English
In hot-dip galvanizing, the zinc bath contains various elements apart from the zinc matrix. These elements originate from three sources: dissolution from workpieces or self-generation in the zinc kettle, intentional additions, and inherent constituents of zinc ingots. By altering the melting point, viscosity and surface tension of the zinc bath, different alloying elements interfere with iron-zinc interdiffusion and the growth of intermetallic compounds, and ultimately change the thickness, microstructure, appearance and corrosion resistance of the coating. The functions of each element are summarized as follows:
## I. Naturally Enriched Elements
1. Iron (Fe): Iron has a maximum solubility in molten zinc at 450°C. Zinc dross forms once saturation is reached, increasing zinc consumption. Higher iron content raises the viscosity of molten zinc, thickens the coating, and impairs the appearance and ductility of the coating.
2. Silicon (a matrix element of steel, regarded as a solute element in the zinc bath in this section): Silicon in the zinc bath mainly dissolves from workpieces and interacts with other elements to affect the Sandelin effect.
## II. Commonly Used Artificially Added Elements / Associated Elements in Zinc Ingots
1. Aluminum (Al)
- Low content (0.005%~0.02%): An aluminum oxide film forms on the zinc surface to restrain zinc oxidation and brighten coatings, widely adopted for galvanizing bulk workpieces.
- 0.1%~0.15%: A Fe₂Al₅ barrier layer generates on the steel surface to retard iron-zinc reactions and inhibit excessive thickening of alloy layers.
- >0.3%: The barrier layer tends to crack and a large amount of floating dross is produced. This concentration range is rarely used in conventional hot-dip galvanizing but extensively applied for continuous strip steel galvanizing.
2. Lead (Pb)
Derived from zinc ingots, lead can lower the melting point and surface tension of the zinc bath, improve wettability and fluidity, and facilitate spangle formation. Saturated lead protects the zinc kettle and facilitates separation of zinc dross. Excessive lead easily causes defects such as runs and burrs. Subject to environmental protection regulations, lead is gradually replaced by bismuth. Lead barely changes the growth pattern of iron-zinc alloy phases.
3. Tin (Sn), Antimony (Sb)
Both elements improve the fluidity of the zinc bath. Antimony tends to cause later blackening of coatings. The combination of tin, antimony and lead favors spangle formation, while spangles reduce coating corrosion resistance. Low tin content exerts negligible influence, whereas high tin content can suppress excessive coating thickening on reactive steel.
4. Copper (Cu)
Low copper content has minor effects. Increased copper concentration continuously thickens coatings, modifies the microstructure of ζ and δ phases, and accelerates zinc dross formation. Copper and aluminum counteract each other’s effects.
5. Cadmium (Cd)
Cadmium accelerates interfacial iron-zinc reactions, thickens alloy layers and increases coating brittleness. It promotes spangle growth, significantly facilitates the development of ζ phase and restrains δ phase. Excessively high cadmium content triggers drastic structural changes of alloy layers.
6. Germanium (Ge)
Germanium accelerates iron-zinc interdiffusion and overall coating growth and inhibits δ phase formation. Its promotion effect on coatings of reactive steel is more prominent.
## III. Functional Alloying Elements (Mainly for Regulating the Sandelin Effect of Reactive Steel)
1. Nickel (Ni)
A suitable nickel content (0.04%~0.12%) eliminates the Sandelin effect of silicon-containing reactive steel, restrains abnormal growth of ζ phase and yields bright coatings, and improves the fluidity of molten zinc. Excessive nickel generates ternary zinc dross and causes granular defects on coatings, and its inhibitory effect weakens for high-silicon steel.
2. Manganese (Mn)
Manganese preferentially combines with silicon to form manganese-silicon compounds, preventing silicon in steel from triggering the Sandelin effect and eliminating the problem of overly thick coatings on reactive steel. It facilitates the growth of uniform and dense alloy phases and improves the corrosion resistance and formability of coatings.
3. Magnesium (Mg)
Magnesium forms stable magnesium-silicon compounds with silicon to indirectly suppress iron-zinc reactions and adjust zinc bath fluidity. It has a narrow effective addition range; both insufficient and excessive magnesium alter coating thickness. Its performance is restricted by steel composition and requires precise control.
4. Bismuth (Bi)
As an environmentally friendly alternative to lead, bismuth improves zinc bath fluidity, reduces zinc dross and zinc consumption and produces bright coatings, and can regulate iron-zinc reactions on reactive steel. The action mechanism of bismuth inside coatings has not yet been fully clarified.
5. Titanium (Ti)
Titanium suppresses the hot-dip reactivity of silicon-containing steel, stabilizes ζ phase, promotes the formation of dense δ phase and effectively alleviates the Sandelin effect. It enhances the passivation capacity and corrosion resistance of coatings, and its effectiveness is highly correlated with the silicon content of steel.
6. Rare Earth (RE)
Rare earths isolate molten zinc from air to reduce zinc oxidation, lower the surface tension of the zinc bath and refine coating grains, optimize the structure of corrosion products and greatly boost corrosion resistance. There exists an optimal addition range (approximately 0.1%); excessive rare earths degrade corrosion resistance and corrode the zinc kettle.
## IV. Other Minor Elements
Silver: Accelerates coating growth to produce thick coatings with good corrosion resistance.
Chromium, Vanadium, Zirconium: These elements form ternary compounds at the interface between zinc and alloy phases, hinder the development of ζ phase, thin the ζ phase and smooth phase interfaces.
## General Conclusions
Various alloying elements inside the zinc bath demonstrate vastly different functions. The core regulation objectives include: suppressing or mitigating the Sandelin effect of reactive steel, optimizing the fluidity and wettability of the zinc bath, regulating the growth of Fe-Zn intermetallic compounds (ζ and δ phases), improving coating appearance and corrosion resistance, and cutting zinc consumption. The effects of elements are strongly related to their addition concentrations; most elements have a suitable concentration window, and excessive addition easily induces defects. Meanwhile, synergistic and antagonistic interactions exist among elements (e.g., aluminum-copper), so comprehensive evaluation is required for combined addition. Some toxic elements (lead) are gradually replaced by environmentally friendly alloy systems such as bismuth.