Corrosion Properties ofThorium

As mentioned before, freshly cut thorium is bright with silvery luster, but darkens quickly on exposure to air. The oxidation creates protective oxide film of thorium oxide (ThO2) up to a temperature of ~350 °C. At further higher temperatures, the breakaway transition occurs when the oxide film cracks and the oxidation proceeds almost linearly. At about 1100 °C, the oxidation rate becomes parabolic again. Tho­rium corrodes at a slow rate at around 100 °C in high-purity water with the forma­tion of an adherent oxide film. In the temperature range of 178-200°C in water, the oxide growth rate becomes rapid and eventually starts to spall (i. e., break up). The reaction becomes very rapid at 315 ° C. Thorium has good resistance against most metals barring aluminum below 900 °C.