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 SO 2 . 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 r crit is the corrosion rate at the critical relative humidity RH crit and b″ is the normalized corrosion rate at RH = 100%. While a large effect of NaCl impurities in rust was found, no effect of SO 2 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%.