The Pipe Fabrication Institute has issued Engineering Standard ES-50, Internal Oxidation for Piping Welds.
When welding stainless steel piping, it is normal to purge air from the root side of the pipe and replace it with argon to prevent oxidation. Without purging, the root side of the weld can be unacceptably coarse and rough, but even with purging, the oxides formed can reduce corrosion resistance. The standard provides color reference photographs showing a numbered sequence of discolorations that engineers can select from and incorporate into their contract documents. It also provides some guidance on selection of appropriate levels of discoloration.
While this standard is similar to the weld discoloration figures in AWS D18.1 and the ASME Bioprocessing Engineering Code, these photographs were made using common pickled pipe rather than polished tube.