The unexpected prime suspects for the corrosion that threatens to
cut off 8% of US oil production are bacteria that colonize the sludge
that collects in pipes. BP officials say the best guess so far is that
bacteria colonized part of the pipe and corroded the metal. A similar
problem probably led to a 750,000-litre oil spill in another part of
BPs Alaska pipeline in March 2006.

Read (New Scientist)