Electrode reactions inhibitors

The effects of adsorbed inhibitors on the individual electrode reactions of corrosion may be determined from the effects on the anodic and cathodic polarisation curves of the corroding metaP . A displacement of the polarisation curve without a change in the Tafel slope in the presence of the inhibitor indicates that the adsorbed inhibitor acts by blocking active sites so that reaction cannot occur, rather than by affecting the mechanism of the reaction. An increase in the Tafel slope of the polarisation curve due to the inhibitor indicates that the inhibitor acts by affecting the mechanism of the reaction. However, the determination of the Tafel slope will often require the metal to be polarised under conditions of current density and potential which are far removed from those of normal corrosion. This may result in differences in the adsorption and mechanistic effects of inhibitors at polarised metals compared to naturally corroding metals . Thus the interpretation of the effects of inhibitors at the corrosion potential from applied current-potential polarisation curves, as usually measured, may not be conclusive. This difficulty can be overcome in part by the use of rapid polarisation methods . A better procedure is the determination of true polarisation curves near the corrosion potential by simultaneous measurements of applied current, corrosion rate (equivalent to the true anodic current) and potential. However, this method is rather laborious and has been little used. 

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 situation is more complex, however, than it appears at first sight. As D(M-B) increases, the surface of M will be more and more occupied by adsorbed B, which in turn will subtract part of the electrode surface to further discharge of A. In other words. Bads acts as a sort of inhibitor for the electrode reaction. Since the amount of Bads ( B = coverage) is governed by an adsorption isotherm, that is, Bb is a function of D(M-H), logj turns out also to be alinear function of-D(M-B), that is, it is depressed by an increase in coverage with the intermediate [24]. 

The role of the steric factors on the electrode reaction occurring in the presence of adsorbed inhibitors was investigated. 

O Brien. 1235 Ohmic drop, 811, 1089, 1108 Ohmic resistance, 1175 Ohm s law, 1127. 1172 Open circuit cell, 1350 Open circuit decay method, 1412 Order of electrodic reaction, definition 1187. 1188 cathodic reaction, 1188 anodic reaction, 1188 Organic adsorption. 968. 978. 1339 additives, electrodeposition, 1339 aliphatic molecules, 978, 979 and the almost-null current test. 971 aromatic compounds, 979 charge transfer reaction, 969, 970 chemical potential, 975 as corrosion inhibitors, 968, 1192 electrode properties and, 979 electrolyte properties and, 979 forces involved in, 971, 972 977, 978 free energy, 971 functional groups in, 979 heterogeneity of the electrode, 983, 1195 hydrocarbon chains, 978, 979 hydrogen coadsorption and, 1340 hydrophilicity and, 982 importance, 968 and industrial processes, 968 irreversible. 969. 970 isotherms and, 982, 983... 

The introduction of 0 in the equations for current density need by no means refer only to the adsorbed intermediates in the electrode reaction. What of other entities that may he adsorbed on the surface For example, suppose one adds to the solution an oiganic substance (e.g., aniline) and this becomes adsorbed on the electrode surface. Then, the 0 for the adsorbed organic substance must also be allowed for in the electrode kinetic equations. So, in Eq. (7.149), the value of 0 would really have to become a 0, where the summation is over all the entities that remain upon the surface and block off sites for the discharging entities. Many practical aspects of electrodics arise from this aspect of the Butler-Volmer equation. For example, the action of organic corrosion inhibitors partly arises in this way (adsorption and blocking of the surface of the electrode and hence reduction of the rate of the corrosion reaction per apparent unit area).67... 

On the other hand, electroinactive but surface active substances (SAS) adsorbed at the electrochemical interface affect the rate of electrode reactions and clear examples are organic additives used in metal deposition and inhibitors of metal corrosion. In a few cases, these substances accelerate the rate of electrode reactions [118, 119]. 

Inhibitor (of an electrode reaction) — is a substance that added to the electrolyte solution causes a decrease in the rate of an electrochemical process by a physical, physicochemical, or chemical action and, generally, by modifying an electrode surface. This modification is due to adsorption of the inhibitor. The inhibitor may play no direct role in the electrochemical reaction or it can be a reaction intermediate. 

In Table 12 we give the values of a for several reactions. Inspection of those values reveals that they are similar for different solvents (see Zn and Mn in Table 12). They are also similar to the r values found [290, 291] for several electrode reactions in the presence of inhibitors analyzed by means of Eq. (69) or (69 a). 

The application of Eq. (75) to the analysis of the Zn(II)/Zn(Hg) electrode reaction in several mixed solvents, shown in Fig. 18, illustrates a good way of determining the mixed solvent composition range, where the organic solvent acts as an inhibitor of the electrode reaction (see Eq. (68)). 

In spite of such experimental difficulties the Evans diagram has been extremely useful in determining certain characteristics of inhibitors. In acid solutions for example, the electrode reactions follow a behavior which is quite predictable on the basis of the electron transfer being the rate determining step. 

Kaesche and Hackerman (13) have investigated the inhibition of several aliphatic and aromatic amines on pure iron corroding in IN hydrochloric acid. These authors observed in thirteen out of fourteen cases that the inhibition was both anodic and cathodic, albeit predominantly anodic. The exception was methylamine which acted only cathodically. In the case of the corrosion inhibition on pure iron by B-naphthoquinoline in sodium sulfate/sulfuric acid solution (13). one observes a simple parallel shift of the anodic and cathodic Tafel lines towards smaller values of current density. Here the effect is almost symetrical, indicating that this inhibitor acts to the same extent upon anodic and cathodic reaction rates. Therefore, the effect of B-naphthoquinoline can be explained on the basis that its adsorption blocks a fraction 0 of the metal surface for all electrode reactions. If equation 9 describes the external polarization behavior in terms of a function of the partial current potential relationship for the anodic and cathodic reactions in the usual terms ... 

Electrochemical inhibitors retard or prevent the anodic and/or cathodic partial reactions (i.e they influence the reaction at the metal/corrosive medium interface). Chemical inhibitors can react both with the material and form protective coatings and with the medium itself or its constituents and thus diminish its aggressiveness. Physical inhibitors form adsorption layers on the metal surface, which block the corrosion reaction. Inhibitors that influence the electrochemical electrode reactions are subdivided according to their mode of action and site of action in the area of the metal/ medium phase boundary, with the subdivision being between interface inhibitors, electrolyte film inhibitors, membrane inhibitors, and passivators. 

The polarization curves for iron, measured in 6M HCl solutions containing different amounts of trimethylene diamine (Figure 12.28), illustrates the described behavior [19]. This compound, whose formula can be found in Figure 12.26, reduces the cathodic partial current density more than the anodic partial current density and its presence lowers the corrosion potential. On the other hand, the slope of the Tafel lines is not affected by the inhibitor, indicating that the mechanism of the electrode reaction remains unchanged. The reduction of the partial currents can be explained by postulating that the inhibitor adsorbs on the metal and thus reduces the surface area available for the corrosion reactions. 

Inhibitors may act on both cathodic and anodic partial electrode reactions (Fig. 9-3), termed accordingly as cathodic or anodic inhibitors. The cathodic inhibitor (Fig. 9-4 b) shifts the corrosion potential in a negative direction, while the anodic inhibitor causes a positive change (Fig. 9-4 c). Very often a passive layer is formed by the anodic inhibitor, while the inhibitor molecule is reduced. On the other hand, an insulating layer is generally formed by the cathodic inhibitor on the cathodic part of the corroding surface. This type of inhibitor is often a cation that is reduced. 

Since pitting is an electrochemical process, it can be stopped by cathodic protection. It can also be prevented by the use of inhibitors to alter the electrode reactions of the local cell and remove their driving force. In some cases, agitation of the environment will prevent environmental differences from developing and will prevent pitting that otherwise would occur. 

Fig. 10.24 Modes of inhibition, shown by the effect on polarization for charge-transfer-controlled electrode reactions, (a) Decrease in the apparent anodic Tafel slope, (b) Decrease in the anodic exchange current (in the case of anodic inhibitors, passivation may also occur), (c) Decrease in the apparent cathodic Tafel slope, (d) Decrease in the cathodic exchange current.

The simplest way to illustrate the influence of the availability of suitable sites on the electrode on an electrode reaction is to add to the solution a known amount of material which adsorbs to a known extent and study the reaction with and without the presence of this known surface active substance. The inhibiting effect of the adsorbed film is shown in Fig. 38. This curve shows the effect of adsorption of cyclohexanol on the reduction of zinc at a zinc amalgam electrode. One clearly sees that when 6 reaches 0 = 0 5, the inhibiting effect of the cyclohexanol is total, i.e. no current is observed. On the other hand, adsorbed species can provide a way for an alternative path for the mechanism of the reaction studied and may, as a result, act as a catalyst rather than as an Inhibitor. The catalytic effect of diphenylamine on the reduction of water is shown in Fig. 39 hydrogen is... 

Almost always, foreign species not involved in a given electrochemical reaction are present on the surface of catalytic electrodes. In some cases these species can have a strong or even decisive effect on reaction rate. They may arrive by chance, or they can be consciously introduced into the electrocatalytic system to accelerate (promoters) or retard (inhibitors) a particular electrochemical reaction relative to others. 

An examination of the theoretical models proposed for metal dissolution and for the general Impedance behavior of electrodes enables the rate-determining step of the corrosion reaction to be Identified. It Is then possible to separately study the rate determining step In order to find a suitable Inhibitor or a suitable surface coating. 

The above results show that BTA, PVI-1 and UDI behave differently in the two pH solutions tested in this study. The differences are probably related to the form of the Inhibitor, for example BTA takes on a cationic, anionic, or neutral form depending on the pH [333 This may affect the solubility of the cast films, adsorption behavior, reactions of the inhibitor at the electrode, and the solubility of any subsequent reaction products. The BTA molecule (pK.sO.44, pK s8.2) is soluble in all pH solutions in all forms l33.]. The neutral form is predominant in both 0.1M... 

The most efficient system devised by Monsanto uses electrodes fabricated from carbon steel plate, electro-coated on one face with cadmium. These are stacked in parallel so that the electrolyte can be pumped through the gap between successive plates. Overall tire system forms a series of electrochemical cells with a cadmium cathode and a carbon steel anode. Each plate of metal forms the cathode of one cell and the anode of the next in the stack. Electric current is passed across the stack. The electrolyte contains phosphate and borate salts as corrosion inhibitors, EDTA to chelate any cadmium and iron ions generated by corrosion together with hex-amethylenebis(ethyldibutylammonium) phosphate to provide the necessary telraal-kylammonium ions. This electrolyte circulates through the cell from a reservoir and there is provision for the introduction of acjylonitrile and water as feedstock. The overall cell reaction is ... 

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