Anodic polarization curves galvanic couple

Fig. 4.28 Idealized anodic polarization curves for metals A and B and for hydrogen and oxygen reduction. An explanation for the use of these curves for estimating the corrosion potentials, currents and rates for aerated and deaerated environments and for galvanic coupling can be found in the text.

Galvanic current, fcoupie. and corrosion potential, Ecoupie, are located at the intersection of the cathodic and anodic polarization curves shown in Fig. 6.2. In the absence of polarization exerted by an external power source, galvanic current polarizes the metal surface. Mixed potential theory applies to galvanic couples as in the case of single metal polarization. Tcorr.A and Ecorr.B represent the uncoupled anode and cathode corrosion potentials. 

The concepts in Chapters 2 and 3 are used in Chapter 4 to discuss the corrosion of so-called active metals. Chapter 5 continues with application to active/passive type alloys. Initial emphasis in Chapter 4 is placed on how the coupling of cathodic and anodic reactions establishes a mixed electrode or surface of corrosion cells. Emphasis is placed on how the corrosion rate is established by the kinetic parameters associated with both the anodic and cathodic reactions and by the physical variables such as anode/cathode area ratios, surface films, and fluid velocity. Polarization curves are used extensively to show how these variables determine the corrosion current density and corrosion potential and, conversely, to show how electrochemical measurements can provide information on the nature of a given corroding system. Polarization curves are also used to illustrate how corrosion rates are influenced by inhibitors, galvanic coupling, and external currents. 

The couple corrosion potential and galvanic current is calculated at the anodic and cathodic polarization curve intersection. From Eqs. (6.9) and (6.10) ... 

Overlaying of polarization curves of the different materials in a galvanic couple must be done by first converting the current densities into total currents by multiplying the polarization curves by the exposed wetted surface area for each material. The curves then may be overlaid, and if IR drop is not important, the couple current and potential may be predicted. If the coupled potential deviates by 50-100 mV from the open-circuit potential of the anode, then the current may be converted by Faraday s Law into a corrosion rate. If the coupled potential deviates by less than 50-100 mV from the open-circuit potential of the anode, then it is conservative to add the open-circuit corrosion rate of the anode to the corrosion rate calculated from Faraday s Law to get the total anodic corrosion rate. Neither of these methods is exactly accurate, but both will give a reasonable approximation to the true corrosion rate of the anode in the couple. 

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