The rusting of type 304 and 430 stainless steels due to occurrence of pitting corrosion has been monitored in marine and rural atmospheric environments using a remote-controlled electrochemical impedance corrosion monitoring system. The monitoring results from the marine environments showed that the rusting of type 304 progressed only under a relative humidity range from 35 to 75%, while the rusting of type 430 takes place at relative humidities over 35%. The lower limit (35%) and upper limit (75%) correspond to the relative humidities in equilibrium with the saturated MgCl2 and 3 M MgCl2 [(Cl-) = 6 M] solutions, respectively. The concentration is identical with a critical chloride concentration of occurrence of pitting corrosion reported in previous studies. In the rural atmosphere, the monitoring results showed that both stainless steels are always in passive state. (c) 2006 The Electrochemical Society.