Welding on Zinc coated plates

For zinc coated plates, we should always use a low silicone type consumable for welding. Some manufacturer have wires that is especially built for welding on these primed plates, however they are not too popular. Safro and Oerlikon has got the product used for welding of zinc surfaces, they are the Safdual Zn, or Carbofil Galva.

Or in anycase if you have difficulties looking for these wires, you could opt for AWS: 5.18:ER70S-3 designated wires. With this general wires, expect porosity in the welds due to vaporisation of zinc which forms gas bubles in the weld.

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.