1.How to test and judge the salt spray test?
Test Standards
Performed in accordance with GB/T 10125 "Artificial Atmosphere Corrosion Tests - Salt Spray Test" or ISO 9227, using a neutral salt spray (NSS): 5% sodium chloride solution (pH 6.5-7.2), temperature 35°C, and salt spray deposition rate of 1-2 mL/h/80 cm².
Testing Procedure and Evaluation: ◦ The specimens are exposed for 24, 48, or 96 hours (depending on the application scenario). Observe the surface corrosion products: ◦ White rust: Zinc oxide/zinc hydroxide (a white, loose substance) formed by corrosion of the zinc layer, a sign of early corrosion. ◦ Red rust: Iron oxide (reddish-brown) formed by corrosion of the substrate (steel plate), indicating that the zinc layer has penetrated and lost its protective properties. ◦ Acceptance Criteria (using common SECC as an example): ◦ SECC with a chromate passivation layer: No white rust should appear within 48 hours; within 96 hours, the white rust area should be ≤5%, and no red rust should be present. Fingerprint-resistant SECC (SECC-N): Due to the fingerprint-resistant coating on the surface, it offers superior corrosion resistance. Typically, white rust is eliminated within 96 hours, and the white rust area is ≤10% within 168 hours. If red rust appears prematurely (e.g., within 24 hours), it indicates that the zinc layer is too thin, plating is missing, or the passivation layer has failed, resulting in extremely poor corrosion resistance.
2.What are the experimental conditions and standards for wet heat testing?
Test conditions: Temperature 40°C ± 2°C, relative humidity 95% ± 3%, for 72 hours or 168 hours (depending on the application).
Judgment Criteria:
High-quality SECC surfaces should be free of white rust, mildew, or blistering of the zinc layer.
Localized accumulation of white rust indicates defects in the zinc layer or passivation layer (e.g., pinholes or scratches).
If the zinc layer is also detached, poor adhesion may be the cause of protective layer failure.
3.How to perform the fingerprint resistance test?
Fingerprint Residue and Corrosion Test: After pressing the surface with a sweaty finger and placing it in a 40°C hot and humid environment for 24 hours, high-quality SECC-N should show no corrosion (white rust) at the fingerprint site, and fingerprints can be removed with an alcohol wipe.
Solvent Resistance: Wipe the fingerprint-resistant film 100 times with alcohol or isopropyl alcohol. The film should not peel off, otherwise the protective effect will be lost, resulting in a decrease in corrosion resistance.
4.How to perform the drip and immersion test?
Copper Sulfate Spot Test:
A 10% copper sulfate solution is dripped onto the surface of the SECC and the time it takes for the zinc layer to be displaced by copper (disappearing red) is observed.
High-quality SECC (zinc layer ≥ 16g/m² + passivation layer) should not turn red for more than 60 seconds.
If it turns red within 30 seconds, it indicates that the zinc layer is too thin or the passivation layer is damaged, resulting in poor corrosion resistance.
Lead Acetate Immersion Test:
The specimen is immersed in a 5% lead acetate solution and observed for the formation of black lead sulfide precipitate (a byproduct of zinc corrosion).
High-quality SECC should show no noticeable precipitation within 2 hours.
If black floccules quickly appear, the zinc layer is susceptible to corrosion.
5.How does surface condition assist in judgment?
SECC with poor corrosion resistance often exhibits surface defects, which can be observed visually or with a magnifying glass:
Passivation layer defects: The chromate passivation layer should be a uniform pale yellow. Rainbow-colored spots or exposed bottom layers (showing the original color of the zinc layer) indicate uneven passivation and localized weak corrosion resistance.
Pinholes and scratches: Small pinholes (from bubbles not removed during electroplating) or deep scratches that expose zinc can become the starting point of corrosion, leading to rapid localized rusting.