Is there any way for a gas to pass through a solid metal?
Yes, some gases can diffuse into and through metal. It is the bane of the high-vacuum engineer's life. Hydrogen is the worst because it tends to dissociate into atoms at the surface and the nucleus, a single proton, can then leave its electron behind and wander through the metal lattice until it picks up another electron when it leaves.
For example Mu-metal, favoured for some applications, typically has to be annealed in hydrogen at high temperature. Once that is over, it can take weeks or months for the residual hydrogen to diffuse out of the metal before a high enough vacuum can be achieved and the work proceed.
A "virtual leak" occurs where a small bubble of gas is embedded in the material inside a vacuum chamber. The leak usually happens because a tiny hole exists for the gas to diffuse out through, but sometimes the "hole" is no more than an ultra-thin skin of metal (invisible to the frustrated technician) and the gas diffuses through it. These little horrors can keep going for months or even years and generally mean replacing suspected parts and pumping down over and over again until the dodgy one is finally stumbled on.
Helium is both monatomic and the physically smallest atom. It can diffuse more easily than any other neutral atom or molecule, making certain metal foils unsuitable as say gas-tight liners for airships. As noted in another answer, in quantity it can also affect the bulk properties of the metal.
On a more energetic scale, hydrogen and helium nuclei (protons and alpha particles) can pass through thin metal foils if fired with sufficient energy, and this has been used to establish the crystalline structures of some metals and alloys (where, for whatever reason, electrons were unsuitable).
Other gases have much larger atoms (neon and other noble gases) or molecules (nitrogen and other diatomic molecules, water and other hydrides), but they can still diffuse extremely slowly through some metals. This can limit the lifetime of some microchips. A related phenomenon occurs where there is a defect in the lattice at the surface, such as a grain boundary, and a gas atom attaches to it. Defects are sometimes quite mobile and can migrate through the lattice; the gas atom will stabilise the defect and may be able to hitch a ride.
Quantum processes such as tunnelling are not really relevant, as they work over distances smaller than the atomic wavelength, which in turn is typically far smaller than the thickness of any metal atom or foil. The probability of a gas atom tunnelling across is so infinitesimal as to be effectively zero.
Not really, but sort of.
Helium atoms do not form molecules, and the atoms are small. They fit between the spaces in iron, and can diffuse around inside.
This is not a fast process. That is to say, it does not create a measurable leak.
I have heard of one case where that actually created a problem, though I have forgotten most of the details. Some facility had pipes with high pressure He. There was a port for an instrument. The port had thin steel bellows on which the instrument we mounted to allow for some movement.
Over the years, He had diffused into the steel. It had stiffened the bellows and made them brittle. They broke, badly injuring someone.
Permeation of atomic hydrogen through metal has been performed in a study(ref.1):
[...] atomic hydrogen is supplied to the metal surface by reaction with an acid, by electrolysis, or by ionization, rapid permeation occurs even at room temperature . Hydrogen atoms can easily dissolve in metal, diffuse, and readily leave the exit surface to recombine as molecules. Dissociation of molecules into atoms on the entrance surface is, for this low-temperature example, the process which determines the permeation velocity of molecular hydrogen.
The rate of permeability is expressed in terms of membrane thickness and pressure differential across the membrane according to Fick's law, and to temperature by the Arrhenius rate theory. You can find the calculation in the paper.
Also research on metal membranes to absorb hydrogen gas has also been discussed(Ref.2,3).
References:
- Webb, R W. PERMEATION OF HYDROGEN THROUGH METALS. United States: N. p., 1965. Web. doi:10.2172/4583045.
- Hydrogen-permeable metal membranes for high-temperature gas separations David Edlund, Dwayne Friesen, Bruce Johnson, William Pledger, Gas Separation & Purification, Volume 8, Issue 3, 1994, Pages 131-136, DOI: 10.1016/0950-4214(94)80023-5
- Shigeyuki Uemiya (1999) State-of-the-Art of Supported Metal Membranes for Gas Separation, Separation and Purification Methods, 28:1, 51-85, DOI: 10.1080/03602549909351644