Corrosion potential current diagrams

The description of corrosion kinetics in electrochemical terms is based on the use of potential-current diagrams and a consideration of polarization effects. The equilibrium or reversible potentials Involved in the construction of equilibrium diagrams assume that there is no net transfer of charge (the anodic and cathodic currents are approximately zero). When the current flow is not zero, the anodic and cathodic potentials of the corrosion cell differ from their equilibrium values the anodic potential becomes, more positive, and the cathodic potential becomes more negative. The voltage difference, or polarization, can be due to cell resistance (resistance polarization) to the depletion of a reactant or the build-up of a product at an electrode surface (concentration polarization) or to a slow step in an electrode reaction (activation polarization). 

The sohd line in Figure 3 represents the potential vs the measured (or the appHed) current density. Measured or appHed current is the current actually measured in an external circuit ie, the amount of external current that must be appHed to the electrode in order to move the potential to each desired point. The corrosion potential and corrosion current density can also be deterrnined from the potential vs measured current behavior, which is referred to as polarization curve rather than an Evans diagram, by extrapolation of either or both the anodic or cathodic portion of the curve. This latter procedure does not require specific knowledge of the equiHbrium potentials, exchange current densities, and Tafel slope values of the specific reactions involved. Thus Evans diagrams, constmcted from information contained in the Hterature, and polarization curves, generated by experimentation, can be used to predict and analyze uniform and other forms of corrosion. Further treatment of these subjects can be found elsewhere (1—3,6,18). 

The corrosion reaction may also be represented on a polarisation diagram (Fig. 10.4). The diagram shows how the rates of the anodic and cathodic reactions (both expressed in terms of current flow, I) vary with electrode potential, E. Thus at , the net rate of the anodic reaction is zero and it increases as the potential becomes more positive. At the net rate of the cathodic reaction is zero and it increases as the potential becomes more negative. (To be able to represent the anodic and cathodic reaction rates on the same axis, the modulus of the current has been drawn.) The two reaction rates are electrically equivalent at E , the corrosion potential, and the... 

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...

Evans Diagram diagram in which the E vs. I relationships for the cathodic and anodic reactions of a corrosion reaction are drawn as straight lines intersecting at the corrosion potential, thus indicating the corrosion current associated with the reaction. 

This can be elucidated by a corrosion diagram (Fig. 12), which shows in semilogarithmic coordinates current-voltage characteristics for two conjugated reactions. Using condition (43) and neglecting ohmic potential drop in the system, one can find from the intersection of those characteristics the steady state corrosion current icorr and corrosion potential [Pg.283]

Fig. 12.16. The potential-current relation for the two reactions occurring at a corroding interface. The corrosion current and corrosion potential are defined by the point on the diagram at which the two currents /M and /so are equal.

Fig. 12.17. The Evans diagrams are plots of the potentials of the two reactions (a) vs. the magnitude of the two currents or (b) vs. their logarithms. The intersections of the curves define the corrosion current and corrosion potential.

The above two methods ofpreventing corrosion can be understood easily with an Evans diagram (Fig. 12.39 see Section 12.19). (These diagrams it will be recalled, result from the superposition of the potential-current curves of the electronation and... 

With the help of the Evans diagram (Fig. E12.1) explain the influence of an inhibitor on the corrosion potential and corrosion current. Assume that the inhibitor decreases the exchange current density for the cathodic reaction. (Contractor)... 

The understanding gained by considering the Evans diagrams allows us to measure the corrosion current in a straightforward manner. First we must realize that the corrosion potential is in fact the open-circuit potential of a system undergoing corrosion. It represents steady state, but not equilibrium. It resembles the reversible potential in that it can be very stable. Following a small perturbation, the system will return to the open-circuit corrosion potential just as it returns to the reversible potential. It differs from the equilibrium potential in that it does not follow the Nemst equation for any redox couple and there is both a net oxidation of one species and a net reduction of another. 

After a first sweep towards the positive which is not shown in the diagram and which is dominated by the dissolution of the airfoimed oxide layer, a sweep in the positive direction starts at the negative potential end of the cathodic part of the curve. In the first part, from A to the corrosion potential B where the curve becomes anodic, Hj evolution is the most important process. In this region both samples are very similar. The corrosion potential at B is nearly the same for unimplanted, with Cr implanted and with Ar bombarded iron. From B to C the anodic dissolution of the metal takes place and at C the active to passive transition starts. Here one observes the most significant difference between the two samples. The critical current density for passivation of implanted iron is more than one order of... 

Further oxidation results in the formation of hydrated ferric oxide or Fe(ni) hydroxide, i.e. rust. The corrosion potential (Ec) and corrosion current (/c) for the cathodic and anodic reaction can be represented by an Evans-type polarisation diagram, Eig. 6.6. Corrosion inhibitors interfere with the anodic or the cathodic partial reaction, or with both, resulting in a reduction in the corrosion current. 

FIGURE 22.36 Schematic polarization diagrams for the corrosion potential of a passive metal electrode (a) alone and (b) in contact with an n-type metal oxide under photoexcitation / (ox> = anodic oxygen current at photoexcited oxide. 

---