How can I have iron powder without a fast reaction with oxygen?
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 &quot;naked iron&quot;, 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.
$$\ce{FeC2O4  -&gt; FeO  +  CO2  +  CO}$$
$\ce{FeO}$ is thermodynamically unstable below $\pu{575 °C}$, tending to disproportionate to metal and $\ce{Fe3O4}$:
$$\ce{4 FeO(s) -&gt; Fe3O4(s) + Fe(s)}$$

With the following oxidation by air;
$$\ce{3 Fe(s) + 2 O2(g)-&gt; Fe3O4(s)}$$
respectively;
$$\ce{6 FeO(s) + O2(g)-&gt; 2 Fe3O4(s)}$$