adsorption of the HER intermediate (OPD H) [75,117,118]. They lower the H
adsorption capacity and are likely to lower the M–H
ads
bond strength [41,42,101].
Explanation 2 was proposed in Ref. 31 for a coupled Volmer-Tafel path and is
valid at constant current density, assumed to be due mainly to the combination rate
(see Table 2). However, a lowering of the rate constant for surface-bulk transfer
cannot by itself explain why H permeation is increased under potentiostatic
conditions [113]. At fixed potential, the weakening of the M–H
ads
bond strength,
associated with a strong reduction of the number of surface adsorption sites, should
lead to a reduction of the H coverage, which would probably be the determining factor.
Explanation 3, rather phenomenological, is often given for an increase
of permeation current under gavanostatic conditions, for an HER mechanism
assumed to occur only by the Volmer-Tafel path. In this case, the constant cathodic
current is related only to the electroadsorption rate, which is equal to the
sum of the combination rate plus the permeation rate [see Eq. 34]. If the
promoter inhibits the chemical combination step, the permeation rate is increased
[33,111]. This trivial explanation fails to explain promoter effects occurring
under potentiostatic conditions.
Explanation 4 is valid if the reaction of consumption of H
ads
is solely the
combination step: in addition to assuming the Volmer-Tafel path for the HER,
the permeation rate must be negligible to allow an increase of the H coverage
[see Eq. 33]. This classic explanation is apparently not self-consistent because it
proposes that a high H coverage may appear whereas the H sites are occupied by
poison, unless the sites for electroadsorption are supposed to be different from sites
for combination, which would conflict with the present view [27,53,58,59].
However, a modeling of promoter effects shows that the relative coverage in the
sites left active may well increase while, due to the blocking effect, the overall
coverage decreases [53,61].
There are two recent significant contributions to the understanding of the
mechanism of action of promoters on H permeation into iron:
1. Some authors, using the rather rough assumption that the mechanism of HER
on iron in acid is only coupled Volmer-Tafel but taking the permeation
current into account in the establishment of the steady-state equations
(so-called IPZ model), analyzed their data on iron in acid solutions
containing H
2
S and concluded that H
2
S increases the proton discharge rate
constant and lowers the combination rate constant, leading to a coupled
Volmer-Tafel mechanism at low overpotentials, a decreased overvoltage, an
increased H coverage, and an increased permeation rate [112]. Their
conclusions are essentially similar to explanation 4.
2. This explanation is contested in the latest permeation study [34], in which the
promoting effects of dissolved species of As on iron are analyzed with accurate
fitting of the experimental current-potential curves with the complete steady-state
equations and taking into account the possibility of reduction of adsorbed As at
cathodic potentials into the hydride AsH
3
; yet a simplifying choice of parameters
could not be avoided. It is concluded that the mechanism of HER cannot be or
become the coupled Volmer-Tafel one. It is once again observed that the H
entry is inhibited a low overpotentials, which is attributed to deposition of bulk
As, and that promoting effects of As occur only at relatively high overpotentials,
where As is likely to desorb into AsH
3
. From the fit, the authors deduce that the
Surface Effects on Hydrogen Entry into Metals 85
Copyright © 2002 Marcel Dekker, Inc.