Mixed potentials and corrosion

The absence of a net current does not necessarily mean that the interface is in equilibrium. In fact, several reactions may proceed in such a way that the total current vanishes. We consider the case where two reactions, an anodic and a cathodic one, balance. The reaction scheme is  

We assume that both reactions obey the Butler-Volmer equation, and denote the corresponding transfer coefficients by a and 2, the exchange current densities by jo,i and jo,2 and the equilibrium potentials by j) and 4%. Since the total current density is zero we have  

Each reaction proceeds with a current density of  

Of course, one can substitute pm from Eq. (11.41), but the resulting expression is complicated. The mixed potential and the two partial currents are illustrated in Fig. 11.4. 

An important example is the corrosion of metals. Most metals are thermodynamically unstable with respect to their oxides. In the presence of water or moisture, they tend to form a more stable compound, a process known as wet corrosion (dry corrosion is not based on electrochemical reactions and will not be considered here). Moisture is never pure water, but contains at least dissolved oxygen, sometimes also other compounds like dissolved salt. So a corroding metal can be thought of as a single electrode in contact with an aqueous solution. The fundamental corrosion reaction is the dissolution of the metal according to  

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Fig. 4. Effect of the exchange current on the mixed potential and corrosion current (schematic).

Fig, 1,26 E Vi, log (curves for the corrosion of a metal in a reducing acid in which there are two exchange processes (c,f. Fig, L24) involving oxidation of M—are reduction of —vH2. Note that (o) the reverse reactions for exchange process are negligible at potentials removed from E, (b) the potential actually measured is the corrosion potential E , which is mixed potential, and (c) the E vs. (,pp curves (where ijppi is the applied current density) when extrapolated intersect at corr. 

Fig. 1.71 How alloying with a noble metal produces a passive mixed potential and a marked reduction in corrosion rate (after Stern and Wissenberg )...

The successful clinical use of titanium and cobalt-chromium alloy combinations has been reported Lucas etal. also investigated this combination using electrochemical studies based on mixed potential and protection potential theories. Verification of these studies was made by direct coupling experiments. The electrochemical studies predicted coupled corrosion potentials of -0.22 V and low coupled corrosion rates of 0.02 ft A/cm. Direct coupling experiments verified these results. The cobalt-titanium interfaces on the implants were macroscopically examined and no instances of extensive corrosion were found. Overall, the in-vitro corrosion studies and the examination of retrieved prostheses predicted no exaggerated in-vivo corrosion due to the coupling of these cobalt and titanium alloys. 

The effect of (10)h on a given metal, M, is shown schematically in Fig. 4. Curves A and B represent the anodic and cathodic Tafel lines for the metal, respectively. The curves C and D, and E and F show two possible sets of Tafel lines for the hydrogen reactions on the metal M. The former set corresponds to a lower value of (i0)H2 t 3an the latter. It will be seen that the value of the mixed potential and the corrosion current depend on the value (iQ)n2 a d that icor increases as (10)h increases while the mixed potential becomes more... 

While an ovapotential may be applied electrically, we are interested in the overpotential that is reached via chemical equilibrium with a second reaction. As mentioned previously, the oxidation of a metal requires a corresponding reduction reaction. As shown in Figure 4.34, both copper oxidation, and the corresponding reduction reaction may be plotted on the same scale to determine the chemical equilibrium between the two reactions. The intersection of the two curves in Figure 4.34 gives the mixed potential and the corrosion current. The intersection point depends upon several factors including (the reversible potential of the cathodic reaction), cu2+/cu> Tafel slopes and of each reaction, and whether the reactions are controlled by Tafel kinetics or concentration polarization. In addition, other reduction and oxidation reactions may occur simultaneously which will influence the mixed potential. 

As an alternative to generating an entire polarization diagram, we can use the exchange current densities and the equilibrium potentials of the anodic and cathodic reactions to estimate the corrosion potential and corrosion current by extrapolating the cathodic and anodic polarization lines of the corroding system. At the corrosion potential, the anodic and cathodic currents are equal. The schematic shown in Fig. 3.6 represents a case for which the anode and the cathode area are the same once the corrosion current is known, the rate of deterioration of the electrode can be estimated. The accurate prediction of the corrosion (mixed) potential depends on the polarization behavior of the specific electrode. 

The potential and the current density at the intersection point are called corrosion potential and corrosion current density. The potential is also called the mixed potential. 

In a PEFC, oxygen and hydrogen crossover is important because of the obvious performance loss, the development of a mixed potential, and even durability issues due to hydrogen peroxide generation platinum migration, and possible carbon corrosion [69]. Furthermore, crossover becomes increasingly important as the membranes used become thinner. Presented in this section are the parameters and governing equations to model this phenomenon. 

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. 

There has been a tendency to correlate half cell potentials with corrosion rates. The half cell potential is a mixed potential representing anodic and cathodic areas on the rebar. It is not the driving potential in the corrosion cell. Any correlation between potential and corrosion is fortuitous and is often due to holding other variables constant in laboratory tests. 

Bubbles of hydrogen are observed from the surface of zinc electrode, and formation of bubbles of hydrogen is a cathodic reaction. Hydrogen is reduced and not oxidized. Similarly, zinc is oxidized and not reduced. Hence, only the two reactions (a) and (b) proceed. Under the condition of rest (no outside current), the potential of the electrode cannot be computed by the Nernst equation as it is not reversible. Also, the above electrode would not corrode in the absence of an external current. The potential assumed by the electrode under the above condition is the mixed potential and its value lies between the value of equilibrium potential of hydrogen and zinc. The value of the potential would depend on the metal and the environment. It is to be observed that the corrosion potential (Ecorr) is not the equilibrium potential of either of the reactions, but some intermediate potential determined by the two partial anodic and cathodic reactions. Both the reactions... 

When a metal, M, corrodes in a solution, there must be at least one oxidation and one reduction process. What is measured is the sum total of all partial cathodic processes and partial anodic processes occurring during corrosion of a metal. An anodic curve represents the sum total of all partial oxidation processes and a cathodic curve, the sum total of all partial reduction processes. The point of intersection of anodic and cathodic polarization curves in an Evans diagram gives the mixed potential Ecorr (corrosion potential), also called the compromise potential, or mixed potential, or free corrosion potential, and the corrosion current (icorr)-... 

Figure 19.4 Similarity between open-circuit corrosion and electroless plating at open circuit (a) Mixed potential in corrosion (same as in Figure 18.1). (b) Mixed potential in electroless plating. CUSO4 0.1 M, ethylenediamine tetraacetate (EDTA) 0.1 M, formaldehyde (FA) 0.05 M, pH 12.5.

Different microstructural regions in a material which has an almost uniform composition can also lead to the formation of corrosion cells (e.g., in the vicinity of welds). Basically, corrosion cells can be successfully overcome by cathodic protection. However, in practice, care has to be taken to avoid electrical shielding by large current-consuming cathode surfaces by keeping the area as small as possible. In general, with mixed installations of different metals, it must be remembered that the protection potentials and the protection range depend on the materials (Section 2.4). This can restrict the use of cathodic protection or make special potential control necessary. 

The above considerations show that the rate of a corrosion reaction is dependent on both the thermodynamic parameter and the kinetic parameters rjj and rjj. It is also apparent that (q) the potential actually measured when corrosion reaction occurs on a metal surface is mixed, compromise or corrosion potential whose magnitude depends on E, and on the Ej, -I and Ej, -I relationships, and (b) direct measurement of 7 is not possible when the electrodes are inseparable. 

Equation 10.2, which involves consumption of the metal and release of electrons, is termed an anodic reaction. Equation 10.3, which represents consumption of electrons and dissolved species in the environment, is termed a cathodic reaction. Whenever spontaneous corrosion reactions occur, all the electrons released in the anodic reaction are consumed in the cathodic reaction no excess or deficiency is found. Moreover, the metal normally takes up a more or less uniform electrode potential, often called the corrosion or mixed potential (Ecotr)-... 

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... 

Stern, eta obtained potentiostatic polarisation curves for titanium alloys in various solutions of sulphuric acid and showed that the mixed potentials of titanium-noble metal alloys are more positive than the critical potential for the passivity of titanium. This explains the basis for the beneficial effects of small amounts of noble metals on the corrosion resistance of titanium in reducing-type acids. Hoar s review of the work on the effect of noble metals on including anodic protection should also be consulted... 

Corrosion or mixed potentials (a) Active corrosion in acid solutions (b) Passive metal in acid solutions Potential dependent on the redox potential of the solution and the kinetics of the anodic and cathodic reactions. Potential dependent on the kinetics of the h.e.r. on the bare metal surface. Potential is that of an oxide-hlmed metal, and is dependent on the redox potential of the solution. Zn in HCI Stainless steel in oxygenated H2SO4... 

Corrosion Potential (mixed potential, compromise potential) potential resulting from the mutual polarisation of the interfacial potentials of the partial anodic and cathodic reactions that constitute the overall corrosion reaction. 

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