Anodizing is the rapid controlled oxidation of the surface of aluminum.  As electric current is passed through the anodize solution, the surface of an aluminum piece is converted to a form of its oxide.

TYPES OF ANODIZING PROCESSES:
There are now four types of anodizing referred to as chromic acid anodizing, sulfuric acid anodizing, hard coat or hard anodizing and PTFE (Teflon) anodizing.  Each coating differs in in their basic properties as well as their end uses.

MIL-A-8625: Anodic Coatings for Aluminum and Aluminum Alloys

• Type I - Conventional chromic acid anodize

• Type II - Conventional sulfuric acid anodize
  • Class 1 - Non-dyed coatings
  • Class 2 - Dyed Coatings

• Type III - Hard Coatings
  • Class 1 - Non-dyed coatings
  • Class 2 - Dyed Coatings

MIL-A-63576: Aluminum Oxide Coatings, Lubricative, for Aluminum and Aluminum Alloys

• Type I - Anodic coating with the unsealed surface impregnated with PTFE

• Type II - Anodic coating which has the unsealed surface coated with a thermoplastic resin containing PTFE

• Type III - Anodic coating which has the unsealed surface coated with a thermosetting resin containing PTFE

AMS-2468: Hard coating treatment of aluminum alloys

AMS-2469: Process and performance requirements for hard coating treatment of aluminum alloys

AMS-2470: Anodic treatment of aluminum alloys, chromic acid process

AMS-2471: Anodic treatment of aluminum alloys, sulfuric acid process, undyed coating

AMS-2472: Anodic treatment of aluminum alloys, sulfuric acid process, dyed coatings