The use of zinc chloride as a flux, sometimes in a mixture with ammonium chloride (see also Zinc ammonium chloride), involves the production of HCl and its subsequent reaction with surface oxides.
Zinc chloride reacts with metal oxides (MO) to give derivatives of the idealized formula . This reaction is relevant to the utility of  solution as a flux for soldering — it dissolves passivating oxides, exposing the clean metal surface. Fluxes with  as an active ingredient are sometimes called "tinner's fluid".
Zinc chloride forms two salts with ammonium chloride:  and , which decompose on heating liberating HCl, just as zinc chloride hydrate does. The action of zinc chloride/ammonium chloride fluxes, for example, in the hot-dip galvanizing process produces  gas and ammonia fumes.