Sulfuric Type II Class 1 and 2 Finishes
Dyed and Undyed

For this technical lesson we will deal strictly with aluminum and its anodizing properties. Anodizing involves the immersion of aluminum into a suitable electrochemical solution which is subjected to a current flow. The electrochemical reaction between the solution and the aluminum forms an oxide film on the surface of the aluminum. The amount of oxide produced can be controlled with great accuracy.

Developed in the early 1930’s, anodizing greatly extends the life of the aluminum, therefore, allowing its use in products where the metal might not have otherwise been utilized.

Anodizing also can alter and improve the aluminum’s appearance by using dyes and special anodizing procedures. The finish can make aluminum look like pewter, stainless steel, copper or brushed bronze.

Anodizing is an electrochemical conversion process, not an applied coating like paint. The surface of the metal is converted to an aluminum oxide as a result of reactions occurring in an acidic solution. The thickness and properties of the coating will vary with alloy, time in solution, and process employed.

The coating is 30 to 50% thicker than the metal it replaces, since the volume of the oxide is greater than that of the metal replaced.

Anodic coatings can be formed in a number of chemical solutions although only a handful are in industrial use. Anodizing Specialists, Inc. provides two of these processes. The sulfuric acid process is the most predominant and is referred to as TYPE II by the military specification MIL-A-8625. This coating is commonly dyed and provides the very basic in corrosion resistance, abrasion resistance, and dielectric properties. The sulfuric/oxalic acid process known often as TYPE III, produces very thick and very dense or ‘hard coatings’. This hard coating provides the very best in abrasion resistance and corrosion protection, but is often difficult to color.

Since anodizing is a process which adds a coating to the surface of the aluminum, it is necessary to understand how this process works.

Anodizing is an anodic process which will penetrate 50% and build-up 50%. Therefore, your part dimensions will change by .002" on a .002" finish thickness. The finish thickness (penetration and build-up) can vary allowing a finish thickness from a few tenths to .003". Your particular requirements depend on a number of factors such as engineering requirements. Therefore, it is essential to allow for the hardcoat build-up changes on blueprints and/or purchase orders.

Our process conforms to a number of standards and specifications. Please check with us to see if your needs can be met. The most common standards which we conform to are Mil-A-8625 Type II Class 1 and 2, Mil Std-171 Finish , AMS 2471, and 2472. Other corporate standards can be
met as well.

In summary, anodizing is one of the most popular choices of finishing for aluminum because it protects without changing appearance; it can be colored; it is extremely adherent; it provides excellent abrasion resistance; it is inert in most environments; it is readily available, and it is relatively inexpensive compared to some of the other finishes.

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