Polarization anode-cathode areas

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. 

Referring to Fig. 5.2, we can calculate the corrosion rate of a metal if data are available for the corrosion potential and for the polarization behavior and thermodynamic potential of either anode or cathode. In general, the relative anode-cathode area ratio for the corroding metal must also be known, since polarization data are usually obtained under conditions where the electrode surface is all anode or all cathode. 

Increase of hydrogen overpotential normally decreases the corrosion rate of steel in acids, but presence of sulfur or phosphorus in steels is observed instead to increase the rate. This increase probably results from the low hydrogen overpotential of ferrous sulfide or phosphide, either existing in the steel as separate phases or formed as a surface compound by reaction of iron with H2S or phosphorus compounds in solution. It is also possible [4] that the latter compounds, in addition, stimulate the anodie dissolution reaetion, Fe -> Fe + 2e (reduce activation polarization), or alter the anode cathode area ratio. 

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... 

Correspondingly, an anode-cathode area ratio exists, and since the observed polarization of anodic or cathodic sites depends, in part, on the area over which oxidation or reduction occurs, the anode-cathode area ratio is an important factor in the observed corrosion rate. 

A variety of standard electrochemical methods may be used to probe the corrosion behavior of electroplated coatings with a particular focus on assessing the effects of galvanic coupling between the coating and the substrate (ASTM G 5, G 59, G 61 , G 82 °, G 102", and G 106 ) [22], In accordance with the mixed potential treatment of galvanic corrosion, the corrosion potential and polarization resistance are expected to be a sensitive function of the anode/cathode area, i.e., p>orosity [13,14,20,23],... 

Figure 4-458. Diagram of polarization of local cathode by a film of hydrogen gas bubbles (cathodic area to right of anode is polarized). (From Ref. [208].)...

Figure 32 Shift in polarization curves on a current basis for cases in which (a) the anode area is considered to be 10 cm2 and the cathode area 1 cm2, (b) the cathode area is considered to be 10 cm2 and the anode area 1 cm2.

Figure 8.27. Polarization curves for various concentrations of Fe and Fe. Solid lines are polarization curves (electrode area = 1 cm ) I o, exchange current. Dashed lines are hypothetical cathodic (-i) and anodic ( + /) currents. Curves are schematic but based on experimental data at relevant points.

Another governing relationship, however, is Ohm s law, which leads to a dependency of the corrosion current on both the polarization characteristics of the anodic and cathodic reactions and on the total electrical resistance of the system, Rtotal. Rtotal includes the resistance in the metal between anodic and cathodic areas, RM a metal junction resistance if different metals are associated with the two areas, Rac any anode- or cathode-solution interface resistance, Rai and Rci and the solution resistance, Rs. The latter depends on the specific resistivity or conductivity of the solution and the geometry of the anode-solution-cathode system. 

The distribution of potential in the solution along the solution/metal interface is shown in Fig. 4.6. If the anode and cathode areas are not connected, they will exhibit their thermodynamic or open circuit potentials, with the potentials in the solution at the anode and cathode being equal to +1000 mV and 0 mV, respectively. When the anode and cathode areas are in contact, current will pass causing polarization of the interface reactions. With a solution-specific resistivity of 1000 ohm-cm, the solution potential at the center of the anode is decreased... 

The processes in real corroding systems are obviously more complicated than represented by this model. Useful quantitative calculation of the distribution of current density, and hence corrosion rate along the surface, based on the polarization curves for the anodic and cathodic reactions and on the geometry of the anodic and cathodic sites is very complex. In principle, computer-based techniques can be used if exact polarization curves and the geometry of the anodic and cathodic areas are available. For most industrially important situations, this information is not available. Also, time-dependent factors, such as film formation, make quantitative calculations of long-time corrosion rates very uncertain. The theory underlying these calculations, however, has been useful in interpreting observations in research and in industrial situations. 

The method is schematically illustrated in figure 2. The operational amplifier A, working in the follower configuration, is used to apply, between the points D and T, the potential difference V present at the terminals of the generator G, which is assumed to be positive. This hypothesis simplifies the analytic expression of the potential difference V, because its polarity determines the behaviour of the two electrodes. The differential-input voltmeter Q determines the intensity of the current that, flowing through the electrochemical cell C from D to T, polarizes the electrodes Wi and W. These electrodes, made of the same material, are identical. Their surface areas are equal to S and, conventionally, they are polarized anodically and cathodically. 

The most frequent and also most favourable case is that in which the ratio r between the anodic and cathodic areas is near unity. The cathodic polarization, ip (i r), is prevalent with respect to other contributions. In facL the cathodic polarization of passive steel reaches values of about 200-300 mV even for a current density of about 1-2 mA/m (Figure 8.5) which is usually sufficient to dissipate most... 

The corrosion rate of metals is determined by estabhshed potential difference, soil conductivity, and relative anodic and cathodic areas [18—20]. Composite polarization diagrams are used to predict galvanic current. They consist of potentiostatic cathodic and anodic polarization curves for different metals and alloys in deaerated 1 N H2SO4 and aerated 3% NaCl. Galvanic corrosion prediction for longer time periods from data obtained in short time periods is not accurate due to surface conditions and impurities. 

As an alternative to generating an entire polarization diagram, we can use the exchange current densities and the equilibrium potentials of the anodic and cathodic reactions to estimate the corrosion potential and corrosion current by extrapolating the cathodic and anodic polarization lines of the corroding system. At the corrosion potential, the anodic and cathodic currents are equal. The schematic shown in Fig. 3.6 represents a case for which the anode and the cathode area are the same once the corrosion current is known, the rate of deterioration of the electrode can be estimated. The accurate prediction of the corrosion (mixed) potential depends on the polarization behavior of the specific electrode. 

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