I've seen this several times over the years and have received numerous reason for the residue. With SMAW you will see this residue when welding with a long arc. Was told that this was from the Cr electrode and others from the flux. the remedy was to shorten the air, let the coating almost drag on the weld puddle. In TIG and MIG it is supposed to be because you loose the shielding gas for some peroid of time. This happens a lot on SS, again it the oxidation of the Cr. It is the same for the yellow fumes you sometimes see around welding. I've never seen anybody that seem to worry about it except to clean it off during after the weld was completed..
Though I've never analysed the residue I think the Cr idea is approaching the cause though I believe Fe enters into the picture because in a Steel mill a lot times you see yellow fumes prior to red fumes.
When we used to cut SS with an iron powder rig you would see yellow appearing fumes before the red cloud appeared.
I agreed that the prevalent color is green as in Chrome Green. Chrome Green will turn brown or black on heating then reverting back to green when it cools.
Chromium Trioxide is a orange brownish color which I believe will form in an arc. Taking into account that you are going to have other metals present in the fumes, I don't think orange/yellow fumes or yellowish deposit are out of the question. As you always lose Cr when welding an alloy containing CR the little information that I posted may not be too far fetched even if some is caught in the slag..
I could be way out in left field, they want let me play center field.
i will assume that you have a mixed bag of tubes in various locations in the boiler other than T22 and you are burning coal.
the oxide film from the older tubes and those non-T22 and the firing of coal (even after a power washing) is leaching (for lack of a better word) into the new welds.
as said above just wire brush after each pass and do not worry about it.
