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Forming Gas I: What is it?

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  • Forming gas is a complicated topic, so I will provide some preliminary background in this section, then get into the soldering part next time.

    Don't you mean "formic"?

    Forming gas is a mixture of hydrogen (H2) and an inert gas (usually nitrogen, N2) that is used to reduce oxides on metal surfaces to water. Please don’t confuse this with formic acid (HCO2H), which I hope to touch on in another posting later this year.


    The reason for the dilution of hydrogen by the inert gas is to keep the hydrogen below 5.7% (by volume), as this is the point above which the hydrogen can spontaneously combust. Gas companies such as Linde and Air Products consider forming gas at less than this level to be an inert mixture, so the fittings used for gas cylinder attach are the standard CGA580 type used for nitrogen, argon, helium and so on. Depending on the gas supplier, they may allow a maximum of either 5.0% or 4.0% hydrogen, to ensure they are within safety margins.


    All this notwithstanding, 100% hydrogen furnaces are used around the world in a variety of different processes, and I have also seen soldering processes around the world where 10% and even 20% hydrogen/nitrogen forming gas is in use. I am not saying that >5% H2/N2 can not be safely used, but you have to be careful when using it.

    Gas Supply

    There are three ways of supplying gas for forming gas-based soldering processes:

    1/ Mixing hydrogen and nitrogen in a special panel. Sometimes this may also incorporate a catalytic reactor that reacts ppm traces of oxygen, with hydrogen to form water: the water  is then removed by adsorption. This process makes a very "clean" forming gas that will have optimal reducing properties. Usually, the nitrogen source is from vaporised cryogenic N2, and the hydrogen is from a cylinder or "tube"-based sources.

    2/ Cylinder supply. A single cylinder, or a manifolded bank of cylinders may be used to provide the gas as a mixture. Usually, this is used as-received without being cleaned up.

    3/ Ammonia cracking. Basically, NH3 -> 3N2 + H2. This is feasible, but results in a fixed 3:1 ratio of N2 to H2, and is never used (to my knowledge) in soldering. It is also massively inefficient in terms of costs and power usage to make the ammonia, plus the ammonia usually has a much higher moisture content than a nitrogen plus hydrogen gas mixture.

    What does it do in soldering? I’ll get into that next time: I'll be talking thermodynamics and kinetics, and there WILL be a test.


    Cheers! Andy