The actual difference
Galvanized sheet has a zinc coating. Galvalume sheet uses a 55% aluminum-zinc alloy coating. The choice changes corrosion and forming behavior; it does not choose the strength of the underlying steel. Grade and base-metal thickness still need to match the component.
For many roof and wall panels, 55% Al-Zn is a strong candidate because of its atmospheric barrier protection. For demanding drawing operations, zinc-coated sheet deserves particular attention for coating adhesion. In aggressive or unusual exposures, the product’s exclusions can decide the choice before price or appearance enters the comparison.
Set the comparison basis first
Compare the same base grade and BMT, the same component/profile, intended processing and environment. Identify each coating chemistry/type and designation, mass basis, surface treatment and any paint. This comparison concerns conventional Al-Zn-Si Galvalume; A792/A792M-25a also covers Mg-containing Type 2, which must be identified separately.
Equal g/m² does not mean equal coating thickness, corrosion reserve or service life. Mass divided by density gives volume, so different alloy densities change the thickness represented by the same mass. Chemistry, coating structure, corrosion products and exposure then influence protection. Do not use the zinc converter’s density as if it were an Al-Zn density.
Coating and designation comparison
| Consideration | Galvanized sheet | Galvalume / 55% Al-Zn sheet |
|---|---|---|
| Coating chemistry | Predominantly zinc. | Aluminum-zinc alloy with a small silicon addition. |
| Protection | Barrier plus sacrificial zinc protection. | Aluminum-rich barrier regions with zinc-rich sacrificial regions. |
| ASTM product family | A653/A653M; distinguish zinc from galvannealed zinc-iron. | A792/A792M-25a Type 1 for the conventional Al-Zn-Si comparison here; Type 2 is a separate Al-Zn-Mg-Si option. |
| Designation example | G90 or Z275 with the specified system. | AZ50 or AZM150 in ASTM; EN uses its own AZ designations. |
| Cut edges | Adjacent zinc can protect exposed steel as it is consumed. | Zinc-rich portions also protect edges; thickness and exposure affect edge staining. |
| Severe forming | Often favored where coating adhesion under deep drawing is central. | Confirm coating response and galling control for the particular forming route. |
| Painted version | PPGI: galvanized substrate plus the selected paint stack. | PPGL: Al-Zn substrate plus the selected paint stack. |
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The coating/formation distinctions follow GalvInfoNote 1.4, December 2017. Designation units and minimum-test context are detailed in the zinc converter and Al-Zn product reference .
Corrosion and cut edges
An intact coated face and a sheared edge present different problems. The face has a continuous coating; the edge exposes a thickness of steel bordered by coating. Sheet thickness, edge geometry, wetting and drainage influence how well adjacent sacrificial material can protect that edge.
A more durable face does not remove the need to check cut-edge requirements. Record whether the roof-system supplier requires edge treatment and how cuts, drilled holes and swarf are handled. Compare actual coating designations and the same exposure, rather than matching unlike coatings simply by g/m².
Exposure and roofing decisions
Steelscape’s product FAQ identifies prolonged wet-concrete/high-alkalinity exposure and animal confinement as restrictions for its ZINCALUME product. These are specific selection signals, not a blanket approval of galvanized sheet for every aggressive environment.
| Situation | Priority decision |
|---|---|
| Ordinary exterior roof or wall | Check the approved substrate, coating mass and paint option within the panel system. |
| Animal-confinement building | Obtain written product suitability for the actual internal atmosphere and underside exposure. |
| Concrete or alkaline contact | Resolve direct contact, runoff and isolation details before selecting bare Al-Zn. |
| Deep-drawn component | Evaluate adhesion, powdering/galling, lubrication and final surface on a forming trial. |
| Marine or industrial site | Use supplier exposure limits for distance, deposits, washing and warranty scope. |
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For roof quantities, effective cover width matters separately from substrate selection. Neither coating determines span capacity or the fixing layout.
Painting and forming
Both substrates can be used in prepainted steel . The pretreatment, primer and topcoat must suit the metallic substrate and intended forming. A resin-treated bare product and a coil supplied specifically for painting can require different preparation.
A useful comparison request therefore includes the same base grade, BMT, finished profile and exposure, then names each proposed metallic coating and paint stack. It also records bend geometry and the appearance required after forming. This produces a technical comparison that can support a purchasing decision without inventing a universal winner or a fixed service-life multiplier.