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I need to build an air tight system that can hold gas (Argon Gas). I see that most pipes use steel. Is steel tight enough to hold that gas? Argon is very dense, so I think it should have no problem.

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    $\begingroup$ There is no problem with steel being gas-tight, but any locations where two steel pieces are joined together, either by welding or by the use of some mechanical joint, need to be carefully designed and tested. $\endgroup$ – user16622 Jun 28 '16 at 10:30
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    $\begingroup$ Density is irrelevant. The energy gap of the outer electrons and the overall atomic or molecular size are what matters when it comes to porosity. Holding against pressure is not dependent on gas type, just the pressure. $\endgroup$ – Carl Witthoft Jun 28 '16 at 13:18
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    $\begingroup$ For what it's worth, in hose fittings, brass is usually used for argon as the material is softer and more likely to form a gas-tight seal. Steel taper fittings may well work too, but I haven't seen many of them. $\endgroup$ – Ethan48 Jun 28 '16 at 15:50
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    $\begingroup$ Also worth noting that steel tanks are sued to store argon at very high pressures, so steel is definitely able to contain argon, but it may not be optimal in piping situations for considerations of joining, forming, or assembly. $\endgroup$ – Ethan48 Jun 28 '16 at 15:52
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Steel is routinely used for high pressure storage of industrial argon. For practical purposes it is certainly 'gas tight' although there may be a certain amount of atomic level diffusion into the material in the long term but this is more about contamination than actual leakage.

As mentioned in comments the quality of mechanical and welded joints is also critical for pressurised gas seals. There are numerous standards which deal with this in a variety for circumstances. In the UK BSP threaded fittings are commonly used for most gasses, including argon as well as a variety of compression and quick release fittings for low pressure applications.

For prototype applications coded TIG welding will generally be preferred for fabrication of the vessel itself and attachment points for fixtures and fittings in terms of both gas tightness and structural integrity. Mass produced gas containers usually use coded automated processes for pressure vessel fabrication in conjunction with in-line quality control testing.

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Just adding on to Mr.Johns's answer, do try to make your threaded fittings according to NPT (National Pipe Thread) or BSPT (British Standard Pipe Taper). Both have a tapered profile which will significantly reduce leakages.

Learned it the hard way here. All the best!

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