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.