ammonium chloride is said to convert solid oxides on metal surface to volatile chlorides (ammonia is a gas), but what about the Oxygen that is taken out of oxide? Does the "ammonium oxide" break down to NH3 and H2O simply, or is there deeper mechanism? Zn and Cu chlorides are of quite high boiling point, so i think soldering them would require other fluxes. What would be the reaction mechanism of those fluxes with constituent metals? And How does ZnCl2 does fluxing while its oxides are not volatile?
(Fe, Al, Ti, Si, P, S, C has low boiling point chlorides but Mn, Cr, Ni have not).
When it comes to resin acids, how thay solubilize metal oxides? Do they retain these in a liquid state or boil up as organometallics? resin acids being much weaker than carboxylic acid, how do they dissolve the oxides in hot state? Do they form any organometallic complex involving their sterol/sterane moieties?
My understanding of fluxes is that in general they acted more as a solvent and simply physically removed the oxide layer, then provided an anaerobic/reducing layer to protect the metal briefly.
The actual mode of action is surely very complex and different for different fluxes and metals.
But at least some do not vaporise the oxide layer and simply move it out of the area being soldered.
I suspect you have been given a very oversimplified mode of action and the reality is much more empirical (if it works, use it as a flux).
Re ZnCl2, it will react with some metal oxides forming basic zincates which would have lower melting points than the oxide and be soluble in many more substances than the oxide at elevated temperatures.
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