How to know if chemical stripping of hardcoat attacked my aluminum

What is the alloy?

Do you understand that anodizing is 1/2 under the original surface and 1/2 on top of the original surface, therefore when you strip the aluminum oxide you are under the original surface by 1/2 the thickness of the oxide? You can never get back to the same original surface again.

If you were 0.002 on top of the original surface and you stripped, then you would be 0.002 under the original surface and would have to apply 0.004 to get back again.

I don't use that particular etching formulation, so I don't know that this is the case, but... It sounds like you just developed some smut on your parts. A proprietary desmutter might be a good bet, otherwise I believe straight nitric CAN work (although not as well). With regard to the straight nitric - I don't know for certain that it will do the trick nor what else it might do; I use a proprietary blend for desmutting and always recommend the same. But, it may be worth trying on a piece or two to see what happens.

Also, you may want to search the archives for desmutter formulas.

Good luck!

Sounds like the impurities in the aluminum are surfacing after the stripping. Try etching the parts briefly (2 minutes) and then using your deoxidizer afterwards @ 5-7 minutes. after the deox the parts should be white in appearance and ready to plate. You can also used a nitric acid stripper to strip anodic coating without attacking the aluminum surface. I find nitric a better beast to play with than Phosphoric. The makeup for Nitric stripper is 50% 42 deg. Nitric and 50% DI water. It takes a bit longer. 15-30 minutes roughly. But I like it better than the alternative.

First guess: A Cu- & Fe-containing smut, similar to one formed in phosphoric-nitric bright dip, from dissolution and precipitation into the low spots of the blasting. Will remain dark in caustic, come clean in nitric.

Second guess: A chromium phosphate conversion coating, with the high temperature compensating for the lack of fluoride. Might come off in caustic, leaving the normal smut per the alloy to remove in the deox/desmut.

Third guess: Si alloying left after galvanic attack of the aluminum. Use fluoride to remove.

For future anodize stripping, add phosphoric and DI water to approximate the standard formula: 20 g/L chromic acid + 35 ml/L of phosphoric acid (85%, SG 1.69).