Weight loss test of sintered neodymium iron boron (HAST, PCT)

Sintered NdFeB permanent magnets are magnetic materials with the highest magnetic properties discovered so far, and are widely used in various fields such as wind power generation, new energy vehicles, maglev trains, and intelligent robots. Sintered NdFeB magnets adopt the powder metallurgy process, and the raw materials contain highly reactive rare earth elements, which deteriorates their corrosion resistance in high-temperature and high-humidity environments, greatly limiting their use in various complex working conditions. This puts forward higher requirements for the comprehensive performance of NdFeB magnets in practical applications.

In 2019, China has formulated the national standard of "Test Method for Weight Loss of Sintered Neodymium-Iron-Boron Magnets".

Currently, it is in the submission review stage and has not been implemented yet. Today, based on the submission draft of this test plan, the "Magnet Master" will introduce to you the weight loss test of sintered neodymium-iron-boron magnets (also known as high-temperature, high-pressure and high-humidity accelerated aging test).

The weight loss test is used to test the corrosion resistance of the neodymium-iron-boron substrate, and the test sample is the magnet base material without any protective layer. The test sample is exposed to a high-temperature and high-pressure water vapor environment. Weight loss refers to the mass loss per unit surface area of the magnet's oriented surface due to corrosion, with the unit being mg/cm².

Hast test and PCT test

Weightlessness tests can be carried out in Hast mode and PCT mode. Many people can't figure out the differences between Hast tests and PCT tests. Let's first introduce the differences between these two types of tests.

1. Different test conditions:

PCT is the English abbreviation of "pressure cooker test", referring to the high - pressure accelerated aging life test. It is of the saturated type, with a default humidity of 100%, and the temperature, humidity, and pressure rise or fall simultaneously. It is used to test the sealing performance and aging performance of magnetic material products.

HAST is of the non - saturated type. The temperature, humidity (70% - 100%), and pressure can be adjusted according to requirements. It can conduct tests such as high - temperature and high - humidity tests, high - low temperature cycling tests, double 95 tests, and high - temperature, high - humidity, and high - pressure tests. It is also called the aging test.

2. Different test instruments and equipment

Both PCT tests and HAST tests have dedicated testing equipment and cannot be used interchangeably.

3. In addition to testing the corrosion resistance of the magnet base material, the PCT test can also test the corrosion resistance of the finished neodymium-iron-boron products (products after processing and surface plating treatment). The evaluation index is whether the surface of the test sample can be guaranteed to be free of corrosion within a certain period of time (such as 12h, 24h, 72h, etc.).

Methods for Weightlessness Tests (Draft for National Standard Submission, Recommended Standard)

The weight loss test has currently become the most direct reference index for evaluating the corrosion resistance and long - term stability of sintered NdFeB permanent magnets. It is commonly used in mainstream NdFeB manufacturers at present 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 upon with customers. In particular, there is a lack of standardization in important aspects such as the size of the sample, the placement method, and the surface treatment method before the test, resulting in poor comparability of a large amount of test data. The compilation and implementation of national standards can effectively change this chaotic situation.

1. Test method

Put the sintered neodymium-iron-boron magnet samples that have been accurately weighed and measured in size into a high-pressure vessel. Under the action of high-temperature and high-pressure water vapor, the samples are continuously tested for a certain period of time. Then, an electrochemical reaction occurs between the surface grains and the water vapor, resulting in crystal peeling and surface corrosion and pulverization. After the test, take out the samples, remove the loose corrosion products on the surface, and then weigh them. Calculate the mass loss (i.e., weight loss) corresponding to the unit magnetic orientation surface area based on the mass change of the samples before and after the test.

2. Test samples

The test specifies three durations, and the selection should be made based on the corrosion resistance of the sample and customer requirements. The test has confirmed that for most sintered neodymium-iron-boron permanent magnet substrate samples, the weight loss is close to zero after 480 hours under the HAST condition of 130°C temperature and 95% relative humidity. Therefore, it is more scientific to use the more severe 120°C saturated mode PCT condition to evaluate the weight loss of the magnet substrate, which can significantly shorten the time required for evaluation.

During the weight loss test, once corrosion pits form on the surface of the sintered neodymium-iron-boron permanent magnet sample, with the expansion of corrosion, after reaching a certain critical point, the corrosion weight loss caused by the spalling of surface grains of the sample almost shows a linear increase with the increase of test time. The weight loss critical point is approximately 5mg/cm2.