Study of the redox and acid-base properties of soda-lime silicate glass: application to the high temperature corrosion of nickel-based alloys and ceramic materials

The corrosion behaviour of chromia (Cr₂O₃) and alumina (Al₂O₃) forming alloys in soda-lime silicate melts was studied by using 'raw immersion' technique which was coupled with electrochemical measurements. The combination of both techniques leads to the determination of the general aspects of the corrosion by the molten glass as well as the redox reactions and the kinetics that rule the corrosion process. The results revealed that Al₂O₃ is not able to provide protection against corrosion by molten glass since it has a high dissolution in the melt. However Cr₂O₃ forming alloy could resist against corrosion by molten glass if the alloy was subjected to a pre-oxidation treatment in air before the immersion. As the durability of the Cr₂O₃ layer is linked to the competition between the oxide growth and its dissolution in the melt, the physicochemical properties of Cr₂O₃ in molten glasses were then thoroughly studied in order to understand the influence of different parameters on the limit of Cr₂O₃ solubility. The works were conducted by taking into account the influence of temperatures (T), oxygen fugacities (fO₂) and the glass compositions on the solubility of Cr₂O₃ in the melts. The preliminary kinetics study revealed that the time to reach equilibrium varies with the parameters (T, fO₂ and glass compositions). However, a compromising equilibrium time was chosen in order to satisfy all the problems encountered. The thermodynamic study showed that the influence of T and fO₂ on the Cr₂O₃ solubility in ternary soda-lime silicate melts (Na₂O-CaO-xSiO₂) seems to be in coherence with the results in binary melts (Na₂O-xSiO₂). Nevertheless, the Cr₂O₃ solubility in ternary melts is lower than in binary melts. The Cr solubility values in Na₂O-CaO-xSiO₂ (x = 3, 4, 5 and 6) for different temperatures (T = 1200 deg. C, 1300 deg. C and 1350 deg. C) and different oxygen fugacity (-12 ≤ log fO₂ ≤ -0.6) lie between 0.23 at.% to 1.09 at.%. The influence of basicity was studied in these ternary and binary melts by taking into account the theoretical optical basicity (Λ_th) and the activity of Na₂O (aNa₂O). The redox ratios (Cr^II/Cr^III and Cr^VI/Cr^III) were determined through a mathematical refinement. The comparison with the results obtained in binary melts and the deviation from the theoretical values expected when speciation of Cr is involved, induce the assumption of the role played by CaO in influencing the reactions involving species from O₂^-/O₂ couple. (author)