Final answer:
Deposits contribute to concentration cell corrosion by creating localized differences in ion concentration, towards setting up anodic and cathodic areas, and to pitting by concentrating aggressive agents and restricting oxygen diffusion at coating defects, resulting in deep corrosion pits.
Step-by-step explanation:
Deposits contribute to concentration cell corrosion and pitting by creating localized environments where corrosive processes can occur at an accelerated rate. In concentration cell corrosion, differences in concentration of ions lead to the formation of a galvanic cell with anodic and cathodic regions. Deposits such as dirt or salts can trap moisture and lead to differences in oxygen concentration; under a deposit, oxygen levels could be lower (anodic), while areas with free access to oxygen remain high (cathodic). This sets up an electrochemical cell where metal can dissolve at the anode.
Pitting occurs when small defects in protective coatings, like paint, allow aggressive agents such as chlorides to reach the underlying metal, forming pits. Deposits exacerbate pitting by concentrating these corrosive agents and by restricting the diffusion of oxygen, leading to a localized aggressive attack that creates deep, hard-to-detect pits that can compromise the structural integrity of the metal.