How does hot dip galvanizing work?

Hot-Dip Galvanizing (HDG) is an anticorrosive process in which an intermetallic compound coating is formed by immersing a steel workpiece in a molten zinc solution. The core principle is metallurgical bonding and sacrificial anode protection, the following is the detailed workflow and mechanism:

First, basic principles

         Metallurgical bonding

         At high temperature (about 450℃), iron (Fe) diffuses with molten zinc (Zn) to form an iron-zinc alloy layer (such as FeZn₇, FeZn₁₃, etc.). This alloy layer is tightly bound to the steel matrix to provide a physical barrier.

         Sacrificial anode protection

         Zinc has a lower electrode potential (-0.76V) than iron (-0.44V) and is preferently oxidized in a corrosive environment, delaying the corrosion of the steel matrix (even if the coating is damaged, zinc can still protect the iron).

Second, detailed explanation of the process flow

         1. Pre-treatment: Clean the surface and ensure that the zinc liquid is in full contact with the steel

         Degreasing:

         Use alkaline solution (such as NaOH, 10-20% concentration) to remove the grease and dirt on the surface of the workpiece, the temperature is 60-80℃, and the time is 10-30 minutes.

         Pickling:

         Immersed in hydrochloric acid (10-20% concentration) or sulfuric acid, dissolved iron oxide (Fe₂O₃, Fe₃O₄, etc.). Reaction:

         The pickling time needs to be precisely controlled to prevent overcorrosion (surface roughness).

         Plating aid (Fluxing) :

         Immersion in solution of zinc ammonium chloride (ZnCl₂·NH₄Cl) (pH 3-4). This removes the residual oxide and forms a protective film on the surface to prevent secondary oxidation. Then dry at 80-120℃.

Two. Hot dip galvanizing: forming alloy coating

         Zinc liquid composition:

         Pure zinc (≥98%), containing a small amount of aluminum (Al ≤0.02%), lead (Pb ≤1%) to improve the gloss of the coating and reduce zinc slag.

         Temperature control: 445-465℃ (too high leads to increased zinc oxidation, too low affects fluidity).

         Dipping process:

         The workpiece is slowly immersed in zinc solution to avoid air retention.

         The zinc liquid penetrates into the surface of the steel and the iron reacts with the zinc:

         Fe+Zn→Fe-Zn alloy layer (Γ, δ, ζ phases)

         Dipping time: 1-10 minutes, depending on workpiece thickness and structural complexity.

Three. Post-treatment: Optimize coating performance

         CD:

         Air cooling: natural cooling to form a dense coating.

         Water cooling (rapid quenching) : refine the grain and improve the hardness of the coating.

         Passivation:

          Use chromates or environmentally friendly chromium-free passivators (such as silicates) to form passivating films to enhance corrosion resistance.

         Trim:

         Zinc nodules and burrs should be removed, and zinc should be added to the local missing plating area.

Four, coating structure

         Divided into four layers from inside to outside:

         Γ phase (Fe Zn₁₀) : The thinest (1-2μm), high hardness, and most closely bound to the matrix.

         δ phase (FeZn₁₀) : dense cylindrical crystal, thickness about 50-80μm, main corrosion protection layer.

         ζ (FeZn₁₃) : A loose layer containing free zinc, 20-40μm thick.

          Pure zinc layer (η phase) : the outermost layer, providing sacrificial protection.

Five. Application scenarios and advantages

         Typical applications:

         Power tower, bridge steel structure, highway guardrail (life up to 50 years).

         The plating thickness (≥85μm) should be increased in Marine environment.

         Compare other processes:

         Electrogalvanizing: thin coating (5-25μm), no alloy layer, weak corrosion resistance.

         Zinc spraying: poor adhesion, can not form sacrificial protection.

Six. Common problems and solutions

         Leakage plating: incomplete pre-treatment (residual grease or oxidation) → enhanced degreasing and pickling.

         Zinc nodules: too fast dipping speed or zinc liquid impurities → optimize the lifting speed, regular slag removal.

         Coating cracking: the cooling speed is too fast → Use air cooling or adjust the water cooling temperature.

         Seven. Environmental Protection and safety

         Waste gas treatment: The HCl produced by pickling is neutralized with NaOH; An air extraction system is arranged above the zinc pan.

         Zinc slag recovery: the precipitation is separated and reused in the furnace to reduce the waste of resources.

         Hot dip galvanizing provides long-term corrosion protection for steel through the dual mechanism of metallurgical reaction and electrochemical protection, and the scientific control of its process parameters is the core of ensuring quality.