What is the standard method for testing zinc coating adhesion? How is compliance determined?
Commonly used methods include the "bend test" and "cross-cut test":
Bend test: Bend the galvanized sheet 180° around a cylindrical axis of a specified diameter (as per GB/T 2518). A pass is obtained if the zinc coating shows no peeling or cracking after bending.
Cross-cut test: Use a scribing knife to scratch a 1mm x 1mm grid (100 small grids) on the surface. Apply tape and quickly remove it. A pass is obtained if ≤5 small grids are removed (applicable to galvanized sheets that will be subsequently painted).
12. What causes "missing plating" on galvanized sheets? What are the potential hazards?
Missing plating refers to areas on the substrate surface where the zinc layer is not applied, revealing the original steel color. Causes include:
Oil and scale on the substrate surface have not been cleaned (incomplete pre-treatment);
Excessive aluminum content in the zinc bath (resulting in poor zinc bath fluidity);
Lower temperatures in certain areas of the substrate during galvanizing, preventing the zinc bath from fully adhering.
Hazards: Missing plating areas, unprotected by the zinc layer, will rust first, accelerating corrosion of the surrounding zinc layer (the "small anode effect"), seriously impacting the overall service life.
13. What causes "black spots" on the surface of galvanized sheet metal? Do they affect performance?
Black spots typically have two causes, each with different effects:
Zinc oxidation: Storage in a humid environment causes zinc oxide (dark gray spots) to form on the zinc surface. These spots do not shed powder and do not affect corrosion resistance (the oxide layer protects the internal zinc layer). They can be removed by wiping.
Substrate rust: Damage to the zinc layer causes rust on the substrate, forming reddish-brown spots accompanied by flaking powder. This indicates substrate corrosion and requires immediate attention (such as repainting). Otherwise, corrosion will spread and affect performance. 14. What is the weldability of galvanized sheet? What precautions should be taken during welding?
The weldability of different types of galvanized sheet varies significantly:
GI (Galvanized Sheet) has poor weldability: During welding, the zinc layer evaporates and produces zinc vapor, which can easily form pores and spatter. It can also cause "zinc explosion" (a sudden expansion of zinc vapor that causes cracking in the weld). Low current and fast welding speeds are required, or the zinc layer in the weld area must be removed in advance.
GA (Galvanized Sheet) has excellent weldability: The zinc layer is an alloy structure, preventing pure zinc evaporation. The weld quality is high and no special treatment is required. It is suitable for continuous welding of automobile bodies.
EG (Galvanized Sheet) has moderate weldability: The zinc layer is thin and has few pores, but care must be taken to prevent oxidation of the zinc layer in the heat-affected zone.
15. What factors determine the stamping formability of galvanized sheet? Stamping formability (e.g., stretching, bending, and deep drawing) depends primarily on three factors:
Mechanical properties of the substrate: The higher the elongation and the lower the yield strength, the better the formability (e.g., a DC51D substrate with an elongation ≥30% is suitable for shallow drawing; a DC53D substrate with an elongation ≥34% is suitable for deep drawing);
Toughness of the zinc layer: GI zinc layers are tougher than EG, while GA zinc layers have moderate toughness. Zinc layers with poor toughness are prone to cracking during stamping;
Surface lubricity: A surface free of oil and scratches, or application of a stamping lubricant, can reduce die wear and prevent zinc layer flaking.