Field tests of stainless steels were carried out in six wastewater treatment plants. Welded coupons were tested in the final settling tanks in the plants. Ennoblement of the OCP, similar to that reported from
investigations in seawater, was observed at two plants. Pitting corrosion was observed under deposits after the field test on steel grade AISI 304 (UNS S30400) and on AISI 316 (UNS S31600) in one plant. No
corrosion was found on 2205 (UNS S31803) after the field tests. Laboratory measurements were also carried out to predict MIC in the final stages of aerobic wastewater treatment systems. The breakdown
potential was measured in distilled waters with various amounts of chlorides, nitrates and sulfates according to a factorial design from maximum and minimum values in two levels measured in six plants. A risk window for MIC in wastewater treatment plants was designed, which was in good agreement with the field tests.