Metal finishing Q&As since 1989

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Controlling weld discoloration on 304L stainless

2003

Hi there,

I am new to this q&a board so please forgive me if this question is redundant. I'm looking for a way to fix the discoloration/burn through of some tig weld through the back of some 304L ss sheet metal. For the longest time, we have simply wire brushed or fine grit scrubbed the welds if they are visible. This still leaves a scratched surface which is noticeable, which I would like to get rid off.

If anyone has any suggestions I would greatly appreciate it.

Peter Dietrich
- Ontario, Canada

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2003

Options for removing discoloration:
1) Modify present method to make smaller initial scratches. Use a finer SS wire (0.006‡ dia.) brush with less pressure, switch to a flapper wheel or glass bead blasting, etc. Finish with successively finer grit nonwoven wheels.
2) Use a nitric acid/fluoride pickling solution or paste to remove the discoloration.
3) Combine light mechanical abrasion with final removal by the acid solution or paste (very common).
4) Use brush electrocleaning or electropolishing (same set-up for different times).
What is the original finish (No. 2B, satin No. 6, polished No. 7 or 8, grained, etc.)? The rougher the surface, the more advantageous to chemically or electrochemically clean to avoid leaving a depression. For grained sheet stock, follow cleaning with a handheld belt sander to restore the grain. Pickling may be best for a satin or matte finish, electrocleaning for a polished surface, electropolishing for a No. 8 mirror polish, etc. The pickling and electro methods have the advantage of passivating the surface. See "Surface Engineering of Stainless Steels," in ASM Handbook Vol. 5 (1994) for details, matching mill finishes, etc. You can find chemicals, consultants, equipment and supplies via links at the bottom of the page.

Try to minimize formation of the discoloration using an inert gas or a copper chill block/backing plate to shield the surface from air and cool the surface. Keeping the sheet flat against a (clean) welding tabletop would have a similar effect. If welding onto the front (not doing through welds) and the above methods are impractical, try covering with a brush-on protective flux or SS foil, keeping any adhesive away from the heat (the latter are ideas of mine; haven't seen).

The American Welding Society (www.aws.org) and ASM (www.asm-intl.org) have many useful publications, as do the manufacturers of filler metals and SS stock.

Also search recent letters (18015, 18215, 18732) and Archives for similar problems and responses.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.