The influence of the size of corundum particles on the properties of chemically resistant porcelain enamels

Porcelain enamels are silica-based inorganic coatings, which are successfully applied to different kind of metallic substrates in applications where high chemical resistance and corrosion protection are needed. The main weakness of enamel coatings is represented by the glassy and brittle nature of the matrix: therefore, it is necessary to find some solutions to address this issue effectively and improve their resistance to abrasion. Some recent papers about the production of composite enamels with Al2O3 particles showed that a high concentration of corundum particles (10–20 wt%) could improve the abrasion resistance of these coatings. This work is focused on studying the influence of micro and macro sized corundum particles on the abrasion and chemical resistance of composite enamel coatings. The Al2O3 particles are added in 20 wt % concentration and the firing treatment is optimized for every sample type. The coatings topography and morphology are mainly characterized by scanning electron microscopy measurements and Energy Dispersive X-ray (EDX) microanalysis. The abrasion resistance of the coatings is evaluated by P.E.I. abrasion test. Mass loss and roughness measurements are used to assess the coatings surface properties during the mechanical damage; in addition, Electrochemical Impedance Spectroscopy (EIS) measurements and electron microscopy observations are carried out after 5000 P E.I. abrasion cycles to evaluate the decrease of the coatings protection properties due to mechanical damage. Finally, the coatings chemical resistance is evaluated by exposure to an aggressive fog with the presence of SO2 (Kesternich test), simulating an industrial environment. The addition of macro sized alumina particles leads to an improvement of the coating abrasion resistance, without a loss in the protection properties. Differently, the addition of micro sized alumina particles produces an increase of the coating surface roughness and results to have detrimental effects on the behaviour of the coating in aggressive environments.