The Hydrogen-Evolution Reaction

Using this mechanism, explain why the current density for the reduction of H+ varies widely with the nature of the metal electrode. For example, the rate on a Pt electrode is nine orders of magnitude higher than on a Hg electrode, and five orders of magnitude higher than on a Ta electrode. Explain these observations. 

Problem 2.6 demonstrated that for the Tafel analysis of a multi-electron system we expect to measure a gradient which is proportional to n + otrosj where n is the number of electrons transferred prior to the rate-determining step and ctros is the transfer coefficient for the rate-determining step, note that if all electron transfers are reversible and highly driven then the gradient is proportional to n (the number of electrons). 

The rates of electron transfer associated with each metal and the cause of the change in mechanism maybe understood through studying the estimated enthalpy of adsorption of H on the metal surfaces. 

For metals upon which the adsorption of H is weak, Eq. 2.10 is the ratedetermining step. As the strength of the binding increases the rate of the first electron transfer increases and hence the current density increases. As the strength 

The discharging particles producing H2 evolution at a cathode are H2O molecules in alkaline solutions and protons in acidic solutions [27]. Thus, at intermediate pH there may be a transition from to H2O vith increasing current density, vhich manifests itself as a maximum in overpotential. Usually H2 evolution is carried out in strongly alkaline or strongly acidic environments. Since H2 evolution produces alkalinization of the catholyte  

The hydrogen evolution reaction is the most studied electrode process. In spite of these efforts, essential features are not understood indeed, it is sometimes suggested that focusing on hydrogen evolution has delayed the development of modern electrochemistry by years, if not decades. The overall reaction in acid media is  

We discuss acid solutions in greater detail. Two different mechanisms have been established. The first is the Volmer- Tafel mechanism, which consists of a proton-transfer step followed by a chemical recombination reaction  

In the Volmer-Heyrovsky mechanism the second step also involves a charge transfer and is sometimes called electrochemical desorption  

Both schemes have been observed in various systems. We consider hydrogen evolution on platinum from an aqueous solution in greater detail. In this system the Volmer-Tafel mechanism operates, the Volmer reaction is fast, the Tafel reaction is slow and determines the rate. Let us denote the rate constant for the Volmer reaction as ki(rj), that of the back reaction as k i(rj). Since the Volmer reaction is fast and in quasiequilibrium, we have  

Denoting the forward rate constant for the Tafel reaction by k2 and that for the back reaction by fc 2, we can write the current density in the form  

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Adequate ventilation is necessary for aH process lines to ensure worker safety. Electroless copper baths must have good ventilation to remove toxic formaldehyde vapors and caustic mist generated by the hydrogen evolution reactions and air sparging. Electroless nickels need adequate ventilation to remove nickel and ammonia vapors. Some states and municipalities requite the removal of ammonia from wastewaters. A discussion of printed circuit board environmental issues and some sludge reduction techniques is avaHable (25). 

The hydrogen evolution reaction (h.e.r.) and the oxygen reduction reaction (equations 1.11 and 1.12) are the two most important cathodic processes in the corrosion of metals, and this is due to the fact that hydrogen ions and water molecules are invariably present in aqueous solution, and since most aqueous solutions are in contact with the atmosphere, dissolved oxygen molecules will normally be present. 

If it is assumed that anodic polarisation of the more negative metal of the couple is insignificant, then it can be shown from the Tafel relationship for the hydrogen evolution reaction that... 

The standard electrode trotential, Ep, 2+ Pb = —Q.126V . shows that lead is thermodynamically unstable in acid solutions but stable in neutral. solutions. The exchange current for the hydrogen evolution reaction on lead is very small (-10 - 10"" Acm ), but control of corrosion is usually due to mechanical passivation of the local anodes of the corrosion cells as the majority of lead salts are insoluble and frequently form protective films or coatings. 

The corrosion of tin by nitric acid and its inhibition by n-alkylamines has been reportedThe action of perchloric acid on tin has been studied " and sulphuric acid corrosion inhibition by aniline, pyridine and their derivatives as well as sulphones, sulphoxides and sulphides described. Attack of tin by oxalic, citric and tartaric acids was found to be under the anodic control of the Sn salts in solution in oxygen free conditions . In a study of tin contaminated by up to 1200 ppm Sb, it was demonstrated that the modified surface chemistry catalysed the hydrogen evolution reaction in deaerated citric acid solution. 

However, when the second stage in the hydrogen evolution reaction is electrochemical desorption, the rate of this reaction is increased as the potential falls, and the adsorbed hydrogen concentration may remain constant or fall, according to the detailed electrochemistry. This results in curves such as that shown in Fig. 8.38 for steel in sodium chloride solution. 

It was indicated earlier that the cathodic current was a poor indicator of adequate protection. Whilst, to a first approximation the protection potential is a function of the metal, the current required for protection is a function of the environment and, more particularly, of the cathodic kinetics it entails. From Fig. 10.4 it is apparent that any circumstance that causes the cathodic kinetics to increase will cause both the corrosion rate and the current required for full (/") or partial (1/ — /, ) protection to rise. For example, an increase in the limiting current in Fig. 10.5 produced by an increase in environmental oxygen concentration or in fluid flow rate will increase the corrosion rate and the cathodic protection current. Similarly, if the environment is made more acid the hydrogen evolution reaction is more likely to be involved in the corrosion reaction and it also becomes easier and faster this too produces an increased corrosion rate and cathodic current demand. 

If the acid contains certain impurities such as arsenic, the arsenic raises the overvoltage for the hydrogen evolution reaction. Consequently, the amount of atomic hydrogen diffusing into the steel, and the brittleness, increase. 

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. 

The present Section, which provides an outline of selected relevant topics in electrochemistry, is intended primarily as an introduction to aqueous corrosion for those readers whose basic training has not involved a study of electrochemistry. The scope of electrochemistry is enormous and cannot be treated adequately here, but there are now a number of excellent books on the subject, and it is hoped that this outline will serve to stimulate further study. The topics selected are as follows a) the nature of the electrified interface between the metal and the solution, (b) adsorption, (c) transfer of charge across the interface under equilibrium and non-equilibrium conditions, d) overpotential and the rate of an electrode reaction and (e) the hydrogen evolution reaction and hydrogen absorption by ferrous alloys. For reasons of space a number of important topics, such as the electrochemistry of electrolyte solutions, have been omitted. 

The parameters included in the transfer coefficient will be considered subsequently in relation to the r.d.s. of the hydrogen evolution reaction. 

The hydrogen evolution reaction (h.e.r.) is of particular importance in corrosion for a number of reasons. Firstly, the reduction of the HjO ion in acid solutions or the H2O molecule in neutral and alkaline solution is a common cathodic reaction for the corrosion of metals in acid, neutral and alkaline solutions the fact that iron will corrode in neutral water free from dissolved... 

Table 20.3 Mechanism of the hydrogen evolution reaction at different metals (data after...

Table 21.13 Exchange current densities for the hydrogen evolution reaction...

In the past, elevated voltages in electrolysis cells (a cell overvoltage) had been attributed mainly to polarization of the hydrogen evolution reaction. Hence the term hydrogen overvoltage became common for this kind of polarization. 

Chen, Y. X. and Tian, Z. Q. (1997) Dependence of surface enhanced Raman scattering of water on the hydrogen evolution reaction. Chem. Phys. Lett., 281, 379-383. 

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