Autocatalytic pitting, mechanism

The insoluble corrosion product Fe(OH)2 can help bacterial film to control the diffusion of oxygen to the anodic sites in the pit. This forms a typical tubercle. If chlorides are present in the aqueous solution, the pH of the solution trapped in the tubercle can become very acid due to the autocatalytic propagation mechanism of localized corrosion due to deposit formation and generation of hydrochloric acid. 

The crevice corrosion mechanism is very similar to that of pitting with respect to the autocatalytic propagation. However, the causes of initiation, the morphology and the penetration of pitting are quite different from that of crevice corrosion (Figure 6.21).16... 

Furthermore, the secondary mechanism then causes the pit propagation to become autocatalytic where one of the products formed in the reaction serves to accelerate it. Figure 7.104 describes the following mechanism. Pitting occurs in three stages initiation, propagation and termination. 

Autocatalytic mechanism of pit growth 7.3.2.1 Formation of nucleated pits... 

The reaction at the propagation stage is relatively fast, because Eq. (3-9) is a solid-ion reaction. Hence, the essential mechanism of the autocatalytic process is the generation of galvanic couples by which aluminum reacts with the hydrogen ions in the pitting cavities to form aluminum ions and hydrogen gas. 

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