Zinc Chloride — Frequently Asked Questions

Solder will not wet a metal carrying an oxide film, and heating the joint makes that film grow faster. Zinc chloride hydrolyses in the presence of moisture at soldering temperature to release hydrochloric acid, which strips the oxide just as the solder flows, and the molten salt then blankets the surface and stops it reoxidising before the joint is made.

Yes. The same acidity that cleans the joint will corrode it afterwards, and zinc chloride residue is hygroscopic so it draws moisture and keeps corroding. Joints made with an active flux of this type must be washed thoroughly after soldering, which is why electronics work generally uses milder rosin-based fluxes instead.

It is among the most water-soluble salts known and is strongly deliquescent, absorbing enough atmospheric moisture to dissolve in it. Containers must be kept sealed and the material stored dry, because exposed zinc chloride will liquefy into a corrosive solution. That same affinity for water underlies its use as a dehydrating and condensing agent.

As a Lewis acid catalyst. It accepts an electron pair and thereby activates substrates, which is used in Friedel-Crafts reactions and in the Lucas reagent for distinguishing and converting alcohols. It is cheaper and easier to handle than aluminium chloride for many of these transformations, though it is a weaker Lewis acid.