Galvanized versus Zinc-Aluminium-Magnesium Panels: Composition, Corrosion and Selection
Nov 01, 2023
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Two Coating Families, One Purpose
Both galvanized and zinc-aluminium-magnesium panels exist to do the same job: keep steel from rusting in roofing, cladding, ducting and appliance housings. The difference lies in the coating chemistry and in how that chemistry behaves at a cut edge, in a salt spray chamber and after forming. Galvanized sheet carries a nearly pure zinc coating. Zinc-aluminium-magnesium sheet carries a ternary alloy coating that adds aluminium and magnesium to the bath, and the magnesium fraction changes the corrosion mechanism rather than just slowing it.
Hot-dip zinc-coated steel sheet is covered by ASTM A653/A653M and by EN 10346, which also covers the alloy-coated grades. Galvalume-type sheet with a nominal coating composition of about 55 percent aluminium, 43.5 percent zinc and 1.5 percent silicon is standardised in ASTM A792/A792M. Zinc-aluminium-magnesium coated sheet is standardised in ASTM A1046/A1046M, and in EN 10346 it appears under the ZM designation alongside the Z and AZ designations for zinc and aluminium-zinc coatings.
Coating Composition and Designation
Coating mass is quoted as total mass on both surfaces, in grams per square metre, or in ounces per square foot in the imperial editions of the same standards. A Z275 grade therefore carries 275 g/m2 of zinc in total, nominally 137.5 g/m2 per side, and an AZ150 grade carries 150 g/m2 of the aluminium-zinc alloy. Zinc-aluminium-magnesium grades are commonly supplied in the ZM80 to ZM200 range depending on the required service life.
| Feature | Galvanized (Z) | Zinc-aluminium-magnesium (ZM) |
|---|---|---|
| Coating chemistry | Zinc, typically above 99 percent | Zinc with aluminium and magnesium additions |
| Standard reference | ASTM A653/A653M, EN 10346 | ASTM A1046/A1046M, EN 10346 |
| Typical mass designation | Z275 total, both surfaces | ZM120 total, both surfaces |
| Cut-edge behaviour | Galvanic protection, limited lateral throw | Slower red rust at sheared edges |
| Relative corrosion life | Reference baseline | Longer in the same environment |
| Forming and welding | Easy to weld and form | Coating is harder, weld parameters need adjustment |
Corrosion Resistance in Real Environments
The magnesium addition is the key variable. When a ZM coating is exposed, magnesium oxide and hydroxide species form in the corrosion product layer and buffer the local chemistry, which suppresses the cathodic reaction and slows coating consumption. The practical consequence is that a ZM coating of lower mass can deliver a longer time to red rust than a plain zinc coating of higher mass in the same chloride-bearing or humid environment.
For selection, the relevant inputs are the corrosivity category of the site, the expected service life, and whether the panel will be cut and left uncoated at the edge. Industrial and coastal installations, livestock buildings with ammonia exposure, and unwashed roofing in humid climates are the cases where the alloy coating most clearly earns its higher price.
Cut Edge, Forming and Joining Considerations
A sheared edge exposes bare steel. Zinc protects it by sacrificial action, but only within a limited lateral distance, so narrow strips and heavily perforated parts lose protection quickly. ZM coatings extend that distance because the corrosion products are less conductive and less soluble, which is why the material is often chosen for roll-formed profiles that are cut to length on site.
Forming behaviour also differs. Zinc coatings are relatively soft and lubricious, and the surface tolerates tight bend radii with conventional tooling. ZM coatings are harder and more abrasive, so tool wear increases and drawing lubricants may need to be reviewed. Welding is possible in both cases, but the alloy coating has a higher electrical resistance at the interface, which raises electrode wear in resistance spot welding.
Specifying the Right Panel
Specification should state the base steel grade, the coating type and the coating mass designation, the surface treatment or passivation, and the intended corrosivity category. Stating only the thickness of the base metal leaves the corrosion performance undefined, which is the most common source of disputes after delivery. For painted systems, the pretreatment layer also matters, since the phosphate or chromate-free conversion coating is what anchors the organic topcoat.
Where a project requires documented performance, it is more reliable to specify the coating mass and the applicable standard together, and to ask for a coating mass test report rather than to rely on a commercial description of the product.
Frequently Asked Questions
Q: Is zinc-aluminium-magnesium sheet always better than galvanized sheet?
A: No. It offers longer corrosion life in most aggressive environments, but it costs more and is harder to weld. For dry indoor applications with short service life expectations, a conventional zinc coating remains the more economical choice.
Q: What does the number in Z275 or ZM120 mean?
A: The number is the total coating mass in grams per square metre on both surfaces combined. Z275 means 275 g/m2 total zinc, nominally 137.5 g/m2 per side, while ZM120 means 120 g/m2 of zinc-aluminium-magnesium alloy in total.
Q: How do I compare a zinc coating with an aluminium-zinc coating by mass?
A: You cannot compare them by mass alone, because the alloys have different densities and different corrosion mechanisms. Compare them by the corrosion performance required in the intended environment, and specify the coating designation together with the standard that defines it.
Q: Does the coating affect the strength of the panel?
A: The coating does not change the mechanical properties of the base steel, which are set by the steel grade and its annealing cycle. However, forming limits are affected: a harder alloy coating is less tolerant of tight bend radii and heavy drawing than a soft zinc coating.
Q: Can both materials be painted?
A: Yes. Both accept organic coatings, but the pretreatment must match the substrate. Zinc-aluminium-magnesium surfaces generally need a conversion coating tailored to the alloy chemistry so that the paint system adheres and the cut-edge protection is not compromised.
Q: What should a purchase order include for these panels?
A: Base steel grade and thickness, coating type and mass designation, applicable standard, surface treatment, coil or sheet dimensions, and the corrosivity category the material must satisfy. Adding the coating mass test requirement removes most ambiguity at delivery.