Corrosion process polarization curves

Considering the electrochemical nature of corrosion processes, the potentiostatic technique was successfully applied in the evaluation of corrosion processes. Polarization curves were recorded for stainless steel in aqueous model solutions with or without inhibitors (Fig. 9-11). The results of these experiments are summarized in Table 9-5. The polarization resistance (/ p) values are representative for general corrosion (Teleg-... 

Electrochemical corrosion processes also include a number of processes in organic chemistry, involving the reduction of various compounds by metals or metal amalgams. A typical example is the electrochemical carbonization of fluoropolymers mentioned on p. 316. These processes, that are often described as purely chemical reductions, can be explained relatively easily on the basis of diagrams of the anodic and cathodic polarization curves of the type shown in Fig. 5.54. 

Though processes occurring under photopassivation have not so far been understood in detail, they may be related with certainty (Izidinov, 1979) to the acceleration, under illumination, of one of the two conjugated reactions, which constitute the overall process of electrochemical corrosion. Depending on the initial state of corroding silicon, either the anodic (at the active surface) or the cathodic (at the passive surface) partial reaction is accelerated. This leads to the shift of the potential, and the system jumps over the maximum of the polarization curve from one stable state to the other. 

Given sufficient quantitative information about the electrochemical processes occurring, mixed potential theory can be used to predict a corrosion rate. Unfortunately, in the vast majority of cases, there are few data that can be applied with any confidence. In general, experimental measurements must be made that can be interpreted in terms of mixed potential theory. The most common of these measurements in electrochemical corrosion engineering is the polarization curve. 

Armed with an understanding of the underlying physical processes, the electrochemical phenomenology of localized corrosion can be better understood. Figure 23 shows three schematic polarization curves for a metal in an environment in which it spontaneously passivates and (1) can be anodized, (2) transpassively dissolves at higher potentials, and (3) pits upon further anodic polarization. We have discussed cases 1 and 2 in the section on passivity. For case 3, the region of passivity extends from to a potential labeled EM at which point the current increases dramatically at higher potentials. 

An -> ideal nonpolarizable electrode is one whose potential does not change as current flows in the cell. Much more useful in electrochemistry are the electrodes that change their potential in a wide potential window (in the absence of a - depolarizer) without the passage of significant current. They are called -> ideally polarized electrodes. Current-potential curves, particularly those obtained under steady-state conditions (see -> Tafel plot) are often called polarization curves. In the -> corrosion measurements the ratio of AE/AI in the polarization curve is called the polarization resistance. If during the -> electrode processes the overpotential is related to the -> diffusional transport of the depolarizer we talk about the concentration polarization. If the electrode process requires an -> activation energy, the appropriate overpotential and activation polarization appear. 

From the slope of the polarization curve and its variation with time (exposure time of the iron electrode), information on the kind of inhibition can be gained. An inhibition of anodic processes decreases the ia versus E current density and increases the corrosion potential correspondingly, an increase in cathodic inhibition causes a decrease in the i. and lowers the corrosion potential. 

The effect of ultrasound on the process of tellurium anodic dissolution in alkaline solutions was studied by the method of plotting polarization and galvanostatic curves [148]. Tests were made in NaOH solutions (concentrations of 0—20 g/L), subjected to the action of ultrasound at a frequency 17.5 kHz and using Te electrodeposited under ultrasound. The anodic polarization curves plotted without ultrasound and in its presence shifted with increased NaOH concentration towards negative values as a result of the increasing rate of Te anodic dissolution. The presence of ultrasound inhibited the process of Te anodic dissolution, probably due to the desorption of OFT anions from the anode surface. This sonoelectrodeposited Te thus showed greater corrosion resistance in alkaline solution than that deposited... 

The processes in real corroding systems are obviously more complicated than represented by this model. Useful quantitative calculation of the distribution of current density, and hence corrosion rate along the surface, based on the polarization curves for the anodic and cathodic reactions and on the geometry of the anodic and cathodic sites is very complex. In principle, computer-based techniques can be used if exact polarization curves and the geometry of the anodic and cathodic areas are available. For most industrially important situations, this information is not available. Also, time-dependent factors, such as film formation, make quantitative calculations of long-time corrosion rates very uncertain. The theory underlying these calculations, however, has been useful in interpreting observations in research and in industrial situations. 

Tafel Curve Modeling (Ref 4, 5). Equation 6.5 provides the form of the experimental polarization curve when the anodic and cathodic reactions follow Tafel behavior. The equation accounts for the curvature near Ecorr and Icorr, which is observed experimentally. Physically, the curvature is a consequence of both the anodic and cathodic reactions having measurable effects on Iex at potentials near Ecorr. Tafel-curve modeling uses experimental data taken within approximately 25 mV of Ecorr where the corrosion process is less disturbed by induced corro-... 

Figure 7, concerning the behaviour of the SA 213 grade T22 low alloy steel in a solution containing 100 g/1 of EDTA at pH 6 and a temperature of 100 °C, gives a very effective representation of this mathematical concept because it clearly shows the difference in shape between the experimental polarization curve (Ra—Q ft) and the curve closest to the ideal evolution of the corrosion process (1 ,=1.1 ft). In the case under examination... 

The anodic partial process. Equation 46, generates the electrons which are used in the cathodic partial process, Equation 47. This model of corrosion processes is based on the theory of mixed potentials (11) and is shown schematically in Figure 9. The original theory of mixed potentials was based on the "superposition" of polarization curves for the respective partial processes (11-13). However, since many mixed potential systems (particularly corrosion processes) involve interactions among the reactants, the presentation of mixed potentials given here will consider the more recent approach considering these interactions (14). 

Polarization curves. The rate at which the anodic or the cathodic process takes place depends on the potential ( ). The corrosion behaviour of the reinforcement can be described by means of polarization curves that relate the potential and the anodic or cathodic current density. Unfortunately, determination of polarization curves is much more complicated for metals (steel) in concrete than in aqueous solutions, and often curves can only be determined indirectly, using solutions that simulate the solution in the pores of cement paste. This is only partly due to the difficulty encountered in inserting reference electrodes into the concrete and positioning them in such a way as to minimize errors of measurement. The main problem is that diffusion phenomena in the cement paste are slow (Chapter 2). So when determining polarization curves, pH and ionic composition of the electrolyte near the surface of the reinforcement may actually be altered. 

When one is conducting an experiment more or less data are collected usually in the form of numbers. These raw data are only a sequence of numbers without any value, which require a proper meaning to become information. A series of potential and current density values is raw data, but can be processed to give polarization resistance values or anodic polarization curves which provide valuable information. en the same type of analysis is used for samples of similar nature, the evaluation and interpretation of a measurement can be totally automated. This situation occurs mostly in corrosion monitoring. In research laboratories a changing variety of samples requires flexible evaluation procedures and a more active role of the human operator. 

The dependence of the rate of oxidation upon the thickness of the Pb02 corrosion layer is the first important relationship of the anodic kinetics. The correlation between corrosion rate and time of polarization of a lead electrode has been smdied. The relationship between the weight of the oxidized lead (AP) per unit area has been determined as a function of the time of polarization at constant current density (6.5 mA cm ) [114—116]. The obtained results are given in Eig. 2.39. The shape of the curve suggests that the corrosion process comprises two stages ... 

At significantly large overpotentials (>100 mV) the anodic and the cathodic polarization curves become linear. Linear extrapolation of the curves will yield a point of intersection at the corrosion potential with the corresponding current being the corrosion current. The experimental procedure above can also be performed potentio-statically using modem potentiostats that are capable of automatically handling the process. 

Once the corrosion (mixed) potential is known, the estimation of the cathodic protection current is relatively simple the cathodic Tafel line is extended until the ordinate reaches the anode equifibrium value. The current corresponding to that ordinate value is the minimum value of the external current that must be suppfied to stop the corrosion process. For processes in which there are multiple species undergoing cathodic or anodic reactions, the resultant cathodic and anodic Tafel curves are calculated by adding the individual polarization curves within the respective potential range. 

The anodic polarization curve for a specimen with an active crevice will be in principle as shown in Figure 7.17. In this case a very small free external surface is assumed, and any internal hydrogen reduction is disregarded. is the potential as measured with the reference electrode positioned outside the crevice. As explained above, the real potential in the crevice, Ei , is more negative. The lower limit for corrosion in an active crevice is the protection (or repassivation) potential Epr. However, the critical potential that must be exceeded for initiation of the ereviee corrosion process, the crevice corrosion initiation potential, is higher than the protection potential. 

The rate of corrosion process will depend on the conductivity of electrolyte and the difference of potential between the anode and cathode. Particularly the oxygen access, necessary for the cathodic reaction, can be the factor limiting the rate of corrosion [98]. Simultaneously, as a result of corrosion current, the polarization of electrodes occurs (their potentials increase in respect to the equilibrium potential values) and the dynamically maintained potential value has the deciding effect on the corrosion rate. In the case of steel in paste environment strong polarization of anodic microareas occurs, which increase anodic potential, decreasing the difference of potential in respect to cathode therefore, as it results from the curves in E -pH system, the passivation of steel due to the oxides film occurs [98]. 

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