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Spot welding smut

We are clear anodizing a part made of two 5005 pieces spot welded together. Appearance is critical. Occasionally, caustic etching forms an unacceptable tenacious black smut in the spot weld area. We have tried to remove it with ammonium hydroxide and various concentrations and mixtures of nitric, hydrochloric, and hydrofluoric acids with no success. Mechanical cleaning is unacceptable due to the marks. A metallurgical analysis indicates high levels of copper. Does anyone have any ideas on how to prevent and/or remove this smut?

Chris Jurey, Past-President IHAA
Luke Engineering & Mfg. Co. Inc.

Wadsworth, Ohio

1999

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Chris, My experience with high copper alloys is that they respond well to a post etch treatment in what is commonly referred to as a 'tri-acid' de-smut. I use a home brew of 25% BV sulfuric acid, 25% BV water, 50% BV nitric acid and 50g/l ammonium bifluoride. A 10-30 second dip usually does the job. This also works well for high silicon alloys like castings, etc. Good luck..

Keith Wicklund, CEF-SE
avionics - Minneapolis, Minnesota
1999

YOU DID NOT SAY WHAT MATERIAL YOU WERE SPOT WELDING. ? WHICH IS A FACTOR . THERE ARE SEVERAL TRICKS TO SPOT WELDING AND HIDING YOUR TRACKS. AS BEST AS YOU CAN. SUGGESTION ONE WOULD BE FOCUS ON PREVENTING OR LIMITING THE SMUT . NOT REMOVING. NOW ALSO YOU DID NOT SAY WHAT GAGE MATERIAL ? WELL ANY WAY, IN MY EXPERIENCE WITH LIGHT SHEET METALS 26 GAUGE TO 16 GAUGE YOU CAN TRY PUTTING aluminum pieces OVER THE ELECTRODES (BETWEEN THE TIP AND THE OBJECT) FLAT STRIPS WORK WELL AND ALSO HELP PREVENT INDENTS. I HAVE ALSO SEEN A MECHANIC SWIPE THE TIPS WITH MURIATIC ACID BEFORE WELDING S.STEEL TO CLEAN THE TIPS OF RESIDUE FROM PRIOR WELDS BUILD UP. ALSO COPPER STRIPS ON S.STEEL WORKS.TO PREVENT THE CARBON DEPOSIT (FROM THE TIPS BURNING) AFTER ALL THEY ARE CONSUMABLES. AND S.S. WELDS QUICK .I HOPE I HAVE HELPED IN YOUR QUEST. MND

MIKE DAVIE
- SANTA MARIA CA
2000