Anode-cathode area ratio

Steel socket welds is a good example of rapid local corrosion. The potential difference between the anodic and the cathodic states drives the corrosion cells (this is an example of galvanic corrosion). Corrosion due to adjacent active-passive sites can be particnlarly rapid if the corrosion cell has an unfavorable anode/cathode area ratio. 

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. 

Maximum /err occurs at d log I oJdAc = 0—that is, when the numerator of (5.16) equals zero. This occurs when A = Pc/(Pc + P ). If, as is frequently observed, Pc = Pa, then the maximum corrosion rate occurs at Ac = that is, at an anode-cathode area ratio of unity. At any other anode-cathode area ratio, the corrosion rate is less, reaching zero at a ratio equal to either zero or infinity. 

Figure 7.7. Effect of anode-cathode area ratio on corrosion of gaivanic coupies in deaerated nonoxidizing acids, (a) Large cathode coupied to smaii anode, (b) Large anode coupied to smaii cathode.

In general, the copp>er-base alloys are galvanieally compatible with one another in seawater. While the copper-nickel alloys are slightiy cathodic (noble) to the nickel-fiee copper btise edloys, the small differences in corrosion potential generally do not lead to serious galvanic effects unless unusually adverse anodic/cathodic area ratios are involved. 

In the contact corrosion tests, samples with anode cathode area ratios of 10 1, 4 1 and 1 1 were used. The dimensions of the cathodic sample were 20 cm x 40 cm. A further varied test parameter was the distance (0.2 cm, 15 cm, 100 cm) between anode and cathode. The following measured values were registered continuously ... 

Another challenge are the low reaction kinetics of oxygen. Performance of air-cathode MFCs is mostly limited by the air-cathode, which degrades the performance of the MFC. Fan et al (2007) changed the size of the air-cathode, and reported that as anode/cathode area ratio reduces from 1 1 to 1 7, the areal power density increased from 1.04 W m to 6.71 W m (normalized to the anode area). Thus, enhancing the reaction kinetics of oxygen is critical. 

The effect of anode-cathode area ratios has an important bearing on the rate of corrosion. This can be explained by a plot of log J vs . In the plot (Fig. 3.13) current I is plotted vs E and not i (current density) to establish the effect of area ratio. To define the conditions, the reversible potential ° of Zn (—0.760 V) and hydrogen (0.00 V) are located in the diagram. The values for hydrogen reduction on zinc, io (H) on zinc (—1 cm ). 

The geometry determines the galvanic current. Geometric factors include the anode/cathode area ratio, insulation distance (5) between the anode and cathode, electrolyte him depth (d) and the shape of the anode and cathode... 

The measured currents may not represent actual galvanic corrosion rates, as this form of corrosion is highly dependent on the anode cathode area ratio. An increase in current readings is not always directly associated with an actual increase in corrosion rates. 

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