A small amount of mercury amalgamates itself into an aluminum I-beam and destroys it from within. Gallium scratched into the surface allows the mercury to penetrate the protective oxide layer that normally surrounds anything made of aluminum.
This is a time-lapse video, the action takes about two hours in real time. The powdery oxide is falling off, what you don’t see is a significant pile of it building up underneath.
This is the original Pop Sci article on the topic: http://periodictable.com/PopSci/2004/10/1/index.html