If there is copper in the aluminum alloy, then you may have to be concerned, but otherwise nitromethane poses no imminent risk to aluminum or steel for that matter. Not in its unburned form anyway. Methanol is far more caustic to aluminum than nitromethane is. The risk lies in the combustion byproducts/residues left behind can be a problem. This is why it's so important to run the engine dry at a modest rpm to evacuate as much residue (unburned fuel) as possible. THAT is the key. Leaving any unburned fuel residue in the engine will attract moisture and rust/corrode the internal parts.