AbstractThe 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:\documentclass{article}\pagestyle{empty}\begin{document}$$ \log {\rm\; r = log\; r}_{{\rm crit}} + {\rm b''}\left({\frac{{{\rm RH } - {\rm RH}_{{\rm crit}} }}{{100 - {\rm RH}_{{\rm crit}} }}} \right) $$\end{document}describes the experimental results, where rcritis the corrosion rate at the critical relative humidity RHcritand b″ is the normalized corrosion rate at RH = 100%. While a large effect of NaCl impurities in rust was found, no effect of SO2was 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