Titanium Passive Oxide Barrier
The spontaneous formation of an extremely adherent titanium dioxide layer (TiO2) upon contact with oxygenous media. It imparts outstanding chemical resistance to the material in aggressive environments.
Titanium immediately forms a dense, tightly adhering oxide layer in air. That's why titanium normally doesn't corrode further – the layer blocks oxygen. When welding, you must consider this layer: as soon as you heat the material above the recrystallization temperature, it eagerly absorbs oxygen, nitrogen, and hydrogen from the environment. This makes the weld brittle and prone to cracking.
Therefore, you must shield the weld zone and the cooling material with shielding gas (argon) – also on the back side of the weld. The workpiece surface must be free of grease and oxides before welding, otherwise you'll pull impurities into the weld. After welding, you can recognize a clean weld by its silvery shiny color. Blue or gray discolored areas mean: there was too little shielding gas, the layer is destroyed, and the material has already been damaged.