Galvanized Layer Formation Process: Metallurgy of Hot-Dip Zinc Coatings

Overview of the Galvanizing Reaction

The hot-dip galvanizing process forms a metallurgically bonded zinc coating on steel through a series of diffusion reactions between iron and molten zinc. When the steel workpiece is immersed in molten zinc at about 450 °C, the zinc first wets the clean steel surface and dissolves a small amount of iron, then iron and zinc interdiffuse to grow a sequence of iron-zinc intermetallic layers. On top of these grows the outer layer of nearly pure zinc that gives galvanized products their characteristic appearance.

Stage 1: Formation of the Iron-Zinc Solid Solution

At the moment of immersion, zinc atoms dissolve into the surface of the steel, forming an alpha-iron solid solution in which zinc is dissolved in the body-centred cubic iron lattice. This thin layer is the foundation of the coating: the fusion between the two metals is not a simple mechanical bond but a true metallurgical bond, which is why the coating adheres so strongly and survives bending and impact.

Stage 2: Growth of Intermetallic Phases

Once the solid solution is saturated with zinc, further reaction proceeds by diffusion. Zinc atoms migrate into the iron lattice while iron atoms diffuse outward into the molten zinc, and a series of intermetallic compounds forms at the interface. In a normal hot-dip coating the structure consists of the gamma and delta phases close to the steel (rich in iron) and the zeta phase further out, each with a characteristic hardness and composition. The overall intermetallic layer is hard and wear-resistant, while the outer layer remains soft and ductile.

Stage 3: Formation of the Pure Zinc Layer

When the workpiece is withdrawn from the bath, it carries a layer of molten zinc that solidifies into the outer eta phase: a hexagonal close-packed structure of nearly pure zinc with an iron content of no more than about 0.003 %. The eta layer is the visible, reflective surface of the coating. Its thickness is controlled by the withdrawal speed and by gas wiping in continuous lines; in batch galvanizing it is governed by the immersion time, the bath temperature and the steel chemistry.

Zinc Dross Formation

Not all the iron that dissolves in the bath reaches the surface of the workpiece. A portion reacts with zinc in the melt to form intermetallic particles that are denser than the molten zinc and sink to the bottom of the galvanizing pot, where they accumulate as bottom dross. Top dross is formed by oxidation at the bath surface. Both forms remove zinc from useful circulation, so bath management, including temperature control, aluminium addition and regular skimming, is essential to keep coating quality stable and zinc consumption economic.

Effect of Steel Chemistry and Process Variables

The reaction rate depends strongly on the silicon and phosphorus content of the steel, because these elements accelerate iron-zinc interdiffusion. Modern practice adds a small amount of aluminium to the bath, typically 0.1-0.3 % for batch work, which forms a thin inhibiting layer that slows the iron-zinc reaction and produces a thinner, more ductile coating. Bath temperature, immersion time, withdrawal speed and the roughness of the steel surface all influence the final coating thickness and structure.

Coating Structure and Properties

The complete coating is therefore a layered composite: a thin alpha-iron solid solution, a hard intermetallic region (gamma, delta and zeta phases) that provides adhesion and wear resistance, and a soft pure-zinc eta layer that gives corrosion protection and appearance. This composite behaves differently from a single-phase coating: the intermetallic layers are hard and brittle, while the eta layer bends and deforms with the steel. For sheet products a thin coating with a small intermetallic fraction is preferred for formability; for structural sections a thick coating is chosen for maximum corrosion life.

Standards and Verification

Coating thickness and mass are verified against the product standards: ISO 1461 for hot-dip galvanized coatings on fabricated iron and steel articles, GB/T 13912 as the Chinese equivalent, and EN 10346 or ISO 3575 for continuous galvanized sheet. Coating thickness is measured by magnetic gauges or by the gravimetric method, and adhesion is assessed by bend and impact tests. The layered structure can be examined metallographically to confirm that all phases are present in the correct proportions.

FAQ

Why does galvanized steel show a spangled appearance?

The spangle forms when the molten zinc layer solidifies and zinc crystals grow outward from nucleation points. Spangle size depends on the bath composition, cooling rate and the presence of spangle-promoting elements in traditional baths.

What is the difference between the eta layer and the alloy layers?

The eta layer is nearly pure zinc, soft and ductile. The alloy layers below it are iron-zinc intermetallic phases that are harder and more brittle, and they provide the metallurgical bond to the steel.

Why is aluminium added to the galvanizing bath?

Aluminium forms a thin inhibiting layer at the steel surface that slows the iron-zinc reaction, producing a thinner, more uniform and more ductile coating. This is essential for galvanized sheet that must be roll-formed or deep-drawn.

How is zinc dross removed from the pot?

Bottom dross is removed periodically by a skimming or pumping operation when the pot is serviced, and top dross is skimmed regularly during production. Good temperature control reduces dross formation.

Can galvanized coating thickness be specified by the customer?

Yes. Product standards define coating classes by mass per square metre, and the customer specifies the class appropriate to the environment and service life, for example Z275 for general outdoor building use.

Why does the coating on batch-galvanized steel look thicker than on sheet?

Batch galvanizing of fabricated articles uses longer immersion times without wiping, so the intermetallic and eta layers grow thicker. Continuous sheet lines control thickness precisely with air knives, giving a thinner, more uniform coating.