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Black Stain in 316L Weld Heat Affected Zone

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Hello,

I'm with a company that builds high purity filtration skids for pharmaceutical companies. All of our systems consist of 316L stainless steel tubing that meets the ASME BPE-2002 material spec and is welded with an automatic orbital welder using the GTAW process, backed and purged with ultra-high purity Ar. Once all of our manifolds are welded and inspected, we send them out for an outside surface electropolish (all outlets are capped to prevent anything from touching the inside) and an ID/OD nitric passivation. When our parts come back from our vendor, we occasionally see a black stain on the edge of the heat-affected zone of some welds inside the tubing. This only appears on about 5% of the parts we send out, and usually only on 1 or 2 welds, and usually on welds near an opening in the tubing. I've monitored the various steps to our manufacturing process, and also our vendor's passivation/EP process, and it seems that the black staining occurs during the electropolish stage. The thing that troubles me is that our parts are sealed - the EP should not be affecting the inside of the tubes at all. The stain also never shows up on any welds that have been polished - whether it's been polished completely out or just lightly scotch-brited. The black stain doesn't wipe out with the chemicals we�ve tried (acetone and sodium hypochlorite), so the corrective action is not simple--anything we do to remove it requires another trip to be EP'd and passivated again. Any ideas on what the staining might be and how we can get rid of it? I'm attaching pictures, but the stain is difficult to get a picture of with the equipment available to me.

If any more information is needed, please let me know - I didn't want to get too exhaustively specific unless it's necessary.

Thanks in advance for any help you can give!

Anthony B [last name deleted for privacy by Editor]
- Uxbridge, Massachusetts

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