Overpotential in corrosion

Resistance overpotential i/r Since in corrosion the resistance of the metallic path for charge transfer is negligible, resistance overpotential ijr is determined by factors associated with the solution or with the metal surface. Thus resistance overpotential may be defined as... 

The importance of the method in corrosion testing and research has stimulated other work, and since Stern s papers appeared there have been a number of publications many of which question the validity of the concept of linear polarisation. The derivation of linearity polarisation is based on an approximation involving the difference of two exponential terms, and a number of papers have appeared that have attempted to define the range of validity of polarisation resistance measurements. Barnartt" derived an analytical expression for the deviations from linearity and concluded that it varied widely between different systems. Leroy", using mathematical and graphical methods, concluded that linearity was sufficient for the technique to be valid in many practical corrosion systems. Most authors emphasise the importance of making polarisation resistance measurements at both positive and negative overpotentials. 

One of the questions in the mechanism of hydrogen evolution, and ancillary topics connected with the surface hydrogen involved in corrosion, is the dependence of the surface hydrogen upon overpotential. This topic has importance not only because of the... 

The difference between the potential applied and the reversible potential for a reaction is known as the overpotential. It represents the driving force for the kinetics of the reaction. Anodic overpotentials are associated with oxidation reactions, and cathodic overpotentials are associated with reduction reactions. The relationship between the overpotential and the reaction rate defines the kinetics. Mathematical relationships exist for many instances, but in corrosion situations, the data are generally experimentally derived. 

The EV range is obviously affected by the level of selfdischarge. The latter is mainly due to the reduction of Pb02 by lead of the grid and to the formation of anodic oxidation products (e.g., of Sb) which diffuse to the negative plate. Their deposition decreases the hydrogen overpotential and results in corrosion of... 

In order to minimize the hydrogen evolution corrosion reaction that occurs at the open circuit potential and on the charge, which lowers the energy and ciurent efficiency, electrolyte additives are used in order to increase the hydrogen overpotential. In particular, sulphide ions have favourable effects. 

The results of such measurements are known as current density-potential curves. They represent cumulative curves given by the superimposition of the current density-potential curves of the individual reactions. For simple electrodes with defined electrode processes, these are the overpotential curves. For metals exposed to electrolytic attack, superimposition of several overpotential curves gives the actual current density-potential curves that are of significance in corrosion testing and research. Figure 20.9 shows the superimposition of the overpotential curves of a hydrogen electrode... 

The occurrence of a concentration overpotential in oxygen corrosion is clearly shown by the cathode overpotential curve. As shown in Figure 20.15, a diffusion-limited current density appears... 

The polarization term that controls the corrosion rate of many metals in deaerated water and in nonoxidizing acids is hydrogen overpotential. In accord with the previously discussed definition of polarization, hydrogen overpotential is the difference of potential between a cathode at which hydrogen is being evolved, ( )measured, 3nd a hydrogen electrode at equilibrium in the same solution that is. 

Both alloyed sulfur and phosphorus markedly increase the rate of attack in acids. These elements form compounds that apparently have low hydrogen overpotential in addition, they tend to decrease anodic polarization so that the corrosion rate of iron is stimulated by these elements at both anodic and cathodic sites. Rates in deaerated citric add are given in Table 7.5 [43]. In strong acids, the effects of these elements are still more pronounced [44] (see Fig. 7.13 and Table 7.5). 

Cathode mass transport Higher mass-transport overpotentials in start/stop-cycled ceU Electrolyte fiooding due to carbon corrosion and resulting changes in hydrophobicity Significant increase of mass-transport overpotentials Severe corrosion of carbon results in loss of structural integrity of cathode catalyst layer and void volume loss... 

The electrolyte must be specially purified because (a) the lead anodes are attacked by an electrolyte containing more than about 50 mgdm" of chloride ions and (b) almost any other metal present in the electrolyte would be deposited on the cathode, and would then tend to lower the necessary high hydrogen overpotential (see corrosion). The current efficiency is about 90%, an inevitable loss being the chemical attack of the zinc by the acid electrolyte. 

There is great interest in electrochemical treatment of waste water using diamond electrodes. In addition to the stability of these electrodes in corrosive electrolytes, they also exhibit a large overpotential for oxygen evolution. This permits the production of strong oxidants, such as ozone and hydroxyl radical [32-36]. 

The various types of overpotentials are dealt with in more detail in Section 20.1 but it is appropriate here to outline the significant factors in relation to their importance in controlling the rate of corrosion reaction. 

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