Cathode:anode ratio localized corrosion

In all cases of localized corrosion, the ratio of the cathodic to the anodic area plays a major role in the localized dissolution rate. A large cathodic area provides high cathodic currents and, due to electroneutrality requirements, the small anodic area must provide a high anodic current. Hence, the local current density, i.e., local corrosion rate, becomes higher with a larger cathode/anode-ratio. 

Pits occur as small areas of localized corrosion and vary in size, frequency of occurrence, and depth. Rapid penetration of the metal may occur, leading to metal perforation. Pits are often initiated because of inhomogeneity of the metal surface, deposits on the surface, or breaks in a passive film. The intensity of attack is related to the ratio of cathode area to anode ai ea (pit site), as well as the effect of the environment. Halide ions such as chlorides often stimulate pitting corrosion. Once a pit starts, a concentration-cell is developed since the base of the pit is less accessible to oxygen. 

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. 

Crevice corrosion is one of the forms of localized metal corrosion, in which the anodic metal dissolution inside a crevice is coupled with a cathodic reaction outside the crevice [58], This form of localized corrosion occurs only if the structural crevice is thinner than a certain width, for example, 30-40 p,m for stainless steels [59], and thus restricted mass transport through the crevice is responsible. For crevice corrosion to occur, a certain induction period of time is required, during which a local cell has formed between the inside and the outside of the crevice. It was also shown that scaling factors in crevice corrosion may be characterized by the aspect ratio, L/a, where L is the depth of crevices and a is the crevice opening. If the aspect ratio is greater than its critical ratio, crevice corrosion will occur, whereas no crevice corrosion will occur if the aspect ratio does not exceed its critical ratio [60]. 

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. 

Cyclic anodic polarization procedures based upon ASTM G5 (Reference Test Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements) have been used to evaluate the localized corrosion resistance of stainless steel alloys in paper machine white waters. The difference between the open circuit or naturally occurring corrosion potential and the pitting breakdown potential of various materials has been reported by many investigators [lO-Id]. Bowers [14] called the difference between the breakdown potential and the potential of the cathodic/anodic current reversal the margin of safety. He also noted the effect of the sulfate to chloride concentration ratio on localized corrosion of Types 304 and 316L stainless steels, These results permitted alternative materials of construction to be ranked and their limits of resistance to be defined. 

The use of multielectrode array systems (CMAS) for corrosion monitoring is relatively new. The advantages of using multiple electrodes include the ability to obtain greater statistical sampling of current fluctuations, a greater ratio of cathode-to-anode areas in order to enhance the growth of localized corrosion once initiated. CMAS also provide the ability to estimate the pit penetration rate and obtain macroscopic spatial distribution of localized corrosion [25]. 

It is also important to realize that most textbooks present corrosion current data as current densities. The main reason for that is simple Current density is a direct characteristic of interfacial properties. Corrosion current density relates directly to the penetration rate of a metal. If one assumes that a metallic surface plays equivalently the role of an anode and that of a cathode, one can simply balance the current densities and be done with it. In real cases this is not so simple. The assumption that one surface is equivalently available for both processes is indeed too simplistic. The occurrence of localized corrosion is a manifest proof that the anodic surface area can be much smaller than the cathodic. Additionally, the size of the anodic area is often inversely related to the severity of corrosion problems The smaller the anodic area and the higher the ratio of the cathodic surface Sc to the anodic surface Sa, the more difficult it is to detect the problem. 

Fig. 10.21 Corrosion at damaged metal coatings on iron or steel, (a) The zinc coating is anodic with respect to the steel and dissolves preferentially, (b) The tin coating is cathodic damage to the coating or the presence of imperfections in it may give rise to a small exposed area of steel. The resultant attack may be localized and rapid due to the adverse situation of a high ratio of cathode to anode area.

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