Type I

Thin chromic acid anodizing for aluminum providing high corrosion resistance, minimal dimensional change, and excellent paint adhesion, ideal for fatigue-critical and tight-tolerance parts.

Overview

Type I anodizing (chromic acid anodizing) forms a very thin, dense oxide layer on aluminum, typically 0.00002–0.0001" thick. It provides strong corrosion resistance and an excellent base for paint or primer while adding almost no measurable thickness, so tight machined tolerances are easier to maintain than with thicker anodic coatings.

Choose Type I when you need corrosion protection on precision, fatigue-critical, or complex geometry parts—especially in aerospace—or where many small holes, threads, or internal passages make thicker anodize risky. The tradeoffs: limited cosmetic options (dull gray appearance), lower wear resistance than thicker anodize, and environmental controls due to hexavalent chromium chemistry. Shops often use it where specs call out MIL-A-8625 Type I or where structural integrity takes priority over abrasion resistance or decorative finish.

Common Materials

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

Tolerances

Coating thickness typically 0.00002–0.0001" with about ±0.00002" variation; dimensional change is usually negligible for most machined tolerances.

Applications

  • Aircraft structural fittings and brackets
  • Aerospace fasteners and hardware
  • Hydraulic and pneumatic valve bodies
  • Avionics and electronics housings
  • Adhesive-bonded aluminum assemblies
  • Precision machined parts with tight fits and threads

When to Choose Type I

Use Type I anodizing when corrosion resistance and paint/primer adhesion are critical, but you cannot afford dimensional growth from thicker anodize. It suits precision, fatigue-sensitive, and complex parts where small features, thin walls, or tight fits would be compromised by thicker oxide layers. Volumes can range from prototypes to high-run production, as long as the chemistry and masking setup are stable.

vs Type II

Choose Type I over Type II when you need minimal dimensional change on tight-tolerance parts or when a spec explicitly calls for chromic acid anodize (common in aerospace). Type I is better for fatigue-critical components and complex internal features where thicker sulfuric films from Type II could cause binding, cracking, or loss of fit, and you do not need bright colors or higher wear resistance.

vs Type III

Choose Type I instead of Type III when corrosion protection and dimensional stability matter more than surface hardness and abrasion resistance. Type I’s thin film avoids the significant dimensional growth and edge buildup of hardcoat, reducing risk on small holes, threads, and thin sections that could seize or crack with a thick Type III layer.

vs Chemical conversion coating (Alodine)

Choose Type I anodizing over chemical conversion when you need a more robust, durable oxide layer and better paint adhesion, but still with tight dimensional control. Type I offers higher corrosion resistance and better long-term stability than conversion coatings, while still being thin enough for precision machined parts.

vs Painting or powder coating on bare aluminum

Choose Type I anodizing before painting when adhesion, corrosion resistance, and long-term durability are critical, especially outdoors or in aerospace environments. The anodic layer provides a much more reliable base than bare aluminum for primers and topcoats, without the thickness and edge rounding issues of standalone paint or powder.

Design Considerations

  • Call out the required spec and class clearly (e.g., MIL-A-8625 Type I, Class 1 or 2) and state if sealing is required.
  • Identify all critical dimensions and fits that must be protected from buildup so the shop can plan masking and fixturing correctly.
  • Avoid extremely sharp edges; add small radii or breaks to reduce the risk of burning and to promote uniform coating.
  • Minimize masked areas and complex masking patterns; masking drives labor time and cost significantly.
  • Provide vent and drain paths for deep pockets or blind holes so chromic solution can flow and rinse out cleanly.
  • Specify alloy and heat treatment on the drawing; coating appearance and performance vary by alloy and affect process settings and expectations.