What is the difference between atmospheric corrosion and immersion corrosion?

• A. Atmospheric corrosion occurs in liquid environments.
• B. Immersion corrosion occurs in air.
• C. Atmospheric corrosion occurs on exposed surfaces in air with humidity; immersion corrosion occurs in liquid environments; rates and mechanisms differ due to electrolyte availability.
• D. They are identical processes.

Answer: (C)

The key idea is the role of the electrolyte in the corrosion process. Atmospheric corrosion happens on exposed metal surfaces in air when there is humidity. The electrolyte is just a thin, patchy film of moisture that forms from the surrounding air, so ion transport is limited and the rate is strongly influenced by how much moisture is present, how oxygen can reach the surface, and what pollutants are there. The resulting corrosion behavior can be gradual or localized, depending on how the thin film forms and breaks down.

Immersion corrosion occurs when the metal is fully submerged in a liquid electrolyte. In this case, the entire surface is in contact with the liquid, allowing continuous ionic conduction, steady supply of reactants (like oxygen and water) and products to be transported away. This generally leads to different rate conditions and often different corrosion morphologies (for example, more uniform attack or pitting driven by aggressive ions such as chlorides) because the electrolyte is readily available throughout the interface.

So, the difference comes down to whether there is an aqueous electrolyte present on the surface (a thin film from humidity in air vs a fully submerged liquid). The availability and transport of electrolytes change both how fast corrosion proceeds and the dominant mechanisms involved.