Weight loss test of sintered neodymium iron boron
In 2019, China has formulated the national standard for the weight loss test method of sintered neodymium iron boron, which is currently in the review stage and has not yet been implemented. Today, DCD will introduce the sintered neodymium iron boron weight loss test (also known as high-temperature, high-pressure, and high humidity accelerated aging test) based on the review of this experimental plan.
The weightlessness test is used to test the corrosion resistance of neodymium iron boron substrate, and the sample is a magnet substrate without any protective layer. The weight loss of the test sample exposed to high temperature and high pressure water vapor environment refers to the mass loss per unit surface area of the magnet orientation surface caused by corrosion, measured in mg/cm2. The weight loss test of Hast test and PCT test can be conducted in Hast mode and PCT mode. Many people are confused about the differences between Hast test and PCT test. Let's first introduce the differences between these two tests. 1. Different experimental conditions: PCT stands for Pressure Cook Test, which refers to the high-pressure accelerated aging life test. It belongs to the saturation type, with a default humidity of 100%, and temperature, humidity, and pressure rise or fall simultaneously. It is used to test the sealing performance and aging performance of magnetic material products. Hast belongs to the unsaturated type, and temperature, humidity (70%~100%), and pressure can be adjusted according to needs. It can be used for high temperature and high humidity, high and low temperature cycling, double 95, high temperature and high humidity high pressure tests, also known as aging testing. 2. Different testing instruments and equipment are used for PCT testing and Hast testing, which have specialized testing equipment and cannot be mixed.
3. In addition to testing the corrosion resistance of the magnet substrate, the PCT test can also test the corrosion resistance of neodymium iron boron finished products (products processed and surface coated), with the evaluation index of whether the sample surface can be guaranteed to not corrode within a certain period of time (12h, 24h, 72h, etc.).
The method of weight loss test (national standard review, recommended standard) has become the most direct reference index for evaluating the corrosion resistance and long-term stability of sintered neodymium iron boron permanent magnets. It is widely used by mainstream neodymium iron boron manufacturers and recognized by customers. There have been no corresponding national or industry standards for the weight loss test method of sintered neodymium iron boron permanent magnets. In the actual experimental process, the main test parameters such as temperature, humidity, and constant temperature time are mostly mutually agreed upon with the customer, especially the lack of standardization in important aspects such as the size, placement, and surface treatment of the sample, resulting in poor comparability of a large amount of test data. The writing and implementation of national standards can effectively change the current chaotic situation. 1. Experimental method: The sintered neodymium iron boron magnet sample, which has been accurately weighed and measured in size, is placed in a high-pressure container. After a certain period of continuous testing under the action of high temperature and high pressure water vapor, the surface grains undergo electrochemical reactions with water vapor, resulting in crystal peeling and surface layer corrosion powdering; After the experiment is completed, take out the sample, remove the loose corrosion on the surface, and weigh it again. Calculate the mass loss (and weight loss) corresponding to the unit magnetic orientation surface area based on the change in sample mass before and after the experiment. 2. Test sample ■ Shape: It is recommended to use slicing processing to process sintered neodymium iron boron wool blanks into blocks. For those that are difficult to process into blocks, they can be processed into cylinders using methods such as wire cutting. If the sample contains a protective layer, it should be removed by grinding or other methods. ■ Size: If conditions permit, the following standard size samples are recommended: block 12 × 12 × 8mm, cylinder 13.5 × 8mm, where the 8mm direction is the easy magnetization direction. Surface cleaning: Dilute nitric acid solution is used to clean the sample. After machining, the surface oil stains of the sample need to be removed first (see standard requirements for specific operation methods). Sample quantity and placement requirements: The same batch of samples should not be less than 3 pieces, with 5 pieces being appropriate. The easy magnetization direction of the sample should be along the horizontal direction, and the easy magnetization surface should not come into contact with the sample frame or bracket. The distance between samples shall not be less than 10mm. 3. The test conditions stipulate three durations, which should be selected based on the corrosion resistance of the sample and customer requirements. Experimental results have shown that most sintered neodymium iron boron permanent magnet substrate samples experience nearly zero weight loss for 480 hours under HAST conditions at a temperature of 130 ℃ and a relative humidity of 95%. Therefore, using the more stringent 120 ℃ saturation mode PCT condition to evaluate the weight loss of magnet substrates is more scientific and can significantly shorten the evaluation time required.
During the weight loss test, once corrosion pits form on the surface of sintered neodymium iron boron permanent magnet samples, as the corrosion expands and reaches a certain critical point, the corrosion weight loss caused by grain peeling on the sample surface almost increases linearly with the increase of test time. The critical point of weight loss is approximately 5mg/cm2.