Coupled discharge-recombination mechanism

A convenient way to examine the amount of hydrogen absorbed by the specimen is to compare the anodic permeation current with the cathodic charging current. The relationship between them depends on the mechanism for hydrogen evolution. In the case of iron, several studies indicate that hydrogen evolution involves a coupled discharge-recombination mechanism. Therefore, the cathodic current is given by... 

Hence, the steady state flux through an iron membrane under pure diffusion control should be proportional to the square root of the cathodic current density. Deviations from the square root dependence have been observed [116, 117], particularly at extreme current densities, but this relationship seems to hold generally [7, 118). The relationship between Joo and rj can also be derived, and values of dr]/d nJcx for coupled discharge-recombination and other mechanisms are shown in Table 8 [6],... 

The path of the HER mechanism taking place on Pd was characterized by a fast discharge step (8) and a slow recombination (9 or 10). On the opposite, the rate of hydrogen evolution at the Ag cathodes in alkaUne solutions is controlled by a slow discharge from water molecules. The behaviour of a Pd-Ag (75 25) alloy tends to approach a coupled discharge-combination mechanism. 

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