Manufacturers who frequently work with metal understand the importance of the anodizing process. Anodizing is an electrochemical procedure in which a metal surface is altered so that it has a durable, corrosion-resistant finish. The protective layer is formed from an anodic oxide that will last much longer than the original surface. Aluminum is one of the materials that is best suited for anodizing because of its particular characteristics.
What is the anodizing process?
The way anodizing works is that an anodic oxide structure forms from the aluminum substrate. This layer is made up solely of aluminum oxide. Unlike a surface layer that has been painted on or plated, the aluminum oxide is actually part of the underlying aluminum substrate. This means that it will never peel or chip. Not only that, but the anodized structure is both highly ordered and porous, so it is ready for follow-up processes to be applied to it, such as coloring or sealing.
The process is known as anodizing because the material that is being treated forms the anode electrode of an electrical circuit. The aluminum (or other suitable metal such as zinc or magnesium) is immersed in an acid electrolyte bath. An electric current is then passed through it. Because a cathode is mounted on the inside of an anodizing tank, the metal becomes an anode, thus releasing oxygen ions that fuse with the surface aluminum atoms. While anodizing is actually a naturally occurring phenomenon, the mechanical process causes it to happen much more quickly and on a wider, uniform scale.
There are a number of reasons why aluminum alloys are anodized, including increased corrosion resistance, easier dyeing, or improved lubrication and adhesion. While improving its durability, anodizing does not affect the strength of an aluminum part and the newly formed layer is non-conductive.
Pure aluminum, when exposed to oxygen at room temperature, forms a surface layer of amorphous aluminum oxide that is up to 3 nm thick. This layer offers good corrosion protection. Most aluminum alloys are able to form even thicker oxide layers, as much as 15 nm thick. However, depending on the alloy, the protection offered against corrosion is lessened compared to pure aluminum. This is why aluminum alloys used in manufacturing are anodized. The process greatly enhances the naturally occurring corrosion resistance, which is one of the great benefits of using aluminum rather than other metal options.
How to select the right aluminum alloy for anodizing
Not all alloys offer the same corrosion resistance. For instance, certain alloying elements, such as copper, iron, or silicon, mean that those are alloys are more at risk of corrosion. This, in particular, applies to 2000-, 4000-, and 6000-series alloys.
Anodizing generally produces a very even and uniform coating. However, extremely tiny fissures in the coating that are invisible to the naked eye often form, and these can increase the potential for corrosion. Another problem is that the coating can be weakened by the presence of high- or low-pH chemistry. This will strip the substrate coating at the molecular level. These are some of the primary reasons that selecting the right alloy for anodizing is so important.
Of course, the type of anodizing is also an important factor. With decorative anodizing, the process produces a softer coating that is more easily used with cosmetic work. Hard anodizing offers a much thicker, more durable coating and is used with applications that require the extra toughness. Sulfuric and chromic anodizing can generally be used with non-decorative applications. Other aspects, such as casting and temper state, also affect the choice of alloy.
Let’s quickly run through the various aluminum alloy series. Starting with 1000, these alloys are relatively soft with high conductivity, and they have clear, bright coating properties. While they are good for decorative work, their other shortcomings often point people in a different direction.
2000 series alloys rely on copper as their alloying agent. They are known to be strong and hard, but their coatings tend to be yellow and offer poor protection. This is due to he high copper content. It is possible to use 2000 series in the hard anodizing process, but it is difficult to get a truly hard surface and so care must be taken.
With 3000 series, the alloying agent is manganese, which allows for strong, small grains. The coating comes out grayish-brown, and it can be difficult to match the color from sheet to sheet. This alloy is used extensively in lighting applications.
4000 series alloys, which are mixed with silicon, produce a strong, fluid metal. The coating properties tend to a dark gray color and, when finishing, there is often dark black smut that is very difficult to remove. These alloys are most often found in architecture and welding applications.
Using magnesium as an alloying agent, 5000 series aluminum is known to be strong, ductile, and fluid. The coating is generally very solid and offers good protection over the lifetime of the part. If using 5005, when finished it is recommended to keep the silicon at less than .1% and you need to watch out for oxide streaks. When used with hard anodizing, certain properties such as wear resistance will be compromised, making it less than ideal, so other anodizing methods are preferred, including non-decorative sulfuric and chromic anodizing.
For 6000 series alloys, both magnesium and silicon are used. These alloys are strong and ductile and offer a clear coating with good protection. When finished, keep the matte-iron at less than .2% and the bright iron at less then .1%. When hard anodizing, the best results are usually found with 6061 and 6082 alloys. 6063 is a favorite alloy for cosmetic work.
Finally, 7000 series alloys use zinc as the alloying agent and they are considered very strong with a clear finish. These alloys are popular in the automotive industry. When finishing, over 5% zinc will result in a brown coating. The flexibility of these alloys also allows them to be used with many different anodizing techniques.
If you’d like more detailed advice about what alloy to use for your anodizing application, contact Clinton Aluminum today. Our motto is the right alloy for the right application!