In the field of material corrosion performance testing, neutral salt spray test (NSS), acetic acid salt spray test (AASS), and copper-accelerated acetic acid salt spray test (CASS) are three commonly used salt spray testing methods. They are both interrelated and distinctly different, together forming a relatively complete salt spray corrosion testing system, which provides important technical support, for evaluating the corrosion resistance of materials in different scenarios.​

1. Basic Overview of the Three Salt Spray Tests​

1.1 Neutral Salt Spray Test (NSS)

The neutral salt spray test is the most basic type of salt spray test. Its test solution consists of sodium chloride with a mass fraction of 5%. And the pH value is controlled between 6.5-7.2, the test temperature is 35°C, and the salt fog deposition rate is required to be 1-2mL per hour per 80cm² area. This test mainly simulates a mild marine or humid atmospheric environment. And is suitable for evaluating the corrosion resistance of materials in ordinary humid and salt-containing environments.​

1.2 Acetic Acid Salt Spray Test (AASS)

The acetic acid salt spray test is developed on the basis of the neutral salt spray test. Its test solution adds glacial acetic acid to the neutral salt spray solution, to reduce the pH value to 3.1-3.3. The test temperature is also 35°C, and the salt fog deposition rate is consistent with that of the neutral salt spray test. This test mainly simulates corrosive environments containing acidic components, such as industrial areas and acid rain-prone areas. And is used to evaluate the corrosion resistance of materials in acidic and humid environments.​

1.3 Copper-Accelerated Acetic Acid Salt Spray Test (CASS)

The copper-accelerated acetic acid salt spray test is a further improvement on the basis of the acetic acid salt spray test. In addition to sodium chloride and glacial acetic acid, the test solution also contains copper chloride with a concentration of 0.26g/L, the pH value is also 3.1-3.3, but the test temperature is 50°C. This test can significantly accelerate the corrosion process, quickly evaluate the corrosion resistance of materials, and is suitable for scenarios with high requirements for test efficiency.​

2. Connections Between the Three Salt Spray Tests​

The three salt spray tests are closely connected in terms of technical foundation. They all use salt spray as the corrosive medium, form a salt spray environment by atomizing the test solution, cause the sample to undergo corrosion reactions under the action of salt spray, and then evaluate the corrosion resistance of the material. Their basic testing principle is consistent: all utilize chloride ions in the salt spray to cause corrosion on the material surface, and judge the corrosion resistance by observing the corrosion degree of the sample within a certain period of time.​

At the same time, the core goal of the three tests is the same, which is to evaluate the corrosion resistance of materials or products, and provide a basis for product quality control, material selection and improvement. Regardless of the type of test, the final result is to judge the durability and reliability of the sample in a specific environment through the analysis of the corrosion situation of the sample.​

In addition, the three tests also have commonalities in equipment and operation procedures. They usually need to use a salt spray test chamber to create a salt spray environment, and strictly control parameters such as temperature and salt fog deposition rate during the test to ensure the accuracy and reliability of the test results.​

3. Differences Between the Three Salt Spray Tests​

3.1 Differences on test solution

The most significant difference between the three salt spray tests lies in the composition of the test solution. The neutral salt spray test solution only contains sodium chloride; the acetic acid salt spray test solution adds glacial acetic acid on the basis of sodium chloride; the copper-accelerated acetic acid salt spray test solution adds copper chloride on the basis of the acetic acid salt spray test solution. The difference in solution composition directly leads to differences in the corrosive environments they simulate.​

3.2 Difference on degrees of corrosion

Due to the difference in solution composition, the corrosiveness of the corrosive environments simulated by the three tests also varies. The neutral salt spray test simulates a relatively mild corrosive environment; the acetic acid salt spray test, because the solution is acidic, simulates a more corrosive environment; in the copper-accelerated acetic acid salt spray test, the addition of copper chloride plays a catalytic role, further enhancing the corrosiveness of the corrosive environment, making it the most corrosive.​

3.3 Difference on test efficiency

Test efficiency is also an important difference between the three. The corrosion rate of the neutral salt spray test and the acetic acid salt spray test is relatively slow, and it takes a long test time to observe obvious corrosion effects; while the copper-accelerated acetic acid salt spray test, due to the catalytic effect of copper chloride, can greatly accelerate the corrosion reaction rate, shorten the test cycle, and improve test efficiency.​

3.4 Different simulated corrosion environments

The difference in applicable scenarios is also one of the main differences between the three. The neutral salt spray test is suitable for evaluating the corrosion resistance of materials in ordinary humid and salt-containing environments, such as ordinary building materials near marine environments and some daily necessities. The acetic acid salt spray test is suitable for evaluating the corrosion resistance of materials in acidic and humid environments, such as metal components in industrial areas and outdoor equipment that often comes into contact with acid rain. The copper-accelerated acetic acid salt spray test is suitable for scenarios with high requirements for test efficiency, such as rapid screening in the process of new product development and quality sampling inspection, which can obtain test results in a short time.​

4. Progressive and Complementary Relationships Among the Three Salt Spray Tests​

The three salt spray tests have a progressive relationship in terms of corrosiveness and test efficiency. From the neutral salt spray test to the acetic acid salt spray test, and then to the copper-accelerated acetic acid salt spray test, the corrosiveness of the simulated corrosive environment gradually increases, and the test efficiency also gradually improves. This progressive relationship enables them to meet different testing needs. For the same material, by conducting different types of salt spray tests, we can fully understand its corrosion resistance in environments with different corrosion intensities.​

At the same time, there is also a complementary relationship between the three. Different materials or products may face different corrosive environments in actual use, and a single salt spray test cannot fully evaluate their corrosion resistance.

5. Selection in Practical Applications​

In practical applications, it is necessary to select the appropriate type of salt spray test according to the specific test purpose and the service environment of the product. Sometimes, to more fully understand the corrosion resistance of products, the three tests can also be used in combination.

To sum up, the neutral salt spray test, acetic acid salt spray test, and copper-accelerated acetic acid salt spray test are all interrelated and distinct. They jointly provide an effective means for evaluating the corrosion resistance of materials through progression and complementarity, and play an important role in different application scenarios.​