What 55% Aluminum-Zinc Alloy Coated Steel Really Is
55% aluminum-zinc alloy coated steel is a continuously hot-dip coated product made by passing cold rolled steel strip through a molten bath of roughly 55% aluminium, 43.4% zinc and 1.6% silicon. The coating is not a simple layered deposit: it solidifies into a fine two-phase structure of aluminium-rich dendrites surrounded by a zinc-rich inter-dendritic network. That microstructure explains why the product behaves differently from plain galvanized steel. The aluminium-rich phase forms a stable barrier against the atmosphere, while the zinc-rich phase continues to provide sacrificial protection at sheared edges, drilled holes and scratches.
The family is standardised in several regions. EN 10346 covers continuously hot-dip coated flat products with aluminium-zinc (AZ) coatings; ASTM A792/A792M describes the same group in North America, where coating weights are designated in ounces per square foot as AZ50, AZ55 and AZ60; JIS G3321 specifies the SGLCC series in Japan. Because the three documents do not use identical designations, buyers should state the required coating mass in grams per square metre, the substrate grade and the surface treatment when ordering.
Why It Suits Building Envelope Systems
Roofs, walls, eaves, soffits and concealed-fix cladding all carry the same duty: shed water, resist atmospheric attack and hold their geometry for decades. Aluminium-zinc coated steel meets that duty on four counts.
Corrosion resistance. In accelerated neutral salt-spray testing, aluminium-zinc coatings typically exceed 1,000 hours to red rust, compared with roughly 500 hours for a pure zinc coating of equal mass. Long-term field exposure in humid, acid-rain and coastal atmospheres is commonly reported as two to six times the service life of galvanized sheet.
Heat and ultraviolet response. The coating tolerates continuous service temperatures of about 315 °C, with short-term excursions to around 400 °C, so dark metal roofs that reach high surface temperatures do not suffer premature coating breakdown. The aluminium-rich surface also reflects a large share of incident radiation, which reduces the heat transmitted into the building.
Strength to weight ratio. The substrate is a low-carbon forming or structural grade, so the finished sheet stays light enough for economical purlin spacing while resisting wind uplift and occasional foot traffic during installation.
Fabricability. The coating survives slitting, roll forming and profiling, which allows one coil to become trapezoidal sheeting, standing-seam panels, liner trays, eaves flashings or ventilation louvres without extra surface preparation.
Typical Specification Window
| Parameter | Typical range |
|---|---|
| Coating composition | About 55% Al, 43.4% Zn, 1.6% Si |
| Substrate grades, EN 10346 | DX51D+AZ (forming), S250GD+AZ and S350GD+AZ (structural) |
| Coating mass | AZ100, AZ150, AZ185 (g/m² total, both faces) |
| Minimum yield strength | S250GD: 250 MPa; S350GD: 350 MPa |
| Thickness for cladding | Commonly 0.30 mm to 1.20 mm |
| Coil width | 600 mm to 1250 mm, slit to order |
| Continuous service temperature | Up to about 315 °C |
Coating mass is the single most important ordering decision. AZ100 is adequate for sheltered internal liners, AZ150 covers most roof and wall applications, and AZ185 is chosen where the envelope faces marine salt spray or industrial pollution. On the same coating mass, a thicker substrate raises the stiffness of the panel but does not by itself extend corrosion life, so the two parameters should be specified separately.
Aluminium-Zinc Coated Steel Compared with Galvanized and Stainless Sheet
| Criterion | 55% Al-Zn coated | Hot-dip galvanized | Stainless steel |
|---|---|---|---|
| Relative cost | Medium | Low | High |
| Expected life, general atmosphere | Approximately 20 to 30 years | Approximately 10 to 15 years | Over 30 years |
| Suitable exposure | Medium to high corrosivity | Low corrosivity, sheltered | Severe corrosivity |
| Cut-edge behaviour | Sacrificial protection | Sacrificial protection | Not applicable |
| Coating formability | Good for profiling | Good for profiling | Excellent |
For most industrial, commercial and residential envelopes, aluminium-zinc coated steel occupies the practical middle ground. It outlasts galvanized sheet wherever moisture, salt or pollutants are present, at a fraction of the cost of stainless steel, and it can be painted when a decorative finish is required.
Fabrication, Fixing and Detailing Rules
Choose a matt or lightly textured surface where panels are directly exposed, so that large roof areas do not create glare.
Where colour and a decorative finish matter, apply a coil coating such as PVDF or polyester over the aluminium-zinc substrate; this adds a further barrier layer and improves weather resistance.
Avoid long-term direct contact between the coated sheet and copper or bare stainless steel components, because trapped moisture can set up galvanic corrosion.
Treat sheared edges, drilled holes and site-cut ends with a suitable zinc-based repair paint, particularly on coastal and heavy industrial projects.
Use fasteners with a compatible coating system and isolate dissimilar metals with sealing washers or sealant.
Store coils and bundles under cover with free air movement, and keep them clear of standing water.
Where It Is Used
The material is specified for stadium and airport terminal roofs, factory and logistics warehouse cladding, shopping centre and exhibition hall curtain walls, and agricultural buildings. Outside construction it appears in appliance parts such as refrigerator back panels, air-conditioner housings and cooker bodies, and in automotive underbody panels where a light, corrosion-resistant sheet is required. In every case the selection logic is the same: match coating mass to the corrosivity of the site, match substrate strength to the span and wind load, then protect the cut edges.
Frequently Asked Questions
Q: What does the 55% in the coating name refer to?
It is the nominal aluminium content of the coating bath. The balance is about 43.4% zinc and 1.6% silicon, and the three elements together form the two-phase coating structure.
Q: Is aluminium-zinc coated steel better than hot-dip galvanized steel for roofing?
For roofs exposed to humidity, salt or industrial pollution, yes. The coating resists red rust roughly twice as long as galvanized steel of equal coating mass in accelerated tests, and field service life is generally longer in aggressive atmospheres.
Q: Which coating mass should be selected for coastal projects?
Specify AZ185 as a minimum for direct marine exposure, or AZ150 where the cladding is set back from the shoreline. Raised coating mass is more effective than extra substrate thickness for corrosion life.
Q: Can aluminium-zinc coated steel be painted?
Yes. It accepts coil-applied PVDF, polyester and other organic coatings, and it can also be painted on site after suitable pre-treatment. Painting adds colour and a further barrier layer but does not remove the need to protect cut edges.
Q: Does the coating survive high roof temperatures?
It withstands continuous service temperatures of about 315 °C and short-term peaks near 400 °C, which covers the surface temperatures reached by dark metal roofs under strong sun.
Q: Why do cut edges need attention?
Shearing exposes bare steel. The surrounding zinc-rich phase provides sacrificial protection, but the effect is limited, so a zinc-based repair paint at exposed edges extends service life appreciably on coastal and industrial projects.