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Volume 38, Issue 4, April 2003

The Aluminum Citings

Aluminum Finishing Alternatives: Presenting a Case in Defense of Anodizing

By Penn McClatchey

Many excellent articles have been published in industry publications concerning the great features and benefits of PVDF paint coatings for aluminum. About half of our company’s revenue comes from the application of PVDF and we value it highly as an architectural product. However, there is another aluminum finish that has not received such favorable treatment in the trade press. Anodizing doesn’t win the same respect as PVDF for many reasons, but its suitability as a building product is proven by years of success.

Advantages of Anodizing

Anodizing is a simple, electrochemical process developed more than 50 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. The coating is integral to the metal and cannot peel or flake.

The advantages of anodizing are as follows:

  1. In general, anodizing is less expensive than painting.
  2. 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.
  3. Anodizing cannot peel off because the coating is actually part of the metal.
  4. Anodizing gives aluminum a deep, rich, metallic appearance. An anodized coating is translucent and one can see the base metal underneath the coating. The translucence contributes to color variation problems, but computerized color matching with quantitative, objective color data is now possible.
  5. Anodizing is unaffected by sunlight, while all organic coatings will fail eventually due to exposure to ultraviolet light.

Avoiding Color Variations

Many users do not know that the most significant reason for color variation experienced in the anodizing process is due to variation in the metal. Most aluminum can be anodized, but most alloys are not attractive after anodizing. Different alloys anodize different colors, therefore some alloys are more suitable for anodizing (5005, 6063 ...) than others (2024, 3003 ...). Even when an anodizing alloy is used there is little hope for color consistency between different alloys. For example, 6063 and 5005 both are considered suitable for anodizing. However, 6063 and 5005 will appear different from one another when anodized, even when clear anodized.

A lot of metal comes from a batch of metal that was produced under identical circumstances at the same time. Even metal of the same alloy may vary because of alloying constituents, metallurgical structure, temper and the method of production for ingots and billets. A lot of sheet coil is rolled from consecutively produced ingots and is produced under nearly identical circumstances. Color consistency may be perfect if the rolling process is executed properly and it is anodized carefully. The coils are then cut to length into sheets, either before or after anodizing.

The only way to get sheet from a single lot is to order it all from one supplier in one shipment and to specify that the metal come from single or consecutive lots. By definition, different alloys cannot be made from the same lot, so sheet and extrusion will always be different in appearance from one another. In addition, a mill may not want to supply different gauges and widths from a single lot of metal. When they set up to roll a 40,000-pound ingot, they will probably want to roll the entire lot into a single gauge and width to minimize their set-up time. Normally one cannot order different gauges and widths from a single lot.

Extrusions are more complicated to buy in large lots because different extrusions probably come from different lots of billet. A billet is a casting of aluminum that is pushed through a die to make an aluminum extrusion. Extruders usually buy their billet on the open market and different shapes may come from billets that were cast in different cast houses. Some extruders buy most of their billet from one source. Anodizing color consistency will probably be better from an extruder like this, but still not perfect because the billet may be cast under slightly different conditions, even from the same casting mill.

Another process that causes color variation in anodizing is heating the metal. Heat bending is a crude process where an extrusion is heated using a torch and wrapped around a steel jig. This process changes the temper of the extrusion and often plays havoc with the color. I have seen curves that would not take an anodized coating at all because the aluminum was heated to a very high temperature during heat bending. Because of heating, welded aluminum is almost impossible to camouflage. Grinding makes it more obvious because of the difference in surface texture the grinding imparts. Welding also creates a halo in the surrounding metal because the temper is changed by the heat.

An analogy might help illustrate the need for using one lot of metal. A coil of aluminum is analogous to a bolt of cloth that a tailor would use to make a suit of clothes. A tailor would be irresponsible to make a suit of clothes from different bolts of cloth that were manufactured at different textile mills. A tailor is expected to manufacture clothing from a single bolt of cloth so that all the components of the suit will have the same appearance. A purchaser of anodized aluminum where color match is critical should do no less.

Alum20C2.gifPenn McClatchey is vice president and marketing manager for Southern Aluminum Finishing, based in Atlanta.