Corrosion total cathodic current

Fig. 10.6 Polarisation diagram showing the limited role hydrogen evolution plays at the corrosion potential of steel in aerated neutral solution, the larger role in determining cathodic protection currents and the dominant role in contributing to current requirements at very negative potenitals. The dotted line shows the total cathodic current due to oxygen reduction and...

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... 

Figure 1.32 shows the E-i curves for a metal corroding in an acid in which both dissolved oxygen and HjO ions act as cathode reactants (note that in order to illustrate the summation of the partial currents to give the total current i, = i hi + 02. I rather than log i has been used). The total cathodic curve ABCD is the sum of the partial cathodic currents, and it can be seen that the corrosion potential due to the total cathodic current is more positive than either co.t..hj or However, whereas in the case of oxygen... 

Figure 10.5 demonstrates that, even when semi-logarithmic cathodic kinetics are not observed, partial or total cathodic protection is possible. Indeed, Fig. 10.5 shows that the corrosion rate approximates to the limiting current for oxygen reduction (/,ij and the current required for protection is substantially lower than when semi-logarithmic cathodic behaviour prevails. 

In the presence of oxidizing species (such as dissolved oxygen), some metals and alloys spontaneously passivate and thus exhibit no active region in the polarization curve, as shown in Fig. 6. The oxidizer adds an additional cathodic reaction to the Evans diagram and causes the intersection of the total anodic and total cathodic lines to occur in the passive region (i.e., Ecmi is above Ew). The polarization curve shows none of the characteristics of an active-passive transition. The open circuit dissolution rate under these conditions is the passive current density, which is often on the order of 0.1 j.A/cm2 or less. The increased costs involved in using CRAs can be justified by their low dissolution rate under such oxidizing conditions. A comparison of dissolution rates for a material with the same anodic Tafel slope, E0, and i0 demonstrates a reduction in corrosion rate... 

In the case of galvanic corrosion, total current, /, is used instead of current density, i. The galvanic current associated with the anode must equal that from the cathode, or if areaA = if areaB. In order for the last term in Eq. (3) to yield volts, RQ must have units of ohms. 

Experimental studies usually yield good agreement between the rates of corrosion obtained from polarization resistance measurements and those derived from weight-loss data, particularly if we recall that the Tafel slopes for the anodic and the cathodic processes may not be known very accurately. It cannot be overemphasized, however, that both methods yield the average rate of corrosion of the sample, which may not be the most critical aspect when localized corrosion occurs. In particular it should be noted that at the open-circuit corrosion potential, the total anodic and cathodic currents must be equal, while the local current densities on the surface can be quite different. This could be a serious problem when most of the surface acts as the cathode and small spots (e.g., pits or crevices) act as the anodic regions. The rate of anodic dissolution inside a pit can, under these circumstances, be hundreds or even thousands of times faster than the average corrosion rate obtained from micro polarization or weight-loss measurements. 

Remember that at the steady-state corrosion potential, the total anodic and cathodic currents must be equal, but the current densities may be quite different. 

A reference electrode scanned along the metal surface will measure the series of (E"x)n and (E"M)n interface potentials. From these values, solution potentials (t))s) at the metal/solution interface may be calculated (< )s = -E") and presented as in Fig. 4.6. When the anodic and cathodic sites are microscopic relative to the size and position of the reference electrode, identity of the anodic and cathodic sites on a macroscale is lost, and a single mixed or corrosion potential, Ecorr, is measured as discussed previously. There is essentially a uniform flux of metal ions from the surface, and cathodic reactants to the surface, which constitute anodic and cathodic currents. Since the relative areas to which these currents apply usually are not known, the total area is taken as the effective area for each reaction. It is these currents, however, that mutually polarize the anodic reaction potential from E M up to Ecorr and the cathodic reaction potential from E x down to Ecorr. 

Consider the effect on A of coupling it to the B in the aerated solution. Because the anodic currents for B are so much smaller than for A, the total anodic curve is essentially equal to the anodic curve for A. Both B and A surfaces are now available as cathodic reaction sites with atotal areaof 20 cm2 (2 x 10-3 m2), and both cathodic reactions occur on these surfaces. The total cathodic curve is now 2(C + D). The new corrosion condition is labeled, A-B Couple,... 

For concrete immersed in water, or in any way saturated with water, the diminished supply of oxygen to the surface of the steel can bring the potential down to values below —400 mV SCE. Finally, when oxygen is totally lacking (a very difficult condition to achieve, even in the laboratory) the potential may even drop to values below —900 mV SCE and the cathodic process will lead to hydrogen evolution. Under all of these conditions, embedded steel is subjected to a corrosion rate that is practically zero. Consequently, the cathodic current density is also very small. 

Equilibrium is disturbed when a net forward or backward reaction occurs, producing current in the external circuit. The current induces a potential change and causes polarization. Charge conservation requires the total rate of oxidation be equal to the total rate of reduction for any corrosion process. To avoid accumulation charge in the electrode, the sum of anodic currents must equal the sum of cathodic currents. 

The overall balance of electric charges between the anodic and the cathodic processes must be equal to zero, expressed in terms of currents and not in terms of current densities. If we consider a hypothetical corrosion process, in which the kinetic corrosion resistances are those related to the cathodic process, and if we consider also that the cathodic surface is coupled with an anodic one of equal surface area, then both currents and the current densities will be equal on both areas. If the cathodic area is kept constant and the anodic surface area is varied, the total anodic current must be maintained constant by assuming cathodic control, it will also render constant the amount of dissolved metal. The anodic current density and, hence, the rate of penetration of the attack of the... 

FIG U RE 19.17 Equivalent current flow loop for internal corrosion protection of a pipe, where = anode potential (V vs. Ag/AgCl) Eq = potential on the pipe surface (cathode) (V versus Ag/AgCl) = anode resistance ( 2) Rs = resistance for current flow in seawater inside the pipe ( 2) Rc = resistance for current entering the pipe surface = resistance for current flowing in the pipe metal ( 2) and = total protection current in the loop (A) [5]. 

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