Grainy Structure in 7075 anodized parts

[mick]

Well I am new to anodizing - have had some of my CNC machined components anodized commercially here in Australia. The last batch have come back (finishes of bright black,blue and gold)with a grainy structure throughout them and the anodizing is very blotchy and uneven. The gold is especially bad. The material we use is 7075 T651. The finish was also a lot duller or matt than on previous times.
Can anyone help us with reasons this may be happening - would it be the structure of the material or the anodizing procedure? We anodize clear small parts in our own workshop very effectively but the effect is still there but not as dominant. I can provide a picture if this helps determine the cause.

[acidrain]

I'm thinking the surface of your parts was overheated during machining. I've seen this before on other parts... not sure how they got that way, but here is what I would do:
Try alternating etch and de-smut until the parts look more even. If the parts are not glossy enough, you may have to buff them. If these parts have machined tolerances, there may be a problem loosing a couple of thousands of an inch during this process.

[mick]

Thanks for that thought - we don't feel that the material would have been over heated in machining though so there must be other reasons, but we won't rule that one out. Yes,tolerances are very important in our parts as bearings are fitted internally. We have heard that the higher zinc content in the 7075 can cause differences in colour take up compared to other grades??

[M_D]

I don't know exactly what your parts look like, so this may or may not apply.

One type of grainy appearance is from metal removed in the etch or bright dip process, which brings out the grain of the metal. There will be very small dark steaks oriented with the grain. The longer the etch, the more the grain will become pronounced. It is more ovious on bright shiny surfaces than on duller surfaces.

Also, the parts may have been stripped and re-anodized. Stripping with a sodium hydroxide (lye) stripper will tend to leave a spotty, galvenized look, which is still obvious after anodizing. Also, etching too long will do the same thing.

[mick]

Hi Guys yes that sound like what it could be thanks

Mick