Types of anodizing:  Type I Anodizing - Chromic Acid also known as CAA or Chromic Acid Anodize.  Type II Anodizing - Sulfuric Acid or Sulfuric Acid Anodize SAA.  Type III Anodizing - Hard coat or  Hardcoat Anodizing HCA.

Aerospace Anodize Finishes

Phosphoric Acid Anodizing was developed to improve bond reliability for “metal-bond” structure parts. The hydration resistant oxides of PAA result in environmentally durable bonded part components. The prescribed surface preparation and anodize process provides a method for producing and controlling the unique oxide characteristics.

Chromic Acid Anodizing has minimal or no effect on the fatigue strength of finished parts. CAA is often used where there is the potential for solution retention in holes, recesses and crevices. The coating has substantial corrosion resistance, but does not provide substantial abrasion resistance.

Boric-Sulfuric Acid Anodizing was developed by The Boeing Company as a safer and more environmentally friendly replacement for CAA. It has equivalent or better fatigue life on aircraft structures. The addition of an organic acid can mitigate bio-contamination.

Sulfuric Acid Anodizing coatings are thick enough that they reduce the fatigue characteristics of aluminum alloys, so they are not used on structural parts. The majority of the anodic coating is aluminum oxide, with approximately 10-15% SO3 in the coating. Consequently, there is a potential for corrosion when there is solution entrapment.

Thin Film Sulfuric Acid Anodizing meets or exceeds MIL-A8625F, Type IIB. Comparatively, SAA (per MIL-A-8625, Type II) is a thick porous coating that is easy to dye, but has relatively poor corrosion resistance and an adverse effect on latent metal flexural fatigue. TFSAA is an improvement over conventional SAA in terms of corrosion resistance and as a base for paint.

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