1.How to judge the quality of galvanized steel by appearance?
Coating Color and Gloss
High-quality galvanized steel: The surface exhibits a uniform silvery-white or light-gray metallic luster, without noticeable color variations, darkening, or localized yellowing (yellowing may be caused by excessive zinc oxidation or improper passivation).
Low-quality galvanized steel: The surface may exhibit mottling (localized zinc coating is too thick or too thin), black spots (zinc inclusions or localized corrosion), peeling (poor adhesion between the zinc layer and the substrate), or a dull luster (severe oxidation).
Surface Flatness and Defects
High-quality steel: The surface is smooth and flat, free of noticeable bumps, scratches, pitting (caused by uneven zinc deposition), bubbles (gas trapped in the substrate during galvanizing), or missing galvanizing (areas not covered with zinc, exhibiting the color of the bare steel and prone to rust).
Low-quality steel: Commonly found are "exposed bottom" (missing galvanizing), "zinc nodules" (protrusions caused by excessive zinc coating), and "scratches" (damage to the coating caused by processing or transportation, which are prone to corrosion).
2.How to detect coating thickness?
Non-professional method:
Comparing the feel: Galvanized steel with a thicker coating feels thicker and has less noticeable sharp edges (thin coatings tend to show through the substrate).
Magnet test: Zinc is a non-magnetic metal. The thicker the coating, the weaker the overall magnetism of the steel (comparisons must be made on the same substrate, as magnetism can vary between substrates).
Professional method:
Use a coating thickness gauge (such as an eddy current thickness gauge, suitable for non-magnetic coatings) and test according to standards (such as GB/T 13912-2022 "Hot-Dip Galvanized Steel Sheet and Strip"):
Hot-dip galvanized steel: Coating thickness ≥ 60g/㎡ (both sides) for normal environments; ≥ 80g/㎡ for harsh environments (such as marine environments and high humidity).
Electrogalvanized steel: Typically ≥ 10g/㎡ for home appliances and electronics, ≥ 20g/㎡ for automotive applications (specific thickness to be confirmed upon request).
3.How to test coating adhesion?
Simple Test:
Bend Test: Bend the sample repeatedly 180° (e.g., around a cylinder with a diameter twice the thickness of the steel). High-quality products will show no cracking or peeling of the coating after bending; only minor scratches are permitted. If the zinc layer peels or flakes at the bend, it indicates poor adhesion.
Scratch Test: Use a utility knife to make a cross-shaped cut (1mm square, scratching through the zinc layer to the substrate). Apply the tape and quickly remove it. High-quality tape will show no zinc residue. If the tape removes a large amount of zinc, it indicates unacceptable adhesion.
Professional Standard: Complies with GB/T 5270-2005 "Metallic Coatings on Metal Substrates - Electrodeposited and Chemically Deposited Coatings - Test Method for Adhesion Strength," with no peeling or cracking.
4.How to test corrosion resistance?
Simple Test:
Salt Spray Test (Home/Small Application): Place the sample above a container filled with salt water (5% sodium chloride solution) (avoid direct immersion). Seal the container and leave it for 24-48 hours. High-quality products should show no visible rust or white spots (normal for minor corrosion of the zinc layer, not rust on the substrate). If red rust (rust on the substrate) appears, it indicates coating failure.
Humid Environment Storage: Place the sample in a damp corner (such as a bathroom) for 1-2 weeks. High-quality products should show no rust spots, while low-quality products are prone to localized rust.
Professional Standard: According to GB/T 10125-2021 "Artificial Atmosphere Corrosion Tests - Salt Spray Test," under the neutral salt spray test (NSS), hot-dip galvanized steel generally needs to be free of red rust for 48 hours or longer, and electrogalvanized steel (with a passivation layer) needs to be free of red rust for 24 hours or longer.
5.How to check the surface quality of substrate?
If the substrate itself has cracks, impurities, or scars, these will "penetrate" the zinc layer after galvanizing, causing the coating to be locally weak. Observe the surface of the galvanized steel. If there are regular defects (such as continuous fine cracks), it may be a problem with the substrate.