SZACC is the JIS G3321 designation for 55% aluminum–43.4% zinc–1.6% silicon alloy-coated steel sheet — the product known internationally as Galvalume (Bethlehem Steel’s original trade name, now generic) or GL steel. The coating composition is not arbitrary: at 55% aluminum, the alloy-rich phase forms a dense, continuous aluminum oxide barrier that is far more protective than the porous zinc hydroxide layer that forms on pure-zinc SGCC. The 43.4% zinc provides galvanic (sacrificial) protection at cut edges and damaged areas where bare steel is exposed. The 1.6% silicon prevents the formation of a thick, brittle Fe-Al intermetallic layer at the steel-coating interface. The result is a coated steel with 2–4× the corrosion life of equivalent-weight hot-dip galvanized SGCC in most atmospheric environments — the specification of choice for long-life roofing, solar panel mounting, agricultural buildings, and any application where re-coating is difficult or costly.
| Property | SZACC (JIS G3321) |
|---|---|
| Standard | JIS G3321:2019 |
| Coating composition | 55% Al, 43.4% Zn, 1.6% Si (by mass) |
| ASTM equivalent | A792 (Galvalume / AZ-coated) |
| EN equivalent | EN 10346 +AZ (AZ100, AZ150, AZ185) |
| Coating weight (common) | AZ90, AZ150, AZ185 (g/m², both sides) |
| Corrosion life vs SGCC Z18 | ~2–4× longer in C3–C4 environments |
| Max service temperature | ~300°C continuous; SGCC limit ~200°C |
| Cut-edge protection | Partial (less than SGCC — aluminum passivates at cut edge, zinc protection only) |
- Why 55% Aluminum — The Coating Science
- JIS G3321 Grade Structure
- Coating Weight Designations (AZ Codes)
- Mechanical Properties
- Surface Conditions and Passivation
- ASTM A792 and EN 10346 Equivalents
- Corrosion Performance vs SGCC
- Limitations — When SZACC Is Not the Answer
- Common Mistakes
- Applications
- FAQ
1. Why 55% Aluminum — The Coating Science
The 55/43.4/1.6 Al/Zn/Si composition was developed by Bethlehem Steel in the late 1960s after systematic trials across the full Al-Zn binary composition range. The composition corresponds to a eutectic region in the Al-Zn phase diagram where the alloy solidifies with a two-phase microstructure:
- Aluminum-rich dendrites (~80% of coating volume): form a continuous barrier network. Al₂O₃ self-heals — when scratched, fresh aluminum oxidizes immediately in air, re-sealing the barrier. This is why the Al-rich phase provides far better barrier protection than pure zinc.
- Zinc-rich interdendritic phase (~20% of volume): positioned between the Al dendrites. At cut edges or coating damage, zinc dissolves preferentially (zinc is anodic to iron at pH >7), providing cathodic protection to exposed steel.
- 1.6% Si: prevents excessive growth of the brittle Fe₂Al₅ intermetallic at the steel-coating interface during bath immersion, maintaining coating adhesion and formability.
At compositions below ~30% Al, the zinc-rich phase dominates — more galvanic protection, less barrier protection, shorter life. Above ~65% Al, the zinc interdendritic phase disappears — maximum barrier protection but essentially no galvanic protection at cut edges. The 55% Al composition optimizes both mechanisms.
2. JIS G3321 Grade Structure
| Grade | Base Formability | Equivalent Uncoated |
|---|---|---|
| SZACC | General purpose | SPCC |
| SZACD1 | Drawing | SPCD |
| SZACD2 | Deep drawing | SPCE |
| SZACD3 | Extra deep drawing | SPCG (IF) |
SZACC is produced on the same continuous hot-dip lines as SGCC — the base steel passes through an Al-Zn-Si bath at ~600°C (higher than the Zn-only bath). The annealing effect during the line process is similar to SGCC; the base steel arrives in a fully annealed, soft condition.
3. Coating Weight Designations (AZ Codes)
JIS G3321 uses AZ## designations where ## is the minimum total coating weight (both sides) in g/m²:
| JIS Code | Min Total (g/m²) | Each Side (g/m²) | Coating Thickness Per Side (μm) |
|---|---|---|---|
| AZ90 | 90 | ≥27 | ~13 μm (Al-Zn density ~3.7 g/cm³) |
| AZ150 | 150 | ≥45 | ~22 μm |
| AZ185 | 185 | ≥56 | ~27 μm |
AZ150 is the most common commercial specification. Despite being lighter than SGCC Z18 (180 g/m²) on a mass basis, AZ150 provides ~2–4× longer corrosion life due to the superior barrier mechanism of the aluminum-rich phase.
4. Mechanical Properties
| Property | SZACC (JIS G3321) |
|---|---|
| Tensile strength | Not specified (base steel; typically 270–380 MPa) |
| Elongation (t ≥ 1.0 mm) | ≥ 30% |
| Minimum bend radius | Slightly larger than SGCC for same thickness (Al-Zn coating more brittle than pure Zn) |
The Al-Zn-Si coating is harder and less ductile than pure zinc. At tight bend radii, the coating can microcrack (crazing). For roll-formed profiles (roofing, wall panels), minimum bend radius recommendations from the manufacturer must be followed — typically 3–5× sheet thickness for AZ150. For deep-drawn parts, SZACD2 (SPCE base) provides better drawing depth before coating failure than SZACC.
5. Surface Conditions and Passivation
| Suffix | Meaning |
|---|---|
| (none) | Regular spangle — visible Al-Zn crystal pattern; characteristic spangled appearance |
| -MC | Minimized spangle (smooth surface for pre-painted/CGCC-equivalent) |
| -P (or -NC) | Non-chromate passivation; standard for RoHS-compliant supply |
| -O | Anti-rust oil applied |
SZACC is very commonly supplied pre-painted (as the base for CGLCC — pre-painted Galvalume, equivalent to CGCC for SGCC). Pre-painted Galvalume with PVDF topcoat is the dominant specification for long-life architectural roofing and wall cladding (50+ year design life in C3 environments).
6. ASTM A792 and EN 10346 Equivalents
| JIS G3321 | ASTM A792 | EN 10346 |
|---|---|---|
| SZACC AZ90 | CS AZM90 (90 g/m²) | DX51D+AZ100 |
| SZACC AZ150 | CS AZM150 (150 g/m²) | DX51D+AZ150 |
| SZACC AZ185 | CS AZM180 (180 g/m²) | DX51D+AZ185 |
ASTM A792 uses the same g/m² total coating weight system as JIS G3321. The trade name “Galvalume” is used for A792-compliant material by licensed producers worldwide (BlueScope, POSCO, Nippon Steel, etc.) — the chemistry is identical across licensed producers. “GL steel” or “AZ steel” are common generic terms for the same product.
7. Corrosion Performance vs SGCC
| Product | Coating | C3 Urban Life Estimate | C4 Industrial/Coastal Life |
|---|---|---|---|
| SGCC Z18 | 180 g/m² pure Zn | ~15–25 years | ~6–10 years |
| SZACC AZ150 | 150 g/m² Al-Zn | ~35–50 years | ~15–25 years |
| Pre-painted SZACC | AZ150 + PVDF paint | 50+ years | 30+ years |
Source: field exposure data (Bethlehem Steel, BlueScope, and independent studies). The life advantage of SZACC over SGCC is largest in urban and industrial environments where SO₂ and NOₓ are present — pollutants that rapidly attack the zinc hydroxide passive layer on pure zinc but have little effect on the aluminum oxide barrier on Galvalume.
8. Limitations — When SZACC Is Not the Answer
9. Common Mistakes
10. Applications
Industrial buildings, warehouses, agricultural facilities. Pre-painted SZACC with PVDF or SMP topcoat achieves 30–50 year design life. The standard product in Japan, Australia, and Southeast Asia for metal roofing where long maintenance-free life is required.
Ground-mounted and rooftop solar racking. SZACC AZ150 is the dominant specification for solar mounting rails and purlins in Japan — the 25–30 year system design life aligns with SZACC’s corrosion performance in C3 environments, and the aluminum-rich coating resists the acidic condensate that forms under solar panels.
Air handling units, ventilation ductwork, refrigeration enclosures. SZACC’s superior corrosion performance over SGCC at the same or lower coating weight reduces ductwork replacement costs. Maximum service temperature ~300°C allows use near heat exchangers where SGCC (limit ~200°C) would require painting or other protection.
11. FAQ
Q: Is Galvalume the same as galvanized steel?
No — “galvanized” conventionally refers to pure-zinc coated steel (SGCC). Galvalume / SZACC has a 55% aluminum–44% zinc–1% silicon alloy coating. The protection mechanisms, corrosion life, cut-edge behavior, alkaline resistance, and paintability all differ significantly. Specifying “galvanized or Galvalume” without distinguishing the two can lead to incorrect material selection, particularly for concrete-adjacent applications.
Q: Can SZACC be welded?
Yes, with similar precautions to SGCC. The Al-Zn fumes released during welding contain aluminum oxide — use adequate ventilation. Weld spatter removes the coating locally, creating bare steel spots with limited cut-edge protection. For structural welds, use zinc-rich primer to touch up welded areas after fabrication.
Summary
- SZACC (JIS G3321) — 55%Al/43.4%Zn/1.6%Si coated steel; “Galvalume” or “GL steel” internationally
- AZ coating codes: AZ90/AZ150/AZ185 = total g/m² both sides; AZ150 is standard commercial grade
- 2–4× longer corrosion life than SGCC Z18 in urban/industrial environments due to Al₂O₃ barrier mechanism
- ASTM A792 / EN 10346 +AZ — direct equivalents
- Critical limitation: alkali (concrete, mortar, pH >10) destroys aluminum passivation — SZACC corrodes faster than SGCC in alkaline contact; always isolate from concrete
- Cut-edge galvanic protection radius (~1–2 mm) is less than SGCC (~3–5 mm) — design for few cut edges or seal after cutting
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