Hydrogen gas is highly flammable & will burn in air at a very wide range of concentrations between 4% & 75% by volume.
It can react spontaneously & violently at room temperature with chlorine & fluorine. It has the highest energy content as compared to any other common fuel by weight & lowest energy content by volume.
It is used as a fuel gas in oxy-hydrogen cutting / welding and for aluminum & underwater welding / cutting.
It helps in scavenge oxygen, helps in faster welding & a bright finish