1.What is Zinc-Aluminum-Magnesium Coated Steel?
Zinc-Aluminum-Magnesium coated steel, also known as ZAM-coated steel, is an innovative steel finish with superior corrosion resistance. Initially, pure zinc alloys were primarily used to enhance the corrosion resistance of metal surfaces. With continuous innovation in steel manufacturing technology, corrosion-resistant coated steels like zinc-aluminum alloys and ZAM alloys continue to emerge in the market, significantly improving the corrosion resistance of steel surfaces.
2. Composition and Classification of Zinc-Aluminum-Magnesium Coating
Zinc-Aluminum-Magnesium coated steel refers to a coated steel where a specific amount of Al and Mg is added to existing hot-dip galvanized coatings, or Mg is added to hot-dip galvanized coatings. It offers high corrosion resistance performance and excellent cut-edge protection.
A broad analysis of commercially available magnesium-containing coated steel shows that the Mg content in most coatings is ≤3%. Thus, based on aluminum content, zinc-aluminum-magnesium coatings are classified as follows:
Low-Aluminum Zn-Al-Mg Coating (Al: 1–3.5%):
Formed by adding aluminum-magnesium and other elements to a hot-dip galvanized base. This zinc aluminum coating has been tested on color coating lines, and low-Al Zn-Al-Mg steel sheets (known as BZM coated steel and its color-coated variants) have been successfully developed. It represents an enhanced corrosion-resistant version of pure hot-dip zinc coating.
Medium-Aluminum Zn-Al-Mg Coating (Al: 5–11%)
High-Aluminum Zn-Al-Mg Coating (Al: 55%):
Formed by adding magnesium and other elements to a hot-dip Al-Zn base. This coating has been tested on color coating lines, and high-Al Zn-Al-Mg color-coated steel sheets (known as BAM coated steel) have been successfully developed. It is an enhanced corrosion-resistant version of hot-dip Al-Zn coating.
Composition of the substrate
| Element | Content (%) | Function |
|---|---|---|
| C | ≤ 0.12 | Ensures ductility and weldability |
| Mn | 0.20-0.60 | Improves strength and hardenability |
| Si | ≤ 0.05 | Deoxidizes, reduces inclusions |
| P/S | ≤ 0.025 | Controls contaminants, prevents brittleness |
Mechanical properties of the substrate
| Tensile Strength | Yield Strength | Elongation | Hardness |
|---|---|---|---|
| 270~550 MPa | 140~350 MPa | 20%~40% | 60~120 HV |
Composition of coatings
| Elemental | content (%) | Function |
|---|---|---|
| Zn | 90-94 | Sacrificial anodic protection, dominant corrosion reaction |
| Al | 4-11 | Formation of oxide film, high-temperature resistance, and barrier protection |
| Mg | 0.2-3 | Promote self-healing, generate an Alkaline protective film |
| Si | 0.1-0.5 | Inhibits the overgrowth of Fe-Al alloy layer |
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3. Comparison Among GI (Galvanized), Aluzinc, and Zn-Al-Mg Coated Steels
Galvanized Coating (GI):
An economical anti-corrosion coating (Zn ≥99%).
Classified as spangled or non-spangled; both offer similar corrosion resistance.
Non-spangled versions provide better aesthetics for painted/post-sprayed applications.
Thick zinc layers typically feature spangles (e.g., 275g/m² coating weight).
Aluzinc Coating:
Alloy metal coating (55% Al, 43.5% Zn, 1.5% Si).
Corrosion resistance is 2–6× better than GI coatings of equivalent weight.
Surface can be modified to produce uniform spangles for construction/exterior use.
Disadvantages:
Inferior weldability compared to GI (welding should be minimized).
Worse cut-edge protection than both GI and Zn-Al-Mg coatings.
| Indicators | GI (Galvanized) | GL (Aluminum-Zinc Plated) | ZAM (Zinc-Aluminum-Magnesium) |
|---|---|---|---|
| Salt Spray Resistance (hrs) | 200-400 (Z120) | 600-1500 (AZ150) | 1500-3000 (ZM150) |
| Cutting Corrosion Resistance | Poor (susceptible to red rust) | Medium (Aluminum Retarded Corrosion) | Superior (Self-healing) |
| Process Hardness (HV) | 70-150 | 200-300 | 250-400 |
| Cost | Low | Medium | High |
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Zn-Al-Mg Coating:
Excellent all-round performance with marginally higher manufacturing costs than Aluzinc.
Significantly superior corrosion resistance and cut-edge protection.
Weldability comparable to GI steels.
Zn-Al-Mg color-coated steel sheets are now mass-produced and widely used.
How to Make Zinc aluminum-magnesium ZAM-coated Steel?
Zinc-aluminum-magnesium steel is a multistep process of coating a steel substrate with zinc, aluminum, and magnesium alloys, starting with the preparation of the steel sheet/coil, followed by hot-dip galvanizing of the aluminum-magnesium coated steel, and then cutting and shaping of the coated steel to finish the quality control.
Substrate pre-treatment: degreasing → pickling → auxiliary plating agent (zinc ammonium chloride) → drying.
Hot-dip plating: immersed in 430-450°C ZAM alloy bath (Zn-Al-Mg-Si).
Air Knife Control: Precise regulation of plating thickness (60-200g/m² on both sides).
Cooling and post-treatment: water cooling → chrome-free passivation → oiling or laminating.
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Future Outlook:
As material requirements in construction and industries increase, Zn-Al-Mg coated steel sheets are expected to gradually replace Aluzinc sheets in the market.
FAQ
1.What is the difference between Galvanized and Galvalume steel?
Galvalume has aluminum and magnesium in the coating, offering better corrosion resistance than pure galvanized steel.
2.What is the typical coating composition of Al-Zn-Mg steel?
Approximately 55% aluminum, 43.4% zinc, and 1.6% magnesium.
3.Can Galvalume steel be used in coastal areas?
Yes, it is ideal for coastal environments due to excellent corrosion resistance.
4.What thicknesses are available for Al-Zn-Mg steel coils?
Commonly 0.2mm to 2.0mm, depending on application.
5.Can Galvalume steel be painted?
Yes, it can be pre-painted or painted after fabrication.
6.Is Al-Zn-Mg steel suitable for roofing?
Yes, widely used for industrial, commercial, and residential roofing.
7.Can Galvalume steel be welded?
Yes, it can be welded, though surface coating may require special handling.
