Zinc Alloys and Galvanising: Hot-Dip Process, Coating Metallurgy, and Corrosion Protection

Zinc is the fourth most consumed metal globally, and the majority of that consumption is dedicated to protecting steel from corrosion. Hot-dip galvanising, continuous strip galvanising, zinc-rich coatings, die casting, and sacrificial anodes collectively represent the breadth of zinc’s engineering utility. This article provides a technically rigorous treatment of the Zn-Fe coating reaction, Fe-Zn intermetallic phases, bath chemistry, advanced coating grades (Zn-Al-Mg), die casting alloy metallurgy, cathodic protection mechanism, and quality assurance requirements.

Zinc — Physical Metallurgy and Key Properties

Zinc (atomic number 30, relative atomic mass 65.38) crystallises in the hexagonal close-packed (HCP) structure with a c/a ratio of 1.856 — significantly above the ideal 1.633. This high c/a ratio limits the number of operative slip systems at room temperature (basal slip on {0001}〈11̅20〉 is dominant), making pure zinc relatively brittle below ~150 °C but permitting rolling at slightly elevated temperatures where prismatic and pyramidal slip becomes accessible.

The melting point of 419.5 °C is low enough to allow energy-efficient casting and hot-dip coating operations. The relatively high vapour pressure of zinc at elevated temperatures (significant sublimation above ~700 °C) requires care in furnace operations and zinc-containing weld fume management. Key physical and mechanical properties of commercially pure zinc (Grade Zn99.995 per EN 1179) are summarised below.

Alloying Effects in Zinc

Small alloying additions profoundly alter zinc’s properties. Aluminium is the most important alloying element: even 0.005 wt% Al in the galvanising bath significantly reduces iron attack on steel by forming an inhibition layer at the steel surface. Higher Al additions (5 wt% in Galfan; 55 wt% in Galvalume/Zincalume) shift the coating microstructure from Fe-Zn intermetallics to Zn-Al eutectic. Magnesium, in combination with aluminium, fundamentally changes the corrosion product chemistry (see Section on Zn-Al-Mg below). Lead was historically added (0.5–1.0 wt%) to reduce bath surface tension and improve spangle appearance on continuous galvanised strip; it is now eliminated from most commercial baths due to environmental regulations.

Hot-Dip Galvanising: Process Metallurgy

Batch (Structural) Galvanising

Batch galvanising, governed by ISO 1461, processes fabricated structural steelwork — beams, columns, grating, fasteners, and assemblies — by immersion in a molten zinc bath. The process sequence is critical to coating quality:

1. Degreasing: Alkali or solvent cleaning removes oil, grease, and drawing lubricants. Inadequate degreasing causes bare spots (“black spots”).
2. Acid pickling: Immersion in 10–18% hydrochloric acid (room temperature) or 10–20% sulfuric acid (60–80 °C) removes mill scale and rust. Inhibited acid minimises base metal attack and hydrogen evolution. Pickling time should be minimised for high-strength steels (>1000 MPa) to limit hydrogen uptake.
3. Fluxing: Immersion in aqueous zinc ammonium chloride (ZnCl₂·2NH₄Cl, ~450 g/L, 60–80 °C) or passage through a dry flux blanket on the bath surface. Flux cleans the steel surface of residual oxides, activates it for reaction with molten zinc, and prevents re-oxidation.
4. Drying: Essential before immersion to prevent steam explosions in the zinc bath and to reduce zinc spattering.
5. Galvanising (immersion): Steel is immersed in molten zinc at 445–455 °C for 3–12 minutes depending on section thickness. Withdrawal speed is controlled to regulate coating thickness; faster withdrawal leaves more zinc on the surface.
6. Cooling and inspection: Air cooling or quenching in water (sometimes with chromate passivation). Visual and thickness inspection per ISO 1461.

Bath Chemistry and Dross Management

The galvanising bath is predominantly zinc (≥98.5 wt%) with controlled additions. Iron dissolved from work pieces accumulates in the bath. At bath temperatures of 445–455 °C, iron solubility in zinc is ~0.03 wt%; excess iron precipitates as hard (bottom) dross (FeZn₇, delta phase) or floating top dross (FeZn₅, with Al). Dross accumulation reduces zinc yield and must be removed periodically. Bath lead content is controlled to <1% to comply with RoHS and environmental standards; tin (0.05–0.3%) is sometimes added to improve wetting on silicon-containing steels.

Continuous Strip Galvanising (CGL)

Continuous galvanising lines process cold-rolled steel strip at speeds of 60–200 m/min. Strip is cleaned, annealed in a reducing atmosphere furnace (typically 5% H₂/N₂), and passed through a ceramic snout into the zinc bath without air contact. After the zinc bath, an air-knife system controls coating weight (g/m²) by directing a curtain of pressurised air or nitrogen at both strip surfaces. Continuous lines produce GI (hot-dip galvanised) and GA (galvannealed) products. GA is produced by passing the freshly galvanised strip through an induction annealing furnace at 490–560 °C immediately after the bath, converting the eta zinc layer into all-intermetallic (predominantly delta) structure for improved weldability and paint adhesion.

Fe-Zn Intermetallic Phases: Structure and Properties

The reaction between liquid zinc and solid steel produces four well-defined intermetallic phases in a sequence that is fundamental to galvanising metallurgy. Each phase has a distinct crystal structure, iron content, hardness, and growth behaviour. Understanding this phase sequence is essential for interpreting coating failures, specifying thickness tests, and explaining the mechanics of corrosion protection.

The gamma phase is thin and very hard; it does not contribute to coating ductility or corrosion resistance but must be present for delta and zeta growth to proceed. The delta phase is the structural backbone of the coating — columnar grains with good adhesion to both the gamma phase below and the zeta phase above. The zeta phase is the thickest layer in batch galvanised coatings and is acicular (needle-like) in morphology; it contributes significantly to total coating thickness and is the phase primarily responsible for the Sandelin effect. The eta layer is soft pure zinc, contributing ductility and the primary barrier/sacrificial corrosion protection.

Coating Growth Kinetics

At standard bath temperatures (445–455 °C), intermetallic growth follows approximately parabolic kinetics for short immersion times, transitioning toward linear kinetics as the zeta layer thickens. The parabolic rate constant for delta phase growth is on the order of 10⁻¹² m²/s at 450 °C. The relevant diffusion-controlled growth equation is:

x² = 2kₚ · exp(−Q/RT) · t

where:
  x  = intermetallic layer thickness (m)
  kₚ = pre-exponential rate constant (m²/s)
  Q  = activation energy for Fe-Zn interdiffusion (~80 kJ/mol for δ phase)
  R  = 8.314 J/(mol·K)
  T  = bath temperature (K)
  t  = immersion time (s)

This relationship explains why even small increases in bath temperature dramatically increase intermetallic thickness and why tight temperature control (±5 °C) is specified. Aluminium additions to the bath suppress this kinetics by forming a thin FeAl intermetallic inhibition layer at the steel surface before zinc diffusion begins.

Cathodic Protection Mechanism of Zinc

Zinc’s effectiveness as a corrosion protectant is rooted in its electrochemical behaviour. In the electromotive force series, zinc (E° = −0.762 V vs SHE) is significantly more electronegative than iron (E° = −0.44 V vs SHE). When both metals are coupled in an electrolyte (moisture), zinc acts as the anode (oxidises preferentially) and steel acts as the cathode (protected).

Anodic reaction (zinc):   Zn → Zn²⁺ + 2e⁻         (E° = −0.762 V)
Cathodic reaction (steel): Fe²⁺ + 2e⁻ → Fe           (E° = −0.440 V)

Driving EMF = E°cathode − E°anode = −0.440 − (−0.762) = +0.322 V

Zinc dissolution rate in neutral NaCl solution (~3.5 wt%):
  ~5–10 µm/year (atmospheric exposure, temperate climate)

A critical advantage over organic barrier coatings is lateral (“throw”) protection: zinc protects bare steel exposed at cut edges, drilled holes, and scratches within a lateral distance of approximately 1–3 mm. The throw distance depends on electrolyte conductivity and geometry; it is greater in low-conductivity environments and shorter in high-conductivity (seawater) environments.

In atmospheric exposure, zinc corrosion products form a protective patina. In the first months, white rust (basic zinc carbonate, hydroxide, and sulfate mixtures) forms, which is soft and powdery if the surface remains wet. Over time, a compact, adherent grey patina of zinc carbonate (ZnCO₃), zinc hydroxide (Zn(OH)₂), and zinc sulfate (ZnSO₄) develops, which substantially reduces the zinc dissolution rate to 0.5–2 μm/year in rural/suburban atmospheres. This underpins the life-to-first-maintenance calculation: for a 85 μm coating in a C3 environment, expected service life before 5% rusting is 50–75 years.

Advanced Coating Systems: Zn-Al-Mg

Conventional hot-dip zinc coatings dissolve uniformly, providing excellent sacrificial protection but relatively modest barrier function. The addition of both aluminium and magnesium to the zinc bath creates a fundamentally different coating microstructure and corrosion behaviour that delivers 3–5 times the service life of equivalent pure zinc coatings.

Microstructure of Zn-Al-Mg Coatings

A typical Zn-1.5Al-1.5Mg coating solidifies to form a ternary eutectic microstructure containing: zinc-rich dendrites, MgZn₂ (Laves phase) intermetallics distributed as lamellae, and a fine Zn-MgZn₂ binary eutectic. The MgZn₂ phase is the key constituent: during corrosion, it dissolves to release Mg²⁺ ions, which react with Cl⁻ and OH⁻ to form simonkolleite [Zn₅(OH)₈Cl₂·H₂O] with MgO and Mg(OH)₂. This compact layered double hydroxide structure acts as a physical diffusion barrier, suppressing further zinc dissolution and dramatically improving cut-edge protection — a key failure mode of Galvalume (55Al-Zn), which has poor cut-edge protection despite its excellent flat-panel corrosion resistance.

Commercial Zn-Al-Mg Grades

Weldability of Zn-Al-Mg coatings is an active area of development. The higher Al and Mg content increases spatter in resistance spot welding and can produce MgO oxide inclusions in welds. Electrode wear is higher than for conventional GI or GA. Process windows must be established for each specific grade, and automotive body-in-white applications typically require Zn-Al-Mg-specific welding schedules. For further context on corrosion mechanisms in coated systems, see the pitting corrosion guide and corrosion mechanisms overview.

Zinc Die Casting Alloys (Zamak)

Zamak alloys (a German acronym: Zink, Aluminium, MAgnesium, Kupfer/copper) are the dominant commercial zinc-based casting alloys. Their combination of excellent die filling, low melting range, dimensional accuracy, and good mechanical properties makes them the material of choice for high-volume precision parts — automotive door handles, carburettors, lock bodies, electrical connectors, and consumer hardware. The die casting process used for Zamak is the hot chamber process, in which the injection mechanism is submerged in molten zinc, enabling rapid cycle times (1–4 seconds for small parts) and tight tolerances (ISO tolerance class IT13–IT14).

Zamak Alloy Compositions and Properties

Impurity Control — The Critical Requirement

Zamak alloys are uniquely sensitive to trace impurities. Iron, lead, cadmium, and tin at concentrations above their respective limits promote intergranular corrosion by segregating to grain boundaries, forming low-melting eutectic phases (Pb-Zn: 318 °C eutectic), and creating galvanic micro-couples. The intercrystalline corrosion mechanism involves penetration of moisture along grain boundaries contaminated with Pb and Cd, causing expansion, cracking, and eventual component disintegration — a failure mode historically observed in non-specification Zamak, colloquially known as “zinc pest.” Alloy sourcing from accredited suppliers with ISO 9002 / ASTM B86 certification is essential. Maximum permissible impurity levels (ASTM B86):

Maximum impurity limits (ASTM B86, Zamak 3/5):
  Fe  ≤ 0.075 wt%  (~750 ppm)
  Pb  ≤ 0.005 wt%  (~50 ppm)
  Cd  ≤ 0.004 wt%  (~40 ppm)
  Sn  ≤ 0.003 wt%  (~30 ppm)
  Cu  ≤ 0.10 wt% (Zamak 3 only; Cu is alloying element in Zamak 5)
  Ni  ≤ 0.020 wt%

For context on the role of aluminium in zinc alloy phase stability, the iron-carbon phase diagram article provides background on how alloying elements modify binary phase diagrams — the Zn-Al binary has its own eutectic at 5 wt% Al, 381 °C, which underpins the Galfan coating system.

Surface Pre-treatment and Post-treatment

Chromate and Chromate-Free Passivation

Freshly galvanised zinc is susceptible to “white rust” (wet storage staining) — a bulky, powdery white deposit of zinc hydroxide and carbonate — if surfaces are stored wet or with restricted air circulation before the stable patina forms. Chromate conversion coatings (hexavalent Cr, per ASTM A780) were historically applied to prevent white rust, but are now restricted under RoHS Directive 2011/65/EU (Cr(VI) limit: 1000 mg/kg). Trivalent chromate (Cr(III)) and chromate-free alternatives (phosphate, silicate, titanate-based passivation, and thin organic clear coats) are standard in the automotive and appliances sectors.

Duplex Systems: Galvanising + Paint

Duplex protection — galvanising as the primary layer plus an organic coating system — delivers synergistic corrosion protection that significantly exceeds the sum of the individual systems. The zinc provides cathodic protection at coating defects and cut edges; the organic coating suppresses the zinc dissolution rate at intact surfaces. ISO 12944 classifies corrosivity categories and specifies minimum coating system requirements. For C5-M environments (offshore, chemical plants), a typical duplex system might be: 85 μm hot-dip zinc + zinc-rich epoxy primer (DFT 60 μm) + epoxy midcoat (80 μm) + polyurethane topcoat (80 μm). For guidance on measuring paint film thicknesses, refer to the materials testing overview.

Coating Thickness Specification and Quality Assurance

ISO 1461 (batch galvanising) and EN 10346 / ASTM A653 (continuous strip galvanising) govern minimum coating thicknesses and testing methods. The key requirements from ISO 1461:2022 are:

Continuous strip products are specified by coating mass per unit area (g/m² per side), designated as Z100 (100 g/m² total, ~7 μm per side), Z275, Z350, etc. up to Z600 in heavy-coat grades. The conversion between coating mass and thickness uses: thickness (μm) ≈ coating mass (g/m²) / (2 × 7.133), where 7.133 g/cm³ is zinc density.

Coating thickness from mass:
  t (µm) = G / (2 × ρₛₙ)

  where:
    G     = coating mass (g/m², total both sides)
    ρₛₙ  = zinc density = 7.133 g/cm³ = 7.133 mg/cm²/µm
    Factor 2 = both sides (for double-sided coating mass)

  Example: Z275 → t = 275 / (2 × 7.133) = 19.3 µm per side

For structural assessment and remaining service life estimation, the corrosion loss model from ISO 9224 can be applied. The materials testing section covers general mechanical test methodologies applicable to assessing coating adhesion through bend testing.

Industrial Applications of Zinc and Zinc Coatings

Construction and Infrastructure

Structural steelwork (beams, purlins, rafters, grating, handrails), transmission towers, highway guard rails, and bridge components routinely specify hot-dip galvanising per ISO 1461 as the primary and often sole corrosion protection measure. Life-cycle cost analysis consistently favours galvanising over paint systems for outdoor infrastructure in C3–C4 environments due to the 50+ year design life with zero maintenance.

Automotive

Galvannealed (GA) sheet is the dominant automotive substrate for body-in-white panels in passenger vehicles. Continental European vehicles typically use ZnFe (GA) on the outer body side for paint adhesion and GI on the inner body side for cut-edge protection. Zn-Al-Mg (ZM) grades are increasingly used for structural parts. Approximately 70% of the zinc consumed in the automotive sector goes into corrosion protection coatings on steel sheet; the remainder is in Zamak die castings (lock mechanisms, bracket castings, electrical housings).

Electrical and Electronics

Zamak die castings are standard in electrical connectors, EMC shielding enclosures, and consumer electronics hardware due to their dimensional precision, electromagnetic shielding properties, and ability to be electroplated (nickel, chrome, gold). Zinc electrodeposited coatings (electroplating from zinc sulfate or zinc chloride baths) are specified per ISO 2081 for small fasteners, springs, and precision parts where the galvanising bath temperatures would cause distortion or temper the spring steel.

Sacrificial Anodes

Zinc sacrificial anodes (UNS Z13001, MIL-A-18001) protect steel hulls, offshore platforms, pipelines, and buried structures in marine and soil environments. Standard anode alloys are Zn-0.1–0.5Al-0.025–0.07Cd (cadmium activates the anode surface; al-ternative Cd-free grades use Zn-0.3–0.6Al-0.025–0.05In). Zinc anodes are preferred in seawater (high conductivity); magnesium anodes are used in low-conductivity soil or freshwater. Closed-circuit potential of a zinc anode coupled to steel: approximately −1.00 to −1.05 V(Ag/AgCl/seawater), well above the −0.80 V protection criterion for steel.

Frequently Asked Questions

Recommended Reading

The following references cover zinc metallurgy, galvanising technology, corrosion engineering, and die casting alloys in depth. All are available on Amazon India.

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