English
## I. Iron-zinc Phase Diagram and Various Intermetallic Compound Phases
Fundamental Function of the Iron-Zinc Phase Diagram: Hot-dip galvanizing is essentially a process of mutual reaction and diffusion between iron and zinc, and the evolution of coating microstructure strictly follows the rules of the iron-zinc phase diagram. The zinc-rich end corresponds to the conventional temperature range of galvanizing processes, where five major phases including Γ, Γ₁, δ, ζ and η can be distinguished. These phases arrange sequentially outward from the steel substrate, with obvious differences in crystal structure, iron content and hardness among each phase.
1. Γ Phase (Fe₃Zn₁₀): Adjacent to the steel substrate, featuring a face-centered cubic structure with the highest iron content and hardness as well as strong brittleness.
2. Γ₁ Phase (Fe₅Zn₂₁): Possesses the maximum hardness and brittleness among all alloy phases, and forms an extremely thin layer under conventional galvanizing conditions.
3. δ Phase (FeZn₇): Has a hexagonal lattice and is divided into loose and dense sublayers; it accounts for the largest thickness proportion within the alloy layer.
4. ζ Phase (FeZn₁₃): Exhibits a monoclinic lattice with relatively low hardness and superior ductility compared to the preceding alloy phases.
5. η Phase: The surface layer consisting of pure free zinc with extremely low iron content and the best ductility.
## II. Formation Mechanism of Iron-zinc Intermetallic Compound Layers
1. Three Fundamental Stages of Coating Formation
- Dissolution of iron substrate into liquid zinc;
- Reaction between iron and zinc to generate iron-zinc intermetallic compounds;
- Precipitation of pure zinc layer on the surface of alloy phases.
2. Three Equilibrium Pathways
Academia has summarized three pathways for iron and zinc to reach equilibrium. Industrial hot-dip galvanizing mainly adopts Path 2 featuring layer-by-layer phase generation. Horstmann established a six-step reaction model that reproduces the full growth process of phase layers from the steel substrate to the surface pure zinc layer: a solid solution of zinc in iron forms first, followed by the sequential generation of Γ, Γ₁, δ and ζ alloy phases, and finally the surface η pure zinc layer is obtained.
3. Interfacial Reactions and Microscopic Growth Characteristics
(1) The rate of interfacial chemical reactions exceeds the atomic diffusion rate, and the overall coating growth is governed by the cross-phase diffusion of iron and zinc atoms.
(2) In the microstructure, the Γ phase exists as a thin layer attached to the substrate, while the δ phase forms columnar crystals perpendicular to the interface. When the iron supersaturation in molten zinc is high, massive nucleation and grain refinement of the ζ phase occur. The dominant growing phase varies with different immersion durations.
## III. Coating Growth Kinetics (Temperature, Time and Theoretical Models)
1. Effects of Temperature and Immersion Time on Coating Growth
(1) Three Categories of Iron-zinc Reaction Rules Classified by Temperature
- Low-temperature range (430∼490°C): Gentle iron-zinc diffusion generates continuous and dense alloy layers.
- Medium-temperature range (490∼530°C): The atomic diffusion rate rises sharply, leading to rapid thickening of alloy layers and significantly increased coating brittleness. Around 490°C, the ζ phase becomes discontinuous and partially disappears, a large amount of iron dissolves into molten zinc, and zinc dross production surges.
- High-temperature range (>530°C): Cracks form inside the alloy layer, and the iron dissolution rate declines.
(2) Process Matching of Immersion Duration
- Short-duration immersion (second scale): Mainly applied to continuous galvanizing of steel strips and steel wires, which primarily forms the surface free zinc η phase.
- Long-duration immersion (minute scale): Used for galvanizing bulk components; alloy phases continuously thicken, while accelerating steel dissolution and shortening the service life of the zinc pot.
2. Mathematical Models for Coating Growth Kinetics
General formula: \(Y=Kt^n\)
Where \(Y\) = coating thickness, \(K\) = rate constant, \(t\) = immersion time, and \(n\) = time exponent.
(1) \(n≈0.5\): Diffusion-controlled growth, where coating thickness increases with the square root of time, representing the mainstream growth mode of galvanizing alloy phases.
(2) \(n≈1\): Interfacial chemical reaction-controlled growth, where coating thickness increases linearly with time.
Derived from Fick’s Law of Diffusion: Thickness is proportional to the square root of time under diffusion-dominated conditions, and proportional to time under interfacial reaction-dominated conditions.
3. Engineering Rules of Time Exponent \(n\)
- Short immersion (<300 s): ζ phase growth dominates, and the time exponent \(n\) of the overall coating approximates the value of the ζ phase.
- Long immersion (>1 h): δ phase growth dominates, and the time exponent \(n\) of the overall coating approximates the value of the δ phase.
Different phases have distinct diffusion capacities. Changes in immersion duration alter the proportional composition of internal coating phases, which ultimately affects coating hardness, ductility and corrosion resistance.
## IV. Core Process Conclusions
1. The temperature should be preferentially controlled within 430∼480°C, avoiding the high-risk range of 490∼530°C to reduce iron dissolution and zinc dross generation, as well as control coating brittleness.
2. Short immersion time is adopted for continuous galvanizing, while immersion time is extended as required for batch galvanizing. Coating performance can be regulated by utilizing the growth characteristics of different phases.
3. The iron-zinc phase diagram and kinetic equations enable quantitative prediction of coating thickness and phase composition, guiding process design for zinc temperature and immersion duration.