Anodizing is a simple electro-chemical process developed more than 75 years ago that forms a protective coating of aluminum oxide on the surface of the aluminum. The lifetime of the finish is proportional to the thickness of the anodic coating applied.
Aluminum oxide is a hard, durable, weather resistant substance that protects the base metal. The coating may be colored by dyeing or may exhibit bronze tones through diffraction phenomena produced by the coating. The coating grows from the base aluminum metal by this electrochemical process.
The coating is integral to the metal and can not peel or flake. The structure of the coating is many small hexagonal pores, which are filled with a “seal” that hydrolyzes these pores to fill them with inert aluminum oxide.
ADVANTAGES OF ANODIZING
- In general anodizing is less expensive than painting with the exception of coil painted products.
- Anodizing is harder than PVDF. Anodizing is better for aluminum in high traffic areas where the coating is subject to physical abuse and abrasive cleaners.
- Anodizing cannot peel off. The coating is actually part of the metal.
- Anodizing gives aluminum a deeper, richer metallic appearance than is possible with organic coatings. This is because an anodized coating is translucent, and one can see the base metal underneath the coating.
This translucence contributes to color variation problems, but anodizers are doing a much better job of controlling the amount of color variation than in the past.
- Computerized color matching with quantitative, objective color data is now commonplace in most anodizing facilities.
- Anodizing is unaffected by sunlight. All organic coatings will eventually fail due to exposure to ultra-violet light.
There is a big difference between good and bad anodizing. Anodizing which is improperly sealed has poor chemical resistance. Brand new anodizing with a thin coating thickness is nearly identical in appearance to an Aluminum Association Class I (very thick!!) anodized finish, but thin (sub-Class II) anodized coatings are unsuitable for use on exterior curtainwall or metal roofing.
The advantage of a thicker anodic coating is its durability and longer life. The Achilles heel of anodizing is it’s chemical resistance. Eventually the surface of an anodic coating may succumb to acidic pollutants in urban environments. Anodized surfaces, like other building components, must be protected from acidic attack during construction. The life expectancy of an anodized coating is determined by its thickness and the building’s environment.
After many years anodized surfaces may accumulate dirt and stains that look similar to chalking paint. This “chalk” can be removed with a mild detergent combined with an abrasive cleaning technique.
A small amount of the anodic coating can actually be removed, leaving behind a renewed anodized finish which can last for another twenty years.
This is why anodizers say their product is “renewable”. Once an organic coating has failed, the only options are to re-coat the surface with another paint or replace the metal. When an anodized coating appears to have failed, cleaning often results in a renewed appearance.
ANODIZING AND THE ENVIRONMENT
Anodizing appears to be compatible with today’s environmental concerns. Though more research needs to be done to determine the total environmental impact of different aluminum finishes, from a finisher’s point of view, compliance with environmental regulations is easier than with other metal finishing processes.
There are anodizers positioned in every major urban area and anodizing may help products qualify for LEED (Leadership in Energy and Environmental Design) points based on regional materials
The chemicals from anodizing can be used by municipal waste water treatment facilities. The aluminum sulfate from anodizing plants’ effluent actually improves the solids settling efficiency of some waste water treatment plants.
Anodizing byproducts are often beneficial, like when aluminum trihydrate is often used as a fire retardant. Anodizing emits no ozone producing solvents (VOCs), and there are no heavy metals involved in the process.