Making Sense of Anodized Aluminum Finish Callouts and Designations

Q. What are the differences between the Aluminum Association Designation AA- A31 and the MIL-A-8625, Type II, Class 1 anodizing finishes? -T.M.

A. The Aluminum Association Publication 45, “Designation System for Aluminum Finishes,” is an organized system for calling out various finishes, appearances and in some cases coating thickness ranges for a wide variety of anodic finishes for aluminum. There are three major categories of finishes:

Mechanical Finishes (M)
Chemical Finishes (C)
Coatings

The last category is divided into five major groups representing Resinous and Organic (R), Vitreous (V), Electroplated (E), Laminated (L) and Anodic (A).

The three categories for anodic coatings on aluminum (that is, anodized aluminum) are broken down further in this manner:

Mechanical Finishes (M) include finishes prior to any chemical or anodic processes. Typical examples include buffed, sanded, scratch brushed and shot blasted finishes.
Chemical Finishes (C) are part of the cleanup process that is done before anodizing. This includes such things as chemical etching, brightening and also conversion coatings.
Anodic Finishes (A) are categorized by the type of anodizing required. Included are Chromic Acid Anodizing, Hard Anodizing and what MIL-A-8625 calls Type II coatings.

The AA system divides what could be called Type II anodic coatings into three general groups differentiated by their coating thicknesses:

Protective and Decorative anodic coatings are less than 0.4 mils (10 microns) thick. This category is designated by the codes A21 through A24.
Architectural Class II anodic coatings are 0.4 to 0.7 mils (10 to 18 microns) thick. This category is designated by codes A31 through A34.
Architectural Class I anodic coatings are greater than 0.7 mil (18 microns) thick. This category is designated by codes A41 through A44.

With this in mind, the AA- A31 finish is a clear anodizing finish that has a coating thickness of between 0.4 and 0.7 mils.

MIL-A-8625, Type II, Class 1 is also a clear anodize finish. There are certain criteria which are called out in the spec concerning the characteristics and the performance of this coating:

Coating weight must be at least 1,000 mg/ft²when tested by IAW ASTM B 137 or the method stated in 8625.
Corrosion resistance must be tested by IAW ASTM B 117 (salt spray).
Other performance criteria may also be required by 8625 if called out in the contract.

So, while the A31 clear anodize finish and the Type II, Class 1 anodize finish could be the very same finish, there are no performance requirements if the contract or purchase order calls for an A31 finish. Anodic coatings called out as IAW MIL-A-8625 are subject to very strict performance requirements spelled out in the specification.