The Three Coating Families Explained
Coated steel sheet for building and fabrication comes in three main coating families, and they are often confused because the sheet looks similar in the bundle. Galvanized steel carries a pure zinc coating, galvannealed steel carries a zinc-iron alloy coating produced by heating the strip after it leaves the zinc bath, and zinc-aluminium coated steel carries a coating that is predominantly aluminium with a substantial zinc content. All three are produced on continuous hot-dip coating lines and all three are ordered to EN 10346.
The differences that matter in selection are how each coating protects a cut edge, how long it lasts in a given atmosphere, and how easy it is to paint, weld or form. Coating mass alone does not tell the whole story, because a kilogram of aluminium-zinc coating behaves very differently from a kilogram of zinc.
Composition and How Each Coating Protects Steel
| Coating | Composition | Protection mechanism | Typical designation |
| Galvanized (GI) | Pure zinc | Barrier plus sacrificial cathodic protection of cut edges and scratches | Z100, Z180, Z275 |
| Galvannealed (GA) | Zinc-iron alloy formed by post-dip heat treatment | Barrier protection with a fully alloyed, matt surface | ZF, zinc-iron alloy coatings |
| Zinc-aluminium (AZ) | About 55 % aluminium, 43.4 % zinc, 1.6 % silicon | Barrier protection mainly from the aluminium-rich eutectic, with limited sacrificial action | AZ150 |
| Zinc-aluminium (ZA) | Zinc with about 5 % aluminium | Barrier protection with improved forming and paint response | ZA coatings to EN 10346 |
Pure zinc corrodes preferentially at exposed steel and keeps the steel below it cathodically protected, which is why a galvanized sheet tolerates cutting, drilling and site damage well. The zinc-iron alloy coating on galvannealed sheet is harder and completely alloyed, so it takes paint and resistance welds exceptionally well, but it offers less sacrificial protection than a pure zinc coating of the same mass. The aluminium-rich coating on zinc-aluminium sheet forms a dense, slow-consuming oxide barrier that lasts a long time in most atmospheres, yet it does not protect a bare cut edge sacrificially, so exposed edges rust unless they are sealed or coated.
Coating Mass and Expected Service Life
For zinc coatings, the relationship between mass and thickness is straightforward: a Z275 coating carries 275 g/m2 over both faces, about 137.5 g/m2 per face, which is roughly 19 micrometres of zinc on each side. Zinc-aluminium coatings are less dense, so AZ150 at 150 g/m2 is about 20 micrometres thick per face even though it carries barely half the mass of a Z275 coating.
| Comparison point | Galvanized Z275 | Zinc-aluminium AZ150 |
| Coating mass, both faces | 275 g/m2 | 150 g/m2 |
| Approximate thickness per face | 19 micrometres | 20 micrometres |
| Relative atmospheric life in rural and industrial sites | Reference | Approximately 2 to 4 times longer |
| Cut edge protection | Good, sacrificial | Limited, edges need sealing |
| Paint adhesion | Good with suitable pretreatment | Good, needs a system designed for aluminium-rich surfaces |
| Relative cost per square metre of coating | Lower | Higher |
The widely quoted advantage of zinc-aluminium coatings is that they last roughly two to four times as long as a zinc coating of equal mass in most atmospheric exposures, and that advantage is strongest in rural, industrial and inland building service. In severe marine exposure that advantage narrows and edges become the weak point, so galvanized sheet is often preferred where sections are cut and left unprotected.
Matching the Coating to Environment and Design Life
Selection works backwards from the design life and the exposure, then forwards through the fabrication route:
Dry indoor service with a short design life: a light zinc coating such as Z100 is the economical choice, and galvannealed sheet is a good option where the part will be painted or welded.
Outdoor building products with a 20 to 30 year expectation: Z275 galvanized sheet or AZ150 zinc-aluminium sheet both work in rural and industrial atmospheres, with the zinc-aluminium coating favoured where the surface will be left unpainted and edges can be detailed.
Coastal or high-salinity sites: galvanized sheet with a heavy coating and good edge detailing is normally preferred, because sacrificial edge protection matters more than coating mass alone.
Painted automotive and appliance panels: galvannealed sheet is the standard choice because its alloyed surface gives the most consistent paint adhesion and weldability.
Long-life unpainted roofing and wall cladding: zinc-aluminium coated sheet, provided cut edges are hemmed, sealed or covered by the fixing detail.
Alkaline contact and wet service: neither aluminium-rich nor zinc coatings tolerate strongly alkaline conditions well, so a barrier coating system is applied over the metallic coating in these cases.
Cut Edges, Forming, Welding and Painting
Cut edges decide more coating selections than any other single factor. A galvanized sheet protects a sheared edge by sacrificial action, so a narrow bright edge will weather to a stable zinc corrosion product. A zinc-aluminium sheet does not behave in the same way, so slit edges, punched holes and drill swarf sites should be treated with a zinc-rich repair coating, or the edge should be covered by a hem, a sealant or an overlapping detail.
Both coatings form and bend without flaking provided the coating mass is matched to the bend radius, but the harder alloyed galvannealed surface is the best of the three for severe drawing and for parts that will be spot welded. Welding of any coated sheet requires the coating to be removed from the immediate weld zone to prevent porosity and zinc fume, and the joint should be protected afterwards. For painting, galvanized and galvannealed surfaces accept suitably primed systems after cleaning; zinc-aluminium surfaces need a pretreatment and primer designed for aluminium-rich substrates to obtain a durable bond.
Whatever coating is chosen, the order should state the coating designation and mass, the surface treatment, the tolerance class and the required inspection document, so that the delivered coil can be checked against the specification.
Frequently Asked Questions
Q: Which lasts longer, galvanized or zinc-aluminium coated steel?
Zinc-aluminium coating generally lasts two to four times longer than a zinc coating of equal mass in rural and industrial atmospheres. In severe marine service the gap narrows, and unprotected cut edges can shorten the life of the zinc-aluminium product.
Q: Is galvannealed the same as galvanized?
No. Both start in the zinc bath, but galvannealed sheet is then heated so that the coating converts to a zinc-iron alloy. That gives a dull, matt surface with excellent paint adhesion and weldability but slightly less sacrificial protection.
Q: Can zinc-aluminium coated sheet be used at the coast?
It can be used, but edges and fixings must be protected and the design should avoid trapped moisture. Many coastal projects still specify a heavy zinc coating for that reason.
Q: Why do cut edges matter so much?
Because a zinc coating protects exposed steel sacrificially while an aluminium-rich coating does not, so on zinc-aluminium sheet a bare edge is the first place where rust appears.
Q: Which coating should be specified for parts that will be painted?
Galvannealed sheet for the most demanding paint and weld requirements, otherwise a galvanized sheet with a suitable pretreatment and primer, matched to the exposure and the required colour and gloss.
Q: What should be written on the order?
The coating designation and mass, the base steel grade, dimensions and tolerances, the surface treatment, and the inspection document type, so that the coated coil can be verified on delivery.
