Copper alloys which contain oxygen as an impurity (in the form of residual oxides present in the metal matrix) can be embrittled if exposed to hot hydrogen. The hydrogen diffuses through the copper and reacts with inclusions of Cu2O, forming H2O (water), which then forms pressurized water steam bubbles at the grain boundaries. This process can cause the grains to be forced away from each other, and is known as "steam embrittlement" (because steam is produced, not because exposure to steam causes the problem).