What is Anodizing?
Anodizing is an electrochemical process that creates a thick protective finish on a piece of metal in a rapid and controlled manner.
This finish is strong, durable, corrosion resistant, and highly porous. The porosity also allows it to be used for decorative purposes. Dyes and sealants easily seep into the anodized surface and stay there. This is how you get all of those brilliantly colored aluminum products, for example.
Japanese scientists developed and patented the process in 1923, initially using it to protect aluminum-copper alloy parts on seaplanes from corrosion.
What can be anodized?
While any nonferrous metal can be anodized, aluminum is the most common. Titanium is the second most common, but it is nowhere near as popular as aluminum. If you try to anodize ferrous metals, such as alloy steel, they will just corrode and damage the material.
How does the process work?
Without being overly technical, the metal is submerged in a tank filled with an acid electrolytic solution. Next you pass an electrical current through the aluminum, which serves as an anode. Then place a cathode into the tank, usually made from aluminum or lead. The resulting electrical current pulls oxygen ions out of the electrolytic liquid. This causes rapid, controlled, oxidation on the surface of the piece.
Aluminum and titanium do naturally oxidize, but the layer created through anodiziation is thicker than what happens in nature.
There are numerous methods and typed of acids used for the process. However, those are a bit complicated and beyond the scope of this blog post. If your curiosity is piqued, check out the Aluminum Anodizers Council. They have much more technical explanations.
What is anodizing used for?
Anodized metal, especially aluminum, is used for all sorts of applications, Some are literally out of this world.
NASA uses anodized aluminum on spacecraft and satellites. Spacecraft see massive temperature swings in space, depending on if they are exposed to sunlight or not. But anodized metal is less thermally conductive, reducing the impact of these rapid and dramatic changes. NASA also anodizes its handrails a bright gold color to help them visually stand out to astronauts.
More down to earth uses include architectural structures, appliances, interior decoration, food preparation equipment, sporting goods, boats, oil rigs, docks, motor vehicle components, firearm components, jewelry, and artwork.
In daily life, you are most likely to encounter anodized aluminum smartphones, multitools, flashlights, cookware, cameras, window frames, roofs, pocket knives, and jewelry.
With excellent durability, low maintenance, superb corrosion resistance, and ease of applying cosmetic features, anodized metal has countless uses.
Why is it so durable?
Largely this is because it isn’t a clear separate layer. Consider paint or plating, where you have a thin layer of material attached to an underlying layer of metal. This can be scratched, scraped, or flake off. Anodizing physically changes the outer layer of the material, so it is melded to the rest of the metal. It therefore cannot be scratched or scraped off since it is part of the metal. The only way to remove the anodizing is to gouge the metal straight through the oxide. And if that is happening, you’ve got bigger problems than flaking or peeling.
The anodized layer is an oxide too, so it won’t oxidize further, which is what makes it so corrosion resistant. And due to the porosity of the anodizing layer, its both durable and can be given sealants, paints, and other hardening surfaces to further enhance toughness. These sealants seep into the metal and bond with it it in a much stronger fashion then just putting a layer on top.
Any other benefits?
Beyond durability of both the finish and any applied colors, there are numerous other reasons to choose anodized metals. Generally, components with an anodic oxide finish wind up having a lower cost. The process is itself inexpensive, and the durability of the finish results in lower maintenance costs and superior long-term value. Further, it is chemically stable and will therefore not decompose. The finished metal is also non-toxic, and heat-resistant.
What about welding?
Welding is the only real weak spot for this process. Because the anodic oxide layer is very hard and not thermally conductive, you can’t really weld it easily. Furthermore, the melting point for the oxide is typically higher than that of the base aluminum below. This adds extra complications with welding. Lastly, due to the rigidity of the material, it is more likely to crack from the heat than melt and weld. As such, it is inadvisable to attempt to weld anodized aluminum.
What’s the environmental impact?
This is another great benefit for the anodizing process! It is one of the most environmentally friendly metal finishing processes available. A little bit of aluminum hydroxide and aluminum sulfate are produced, both of which are easily recycled. This leaves a small amount of waste, typically halogens and volatile organic compounds, that are easily disposed of.
We hope this answers your questions about anodizing and helps inspire some ideas about what this fascinating finish can do for you!