Avaliação de revestimentos galvanizados industriais com e sem cromatização para avaliação das propriedades anticorrosivas

Abstract

The hot-dip galvanizing process is widely used as a form of anti-corrosion protection for metallic structures, mutually promoting a physical barrier and cathodic protection of the substrate, acting as a “sacrificial electrode” if the base metal is exposed. However, the zinc layer, when exposed to humid and aggressive environments, is quickly subjected to the white zinc corrosion process. In order to minimize this process, the chromatization process is commonly used in combination, which consists of immersing the galvanized part in a bath containing hexavalent and trivalent chromium ions. Despite the protective nature, the biggest limitation of this process is the toxic and carcinogenic effects of Cr+6. The present work aims to investigate the effectiveness of anti-corrosion protection of the chromatization layer applied to galvanized carbon steel obtained through an industrial process, when exposed to NaCl. In the present study, 3 conditions of galvanized coatings obtained through an industrial process on ASTM A36 steel were investigated: G (galvanized steel); C10 (galvanization + chromatization for 10s) and C30 (galvanization + chromatization for 30s). The samples were subjected to immersion in molten zinc for 2 minutes and the chromatizing bath had a chromium concentration of 0,46% m/v de Cr3+, with the entire process carried out at the company Galvanisa (Igarassu-PE). The thickness of the coatings and the roughness of the surfaces were monitored. To evaluate the microstructure of the coating, micrographs were obtained by SEM and compositional analysis was carried out by EDS. To analyze corrosion resistance, linear polarization tests were carried out in a 3.5% NaCl solution and the morphology and composition of the oxidized surfaces were subsequently analyzed using SEM and EDS, respectively. The results indicated that the coatings obtained present heterogeneity in terms of thickness, roughness and composition. The C10 and C30 coatings did not present a uniform distribution of Cr on the galvanized surface, with sections more susceptible to corrosion being observed, due to the absence of this element. The C30 condition showed lower anodic current density, indicating a behavior of reduced corrosion kinetics.