1.How to detect the thickness of zinc layer?
Standard requirements: SECC zinc coating weight is typically 10-50g/m² (single-sided), corresponding to a thickness of 1.4-7.0μm (zinc density 7.14g/cm³). Common grades include E16 (16g/m², approximately 2.2μm) and E24 (24g/m², approximately 3.4μm).
Testing Method:
Use a magnetic thickness gauge (suitable for non-magnetic coatings on magnetic substrates) to measure at different locations on the specimen (at least five points, including the edges and center). The deviation should be ≤±10% (e.g., the acceptable range for E24 is 21.6-26.4g/m²).
Excessive deviation (e.g., locally <10g/m²) indicates uneven plating current distribution and localized weak corrosion protection.
2.How to judge the uniformity of zinc layer?
Cross-section observation: Observe the cross-section of the coating using a metallographic microscope. High-quality SECC zinc coatings should be continuous and uninterrupted, with minimal thickness variation. If there are areas of excessive thinness, missing plating (exposed substrate), or nodules (locally excessive thickness), the coating is unqualified.
Drop test: A 10% copper sulfate solution is dripped onto the surface. A high-quality coating should not turn red (copper precipitation) for more than 30 seconds, with minimal variation in reaction time between locations. Rapid reddening of a specific area indicates that the zinc coating is too thin.
3.How to perform zinc layer adhesion test?
Bend Test
Bend the specimen 180° (bend radius = plate thickness). Tape the bend and quickly remove the tape. High-quality SECC should exhibit no flaking of the zinc layer, and no visible zinc residue should remain on the tape surface. If the zinc layer flakes off, it indicates poor adhesion.
Scratch Test
Use a scribing knife to scratch 1mm x 1mm squares on the surface (reaching the substrate). Apply the tape and remove the tape. High-quality coatings should exhibit no zinc flakes within the squares. If the flakes exceed 5%, the adhesion fails.
Heat Shock Test
Bake the specimen at 100°C for 1 hour, then cool to room temperature. Repeat this three times. Observe the surface for blistering or peeling of the zinc layer. High-quality SECC should exhibit no noticeable changes.
4.How to evaluate surface condition?
Defect Inspection
Visual inspection or observation with a 10x magnifying glass indicates that high-quality SECC surfaces should be free of the following defects:
Misplating (localized absence of zinc, revealing the base material color), pinholes (small holes), scratches (deep exposure of the base material), and oxidation spots (gray-black spots).
Zinc nodules (localized raised zinc layer) and burrs (extension of the zinc layer at the edge) can cause stress concentration during processing or uneven coating.
Roughness and Gloss
Using a roughness meter, the Ra of the SECC's shiny surface should be ≤0.8μm, and the Ra of the matte surface should be ≤1.6μm. Extensive fluctuations in roughness indicate poor contact between the plating drums or uneven current flow.
Using a gloss meter, the gloss deviation of samples within the same batch should be ≤5% to avoid areas of dark or excessive brightness.
5.How does the corrosion resistance test work?
Salt Spray Test
According to GB/T 10125, a neutral salt spray test is conducted in a 5% NaCl solution at 35°C.
High-quality SECC (with a passivation layer) should show no white rust (zinc corrosion products) within 48 hours, and the white rust area should be ≤5% within 96 hours.
If large areas of white rust or red rust (base material corrosion) appear within a short period of time, it indicates that the passivation layer has failed or the zinc layer is too thin.
Fingerprint Resistance
High-quality fingerprint-resistant SECC (SECC-N) should show no visible fingerprint residue after pressing the surface with a finger, or completely remove it with alcohol. If fingerprints are difficult to remove, the fingerprint-resistant film is of poor quality.
Damp Heat Test
After 72 hours at 40°C and 95% relative humidity, the surface should show no white rust or mildew. Otherwise, the zinc layer or passivation layer should be inadequately protected.