some type of ionic solution must be present, salt is a good example, but anything that has dipoles as opposed to non-polar solvents (oils example), can cause "migration" and coating.
a "dirty" gun would have some oils left in the grime, and as long as it broke up the polar solvent, the "battery" effect would be stopped.
the metals "coating" the Fe and preventing iron rust are really the oxides of that metal, ZnO, zinc oxide, aluminum oxide etc. and are only on the surface of that metal, are very hard, and prevent the oxidation of the Fe
The polar salts do not have to be NaCl but can come from primer mixtures. Lead styphanate, sodium nitrate others, military surplus contained large amounts in the primers, and that is why military barrels had a reputation for corrosion and the subsequent "need" to have them chrome lined. Because the residue was so "polar" ordinary solvents wouldn't work thus the need for a hot water flush.
