Discolourtion of HAZ

I was working on some PLC tank welding of 304 stainless steel when i found this. A welder asked me why is there traces of blue in their welds?.

As you could see from the diagram below, blue HAZ means the thickness of the oxide layer measures at 170nm (1 x 10^-6 milimeter). At this scale its thickness enough to get a fail from the welding inspector.

Temperature	Colour	Oxide thickness
500	Jaune	50nm
750	Violet	100nm
900	Bleu	170nm
1050	jaune-vert	280nm

The reason why discolouration happens in a weld is because of oxidation that occurs in HAZ.Oxygen as much as 50ppm could result in loss of corrosion resistance. While in Titanium alloys, less than 20ppm is desirable.

If ever oxide thickness exceeds 50nm, recheck on the purging gas as there might have been an oxygen contamination either in the gas supply, or there's a leak in the purging system.