And in what world does boiling water turn it into oxygen and hydrogen? It turns into steam. Do you have a teapot?
No the intense heat right at the welding site actually separates the two gasses on a molecular level into Hydrogen and Oxygen creating “Hydroxy” gas, or “Brown’s gas”. It is flamable on it’s own, but in this case it also created Chlorine gasses and added them to the mix. So it created a gas bomb with fuel and an Oxidizer.
Arc welding emits intense UV light. UV light causes Hydrogen and Chlorine combined to have an extremely intense explosive reaction. The Oxygen just added more intensity as an Oxidizer.
I think if they had fan forced fresh air in the top and displaced those gases from the tank it would not have happened.
Hydroxy, or what is also called Brown’s gas is what they create with electric Hydrogen fuel cells. Like those that they add to cars... they boil the water into the two gases Hydrogen and Oxygen which is flammable in a gaseous state.