What conditions are necessary for filiform corrosion to occur?

Filiform corrosion requires a specific set of environmental conditions to initiate and propagate effectively. In particular, high relative humidity is a crucial factor, with the most conducive range being between 65% and 90%. This level of humidity allows for the formation of moisture under coatings, which is essential for the corrosion process.

Additionally, the temperature range of 70°F to 100°F is optimal for filiform corrosion because it enables the corrosion reactions to occur at a pace conducive to the development of filiform threads. These conditions create an environment where localized anodic and cathodic reactions can take place, fueling the corrosion mechanisms and leading to the characteristic filament-like structures.

Other conditions mentioned in the question, such as low humidity or constant wet conditions, either do not support the growth of filiform corrosion or represent entirely different forms of corrosion processes. Understanding the specific conditions that promote filiform corrosion is critical for effective prevention and control measures in materials and coatings exposed to potential corrosion environments.