1.What are the classification and principles of salt spray test?
Neutral salt spray test
Principle: 5% NaCl solution is atomized at 35℃ to form a salt spray environment to simulate neutral atmospheric corrosion.
Corrosion mechanism: Cl⁻ penetrates the pores of the coating and forms galvanic corrosion with the substrate, causing the coating to fall off or the substrate to rust.
Acetic acid salt spray test
Improvements: Add 3% acetic acid to the NSS solution to accelerate the corrosion rate and simulate the industrial acid rain environment.
Copper accelerated acetic acid salt spray test
Strengthening conditions: add 0.26g/L CuCl₂・2H₂O, the temperature rises to 50℃, the corrosion rate is 8 times that of NSS, suitable for high-end coatings.
2.What are the key factors that interfere with the experiment?
Uneven salt spray deposition: The difference in deposition in different areas of the chamber is greater than 50%, resulting in deviation in local corrosion rate (e.g., faster corrosion near the nozzle);
Sample placement direction: When placed vertically, the amount of salt spray deposition is 30% less than when placed at a 30° angle, and it must be placed strictly at the standard angle;
Temperature fluctuation: When the temperature in the chamber exceeds 35°C±1°C, the corrosion rate doubles for every 10°C increase (e.g., the NSS test is equivalent to 10 times acceleration at 45°C).
3.What are the quality control measures?
Calibrate the salt spray precipitation weekly (use at least 2 collectors, located ≥15cm from the box wall);
Place standard samples (such as hot-dip galvanized sheets with known corrosion resistance) in each batch of tests, and retest when the deviation is ＞10%;
The interval between samples is ≥2cm to avoid blocking each other's salt spray deposition.
4.What are the directions for coating optimization?
Alloying treatment: alloying annealing is performed after hot-dip galvanizing to transform the ζ phase into the δ phase, and the corrosion resistance is improved by 20%;
Pore sealing treatment: chromate passivation is performed after electrogalvanizing, the porosity is reduced from 5% to 1%, and the time for white rust to appear in the NSS test is extended from 100h to 300h.
5.How is the equivalence of the test to the actual environment?
Acceleration factor conversion: NSS test 1h ≈ 1 day in marine atmosphere (needs to be corrected according to Cl⁻ concentration);
Outdoor verification: The coating that passes the salt spray test needs to undergo a 1-year outdoor exposure test in typical corrosion areas such as Hainan and Qingdao, and the red rust area ≤5% is considered to be qualified.