Understanding and Specifying Anodizing:

Why Anodizing?

Anodizing is likely the most commonly specified finish for aluminum machined parts. Advantages of Anodizing include:

  1. A very thin coating (.0002-.0008" for Type II) compared to paints and powder coat. Coating thickness can often be ignored for machined parts.

  2. Extremely durable, hard, abrasion resistant and long lasting. Coating does not peel or chip. Much, much harder surface than paint (harder than tool steel) and the coating lasts indefinitely.

  3. Some types (e.g., architectural anodizing) have colors which are fade resistant in sunlight nearly indefinitely.

  4. Excellent corrosion protection. Withstands salt-spray and other tests extremely well.

  5. Environmentally friendly finish. Can be readily recycled. Production involves simple inorganic chemicals which have minimal environmental impact

  6. Good electrical insulator. Combined with other coatings can be used for selective masking of low-voltage currents.

  7. Inexpensive. Very price competitive with painting and powder coating.

Conversion Coating: what it means

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.

Types of Anodizing

Type II (or "regular") sulfuric anodizing and Type III Hardcoat or "hard" sulfuric anodizing. The Type II & III designators come from the Military specification, MIL-A-8625.

Both Type II and Type III "hard" anodizing are hard coatings. Type III hard anodizing is done at a lower temperature, is more expensive, and harder than Type II, and is thicker than Type II, typically 0.002" vs. 0.0006" respectively, which makes it more resistant to scratching and heavy wear.

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. 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.

Manufacturing Tolerancing

With common Type II anodizing, the coating thickness will usually vary from .0002" to .0008". The thinnest coatings are usually seen in clear anodizing, since anodizing dyes require at least .0004"- .0008" of coating thickness to dye properly. With an average coating thickness of .0006", the build-up will typically be .0002"-.0003", which is small enough that it can be effectively ignored in most machined and fabricated parts. This is not the case, however, with Type III, Hardcoat anodizing, which can have coating thicknesses of up to .004", and build-up to .002".

Cost factors in Anodizing

Hard vs. regular anodizing: Since hard Type III anodizing is performed at a much lower temperature than regular Type II anodizing, and at higher current and voltage, it requires much higher energy use, and is usually considerably more expensive.

Masking: Masking usually involves a significant amount of hand work, which adds to cost. Masking materials often cannot be reused and add to costs as well.

Racking: Difficult-to-rack parts can add to cost. If needed, discuss adding racking features with your anodizer when needed on high-volume, low-cost parts.

Small features which trap chemistry: Small holes, especially blind holes, as well as small, deep pockets or other enclosed features force extensive rinsing of parts and raise labor costs.

Poor Specification: Poorly written or missing specifications on drawings mean more time spent on clarification and more cost. The more clearly the anodizing is defined on a drawing, with clear notes on racking, masking and other desired specifications, the better.

Mil Spec MIL-A-8625

US Military Specification MIL-A-8625 is the most common specification for anodizing in use. Although the specification is somewhat out of date, especially in regards to the environmental impact of some of the processes (e.g., Type I chromic acid anodizing), it still is used as a model for much of the anodizing specified for military and aerospace products, and forms the basis for many proprietary company specifications.

The most common types and classes of anodizing found in MIL-A-8625 are:

  • Type II Sulfuric Acid Anodizing (common or "standard" anodizing)

  • Type III Sulfuric Acid Anodizing (hard or "hardcoat"anodizing)

  • Class 1 – Undyed (clear)

  • Class 2- Dyed (colored)

Note that MIL-A-8625 does not cover architectural anodizing, which is usually covered by a designation system provided by the Aluminum Association (e.g., AA-M12-C22-A34, for electrolytically dyed architectural anodizing).

As an example, a standard black part designated by MIL-A-8625 would be called out on a drawing as follows:

"MIL-A-8625, Type II, Class 2, black"