Abstract
In order to make clear the effects of alloying elements, especially chromium and silicon, on the corrosion of steels by V2O5 and V2O5-Na2SO4 mixtures, the corrosion resistance of Cr steels, Cr-Ni steel, Cr-Si steel and Cr-Ni-Si steel was examined by weight loss, metallography, X-ray diffraction and electron beam X-ray microanalysis. The main results obtained are as follows:
1. The corrosion resistance of the steels against molten V2O5 at 850°Cwas increased by the addition of more than 18% Cr, but less improved by the addition of 12% Cr. This was explained to be due to the differences in properties and constitutions of the scales. The corrosion resistance of Cr steels was increased by the addition of 3% Si. This effect was considered to be due to the formation of SiO2 layers in the scales. The enrichment of Ni was observed at the base metal-scale interface of corroded 18% Cr-8% Ni steel, which showed a fairly good corrosion resistance. The existence of CrχFe1-χVO4solid solution in Fe2O3-Cr2O3-V2O5 ternary system was suggested by X-ray diffraction. This solid solution was found in the corrosion products of Cr-Si steels and 18% Cr-8% Ni steel.
2. The corrosion resistance of steels against molten V2O5-Na2SO4 mixture (10: 1 by weight ratio) at 850°C was less improved by the addition of 18-21% Cr, but increased by the addition of about 60% Cr. The resistance of Cr steels was improved by the addition of about 3% Si. The effect was considered to be due to the formation of SiO2 layers in the scales.
8% Cr-3% Si steel, which showed a fairly good resistance against the molten mixtures containing 0-30mol% Na2SO4, showed less resistance against the mixtures more than 40 mol% Na2SO4. This difference was considered to be due to the lack of protective SiO2 layers.
