1. Oxygen Depletion:
- At the initiation stage of crevice corrosion, oxygen is consumed by the corrosion reactions occurring on the metal surface within the crevice. This leads to the depletion of dissolved oxygen within the crevice, creating a localized environment with limited oxygen supply.
2. Acidification:
- As oxygen is depleted, the crevice becomes an anaerobic environment, allowing for the growth of acid-producing bacteria. These bacteria consume organic matter and produce acidic metabolites, such as hydrochloric acid (HCl) and organic acids. This acidification lowers the pH within the crevice, further accelerating the corrosion process.
3. Metal Dissolution:
- The acidic environment created by the bacteria initiates the dissolution of the metal at the crevice site. The dissolved metal ions can then react with the available oxygen at the crevice entrance, forming metal oxides or hydroxides. This process continues as long as there is a supply of dissolved oxygen at the crevice entrance.
4. Propagation:
- The metal dissolution and the formation of metal oxides or hydroxides result in the accumulation of corrosion products within the crevice. These corrosion products can block the crevice entrance, limiting the diffusion of oxygen and other species into the crevice. As a result, the crevice environment becomes even more aggressive and acidic, promoting further metal dissolution and corrosion.
5. Cathodic Reaction:
- Dissolved oxygen plays a role in the cathodic reaction that occurs at the outer surface of the metal, outside the crevice. Oxygen reduction takes place at this surface, consuming electrons and generating hydroxyl ions. The hydroxyl ions can then migrate into the crevice and react with the metal ions, contributing to the formation of metal oxides or hydroxides.
6. Oxygen Concentration Gradient:
- The presence of dissolved oxygen in the bulk solution outside the crevice creates a concentration gradient of oxygen across the crevice. This gradient drives the diffusion of oxygen into the crevice, replenishing the oxygen consumed in the corrosion reactions and perpetuating the crevice corrosion process.
Therefore, dissolved oxygen plays a crucial role in the initiation, propagation, and continuation of crevice corrosion by influencing the local environment within the crevice, promoting acidification, metal dissolution, and the formation of corrosion products.
