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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.

> 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.

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-b...

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.

I agree, the point is you’re not risking something like:

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.”

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... 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.

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.

Yes, that's why I said, "dry magnesium, no". A pile of dry magnesium powder only burns at the surface as air diffuses into it. If you have it mixed with the oxidizer so that the flame can propagate through the whole mixture, it will, and the propagation speed is determined by factors like the reaction speed, gas production, and thermal conductivity. The reaction speed in turn is governed by the particle size, since the reaction only takes place at particle surfaces; it goes to completion faster when particle size gets smaller.

Small hydrogen balloons do not in fact go bang; they just create small conflagrations. What goes bang are small balloons filled with a near-stoichiometric mixture of hydrogen and oxygen, such as you get from the simplest forms of water electrolysis.

The stoichiometric mixture of magnesium with water is 1.36 grams of water per gram of magnesium (which is 1.74g/cc, so this 58-wt%-water mixture is 70% water by volume), the enthalpy of formation of H₂O is -285.83kJ/mol, and the enthalpy of formation of MgO is -601.6kJ/mol. So this reaction:

    Mg + H₂O → MgO + H₂

yields 315.8kJ/mol, which is to say, 315.8kJ per 24.3 grams of magnesium, or per 58 grams of mixture, about 5.4MJ/kg, about an 18% higher energy density than TNT. And the hot hydrogen gas produced will carry the heat produced by the reaction into nearby areas, igniting them and resulting in a flame propagation velocity that's higher than thermal conduction alone.

For a large enough particle size, you won't get an explosion, and you may even lose most of your water as steam; but for a small enough particle size and an oxidizer concentration close enough to stoichiometric, you will. Some nanothermites consisting of magnesium nanoparticles with an oxidizer such as iron oxide even reliably detonate.

So, it's a potential safety hazard, but it seems like one that should be easy enough to guard against.

> for a small enough particle size and an oxidizer concentration close enough to stoichiometric, you will.

Sure but nobody is going to ship large quantities of magnesium like that: “Use proper packaging: Ensure the magnesium is sealed in moisture-proof, airtight containers.” https://www.freightamigo.com/blog/hs-code-for-containing-at-...

> Small hydrogen balloons do not in fact go bang

It’s not a supersonic detonation but even normal balloons pop with a small bang, pure hydrogen balloons are louder. Though a you mention hydrogen + oxygen is significantly more extreme.