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Caracterização de fases intermetálicas presentes em aço galvanizado por imersão a quente com tratamento térmico do revestimento metálico
Rosangela Maria Vitor Paranhos
DATA DE PUBLICAÇÃO
Hot dip galvanized steels with thermal treatment, or galvannealed steels, are used in automotive industries because of their properties of weldability, workability, and corrosion resistance. The hot dip galvanized steel with thermal treatment is called as galvannealed. The zinc layer of galvannealed steels have iron-zinc intermetallic phases, depending on the composition and texture of substrate, the composition of zinc bath, and the operational conditions. The properties of galvannealed steels are related to their phases. The characterization of iron-zinc phases is relevant and allows the establishment the relation between the phases and the steel performance. The alteration of industrial process, controlling the iron-zinc phase formation, produces a higher quality of the steel products. In the present work, a 40/40 g.m-2 zinc layer was produced on a interstitial free steel substrate chemical composition and mass of zinc layer were evaluated, and the intermetallic phases were characterized in relation to composition, morphology and structure, before and after the electrochemical dissolution. The optimization of electrochemical dissolution was performed, varying the chemical composition of solution, pH, and current density. Hot dip galvanized without thermal treatment and electrogalvanised steels were submitted to the electrochemical dissolution and the results were compared with the obtained for the galvannealed steels. The optimal conditions of electrochemical dissolution of galvannealed steels were: electrolyte of 0.35 mol.L-1 zinc sulphate heptahydrate and 3.42 mol.L-1 sodium chloride, current density 2 mA.cm-2, pH 4,5. Mossbauer spectroscopy and X-ray diffraction analysis identified the delta phase as the predominant phase of the galvanized layer of galvannealed steel.