Problems with cast alum

[sswee]

I just wanted to see if anyone was doing much with cast alum. and was getting any consistant results. If so what are you doing to achieve them. The only thing I have trouble with is cast. Excessive smut and turning medium to dark gray in the anod. tank.

[Tim Wiltse]

sswee,

Castings are a whole different animal. Do you by any chance happen to know the alloy type. I believe the best of the casting types is 356 alloy. I am working on a project that are made of castings myself. I am working on getting my Desmut made up now. The hard part is tracking down one of the needed chemicals for it.

Later,
Tim

[sswee]

I haven't a clue on the grade of cast. I've encountered the same problem on automotive, motorcycle and sporting parts that were cast alum. The parts were beaded and polished, etched, DeOx/DeSmut. When the parts come out of the anod. tank, they are med. to dark gray and you can see what looks like something that was not mixed good in the metal. I am still trying different processes but haven't found anything to give the desired results.

[Tim Wiltse]

For Die cast parts I believe the right Desmut bath is 50%vol Nitric acid and 1lb ammonium bifluoride, that seems to be the pain to find. This will be the mix I use once I get my paws on the ammonium bifluoride.

Later,
Tim

[sswee]

Are you saying to combine the Nitric acid of 50% by volume to 1 lb. ammonium bifluoride. If so what volume of Nitric to the 1 lb. ammonium bifluoride? Have you talked to anyone using this mix? I found a few listings for distributors but most are far from me.

[Techno]

I'm not much up on this but read that cast aluminum is 'contaminated' with silicon, I think it was silicon. 'cantaminated' for anodizing.
Makes it blotchy. It was one of the elements used in cast aluminum VS non casting aluminum. I don't think its the casting process itself.
Pity since I cast aluminum.