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I've seen this video in which fine iron powder reacts immediately with oxygen when exposed to the air, turning into rust.

Now, will this reaction always occur with iron powder? Or is there something special with this specific sample?

I'm writing a steampunk story where people are liberating iron powder in the air. Now, I know that iron powder exists without creating sparks, but I don't want to make some huge mistake, so I'd like to understand a little more about this reaction.

Also, the presence of coal combustion smoke or water steam would have some influence in the reaction?

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All is matter of how fine the powder is ( surface per mass ratio ) and what is the level of prior surface reaction with oxygen.

If the powder is very fine ( I mean very ) and the surface is "naked iron", it would burn in a small firework of sparks.

If the powder is course or the surface is already passivated by oxygen because of the powder history of preparation, it would not provide noticeable reaction.

I remember some old book of chemical experiments, preparing pyrophoric powder by pyrolysis of ferrous oxalate, but it was not iron powder, but pyrophoric ferrous oxide $\ce{FeO}$:

Preparation
$\ce{FeO}$ can be prepared by the thermal decomposition of iron(II) oxalate. $$\ce{FeC2O4 -> FeO + CO2 + CO}$$

$\ce{FeO}$ is thermodynamically unstable below $\pu{575 °C}$, tending to disproportionate to metal and $\ce{Fe3O4}$:

$$\ce{4 FeO(s) -> Fe3O4(s) + Fe(s)}$$

With the following oxidation by air;

$$\ce{3 Fe(s) + 2 O2(g)-> Fe3O4(s)}$$ respectively; $$\ce{6 FeO(s) + O2(g)-> 2 Fe3O4(s)}$$

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  • $\begingroup$ Are you sure that the substance obtained by pyrolysis of ferrous oxalate is FeO ? I remember having read that it was pure iron. $\endgroup$ – Maurice May 14 at 20:21
  • $\begingroup$ I remember reading it in that book, and independently it is also listed in the ferrous oxide Wikipedia page, so I would take it as OK. $\endgroup$ – Poutnik May 14 at 20:23
  • $\begingroup$ It may be, that in the end there is pyrophoric $\ce{Fe}$ carried by $\ce{Fe3O4}$ $\endgroup$ – Poutnik May 14 at 20:33
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Iron will react with the oxygen in air, just at a very slow speed. When you make the particles really fine, however, their surface to volume ratio becomes bigger, the energy that is set free by the reaction is enough to heat up the small iron particles and the reaction will increase in speed due to the temperature increase, resulting in a thermal runaway. Finally, you'll have glowing particles which even may start a fire. For more insights, do some research on "pyrophoric iron", which is the technical term for it.

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