Galvalume vs Galvanized – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Galvalume and galvanized steels are two of the most common coated carbon-steel products used in construction, appliance manufacture, and general fabrication. Engineers, procurement managers, and manufacturing planners routinely weigh corrosion protection, cost, formability, and joining performance when choosing between them. Typical decision contexts include: selecting a roof panel for a coastal building (corrosion resistance vs edge protection), specifying structural sheet for fabrication lines (weldability and paintability), and choosing material for long-life cladding (upfront cost vs lifetime maintenance).
The principal distinction between the two lies in the coating system applied to the base carbon steel. One uses a zinc-only coating that provides sacrificial (galvanic) protection, while the other uses an aluminum–zinc alloy coating that emphasizes barrier protection supplemented by galvanic action. Because both products are coated versions of similar carbon-steel substrates, comparisons emphasize coating chemistry and performance rather than a change in steel metallurgy of the substrate.
1. Standards and Designations
Representative international standards and specification families that govern coated flat steel products include:
- ASTM/ASME
- ASTM A653 — Hot-dip galvanized and galvanized-aluminized (Z) and galvannealed coatings on cold-rolled sheet (Z, AZ, GA variants).
- ASTM A792 — Steel sheet, 55% Aluminum-Zinc alloy-coated (commonly referenced for Galvalume/AZ55).
- EN (European)
- EN 10346 — Continuously hot-dip coated steel flat products (covers Zn and Al–Zn coatings and their classifications).
- JIS (Japanese)
- JIS G3302 — Hot-dip galvanized steel sheet and strip (zinc coatings).
- JIS G3321 — Aluminum-zinc alloy coated steels (Al–Zn coatings).
- GB / Chinese national standards
- GB/T series specifications for hot-dip coated steels (cover both Zn and Al–Zn coated products).
Classification: Both Galvalume and galvanized products are coated carbon steels (not stainless, tool, or HSLA in the alloy-designation sense). Base substrates are generally low-carbon cold-rolled or hot-rolled steels; higher-strength coated products can be produced using HSLA or microalloyed substrates but remain coated carbon-steel products.
2. Chemical Composition and Alloying Strategy
Coated steels are specified by substrate chemistry and by coating composition. The coating chemistry is the defining difference:
- Galvanized: coating is essentially metallic zinc (Zn) or zinc with small additions/variations (e.g., galvannealed is Zn–Fe alloyed at the surface).
- Galvalume (typical AZ55): nominally 55% Al / 43.4% Zn / 1.6% Si by weight in the coating alloy (the “AZ” family; Al-rich alloy forms stable Al2O3 barrier).
Table — Typical substrate chemistry ranges (wt%) for commercial coated carbon steels (note: exact composition depends on grade and mill practice):
| Element | Typical range (wt%) | Comment |
|---|---|---|
| C | 0.02 – 0.12 | Low carbon to preserve formability and weldability |
| Mn | 0.10 – 1.50 | Strength and hardenability control |
| Si | ≤ 0.30 (often <0.10) | Deoxidation; higher Si can affect coating adhesion |
| P | ≤ 0.04 | Impurity; kept low for ductility and forming |
| S | ≤ 0.05 | Controlled for machinability; sulfides affect surface quality |
| Cr | trace – 0.30 | Minor alloying in some grades |
| Ni, Mo, V, Nb, Ti, B, N | trace – small additions | Used in HSLA or microalloyed steels; often not present in standard commercial substrates |
How alloying strategy affects performance: - Carbon and manganese primarily set base strength and hardenability; higher C and Mn increase strength but reduce weldability and formability. - Silicon and phosphorus influence surface chemistry and coating adhesion; very high Si can produce “silicon-killed” steel that alters coating wetting. - Microalloying (Nb, V, Ti) raises strength via precipitation and grain-size control but can complicate forming and welding if used without process controls.
3. Microstructure and Heat Treatment Response
Microstructure: - Typical coated products use low-carbon ferrite–pearlite or ferrite–bainite substrates depending on strength level and processing. Cold-rolled substrates intended for good formability are usually fully ferritic with polygonal ferrite and very fine pearlite, or fully ferritic in some low-strength grades. - For higher-strength coated products, microalloying and controlled rolling can produce refined ferrite with dispersed carbides/nitrides (HSLA characteristics).
Heat-treatment and processing effects: - Coating application is usually by continuous hot-dip coating (for both Zn and Al–Zn) where the strip passes through a molten bath and then air- or forced-cooled. Bath chemistry and cooling control intermetallic layer growth. - Annealing and tempering prior to coating set the substrate microstructure and mechanical properties; post-coat thermal exposure (e.g., galvanneal) can produce localized diffusion layers (Zn–Fe intermetallics). - Normalizing is uncommon for thin-gauge coated sheet; quenching & tempering or heavy heat treatments are typical only when the substrate grade demands higher strength, in which case coating considerations (thermal stability of coating, diffusion) must be managed. - For Galvalume, the Al content forms a protective Al–Fe intermetallic/intermixed layer at the steel/coating interface; for galvanized coatings, Fe–Zn intermetallic layers (e.g., Gamma, Delta phases) form and influence adhesion and brittleness.
4. Mechanical Properties
Because Galvalume and galvanized products share similar substrates, mechanical properties are primarily set by the base steel and processing. Coatings contribute minimally to bulk tensile/yield but influence surface-related fracture initiation (e.g., coating cracks during forming).
Table — Typical mechanical property ranges for commercial coated cold-rolled carbon steels (indicative; depends on substrate grade and temper):
| Property | Typical range | Notes |
|---|---|---|
| Tensile strength (Rm) | 270 – 450 MPa | Higher values possible for HSLA substrates |
| Yield strength (Rp0.2 or ReH) | 140 – 350 MPa | Dependent on grade (commercial vs high-strength) |
| Elongation (A%) | 15 – 40% | Coating type has minimal direct effect |
| Impact toughness | moderate to good (temperature dependent) | Determined by substrate microstructure |
| Hardness | low to moderate (HV relative to substrate) | Coating hardness varies; Galvalume coating can be harder than pure Zn |
Interpretation: - Neither coating substantially changes core strength; choose substrate grade to meet structural requirements. - Coating choice affects surface toughness and susceptibility to coating cracking during bending — Galvalume coatings are typically harder and may show more visible cracking at sharp bends than ductile Zn coatings.
5. Weldability
Weldability considerations are dominated by substrate chemistry and coating behavior under heat:
- Coating removal at the weld zone: both coatings burn off or are displaced in the fusion zone and can produce fumes; pre-cleaning and appropriate fume extraction are required.
- The substrate carbon level and alloying determine hardenability and cold-cracking susceptibility. Use accepted carbon-equivalent metrics to assess preheat/heat-input requirements.
Useful weldability indices (examples): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - Low-carbon, low-alloy substrates used for coated sheet usually yield low $CE_{IIW}$ and $P_{cm}$ values, indicating good general weldability with standard processes (GMAW, SMAW, resistance welding). - Galvalume’s Al–Zn coating can create refractory aluminum oxides and increased spatter/fume; welding parameters and cleaning differ slightly from galvanized (Zn) products. - Galvanized coatings provide more sacrificial corrosion protection at cut edges but can increase porosity risk if Zn vaporizes during high heat processes; appropriate welding procedures and ventilation are required.
6. Corrosion and Surface Protection
Protective mechanisms: - Galvanized (Zn coating): primary protection is galvanic (sacrificial) — zinc corrodes preferentially and provides cathodic protection to exposed steel at scratches and cut edges. Over time, zinc corrosion products adhere and provide some barrier. - Galvalume (Al–Zn alloy coating): primary protection is a dense Al-oxide barrier formed on the surface, which resists corrosion; the Zn component offers secondary galvanic protection where the coating is breached.
When stainless properties are relevant, PREN is used to estimate pitting resistance: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ Note: PREN is not applicable to non-stainless coated carbon steels; it is included as a stainless-reference indicator.
Practical implications: - Atmospheric exposure: Galvalume typically exhibits superior general atmospheric corrosion resistance and longevity for roof and wall cladding in many environments. - Edge and mechanical damage: Galvanized coatings often afford better sacrificial protection at cut edges and deep scratches because zinc is more anodic. - Marine and highly corrosive environments: neither coating equals stainless steel; Galvalume often outperforms Zn in some marine atmospheres but localized galvanic effects and mechanical damage must be considered. Material selection should be based on specific corrosion testing or service history.
7. Fabrication, Machinability, and Formability
- Cutting and shearing: both perform similarly; Galvalume’s harder coating can produce slightly increased tool wear and different burr formation.
- Forming and bending: Galvanized (pure Zn) coatings are typically more ductile and resist visible cracking at acute bends better than Al–Zn coatings. Tight-radius bending may require lubrication and careful die design for Galvalume to avoid coating fracture.
- Punching and stamping: Galvalume may produce more pronounced coating flaking if tooling is not optimized; galvanneal (zinc iron alloy) can provide better paint adhesion for subsequent finishing.
- Painting and finishing: paint adhesion depends on surface pretreatment. Galvanneal and pretreated zinc surfaces often provide superior paint bonding; Galvalume typically paints well if properly pretreated and may require different pretreatment chemistry.
8. Typical Applications
| Galvalume (Al–Zn coated) | Galvanized (Zn coated) |
|---|---|
| Roofing and siding panels where long life and barrier protection are valued | Structural members, framing, fasteners, guttering where sacrificial edge protection is important |
| High-temperature service (heat-reflective properties of Al layer) | Fencing, purlins, studs, and cold-formed sections |
| Appliance housings and HVAC ducts where corrosion resistance and appearance matter | Automotive inner panels, chassis components (often galvannealed for paintability) |
| Industrial buildings, warehouses, and agricultural structures | General-purpose sheet metal, pipes, and tubing that are economical and widely available |
Selection rationale: - Choose Galvalume where long-term atmospheric corrosion resistance and thermal reflectivity matter and where edges can be detailed to avoid rapid exposure to corrosive elements. - Choose Galvanized where cost and sacrificial protection at cut edges, fasteners, or areas subject to mechanical damage are dominant considerations.
9. Cost and Availability
- Relative cost: Galvanized (Z) products are generally less expensive per unit area than Galvalume (AZ55) due to the cost of aluminum in the coating. Actual prices fluctuate with commodity Zn and Al markets.
- Availability: Both coatings are widely available in coils, sheets, panels, and prepainted products worldwide. Galvanized is historically the most common and may be more readily stocked in some commodity forms; Galvalume is commonly available for roofing and cladding markets.
- Product forms: coils, sheets, prepainted coil-coated panels, roofing coils, and formed profiles. Lead times are typically short for standard gauges and widths, longer for specialty alloys or prepainted finishes.
10. Summary and Recommendation
Table — Comparative summary (qualitative)
| Metric | Galvalume (Al–Zn) | Galvanized (Zn) |
|---|---|---|
| Weldability | Good (requires attention to Al-related fumes) | Good (Zn vapor/fume management needed) |
| Strength–Toughness (substrate-controlled) | Similar (substrate choice governs) | Similar (substrate choice governs) |
| Corrosion Resistance (general atmospheric) | Higher (barrier + galvanic) | Good (strong sacrificial protection) |
| Edge/cut protection | Moderate (less sacrificial at edges) | Better (sacrificial protection at exposed edges) |
| Formability / bendability | Moderate (coating harder; watch tight radii) | Better (coating more ductile) |
| Cost | Higher (Al in coating) | Lower (generally economical) |
Choose Galvalume if: - Long-term atmospheric corrosion resistance and aesthetics for roofing, siding, or appliance skins are priority. - Thermal reflectivity or superior barrier protection is desired and detailing minimizes prolonged exposure of cut edges. - You accept modestly higher material cost for extended service life.
Choose Galvanized if: - Cost sensitivity, sacrificial protection at cut edges, or heavy mechanical handling dominate the selection criteria. - Applications include structural framing, fasteners, or general-purpose sheet where formability and edge protection are important. - Supply-chain ubiquity and low capital cost are significant drivers.
Final note: Because mechanical performance is dominated by the substrate, always specify the correct base steel grade and temper for structural requirements, and specify coating weight/thickness and post-coat treatments (paint, passivation, galvannealing) to meet corrosion and fabrication needs. When in doubt, request comparative corrosion testing for the intended exposure class and confirm weld/process compatibility with your fabricator.