Corrosion of stainless steel pipes in sewage treatment systems has been studied as part of a Danish research programme. The pipes were attacked by localized corrosion in locations near the final stage of the treatment process, where the water is practically free of organic substances. Since the temperature and chloride content is quite low, it has been difficult to explain the reason for corrosion. An extensive series of field-tests and inspections showed that the failures in most cases could be explained by bacterial deposition of manganese dioxide, which results in potential ennoblement due to its effective cathode properties. Preventive measures for this form of corrosion were evaluated with special focus on cathodic protection. Finally, revised design diagrams were established to support the selection of resistant stainless steel grades for low chloride environments affected by highly oxidizing conditions caused by bacteria.
Keywords: sewage treatment, AISI 304, AISI 316L, EN 1.4435, MIC, potential ennoblement, manganese bacteria, field tests, cathodic protection, CPT