Sintered NdFeB magnet adopts powder metallurgy process, and the raw materials contain highly active rare earth elements, which makes its corrosion resistance performance worse in high temperature and high humidity environment, which greatly limits its use in various complex working conditions. This makes higher requirements for the comprehensive performance of NdFeB magnets in practical applications.
The weight loss test is used to test the corrosion resistance of the NdFeB substrate, and the sample is a magnet substrate without any protective layer. The test sample is exposed to high temperature and high-pressure water vapor environment, weight loss refers to the mass loss per unit surface area caused by corrosion on the orientation surface of the magnet, the unit is mg/cm2. Hast test and PCT test.
Weightlessness test can be carried out in Hast mode and PCT mode. Many people don't know the difference between Hast test and PCT test. Let's introduce the difference between these two tests first.
1. Different test conditions:
PCT is the English abbreviation of pressure cooker test, which refers to the high-pressure accelerated aging life test. It is a saturated type, and the humidity is 100% by default, and the temperature, humidity, and pressure rise or fall at the same time. It is used to test the sealing performance and aging performance of magnetic material products.
Hast belongs to the unsaturated type, temperature, humidity (70%~100%), and pressure can be adjusted according to needs, and can be used for high temperature and high humidity, high and low temperature cycle, double 95, high temperature, high humidity and high-pressure tests, also called aging tests.
2. The test equipment is different
Both PCT test and Hast test have special testing equipment and cannot be mixed.
3. In addition to testing the corrosion resistance of the magnet base material, the PCT test can also test the corrosion resistance of NdFeB finished products (products after processing and surface coating treatment), within a certain period of time (12h, 24h, 72h, etc.) Whether it can ensure that no corrosion occurs on the surface of the sample is the evaluation index.
Method of weight loss test (recommended standard) Weight loss test has become the most direct reference index to evaluate the corrosion resistance and long-term stability of sintered NdFeB permanent magnets. It has been widely used by mainstream NdFeB manufacturers and has been recognized by customers.
For a long time, there have been no corresponding national and industry standards for the weight loss test method of sintered NdFeB permanent magnets. In the actual test process, the main test parameters such as temperature, humidity, and constant temperature time are mostly agreed with the customer, especially the lack of specifications for important links such as the size of the sample, the placement method, and the surface treatment method before the sample, resulting in a large amount of test data. Poor sex. The writing and implementation of national standards can change the current chaotic situation very well.
Test method
Put the sintered NdFeB magnet sample that has been accurately weighed and dimensionally measured into a high-pressure container. After the sample is continuously tested under the action of high-temperature and high-pressure water vapor for a certain period of time, the surface grains and water vapor will undergo an electrochemical reaction, resulting in crystallization. Peeling off, surface corrosion and powdering; after the test, take out the sample, remove the loose corrosion on the surface and then weigh it, and calculate the mass loss (and weight loss) corresponding to the unit magnetic orientation surface area according to the mass change of the sample before and after the test.
Test sample
Shape: Slicing is recommended to process sintered NdFeB blanks into cubes, and if it is inconvenient to process into cubes, wire cutting can be used to process them into cylinders. If the sample contains a protective layer, the protective layer should be removed by grinding, etc.
Size: If conditions permit, the following standard size samples are recommended: square 12×12×8mm, cylinder 13.5×8mm, and the direction of 8mm is the direction of easy magnetization.
Surface cleaning: Use dilute nitric acid solution to clean the sample. After machining, the surface of the sample needs to be cleaned of oil stains (see the standard requirements for specific operation methods).
Sample quantity and placement requirements: The same batch of samples shall not be less than 3 pieces, preferably 5 pieces. The easy magnetization direction of the sample should be along the horizontal direction, and the easy magnetization surface should not be in contact with the sample frame or support. The distance between samples is not less than 10mm.
Test conditions
The test specifies three durations, which should be selected in combination with the corrosion resistance of the sample and customer requirements. Tests have confirmed that most samples of sintered NdFeB permanent magnet substrates have a weight loss close to zero for 480 hours under HAST conditions of 130°C and 95% relative humidity. Weightlessness is more scientific and can significantly shorten the time required for evaluation.
During the weight loss test, once corrosion pits are formed on the surface of the sintered NdFeB permanent magnet sample, as the corrosion expands, after reaching a certain critical point, the corrosion weight loss caused by the exfoliation of the sample surface grains increases almost linearly with the increase of the test time. The critical point of weight loss is about 5mg/cm2.