Active Protection of Magnesium Alloy AZ91D Using Corrosion Inhibitor Encapsulated Halloysite Nanoclay-Based Smart Sol-Gel Coatings

Abstract

Abstract Magnesium alloys are preferred extensively in automobile and aerospace industries, because of their very low density and high-specific strength. However, magnesium alloys are unstable in highly corrosive environments because of their extremely low standard electrode potential (−2.37 V). One of the most promising approaches to overcome this limitation is the use of eco-friendly and versatile sol-gel coatings. In the present work, corrosion-inhibition ability of various corrosion inhibitors, such as Ce3+-Zr4+, 8-hydroxyquinoline and 2-mercaptobenzothaizole, was evaluated on magnesium alloy AZ91D. For this purpose, inhibitors were loaded in as-received halloysite nanotubes (HNTs), which were then end-stoppered using polymeric microcapsules and dispersed in a hybrid silane matrix. The surface morphology of raw and inhibitor-loaded HNTs was examined with transmission electron microscopy analysis, whereas Brunauer-Emmett-Teller and scanning electron microscopy/energy-dispersive X-ray spectroscopy analyses were carried out to verify the encapsulation of inhibitors. The coatings were developed on coupons of AZ91D by the dip-coating technique, which were then thermally cured for 1 h at 130°C. The anticorrosion ability of inhibitor-encapsulated, HNT-based sol-gel coatings on AZ91D was evaluated using electrochemical impedance spectroscopy and potentiodynamic polarization for different durations of exposure to sodium chloride solution of 0.6 M concentration. Salt spray analysis was also carried out according to ASTM B117, Standard Practice for Operating Salt Spray (Fog) Apparatus, to examine the anticorrosion ability of coatings for a prolonged duration of exposure, 168 h. Electrochemical measurements and salt spray analyses have revealed that Ce3+-Zr4+-loaded HNT-based coatings were found to give better anticorrosion properties during prolonged durations of exposure to extremely corrosive environments, such as a 0.6 M sodium chloride solution.