Cathodic process

When hydrogen ions are reduced to their atomic form they often combine, as shown earlier, to produce hydrogen gas through reaction with electrons at a cathodic surface. This reduction of hydrogen ions at a cathodic surface will disturb the balance between the acidic hydrogen (H ) ions and the alkaline hydroxyl (OH ) ions and make the solution less acidic or more alkaline or basic at the corroding interface. 

In neutral waters the anodic corrosion of some metals like aluminum, zinc, or magnesium develop enough energy to split water directly as illustrated in Fig. 3.4 and Eq. (3.15). 

The change in the concentration of hydrogen ions or increase in hydroxyl ions can be shown by the use of pH indicators, which change color and thus can serve to demonstrate and locate the existence of surfaces on which the cathodic reactions in corrosion are taking place. There are several other cathodic reactions encountered during the corrosion of metals. These are listed below  

Hydrogen ion reduction, or hydrogen evolution, has already been discussed. This is the cathodic reaction that occurs during 

Note that all of the above reactions are similar in one respect—they consume electrons. All corrosion reactions are simply combinations of one or more of the above cathodic reactions, together with an anodic reaction similar to Eq. (3.10). Thus, almost every case of aqueous corrosion can be reduced to these equations, either singly or in combination. 

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The environment plays several roles in corrosion. It acts to complete the electrical circuit, ie, suppHes the ionic conduction path provide reactants for the cathodic process remove soluble reaction products from the metal surface and/or destabili2e or break down protective reaction products such as oxide films that are formed on the metal. Some important environmental factors include the oxygen concentration the pH of the electrolyte the temperature and the concentration of anions. 

The main anodic reaction in acid solutions is given in Reaction 7.1 iron is dissolved at exactly the rate of the cathodic process ... 

A key criterion for selection of a solvent for electrochemical studies is the electrochemical stability of the solvent [12]. This is most clearly manifested by the range of voltages over which the solvent is electrochemically inert. This useful electrochemical potential window depends on the oxidative and reductive stability of the solvent. In the case of ionic liquids, the potential window depends primarily on the resistance of the cation to reduction and the resistance of the anion to oxidation. (A notable exception to this is in the acidic chloroaluminate ionic liquids, where the reduction of the heptachloroaluminate species [Al2Cl7] is the limiting cathodic process). In addition, the presence of impurities can play an important role in limiting the potential windows of ionic liquids. 

It follows from equation 1.45 that the corrosion rate of a metal can be evaluated from the rate of the cathodic process, since the two are faradai-cally equivalent thus either the rate of hydrogen evolution or of oxygen reduction may be used to determine the corrosion rate, providing no other cathodic process occurs. If the anodic and cathodic sites are physically separable the rate of transfer of charge (the current) from one to the other can also be used, as, for example, in evaluating the effects produced by coupling two dissimilar metals. There are a number of examples quoted in the literature where this has been achieved, and reference should be made to the early work of Evans who determined the current and the rate of anodic dissolution in a number of systems in which the anodes and cathodes were physically separable. 

The limiting current density (the maximum possible rate/unit area under the conditions prevailing) for a cathodic process is given by... 

Over the years the original Evans diagrams have been modified by various workers who have replaced the linear E-I curves by curves that provide a more fundamental representation of the electrode kinetics of the anodic and cathodic processes constituting a corrosion reaction (see Fig. 1.26). This has been possible partly by the application of electrochemical theory and partly by the development of newer experimental techniques. Thus the cathodic curve is plotted so that it shows whether activation-controlled charge transfer (equation 1.70) or mass transfer (equation 1.74) is rate determining. In addition, the potentiostat (see Section 20.2) has provided... 

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. 

Corrosion reactions involving two simultaneous cathodic processes have already been referred to, and it is now appropriate to consider the graphical method of representing the corrosion rate. It should be noted that although the simultaneous reduction of HjO and dissolved oxygen occurs frequently this does not exhaust the possibilities, and reactions such as Fe - Fe, -> Cu, CI2 Cl may accompany either or both of the... 

Thus localised attack usually involves a corrosion cell consisting of a large cathodic area and a small anodic area, and since must equal /, the effect will become more pronounced the higher the rate of the cathodic process and the larger the effective area of the cathode. 

It is appropriate to consider first the crevice corrosion of mild steel in oxygenated neutral sodium chloride, and then to consider systems in which the metal is readily passivated. Initially, the whole surface will be in contact with a solution containing oxygen so that attack, with oxygen reduction providing the cathodic process, occurs on both the freely exposed surface and the surface within the crevice (Fig. 1.50). However, whereas the freely exposed surface will be accessible to dissolved oxygen by convection and diffusion, access of oxygen to the solution within the crevice can occur only... 

The effects of concentration, velocity and temperature are complex and it will become evident that these factors can frequently outweigh the thermodynamic and kinetic considerations detailed in Section 1.4. Thus it has been demonstrated in Chapter 1 that an increase in hydrogen ion concentration will raise the redox potential of the aqueous solution with a consequent increase in rate. On the other hand, an increase in the rate of the cathodic process may cause a decrease in rate when the metal shows an active/passive transition. However, in complex environmental situations these considerations do not always apply, particularly when the metals are subjected to certain conditions of high velocity and temperature. 

With regard to the anodic dissolution under film-free conditions in which the metal does not exhibit passivity, and neglecting the accompanying cathodic process, it is now generally accepted that the mechanism of active dissolution for many metals results from hydroxyl ion adsorption " , and the sequence of steps for iron are as follows ... 

Oxygen from the atmosphere, dissolved in the electrolyte solution provides the cathode reactant in the corrosion process. Since the electrolyte solution is in the form of thin films or droplets, diffusion of oxygen from the atmosphere/electrolyte solution interface to the solution/metal interface is rapid. Moreover, convection currents within these thin films of solution may play a part in further decreasing concentration polarisation of this cathodic process . Oxygen may also oxidise soluble corrosion products to less soluble ones which form more or less protective barriers to further corrosion, e.g. the oxidation of ferrous species to the less soluble ferric forms in the rusting of iron and steel. 

Rosenfel d" considers that SO2 can act as a depolariser of the cathodic process. However, this effect has only been demonstrated with much higher levels of SO2 (0-5%) than are found in the atmosphere (Table 2.4) and the importance of this action of SO2 has yet to be proved for practical environments. However, SO2 is 1 300 times more soluble than O2 in water" and therefore its concentration in solution may be considerably greater than would be expected from partial pressure considerations. This high solubility would make it a more effective cathode reactant than dissolved oxygen even though its concentration in the atmosphere is comparatively small. 

The cathodic process in atmospheric corrosion is often stated to be oxygen reduction, and indeed in many cases the evidence is that this is so , i.e. 

Fig. 10.5 Polarisation diagram representing corrosion and cathodic protection when the cathodic process is under mass transfer control. The values of fcorr and /cor, are lower than when there is no mass transfer restriction, i.e. when the cathodic kinetics follow the dotted line...

The metal pretreatment process requires ceneful selection, particularly with the cathodic process. BS4479 1990, Part 3, Recommendations for Conversion Coatings should be consulted. 

The solution of iron represented in equation 15.1 takes place at local anodes of the steel being processed, while discharge of hydrogen ions with simultaneous dissociation and deposition of the metal phosphate takes place at the local cathodes. Thus factors which favour the cathode process will accelerate coating formation and conversely factors favouring the dissolution of iron will hinder the process. 

Since for the cathodic process the fraction of the energy was 0EF it is reasonable to assume that when the ion travels in the reverse direction during an anodic process the fraction of the energy will be (1 - 0)EF. Thus for the anodic process... 

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