Investigation of Metal-H₂O Systems at Elevated Temperatures: Part 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 CANadian Deuterium Uranium^TM (CANDU) reactor are exposed to an aqueous phase coolant in the range 250 °C (523.15 K) to 310 °C (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 85 °C (358.15 K) alongside the reconstructed 25 °C (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 25 °C and 85 °C diagrams.