In my admittedly limited experience working with plumbers, natural gas pipes are typically made from galvanized steel for interior work, or from black steel pipe for underground. Last-leg fittings and tubes which connect the stub-outs to various appliances are sometimes made from copper and brass. Other materials are possible, as noted here. Some things to consider:
- When joining two pipes made from dissimilar metals (including steel and cast iron), always use a dielectric union to decrease the risk of galvanic corrosion between the two metals.
- In US jurisdictions, exterior pipes must be painted to reduce the risk and rate of weathering.
- In the US, natural gas typically contains a small amount of methyl or ethyl mercaptan. The additive has a foul smell which makes leaks readily detectible by most people. These compounds contain sulfur and will react with copper to form copper sulfide, eventually corroding through the pipe. Some copper pipes of types K and L are approved for use with gas. Only properly approved copper pipes should be used with natural gas, with appropriate fittings.
For argon, the first point is still relevant, though the last is likely irrelevant if the argon is pure. The second point may be relevant if you need any pipe runs outdoors.
As for leaking, the pipes themselves should not leak, typically it is joints that leak. If you need high pressure, ensure that the schedule of your pipe is sufficiently high (i.e. wall thickness) and that it is rated for the required pressure. As with all pressure vessels, ensure you have appropriate mechanical safety valves and a pressure regulator to decrease the risk of explosion. If you are worried about diffusion through the pipe, unless you plan to heat the argon to many hundreds of degrees C (which you will need specialized pipe for), the diffusion rates will probably be trivial. Focus on the pipe joints first for leak reduction.
If you only need to transport small quantities of argon over short distances, as in a lab setting, copper and brass flare fittings are most likely sufficient if the pressures aren't too high. If you need to transport large quantities, you should consult an professional engineer.