What AZ150 Actually Means
AZ150 is a coating designation, not a steel grade. The letters identify the coating type - aluminium-zinc alloy - and the number identifies the coating mass in grams per square metre, measured as the total on both surfaces. AZ150 therefore carries 150 g/m2 in total, which is approximately 75 g/m2 per side.
Converting that to thickness requires the coating density. A 55 % aluminium-zinc alloy has a density of about 3.7 g/cm3, so 75 g/m2 per side corresponds to roughly 20 µm of coating on each face. That thickness figure, rather than the mass figure, is what determines barrier performance in a corrosive environment, which is why buyers should compare coating designations on a mass basis and not on an assumed thickness.
The coating itself is an alloy of approximately 55 % aluminium, 43.4 % zinc and 1.6 % silicon by weight. The silicon addition is not decorative: it controls the growth of the brittle intermetallic layer between the coating and the steel substrate, which preserves the formability of the finished product.
Specifications and Substrate Grades
Product supplied as 55 % aluminium-zinc coated steel sheet is specified to ASTM A792/A792M for the coating and product requirements, with ASTM A924/A924M providing the general requirements for steel sheet. The European equivalent is EN 10346, which designates such material with an AZ coating suffix, and the Japanese equivalent is JIS G3321. The domestic equivalent route is GB/T 14978 for continuously hot-dip aluminium-zinc alloy coated steel sheet and strip.
| System | Standard | Typical structural designations |
|---|---|---|
| ASTM | A792/A792M, A924/A924M | Grades 33 to 80 by minimum yield strength |
| EN | EN 10346 | S250GD+AZ, S350GD+AZ, S550GD+AZ |
| JIS | JIS G3321 | SGLCC, SGLCD, SGLCH |
| Domestic | GB/T 14978 | Corresponding commercial and structural grades |
Substrate selection follows the application. Commercial forming grades are used for roofing and cladding profiles, while structural grades with minimum yield strengths of 250, 350 or 550 MPa are used for purlins, decking, load-bearing panels and frames. The highest-strength substrate is normally supplied in the lightest gauges, where the profile geometry rather than the material thickness provides the stiffness.
How the Coating Performs
The 55 % aluminium-zinc coating is deliberately a compromise between two mechanisms. Aluminium provides a stable barrier oxide that resists atmospheric attack, and the zinc fraction provides sacrificial protection at cut edges, scratches and fastener holes, where the steel would otherwise be exposed.
Atmospheric corrosion life: in the same coating mass, aluminium-zinc coatings are widely reported to outperform conventional zinc coatings by a factor of two to four in industrial, rural and marine atmospheres, largely because of the barrier effect of the aluminium-rich oxide.
Heat reflectivity: the coating reflects roughly 75 to 80 % of incident solar radiation, which is the reason it is used for roofing on buildings with cooling loads and for appliance components.
High-temperature oxidation: aluminium-zinc coated steel resists discolouration and oxidation at temperatures up to about 315 °C in continuous service, against roughly 230 °C for conventional zinc coatings, so it suits exhaust-adjacent panels, oven casings and heat shields.
Cut-edge protection: the zinc phase protects exposed edges, but edge creep resistance is inferior to a pure zinc coating in some aggressive environments, so cut edges should be protected in long-life installations.
Formability: the coating withstands roll forming and bending to normal radii without flaking, provided the silicon content is controlled and the forming is done at moderate speed.
Product Range, Delivery Condition and Processing
Coils are commonly supplied in thickness from 0.20 mm to 1.5 mm, width from 600 mm to 1500 mm, with 508 mm or 610 mm inside diameter and coil weights typically between 3 t and 15 t. Surface condition is defined by spangle - regular or minimised - and by surface treatment.
Available surface treatments include chromate-free passivation, anti-fingerprint treatment, and light oiling, each selected according to the downstream process. Anti-fingerprint treatment is specified when panels will be handled and visible before painting; passivation is specified when the surface will be painted or adhesively bonded and must remain chemically clean.
Fabrication follows established rules for coated sheet. Bending radii should be generous enough to avoid coating cracking on the outside of the bend, roll forming should be set with smooth, clean rolls, and shear and punch tools should be sharp so that the cut edge is clean rather than torn. Where welding is required, the coating must be removed from the weld zone and the joint restored with a zinc-rich repair coating afterwards, because the aluminium-zinc coating does not permit sound fusion welding as supplied. Fasteners and fixings should be selected for compatibility with the coating so that galvanic corrosion does not develop at the connection.
Inspection, Tolerances and Ordering Information
Coating mass is verified by a standard test method such as ASTM A754 for coating weight by X-ray fluorescence, or by gravimetric stripping methods, and coating thickness can be checked non-destructively by magnetic or eddy current gauges to ASTM E376. Adhesion is assessed by bend and impact tests, and corrosion performance by salt spray testing to ASTM B117 for comparative purposes only, since salt spray ranking does not reproduce real atmospheric exposure.
Dimensional inspection covers thickness across the width, width and camber, flatness and coil build. Mechanical properties are verified by tensile testing per ASTM E8/E8M, and the specification should state the required minimum yield strength and the tensile range, because some structural grades are supplied with a maximum as well as a minimum.
A complete order description includes the product standard, the substrate grade, the AZ coating designation, the surface treatment, the spangle type, thickness, width, coil inside diameter, coil weight and the required test certificate. That last item matters more than it appears: a 55 % aluminium-zinc product supplied without a coating mass result is impossible to verify downstream, and an underweight coating is the most common way that corrosion performance falls short of the design assumption.
Frequently Asked Questions
Q: What does AZ150 mean on a steel coil?
A: It is a 55 % aluminium-zinc alloy coating with a total coating mass of 150 g/m2 across both surfaces, equivalent to about 75 g/m2 per side or roughly 20 µm of coating per face.
Q: Which standard covers AZ150 coated steel?
A: ASTM A792/A792M with general requirements in ASTM A924/A924M, EN 10346 with the AZ suffix, JIS G3321, and GB/T 14978 for continuously hot-dip aluminium-zinc alloy coated sheet and strip.
Q: How does 55 % Al-Zn compare with ordinary galvanised coating?
A: In the same coating mass it typically gives two to four times the atmospheric corrosion life, because the aluminium-rich oxide acts as a barrier, and it tolerates continuous service temperatures up to about 315 °C against about 230 °C for zinc.
Q: Can AZ150 coil be welded?
A: Not as supplied. The coating must be removed from the weld zone, the joint welded, and the coating restored with a zinc-rich repair product, because aluminium-zinc coatings interfere with sound fusion welding.
Q: How is the coating mass verified?
A: By X-ray fluorescence to ASTM A754 or by gravimetric stripping, with thickness verified non-destructively to ASTM E376. Adhesion is checked by bend and impact tests.
Q: What coil sizes are available?
A: Thickness typically 0.20 mm to 1.5 mm, width 600 mm to 1500 mm, coil inside diameter 508 mm or 610 mm, and coil weight between about 3 t and 15 t depending on the line.
