In the hot-dip galvanizing process, Iron removal (i.e., removing iron impurities from the zinc solution) is a critical step in ensuring the quality of the coating and extending the life of the zinc pot.
Author: Anna
In the hot-dip galvanizing process, **Iron removal (i.e., removing iron impurities from the zinc solution)** is a critical step in ensuring the quality of the coating and extending the life of the zinc pot. The following are specific reasons and implications:
1. Sources of iron impurities
Substrate dissolution: When steel fasteners are immersed in the zinc solution (about 450°C), iron will dissolve into the zinc solution in small amounts, forming iron-zinc alloys (e.g. FeZn₇, FeZn₁₃).
Zinc pot corrosion: Long-term contact of zinc liquid with steel zinc pots will slowly corrode the inner wall of the pots, releasing iron ions.
Fluxing agent residue: If the pretreatment is not thorough, iron salts in the fluxing agent may enter the zinc liquid.
2. Effects of iron impurities
(1) Decrease in plating quality
Roughness and grayness: Iron and zinc form hard and brittle zinc-iron alloy particles (bottom slag), resulting in rough and lusterless plated surface.
Poor adhesion: Excessive iron interferes with the normal reaction between the zinc solution and the substrate, forming an uneven alloy layer (Γ-phase, δ-phase) and reducing the bonding force.
Leakage plating defects: Iron impurities may adhere to the surface of the workpiece, preventing the zinc liquid from wetting and forming unplated areas.
(2) Reduced process efficiency
Increase in zinc slag: Iron reacts with zinc to form bottom slag (FeZn₇ sinks to the bottom of the zinc pot) and floating slag (Fe₂Al₅ suspends on the surface), which need to be cleaned up frequently, increasing zinc consumption.
Increased viscosity of zinc liquid: high iron content will make the liquid fluidity of zinc worse, affecting the uniformity of the coating.
(3) Increased equipment loss
Accelerated corrosion of zinc pots: Iron impurities promote the erosion of zinc liquid on zinc pots, shortening the service life of zinc pots (especially steel pots).
3. Necessary measures to remove iron
(1) Control of iron content
Safety threshold: iron content in hot-dip galvanizing solution should be controlled below 0.03%~0.05% (more than 0.1% will significantly deteriorate the coating quality).
Detection method: Take samples regularly and test the iron concentration by chemical analysis or spectrometer.
(2) Iron removal methods
Mechanical deslagging: Regularly fish out the floating slag on the surface of the zinc solution (Fe₂Al₅) and the bottom slag (FeZn₇).
Add Aluminum: Aluminum can generate lower density Fe₂Al₅ slag with iron (Aluminum content is controlled at 0.005%~0.02%), which is easy to fish out.
Filtration and purification: ceramic filters or centrifugal separation techniques are used to remove suspended iron particles.
Zinc pot protection: use ceramic coated or corrosion resistant alloy (e.g. high silicon steel) zinc pots to reduce iron leaching.
(3) Process optimization
Temperature control: avoid high zinc solution temperatures (>480°C) to accelerate iron dissolution.
Shorten the zinc dipping time: reduce the amount of iron dissolved in the base material, especially for thin-walled parts.
4. Consequences of not removing iron
If iron removal is neglected, it will result in:
particles and burrs on the plated surface, requiring additional grinding and increased costs.
Zinc dross clogging of galvanizing equipment (e.g. nozzles, rollers), affecting continuous production.
Risk of zinc liquid scrapping (when iron content is too high, the whole zinc liquid needs to be replaced, which is a big economic loss).
The core objective of hot-dip galvanizing iron removal is to maintain the purity of the zinc bath to ensure a smooth, dense, and highly adhesive coating, while reducing zinc consumption and equipment maintenance costs. This aspect is an important safeguard for the stability and economy of the hot-dip galvanizing process.