Corrosion potential example calculations

The Tafel expressions for both the anodic and the cathodic reaction can be directly incorporated into a mixed potential model. In modeling terms, a Tafel relationship can be defined in terms of the Tafel slope (b), the equilibrium potential for the specific half-reaction ( e), and the exchange current density (70), where the latter can be easily expressed as a rate constant, k. An attempt to illustrate this is shown in Fig. 10 using the corrosion of Cu in neutral aerated chloride solutions as an example. The equilibrium potential is calculated from the Nernst equation e.g., for the 02 reduction reaction,... 

In the previous discussion, Faraday s law was derived on the basis that the net metal oxidation current, Inetox, was equal to the corrosion current, Icorr, at the corrosion potential, Ecorr. Although this is by far the most common way in which Faraday s law is applied in the analysis of corrosion, it should be noted that the law is quite general in terms of relating currents to electrochemical reaction rates. For example, in Eq 4.30 and 4.31, if icorr is replaced with inet ox (or iox M if ired M is negligible), the equations allow calculations of Cl and CPR at any potential. Alternately, the net reduction rate at any potential (including Ecorr) can be obtained from Eq 4.27 upon replacement of Icorr with Inet red. 

Example Calculations of Corrosion Potentials, Corrosion Currents, and Corrosion Rates for Aerated and Deaerated Environments, and the Effects of Galvanic Coupling... 

For example, the corrosion current and corrosion potential of Fe in a solution of pH 7 saturated with oxygen (1 atm) can be calculated graphically or analytically if the following electrochemical kinetic parameters are known ... 

In this section, the behavior of a redox system at the equilibrium potential has been discussed. It should, however, be noted that impedance spectroscopy of irreversible systems can also yield useful information. For example, the charge-transfer resistance determined at the corrosion potential corresponds to the slope of the current-potential curve (/ ct = dV(t)/dI (t) at that potential and allows calculation of the rate of corrosion [1]. 

The corrosion current can be calculated from the corrosion potential and the thermodynamic potential if the equation expressing polarization of the anode or cathode is known, and if the anode-cathode area ratio can be estimated. For corrosion of active metals in deaerated acids, for example, the surface of the metal is probably covered largely with adsorbed H atoms and can be assumed, therefore, to be mostly cathode. The thermodynamic potential is -0.059 pH, and if icon is sufficiently larger than io for 2 - e, the Tafel equation expresses... 

Now let us recall the two examples of copper and gold, which were shown to be feasible, and not feasible, respectively, with respect to corrosion, based on the free energy values, calculated for the respective reactions. The same conclusion can be reached based on the electrochemical potentials for the reactions of copper and gold. 

Most electrochemical testing conducted to date has used various DC approaches. The most common methods involve linear polarization (to determine the polarization resistance for calculation of corrosion current via the Stem-Geary equation) [44] and potentiodynamic polarization (to determine breakdown and repassivation potentials). Other tests are also conducted, however. For example, long-term open circuit potential versus time measurements, potentiostatic chronoamperometry, and galvanostatic measurements are occasionally conducted for specialized purposes. 

Metal coupon data are relatively easy to use for materials selection purposes from calculation of corrosion rate and observations on the appearance of the coupon. For nonme-tallic materials, the problem is to define what constitutes failure. For example, when is color change, flaking, swelling, or tackiness acceptable, and when do they indicate potential failure ... 

The present section illustrates how calculations from basic thermodynamic data can lead to open-circuit cell potential in any condition of temperature and pressure. Chemical power sources, with the exception of fuel cells, are all based on the corrosion of a metal connected to the negative terminal. The aluminum-air power source, that owes its energy to the corrosion of aluminum in caustic, was chosen for this example because of the relative simple chemistry. 

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