Grüss Dich Andrea!
- Yes, some gases (the smallest molecules) pass through some solids...
- But normal gases don't pass through normal solids under normal conditions in normal amounts!
- In your case, only holes, cracks, crevices are relevant, diffusion is not.
Examples where diffusion can be noticed :
- A helium balloon, because He diffuses so quickly, and macromolecules, especially elastomers, are so permeable.
- A silicon chip in an epoxy package, because vapour diffuses quickly (less so than He and H2), epoxy is a polymer, and minute amounts of vapour over years corrode <1µm of metal thickness on a chip.
- Hydrogen (of course!) in some metals like palladium in thin film or powder form, and then it may be adsorption (of individual protons in the metal).
- Hydrogen embrittlement of steel. Put 1/4h at 180°C (kitchen oven), the hydrogen is gone.
- Water purification by reverse osmosis. Again, chemical or Van der Waals forces act. And just check the surface and the thickness of the polymer required to get a little throughput!
- Washing water in polyamide clothes
- Semiconductors, because the transistors are in the first 100nm depth, manufacturing involves heat, and the slightest impurity has tremendous consequences.
Normal gases like N2, CO, CO2... do not diffuse through metals, compact ceramic... under normal conditions. You would need your bricks to be porous, and then a diffusion equation won't tell it, because it's not a property of the compact material.
So much that people willing to test the hermeticity of a ceramic package for electronic chips heat then, use helium, wait for long, and have an ultra-sensitive helium detector.
Fick's law would have been the proper one and you used it properly (and its answer, zero, would be right as well...). You need to complete it with one experimental data that tell the equilibrium ratio of gas concentration in the air versus in the solid - hard to find.
My suggestion: CO permeability is much lower, so use vapour permeability to prove that diffusion is negligible. Or neglect the whole hypothesis anyway.