In hot-dip galvanizing, the flux bath is the final chemical treatment step before immersion in molten zinc. Its purpose is to dissolve any residual oxide films on the steel surface and to create a protective layer that prevents re-oxidation during the transfer from flux bath to zinc bath. The flux essentially guarantees that the molten zinc can wet the steel surface completely, which is the prerequisite for a uniform, adherent zinc coating.
The standard galvanizing flux is a double salt of zinc chloride and ammonium chloride (ZnCl₂·2NH₄Cl), available as either a dry powder for top-flux (dry galvanizing) or as an aqueous solution for wet-flux processes. The zinc-to-ammonium chloride ratio is critical: too much ammonium chloride produces excessive fuming and can cause bare spots from gas entrapment; too little results in poor oxide dissolution and skip-coating. Most operations target a ratio of 1:2 to 1:2.5 (ZnCl₂:NH₄Cl by weight).
Flux bath maintenance is as important as initial formulation. Key parameters to monitor include: Baumé density (target 12–16°Bé for solution flux), pH (target 4.5–5.5), iron contamination (keep below 3 g/L Fe²⁺), and temperature (60–80°C for optimal performance). Iron contamination accumulates from dissolved steel surfaces and must be controlled by regular filtration or treatment with hydrogen peroxide to oxidize Fe²⁺ to Fe³⁺, which precipitates as insoluble ferric hydroxide.
For high-speed galvanizing lines, specialized flux formulations with additional wetting agents and corrosion inhibitors can improve throughput and coating quality. Pre-flux drying temperature and time also significantly affect results — the flux layer should be dried at 120–150°C until a white, crystalline film forms. Inadequate drying leads to spattering when the part enters the zinc bath. Overdrying above 180°C decomposes the ammonium chloride, reducing fluxing effectiveness.