ANS:C - pitting

Pitting corrosion is a localized form of corrosion that occurs on metal surfaces, characterized by the formation of small pits or cavities. These pits can penetrate deeply into the metal, leading to significant material loss and structural damage. Pitting corrosion is often considered more damaging than uniform corrosion because it can cause rapid and unpredictable failure of components, even in the presence of relatively low corrosion rates. Here's an explanation of how pitting corrosion occurs:

1. Initiation: Pitting corrosion typically starts with the formation of small defects or imperfections on the metal surface. These defects can be microscopic in nature and may result from factors such as surface contamination, scratches, or localized differences in composition.
2. Local Anodic Areas: Once the surface defects are present, they create sites with higher local anodic activity compared to the surrounding area. This can occur due to differences in chemical composition, microstructure, or mechanical stress.
3. Anodic Dissolution: At the sites of increased anodic activity, metal ions are released into the surrounding electrolyte (e.g., a corrosive solution). This process is known as anodic dissolution. As metal ions are released, electrons are also generated, leading to the formation of positively charged metal ions (cations) and negatively charged electrons.
4. Cathodic Reaction: Concurrently, a cathodic reaction occurs on nearby unaffected areas of the metal surface, where oxygen reduction or another reduction reaction takes place. This cathodic reaction consumes the electrons generated during anodic dissolution, maintaining electrical neutrality in the system.
5. Local Cell Formation: The combination of the anodic dissolution at the pit sites and the cathodic reaction on adjacent areas leads to the formation of localized electrochemical cells, known as corrosion cells. These cells concentrate the corrosive attack, accelerating the dissolution of metal ions at the pit sites.
6. Pit Growth: As the corrosion process continues, the pits deepen and widen, often extending below the original surface of the metal. This progression can occur rapidly, and the pits may grow unpredictably in size and shape, leading to structural weakening and eventual failure of the material.

Pitting corrosion is particularly problematic because it can occur even in environments where the overall corrosion rate is relatively low. Additionally, the presence of pits can act as stress concentrators, increasing the susceptibility of the material to other forms of mechanical failure, such as stress corrosion cracking or fatigue failure. Therefore, preventing and mitigating pitting corrosion is essential for maintaining the integrity and reliability of metal components exposed to corrosive environments.