Which factor primarily contributes to localized corrosion under polymer layers?

Localized corrosion under polymer layers is primarily influenced by the retention of moisture by certain polymers. When polymers are applied over metallic substrates, they can form a barrier that traps moisture underneath. This trapped moisture can create an environment that is conducive to corrosion, especially if it contains contaminants such as salts or other corrosive agents.

The moisture can lead to the initiation of localized corrosion mechanisms, such as pitting or crevice corrosion, which can occur in the presence of an electrolytic solution. This happens because the trapped moisture disrupts the protective oxide layer on the metal surface, allowing for electrochemical reactions that favor localized corrosion rather than uniform corrosion.

In contrast, factors like variability in metal density or external temperature fluctuations do not play a direct role in the localized corrosion phenomenon under polymer layers in the same manner. While they may influence general corrosion rates or the overall integrity of the metal, they do not specifically account for the moisture retention effect that can occur beneath a polymer coating. Air exposure, while important in some contexts, does not target the localized effects brought about by moisture retention as significantly as this factor does.