Hot-dip coatings are the main corrosion-protection system for flat steel, and the classification in the major standard systems (European, American, Japanese and Chinese) is broadly similar. The European standard EN 10346:2015, which governs continuously hot-dip coated steel flat products, recognises six coating families. Understanding them prevents both under-specification and the common habit of paying for a premium coating where a standard one is enough.
The Six Coating Families
| Designation | Coating metal | Key characteristics | Typical use |
|---|---|---|---|
| Z | Pure zinc | Sacrificial protection; cheapest; predictable | Building profiles, ducts, general galvanizing |
| ZF | Zinc-iron alloy (galvannealed) | Dull grey, weldable, paintable; formed in the annealing step | Automotive body parts, painted components |
| ZA | Zinc-aluminium, about 5% aluminium | Better corrosion than Z, good formability | Roofing, agricultural buildings |
| AZ | Aluminium-zinc, about 55% aluminium, 43.4% zinc, 1.6% silicon | Barrier plus sacrificial behaviour; heat reflectivity; long cut-edge protection | Roofing, cladding, HVAC, heat-exposed panels |
| AS | Aluminium-silicon (about 90% aluminium) | Heat and oxidation resistance up to high service temperatures | Exhaust systems, heat shields, ovens |
| ZM | Zinc-aluminium-magnesium | Fine protective corrosion products; self-healing at cut edges; 2-4 times Z life in salt spray | Coastal structures, solar frames, agricultural, cut-edge parts |
How the Coating Families Protect Steel
Pure zinc (Z) protects sacrificially: it corrodes in place of the steel at scratches and cut edges. ZF keeps the same chemistry but converts the layer to an alloy for weldability and paint adhesion. ZA adds aluminium to strengthen the barrier. AZ flips the balance to aluminium, giving a dense oxide barrier plus enough zinc to offer sacrificial protection at edges. AS is essentially an aluminium coating for heat service. ZM adds magnesium so that corrosion products form a dense, stable film that covers scratches and cut edges and slows lateral corrosion, which is why it is the fastest-growing family for exposed structures.
Coating Mass Designations
Each family is ordered by coating mass, for example Z100 (100 g/m2 total), AZ150 (150 g/m2 total) or ZM130. The number is the minimum total mass on both surfaces measured by the triple-spot method. The same physical coating is designated differently across standards: EN 10346 uses Z100/Z275, JIS G3302 uses classes such as Z12 and Z27 (multiply by ten), and ASTM A653 uses G60 and G90 in ounces per square foot (183 and 275 g/m2). Always convert to grams per square metre before comparing offers.
How to Choose
Dry indoor, cost-sensitive: Z with the lowest adequate coating class.
Standard outdoor building: Z200-Z275 or ZA; AZ for painted roofing in coastal areas.
Heat-exposed components: AS or AZ, depending on the service temperature.
Coastal, agricultural, chemical-adjacent or cut-edge-critical parts: ZM, or AZ where 55% aluminium behaviour is preferred.
Automotive painted parts: ZF, which welds and paints better than pure zinc.
Frequently Asked Questions
1. Which coating family is cheapest?
Pure zinc (Z) is the least expensive because zinc is cheaper than aluminium and the process is the most established. Premium families such as AZ and ZM carry higher alloy and processing costs.
2. Is ZM the same as AZ?
No. AZ is aluminium-rich (about 55% aluminium); ZM is zinc-rich with small additions of aluminium and magnesium (commonly 1-3.5% aluminium, 1-3% magnesium). They protect by different mechanisms.
3. Can all six families be welded?
Yes, with process adjustments. ZF welds most easily; AZ and ZM produce more fume and require ventilation, and welds should be re-protected afterwards.
4. Does AZ coating resist heat better than Z?
Yes. AZ keeps its protective behaviour up to higher temperatures and reflects heat better, which is why it is used for roofs and heat-exposed panels; AS goes further for exhaust and oven parts.
5. Which coating is best for cut edges?
ZM has the best cut-edge behaviour because its corrosion products seal the exposed steel. AZ also performs well; pure zinc offers limited sacrificial protection at edges.
6. Where is EN 10346 used?
EN 10346 is the European standard for continuously hot-dip coated steel and is widely referenced in Asia, the Middle East and Africa. American projects more often reference ASTM A653, A792 or A875.