Galvanizing flux is a double salt of zinc chloride and ammonium chloride (ZnCl₂·2NH₄Cl) applied to steel as the last chemical step before immersion in molten zinc. It dissolves the residual oxide film left after pickling and rinsing, and it shields the surface from re-oxidation during the transfer into the zinc bath, so the molten zinc can wet the steel completely — the prerequisite for a uniform, adherent coating. Selecting a flux comes down to three decisions: the process form (dry powder or solution), the ZnCl₂:NH₄Cl ratio, and the impurity and fuming specification. Keeping it working comes down to controlling four bath parameters: density, pH, iron, and temperature.
What the flux actually does
Between the rinse tank and the zinc kettle, freshly pickled steel starts re-oxidizing within minutes — faster in a humid galvanizing shop. Wherever an oxide patch survives on the surface, the molten zinc cannot alloy with the iron beneath it, and the part comes out with a bare spot or a skip-coated area that no amount of extra immersion time repairs. The flux layer does two jobs at once: its ammonium chloride fraction chemically cleans residual oxides on contact with the hot zinc, and the fused salt film physically excludes air until the moment of immersion. A well-run flux bath is therefore the cheapest insurance in the plant against re-dips and rework.
Double-salt chemistry and the ZnCl₂:NH₄Cl ratio
The standard flux is the double salt ZnCl₂·2NH₄Cl, and the zinc-to-ammonium chloride ratio is the single most important formulation parameter. Too much ammonium chloride produces heavy fuming at the kettle and can cause bare spots from gas entrapment; too little leaves oxides undissolved and shows up as skip-coating. Most general galvanizing operations target a ratio of about 1:2 to 1:2.5 (ZnCl₂:NH₄Cl by weight). Lines with strict fume limits often move toward the zinc-chloride-rich end of that window or to formulations with specialized additives — a trade-off that should be verified on the line's own work mix rather than assumed.
Dry fluxing vs wet fluxing
In dry fluxing — the dominant method in general (batch) galvanizing — parts pass through an aqueous flux bath, are dried, and then enter a clean zinc surface: the flux arrives on the part. In wet fluxing, a molten flux blanket floats on part of the zinc bath and the work passes down through it; drying is unnecessary, but ash and dross management are harder and fume loads are higher. CHIMI ART supplies both forms: CHIMI FLUX Powder, a high-speed galvanizing grade formulated for low fuming and uniform zinc coats, dissolved on site to prepare or replenish the bath, and CHIMI FLUX Solution, a ready-made aqueous concentrate at 25–30% active content and pH 4.5–5.5, used for galvanizing pretreatment and oxidation prevention.
The four bath parameters to control
- Baumé density: target 12–16 °Bé for solution flux — too thin leaves insufficient salt on the part, too thick wastes chemical and slows drying
- pH: hold 4.5–5.5 — more acidic baths accelerate iron pick-up, more alkaline baths precipitate zinc compounds and weaken the cleaning action
- Iron: keep dissolved Fe²⁺ below about 3 g/L — the main contamination limit, and the main driver of dross
- Temperature: 60–80 °C gives the best salt deposition and speeds the subsequent drying step
None of these parameters holds itself. Density falls as drag-out removes salt and rises as water evaporates; pH drifts downward as acid is dragged in from a poorly rinsed pickle line; iron climbs continuously. A weekly titration and density check, logged against throughput, is the minimum control routine for a bath that behaves consistently.
Managing iron contamination
Iron enters the flux from acid drag-in and from slow dissolution of the steel being processed, and it is the costliest impurity in the plant: iron carried on the fluxed part into the kettle reacts with molten zinc to form dross, and each unit of iron is commonly cited as converting many times its own weight of zinc into dross at the bottom of the bath. Control it in two ways. First, cut the input — improve rinsing after pickling so acid and dissolved iron are never carried into the flux tank. Second, treat the bath: oxidize Fe²⁺ to Fe³⁺ with hydrogen peroxide (or by aeration, more slowly), let ferric hydroxide precipitate, then settle or filter the sludge out and re-correct the pH. A side-stream trial establishes the peroxide demand before the whole tank is treated.
Drying: where flux problems become visible
After the flux bath, the part must be dried until a white, crystalline salt film forms — typically at 120–150 °C. Under-dried parts carry water into the kettle and cause spatter and zinc splash the moment they enter the bath, which is a safety issue as much as a quality one. Over-drying above about 180 °C decomposes the ammonium chloride fraction and quietly destroys the flux's cleaning power, producing bare spots that get misdiagnosed as a bath problem. If coating defects appear when nothing has changed in the flux tank, check the dryer first.
Selection checklist and next steps
- Process form: powder for on-site make-up and top-flux use; solution concentrate where simple dilution and consistent strength matter
- ZnCl₂:NH₄Cl ratio matched to your fume constraints and work mix, starting near 1:2 to 1:2.5 by weight
- Impurity specification: low iron and low insolubles in the delivered product, so the bath starts clean
- Fuming behaviour: low-fume grades such as CHIMI FLUX Powder for enclosed or ventilation-limited kettles
- Supplier support: bath analysis and adjustment guidance, not just a delivery
The ranges in this guide are sound starting points, but every line's steel mix, rinsing discipline, and kettle practice shift the optimum. The reliable path is a bath audit — density, pH, iron, and salt ratio — followed by a controlled trial of the corrected or replacement flux on your own production, the galvanizer's equivalent of a jar test. CHIMI ART's technical team runs exactly this exercise with galvanizing plants: send us your current bath parameters and defect pattern, and we will recommend a flux grade and a maintenance schedule to match.