Black Powder from TIG Welded SS surface

by RS Reddy
(India)

My question relates to TIG welding of AISI 316 ASTM 270 Sanitary Dairy Pipes. After the welding of all the pipe joints, we have to clean the Piping by circulating cleaning solution of 1.5% NaOH followed by 0.8% HNO3. this is done to clean the welded surfaces and also the internal pipe surfaces fully clean

What we have found that after the above cleaning repeated three times, when the welded surface is wiped with a cotton swab, a black colour appears on the cotton

We had not noticed this earlier. Can you please help with the diagnosis of this problem and how to remove all the black coated that the weld surface may still have

RS Reddy
India

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My suggestion is to observe the welding process of a joint done properly vs one that is overheated and turned gray or black from oxidation and test to see if there is black soot after cleaning on both or just one.

I suspect that since you say you have not seen this before, the welds may have been done differently, possibly by a different welder.

It sounds like the HNO3 is able to have an effect on the weld possibly due to loss of chromium during welding.

regards,

jody