Iron, hydrogen diffusion

The remarkable solubility of hydrogen in palladium discovered by T. Graham following the observation of hydrogen diffusion through red-hot platinum and iron by H. St. C. Deville and L. Troost, 1863. 

As a small interstitial atom, hydrogen diffuses rapidly in iron, the diffusion rate being of a similar order to that of solutes in aqueous solution. 

W. Geller and T. Sun, Influence of Alloy Additions on Hydrogen Diffusion in Iron and Contribution to the System Iron-Hydrogen, Arch. Eisenhuttenw 21, pp 423-430,1950. 

In the case of iron, high diffusivity allows hydrogen to rapidly diffuse even at room temperature. This can lead to well-known hydrogen embrittlement of bee iron and its alloys. However, as shown in Fig. 8.1, the hydrogen permeability of iron is relatively poor despite its exceptionally high diffusivity. This is attributed to the relatively low hydrogen solubility of iron [27]. 

As shown in Figure 10, hydrogen diffuses along the grain boundaries and combines with the carbon (C), which is alloyed with the iron, to form methane gas. The methane gas is not mobile and collects in small voids along the grain boundaries where it builds up enormous pressures that initiate cracks. Hydrogen embrittlement is a primary reason that the reactor coolant is maintained at a neutral or basic pH in plants without aluminum components. 

Zhang et al. [31] used constant concentration (CC), constant flux (CF), flux continuity (FC), and flux continuity-constant concentration (FCCC) hybrid boundary conditions to evaluate hydrogen diffusivity from permeation tests on commercial pure iron... 

W. Raczynski, Hydrogen diffusion through iron sheets at 20-90°, Arch. Hutnictwa 3 (1958) 59—78. 

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