The rate of permeation of hydrogen through a highly purified iron (Ferrovac E) was measured over a pressure range of 1/30 to 300 atm. and a temperature range of 126' to 693OC. The effect of cold working of the annealed metal and of hydrogen purity were also studied. Permeability, diffusivity, and solubility were obtained from the data. Permeation rate was linear with square root of fugacity rather than pressure. No difference in @ or D between the annealed and cold worked tube was observed. Log D vs. 1/T was linear over the whole range of temperature, in disagreement with recent work showing a break in the plot a t about 400OC. The calculated activation energy was lower than values reported in the literature. Solubility agreed well with most previous work and did not show the break in the solubility vs. temperature curve found by some recent investigators. No ageing effect was observed nor any difference between the two grades of hydrogen. All the evidence shows that diffusion through the metal is the controlling rate process in this case.The permeation of gases through metals is a phenomenon of considerable practical importance. The special case of hydrogen and steels has been extensively studied because of the attack by hydrogen at elevated temperatures and the embrittlement caused by it even at ambient temperatures. Two papers dealing with the effect of hydrogen on the structure and the tensile properties of a wide variety of metals have been published from this laboratory (22, 27). The present paper is concerned with the more fundamental question of mechanism and rate of penetration of hydrogen through a metal. DEFINITION OF TERMS AND QUANTITIESTo avoid confusion it is well at this point to define some of the terms and quantities commonly but often loosely used in describing the movement of gases through metals. The term "permeation" will be used to describe the overall process of passage of the gas from one side of a metal barrier to the other.A constant known as the permeability has been defined in various ways. is defined: D can be determined from this equation but C is difficult to measure, and other measures have been developed to circumvent this difficulty. In this work the time-lag method of Barrer ( 4 ) was used. The tirne-lag equation for hollow-cylinder geometry isOr, is the intercept on the time axis of the steady state portion of a graph of quantity diffusing Q vs. time.The solubility of a diatomic, elemental gas in a metal is expressed by c = s dF (4) an equation often referred to as Sievert's law. S is a constant whose value depends on the temperature and the units. If equilibrium can be assumed between the molecular gas and the concentration of gas atoms in the metal (this assumes of course that the surface reactions are rapid relative to the diffusion so that the latter is controlling), one may combine ( 2 ) and (4) to giveand this offers a simple means of obtaining D.Now by analogy to the diffusivity define the steady state permeability for the case of the hollow cylinder by the equation (6)...