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**Introduction to Salt Spray Test**
Salt spray corrosion is a common and destructive form of atmospheric corrosion, primarily caused by chloride ions found in salt spray environments. This type of corrosion typically occurs in marine or inland saline areas, where sodium chloride is the main component. The corrosive effect on metal surfaces results from electrochemical reactions between chloride ions penetrating the metal and its protective layer. Chloride ions have high hydration energy, making them easily adsorb onto surface pores and cracks, displacing oxygen and converting insoluble oxides into soluble chlorides, which activates the surface and leads to further damage.
The salt spray test is an environmental test that simulates real-world conditions using specialized equipment. It helps evaluate the corrosion resistance of products or materials. There are two main types: natural exposure tests and artificial accelerated tests. Artificial tests use salt spray chambers to create highly concentrated salt environments, significantly increasing the corrosion rate compared to natural conditions. For example, while natural exposure might take one year for noticeable corrosion, an artificial test can achieve similar results in just 24 hours.
Common types of salt spray tests include:
1. **Neutral Salt Spray Test (NSS)**: Uses a 5% sodium chloride solution with a neutral pH (6–7), at a temperature of 35°C. The salt spray deposition rate is 1–2 ml per 80 cm² per hour.
2. **Acetic Acid Salt Spray Test (ASS)**: Adds glacial acetic acid to the solution, lowering the pH to about 3, making it acidic and accelerating corrosion three times faster than NSS.
3. **Copper Salt Accelerated Acetic Acid Spray Test (CASS)**: Introduces copper chloride to increase the corrosion rate eight times faster than NSS, conducted at 50°C.
4. **Alternating Salt Spray Test**: Combines salt spray with damp heat cycles, testing both surface and internal corrosion, often used for cavity-type products.
Standards such as GB/T 2423.17-1993, GB/T 2423.18-2000, GB 5938-86, and GB/T 1771—91 provide guidelines for conducting and evaluating these tests. Results can be determined through rating systems, weight loss calculations, visual inspection, or statistical analysis of corrosion data.
In practice, the salt spray test is essential for assessing product durability in harsh environments. Proper setup includes maintaining correct temperatures, ensuring uniform salt distribution, and avoiding direct spraying on test samples. These factors help ensure accurate and reliable results, enabling manufacturers to improve material quality and design.