Hydrogen evolution inhibitors

Organic substances are obvious poisons for hydrogen evolution (inhibitors) [154], but the most common poisons for industrial cathodes are the metallic species present in solution as a result of corrosion of the cell hardware and of other components of the electrochemical reactor. As a matter of fact, the most common impurity is Fe coming from the steel employed in manufacturing cells, which can be cathodically deposited on the active layer [34, 155]. Other impurities which have been considered are Cr, Ni, Hg and Cu [156, 157], In other cases, and under different conditions, S [158-160] and CO [161] have also been considered as poisons for Pt. 

Benzaldehyde, benzoic acid and benzene also exhibit substantial inhibiting effect on the hydrogen reaction [45—47]. The hydrogen evolution inhibitors are adsorbed at the protons that are mainly present on the antimony sites (catalytic centres) of the electrode surface. Boehnsted et al. [45] propose a mechanism of the specific adsorption of aromatic aldehydes on these sites [45]. 

The inhibitor should not decompose during the life of the pickle nor decrease the rate of scale removal appreciably. Some highly efficient inhibitors, however, do reduce pickling speed a little. It would be expected that since the hydrogen evolution is reduced the amount of hydrogen absorption and embrittlement would also be reduced. This is not always the case thiocyanate inhibitors, for example, actually increase the absorption of hydrogen. 

Surface-active agents are often added to the pickle if the inhibitor has no surface-active properties. They assist the penetration of the acid into the scale, reduce drag-out losses, and form a foam blanket on the pickle. This blanket reduces heat losses and cuts down the acid spray caused by the hydrogen evolution. 

Participation in the electrode reactions The electrode reactions of corrosion involve the formation of adsorbed intermediate species with surface metal atoms, e.g. adsorbed hydrogen atoms in the hydrogen evolution reaction adsorbed (FeOH) in the anodic dissolution of iron . The presence of adsorbed inhibitors will interfere with the formation of these adsorbed intermediates, but the electrode processes may then proceed by alternative paths through intermediates containing the inhibitor. In these processes the inhibitor species act in a catalytic manner and remain unchanged. Such participation by the inhibitor is generally characterised by a change in the Tafel slope observed for the process. Studies of the anodic dissolution of iron in the presence of some inhibitors, e.g. halide ions , aniline and its derivatives , the benzoate ion and the furoate ion , have indicated that the adsorbed inhibitor I participates in the reaction, probably in the form of a complex of the type (Fe-/), or (Fe-OH-/), . The dissolution reaction proceeds less readily via the adsorbed inhibitor complexes than via (Fe-OH),js, and so anodic dissolution is inhibited and an increase in Tafel slope is observed for the reaction. 

Preliminary results showed that these types of compounds are possible inhibitors of the corrosion of iron in acids. This anticorrosion behavior is believed to arise from the fact that a dithiocarbamate-substituted cobalt cyclam can affect the hydrogen evolution reaction within the system. A... 

For dibenzyl sulfoxide on iron, it turns out that indeed it is the hydrogen evolution reaction, the partner reaction to the anodic dissolution reaction, which controls the corrosion rate. Because the inhibitor acts cathodically, it must interfere with and slow down the rds of this reaction, i.e., make it more difficult for the H to be desorbed. 

The inhibitor withdraws electrons from the Fe. This has the effect of increasing the bond of the adsorbed H to the metal and reduces the rate of the cathodic evolution of H2 and therefore that of the partner anodic dissolution, which must function at the same rate as that of hydrogen evolution. 

Since the process is cathode limited, it is possible to slow it down by inhibiting the rate of hydrogen evolution. Many commercial corrosion inhibitors function in this manner. Considering Fig. lOM, it is easy to see that decreasing the exchange current density of hydrogen evolution by the addition of a suitable corrosion inhibitor is equiva-... 

Figure 6.3 shows the effect of different metal deposits (as islands equivalent to a few monolayers with about 90% surface coverage) on the photocurrent ofThe shift of the i-V curve from that of bare material is due to the catalytic effect of the metal on hydrogen evolution. For a metal deposit the photocurrent is parallel to the exchange current density for the dark evolution of H2 Pt, as a catalyst, has the highest exchange current whereas Pb, as an inhibitor, has a very low exchange current. 

The electroless deposition of metals on a silicon surface in solutions is a corrosion process with a simultaneous metal deposition and oxidation/dissolution of silicon. The rate of deposition is determined by the reduction kinetics of the metals and by the anodic dissolution kinetics of silicon. The deposition process is complicated not only by the coupled anodic and cathodic reactions but also by the fact that as deposition proceeds, the effective surface areas for the anodic and cathodic reactions change. This is due to the gradual coverage of the metal deposits on the surface and may also be due to the formation of a silicon oxide film which passivates the surface. In addition, the metal deposits can act as either a catalyst or an inhibitor for hydrogen evolution. Furthermore, the dissolution of silicon may significantly change the surface morphology. 

The isolated Fe-Mo cofactor thought to be the active site of N-ase catalyzes hydrogen evolution at high potentials,53 and H2 is in fact an inhibitor of N2 reduction. H2 reduction is also proven by the formation of HD from D2 gas and protons derived from H20, which occurs only in the presence of N254 ... 

This simple model of the inhibitor action which is based essentially on the potential independence of the inhibitor adsorption is, however, often not applicable. Kaesche (15) indicates that the corrosion inhibition of pure iron in sulfuric or perchloric acid by phenyl-thiourea strongly affects the slopes of the polarization curves, leaving the corrosion potentials almost unchanged Fig.7. In fact, the polarization curves for the inhibited situation do not exhibit real Tafel behavior. This behavior finds a partial explanation in the fact that the mechanism of the hydrogen evolution appears to be changed in the presence of... 

The effect of a wide variety of organic substances, derivatives of benzaldehyde, benzoic acid and benzene, which act as inhibitors of hydrogen evolution, has been studied. Dietz et al. 

Inhibitors of hydrogen evolution slow down the self-discharge processes and improve the charge acceptance of the negative plates. 

Anisaldehyde, in combination with Indulin-C and Na-l-naphthol-1-sulfonate in amounts of 700 ppm, is a highly efficient inhibitor of the hydrogen evolution reaction and increases the capacity of the negative plates. It can be introduced during operation of the battery, when the water loss increases [44]. 

Vanilline has been tested as inhibitor of the hydrogen evolution reaction on lead—antimony alloys at 47 °C. The water loss is reduced by 50% on battery cycling [48]. 

Parallelism in the specific inhibition of electrocatalytic and enzymatic activity. The specific inhibitors of a particular enzyme are observed also to suppress its electrocatalytic activity in the adsorbed state. Experimental data demonstrate that a,a -dipyridyl completely suppresses the reaction of hydrogen evolution by immobilized hydrogenase fluorine ions inactivate laccase in the reaction of oxygen electroreduction and diphenylhydrazine has the same effect on peroxidase in the reaction of hydrogen peroxide electroreduction. A complete parallelism is also observed in the inactivating effect of hydrogen peroxide on peroxidase in the electrochemical reaction and enzymatic oxidation of o-dianisidine. 

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