Atmospheric corrosion rates, time‐of‐wetness and relative humidity

The effect of relative humidity (RH) on atmospheric corrosion rates r has been studied in laboratory experiments for rust covered steel in air and air + 1 ppm SO2. The expression: describes the experimental results, where rcrit is the corrosion rate at the critical relative humidity RHcrit and b″ is the normalized corrosion rate at RH = 100%. While a large effect of NaCl impurities in rust was found, no effect of SO2 was observed. Time-of-wetness was determined in outdoor exposure using the previously described atmospheric corrosion monitor (ACM). It was found that for exposure at a site in Thousand Oaks, California, the time-of-wetness corresponds to RH > 40%.