What is Anodizing?

Anodizing is a process for finishing aluminum alloys that employs electrolytic oxidation of the aluminum surface to produce a protective oxide coating. The anodic coating consists of hydrated aluminum oxide. It is considered resistant to corrosion and abrasion. Conventional coatings are 0.1 to 1.0 mils thick and are mostly transparent, but may be colored.

An assortment of anodized parts.
An assortment of anodized parts.

 What is Chemical Conversion?

Chemical conversion coatings are intended to prevent corrosion, improve adhesion of paint finish or other coatings, and for improved electrical and electronic applications where low resistance contacts are required. The primary difference between a Class 1A and Class 3 coating is thickness, since current passes more readily through a thinner current resistant barrier (coating). Class 3 is thinner.

Design Considerations

Avoid the following when designing for an anodized finish:

  • blind holes
  • hollow weldments
  • steel inserts
  • sharp corners
  • heavy to thin cross sections

Weldments and Welded Aluminum Parts

When two or more parts are welded together, acid is entrapped in the weld and the area around the weld.  Color variations exist when a welding rod alloy is vastly different from the alloy used to make the part.  Halos appear around welds because of the high temperature used in the welding process.  The area around the weldment will be slightly lighter in color, causing the welded area to appear larger than it is.

Weldments and Welded Aluminum Parts
Weldments and Welded Aluminum Parts


Masking is required where no build up is desired or when a part needs both anodizing & chemical conversion. All threaded holes, 1/4″ or smaller, are typically masked when hardcoating unless otherwise specified. Holes with heli-coils must be masked. Any dissimilar metal (steel, brass, bronze) or any form of plating will burn off in the anodizing tank unless masked.