Q:What are the requirements of national and American standards for zinc spangles on galvanized steel surfaces?
A:The national standard GB/T 2518-2019 divides zinc spangles into normal zinc spangles, small zinc spangles and zero zinc spangles. Zero zinc spangles are required to be invisible to the naked eye and are achieved by controlling the composition of zinc liquid or the cooling process.
The American standard ASTM A653 is also divided into three categories, but the definition of zero zinc spangles is stricter. It requires a special process to completely suppress the generation of zinc spangles, which is suitable for high-end fields such as automotive exterior panels.
Q:What are the differences in zinc flower size and uniformity requirements?
A:The American standard ASTM A526 clearly classifies the size of zinc spangles. For example, the diameter of small zinc spangles is required to be ≤2mm, while the national standard only describes it as "fine" and lacks quantitative indicators. In addition, the American standard has more detailed requirements for the uniformity of zinc spangle distribution. For example, ASTM A653 stipulates that there should be no local aggregation of zinc particles on the surface of zero zinc spangles, while the national standard only requires "uniform and continuous".
Q:How do the requirements for tolerable surface defects differ?
A:National Standard: GB/T 2518-2019 requires the surface to be smooth and clean, without obvious zinc particles, zinc slag, scratches, cracks and other defects, but does not specify the specific size or quantity limit of defects.
American Standard: ASTM A526 has quantitative standards for surface defects, such as the area of a single black spot ≤ 2mm², and no more than 3 per square meter; the scratch depth shall not exceed 1/3 of the coating thickness, and the length shall be ≤ 50mm.
Q:How do surface cleanliness requirements differ?
A:The American standard ASTM A123 requires that passivation treatment (such as chromate or chromium-free passivation) must be performed after galvanizing to enhance corrosion resistance and prevent white rust.
The national standard GB/T 2518-2019 allows passivation, but does not require it. It only recommends the use of trivalent chromium or chromium-free passivation in some high-end applications (such as automobiles).
Q:How do the roughness requirements differ?
A:The national standard GB/T 2518-2019 adds new surface roughness regulations, but does not specify specific parameters, only requiring "meeting use requirements".
The American standard ASTM A526 specifies specific roughness ranges for different uses (such as stamping and painting). For example, the automobile outer panel requires Ra≤1.6μm to ensure coating adhesion.