Overvoltage curves

Figure 4.5 Overvoltage curve for pure concentration polarization.

The complete determination of position and shape of overvoltage curves requires knowledge about the quantities io, it, be, and ba, of which the first one is given by Nemst s equation or the Pourbaix diagram. Through such knowledge, the best tool we have for description of the kinetics of electrode reactions is available. With a reservation for passivation effects (which we shall return to) we can with this background obtain a quantitative description of any corrosion process, in principle as shown in Section 4.9. 

It is seen that there is only one point in the diagram (Figure 4.7) that satisfies these two conditions, namely the intersection point between the anodic and cathodic overvoltage curves (the other two Tafel lines, i.e. the broken lines for the cathodic iron reaction and the anodic hydrogen reaction, contribute extremely little to the net cathodic respective anodic current density, so that in this case we can neglect both of them). 

As we have seen, if the equilibrium potentials, the exchange current densities, and the slope/shape of the overvoltage curves of the involved reactions are known, the corrosion current density and the (corrosion) potential can be determined by means of the E-log i diagram. 

As we have seen, overvoltage curves can be used to determine corrosion rate. In order to draw the overvoltage curves, one must, however, know io, ba, bo and possibly 1l. Eq can be calculated by thermodynamics (or possibly measured on a... 

We assume that fliere are two possible electrode reactions on die working electrode, wifli overvoltage curves (potential-log current curves) and corrosion potential as shown by the solid curves in Figure 4.10. (We neglect still the dotted curves to the left in flic diagram.)... 

It is seen that the overvoltage curves (the Tafel lines) are asymptotes to the polarization curves. When E deviates much from the corrosion potential, the polarization curves and the overvoltage curves merge, and this fact is utilized. In Figure 4.10 we started with the overvoltage curves and drew the polarization curves on this basis. But, as mentioned previously, the purpose is to determine the overvoltage curves from the polarization curves, i.e. to go the opposite way. We record le as a function of potential by means of the potentiostat, i.e. we record the polarization... 

F igure 4.10 Overvoltage curves and corresponding polarization curves. 

Conditions That Affect Polarization Curves, Overvoltage Curves and Corrosion Rates... 

Which four quantities are necessary for determination of (the shape and position of) a cathodic overvoltage curve in a case with activation polarization at small overvoltages (low numerical values of ri) and concentration polarization at higher values of ri ... 

Construct the cathodic and anodic polarization curves from the overvoltage curves in Exercise 5. How are polarization and overvoltage curves usually determined ... 

Passivation and Passivity Described by Anodic Polarization and Overvoltage Curves... 

The apparent deviation from a linear overvoltage curve is somewhat larger than the real one because there is a voltage drop between the reference electrode and the iron sample due to the high current density. 

Figure 5.1 Anodic polarization and overvoltage curve for iron in 1 N H2SO4, mainly based upon data from Franck, Franck and Weil, and Hersleb and EngeU, collected by Kaesche [5.1]. Overvoltage curve for hydrogen reduction estimated from other data [5.2, 5.3].

As we have seen, we can show the regions of immunity, activity (corrosion) and passivity as potential regions by means of die anodie overvoltage curve and as pH-... 

Figure 5.2 Pourbaix diagram and anodic overvoltage curves for iron in water.

Figure 5.3 Anodic polarization curve (identical with the anodic overvoltage curve for metal dissolution in the potential range from -0.2 to +1.6 V) for an 18-8 CrNi steel in 1 N H2SO4 without oxygen at 50 C (from Kaesche [5.1] after Engell and Ramchandran).

Faraday s experiment can be explained by overvoltage curves as shown in Fig. 5.5. The intersection between the anodic and the cathodic curve shows that there is only one stable state (1) in concentrated acid (stable passive state). In diluted acid the reversible potential of the cathodic reaction (Eq) is more negative because of a lower concentration of oxidizer, i.e. nitric acid (in agreement with Nemsf s equation (3.9), see also p. 65-66). The result is that there are three intersection points between the anodic and the cathodic curve in this case, giving the possibility of an unstable passive state (2) or an active state (3). For comparison, a case (4) is shown where the active state is the only possible one. 

Figure 6.7 Schematic overvoltage curves corresponding to Figure 6.6 (active metal).

The reversible potentials of reactions (4.2a) and (4.2b) are defined by the same line in the Pourbaix diagram. However, the nature of hydrogen ion access affects the electrode kinetics, thus the overvoltage curve of the two reactions are different. This... 

The lower straight line is the overvoltage curve of reaction (4.2b), while the cathodic curves above fliis line represent reaction (4.2a). It appears that (4.2b) has a stronger activation polarization flian (4.2a). On the other hand, reaction (4.2a) is limited by concentration polarization (diffusion control) at higher current densities. The consumption of is in fliese cases determined by the diffusion rate of ions to the metal surface. 

Figure 6.16 Smoothed cathodic overvoltage curves for a high-alloy stainless steel in aerated and chlorinated seawater at 25 C (From Gartland and Drugli [6.34]).

Figure 6.16 shows how different concentrations of chlorine can affect cathodic overvoltage curves for stainless steel in seawater. The corrosion risk for stainless steel at higher CI2 concentrations arises because the increased cathodic reaction rate lifts the potential so that critical potentials for local corrosion arc exceeded (see also Section 8.3). Chlorine may cause corrosion on several other materials as well. 

Deseription of uniform corrosion by means of polarization and overvoltage curves is relatively simple, since we can consider both the anodic and the cathodie area equal to the total area. 

Parallel use of the Pourbaix diagram and overvoltage curves is sometimes useful when evaluating how typical and stable the uniform corrosion is under different conditions. Let us consider a case with iron or unalloyed steel in two different environments ... 

A relevant series of this type is a more adequate basis for prediction of galvanic corrosion risks than is the standard reversible potential series. However, the practical galvanic series provide only qualitative and incomplete information. To explain what conditions determine the rate of galvanic corrosion, and to predict the rate, we must study the overvoltage curves of the actual reactions that take place (see Figure 7.7). 

Figure 7.7 Cathodic and anodic overvoltage curves for a galvanic element of the metals A (less nohle) and B (more nohle).

Figure 7.27 Anodic overvoltage curve for an active-passive metal in an environment causing pitting corrosion. Ep = pitting potential, Ep = passivation potential, i r = critical current density and ip = passive current density.

Figure 7.40 Overvoltage curves at a) low velocity and formation of a layer of corrosion product on the surface (—), b) high flow velocity and removed corrosion product, c) increased cathodic reaction as a result of galvanic contact with a more noble metal, and d) flow velocity high enough to cause passivity.

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