In the hot-dip galvanizing process, Fluxing is a key step between pre-treatment (pickling, washing) and dip plating, and its role directly affects the quality, uniformity and adhesion of the coating.
Author: Anna
In the hot-dip galvanizing process, **Fluxing** is a key step between pre-treatment (pickling, washing) and dip plating, and its role directly affects the quality, uniformity and adhesion of the coating. The following are the specific roles and principles of auxiliary plating:
1. The core role of auxiliary plating
(1) Remove residual oxides
After pickling, trace iron oxide (Fe₂O₃) or pickling salt (such as FeCl₂) may remain on the surface of fasteners. The fluxing agent further cleans the surface through a chemical reaction and ensures that the zinc liquid is in direct contact with the base metal (iron).
(2) Prevention of secondary oxidation
Steel parts exposed to air after pickling oxidize rapidly (formation of a new oxide layer). The fluxing agent forms a protective film on the surface, insulating it from oxygen and preventing re-oxidation before zinc dipping.
(3) Promotion of zinc wetting
Fluxing agent can reduce the surface tension of zinc liquid, so that it is easier to spread and moisten the surface of steel parts, reducing defects such as leakage and false plating.
(4) Activating the surface of the substrate
The active ingredients in the fluxing agent (such as ammonium chloride) react with the substrate to remove the passivation film and expose the active iron atoms, accelerating the formation of iron-zinc alloy layer (Γ phase, δ phase).
2. Typical composition of fluxing agents
The commonly used fluxing agent is a mixed aqueous solution of zinc chloride (ZnCl₂) and ammonium chloride (NH₄Cl), the ratio is usually:
ZnCl₂:NH₄Cl = 3:1 to 4:1 (concentration about 20% to 30%).
A small amount of surfactant or corrosion inhibitor (e.g. glycerin, thiourea) may be added.
3. Key points of the plating aid process
(1) Process flow
Acid washing and then water washing: Thoroughly remove the acid (to avoid contamination of fluxing agent).
Fluxing agent immersion: the workpiece is immersed in 60~80℃ fluxing agent for 1~3 minutes.
Drying: dry at 100~150℃ to form a uniform fluxing agent film (to avoid moisture brought into the zinc solution to trigger zinc burst).
(2) Key parameters control
pH value: 4.5~5.5 (too much acid will corrode the substrate, alkaline is easy to produce precipitation).
Iron ion content: <1g/L (too high will lead to failure of fluxing agent, need to be replaced regularly).
Drying degree: the surface is dry but not burnt (residual moisture will trigger zinc splash).
4. Consequences of poor plating aid
Leakage: Incomplete coverage by the fluxing agent, resulting in localized unplated zinc.
Zinc lumps/roughness: Excessive residual fluxing agent, uneven reaction of the zinc bath.
Increased zinc dross: Impurities in the fluxing agent (e.g. Fe²⁺) enter the zinc bath, generating additional zinc-iron alloy dross.
Poor adhesion: surface oxidation is not completely removed, the alloy layer is not firmly combined.
5. Special auxiliary plating process
Ammonium-free fluxing agent: zinc chloride + hydrochloric acid (environmentally friendly scenario).
Solvent method hot-dip galvanizing: first dip-coated with molten ammonium chloride (NH₄Cl), then galvanized (for continuous galvanizing of steel wires).
6. Correlation with the quality of hot-dip galvanizing
Salt spray test performance: Incomplete auxiliary galvanizing accelerates corrosion of the plated layer.
Appearance quality: affects the smoothness and color uniformity of the coating.
Zinc consumption control: good auxiliary galvanizing reduces the waste of zinc solution (reduces rework).
The secondary plating is the “last layer of insurance” for hot-dip galvanizing, ensuring that the metal surface after pickling remains clean and activated before zinc dipping. Its effect directly determines the integrity of the coating, adhesion and production efficiency, and requires strict monitoring of composition, temperature and process parameters.