Metakaolin-based geopolymer coatings on metals by airbrush spray deposition

Metakaolin-based geopolymer coatings with a thickness of about 60 μm were prepared on 2024 Al alloy and low carbon steel substrates by airbrush spray deposition. Curing in a sealed condition led to a limited geopolymerization reaction and corrosion of the uncoated surface in the metal substrates. Fourier-transform infrared spectroscopy and X-ray diffraction measurements of the coatings cured in an unsealed condition at 40–150°C confirmed the formation of the geopolymer network. Coatings cured at 80°C possessed a favorable microstructure, while some microcracks were present in the coatings after curing at 150°C due to shrinkage and residual stresses. The adhesion strength, measured by scratch tests, showed high values of critical load for samples cured at 80°C. For samples cured at 150°C, the coating was completely removed upon loading when the geopolymer was deposited on Al but not when on mild steel. Immersion tests demonstrated that the stability of the coatings in distilled water increased with increasing curing temperature.