All fluxes, by their nature, are reactive, they have to be, in order to remove oxide layers from whatever it is that you’re soldering. Only electronics fluxes will be truly non corrosive, and even some of them have to be washed off or they can be sufficiently conductive to cause circuit malfunctions.
I’d suggest that doing the soldering and washing after every session, and not putting the steel bits (or motors & gearboxes) anywhere near the model until the soldering is finished, washed, dried and cleaned is the best way to go. I do recognise that might be a lot more difficult in smaller scales than in 7mm.
if you want a cheap, washable flux, citric acid mixed with water (and ipa, optionally) is easy to get, eg at Wilco, in the home brew aisle. It goes a bit sticky, but washes off easily with a drop of hot water.