The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Red smut after bright dipping

I have recently developed a red/rust smut on batch anodized sheets after bright dipping. The smut does not become visible until after a deox rinse (HNO3 + water). It does not rinse off, even if I use a high pressure spray rinse. I have to actually wipe it off by hand. The smut is not removed by the anodize bath. If I anodize the piece without removing the smut, this smut ends up under the coating, adding a red tint to the piece.

Any clues?

Gerald Janssen
aluminum coil anodizing - Streamwood, Illinois
1998

---

1998

Gerald,

I have seen the problem you refer to before. When aluminum is cooled after the manufacturing process, if the cooling is uneven Silicon will migrate to the last areas to cool. These high Silicon areas will develop the smut you refer to. You need to watch the manufacturing process for areas where the aluminum overlaps during the cooling phase of the operation. Any touching of sheets or stacking on uneven substrate may cause the same effect.

The problem is in the operations prior to anodize line.

Good Luck,

Mike McDonald
mack products - Jefferson, Wisconsin

Gerald:

If the alloy contains Manganese (Mn), there is a good chance it is MnO3. try a sulfuric acid dip after the nitric desmut.

good luck,

Paul Stransky
- Putnam, Connecticut
1998

---

We are a fellow job shop anodizer & also do bright dipping. We do not do coil work, BUT I think I've recommended you before.

My boss would like to know the alloy before he can help you BUT he said it may be a buildup of copper in your bright dip causing the red color.

David A. Kraft
- Long Island City, New York
1998

We recently had a red smut issue after our deoxidizer process as well. Upon my research, if the deoxidizer gets more then 200 PPM of copper into the tank, then the copper will re-deposit itself back onto the parts. We pulled off about 1/4 of our tank and remade. This addressed our red smut issue.

Mervin Archer
- Grove, Oklahoma, USA
August 3, 2011