Using a metal radio tube having a steel shell as an ionization gage, the diffusion of hydrogen through the steel into the interior was measured. This results from the fundamental corrosion reaction on the exterior, when it is dipped into water:Fe+H2O=FeO+H2.The protective paint on the tube prevents this, but a bare sandblasted iron surface admits hydrogen at 0.13 micron per hour with the shell in water at 25°C. Hydrogen does not diffuse into the tube when water vapor alone surrounds it. Rates of diffusion were measured through plain and composite metal shells, varying surface treatment and temperature.