Estudo da oxidação em altas temperaturas do aço inoxidável ferrítico aisi 441, em atmosferas de argônio e ar sintético

Abstract

Stainless steels play an important role in industry and society, especially because of their corrosion properties, which, even in oxidizing atmospheres and high temperatures, give the steel excellent resistance to oxidation. This is due to the presence of chromium as a component in the alloy that forms an oxide film on steel. Whenever formed, this film exhibits high stability and adhesion and does not come off easily being present on the entire surface of the steel. In order to investigate and compare the kinetics of oxidation, microstructure and chemical composition of the oxide films formed on the AISI 441 ferritic stainless steel, at temperatures of 850 °C, 900 °C and 950 °C, in atmospheres of Argon with 1 ppm of O2, in thermobalance for 50 hours and also in atmosphere of Synthetic Air containing 20% O2, in a tubular oven, with the times of 2 h, 4 h, 8 h, 16 h, 32 h and 50 h. The oxidation kinetics were established by measuring the mass gain per unit area versus oxidation time. The microstructure and the chemical composition of the oxides were analyzed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The oxidation kinetics presented a parabolic behavior for the atmosfere. At 850 °C in Argon the steel presented a higher oxidation rate than at 900 and 950 ºC, in relation to oxidation in Synthetic Air the mass gain was greater to the temperature of 900 °C and lower to 950 °C. The chemical analyzes describe that Chromium is the main element of the film formed on steel in Argon and Synthetic Air. The main phases identified in Argon atmosphere were chromium oxide and Cr / Ti, Cr / Mn, Mn / Ti spinels and others. For the atmosphere of Synthetic Air at 850 °C, 900 °C and 950 °C, the most notable were chromium oxide, titanium dioxide and Cr / Mn and Cr / Fe spinel, as well as other Fe / Ti / Cr.