Solubility of Metal Oxides in Na2o-B2O3 Melts Associated with Basicity
Authors: Toki Ryutaro , Doi Takashi 
Published: February 2020

1. Introduction
Molten salts have been used as slag, and as an oxidation inhibitor in the steel production process at high temperature. Steels and refractories are corroded by adherence of fused salts such as molten oxide to their surfaces at the solid-liquid interface. For example, casting nozzles made of refractory oxides are corroded by molten oxide used in continuous casting molds. Dissolution of oxides on the surface of materials in molten salts is thought to be the cause of hot corrosion1). So, it is necessary to understand the corrosion behavior to clarify the dissolution behavior of oxides in melts. The basicity of the fused salts, which is similar to pH in aqueous solution, is one of the dominant influencing factors in the dissolution behavior of metal oxides. In the range of basic of fused salts, metal oxides are dissolved via a basic dissolution reaction, and in the range of acidity, an acidic dissolution reaction dominates in melts1). Dissolution of metal oxides in molten Na2SO4 at 1173 K associated with basicity defined by Na2O activity is well known as an example of reaction of metal oxides with fused salts1). To clarify the dissolution behavior of protective oxide scales in a molten sulfate film under a corrosion environment of gas turbine blades and vanes, the dissolution behavior of metal oxides such as Fe2O3 and Cr2O3 in molten Na2SO4 was investigated. In the case of molten oxides, dissolution reactions of metal oxides in molten sodium silicate used in the steel process were investigated2). However, researches on the dissolution reaction of metal oxides in borate melt, which steel slag comprises, are not sufficient. In this study, the correlation between the basicity and the solubility of Fe2O3 and Cr2O3 in Na2O-B2O3 melt was investigated using the method applied to molten Na2SO4
3).
2. Experimental
In order to measure the solubility of Fe2O3 and Cr2O3 powders in binary fused Na2B4O7-B2O3 melts at 1173 K and 1373 K in air, salts and oxide powders were placed in a platinum crucible and heated at 1173 K and 1373 K. The temperature of the platinum crucible was measured by an R-type thermocouple with a wire diameter of 0.2 mm. The basicity of the melt was measured by a basicity sensor using a solid electrolyte, and a coil of platinum wire was immersed and drawn out of the melt to quench the melt sample. To determine the quantity of soluble Fe or Cr in the melt, the quenched samples were analyzed by ICP-AES. To secure the chemical equilibrium of the melts, measurements were conducted after heating at 1173 K and 1373 K in air for more than 86.4 ks.
3. Results
For the entire range of the tested basicity, the solubility of Fe2O3 was one order of magnitude higher than that of Cr2O3. The solubility of both Fe2O3 and Cr2O3 decreased as the basicity of the melt increased. From the oxides’ solubility dependence on the melt basicity, both Fe2O3 and Cr2O3 are suggested to dissolve by the basic dissolution reaction. Comparing the solubility of oxides with the same basicity at 1173 K and 1373 K suggests the possibility of organizing the solubility of oxides in Na2O-B2O3 melts in air by their basicity.
References
1) R. A. Rapp et al.: JOM 1994. December 47-55
2) H. Khedim et al.: J. Non-Crysta. Solids. 356 (2010) 2734-2741
3) R. Toki et al.: Mater. Trans. 57 (2) (2016) 143-147