The salt spray test is an important standard for evaluating the corrosion resistance of powerful magnets. The corrosion resistance of magnets directly determines the product’s lifespan and reliability.
Definition of Salt Spray Test
The salt spray test (Salt Spray Test) is an environmental test that uses artificial salt spray conditions created by salt spray testing equipment to evaluate the corrosion resistance of products or metal materials. It is divided into two main categories: natural environmental exposure tests and accelerated artificial simulation salt spray tests. The accelerated artificial simulation salt spray test utilizes a salt spray chamber, a testing device with a certain volume, to artificially create a salt spray environment within its volume to assess the quality of the product’s salt spray corrosion resistance.
Compared with natural environments, the salt concentration of chloride in artificial salt spray environments can be several or even dozens of times higher than that in natural environments, significantly increasing the corrosion rate. As a result, the time required to obtain results from a salt spray test is greatly reduced.
Core Indicators of Salt Spray Test
Surface Corrosion Area: Whether rust spots, oxidation, or coating peeling occur.
Performance Stability: Whether the magnetic force decreases due to corrosion.
If the product is intended for use in high-humidity and high-salt environments (such as offshore wind power and automotive electronics), a 72-hour salt spray test is a mandatory step to ensure long-term reliability!
Principle of Salt Spray Test
The corrosion of metal materials by salt spray is primarily due to the penetration of conductive salt solutions into the metal, causing electrochemical reactions. Chloride ions have strong penetrating power, easily penetrating the metal oxide layer and entering the metal interior, disrupting the passive state of the metal. At the same time, chloride ions have low hydration energy, making them easy to adsorb on the metal surface and replace oxygen in the protective oxide layer, thereby damaging the metal.
Conditions and Parameters of Salt Spray Test
Temperature: Typically set at 35℃ ± 2℃.
Salt Solution Concentration: 5% ± 1% sodium chloride solution (by mass).
pH Value: The pH value of the salt spray deposition solution should be between 6.5 and 7.2.
Test Duration: Can be set according to customer requirements, with common durations including 24 hours, 48 hours, and 72 hours.
Types of Salt Spray Tests
| Neutral Salt Spray Test (NSS): This is the most commonly used method. During the test, continuous salt spray is provided, with a temperature around 35℃, humidity above 95%, and a deposition rate of 1–2 mL/(h·80cm²) using a 5% sodium chloride solution. The test duration is determined based on the actual needs to assess corrosion.
| Acetic Acid Salt Spray Test (AASS): This test is developed based on the neutral salt spray test. In addition to the basic conditions of the NSS test, a certain amount of glacial acetic acid is added to adjust the pH value, quickly making the laboratory pH acidic (around pH 3). This changes the nature of the salt spray, placing the tested object in an acidic salt spray environment, which accelerates corrosion by about 3 times compared to the NSS test.
| Alternating Salt Spray Test: This is a comprehensive test that combines different testing conditions in a laboratory cycle to simulate real-world environments more closely. The tested components are exposed to a humid environment, causing both surface and internal corrosion. During the alternating salt spray test, the environment alternates between a salt spray environment and a wet-heat environment.
Evaluation Standards for Salt Spray Test
After the experiment, observe whether the magnet’s surface darkens or turns black. This usually indicates a pass. If yellowish rust, blistering, cracking, peeling, or powdering occurs, it indicates a failure.
Coating Selection for Powerful Magnets
Neodymium magnets, though excellent in performance, are prone to oxidation and corrosion due to their high iron content when exposed. Coating processes are a core method to enhance salt spray resistance, but the protective effects of different coatings vary significantly:
| Bright Zinc: Resists salt spray for 16 hours, suitable for indoor dry environments.
| Colored Zinc: Resists salt spray for 48 hours, a cost-effective choice.
| Black Zinc: Only 12 hours, mainly used for decorative purposes.
| Nickel Plating: Nickel plating (three-layer nickel-copper-nickel process) is the mainstream protective solution for NdFeB magnets, showing stable performance with 48 hours of salt spray resistance and a smooth, aesthetically pleasing surface. However, if the customer requires a 72-hour salt spray test, conventional nickel plating may not meet the standard—coating may fail due to pores or localized corrosion. A composite coating, such as nickel plating followed by epoxy resin or Parylene, can form a dense protective film, achieving over 72 hours of salt spray resistance.
| Parylene Coating: Parylene is a polymeric material deposited via vapor phase deposition. Its advantage lies in its strong adhesion, allowing it to penetrate the micro-pores of the magnet surface and form a dense protective layer. It can easily pass a 72-hour salt spray test. Parylene coatings are resistant to acids and alkalis and can withstand high temperatures, making them suitable for extreme environments such as chemical industries.
Magnets Manufacturer
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