Why Does Molten Aluminum Explode at Underwater or Wet Surfaces?

Molten Al sometimes explodes when it inadvertently contacts a wet or underwater solid surface. With laboratory experiments, we investigated these initiations at seventeen different submerged surfaces. Surfaces nonwettable by liquid water were inert, while water-wettable surfaces initiated explosive molten Al-water interactions. We hypothesize that the water-wetted surfaces provide a thin layer of liquid water beneath the molten Al that can heat rapidly during transient liquid-liquid contact between melt and water. Because the surfaces of the wettable solid and the molten metal are both presumed to be poor nucleators of bubbles, substantial superheating of the water layer may occur. The resultant vaporization may be explosive, causing local fragmentation of the melt and pressure disturbances that initiate steam explosions elsewhere where melt and water are adjacent. Nonwetted surfaces, on the other hand, are essentially dry beneath the melt and produce little assistance to the explosive interaction.