Type I

Type I (chromic acid) anodizing forms a thin, corrosion-resistant oxide on aluminum with minimal dimensional change, commonly specified for aerospace and bonding prep.

Overview

Type I anodizing (chromic acid anodizing) creates a thin aluminum-oxide conversion layer, typically to MIL-A-8625 Type I. It improves corrosion resistance and paint/primer adhesion with very low thickness buildup, so it’s a good choice when dimensions are tight or fatigue performance matters.

Pick Type I for aerospace and defense hardware, bonded assemblies, and parts that will be painted rather than dyed. Expect a natural gray to light gray appearance; cosmetic uniformity depends strongly on alloy and heat treat.

Tradeoffs: the coating is thin and not a wear surface, so it won’t handle abrasion like hardcoat. The process uses hexavalent chromium chemistry, so availability and lead time can be impacted by permitting and compliance requirements. Masking, racking contact marks, and alloy-to-alloy color variation should be planned into the print.

Common Materials

  • Aluminum 2024
  • Aluminum 6061
  • Aluminum 7075
  • Aluminum 5052
  • Aluminum 6063

Tolerances

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Applications

  • Aerospace structural brackets (painted)
  • Bonded aluminum assemblies (adhesion prep)
  • Riveted/sheet metal airframe details
  • Avionics chassis and instrument housings
  • Corrosion-protection pretreat before primer/topcoat
  • Helicopter gearbox accessory covers (painted)

When to Choose Type I

Choose Type I when you need corrosion protection and strong paint/primer or adhesive bonding performance with minimal dimensional change. It fits low to medium volumes and complex geometries where a thin, conformal coating is preferred over a thick wear layer. Best for parts that will be painted and don’t see sliding wear.

vs Type II anodizing

Choose Type I when you want a thinner oxide with less dimensional impact and you care about fatigue performance or bonding/paint adhesion on aerospace alloys. Type II is more common for general-purpose corrosion protection and offers broader dye colors, but it typically builds more thickness.

vs Type III anodizing

Choose Type I when you do not need wear resistance and you must minimize thickness buildup on precision features or fits. Type III hardcoat is thicker and harder for abrasion and bearing surfaces, but it drives more dimensional change and can add risk on tight-tolerance assemblies.

vs Alodine / chromate conversion coating

Choose Type I when you need an anodic oxide layer with improved durability and paint adhesion beyond a simple conversion coat, while still keeping thickness low. Chromate conversion is thinner and very conductive, but it’s generally less robust than anodizing for long-term corrosion protection.

vs Zinc plating (on steel parts)

Choose Type I when the base material is aluminum and you need an integral oxide coating rather than a deposited metal layer. Plating is not applicable to aluminum in the same way and introduces different risks (hydrogen embrittlement on steels, adhesion stacks, and thickness control).

Design Considerations

  • Call out the exact spec and class (e.g., MIL-A-8625 Type I) and whether sealing is required
  • Specify thickness requirements only when needed; otherwise allow standard Type I thickness to avoid cost and rework
  • Mask critical electrical contact areas or precision fits; note that even thin anodize can affect interference/press features
  • Provide acceptable racking/contact mark locations and cosmetic zones on the drawing
  • Avoid mixed-alloy assemblies if appearance uniformity matters; different alloys will shade differently
  • Break sharp edges and burrs before anodize to reduce burning, thin spots, and handling damage