Anodize Coatings for Aluminum Alloys

purple anodize aluminum partMore recently, we have been seeing manufacturing prints that call-out an incorrect military spec for anodize on aluminum parts. As a result, we wanted to do this blog post to better inform engineers of calling-out the right classifications for anodize. But first, here is a mistake we have recently seen that is a good example:

Incorrect: “Finish: Anodize MIL-A-8625 Type II, Class 1, Blue”

The issue with this incorrect call-out is the class number and color. According to Military Spec MIL-A-8625, Class 1 is designated as “non-dyed.” This contradicts the “blue” color that is called out. All Class 1 specifications need to be clear color. To designate a color (blue, black, etc.), Class 2 needs to be specified. Here is the proper way to call out the specification above (please note the following call-outs are an example pertaining to the incorrect call-out above. Call-outs can vary depending on the desired Type/Class/Color):Correct (clear anodize): “Finish: Anodize Clear MIL-A-8625 Type II, Class 1”  or Correct (blue anodize): “Finish: Anodize Blue MIL-A-8625 Type II, Class 2”

Now that the Classes have been identified and explained, now is a good time to also define the different Types for anodize. Here is the complete list of the different Types and their meanings according to the military specification MIL-A-8625:

Anodize Types:

Type I:Chromic acid anodizing, conventional coatings produced from chromic acid bath.

Type IB: Chromic acid bath, low voltage process, 22 +/- 2V.

Type IC: Non-chromic acid anodizing, for us as a non-chromate alternative for Type I and Type IB coatings.

Type II: Sulfuric acid anodizing, conventional coatings produced from sulfuric acid bath

Type IIB: Thin sulfuric acid anodizing, for us as a non-chromate alternative to Type I and Type IB coatings.

Type III: Hard anodize coatings

As you can see there are quite a few Types which serve very different purposes depending on what the aluminum part is meant for. The thicker (harder) the anodize, the more durable. Type I anodize is the thinnest anodize, but has equal corrosion protection as Type III which is the thickest anodize. Type I and Type IB are used for corrosion resistance, paint adhesion, and fatigue resistance. Type IC and Type IIB are used for similar attributes. The type of acid that is used also determines the attributes the anodize will have. Here are the differences between chromic acid and sulfuric acid anodize:

Chromic acid anodize appears much grayer in color, and being thinner absorbs less color when dyed. This limits chromic acid anodize as a decorative finish, which can be dyed black for use as a non-reflective, protective coating on housings for optical components, but will result in a lighter black. Here are some benefits to using a chromic acid anodize:

  • Good for tight tolerance parts as it will not change the dimensions of a part
  • Can be black dyed (other colors not practical)
  • Good for bonding
  • Non-conductive
  • Good for welded parts and assemblies

Sulfuric acid anodize is the most common method for anodizing and has a thicker coating than chromic acid anodizng. It is well suited for applications where hardness and resistance to abrasion is required. The biggest advantage to sulfuric acid anodize is the ability to add color to the aluminum. Here are some benefits of using sulfuric acid anodize:

  • Less expensive than other types of anodize with respect to processes and chemicals
  • More alloys can be finished
  • Harder than chromic anodize
  • More variety of dying colors

This pretty much sums up the explanation of the Types and Classes of anodize and how to properly call-out anodize on a print. Thanks for reading along and be sure to stay tuned for our next blog post.


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