Corrosion properties of alumina and titania NF membranes

The chemical stability of multilayer alumina and titania nanofiltration (NF) membranes, prepared by a sol–gel procedure was investigated. The membranes have toplayers of γ-AlOOH fired at 400 °C, anatase–TiO2 fired at 300 °C and anatase–TiO2 fired at 450 °C, respectively. The solvent stability of the membranes was proven in a series of stability tests with different types of organic solvents. In order to provide information about the corrosion properties, the membrane performance (PEG-retention, molecular weight cut-off (MWCO), permeability) was studied in a series of dynamic corrosion tests with aqueous solutions at low pH (HNO3) and at high pH (NaOH). For the membrane with a γ-AlOOH toplayer (low crystallinity), the MWCO increased from 200 to 1700 after 7 days of operation at pH 2, indicating serious membrane corrosion. For the anatase–TiO2 membrane fired at 300 °C (low crystallinity), the MWCO increased from 200 to 800 at pH 2. On the contrary, corrosion tests performed with the highly crystalline anatase–TiO2 membrane fired at 450 °C demonstrated no significant loss in membrane performance in the pH range studied (1.5–13). Long-term corrosion tests (6 weeks) at pH 2 and 1.5 confirmed the high corrosion resistance for this type of NF membrane.