Stainless steels are successfully used in the range of water compositions encountered during wastewater treatment. Austenitic stainless steels are the most often used grades in water treatment applications. S30403 and S31603 are known to be suitable for municipal and industrial wastewater with respectively moderate and increased chloride contents. Considering the comprehension of economic cost, good corrosion resistance and higher mechanical properties, lean duplex stainless steels can be a good alternative to these commonly used austenitic grades but they are still too rarely chosen, probably due to a lack of case studies.

The present paper deals with the localized corrosion performance of lean duplex (S32101, S32202 and S32304) in solutions simulating pre-treatment unit waters, using electrochemical testing. The localized corrosion was compared with austenitic stainless steels (S30403 and S31603) and with more alloyed duplex grades (S82441 and S32205). Pitting corrosion methods were adapted from standard electrochemical test methods ASTM G150 ASTM G61 to assess critical pitting temperature (CPT) and pitting potential (PP) respectively. Environmental parameters (chloride content, pH and temperature) were selected to be representative of the different environments that can be encountered in urban wastewater treatment plants.

All the results permit to have an overview of corrosion properties according to the environmental parameters and to choose properly the best stainless steel grade for urban wastewater treatment units. This study allows also to have a clearer view of the behavior of duplex stainless steel grades versus austenitic grades in such environments. It confirms that lean duplex grades S32101 and S32202 can be used as alternatives to S30403 and S31603 respectively.


Wastewater coming from both municipal and industrial activities present corrosive properties toward metals and can in certain cases exceed the tolerances of the most often used stainless steels such as 304L and 316L, thus resulting in the need of superior alloys. The main factors influencing the corrosiveness of the fluids in wastewater treatment plants are high concentration of halides (more specifically chlorides), H2S, low pH, temperature and their combined action. Corrosiveness of incoming fluids at a municipal wastewater treatment finds its roots in the municipal collection network. In most of the cases, the chloride concentration, around 50-200 mg/L in municipal wastewater streams, of this water doesn’t present major issues for austenitic grade such as 304L. However, in certain conditions, chloride concentrations can reach 3 000 - 6 000 mg/L and in extreme cases 10 000 mg/L for seaside‐located facilities or when de-icing salts are used during winter [1,2], requiring more corrosion resistant alloys.

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