Corrosion current example calculations

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 ... 

Calculate the corrosion currents for the metals in Example 5.1, using the Stearn-Geary equation for cathodic and anodic Tafel slopes of h — 0.1 V and h — 0.1 V. Estimate the corrosion rates in mpy. 

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... 

Although values of P are relatively well known for discharge, they are not as generally available for other electrode reactions. Stern showed, however, that the majority of reported P values are between 0.06 and 0.12 V. If p is known to be 0.06 V, for example, and p is between 0.06 and 0.12 V, the calculated corrosion current is within at least 20% of the correct value. Under other assumptions, the corrosion rate can be calculated to at least a factor of 2. 

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. 

Potentiodynamic polarization measurements are quite appropriate for determination of the pitting susceptibility of aluminum coatings, and/or the corrosion current density/ corrosion rate of coated steel products in general. ASTM G 102, Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements, describes the calculation of corrosion rates and other information from electrochemical measurements. Another example of the use of DC electrochemical methods to examine the corrosion performance of coated sheet materials is a study by D. A. Jones et al. [48]. The study used polarization resistance measurements to examine the mechanism of steel and coated sheet degradation under conditions of alternate immersion. Jones compared the polarization resistance of samples of low-carbon steel, unpainted galvanized, aluminum-coated, and Zn-Ni alloy coated steel during continuous immersion and alternate immersion. Alternate immersion cyclic exposure produced a thick oxide that led to significant underfilm attack. Jones found that phosphate pretreatment tends to increase the resistance of these materials to underfilm attack. This study is an excellent example of the way electrochemical measurements can be used as a complement to other techniques to elucidate mechanistic information. 

If the corrosion current is estimated, and we believe that the assumed electrochemical half-reactions, for example, (9.1) and (9.2), as well as the total reaction, for example, (9.3), take place on the surface of an electrode (or a corroding metal), the rate of the electrochemical corrosion (CR) (as depth of the corrosion penetration per time) can be calculated based on Faraday s law as follows ... 

Equation (1) has a form analogous to the Butler-Volmer equation of electrode kinetics therefore, it is not surprising that the techniques of determinations are analogous in many ways to those for the determination of exchange current density in electrode kinetics (see, for example. Ref. 15 and 16). The corrosion rate measurement techniques can be classified in two ways. First, one can consider the different ways that Eq. (1), or its equivalent, is used to calculate the corrosion current density from measured current-density-polarization data. Second, one can consider the different ways that the current-density-polarization data are measured experimentally. The first classification is used in this chapter. The second classification is discussed briefly in Section II.7. 

The time resolution for the detection of a product at the disk by the ring current is in the range of 0.1 s, which is required for the transport due to the laminar flow of the electrolyte in front of the electrode. Therefore, this method is suited for measurements of potentio-static dissolution transients in this time frame. One example for corrosion is the separation of currents measured during passivation transients at the disc into a part for metal dissolution and another for oxide formation. The ring current allows calculating the dissolution rate i at the disc according to Equation 1.130, whereas the difference to the total disc current foi - ic = K yields the current density of layer formation These investigations may be done with the time resolution of the method of ca. 0.1 s, i.e., x c and may be followed as a function of time. 

Another example for the HMRRD electrode is given in Fig. 9 for Fe in alkaline solutions [12, 27]. The square wave modulation of the rotation frequency co causes the simultaneous oscillation of the analytical ring currents. They are caused by species of the bulk solution. Additional spikes refer to corrosion products dissolved at the Fe disc. This is a consequence of the change of the Nemst diffusion layer due to the changes of co. This pumping effect leads to transient analytical ring currents. Besides qualitative information, also quantitative information on soluble corrosion products may be obtained. The size of the spikes is proportional to the dissolution rate at the disc, as has been shown by a close relation of experimental results and calculations [28-30]. As seen in Fig. 7, soluble Fe(II) species are formed in the po-... 

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 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]. 

A large number of parameters are involved in the choice of the corrosion protection system and the provision of the protection current these are described elsewhere (see Chapters 6 and 17). In particular, for new locations of fixed production platforms, a knowledge of, for example, water temperature, oxygen content, conductivity, flow rate, chemical composition, biological activity, and abrasion by sand is useful. Measurements must be carried out at the sea location over a long period, so that an increased margin of safety can be calculated. 

Aluminum anodes are used for the internal cathodic protection of large crude oil tanks which are susceptible to damage from corrosive salt-rich deposits. In an earlier example [6] 71 anodes were equally spaced in the base area. The base region up to 1 m in height in the region of the water/oil interface had, including the inserts, an area of 2120 m and was protected with 17 A. The protection current density was 8 mA m". With a total anode weight of 1370 kg and with 2pj = 2600 A h kg , the service life was calculated to be 24 years. 

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