Corrosion potential current case

Sufficient temperature differences between sites on the same component can cause a galvanic current flow. In these cases, the site with the higher temperature is usually the corrosion site (see Case History 16.3). Galvanic corrosion of this form can potentially affect heat exchangers and refrigeration equipment. 

Equation (2-38) is valid for every region of the surface. In this case only weight loss corrosion is possible and not localized corrosion. Figure 2-5 shows total and partial current densities of a mixed electrode. In free corrosion 7 = 0. The free corrosion potential lies between the equilibrium potentials of the partial reactions and U Q, and corresponds in this case to the rest potential. Deviations from the rest potential are called polarization voltage or polarization. At the rest potential = ly l, which is the corrosion rate in free corrosion. With anodic polarization resulting from positive total current densities, the potential becomes more positive and the corrosion rate greater. This effect is known as anodic enhancement of corrosion. For a quantitative view, it is unfortunately often overlooked that neither the corrosion rate nor its increase corresponds to anodic total current density unless the cathodic partial current is negligibly small. Quantitative forecasts are possible only if the Jq U) curve is known. 

As discussed in detail in Chapter 2, the corrosion potential is determined by the intersection of the sum of the anodic Evans lines and the sum of the cathodic Evans lines. For active-passive materials, the only new wrinkle is the increased complexity of the anodic line. Since the anodic line is not single-valued with respect to current density, three distinct cases can be considered. In all cases, the condition E /a = X Ic determines the position of the corrosion potential, and the condition im = z a - ic determines the appearance of the polarization curve... 

Open-circuit potential (OCP) — This is the - potential of the - working electrode relative to the - reference electrode when no potential or - current is being applied to the - cell [i]. In case of a reversible electrode system (- reversibility) the OCP is also referred to as the - equilibrium potential. Otherwise it is called the - rest potential, or the - corrosion potential, depending on the studied system. The OCP is measured using high-input - impedance voltmeters, or potentiometers, as in - potentiometry. OCP s of - electrodes of the first, the second, and the third kind, of - redox electrodes and of - ion-selective membrane electrodes are defined by the - Nernst equation. The - corrosion po-... 

Several cases can be distinguished. Line 1 crosses the anodic dissolution curve in the active region, leading to a substantial rate of corrosion. The situation is very similar to that shown in Fig. lOM an increase in the rale of the cathodic reaction leads to a shift of the corrosion potential in the anodic direction and an increase in the corrosion current. Line 2 crosses tlie line for metal dissolution in... 

The potential at which the current for multiple electrochemical reactions is equal to zero is termed the mixed potential or, in the case of metal dissolution, the corrosion potential. Concepts of thermodynamics, kinetics, and transport must be applied to calculate values for the mixed or corrosion potential. 

When one of the redox couples is associated with the dissolution of the electrode, M + mh M" + n- m)e, OCP is also called the corrosion potential and the net dissolution rate at OCP is the corrosion current, 4orr = 4a - 4c- This is generally the case with a silicon electrode at OCP in aqueous solutions because the thermodynamic poten-... 

In the case where the anodic dissolution is inhibited, e.g., by surface adsorption of a chemical species, the anodic curve becomes 2. This will result in a more positive corrosion potential (from E°o to if the cathodic reaction remains unchanged. In such a situation the corrosion current is reduced with a more positive potential relative to the original value. On the other hand, if the anodic dissolution kinetics remains unchanged but the rate of the cathodic reactions is changed from curve Ic to curve 2, the potential also becomes more positive (from 2orrto Ecorr). However, in this case the corrosion current is increased with a more positive potential. 

The constant term depends on the environmental conditions such as temperature, pH, concentration of oxygen and the reference electrode offset. But the differential method without the term has advantages on them, in case that the same reference electrodes are used in the short-time measurement. This formulation easily eliminates the effect of open circuit corrosion potential and reference electrode offset. If the potential or current density are constant in two boundary conditions, the differential boundary conditions are zero according to Eqn. (12) or Eqn. (13). 

The evaluation of field of current density is essential in problems of galvanic corrosion. In many cases the direct measurement of current density is not feasible, while the electric potential can be obtained from experimental measurements. This is particularly true in case of cathodic protection systems in general, where many surveying techniques (for example DCVG and CIS for underground structures) rely in potential measurements at different points at the electrolyte in order to identify the current distribution along the metallic structures. 

The cathode-to-anode area ratio is frequently a critical factor in corrosion. (This is true when well-defined cathodes and anodes exist. With mixed electrode behavior, where cathodic and anodic reactions occur simultaneously, separate areas are not readily distinguishable, and Aa is assumed equal to Ac.) Discussion of the influence of this ratio will be restricted to the case of a small total-corrosion-circuit resistance leading to the anodic and cathodic reactions occurring at essentially the same potential, Ecorr, as described previously. In Fig. 4.12, three different values of corrosion current, Icorr, and corrosion potential, Ecorr, are shown for three cathode areas relative to a fixed anode area of 1 cm2. For these cases, a reference electrode placed anywhere in the solution... 

Interface Potential and Pit Initiation. It is generally accepted that pit initiation occurs when the corrosion potential or potentiostatically imposed potential is above a critical value that depends on the alloy and environment. However, there is incomplete understanding as to how these factors (potential, material, and environment) relate to a mechanism, or more probably, several mechanisms, of pit initiation and, in particular, how preexisting flaws of the type previously described in the passive film on aluminum may become activated and/or when potential-driven transport processes may bring aggressive species in the environment to the flaw where they initiate local penetration. In the former case, the time for pit initiation tends to be very short compared with the initiation time on alloys such as stainless steels. Pit initiation is immediately associated with a localized anodic current passing from the metal to the environment driven by a potential difference between the metal/pit environment interface and sites supporting cathodic reactions. The latter may be either the external passive surface if it is a reasonable electron conductor or cathodic sites within the pit. 

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