Anodizing

Preface
This Wikibook will cover the technical subject of anodizing. It will include the following:
 * 1) An introduction to anodizing including its history and applications
 * 2) The theory of anodizing
 * 3) A list of metals that can be anodized along with recipes for anodization.
 * 4) Coloring of anodized metals
 * 5) The equipment used in anodizing such asː tanks, power supplies, water treatment systems, etc...

Introduction
Anodizing is a method for growing oxide films on metals for the purposes ofː
 * 1) Corrosion protection
 * 2) Scratch resistance
 * 3) To provide a base for coloring or painting.

Aluminum and Its Alloys
The native oxide on aluminum is 40 to 50 Å thick. It provides some protection from corrosion but because it is thin, porous and mechanically fragile, it cannot provide a robust degree of protection. One way of improving corrosion resistance is to grow a thicker oxide film. This can be done in the following waysː
 * Heating in air or oxygen
 * Treating with oxidizing agents
 * Anodic polarization

Oxide films created by heating (thermal treatment) are also thin and weak. Oxide films created by treatment with oxidizing agents do not provide appreciable corrosion protection, but the can provide a good base for paint or varnish. By anodic polarization (also known as anodization) it is possible to grow oxide films up to 200 μm thick which provide good corrosion protection and can improve other properties such as scratch resistance.

A typical aluminum anodizing process uses the following process parameters: A processing time of 10 to 60 minutes will produce an oxide thickness of 2.5 to 25 μm.
 * Bath = 15% solution of concentrated sulfuric acid
 * Current Density = 1.3 A/dm²
 * Voltage = 18 to 24 Volts
 * Temperature = 20 °C

Coloring


There are a few dying options available the main two groups at absorptive and electrolytic.

With electrolytic you can achieve gold bronze and black in a gradient type fashion. Pending on the current density, temperature, and time the part is in the bath for dye determines the color obtained.

For absorptive you must decide between azo dyes, organic dyes with a heavy metal (usually chromium), and heavy metal free organic dyes. The deciding factor is primarily light fastness and if your part or product can contain heavy metals.

Links

 * 1)  Wikipedia article on Anodizing
 * 2)  Aluminum Anodizers Council
 * 3)  International Hard Anodizing Association