In gas-metal permeation, diffusion is normally considered to be the rate-controlling step. conditions, the time lag (a characteristic of the system which depends on the time required to achieve steady state flow) i s found to be (and theoretically should be) independent of the gas pressure applied to the inlet surface of the metal membrane. Furthermore, the time lag can then be directly related to the diffusivity of the system.
In this paper, data on the hydrogen type 321 stainless steel system are presented which show a pronounced effect of pressure on measured values of the time lag. Taken alone, these data can be explained either by a concentration-dependent diffusivity or by resistance to flow at the metal surface. From steady state data it i s shown that surface resistance is the better explanotion. A model of the permeation process is presented which is consistent with the time lag data, and an estimate is made of the diffusivity of hydrogen in type 321 stainless steel.
The variables studied were hydrogen pressure at the entrance surface of the membrane (0.1 to 30.0 atm.), temperature (300" to 8OO0C.), and membrane wall thickness (0.0252 to 0.2475 cm.).
In addition, the diffusivity is usually taken to be independent of Concentration. Under these "Ie