Corrosion potential interpretation

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. 

Since the corrosion potential of a metal in a particular environment is a mixed potential — where the total anodic current is equal to the total cathodic current —the potentiostatic curve obtained by external polarisation will be influenced by the position of the local cathodic current curve. (Edeleanu and Mueller have discussed the details which must be considered in the analysis and interpretation of the curves.) For this reason, residual oxygen in the test solution can cause a departure from the usual curve in such a... 

A different pattern of dissolution was seen with a Zn-Sn alloy containing 26% zinc. In this case the stable dissolution situation established after ca. 90 min showed a ratio of EC to CMT measurements of 1 4. As seen in Fig. 3, this remained fairly constant, though the corrosion potential increased by more than 50 mV. Only selective zinc dissolution took place, and analysis by atomic absorption spectroscopy of the amount of dissolved zinc agreed within 10% with the value according to the titration. This pattern is still difficult to understand. The ratio of ca. 1 4 between EC and CMT measurements could be interpreted in terms of formation of the low-valent zinc species ZnJ, which seems unlikely, or in terms of dissolution of divalent zinc ions accompanied by loss of chunks consisting of precisely three zinc atoms, each time a zinc ion is dissolved. The latter alternative seems to require a more discrete mechanism of dissolution than... 

The interpretation of corrosion potential has always been difficult. Wolstenholme [3] concluded that the corrosion potential was not an unambiguous indicator of the amouunt of corrosion, and Cerisola and Bonora [7] described the measurement as one with no quantitative relationship to amount of corrosion. The results shown in Figure 2 confirm the questionable value of potential measurements in correlations with the corrosion of an underlying substrate. 

However, it should not be surprising if the anodic and the cathodic Tafel plots do not intersect at E = Ecorr as the two reactions participating in the corrosion process are actually studied at potentials far removed from the corrosion potential. Moreover, it is not quite realistic to rely on the very simple model described here. Therefore, it appears more useful to record a complete current-potential characteristic and to attempt its interpretation in terms of simultaneous processes that can possibly be expected. Several practical examples have been extensively reviewed [93]. 

The earlier sections of this chapter discuss the mixed electrode as the interaction of anodic and cathodic reactions at respective anodic and cathodic sites on a metal surface. The mixed electrode is described in terms of the effects of the sizes and distributions of the anodic and cathodic sites on the potential measured as a function of the position of a reference electrode in the adjacent electrolyte and on the distribution of corrosion rates over the surface. For a metal with fine dispersions of anodic and cathodic reactions occurring under Tafel polarization behavior, it is shown (Fig. 4.8) that a single mixed electrode potential, Ecorr, would be measured by a reference electrode at any position in the electrolyte. The counterpart of this mixed electrode potential is the equilibrium potential, E M (or E x), associated with a single half-cell reaction such as Cu in contact with Cu2+ ions under deaerated conditions. The forms of the anodic and cathodic branches of the experimental polarization curves for a single half-cell reaction under charge-transfer control are shown in Fig. 3.11. It is emphasized that the observed experimental curves are curved near i0 and become asymptotic to E M at very low values of the external current. In this section, the experimental polarization of mixed electrodes is interpreted in terms of the polarization parameters of the individual anodic and cathodic reactions establishing the mixed electrode. The interpretation then leads to determination of the corrosion potential, Ecorr, and to determination of the corrosion current density, icorr, from which the corrosion rate can be calculated. 

The sums of the currents resulting from the oxidation and from the reduction reactions are also shown in Fig. 4.13 as a function of potential. The steady-state corrosion condition of ZI0X = ZIred corresponds to the intersection of the XIox and XIred lines, which identifies the corrosion potential, Ecorr. The solution ohmic resistance is assumed to be very small for the present interpretation. 

However, it has to be noted that the physical meaning of the corrosion potential in this case carmot be interpreted by conventional electrochemical kinetics. As long as no currents are present, the information gained on the irmer interface potential could be determined by dipole orientation of segments of the polymer chain. 

Interpretation. Half-cell potential measurements allow the location to be determined of areas of corroding rebars being the most negative zones in a potential field (Figure 16.7). However, the interpretation of the readings is not straightforward because the concrete cover and its resistivity in addition to the actual corrosion potential of the steel (Chapter 7) influence the readings at the concrete surface. 

In the advanced state of delamination, two zones of different activity can be located underneath the organic coating. The shift of the corrosion potential close to the rather anodic potential of the intact interface marks the front of the advancing cathode. Behind this cathodic area the steep slope marks the front of anodic undermining. Fiirbeth and Stratmann proved this interpretation of the SKP data with cross-sectional and surface analysis of the delaminated area [86]. 

Rp measurements, preferably with free corrosion potential monitoring can yield real time corrosion data from which interpretation related to the corrosion regime is possible. 

Very negative potentials can be found in saturated conditions where there is no oxygen to form a passive layer but with no oxygen there can be no corrosion (see Section 2.2). This shows the weakness of potential measurements. It is a measure of the thermodynamics of the corrosion, not of the rate of corrosion. Corrosion potentials can be misleading, their interpretation is based on empirical observation, not rigorously accurate scientific theory. The problem is that the potential is not purely a function of the corrosion condition but also other factors, and that the corrosion condition is not the corrosion rate. 

Electrode kinetics is the study of reaction rates at the interface between an electrode and a liquid. The science of electrode kinetics has made possible many advances in the understanding of corrosion and the practical measurement of corrosion rates. The interpretation of corrosion processes by superimposing electrochemical partial processes was developed by Wagner and Traud [1]. Important concepts of electrode kinetics that wifi be introduced in this chapter are the corrosion potential (also called the mixed potential and the rest potential), corrosion current density, exchange current density, and Tafel slope. The treatment of electrode kinetics in this book is, of necessity, elementary and directed toward application of corrosion science. For more detailed discussion of electrode kinetics, the reader should refer to specialized texts Usted at the end of the chapter. 

Electrochemical methods, such as anodic polarization curves electrochemical impedance spectroscopy (EIS), and measurement of the corrosion potential (open circuit or rest potential) are primarily laboratory tests. They require experience in interpretation of the results, but have the advantage of very short test times. As such, they are important in mechanistic studies, but certain commercial uses also exist. 

The main concept that most of the corrosion data interpretation is based on was first introduced by Wagner and Traud (1938), according to which galvanic corrosion is an electrochemical process with anodic and cathodic reactions taking place as statistically distributed events at the corroding surface. The corresponding partial anodic and cathodic currents are balanced so that the overall current density is zero. This concept has proven to be very useful, since it allowed all aspects of corrosion to be included into the framework of electrochemical kinetics. Directly deduced from this were the methods of corrosion rate measurement by Tafel line extrapolation, or the determination of the polarization resistance Rp from the slope of the polarization curve at the open circuit corrosion potential... 

This equation was first postulated empirically by Wagner and Traud (1938). In Eq. (7-26), b+ and b are the slopes of the Tafel lines of the anodic and cathodic partial reactions. The fundamentals of polarization resistance measurements have been described in more detail by Mansfeld (1976). This concept has also been adopted for the interpretation of EIS (Mansfeld, 1981 Mansfeld et al., 1982). For the simplest case of a purely reaction controlled corrosion process, the Faraday impedance Zp in Fig. 7-3 may be replaced by a potential dependent charge transfer resistance / (( ), which is composed of the charge transfer resistances of the anodic and cathodic partial reactions. At the corrosion potential, the polarization resistance corresponds to Rp = R (Eco ) Th overall impedance of the equivalent circuit in Fig. 7-3 can then be described by... 

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