The ceramic layer produced by a centrifugal-thermit process consists of corundum (α-Al2O3) and hercynite (FeO-Al2O3 spinel) when inducing the thermit reaction with aluminum and iron oxide powders. Although the hercynite in the ceramic layer plays some role in improving mechanical properties of ceramic layer such as toughness and thermal shock resistance, it is not desirable for improving the corrosion resistance, because FeO component included in hercynite would be attacked in highly acidic conditions.
In the present work, in order to make clear the mechanism of FeO remaining in the ceramic products and to reduce its content, the centrifugal-thermit process is performed with Al and Fe2O3 powder mixtures of the mole ratios (Fe2O3/2Al) of 1.1, 1.0, and 0.9 under the environments of nitrogen gas and air flow. It is also performed by adding some amounts of Mg and Si powders to the stoichiometric mixture of Al and Fe2O3 powders.
Thus, it is confirmed that the content of FeO decreases with the increase of the amount of Al. The FeO content in the stoichiometric mixture under nitrogen gas flow is about 11 mass% and almost the same with that under air. Therefore, the predominant factor affecting on the FeO residue is not considered due to the lack of Al caused from its reaction with atmospheric oxygen but to the incomplete reaction. As the ratio of thermit mixtures deviates from the stoichiometric one, the amount of FeO obtained under air flow becomes larger than that under nitrogen gas flow. The incomplete reaction caused from the interaction between atmospheric oxygen and nonstoichiometric mixtures is also discussed. On the addition of Mg or Si powders, the FeO content decreases in the reactant condition of 1800 g in total amount and of 0.1 in mole ratio of 3Mg/Fe2O3 or 3Si/2Fe2O3. When Mg powder is added, most of Mg powders react to produce MgO in the ceramic layer. On the other hand, when Si powder is added, it melts into the metal layer and therefore the amount of produced SiO2 is not so large as that of MgO.
