Produção de revestimentos de conversão para filme de alumina por dip coating em sol-gel

Abstract

Alumina ceramic coatings, produced by sol-gel dip coating, present excellent mechanical properties, such as wear resistance and surface hardness, as well as high corrosion resistance and low thermal and electrical conductivity. When used to produce these coatings, the sol-gel dip coating technique promotes the homogeneity of the coating, a better control of the film thickness and porosity, and the possibility of coating substrates with different forms and natures. However, when the substrate is carbon steel, a weakly bonded and non-protective oxide is formed at the coating/metal interface during deposition, resulting in properties that compromise the quality of the coating. Thus, conversion coatings are applied to the substrate, followed by a heat treatment at high temperatures, which is a necessary step to promote the densification of the film, reducing its porosity. Another effect of the heat treatment is the formation of amorphous or crystalline structures, depending on the temperature. In this dissertation, different conversion coatings (boehmite, silica and two combinations of silica and boehmite) were tested under thermal treatment at different temperatures, between 200°C and 600°C. This temperature range was determined by Thermogravimetric Analysis of the precursors used to produce the sols. Further, the AISI 1020 carbon steel substrates, pre-treated with different conversion coatings, were coated with alumina film, which was also produced by sol-gel dip coating. In order to compare these systems with those produced on zinc phosphate conversion coating, traditionally used for this purpose, the alumina film was also deposited on phosphate treated carbon steel surface. The structural and morphological modifications of the surfaces were analyzed by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM), respectively, while the corrosion resistance was evaluated by Electrochemical Impedance Spectroscopy (EIS), Linear Polarization Resistance (LPR) and Potentiodynamic Polarization (PP) in 3.5% m/v NaCl solution. The results showed that all the conversion coatings studied improved the corrosion resistance of the carbon steel, when compared to the bare and even to the film produced on a conversion coating of zinc phosphate. Considering the type of conversion coating and the heat treatment used, the systems presenting higher resistance to corrosion, were evaluated by EIS for stability, after 240 hours of immersion in the corrosive medium, by Nanoindentation tests for mechanical strength and nanohardness, and by Atomic Force Microscopy (AFM) for the roughness of the intermediate layer. The system that presented the best anticorrosive behavior was composed by alumina film produced on boehmite conversion coating treated at 500°C. This system presented higher resistance to charge transfer and to polarization (139.37 k Ω cm2 e 77778 Ω, respectively) and the lower values of capacitance of the electric double layer, corrosion current density and porosity (2.30 x 10-5 F cm-2, 9.44 x 10-4 A m-2 e 1.01 x 10-5%). Furthermore, also presented the best results, in terms of mechanical strength and hardness.