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Steel strip hot-dip coated with an alloy of about 55 % aluminium, 43.4 % zinc and 1.6 % silicon. The aluminium gives a durable barrier and the zinc gives sacrificial protection at cut edges, so in most atmospheres the coating outlasts a zinc coating of the same thickness several times over. It is the standard base for roofing, cladding and pre-painted coil in hot climates, including the Gulf.
The coating was developed in the United States in the 1960s and 70s by Bethlehem Steel and is made under licence around the world under trade names such as Galvalume, Zincalume and Aluzinc; the standards call it 55 % aluminium-zinc alloy coating (AZ). It is applied on a continuous hot-dip line much like galvanizing, with a bath at about 600 °C; the silicon controls the growth of a brittle iron-aluminium layer at the steel surface so the coating stays formable.
The coating solidifies with a fine aluminium-rich dendritic structure, with zinc-rich areas between the dendrites. The aluminium-rich network weathers very slowly and protects the steel as a barrier; the zinc corrodes preferentially and protects scratches and cut edges. In marine and industrial atmospheres an AZ150 coating typically lasts two to four times as long as Z275 galvanizing, and it also reflects heat better, which matters for unpainted roofs in hot climates. It should not be used in contact with wet concrete, mortar or animal housing, where alkali attacks the aluminium, and it is not used for buried parts.
Coatings are designated by total mass on both sides: AZ150 (150 g/m², about 20 µm per side at 3.75 g/m² per µm) is the usual coating for roofing and cladding, AZ100 for indoor parts and under paint, AZ185 or AZ200 for severe exposure. ASTM A792 uses AZ50 and AZ55 (0.50 and 0.55 oz/ft²), approximately equal to AZ150 and AZ165. Full-hard G550 / S550GD+AZ is common for light-gauge roof sheet and steel framing.
Al-Zn coated sheet is widely used for roofing and cladding in the GCC because of the heat and the aggressive coastal atmosphere, both bare and as pre-painted PPGL. Coating lines operate in the region, and imports come mainly from China, Korea, India, Taiwan and Vietnam. A coated sheet's life also depends on its edges and fixings: cut edges, drilled holes and fasteners of incompatible metals are where corrosion starts.
Ordered by base metal thickness × width, steel grade and coating class. For sheet-per-tonne figures see the galvanized and Al-Zn area-per-tonne table, which has AZ150 and AZ185 columns.
| Region | Standard | What it covers |
|---|---|---|
| Europe | EN 10346 CEN-CENELEC |
Continuously hot-dip coated steel flat products — aluminium-zinc alloy coating designated +AZ (AZ100, AZ150, AZ185) |
| Europe | EN 10215 CEN-CENELEC replaced by EN 10346 |
Continuously hot-dip aluminium-zinc coated steel strip and sheet (withdrawn) |
| USA | ASTM A792/A792M ASTM |
Steel sheet, 55 % aluminium-zinc alloy-coated by the hot-dip process — CS, FS, DS, SS and HSLAS grades |
| Japan | JIS G3321 JSA |
Hot-dip 55 % aluminium-zinc alloy-coated steel sheet and strip — SGLCC and structural grades |
| Australia | AS 1397 SA |
Continuous hot-dip metallic coated steel sheet and strip — G250 to G550 with AZ coatings |
| China | GB/T 14978 SAC |
Continuously hot-dip aluminium-zinc alloy coated steel sheet and strip |
| India | IS 15961 BIS |
Hot-dip aluminium-zinc alloy metallic coated steel strip and sheet (plain) — grades YS 300 to YS 550 |
Grade equivalents and the standards bodies: Steel grades & standards
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