smutting that damn smut!

[kickn]

Okay - anyone got any ideas here? We are having some problems with desmutting. We are using a professional desmut solution (D7 by wirral ofspray here in the UK) just after a caustic etch. We rinse well and desmut for twice as long as we etch. We are still getting a residue left though - the only way to get it 100% is to run it through multiple times between the etch and desmut (just for say 10 secs each) and then whipe clean with some towel. After about the 4th or so times the towel comes clean and it can continue. Obvisouly not right and it did work fine a while ago. We refreshed the etch but that didn't help. The supplier says that the desmut should be fine. We test this with startch iodide papers to check that there is enought oxidiant in the solution and it comes up fine.

You can see the smut fall off the part when imersed in the solution but there still seems to be a very feint film!

Any thoughts?

[sswee]

I take it that the part is cast aluminum. I too have some parts that the smut problem is horrific. I have not found a solution yet. I have gotten it from several sources that nitric acid etch would do the trick but have not acquired any to try. If you find a solution, I would be very grateful if you would share it. SS

[kickn]

These are extruded parts so should be okay.....and I've tried 25% nitric acid and 25% sulphuric acid solution - no real difference.

[bp]

Try adding Hydrogen pyroxide to the solution. About 10% volume. This should help put any iron back in solution and help clean the parts.
If these parts are cast, secondary alloy's or 6063 alloy you still may have a hard time cleaning them.