Endothermic gas is a gas that inhibits or reverses oxidation on the surfaces it is in contact with. This gas is the product of incomplete combustion in a controlled environment. An example is hydrogen gas (H2), nitrogen gas (N2), and carbon monoxide (CO). The hydrogen and carbon monoxide are reducing agents, so they work together to shield surfaces from oxidation.
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Endothermic gas Wikipedia(Text) CC BY-SA