Poultice corrosion mechanism

The mechanism of poultice corrosion is shown diagramatically in Figure 4. Corrosion initially occurs uniformly over the whole metal surface. However, as the process continues, the oxygen reduction cathodic reaction may become restricted to a band near the surface where oxygen is readily available. Corrosion of metal then takes place preferentially slightly below the band. The pH rises in the cathodic reaction area due to an increase in hydroxyl ion concentration while the anodic area decreases in pH due to the hydrolysis of metal chloride reaction products. 

Figure 4. Mechanism of poultice corrosion of auto steel showing initial and advanced stages.

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. 

The chassis and undervehicle is most susceptible to mechanical damage because of its proximity to external road conditions [24] (Fig. 4). Usual corrosion mechanisms apply such as those due to the effect of stone impingement and crevice and uniform corrosion. Coatings failure can enhance deterioration as can poultice built up in entrapment areas. 

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