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Variability in 3005 Anodized Aluminum Finish

1998

Our anodizing facility processes small aluminum parts for the cosmetics industry. We have had an ongoing problem with variability in finish/appearance when using 3005. As our customers are much more concerned about aesthetics rather than functionality, this is a major crisis.

We process 3005 parts by actually bright dipping, followed by a Phos acid bath which results in a satin appearance (we also due a standard caustic etch). What has been observed is some lots of 3005 result in parts that are brighter than other lots of 3005 (in both processes). We have run controls of different lots of metal to eliminate process variability. Chemical analysis has shown a recurring theme of low Fe and Cu levels in brighter 3005 parts. Initial results show no difference in grain structure for the bright/dull finished parts. We are going to attempt to have our metal suppliers provide 3005 with min. of 0.4%Fe and 0.1%Cu in hopes of reducing variability.

Can anyone offer any explanations or solutions to our problem? As I have to provide a technical response (with background info) to our customers, are there any good sources on metallurgy of Al and its affect on anodizing?

Sincerely,

Margaret (Peggy) Scully Wessman
- Newark, Ohio

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Typically, an elevated level of iron will inhibit the brightness of an alloy while higher copper levels will improve brightness (brightness being defined as the specular reflectance off of a reflective surface). This can be seen in the 6XXX series; specifically 6063 vs 6463. 6463 is the primary bright extrusion alloy and is a modification of 6063. In 6463 the iron has been reduced and the copper increased. Silicon has shown to decrease the brightness of aluminum alloys. In the 6XXX series it is alloyed with magnesium and the resulting magnesium silicide is the major alloying element. Excess silicon over the stoichimetric ratio leads to decreased brightness. You may want to contact the metalurgist of the suppliers of your metal.

Lee Branch
Richmond, Virginia
1998