With lower alloying costs and higher mechanical properties, lean duplex stainless steels can be a good alternative to the more commonly used austenitic stainless steels. A study was initiated to define the limits of the use of lean duplex stainless steels for urban wastewater treatment (WWT) units. This paper gives and discusses the corrosion results in an aerated wet atmosphere containing H2S at different levels. Exposures were performed both at laboratory scale and in the field WWT plant for 1 year. A specific probe was also designed to study the corrosion process below water condensate film contaminated with H2S. Under such conditions, the properties of stainless steel were strongly modified with an enhanced risk of localized corrosion. The results obtained on lean duplex materials (UNS S32101, S32202, and S32304) are compared with austenitic UNS S30403 and UNS S31603 and with the more standard duplexes UNS S82441 and UNS S32205. The results show that lean duplexes can be used in aerated wet atmospheres in case of moderate contamination of H2S (<10 ppm) and chloride (<200 ppm). For higher contaminations (e.g., H2S around 100 ppm/chloride around 1000 ppm) the duplex S32205 should be preferred.