Failure of 5000 and 6000 series aluminum alloys in modular wastewater treatment aeration tanks

Abstract

Materials used in construction of wastewater treatment plants vary from reinforced concrete to metallic alloys and polymers. For the purposes of constructing multiple, modular wastewater treatment aeration tanks (biological reactors), 5000 and 6000 series aluminum alloys were chosen primarily for the tanks’ fabrication and transport. After only 20 months of intermittent service, severe localized internal corrosion was observed—including through-wall metal loss in certain locations. Visual inspections and failure analyses were performed in order to identify the corrosion modes present, as well as the factors which caused said damage. Characterization techniques including: macro photography, optical microscopy, metallography, scanning electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction, and microhardness testing were utilized. Results of the investigation suggest the corrosion phenomena observed was largely due to under-deposit corrosion—and in some instances—exacerbated by galvanic coupling to dissimilar metals. Ultimately, aluminum alloys should not have been selected for the service.