Caracterização da camada inibidora e do dross em chapas galvanizadas por imersão a quente através de análise e processamento digital de imagens

Abstract

The hot dip galvanizing process is widely used by the automotive industry in order to obtain a zinc (𝑍𝑛) coating as an anti-corrosion protection for steel sheets. During galvanizing, the diffusion of iron (𝐹𝑒) from the steel due to the high temperature of the zinc bath causes the formation of 𝐹𝑒 - 𝑍𝑛 intermetallic phases. These phases have a high iron content in their chemical composition and have high hardness and low tenacity, making the zinc coating susceptible to detachment during the mechanical forming processes of the sheet, which will, therefore, lose its protection. In order to prevent the formation of the phases, aluminum (𝐴𝑙) is added to the zinc bath, as iron preferentially binds to aluminum than to zinc. An inhibitory layer is then formed, composed of the 𝐹𝑒2𝐴𝑙5 phase which is located between the substrate (steel) and the coating and which prevents the formation of the phases. However, if for some reason the phase lies outside the region between the substrate and the coating, there is what is called Dross, which can be considered a defect if it is apparent on the coated surface of the steel sheet. This work used characterization techniques by Scanning Electron Microscopy (SEM), chemical microanalysis by Energy Dispersive Spectroscopy (EDS) and Analysis and Digital Image Processing (PDI) by color segmentation in order to analyze the intermetallic inhibitor layer of and the presence of Dross defect in hot-dip galvanized steel sheet for use in the automobile industry. The inhibitor layer was 1.347 ± 0.188 μm thick, representing 1.39% of the coating studied, with a few regions with discontinuities and isolated presence of the Dross defect. The color segmentation technique was considered efficient and effective for the characterization of the inhibitor layer for the analyzed material and Dross.