Investigation of Metal-H₂O Systems at Elevated Temperatures: II. SnO_2(s) Solubility Data and New Sn Pourbaix Diagrams at 298.15 K and 358.15 K

The Zircaloy alloy system (1.2-1.7 wt.% Sn) provides an excellent basis to study the effect of temperature on Pourbaix diagrams, as Zircaloy fuel rods in a CANDU reactor are exposed to an aqueous phase coolant in the range 523.15 K to 583.15 K. In addition to its presence in Zircaloy, much of the thermochemical data for tin, particularly for the aqueous species, is unavailable or uncorroborated. This paper presents tin Pourbaix diagrams at 358.15 K alongside the re-constructed 298.15 K diagram. Solid-aqueous phase SnO2 equilibria were used to determine thermochemical data for tin +IV species in a batch vessel with in-situ pH measurement. Solubilities were used to calculate the Gibbs energies of Sn complexes. The SnOH3+ and Sn(OH)_5^- species were incorporated into the 298.15 K and 358.15 K diagrams.