1.What are the core differences in the uniformity standards for hot-dip, electroplating, and alloy galvanizing?
Hot-Dip Galvanized Coating
Core Uniformity Indicator: Based on the "zinc coating weight per unit area," focus on the minimum local thickness and the lower limit of the single-point value.
International Standards: ASTM A653/A653M stipulates that the three-point average of the coating weight must be ≥ the nominal value, and the single-point value must be no less than 80% of the nominal value (e.g., a G60 coating with a single-point value of ≥48g/m²). ISO 1461 requires a thickness deviation of ≤±10% for hot-dip galvanized coatings, and the minimum local thickness must be ≥85% of the nominal value.
Domestic Standards: GB/T 2518-2019 requires the three-point average of the coating weight to be ≥ the nominal value, and the single-point value to be ≥85% (e.g., a Z250 coating with a single-point value of ≥212.5g/m²).
Electro-Galvanized Sheet
Core Uniformity Indicator: Focusing on "coating thickness uniformity," it emphasizes the absolute value of thickness deviation and full surface coverage integrity.
International Standard: ASTM A525 requires that the thickness deviation of the electro-galvanized layer be ≤ ±10%, and the single-point thickness must not be less than 90% of the nominal value (e.g., the single-point value for an E10 coating must be ≥ 9μm). ISO 1461 has even stricter requirements for thickness uniformity of electro-galvanized layers (deviation ≤ ±5%).
Domestic Standard: GB/T 13912-2020 stipulates that the minimum local thickness of the electro-galvanized layer must be ≥ 5μm, with a thickness uniformity deviation coefficient ≤ 10%.
2.What are the requirements for coating uniformity in highly corrosive environments (marine, chemical)?
Standard Features: Emphasis on minimum local thickness and full surface coverage.
For galvanized steel sheets used in ships: ASTM A131 requires a uniformity deviation of the zinc layer thickness of ≤ ±8%, and the edge thickness must be ≥ 90% of the center thickness to prevent "edge corrosion."
For chemical equipment: ISO 1461 requires a minimum local thickness of the hot-dip galvanized layer of ≥ 90% of the nominal value, with uniformity verified by porosity testing (≤ 5 points/dm²).
3.What are the requirements for zinc layer uniformity for high appearance requirements (home appliances, automobile exterior panels)?
Standard Features: Focus on surface roughness and zinc spangle uniformity.
Home appliance housings: GB/T 2523-2020 stipulates that the surface roughness of electro-galvanized sheet metal must be Ra ≤ 1.6μm, and the zinc spangle size deviation must be ≤ ±10% (e.g., small zinc spangle diameter ≤ 2mm).
Automotive exterior panels: GM 9540P requires that the surface roughness of electro-galvanized sheet metal must be Ra ≤ 0.8μm, and the zinc layer thickness deviation must be ≤ ±5% to ensure color consistency after painting.
4.What are the requirements for coating uniformity for high processing demands (stamping, bending)?
Standard Features: Emphasizes zinc coating adhesion and ductility uniformity.
Galvanized sheet for deep drawing: ASTM A653 requires hot-dip galvanized sheet to pass a 0T bend test (bend radius = sheet thickness) without flaking, and the zinc layer thickness deviation must be ≤±10%.
Corrugated steel sheet for construction: GB/T 2518-2019 stipulates a zinc layer weight deviation of ≤±10%, and a cupping test (depth ≥9mm) is required to verify processability.
5.How to choose different detection methods for different processes?
Hot-dip galvanized sheet
Main methods: Magnetic thickness measurement (GB/T 4956) and weight per unit area method (ISO 1460), focusing on the three-point average and single-point minimum.
Auxiliary verification: The copper sulfate test (GB/T 26933) is used to detect missing plating or locally thin areas.
Electrogalvanized sheet
Main methods: X-ray fluorescence spectrometry (XRF) and metallographic microscopy, focusing on thickness deviation and microstructural uniformity.
Auxiliary verification: Bend test (GB/T 232) and tape peel test (ASTM D3359) are used to verify adhesion uniformity.
Alloyed galvanized sheet
Main methods: Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) are used to analyze alloy phase distribution, focusing on the Γ phase ratio and interface continuity.
Auxiliary verification: Salt spray test (GB/T 10125) and weathering test (ISO 9227) are used to verify corrosion resistance uniformity.