Metal finishing Q&As since 1989

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Fungus resistance of anodic coatings

This sounds like a dumb question even to me, but I really need some help.

I'm writing a fungus resistance report for a product being manufactured in accordance with military specifications. The product is made of 6061-T6 aluminum and anodized in accordance with MIL-A-8625 / MIL-PRF-8625 [on DLA] F, Type II, Class 2 and Type III, Class 2 (abrasion prone areas).

"Everyone knows" that anodized parts do nut support growth of fungus and are not susceptible to the effects of fungus (i.e. corrosion caused by fungus byproducts), but I can't find a reference for this anywhere, and I need to cite a respectable reference source in the report. MIL-HDBK-454A, Guideline 4, lists "metals" as fungus-inert materials. Can an anodic coating be considered as a metal?

Thanks,

Yossi David
- Israel
2003

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2003

First, the easy question. "Can an anodic coating be considered as a metal?" --- No. An anodic coating is an oxide, which is an ionic compound without the free valence electrons characteristic of a metal.

Second, can anodized parts support the growth of fungus? --- In answering questions like this, never say never. I consider sealed anodize to be a non-porous, inorganic surface not suitable for yeast. Although I haven't seen fungus growth on anodized aluminum, fungus (yeast) can grow on some of the dyes used for Class 2. If the anodic coating seal was destroyed (e.g., by acid or abrasion), dye molecules in the open pores could conceivably support the growth of fungus. But, it is also conceivable that the small diameter of the pores would prevent entry of the fungus.

The Surface Treatment and Finishing of Aluminum and Its Alloys by S. Wernick, R. Pinner, P.G. Sheasby shows typical pore diameters for Type II and Type III of up to 585 and 170 Angstroms, respectively. A filtration guide shows that these small pore diameters would keep out bacteria and yeast/fungi. (http://www.osmonics.com/library/filspcold.htm) Hence, I would conclude that anodic coatings per MIL-A-8625F, Types II and III, cannot support fungus growth. Even unsealed anodize no longer meeting the spec. is inhospitable to fungi. There is a possibility of dye leaching out of the pores, but that would be equivalent to spreading food over the surface.

Of interest to anodizers, paint pigments and blue indigo dye are too large to enter the pores.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.