Aluminum Anodizing is an electrochemical coating process that creates a thin aluminum oxide film on the surface of aluminum substrates. The resulting anodized film is electrically non-conductive, protects the aluminum against corrosion and is resistant to wear. Aluminum anodizing is used widely in aerospace, automotive, defense, and medical applications.

The most common specification governing aluminum anodizing is Mil-A-8625, which denotes three different finishes: Type I, a thin film generated in chromic acid anodizing and used mostly by the aerospace industry for corrosion protection, Type II, the most common form of anodizing produced in sulfuric acid and often dyed different colors, and Type III, or Hard Anodize, which is a thick film that is highly abrasion resistant due to its surface hardness. Electrolurgy offers Type II Sulfuric Anodize, and Type III, Hard Anodize.

  • Type II Sulfuric Acid Anodizing (clear & dyed)
  • Type IIB Thin Film Sulfuric Acid Anodizing
  • Type III Hard Anodizing
  • Hard Anodizing/Teflon® Seal
  • Selective Masking for Multiple Finishes (such as chem film and type II on the same part)


  • Corrosion Resistance: The aluminum oxide film is resistant to atmospheric and saltwater environments.
  • Electrical Insulation: Aluminum oxide film is dielectric.
  • Paint Adhesion: Aluminum oxide film is a chemically active surface for painting.
  • Adhesive Bonding: Aluminum oxide film improves adhesion and bond strength.
  • Cleanability: Anodized coatings do withstand the hydrogen peroxide commonly used in medical sterilization, but currently Electrolurgy offers only clear or black Type II for this application.

Download our free PDF on Engineering Considerations For Anodizing Aluminum


Hard Anodizing, often called “Type III Anodizing” as denoted by the MIL-A-8625 specification, is an electro-chemical process that creates a controlled oxide film on the surface of aluminum. Hard Anodizing utilizes sulfuric acid, low bath temperatures, and constant current rectification to achieve a very hard coating (60-70 Rockwell C). Hard Anodizing is generally used for applications that need a corrosion, abrasion, or wear resistant coating. The appearance of the coating ranges from light to dark gray, depending on the alloy makeup of the aluminum base material, and it can also be dyed black. Parts can be processed with multiple finishes, such as Hardcoat/Chem Film or Hard Coat/Electroless Nickel Plating.  See our Hardcoat Anodizing Page for more detailed information.


Chem Film, sometimes called Alodine or Iridite, is a chemical conversion coating that is used to essentially passivate aluminum. It protects aluminum from corrosion and serves as a base for organic (paint) coatings while leaving the part dimensions unchanged. Chem Film is applied by dipping, spraying or brushing. It is sometimes applied alongside Type II Anodizing and can be used to repair minor damage to anodized surfaces. The resultant coating is dependent on the aluminum condition.  Chem film is generally not intended for decorative use, and has very little abrasion resistance.

Chem film per MIL-DTL-5541 Class 3 is electrically conductive. The best corrosion protection is provided by chem film per MIL-DTL-5541 Class 1A.  See our ChemFilm Page for more detailed information.