Tropical High-Humidity Environments

Time of wetness drives corrosion

Corrosion needs a water film on the metal. ISO 9223 counts the hours when relative humidity is above 80% and the temperature above 0 °C as time of wetness. In the humid tropics that can be most of the year, compared with a small fraction in dry climates.

Warm temperatures speed up the chemical reactions, and any salt or pollution in the air is dissolved into the water film instead of sitting dry on the fins.

Coastal tropical sites

Many tropical projects — resorts, ports, coastal cities — add airborne salt to the humidity. Those sites behave like coastal sites one step higher, because the salt film on the coil rarely dries out.