Rusting of neodymium magnets is a defect of NdFeB magnets if not properly protected. This article will mainly discuss the detailed reasons for rusting.
Reasons for Rusting of Neodymium Magnets
Components
| Neodymium (Nd): The corrosion susceptibility of NdFeB magnets is partly due to Nd being one of the most chemically active elements, with a standard electrode potential of E0(Nd³+/Nd) = -2.431V. Additionally, the alloy has a multiphase structure with significant electrochemical potential differences between the phases, leading to electrochemical corrosion and consequently rusting of Neodymium magnets.
| Iron: Most magnets, including neodymium magnets, contain iron, which in NdFeB magnets constitutes over 60%. When exposed to moisture and oxygen, iron reacts to form rust, causing a loss of magnetism.
Structure
During the manufacturing process of NdFeB magnets, micro-holes, porous structures, and rough surfaces may arise. The operational environment often includes high temperatures and humidity, which facilitate corrosion of NdFeB. Furthermore, impurities such as O, H, and Cl can significantly impact corrosion. Oxygen can lead to oxidative corrosion, while chlorine compounds accelerate rusting.
Humidity
Humidity has a much more significant impact on the corrosion resistance of magnets compared to temperature. In dry oxidative environments, dense corrosion product films can form to protect the magnets, whereas in humid environments, less dense hydroxides and hydrides cannot effectively prevent further oxidation. Excessive moisture can lead to electrochemical corrosion if liquid water is present on the magnet’s surface.
High-Temperature
In dry conditions, when temperatures are below 150°C, the oxidation of NdFeB magnets is slow. However, at higher temperatures, reactions occur that lead to the breakdown of the Nd2Fe14B phase, resulting in further oxidation products like Fe2O3.
Corrosive Media
Different corrosive substances affect the rusting of Neodymium magnets
differently.
| Hydrochloric acid severely erodes the Nd-rich boundaries, significantly reducing magnetic energy,
| Nitric acid damages the main phase, lowering the intrinsic coercivity.
| Phosphoric acid and oxalic acid can form passive films that slow down corrosion.
Electrochemical Environment
In an electrochemical environment, different phases in NdFeB magnets have varying electrochemical potentials. Nd-rich and B-rich phases act as anodes and corrode preferentially, creating localized corrosion cells. If the magnet has a metallic coating (such as zinc or nickel), defects in the coating can lead to corrosion in electrochemical cells.
Typically, the magnet acts as the anode that preferentially corrodes, while the metallic coating acts as the cathode, which is why rusting of Neodymium magnets often shows blistering.
How to Prevent Rusting of Neodymium Magnets
| Surface Coating Protection: Most neodymium magnets are treated with protective coatings at the factory, with common coatings being nickel and zinc. A nickel-copper-nickel triple layer is most prevalent, forming a dense barrier against moisture. Organic coatings like epoxy are suitable for heavily saline environments. It’s essential to consider coating thickness and salt spray testing when purchasing magnets.
| Controlled Use Environment: Avoid exposing neodymium magnets to high humidity or water vapor during design and use. In unavoidable humid applications, consider additional waterproofing or sealing to prevent the rusting of Neodymium magnets
Magnets should be stored in dry, ventilated conditions, and sealed packaging with desiccants can be used for long-term storage.
| Good Assembly Practices: During processing, transportation, and assembly, avoid impacts or scratches to the magnet’s coating, as even minor abrasions can lead to rusting of Neodymium magnets. Cases have been documented where damaged coatings during installation led to failure months later. Wear gloves, carefully handle magnets during assembly, and promptly replace or recoat damaged areas.
Magnet Manufacturer
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