Hot-dip Galvanized Steel Sheet

Product Knowledge

How the Hot-Dip Galvanizing Process Forms Its Coating — Layer by Layer

The zinc coating on a hot-dip galvanized steel sheet is not simply a surface deposit — it is a metallurgically bonded system of distinct intermetallic layers that form during immersion in molten zinc at approximately 450°C. Understanding this layer structure explains why hot-dip galvanizing outperforms other zinc coating methods in long-term corrosion resistance and adhesion strength. From the steel substrate outward, the coating consists of four zones: a Gamma layer (Γ, ~1 μm, ~75% Zn), a Delta layer (δ, ~10–20 μm, ~90% Zn), a Zeta layer (ζ, ~20–30 μm, ~94% Zn), and finally the eta layer (η) — which is essentially pure zinc at 99%+ and forms the outermost free zinc surface visible to the eye.

The intermetallic Gamma, Delta, and Zeta layers are harder than the base steel itself (Gamma and Delta reach 250–300 HV Vickers hardness), which contributes directly to the abrasion resistance of the coated surface. The eta layer, being pure zinc, is softer and more ductile — it provides the sacrificial corrosion protection and the formability that allows the coated sheet to be bent and shaped without coating delamination in normal fabrication. The thickness ratio of these layers is influenced by the steel's silicon and phosphorus content, the bath temperature, the immersion time, and the withdrawal speed — all variables that our manufacturing process controls precisely to deliver a uniform, well-bonded coating across every sheet.

Zinc Coating Weight Standards and How to Specify the Right Grade

Coating weight — the mass of zinc per unit area of steel surface — is the primary parameter used to specify a hot-dip galvanized steel sheet for a given application. It is expressed in grams per square meter (g/m²) and applies to both surfaces combined unless otherwise stated. Different standards use different notation systems, and misreading them is a common source of specification errors.

A critical point that is frequently misread: the ASTM G-designation number refers to the coating weight in oz/ft² multiplied by 100, not in g/m². So G90 equals 0.90 oz/ft² total for both sides, which converts to approximately 275 g/m² — numerically coinciding with EN Z275 but derived from a different unit system. When comparing specifications across international supply chains, always convert to g/m² as the common basis before evaluating equivalence. Our hot-dip galvanized steel sheets are produced to meet both EN 10346 and ASTM A653 specifications and carry full mill test reports documenting actual coating weights per coil.

Corrosion Mechanisms Zinc Protects Against — and the Ones It Doesn't

Zinc protects steel through two distinct mechanisms operating simultaneously. The first is barrier protection: the zinc coating physically separates the steel substrate from moisture, oxygen, and corrosive ions in the environment. The second is cathodic (galvanic) protection: because zinc is electrochemically more active than steel (lower reduction potential at approximately −0.76 V vs. −0.44 V for iron), zinc preferentially oxidizes when both metals are in electrical contact with an electrolyte. This means that even at cut edges, drilled holes, or scratched areas where the steel is exposed, the surrounding zinc corrodes preferentially while the steel remains protected — a self-healing mechanism that no paint or organic coating system can replicate.

However, galvanic protection has a finite range. The cathodic protection radius from a zinc-coated surface extends approximately 1–3 mm into an exposed steel zone, depending on the electrolyte conductivity and the coating thickness. Damage areas wider than this will not be fully protected at their center. Additionally, galvanic protection is reversed in certain environments: in hot water systems above approximately 65°C, zinc becomes cathodic relative to steel, meaning steel corrodes preferentially — a phenomenon known as polarity reversal. Hot-dip galvanized steel sheet is therefore not recommended for uncoated use in hot water tanks or solar thermal pipework without additional protective measures.

Environments That Accelerate Zinc Coating Consumption

Zinc corrosion rate varies dramatically with environment. The ISO 9223 standard classifies atmospheric corrosivity into five categories (C1 through C5) and two immersion categories (Im1 for freshwater, Im2 for seawater), with corresponding zinc loss rates. In a rural C1 environment, zinc corrosion rates may be as low as 0.1 μm per year; in a C5 coastal-industrial environment, rates can exceed 10 μm per year — a 100-fold difference. Acid rain (pH below 6), ammonia atmospheres (common near agricultural facilities), and marine salt spray environments are particularly aggressive to zinc. Concrete contact is generally benign to galvanized steel since concrete's high alkalinity passivates the zinc surface; however, trapped moisture at the steel-concrete interface without adequate drainage creates an accelerated crevice corrosion condition regardless of coating quality.

Spangle Size, Surface Finish Types, and Their Functional Implications

The characteristic crystalline pattern visible on the surface of hot-dip galvanized steel sheet — known as the spangle — is formed by the solidification of the eta (pure zinc) layer as the sheet exits the zinc bath and cools. Spangle size is controlled by the zinc bath chemistry and the cooling rate: faster cooling and the addition of small amounts of antimony or lead produce larger, more pronounced spangling; slower cooling or lead-free bath chemistries (increasingly standard under RoHS compliance) tend to produce minimized or zero-spangle surfaces.

Spangle size is not merely aesthetic — it has functional implications for downstream processing. Large-spangle material has a more pronounced surface topography, which can create differential zinc thickness across the spangle (thicker at grain centers, thinner at boundaries), resulting in uneven paint adhesion if the sheet is to be painted without pre-treatment. For applications where the galvanized surface will be painted, powder-coated, or further processed, minimized-spangle or skin-passed (temper-rolled) galvanized sheet is specified to provide a flatter, more uniform surface. Skin passing also eliminates Lüders bands (stretcher strain markings) that can appear during subsequent forming operations on non-skin-passed material.

Fabrication Considerations: Cutting, Forming, and Welding Galvanized Sheet

Hot-dip galvanized steel sheet can be cut, formed, and welded using standard fabrication equipment, but the zinc coating introduces specific considerations at each process step that affect both finished product quality and worker safety. Ignoring these considerations leads to coating damage, weld defects, and — in the case of welding — serious occupational health hazards.

Cutting and Edge Treatment

Shearing and plasma cutting are the most common methods for processing hot-dip galvanized steel sheets. Laser cutting is effective but vaporizes zinc in the heat-affected zone, producing zinc oxide fumes that require local exhaust ventilation. At cut edges, the zinc coating is absent by definition, and cathodic protection from the adjacent zinc extends only 1–3 mm into the bare steel zone. For most structural applications, this cathodic protection is sufficient to prevent corrosion at cut edges during service life. For applications in C4–C5 corrosivity environments, cut edges should be treated with zinc-rich primer or cold galvanizing compound before assembly. Deburring cut edges is important not only for safety handling but to prevent sharp edge burrs from acting as stress concentration points during bending.

Bending and Roll Forming

The ductility of the zinc coating — specifically of the pure zinc eta layer — determines the minimum bend radius achievable without coating cracking. For standard hot-dip galvanized sheet with Z275 coating, the minimum bend radius is typically 0t to 1t (where t = sheet thickness) for thinner gauges below 1.5 mm, and 1t to 2t for thicker material. The harder intermetallic layers (Delta and Zeta) do not deform plastically — they can micro-crack at tight bend radii, but these micro-cracks in the intermetallic zone do not propagate to delamination under normal service conditions and do not significantly compromise corrosion performance at the bend. For roll-formed profiles (roofing sheets, purlins, decking), the repeated incremental bending of the roll-forming process is generally compatible with hot-dip galvanized coatings without additional formability grades, provided tooling radius is maintained within specification.

Welding: Fume Hazards and Joint Quality

Welding hot-dip galvanized steel generates zinc oxide fumes when the coating in and around the weld zone vaporizes. Inhalation of zinc oxide fumes causes metal fume fever — a flu-like condition with onset 4–8 hours after exposure — and chronic high-level exposure carries more serious respiratory risks. This is the most critical safety consideration when welding our galvanized steel sheets in structural fabrication. Mandatory controls include local exhaust ventilation directed at the weld point, respiratory protection (minimum P3 FFP3 or equivalent), and where possible, grinding back the zinc coating 20–25 mm from the weld line before welding and re-treating the bare zone after welding with zinc-rich cold spray. MIG welding with silicon bronze filler wire (brazing) generates less fume than fusion welding and produces an acceptably strong joint for many structural connections on galvanized sheet, making it a preferred method for lighter gauge material.

Painting Over Hot-Dip Galvanized Steel: Why Adhesion Fails and How to Prevent It

Painting galvanized steel is a widely used strategy to extend service life beyond what zinc alone provides (a "duplex system") or to meet specific aesthetic requirements. However, paint adhesion failure on galvanized steel is a common problem that is almost always attributable to surface preparation failure rather than paint system selection. The mechanisms and prevention of adhesion failure are well understood but frequently overlooked in practice.

Freshly galvanized steel has a smooth, chemically active zinc surface that initially accepts paint well. Over the first few weeks of outdoor exposure, however, the zinc surface develops a stable zinc carbonate and zinc hydroxide passivation layer — commonly called "white rust" in its early form — which is chemically inert and extremely low in surface energy. Paint applied over this passivated zinc surface has poor mechanical interlocking and chemical bonding to the substrate, which manifests as peeling or blistering within 1–3 years. The solution is surface preparation: either mechanical abrasion (sweep blasting to Sa 1–2 profile) to expose fresh zinc, or chemical treatment with phosphoric acid wash or a T-wash (dilute phosphoric acid with copper sulfate indicator) to etch the surface and create a zinc phosphate conversion coating with better paint adhesion. T-washing is the standard preparation method for site-applied coatings on galvanized steel in bridge and infrastructure applications.

• Weathered galvanized steel (6–12 months outdoor exposure): Sweep blast clean to remove zinc salts and roughen the surface, then apply zinc-compatible primer (epoxy or polyurethane) within 4 hours of blasting. This surface actually offers better paint adhesion than freshly galvanized steel due to the slight surface roughness developed during weathering.
• Newly galvanized steel (less than 48 hours since galvanizing): Apply T-wash or phosphoric acid etch, allow to fully dry, then prime with a zinc-compatible etch primer or direct-to-metal epoxy primer. Avoid alkyd oil-based paints, which saponify (turn to soap) in contact with zinc and fail rapidly.
• Pre-painted (coil-coated) galvanized sheet: Factory coil coating applies primer and topcoat under controlled conditions before roll forming or cutting. This is the most reliable duplex system for architectural and roofing applications and avoids all field painting adhesion variables.
• Avoid incompatible primers: Traditional red lead and chlorinated rubber primers are not suitable for zinc substrates. Zinc chromate primers, while highly effective, are restricted under REACH and RoHS regulations in many markets. Modern chromate-free epoxy zinc phosphate primers offer equivalent performance and are the correct specification for industrial duplex systems on galvanized steel.

Galvanized Steel in Structural Engineering: Connection Details That Matter

In structural applications — building frameworks, bridge components, industrial racking, and transmission towers — the performance of a galvanized steel structure is heavily influenced by connection and detailing decisions that determine both structural integrity and long-term corrosion performance. Structural engineers familiar with bare steel design sometimes overlook the specific implications of zinc coating at connection points.

Bolted connections in galvanized steel require attention to bolt material compatibility and hole clearance. Hot-dip galvanized bolts and nuts are standard for structural connections in galvanized assemblies — using uncoated steel fasteners in contact with zinc-coated structural members creates a galvanic couple that accelerates fastener corrosion. Bolt hole dimensions must account for the zinc coating thickness: a Z275 coating adds approximately 0.04 mm per surface (about 20 μm thickness), which requires hole punch or drill diameters to be increased by 0.5–1.0 mm relative to uncoated steel specifications to ensure specified clearances are maintained after galvanizing.

In high-strength friction grip (HSFG) bolted connections, the slip factor (coefficient of friction) of the contact surfaces must be established for galvanized interfaces. The slip factor for hot-dip galvanized surfaces is typically 0.18–0.20, compared to 0.45–0.50 for blast-cleaned steel surfaces — a significant reduction that must be accounted for in design calculations for slip-critical connections. EN 1090-2 provides specific guidance on surface treatment and testing requirements for galvanized slip-critical joints. Our structural-grade hot-dip galvanized steel sheets are produced with documented surface quality and coating uniformity that support accurate slip factor characterization for engineering calculations in bridge and heavy structure applications.

Storage and Handling of Hot-Dip Galvanized Sheet: Preventing Wet Storage Stain

Wet storage stain — also called white rust — is one of the most commonly encountered quality issues with hot-dip galvanized steel sheet and is responsible for significant commercial disputes between suppliers and end users. Understanding its cause, appearance, and implications prevents unnecessary rejection of serviceable material and informs proper storage practices.

Wet storage stain forms when moisture is trapped between closely stacked galvanized sheets in conditions where the zinc surface cannot access sufficient atmospheric oxygen and carbon dioxide to form its normal stable passivation layer. Instead, zinc reacts with water to form zinc hydroxide (Zn(OH)₂) — a white, bulky, loosely adherent deposit visible on the surface. The conditions that promote wet storage stain are: stacking sheets with tight face-to-face contact without ventilation, condensation in transit containers due to temperature differentials, inadequate drainage gradient in outdoor storage, and high humidity warehousing without dehumidification.

• Assessing severity: Light white staining that can be removed with a damp cloth and has not penetrated below the surface of the zinc coating is cosmetic only and does not affect corrosion performance. Heavy staining with pitting into the zinc layer reduces effective coating thickness and service life. EN 10346 Annex C provides guidance on assessing whether stained material remains within specification.
• Storage best practice: Store sheets on tilt-rack systems or interleaved with ventilation spacers to allow air circulation between faces. Cover outdoor storage to prevent rain and dew contact, but ensure the covering does not trap moisture underneath. Avoid stacking sheets horizontally on flat ground without drainage gradient.
• Transit protection: Coils and sheets intended for sea freight or long-distance transport should be chemically passivated (chromate or chromate-free) at the mill before shipping. This passivation layer extends white rust resistance from days to several months under humid conditions and is a standard service we provide on export orders.
• Remediation of light staining: Light wet storage stain can be removed with dilute ammonia solution (1–2%), rinsed with clean water, and dried. For material that will be painted, the stained area should be treated with T-wash or phosphoric acid etch after stain removal to restore surface condition before primer application.