MIL-A-8625

At AOTCO, we work with a wide variety of clients in the aerospace and military defense industries. Oftentimes, since these industries often manufacture machines of great importance, our customers will require us to follow certain specifications and compliances. These requirements are called “specs”.

MIL-A-8625 is the most common anodizing spec for aerospace and military components. The spec covers a total of six different type options, as well as two class choices. This spec contains all requirements that are to be met while anodizing aluminum and aluminum alloys.

Types

While MIL-A-8625 covers six different types, AOTCO offers the four following types:

Type I

Type I, or Chromic Acid Anodize, was developed specifically for the aerospace industry. Most of the aerospace part manufacturing is done with castings. Aluminum is poured into the casting where it solidifies, leaving small holes scattered throughout the part. If type II sulfuric acid anodize was used, then it can seep through the holes and cause massive amounts of damage to the part and its surrounding components. When Chromic acid is used it still can leak through the holes in the component. However, rather than damage the part, it acts as a protecting agent for the component and improves the strength of the surface. Type I is also a lower stress anodizing than Type II or III, meaning the finishing process causes less stress fractures in the aluminum.

Type II

Type II, or Sulfuric Acid Anodize, is the most common anodizing process in our industry. The process creates a base layer of sulfuric acid around the aluminum, increasing the physical strength of the component. The acid also creates a porous layer, allowing for easy dying if a colored finish is desired.

Type IIB

Type IIB is like Type II for the fact that they are both sulfuric acid coatings. However, Type IIB provides a much thinner layer to the aluminum. For most anodizing types, the dimensions of the original metal are changed through a process of etching and anodizing. This often results in the final product having slightly different dimensions than its original form. For Type IIB this is not the case. While etching still occurs, the layer of sulfuric anodize is applied is so thin that it often results in no dimensional change, making Type IIB an excellent finish for highly specified machine parts or unique geometries.

Type III

Type III, or Hard Coat Anodize, provides the strongest results of all the anodic processes. This is because type III anodizing not only applies build up to the top of the aluminum it is binding with, but also penetrates the underlayers, reinforcing the strength of the aluminum from below its surface. This extra layer greatly increases corrosion resistance as well.

Class

Class is determined by spec and can mean a variety of different things. Under the military spec Mil-A-8625, class is used to determine if the anodize is dyed or not. Dyes are primarily used for aesthetic and cosmetic benefits; however, it also increases wear resistance and makes surfaces easier to sterilize.