Corrosion cathode/anode area ratio

FIG. 4—A schematic Evan s diagram of accelerated substrate corrosion due to coupling of the base metal with a porous noble metal coating. The dramatic effect of the geometry, cathode/anode area ratio, is demonstrated [7. ... 

Figure 3.13 Effect of cathode anode area ratio on corrosion of zinc-platinum galvanic couples...

The rate of galvanic attack depends on the relative anode-to-cathode snrface areas that are exposed to the electrolyte, and the rate is related directly to the cathode-anode area ratio—that is, for a given cathode area, a smaller anode corrodes more rapidly than a larger one because corrosion rate depends on cnrrent density (Equation 17.24)—the current per unit area of corroding siud ace—and not simply the ciurent. Thns, a high cnrrent density results for the anode when its area is small relative to that of the cathode. 

Since copper-sheathed cables are also coated with plastic, the ratio of cathode/anode area (SJSJ is very small so that there is not an increased risk of corrosion of the lead-sheathed cable by the electrical connection between the cable sheathings according to Eq. (2-44). 

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-... 

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. 

In most eases there is a zone of less noble material in/at the grain boundaries, which acts as an anode, while the other parts of the surface form the cathode. The area ratio between the cathode and the anode is very large, and the corrosion intensity can therefore be high. 

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. 

Galvanic corrosion is a form of corrosion whereby two metals of different electrode potential are located in the same electrolyte and connected with each other electrically, Fig. l-9e. It is unimportant whether this electrically conductive connection is inside or outside of the electrolyte. A galvanic element is formed when the anode is attacked more severely and the cathode less severely than in the unconnected state. Under these conditions the less noble metal will becomfe the anode. The extent of galvanic corrosion depends partly on the potential difference between the two metals, the electrical conductivity of the medium, and the anode to cathode surface-area ratio. It is important to know the electrode potential of a metal in the electrochemical series and that this potential is influenced by different conditions (medium, temperature, flow velocity, etc.). 

The metallization for some integrated circuits with submicron features is made up of the Ti/TiN/Al/TiN stack. Lateral undercutting of up to 20 pm of aluminum from the middle of the stack has been observed (Fig. 9-16). Residual chloride from the reactive-ion etch initiates the aluminum corrosion process that is stimulated galvanically by titanium nitride (Jones, 1992). Titanium nitride, being more noble, serves as the cathode in this electrochemically driven process. The corrosion rate of the more active metal increases as the cathode-area/anode-area ratio increases. At the time of the rinse... 

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