The chemical composition of SECC (electrolytic chromate-coated steel) depends primarily on the composition of its substrate (cold-rolled steel) and the surface coating (zinc + chromate). The following is a typical chemical element analysis:
1. Base Material (Cold-Rolled Steel, such as SPCC/JIS G 3141)
The base material for SECC is typically low-carbon cold-rolled steel sheet (such as SPCC). Its chemical composition (mass percentage, wt%) is approximately as follows:
Element Content Range (wt%) Function
C (Carbon) 0.02-0.15% Affects strength and hardness; low carbon improves formability.
Mn (Manganese) 0.10-0.60% Increases strength and improves hot rolling properties.
P (Phosphorus) ≤0.03% Increases strength, but excessive amounts reduce toughness.
S (Sulfur) ≤0.03% Impurity element, affects weldability and stamping properties.
Si (Silicon) ≤0.10% Deoxidizer; small amounts are present.
Al (Aluminum) 0.02-0.07% Deoxidizes and refines grains.
Fe (Iron) Balance Main Base Components
Note: The chemical composition of different grades (e.g., SPCC, SPCE, and high-strength steel) varies; please refer to specific standards (e.g., JIS G 3141).
2. Electrolytic Zinc Coating
SECC is coated with a layer of pure zinc (Zn). The coating weight is typically 10-40 g/m² (per surface). Main composition:
Zn: ≥99.9%
May contain trace amounts of impurities such as Pb (lead) and Fe (iron) (depending on the galvanizing process).
3. Chromate Coating
SECC undergoes a chromate passivation treatment to enhance corrosion resistance. Typical composition:
Cr (Chromium): Exists as Cr³⁺ (trivalent chromium) or Cr⁶⁺ (hexavalent chromium, environmentally restricted).
May contain auxiliary ingredients such as phosphates (PO₄³⁻) and silicates (SiO₃²⁻).
Environmental trends: Modern SECCs often use chromium-free or trivalent chromium treatments to replace toxic hexavalent chromium (restricted by RoHS/REACH regulations).
4. Typical SECC Chemical Composition Example (Reference)
Element Base Material (SPCC) Galvanized Layer Chromate Layer
Fe ~98.5% <0.1% Trace
C 0.02-0.15% - -
Mn 0.10-0.60% - -
Zn - ≥99.9% -
Cr - - 0.1-1.0 mg/m² (as Cr)
5. Related Standards
JIS G 3313 (SECC)
JIS G 3141 (SPCC, Cold-Rolled Base Material)
ASTM A653 (Standard for Galvanized Steel Sheet)