Uranium is very active chemically and rapidly reacts with most environments (air, oxygen, hydrogen, water, water vapor, and others). Freshly polished uranium has a dull silvery color. However, when exposed to air for a few minutes, the surface shows straw-like color and darkens to a blue-black color within a few days. The oxide films formed are not quite protective. At elevated temperatures as the film thickens with time, the characteristic black color of UO2 develops, and it starts cracking and crum­bling exposing fresh uranium metal from underneath the oxide film to be attacked.

Unalloyed uranium reacts with water almost readily. Figure 7.4 shows the corrosion behavior of uranium in aerated distilled water. At 50-70 °C, an ini­tially formed UO2 film provides corrosion protection to the metal with consid­erable incubation period. However, beyond this temperature regime, corrosion rate picks up as the surface oxide film becomes porous and the protection of the oxide film is lost. Also, no incubation period is noted at these

temperatures. Conversely, in hydrogen-saturated or degassed water, the corro­sion rates remain linear with respect to time in the moderate temperature regime. It is postulated that hydrogen diffusion takes place through the thin oxide film to form uranium hydride (UH3) between the oxide and the metal and also into the grain boundaries of uranium.