Copper alloys which contain oxygen can be embrittled if exposed to hot hydrogen. The hydrogen diffuses through the copper and reacts with inclusions of , forming 2 metallic Cu atoms and   (water), which then forms pressurized bubbles at the grain boundaries. This process can cause the grains to literally be forced away from each other, and is known as steam embrittlement (because steam is directly produced inside the copper crystal lattice, not because exposure of copper to external steam causes the problem).