Metal finishing Q&As since 1989

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Problems anodising turned 7075 T6 aluminum

2003

We are having problems anodising parts that are turned from 1 1/4" 7075 T6 bar. The parts have to be colour anodised, as the cosmetic finish is very important, but virtually all of the parts, that were machined from the same batch of 7075, end up looking like they have been coated, not anodised. Even if we clear anodise it, the parts come out in varying shades of grey. Our supplier tells us that the 7075 was sourced from a Mill in Italy, but that the analysis report shows the percentages to be within the acceptable tolerances. Whats strange, is that when we anodise parts that were machined from 7075 produced in the U.S., sent to us by our U.S. affiliate, it anodises fine. We gave the anodiser several turned samples, and asked him to anodise the whole lot together. They were all dipped & removed at the same time. All the U.S. sourced samples were acceptable so we figured it had to be the material, but then we had 2 samples produced from the Italian material that came out ok, the rest was bad. Does this sound like a problem with material, or could it be something to do with the anodising process or contamination? We know that the anodiser has been anodising some 2000 grade aluminum for another customer.

Jackie Sosta
- Colchester, Essex, UK

Assuming your observations are true, this may be a probable case of a material whose alloying elements are not homogeneously mixed.

Dado Macapagal
- Toronto, Ont
2003

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Gents,

We have had similar problems with 7075. We wanted to coat it with red and we ended up getting grey striated red (not a desired result but quite groovy!).

I have spoken to quite a few about this and the problem seems to be many faceted. One problem being the homogeneity and the other significant reason seems to be the surface finish (Not sure about this).

More inputs will be greatly appreciated.


Best

N

Niranjan
- Vienna, Austria
2007