Salt spray test is an accelerated corrosion resistance evaluation method in artificial atmosphere. It atomizes a certain concentration of salt water; then sprays it in a closed constant temperature box, and reflects the corrosion resistance of the tested sample by observing the change of the tested sample after being placed in the box for a period of time. It is an accelerated test Method, the salt concentration of chloride in the salt spray environment is several times or tens of times of the salt spray content in the general natural environment, which greatly increases the corrosion rate, and the time for obtaining the results is also greatly shortened by the salt spray test of the product.
There are four main types of salt spray tests: neutral salt spray test, acetic acid salt spray test, copper accelerated acetic acid salt spray test, and alternating salt spray test.
(1) Neutral salt spray test
Neutral salt spray test is the earliest and most widely used accelerated corrosion test method. It uses a 5% sodium chloride salt solution, and the pH value of the solution is adjusted to a neutral range as a solution for spraying. The test temperature is 35℃, and the sedimentation rate of the salt spray is required to be between 1～2ml/80cm².h. The neutral salt spray test is suitable for: metals and their alloys, metal coatings, conversion coatings, anodic oxide coatings, organic coatings on metal substrates.
(2) Acetic acid salt spray test
It is to add an appropriate amount of glacial acetic acid to the prepared salt solution to ensure that the pH value of the collected solution in the salt spray box is 3.1 to 3.3, the solution becomes acidic, and the finally formed salt spray becomes acidic. Its corrosion rate is about 3 times faster than the NSS test. If the pH value of the initially prepared solution is 3.0 to 3.1, the pH value of the collected solution is generally in the range of 3.1 to 3.3. The pH value should be measured with an acidity meter at 25°C ± 2°C, or a precision pH test paper with a measurement accuracy of not more than 0.1 can be used for daily testing. The pH of the solution can be adjusted with glacial acetic acid or sodium hydroxide. The acetic acid salt spray test is suitable for copper + nickel + chromium or nickel + chromium decorative coatings, and also for anodized films of aluminum.
(3) Copper accelerated acetic acid salt spray test
In the prepared salt solution, copper chloride was added at a concentration of 0.26 g/L ± 0.02 g/L, which strongly induced corrosion, and its corrosion rate was about 8 times that of the NSS test. The copper accelerated acetic acid salt spray test is suitable for copper + nickel + chromium or nickel + chromium decorative coatings, and also for anodized films of aluminum.
(4) Alternating salt spray test
It is a comprehensive salt spray test, which is actually a neutral salt spray test plus a constant damp heat test. It is mainly used for cavity-type complete machine products. Through the penetration of the tidal environment, salt spray corrosion occurs not only on the surface of the product, but also inside the product. It is to alternately convert the product under two environmental conditions of salt spray and damp heat, and finally check whether the electrical properties and mechanical properties of the whole product have changed.
When a product sample is tested in a natural environment, the corrosion time may take a year or even several years, while the test in an artificial simulated salt spray environment can obtain similar results in a few days or even a few hours.