Avaliação do desempenho de um revestimento metálico obtido por aspersão térmica a arco elétrico nas condições com e sem selante

Abstract

A modern and very effective method for corrosion prevention has been the coating application, usually metallic, by Thermal Spray Process. The Thermal Arc Spray Process (ASP) is a low cost and high productivity technique, however it produces coatings with high surface roughness values and some inside defects. The surface finish and the defects presence (pores, cracks and oxide clusters) may cause some decrease in the desired corrosion resistance and abrasive wear. Considering this, the sealants are special paints used to penetrate the surface defects in the coatings decreasing the porosity and increase their action in protecting the base material. The objective of this work was to evaluate a metallic coating based on Fe-Cr-Nb-Ni obtained by Thermal Arc Spray Process, in conditions without sealant and with a silicone resin sealant with alumina pigmentation, in the protection of carbon steel tubes against corrosion and wear in a boiler environment. The tested situations expose the coating / substrate system of the samples with and without sealant, which remained in the boiler environment for seven and twelve months. Metallographic preparation techniques, Scanning Electron Microscopy (SEM), semi-quantitative chemical analysis by Dispersive Energy Spectroscopy (EDS) and Digital Image Analysis and Processing (PDI) were used. The same chemical elementsin the same relative quantities were found in the sprayed coating as the wires used in the spraying process, which showed that the spraying process did not cause chemical changes in relation to the wires originally used. The average thickness of the sealant applied according to the technique recommended by the manufacturer, was 60% less than expected. Although the sealant was removed from the coating by the flow of particles present in theboiler operation water, for all samples with tested sealant, the metallic coating remained integral (same morphological aspect and thickness), being considered an efficient solution for regions most critical of the boiler and subject to corrosion and friction. Iron oxides were formed at the coating / substrate interface after seven months in the boiler environment, which suggests that failures in the proposed coating may cause its detachment from the substrate by oxidation at the system interface.