In general, how does the rate of penetration change as the number of pits increases?

• A. Increases
• B. Decreases
• C. Remains constant
• D. Fluctuates

Answer: (B)

The rate of penetration is driven by electrochemical reactions at the pit and by how fast corrosive species can reach the pit. When there are only a few pits, each pit draws a relatively high local current and can deepen more quickly. As the number of pits increases, the available reactants and aggressive ions must be shared among many sites, and diffusion through the surrounding boundary layer becomes the limiting factor for all of them. This spreads the current over more pits, so the current per pit drops and each pit grows deeper more slowly. In practice, increasing pit density tends to reduce the average rate at which penetration deepens, even though total surface area for dissolution is higher.