Steel coil buyers are frequently offered three visually similar but technically different coated products: hot-dip galvanized coil (pure zinc), 55% aluminum-zinc alloy coated coil (aluminium-zinc), and aluminum(-silicon) coated coil. Although all three are produced on continuous hot-dip coating lines, their corrosion behavior, heat resistance and edge protection differ fundamentally. This article compares the three systems on a like-for-like basis so that specification engineers can select the correct coating for the service environment.
1. Three Coating Systems at a Glance
The substrate of all three products is a cold-rolled steel strip, usually DC01-DC04 grade per EN 10130, that is cleaned, annealed and passed through a molten metal bath at 450-660 °C depending on the coating alloy. The table below summarizes the defining characteristics.
| Coating system | Typical coating composition | Governing standard | Common coating mass | Primary protection mode |
|---|---|---|---|---|
| Hot-dip galvanized (+Z) | Pure zinc (99%+ Zn) | ASTM A653, EN 10346, JIS G3302 | Z100-Z600 (EN 10346); G30-G90 (ASTM A653) | Cathodic (galvanic) + barrier |
| Aluminum-zinc (+AZ) | 55% Al, 43.4% Zn, 1.6% Si | ASTM A792, EN 10346, JIS G3321 | AZ30-AZ185 (ASTM A792), typically AZ150 | Barrier + limited cathodic |
| Aluminum-coated (+AS) | Al + 8-11% Si (Type 1); pure Al (Type 2) | ASTM A463 | Type 1: 20-80 g/m² per side | Barrier only (no cathodic) |
2. Corrosion Protection Mechanisms Compared
Pure zinc coatings protect steel in two ways: they act as a physical barrier, and they corrode preferentially when the coating is scratched or cut, a behavior called cathodic protection. Even if a cut edge or scratch exposes bare steel, the surrounding zinc continues to protect it, which is why galvanized coil is forgiving at sheared edges and drilled holes.
Aluminum-zinc (55% Al-Zn-Si) coatings rely mainly on a dense, stable oxide film for barrier protection. Because the coating contains zinc-rich phases, a limited galvanic effect exists, but edge protection is weaker than pure zinc. However, the aluminum-rich oxide film slows lateral corrosion at cut edges (edge creep), so AZ-coated sheet generally outperforms galvanized sheet in long-term atmospheric exposure, particularly in coastal and industrial environments.
Aluminum-silicon coatings provide barrier protection only; once damaged, the steel substrate rusts because no sacrificial action occurs. They are therefore chosen for heat resistance rather than atmospheric corrosion resistance.
3. High-Temperature Performance
Heat resistance ranking is the reverse of cut-edge protection. Pure zinc begins to oxidize rapidly above roughly 225 °C and melts at 419.5 °C, so galvanized coil is not recommended for sustained service above about 200 °C. Aluminum-zinc coatings tolerate continuous service up to approximately 315 °C. Aluminum-silicon coatings are the most heat-stable of the three and are used for continuous service up to about 480 °C, which is why they dominate automotive exhaust and heat-exchanger applications.
4. Application Guidance by Industry
Construction: aluminum-zinc coated sheet is widely used for roofing, wall cladding, awnings and gutters where long-term atmospheric performance and cut-edge stability matter; galvanized sheet remains the economical choice for structural framing, purlins and ventilation ductwork.
Home appliances: galvanized coil is used for washing machine liners, air conditioner casings and refrigerator components where forming and spot welding are required; aluminum-zinc sheet suits oven housings and heat shields thanks to its reflectivity and moderate heat resistance.
Automotive and HVAC: aluminum-silicon coated steel is specified for mufflers, exhaust pipes and heat exchangers per ASTM A463; galvanized and zinc-aluminum coated steels are used for brackets and underbody parts requiring weldability and corrosion resistance.
Frequently Asked Questions
Which coating gives the best protection at cut edges? Hot-dip galvanized (pure zinc). Its cathodic action continues to protect exposed steel at sheared edges, drilled holes and scratches.
Is AZ150 (aluminum-zinc) equivalent in thickness to Z275 (galvanized)? Roughly yes. Because the 55% Al-Zn alloy is about half the density of pure zinc, AZ150 (about 19-20 µm per side) and Z275 (about 19 µm per side) have similar coating thickness but different protection mechanisms.
Which product resists heat best? aluminum-silicon coated coil per ASTM A463, suitable for continuous service up to roughly 480 °C, followed by aluminum-zinc (about 315 °C) and galvanized (about 200 °C).
Why does aluminum-zinc outperform galvanized in atmospheric exposure? Its aluminum-rich oxide film is denser and more stable, and it suppresses lateral corrosion at cut edges, giving longer service life in coastal and industrial atmospheres despite weaker cathodic action.
Can aluminum-coated coil be used outdoors in coastal areas? Not recommended for bare atmospheric service. Without cathodic protection, any scratch or cut edge will corrode quickly; it is chosen mainly for heat resistance.
Which standard should I specify for 55% Al-Zn coated coil? ASTM A792 (AZ classes), EN 10346 (+AZ) or JIS G3321 (SGLCC/SGLCD/SGLCE), depending on the destination market and the required coating mass.