By Tammy Schroeder, LEED Green Associate
‘All aluminum can be anodized’ is a valid statement, but can be misunderstood and lead to failure in managing expectations for color variation. It would be clearer to state, ‘only aluminum can be anodized,’ while the other metal constituents (i.e. zinc, magnesium, silicon) in the alloy do not respond in the same manner.
The variance of metal constituents in an alloy is the major reason for color variation. Other variables affecting color variation in the anodize process include temper, anodize tank chemistry, shape geometry, and material load size.
Anodizers control their tank chemistry (e.g. temperatures, solution concentrations, and the time material spends in each tank), but they do not control the alloy, temper, or shape of the aluminum parts. These variables can make it extremely difficult to achieve an exact color from run to run and load to load.
To minimize color variation:
- maintain metal consistency—the easiest way
to ensure this is to work with one metal source/extruder per project and request all metal come from one lot of material;
- do not mix aluminum alloys (even mixed tempers will not produce uniform results)—for best results, use 6063 alloys for extrusions and 5005 for flat sheet stock and fabricated parts (when structural alloy is required, 6061 and 5052 can be used, but will not give similarly acceptable results);
- perform as much bending and forming as possible prior to finishing—anodic films are very hard, and as a result most post-production bending causes the film to ‘craze,’ which produces a series of small cracks in the finish, giving it a spider-web like appearance;
- be aware of anodizing’s effect on welds—the heat developed from the welding process changes the metallurgy on nearby metal or heat-affected zones, causing localized discoloration (i.e. halo effect), so one should use the proper 5356 alloy welding wire and lowest heat possible; and
- select an anodizer using automation to reduce inconsistencies in the anodize process.
The number of variables in the anodize process make it necessary to use range samples rather than a single sample as is commonly used with a painted finish. An anodize range sample is a set of two anodize color chips for a single color. The set includes light and dark samples to provide a visual reference to represent the extremes of appearance to be expected on the finished parts.
Range samples are not always from the same alloy, temper, and shape that will be used on the project—they are meant as an illustration of the degree of possible color variation and may not be an exact representation of the color achieved for the actual project.
Most applicators use an automated system to control and monitor the products through the entire anodizing process. The system tracks all aspects of the process (i.e. tank sequencing, voltage, current, time, and temperature), ensuring the most consistent anodize finish.
The American Architectural Manufacturers Association (AAMA) offers a specification stating the acceptable range shall not differ by more than 5 Delta E. However, when various suppliers, alloys, etc. are used, color uniformity may vary in excess of 5 Delta E. The highest-quality anodizers strive to keep the range from 1 to 3 Delta E. The lighter the anodize finish, the more noticeable the range; a champagne color variation is more noticeable than is a dark bronze finish.
The opinions expressed in Failures are based on the author’s experiences and do not necessarily reflect those of The Construction Specifier or CSI.
Tammy Schroeder, LEED Green Associate, is a senior marketing specialist at Linetec and serves as an industry educator on architectural coatings. With 18 years of experience in the finishing industry, she shares her knowledge of liquid paint coatings, powder coat, and anodize with architects, specifiers, and architectural product manufacturers in commercial and residential markets. Schroeder can be reached via e-mail at firstname.lastname@example.org.