Characterization of the properties of thin Al<sub>2</sub>O<sub>3</sub> films formed on structural steel by the sol-gel method
The sol-gel method was used to obtain single and double layer oxide-aluminium films on the surface of 08kp structural steel for protection against corrosion. The sol-gel system was prepared according to the Yoldas method by hydrolytic polycondensation of aluminium isopropoxide in the presence of acetic acid at a temperature of 85-90 °C. The pH, size, and zeta potential of boehmite particles were measured, and the corrosion behaviour of steel in the resulting colloids was studied. At room temperature, a boehmite hydrogel layer was applied by immersing the steel samples in a gel, incubating them in the gel, and drying them in the presence of ammonia vapour for the suppression of corrosion. Heat treatment at 500 °C in an air atmosphere was used for the decomposition of boehmite with the formation of alumina and for the fixation of the film on the surface of the substrate. A second film layer was applied in a similar manner on top of a fully formed first layer. The surface morphology of the obtained films was studied by scanning electron and atomic force microscopy. Electrochemical research methods - potentiometry, voltammetry, and electrochemical impedance spectroscopy were used for a comparative study of the anticorrosion properties of single and double layer aluminium oxide coatings on steel in a 3.5% sodium chloride solution. It was shown that the coatings obtained according to the described scheme have good adhesion to the steel surface. Compared with the control sample, coated steel electrodes were characterized by a shift in the stationary potential by more than 0.6 V, and the onset of active electrochemical oxidation by more than 1 V in the positive direction. The kinetics of the anode and cathode processes changed, leading to a decrease in the corrosion current by more than two orders of magnitude. The application of the method of electrochemical impedance spectroscopy allowed establishing a 2.5-hour high anticorrosive efficiency of a double layer oxide-aluminium coating on steel in a solution of sodium chloride.