3 ms·

magnesium is the most explody of all those

Retric 1y ago
But still not explosive at scale. It’s a surface area issue, a small strip of magnesium explodes when dropped in water but a 100t cargo of magnesium sinking in a harbor would be a huge fire.
- pfdietz 1y ago
  > a small strip of magnesium explodes when dropped in water No it doesn't. Magnesium metal burns because the boiling point of magnesium is just 1091 C, so extremely reactive vapor is readily produced. But it would be very hard to heat it that high in water unless it was ignited first. It will then continue to burn under water.
  - Retric 1y ago
    Maybe I should have clarified burning, as in “Why does burning magnesium explode when sprinkled with water?” https://physics.stackexchange.com/questions/33167/why-does-burning-magnesium-explode-when-sprinkled-with-water https://physics.stackexchange.com/questions/33167/why-does-b...
    - kragen 1y ago
      Yes, safety is a significant disadvantage of the use of magnesium as portable stored energy, but if your ship's payload is already on fire, in most cases the shipment will not be very successful anyway, and loss of the ship is a serious possibility. If a hypothetical ship full of magnesium sinks without catching the magnesium on fire first, the magnesium will probably not catch fire from exposure to water. Perhaps if it's sufficiently finely divided, which seems like a bad idea.
      - Retric 1y ago
        I agree, the point is you’re not risking something like: https://en.wikipedia.org/wiki/Halifax_Explosion https://en.wikipedia.org/wiki/Halifax_Explosion “At least 1,782 people, largely in Halifax and Dartmouth, were killed by the blast, debris, fires, or collapsed buildings, and an estimated 9,000 others were injured.” “Nearly all structures within an 800-metre (half-mile) radius, including the community of Richmond, were obliterated.[3] A pressure wave snapped trees, bent iron rails, demolished buildings, grounded vessels (including Imo, which was washed ashore by the ensuing tsunami), and scattered fragments of Mont-Blanc for kilometres. Across the harbour, in Dartmouth, there was also widespread damage.[4] A tsunami created by the blast wiped out a community of Mi'kmaq who had lived in the Tufts Cove area for generations.”
        kragen 1y ago
        Not with magnesium ingots or dry magnesium, no; but, because the water–magnesium reaction is exothermic, spontaneous, and gas-producing, I'm pretty sure there's a range of ratios where wet magnesium does constitute an explosive if it's finely divided, at least a low explosive like gunpowder, so such an accident could happen. It seems unlikely to happen by accident because at stoichiometry you need more water than magnesium, and I don't think spontaneous explosion is a real risk with magnesium. The International Magnesium Association's safe handling guide https://cdn.ymaws.com/www.intlmag.org/resource/resmgr/safety/Safe-Handlling-of-Mg.pdf https://cdn.ymaws.com/www.intlmag.org/resource/resmgr/safety... does mention that magnesium swarf can spontaneously combust in the presence of water, but I think swarf is too coarse to explode. It recommends keeping wet magnesium swarf under water to prevent it from heating up enough to spontaneously ignite. But presumably you'd be shipping the magnesium in the form of plates, ingots, or rolls rather than powder, swarf, or loose foil.
        Retric 1y ago
        Even magnesium powder wouldn’t detonate when you’re talking tons of the stuff on a boat for the same reason small hydrogen balloons can go bang, but the Hindenburg just created a huge conflagration. You get limited mixing due to the volumes of material involved. Even burning across several seconds is just vastly less dangerous than an actual detonation. Same issue with grain silos exploding because of the mixture of fuel with oxygen, but flour just burns etc.

But still not explosive at scale. It’s a surface area issue, a small strip of magnesium explodes when dropped in water but a 100t cargo of magnesium sinking in a harbor would be a huge fire.

pfdietz 1y ago
> a small strip of magnesium explodes when dropped in water No it doesn't. Magnesium metal burns because the boiling point of magnesium is just 1091 C, so extremely reactive vapor is readily produced. But it would be very hard to heat it that high in water unless it was ignited first. It will then continue to burn under water.
- Retric 1y ago
  Maybe I should have clarified burning, as in “Why does burning magnesium explode when sprinkled with water?” https://physics.stackexchange.com/questions/33167/why-does-burning-magnesium-explode-when-sprinkled-with-water https://physics.stackexchange.com/questions/33167/why-does-b...
  - kragen 1y ago
    Yes, safety is a significant disadvantage of the use of magnesium as portable stored energy, but if your ship's payload is already on fire, in most cases the shipment will not be very successful anyway, and loss of the ship is a serious possibility. If a hypothetical ship full of magnesium sinks without catching the magnesium on fire first, the magnesium will probably not catch fire from exposure to water. Perhaps if it's sufficiently finely divided, which seems like a bad idea.
    - Retric 1y ago
      I agree, the point is you’re not risking something like: https://en.wikipedia.org/wiki/Halifax_Explosion https://en.wikipedia.org/wiki/Halifax_Explosion “At least 1,782 people, largely in Halifax and Dartmouth, were killed by the blast, debris, fires, or collapsed buildings, and an estimated 9,000 others were injured.” “Nearly all structures within an 800-metre (half-mile) radius, including the community of Richmond, were obliterated.[3] A pressure wave snapped trees, bent iron rails, demolished buildings, grounded vessels (including Imo, which was washed ashore by the ensuing tsunami), and scattered fragments of Mont-Blanc for kilometres. Across the harbour, in Dartmouth, there was also widespread damage.[4] A tsunami created by the blast wiped out a community of Mi'kmaq who had lived in the Tufts Cove area for generations.”
      - kragen 1y ago
        Not with magnesium ingots or dry magnesium, no; but, because the water–magnesium reaction is exothermic, spontaneous, and gas-producing, I'm pretty sure there's a range of ratios where wet magnesium does constitute an explosive if it's finely divided, at least a low explosive like gunpowder, so such an accident could happen. It seems unlikely to happen by accident because at stoichiometry you need more water than magnesium, and I don't think spontaneous explosion is a real risk with magnesium. The International Magnesium Association's safe handling guide https://cdn.ymaws.com/www.intlmag.org/resource/resmgr/safety/Safe-Handlling-of-Mg.pdf https://cdn.ymaws.com/www.intlmag.org/resource/resmgr/safety... does mention that magnesium swarf can spontaneously combust in the presence of water, but I think swarf is too coarse to explode. It recommends keeping wet magnesium swarf under water to prevent it from heating up enough to spontaneously ignite. But presumably you'd be shipping the magnesium in the form of plates, ingots, or rolls rather than powder, swarf, or loose foil.
        Retric 1y ago
        Even magnesium powder wouldn’t detonate when you’re talking tons of the stuff on a boat for the same reason small hydrogen balloons can go bang, but the Hindenburg just created a huge conflagration. You get limited mixing due to the volumes of material involved. Even burning across several seconds is just vastly less dangerous than an actual detonation. Same issue with grain silos exploding because of the mixture of fuel with oxygen, but flour just burns etc.