Corrosion of Aluminum Alloys in Wastewater Treatment Aeration Tanks

Abstract

Severe localized corrosion was found throughout an aeration tank of a modular wastewater treatment plant. Specifically, pitting, crevice corrosion, galvanic corrosion, and intergranular corrosion (IGC) were observed on the 5000 and 6000 series aluminum alloys used in the tank’s construction. One of the suspected environmental causes of corrosion was an aeration tank process additive, namely a coagulant – either aluminum sulphate (Alum) or polyaluminum chloride (PACl; Isopac). Laboratory scale corrosion cells were designed to isolate, simulate, and evaluate the corrosivity of the coagulants (in relevant quantities) on the metallic alloys of construction. Additionally, due to presence of deposit found throughout the tank (resulting in under-deposit corrosion), fabricated crevices were installed on the tested alloys to help simulate crevice corrosion environments. The solution conditions in the cells were monitored throughout the duration of the immersion testing in order to assess whether the cells were within normal operating conditions of the corroded aeration tank. A series of material characterizations and mass change measurements were performed on the corrosion coupons post-testing to evaluate and rank the corrosivity of the coagulants. Results from the laboratory scale testing revealed that both coagulants led to localized corrosion of the 5000 and 6000 series aluminum alloys – in the form of pitting, crevice corrosion, and IGC. Therefore, it can be concluded that neither Isopac nor Alum should be used in aeration tanks constructed of the aluminum alloys tested in this study. Whilst the immersion testing did not simulate all conditions observed and measured in the corroded tank, the testing did allow for the study of the isolated, corrosive effects of either Isopac or Alum on the tested alloys. It can be reasoned that even though the coagulants led to similar corrosion observed in the tank, the coagulants themselves are not the sole cause of the aforementioned corrosion; ultimately, additional factors and conditions in the aeration tank could be deemed more corrosive than merely the coagulants.