Release rate kinetics of corrosion inhibitor loaded halloysite nanotube-based anticorrosion coatings on magnesium alloy AZ91D

Halloysite nanotubes were used as nanocontainers to hold corrosion inhibitors such as Ce3+-Zr4+, 2-mercaptobenzothiazole and 8-hydroxyquinoline in their lumen. An acid assisted etching of the nanotubes was carried out with a view to increase the lumen diameter and thereby, increase the amount of loading of the corrosion inhibitor. The morphology of as-received and etched halloysite nanotubes was observed using TEM analysis. The loading of corrosion inhibitors was confirmed using SEM-EDS and BET analysis. Polymeric microcapsules were used as capping agents for the ends of the loaded HNTs following which, they were dispersed into a hybrid sol-gel silica matrix. Dip coating method was used to generate coatings on AZ91D substrates followed by heat treatment at 130 °C for 1 h. The release rate kinetics of corrosion inhibitors from as-received and etched nanotubes was investigated in buffer solutions of 3.5 wt% NaCl at different pH. The release mechanism of corrosion inhibitors from the HNT lumen was validated using various semi-empirical models. Coatings were also evaluated for their corrosion protection ability using electrochemical techniques after exposure to 3.5 wt% NaCl solution for 120 h. Coatings generated using Ce3+-Zr4+ loaded into as-received halloysite nanotubes have shown more effective corrosion protection when compared to other corrosion inhibitors after 120 h exposure to the corrosive medium.