aluminum is converted into aluminum oxide. In the same way that charcoal on a charred fire log is integral to the log, the aluminum oxide layer is integral to the aluminum substrate.

The oxide coating is most commonly created by placing an aluminum part in a sulfuric acid bath while running a low-voltage DC current through the part to cathodes on the side of the tank. The part acts as the anode in the electrical circuit, hence the origin of the term “Anod-izing”.

Understanding coating thickness and build-up

Because anodizing is a conversion coating, the surface of the aluminum actually recedes dimensionally as the aluminum is converted to the anodized oxide layer. The oxide layer grows out from the aluminum at a greater rate than the aluminum is removed, so the anodizing layer will tend to add thickness to dimensioned surfaces.

Build-up coating thickness

When specifying anodizing from a mechanical designer’s perspective, it is extremely important to understand the terms used by the anodizer, since these cause much confusion. Coating Thicknessis the actual thickness of the oxide layer. This is not the same as Build-up, which is a common term for t he dimensional difference between the oxide layer and the originally aluminum surface (see figure above).

Current thickness testing meters are usually used to measure the anodized (oxide) layer. This will measure coating thickness, but not build-up. Build-up must be measured by comparing a finished part to an uncoated sample, or by calculation.

In solar mounting system by Solaracks, average coating thickness is 10-12um/microns.

Within 1000meter distance from the coast line, anodization thickness increase to 15um.

Within 50meter distance from the coast line, anodization thickness increase to 20um.

Calculating Build-up

Anodizers usually use a rule of thumb that the oxide layer penetrates 50% into the part and builds up 50%. The true percentages, according to most sources, are closer to 67% in and 33% out for the common Type II anodizing, and 50% in and 50% out for Type III, hardcoat anodizing. In any case, knowing the coating thickness and using these percentages, a rough calculation of build-up is possible. Heavy etching before anodizing can also reduce buildup, by removing up to a few tenths of aluminum before the anodic layer is formed.


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