Abstract

This original work aimed at quantifying the time-dependent indirect corrosion of high alumina refractories by Al2O3-CaO-SiO2 secondary steelmaking slag. The main objectives were:

i) To determine corrosion kinetics based on time-resolved X-ray diffraction at high temperature combined with Rietveld quantification;

ii) To propose a reaction model based dissolution/precipitation/diffusion mechanism for numerical simulation.

The tests were performed at temperature ranging from 1500 °C up to 1650 °C. Two different tests were performed: Ex situ experiments to obtain long time treatment (up to 24 hours) and in situ experiments to analyse the phase changes in the early first moments (less than 5 minutes).

The ex situ corrosion tests associated with quenching method, XRD and high temperature XRD were performed in order to determine the Al2O3/CA6/CA2/CA contents vs time.

The in situ experiments at high temperature were the most innovative part of this work. The tests were performed on a diffractometer equipped with a dedicated furnace allowing for very fast heating. Alumina powder was mixed with crushed slag and put on the especially designed heating strip. The mixture alumina/slag was heated up to 1600 °C in less than 5 minutes to prevent solid/solid reaction before the slag melts. A full 2θ pattern was recorded every 5 s by means of a Curved Position Sensitive detector (CPS120). Rietveld refinement was carried out and crystallised phases were quantified. The liquid phase content was evaluated from stoichiometric balance. In addition, Scanning Electron Microscopy (SEM) was performed to analyse the microstructure of the corroded grains.

This original approach allowed for quantifying the time-dependent indirect dissolution of the alumina grains while mono-mineral calcium aluminates precipitated as well known in literature. The results show that the corrosion process is very fast and that it involves two mechanisms: i) the solid-state diffusion of calcium cations through the successive aluminate layers observed with SEM analyses; ii) the dissolution of the outer layer at the interface with slag providing slag exists. Due to silica adding, the molten slag viscosity changed and the formation rate of aluminates layers slowed down. As a result the thicknesses of the layers change according to amount of slag. Finally, numerical simulations were carried out taking into account the both mechanism and showed relative good agreements with experimental data.