Humidity and Rainfall

Humidity is also a major factor in determining the corrosion rate of metals, as moisture provides the electrolyte, which is required for corrosion reactions to take place. In general, the corrosion rate increases as humidity increases. The critical level of relative humidity in order for serious corrosion to occur in the absence of other electrolytes is usually taken to be 60%.3 This critical level of relative humidity may vary depending on the impurities present in the atmosphere. Rainfall can increase or decrease corrosion processes. In areas where stagnant water may accumulate, a localized corrosion cell will most likely be the result. However, rain may also wash corrosive deposits off from metal surfaces, decreasing corrosivity.