Thanks to their good corrosion resistance and ease to shape and weld, austenitic stainless steels (e.g. UNS S31603) are used as standard materials for the construction of urban wastewater treatment plants (WWTP). The main factors influencing the corrosiveness of the fluids in WWTP are halide concentration (more specifically chlorides), H2S content, low pH, temperature and their combined effects.

The present paper deals with the localized corrosion performance of lean duplex UNS S32202 in solutions or in atmosphere simulating pre-treatment unit environment. It is compared to the austenitic stainless steel UNS S31603 and the higher alloyed duplex stainless steel UNS S32205. Crevice corrosion testing was performed using crevice formers defined in ISO(1) 18070:2015. The effect of welding was also investigated, with and without post-welding pickling. Both laboratory screen tests and field exposures in urban wastewater treatment plants were performed. Exposures of samples in field municipal wastewater plants were conducted for 1 year in a low chloride content unit (Brussels, Belgium) and in a high chloride content unit (Cap Sicié, France).

The results give an overview of corrosion properties according to temperature, chloride and H2S contents enabling to choose properly the most appropriate stainless steel grade for urban wastewater treatment plants. The results from field exposures (1 year) showed good agreement with laboratory results in simulated environments. This study confirms that lean duplex grade UNS S32202 can be used as an alternative to UNS S31603 in immersion and atmospheric exposure conditions for those two grades when exposed to splash zones (combination of H2S and chloride contamination): H2S (<10 ppm) and chloride (<200 ppm). For higher contaminations (e.g. H2S around 100 ppm and chloride content around 1000 ppm) the duplex grade UNS S32205 should be preferred. Whatever the tested condition and alloy, post-weld pickling shall never be avoided.

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