EDIT/Retraction: This answer was written prior to the publication of the research described in the accepted answer, and is now known to be incorrect. Science!

I assume this is due to mesoscopic defects -- if you think about a piece of metal on a molecular scale, it's not going to be presenting a totally flat sheet of atoms at its surface. Hydrogen gas forming within cracks in the metal can force those cracks to open wider, and this then exposes more surface to react, and produces more cracks.

This process will result in the force of the generated hydrogen ripping the bulk metal apart if it happens quickly enough. If the heat generated from the reactions then also starts boiling water in those same cracks and defects, that'll only add to the effect.

EDIT/Retraction: This answer was written prior to the publication of the research described in the accepted answer, and is now known to be incorrect. Science!

I assume this is due to mesoscopic defects -- if you think about a piece of metal on a molecular scale, it's not going to be presenting a totally flat sheet of atoms at its surface. Hydrogen gas forming within cracks in the metal can force those cracks to open wider, and this then exposes more surface to react, and produces more cracks.

This process will result in the force of the generated hydrogen ripping the bulk metal apart if it happens quickly enough. If the heat generated from the reactions then also starts boiling water in those same cracks and defects, that'll only add to the effect.

I assume this is due to mesoscopic defects -- if you think about a piece of metal on a molecular scale, it's not going to be presenting a totally flat sheet of atoms at its surface. Hydrogen gas forming within cracks in the metal can force those cracks to open wider, and this then exposes more surface to react, and produces more cracks.

This process will result in the force of the generated hydrogen ripping the bulk metal apart if it happens quickly enough. If the heat generated from the reactions then also starts boiling water in those same cracks and defects, that'll only add to the effect.

EDIT/Retraction: This answer was written prior to the publication of the research described in the accepted answer, and is now known to be incorrect. Science!

I assume this is due to mesoscopic defects -- if you think about a piece of metal on a molecular scale, it's not going to be presenting a totally flat sheet of atoms at its surface. Hydrogen gas forming within cracks in the metal can force those cracks to open wider, and this then exposes more surface to react, and produces more cracks.

This process will result in the force of the generated hydrogen ripping the bulk metal apart if it happens quickly enough. If the heat generated from the reactions then also starts boiling water in those same cracks and defects, that'll only add to the effect.