By: Devin Frerichs, Date: 2/13/2018

Anodizing is a process in which a layer of metal oxide is created on a part using electricity and chemicals. It is called anodizing because the part to be anodized is placed into an electrfied chemical bath (most commonly sulfuric acid) with a cathode, and it acts as the anode. This oxide layer serves to protect the underlying metal from corrosion and abrasion. Additionally, the surface created through anodizing is porous so it allows dyes to be applied and sealed into the pores to color the part. Anodizing is most commonly used for aluminum, but can be applied to other metals like titanium, zinc, and some other more exotic metals like tantalum.

The first commercial anodizing process was chromic acid anodizing (type I), patented by Bengough and Stuart in 1923. Sulfuric acid anodization (type II and type III) was patented in 1927.  Anodizing using phosphoric acid is an uncommon but newer form created by Boeing, used for preparing surfaces for adhesion.

Any design that uses aluminum would be a suitable candidate for anodizing. It is used in everything from home appliances and gadgets, to exposed framework in buildings, to parts on airplanes and satellites.

Over other processes, anodizing has the benefit of being stronger than paints or plating because it converts the top layer of aluminum into oxide, so it is fully integrated into the part rather than sitting on the surface. The coating also does not peel or chip, is very hard, and lasts indefinitely.

Tolerances:

• Chromic acid anodizing (type I): Creates a 0.5 μm to 18 μm coating
• Sulfuric acid (type II): Creates a 1.8 μm to 25 μm coating
• Hardcoat sulfuric acid (type III): 13 μm to 150 μm

Shortcomings:

• Hardcoat anodized layers can interfere with tolerancing for things such as precision fits and fine threaded holes
• Color cannot be perfectly matched because it depends on the individual part properties
• Finish cannot be touched-up easily
• Any surface imperfections will be visible on the anodized coating

Step-by-step process: (industrial anodizing)

1. Parts are cleaned to remove dust and oils, then rinsed
2. Parts are submerged into an etch tank to remove surface defects such as die lines and scratches with caustic chemicals
3. Parts are moved to the deoxidizing tank to remove surface residue from etching that would interfere with anodizing
4. Parts are moved to anodizing tank, then power is ramped up to begin anodizing process. The solution in the tank is vigorously agitated for better coverage
5. If applicable, parts are dyed in a tank of appropriate dye
6. Parts are moved into the sealer tank (usually boiling de-ionized water) for 15-30 minutes
7. Parts are dried with air

Mechanism:

The layer of anodized aluminum is created by passing an electric current into an electrolytic bath containing a cathode (positive electrode) and your part (which acts as an anode). The current releases hydrogen at the cathode, and oxygen at the aluminum part which creates the oxidative layer. The thickness of the layer depends on the bath used, as well as how long the process is applied. The layer of oxide is porous, so dye can be applied to rest into the pores or it can simply be sealed for 'clear' anodization.

Skills:

Because the anodizing process is fairly straightforward, it could be done at home by an inexperienced person fairly easily.

The cost of industrial anodizing lines varies based on the size. A small setup could cost between $30,000 and $75,000. A medium or large scale setup might cost $100,000 to $150,000, and could require a hoist to move the parts between tanks. Additional costs may include acquiring permits, and the actual cost of all the chemicals involved.

An estimated cost of having a relatively small or simple part anodized in an industrial scale is about $0.10 to $0.25 per part.