1152Localized Corrosion

Localized corrosion results when the anodic and cathodic reactions are fixed at sites spatially separated from each other. Localized corrosion may occur on both the microscopic and macroscopic scale and is the most destructive type of corrosion. The rate of corrosion can depend upon minute changes in local conditions, for example, MnS inclusions exposed at the steel surface and in contact with an aqueous environment can, via hydrolysis reactions, give rise to highly localized acidic conditions. In the case of localized corrosion, the rate of metal loss is often very unpredictable. This applies particularly to pitting in which the location, distribution, and size of pits depends upon the precise microstructure and environmental conditions prevailing.

A number of factors control the rate of localized corrosion and include:

1. Anode/Cathode Area Ratio The rate of reaction is governed by the relative surface areas of the anode and cathode and will be limited by the smallest surface area. Local attack (dissolution) will therefore be more pronounced when the cathodic area is larger than the anodic area. This occurs because the anodic process is confined to a relatively small area, and therefore a large anodic current can be sustained through the availability of a large cathode area. Impurities in the form of segregation, for example, Cu and Fe in Zn give rise to local cathodes with low hydrogen overpotentials that subsequently affects the rate of the hydrogen reduction reaction. This causes an increase in corrosion rate of the metal in low pH solutions but has little effect in aerated neutral solutions where the cathodic reaction is that of oxygen reduction.

2. Differential Aeration Cells Differential aeration cells can arise when a metal is in contact with a solution in which the concentration of oxygen within the solution differs from one site to another. This may be the result of limited transport of oxygen due to poor solution agitation. The concentration of oxygen determines the corrosion rate and the site at which the cathodic reaction takes place, Note; corrosion rate is proportional to [pO (anode)/pO (cathode)], where pO is the partial pressure of oxygen. As oxygen is easily replaced where the electrolyte is exposed to the atmosphere, this area is favored for the cathodic reduction of O2 to OH-. The site of lower oxygen concentration, that is, below the water line, favors the formation of anodic sites.

3. Changes in Solution pH The pH value of a solution is dependent upon the concentration of H+ (in the form of H3O+) or correspondingly of OH-ions. Where dissolved oxygen or H+ are involved in the corrosion reaction, the rate of the anodic and cathodic reactions will therefore depend upon the pH of the solution. For near neutral solutions the cathodic reaction (reduction of oxygen/water to hydroxide) involves a decrease in acidity, that is, an increase in pH, while the anodic reaction, via hydrolysis, leads to a decrease in pH, that is, an increase in H+ concentration.

4. Corrosion Products and Deposits Corrosion products such as surface films, for example, oxide films/layers, often have the effect of reducing the overall rate of corrosion, for example, stainless steels depend for their corrosion resistance on a thin chromium oxide film. However, if these surface films are disrupted, for example, by cracking under stress, then localized corrosion may result. Where deposits lie on the surface of a metal, cathodic sites are often formed at the outer edge of the deposit. As the degree of aeration decreases away from this location, that is, toward the center of the deposit, corrosion activity (dissolution) is favored beneath the deposit.

5. Active-Passive Cells Passivity is a property exhibited by a material when an insoluble film forms on the corroding surface preventing metal-electrolyte contact, hence greatly reducing the corrosion rate. Where a surface has lost its passive film, that is, is depassivated, the local corrosion rate increases markedly. The electrode potential of the passive and active surfaces differ and an electrochemical cell is formed. The magnitude of corrosion enhancement depends upon the ratio of active and passive surface areas and the nature of the electrolyte. Normally the passive (cathodic) surface is far larger than the active (anodic) surface and rapid localized corrosion results.

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