A roofing contractor ordering "galvalume" and a coil trader quoting "AZ150" are describing different things. The coating on 55% aluminium-zinc steel is an alloy rather than a layer of zinc, and the number after the AZ prefix is a coating mass rather than a thickness. Those two facts decide what the sheet does at a cut edge, on a fold, and after five years on a factory roof.

The coating is an alloy, not zinc

The bath that produces galvalume steel runs roughly 55 percent aluminium, 43.5 percent zinc and 1.5 percent silicon. The silicon is not decoration: it controls the intermetallic layer between steel and coating so the coating stays adherent when the sheet is bent. The result is a two-phase structure, with aluminium-rich dendrites acting as a barrier and a zinc-rich interdendritic network supplying sacrificial protection wherever that barrier is broken.

That structure is why the deformed part of an Al-Zn sheet behaves differently from hot-dip galvanized. Under a bend, plain zinc develops comparatively wide microcracks that expose steel; the Al-Zn coating cracks more finely, so the exposed area on the outside of a fold is smaller and the sacrificial phase sits closer to it. Roofing rolled on site and bent over a ridge tends to hold up better than the same operation in galvanized.

Reading the AZ number correctly

AZ designations are grams of coating per square metre across both sides, not microns per side. AZ30 is a thin coating for sheltered work, AZ150 is a heavy coating for exposed roofs, and AZ120 sits between them. A DX51D+AZ120 aluminium zinc plating steel sheet and a DX51D+AZ150 aluminium zinc plating steel coil share the same commercial forming substrate and differ only in that figure, which is the one that sets the corrosion budget.

One piece of arithmetic is worth knowing. Al-Zn alloy is less dense than pure zinc, so at equal coating weight and equal substrate thickness an Al-Zn sheet covers about 3 percent more area than its galvanized equivalent. Buyers comparing coatings purely on grams per square metre therefore overstate the zinc option slightly. Grade and chemistry for the drawing-quality version is published on the DX52D+AZ Galvalume coil page, where carbon is capped at 0.12 percent, manganese at 0.60 percent and titanium at 0.30 percent, with SGLCC, SGLCH, G350 and G550 as the alternative designations mills quote against.

Where the premium pays, and where it does not

Mills quote around three times the corrosion resistance of pure galvanized sheet, with service life of two to six times ordinary galvanized in the same environment. The atmospheric order that matters is rural, then industrial, then general marine, then severe marine. The coating also tolerates continuous surface temperatures above 300 degrees Celsius, which is why it appears in chimney liners, oven panels and lamp housings, and its thermal reflectance runs about twice that of galvanized, reflecting up to some 80 percent of incident sunlight. On a large metal roof in a hot climate that is a measurable cooling load change rather than a marketing line. The Galvalume versus galvanized comparison is worth reading before assuming the upgrade.

Three limits are routinely understated. Aluminium is amphoteric, so an Al-Zn coating is attacked by strong alkalis and sheet in contact with wet concrete, mortar or fresh render loses coating faster than zinc would. The Al-Zn surface can also develop dark wet-storage staining when coil or bundles ship with condensation trapped between wraps, which is cosmetic rather than structural but is a common cause of rejection. And on thick sheet the cut edge relies on a smaller proportion of sacrificial zinc than galvanized does, so where a heavy cut edge stays permanently wet, plain hot-dip galvanized can still be the better answer. Where the alternative is a hot-dip aluminised product with no sacrificial phase at all, the trade-off differs again, as set out in aluminized steel versus Galvalume.

Forming, spangle and surface treatment

Cold rolled AluZinc coil is supplied from 0.12 mm to 2.0 mm thickness and 30 mm to 1500 mm width, with thickness tolerance held to about plus or minus 0.01 mm and a T-bend result of 3T on the top surface and 4T on the back, which is the practical measure of whether the coating survives a hem. The full dimensional and grade envelope is in the cold rolled AluZinc Galvalume steel coil specification.

Spangle is a buying decision rather than a defect. Regular spangle gives the characteristic crystalline surface, minimum and zero spangle give a smoother appearance that paints better, and big spangle is chosen for visual effect. Surface treatment then decides how the coil survives storage and how paint keys to it: light oil, chromate passivation and anti-fingerprint film. Anti-fingerprint is the one to specify when the coil will be handled and stacked before painting, and it is the purpose of the AFP Galvalume steel range. Where the sheet is sold painted, the substrate runs through a coating line to become pre-painted galvanized aluminium-zinc coated steel coil, whose performance depends heavily on the passivation and primer pairing underneath.

From coil to roof

Most Al-Zn tonnage leaves the mill as roofing and cladding rather than flat sheet, and profile geometry sets coverage and stiffness before coating is discussed. Corrugated sheet Galvalume dimensions control how many linear metres a tonne yields and how far purlins can be spaced, while classic rib steel roof panel in Galvalume and Galvalume corrugated metal roof panels represent the two profile families most projects choose between. Where ventilation or screening is needed, perforated corrugated Galvalume panels come off the same coil without a separate coating step, and painted flat material is stocked as Galvalume steel roofing sheet or, when a specific colour and warranty are required, as prepainted Galvalume corrugated roof.

Coil weights on this product usually run 3 to 5 tonnes, which keeps handling inside a standard forklift and lets a roll former take a full coil without a lifting frame. Orders are normally quoted at 25 tonnes or one container, with lighter quantities priced individually, and the governing standards are JIS G3302 and JIS G3312 on the Japanese route and ASTM A653M with A924M on the American one.

The programme, covering bare coil, anti-fingerprint and prepainted material, is listed under 55% galvalume steel and Galvalume steel coil. On a quotation request, state the AZ coating mass, substrate grade, spangle, surface treatment, thickness and slit or mill width in that order; those six lines move the price far more than tonnage does.