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Classification and Analysis of Three Salt Spray Tests (NSS, AASS, CASS)
Salt spray corrosion test chambers are environmental test chambers that primarily use artificial salt spray environments created by salt spray test equipment to assess the corrosion resistance of products or metallic materials. They are used in various industries, including aerospace, aviation, shipbuilding, hardware, electronics, electrical appliances, chemicals, telecommunications, automobiles, motorcycles, mining, and quality inspection departments. They are also used in heat treatment and surface treatment industries. For example, the QPQ process plan usually requires a salt spray test, so it is used in both heat treatment and surface treatment. Electroplating plants also use salt spray tests.
1. Neutral Salt Spray Test (NSS Test)
The neutral salt spray test (NSS test) is an accelerated corrosion test used to evaluate the corrosion resistance of protective coatings (e.g., zinc and cadmium coatings) and protective-decorative coatings (e.g., nickel-chromium coatings). It uses a 5% sodium chloride solution with a pH of 6.5-7.2. The salt water is sprayed onto the coated parts at a rate of 1-2 mL/h·80cm2 at a temperature of 35±2°C and a relative humidity of >95%. The spray is continuous, and the test duration is typically 24 hours per cycle. For detailed information, refer to the GB/T 6461–2002 national standard.
The pH of the test solution should be adjusted so that the pH of the spray solution collected in the salt spray chamber is between 6.5 and 7.2. The pH should be measured at 25±2°C using a pH meter. Precision pH paper with an accuracy of no less than 0.3 can be used for daily checks. If the pH is outside the specified range, analytical grade hydrochloric acid, sodium hydroxide, or sodium bicarbonate can be added for adjustment.
Loss of carbon dioxide from the solution during atomization may lead to pH changes. Appropriate measures should be taken to prevent this, such as heating the solution to above 35°C before feeding it into the instrument or preparing the solution with freshly boiled water to reduce the carbon dioxide content.
The neutral salt spray test is suitable for:
- Metals and their alloys
- Metallic coatings (anodic or cathodic)
- Conversion coatings
- Anodized coatings
- Organic coatings on metallic substrates
Q-fog CRH salt spray chambers can be used to perform neutral salt spray tests, acetic acid salt spray tests, and CASS tests.
2. Acetic Acid Salt Spray Test (AASS Test)
An appropriate amount of glacial acetic acid is added to the prepared salt solution to ensure that the pH of the collected solution in the salt spray chamber is between 3.1 and 3.3. If the pH of the initially prepared solution is 3.0-3.1, the pH of the collected solution will generally be within the range of 3.1-3.3. The pH should be measured at 25±2°C using a pH meter. Precision pH paper with an accuracy of no less than 0.1 can be used for daily checks. The pH of the solution can be adjusted using glacial acetic acid or sodium hydroxide.
The acetic acid salt spray test is suitable for copper+nickel+chromium or nickel+chromium decorative coatings, as well as for anodized aluminum.
3. Copper-Accelerated Acetic Acid Salt Spray Test (CASS Test)
Copper chloride (CuCl2·2H2O) is added to the prepared salt solution at a concentration of 0.26±0.02 g/L (equivalent to 0.205±0.015 g/L anhydrous copper chloride). The pH of the solution is adjusted in the same way as described in section 3.2.3.
The copper-accelerated acetic acid salt spray test is suitable for copper+nickel+chromium or nickel+chromium decorative coatings, as well as for anodized aluminum.
Test Methods: NSS, AASS, and CASS Tests
| Test Method | Temperature | Average Deposition Rate (80 cm2 horizontal area) | Sodium Chloride Concentration (collected solution) | pH |
|---|---|---|---|---|
| NSS | 35±2°C | 1.5±0.5 mL/h | 50±5 g/L | 6.5-7.2 |
| AASS | 35±2°C | 1.5±0.5 mL/h | 50±5 g/L | 3.1-3.3 |
| CASS | 50±2°C | 1.5±0.5 mL/h | 50±5 g/L | 3.1-3.3 |
The GB/T 10125-2012 standard for “Corrosion tests in artificial atmospheres – Salt spray tests” provides a comparison of the three test methods.
The CASS test is an accelerated corrosion test used to evaluate the corrosion resistance of copper-nickel-chromium and nickel-chromium coatings. Its results correlate more closely with those of atmospheric exposure tests. It involves spraying a 5% sodium chloride solution containing 0.26 g/L cupric chloride (adjusted to a pH of 3.2±0.1 with glacial acetic acid) onto the surface of the coated parts. The other conditions and methods are the same as those for the neutral salt spray test. All reagents used should be of analytical grade.
The CASS test conditions are more severe than those of the neutral salt spray test and are mainly used to evaluate the corrosion resistance of nickel-based composite coatings.