Cathode-to-anode area ratio

For further discussion of cathode to anode area ratio effects see References " and also refer to the section entitled Distribution of Bimetallic Corrosion in Real Systems, p. 1.238. 

Experience shows that increasing the cathode-to-anode area ratio increases the rate of consumption of the anode and decreases the corrosion rate of the cathode, but the galvanic series alone would not allow a quantitative analysis of these effects. Inspection of Fig. 32 reveals that the abscissa has been changed to current from current density. When dealing with unequal areas, such a transfor-... 

Compare values of Ecoupie and /C0Upie for a 1 1 cathode-to-anode area ratio to the values calculated with a 1000 1 copper-to-steel area ratio. What is the dissolution rate of copper at this area ratio ... 

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... 

In review, consider a mixed electrode at which one net reaction is the transfer of metal to the solution as metal ions, and the other net reaction is the reduction of chemical species in the solution such as H+, 02, Fe3+, or N02 on the metal surface. For purposes of the present discussion, no attempt is made to define the individual sites for the anodic (net oxidation) and cathodic (net reduction) reactions. They may be homogeneously distributed, resulting in uniform corrosion, or segregated, resulting in localized corrosion. In the latter case, the cathode-to-anode area ratio is of practical importance in determining the rate of penetration at anodic areas. 

Under-deposit attack or poultice corrosion may occur when a metal is locally covered by foreign, absorbent (organic or inorganic) materials [40,45]. In this case, attack can proceed even when the bulk of the system is dry due to retention of moisture in the poultice. The corrosion mechanism is similar to crevice corrosion in that the deposits act to limit the migration of oxygen to the covered area. This leads to acidic shifts in pH, concentration of Cl ions in the shielded area, and a shift to a more active corrosion potential under the deposit. Local corrosion rates can be very high due to the large cathode-to-anode area ratio. 

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