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Anodize weight versus fatigue performance for thin anodizes

I work for an aircraft manufacturer, and we are evaluating alternatives to chromic acid anodize. I am trying to determine what fatigue data is available to justify the usual coating weight limits for chromic, thin film sulfuric, and boric sulfuric anodizes.

The usual coating weight limits at places I've worked were 200 mg/ft2 minimum, and 600 or 700 mg/ft2 maximum, and as long as the maximum coating weight was not exceeded, no fatigue performance penalty was taken.

As for how the upper limit was established, I do not know anyone who can tell me that. Did any of the government labs do tests at some point in the past? And also did anyone think to do those tests for each type of anodize (chromic, sulfuric, boric-sulfuric) in case there is a difference, or was it assumed that it's OK because it's all 99 percent aluminum oxide anyway? I am interested in data for both 2024 and 7075 alloys.

I already did a search through some threads on finishing.com, and I also tried looking in the ASM handbooks and didn't find anything in any of the sections I thought to check.

Note that I am not looking for any fatigue data on the Type III (hard) anodize. I am just trying to figure out who established (way back whenever) that a 600 or 700 mg/ft2 anodize would not affect fatigue performance.

Thanks if you can point me to any references.

Jim Reynolds
Materials Engineer - Savannah, Georgia, USA
January 12, 2011

I don't know what entities originally performed fatigue testing on anodized aluminum, but I am sure that the major aerospace companies like Boeing did, as well as various government entities (USAF, NASA, etc.). If you really want to dig into this, then you should obtain access to a scientific database like Cambridge Scientific Abstracts and perform a keyword search that is limited to a certain time range, like 1930-1960. However, a quick Google search (limit it to .pdf files) will turn up a number of technical papers that show fatigue data of Al anodized in any number of solutions, with the same general trend: coating thickness greater than ~ 5 micrometers has a larger negative impact on fatigue than those in the range of 1-5 micrometers.

Toby Padfield
Automotive component & module supplier - Michigan, USA
January 18, 2011