1.How to inspect the quality of galvanized layer through appearance?
Normal Appearance: High-quality products should exhibit uniform "zinc spangles" (a unique feature of the hot-dip galvanizing process, characterized by blocky or leaf-like crystal patterns) on the surface. The color should be bright silver-white or slightly bluish, with no localized blackening, yellowing, or darkening.
Defect Inspection: Be wary of three types of issues:
Immediate galvanizing/Exposed iron: Black or gray spots, such as dots or stripes, appear on the surface (exposing the cold-rolled baseplate). These areas will rust first, directly damaging the corrosion resistance.
Zinc peeling/Blistering: Gently scratch the surface. If the zinc layer peels or bubbles, it indicates a weak bond between the zinc layer and the baseplate (mostly due to incomplete baseplate rust removal or abnormal galvanizing temperature).
Black spots/White rust: Irregular black spots (excessive zinc oxidation) or white powder (zinc hydroxide formed when the zinc layer is exposed to moisture) appear on the surface. The former affects corrosion resistance, while the latter, if the base color remains uniform after wiping, can be alleviated by drying. However, if the zinc layer peels, it indicates poor quality.
2.How to check the thickness of the galvanized layer?
Standard Requirements: According to GB/T 2518 "Continuously Hot-Dip Galvanized and Zinc Alloy-Coated Steel Sheet and Strip," the zinc coating thickness of cold-rolled galvanized corrugated sheet for outdoor use must be ≥80g/㎡ (double-sided, approximately 11μm). For high-corrosion applications (such as coastal areas and chemical industrial zones), the thickness must be ≥120g/㎡ (approximately 17μm).
Testing Method:
For professional applications: Use a zinc coating thickness gauge (magnetic or eddy current type) to measure the sheet thickness at different locations (at least 5 points, including corners) and take the average value. If the average value is below the standard, the sheet is considered unqualified.
Simple Judgment: For sheets of the same thickness, high-quality galvanized sheet will feel thicker (with a higher zinc coating content). Alternatively, if the surface is lightly scratched with a hard object, high-quality products will only reveal a very fine layer of base color, and the zinc coating will not easily fall off. (Inferior products will easily reveal iron and the zinc coating will fall off easily.)
3.How to test the adhesion of galvanized layer?
Bend Test: Take a small sample and bend it 180° (folded in half) by hand or with a tool. Observe the zinc coating at the bend for cracks or peeling. High-quality products will show no visible cracks in the zinc coating after bending, or only very fine "hairline" cracks, with no zinc peeling. If bending directly exposes the metal or the zinc coating flakes off, adhesion is poor.
Scratch Test: Use a utility knife to make a cross-shaped cut on the surface (1mm square, scratching through the zinc coating to the substrate). Apply 3M tape and quickly remove it. If no zinc residue remains on the tape, adhesion is acceptable.
4.How to inspect substrate and molding quality?
Baseboard Thickness: No "cutting corners"
Measurement Method: Use a micrometer (accuracy 0.01mm) to measure on a flat surface of the sheet (avoiding raised corrugations). Note: The marked thickness is the total thickness of the "baseboard + zinc layer." Verify that the baseboard thickness meets the agreed-upon specifications (e.g., "0.5mm cold-rolled galvanized sheet"). The baseboard thickness should be ≥ 0.48mm, and the zinc layer should be approximately 0.02mm).
Tip: Be wary of discrepancies between the "nominal thickness" and "actual thickness"-low-quality products may be marked as 0.5mm, but the actual baseboard thickness is only 0.4mm. The zinc layer is also thin, significantly reducing overall strength and making it susceptible to denting and fracture under prolonged stress.
Corrugated Forming: Regular, no deformation
Appearance Inspection: Observe the uniformity of the corrugation height and wavelength (e.g., the common 820 corrugated sheet should have a height of 20mm and a wavelength of 80mm). Ensure there are no obvious "skewed waves," "collapsed waves" (localized lowering of the corrugation height), or "warping" (uneven edges). Flatness Test: Lay the sheet flat on the ground. High-quality products should have no noticeable warping. Press the raised corrugated surface with your hand. It should feel firm and have no noticeable springback (excessive springback indicates insufficient forming pressure and poor structural strength). If there is a dent or deformation after pressing, it indicates that the substrate is too soft or the forming process is poor.
Welding/Jointing Performance: Adapt to installation requirements.
If on-site welding is required, sample testing can be performed. During welding, the zinc layer should not spatter significantly, and the weld should be free of cracks or pores. After cooling, tap the weld with a hammer. If there is no visible cracking or large-scale zinc shedding, the substrate is considered acceptable for weldability. (Poor-quality substrates may contain excessive impurities and be prone to cracking during welding.)
5.How to detect surface quality and dimensional accuracy?
Surface Condition: No damage or contamination.
In addition to defects in the galvanized coating, the following inspections are required:
No scratches or pits: The surface should be free of scratches deeper than 0.1mm (which can easily cause water accumulation and rust), and no pits larger than 5mm in diameter (which can affect appearance and strength).
No oil stains or impurities: The surface should be clean, free of visible oil stains, dust, or zinc residue. (Oil stains can affect the adhesion of subsequent pre-coating or painting, and impurities can cause localized corrosion.)
Dimensional Accuracy: Suitable for installation.
Length/Width: Measured with a tape measure, the deviation should be ≤ ±5mm (e.g., a 6m length should be 5.995-6.005m). Excessive deviation can create gaps during on-site assembly, increasing the risk of leaks.
Diagonal Deviation: Take a rectangular sheet and measure the length of both diagonals. For high-quality products, the deviation should be ≤3mm. (Excessive deviation indicates the sheet is out of shape, making alignment difficult during installation.)