Look Neodymium Magnet’s Rust
While this blog promotes the use of neodymium magnets, we also try to be frank about their limitations. Previous examples include:
How a neodymium magnet doesn’t remove all the data from a hard drive,
How fragile magnets are – Breaking Magnets, and
How Magnetic Paint isn’t very attractive.
In that spirit, let’s look at how most neodymium magnets aren’t quite waterproof.
Cutaway view of a neodymium magnet, with plating thickness exaggerated for clarity
While you may know the chemical formula of neodymium magnets as NdFeB, or Nd2Fe14B, they are mostly iron and neodymium. By mass, they’re roughly two-thirds iron and one-third neodymium. Only a tiny fraction is boron, plus a few other elements.
A raw, unplated magnet is not all that different from a piece of raw iron, at least when it comes to corrosion. Iron rusts in the presence of moisture – that’s why iron pans are usually kept oiled. Neodymium magnets are like iron: without protection, they rust easily.
To prevent corrosion, most neodymium magnets are plated with a three layer, nickel-copper-nickel plating. This particular plating combination has been the preferred corrosion protection for many years. It performs better than zinc plating or other solutions in most applications.
The inside layer of nickel plating is different than the bright nickel above it. The grain structure is different, and no phosphorus is used in the plating process. These differences mean they sit in slightly different positions on the galvanic corrosion chart. The top nickel layer will typically corrode first, acting in a sacrificial way to protect the underlying layer. Corrosion tends to propagate horizontally across the layer, rather than boring down into the magnet. It’s like how a boat’s zinc corrodes sacrificially, before the bronze or aluminum propeller.
The copper layer shares this galvanic protection idea, but also helps in other ways. Copper plating forms a ductile underlay that provides some leveling of the rough surface. It’s a good choice for materials that are harder to bond to, such as the rough surface of a neodymium magnet.
What happens if I place a magnet underwater?
Over the years, we’ve heard many anecdotal stories about what works and what doesn’t in wet applications. Epoxy coating helps some folks, while others say the nickel plating alone is better. While all these stories hold some truth, we always prefer data that’s testable. Today we’re sharing an informal corrosion test we started a few months ago.
We submerged some nickel plated magnets, gold plated magnets and epoxy coated magnets into salt water. We scratched one of each type with a knife, and left a second unscratched. What happened over time?
Only one DC2E epoxy coated magnet survived the 6-7 weeks of salt water submersion. Does that mean using epoxy is OK? No, not really. Our sample size is very small. Also, the scratched epoxy magnet started rusting at the edge, in a completely different location than the scratch. If we conducted this same test on 100 epoxy coated magnets, we suspect some percentage would fail.
There also wasn’t a clear answer on how long it takes before corrosion sets in, despite the identical conditions. The first magnet to fail showed a rust bubble after about 1 week. The last magnet to rust didn’t start showing problems until after a month!
In previous testing, gold plated magnets lasted longer than nickel plated, but that certainly wasn’t true here. The only sure conclusion is that corrosion is coming, though it is hard to predict when.
By the Numbers
We measured these 6 test magnets before and after corrosion testing. Careful measurements with a fluxmeter noted the total magnetic moment of each magnet.
After the testing, we found decreases in strength ranging from 0% to 11%. The epoxy magnet with no visible rust tested just fine. The gold plated magnet with the most rust, pictured above, was the weakest. There is simply less non-rusted magnet material available.
More Test Results
We love this kind of down to earth, hands-on testing. For example, we’ve stuck a number of magnets outside for years, monitoring how corrosion progresses in the sun and rain. It’s an informal test outside of our regular qualification testing, but still interesting. Here are a few examples:
A few of our stainless steel magnets have been outside since late 2013, and still show no signs of rust! We never expected them to hold up this long. Expectations exceeded!
A single, rubber coated ring magnet has not fared well in the sun and rain. Cracks in the rubber has allowed exensive corrosion of the magnet. It still sticks, but the magnet is no longer a complete ring.
Again, the cause isn’t moisture, but UV exposure in the sun.
This block magnet is coated in a rubber-like thermoplastic, which has held up well so far. It has only been outdoors since late 2018, so time will tell.
Our plastic coated magnets have an injection molded layer of plastic. The plastic forms a waterproof shell around the magnet. While the magnet inside has remained free of moisture and rust (see video for a peel-away of the plastic), it has faded in color. The plastic feels more brittle than new magnets, thanks to the UV exposure in the sun. These examples have been outdoors since late 2016, about three and a half years.
Seal magnets from moisture in applications where it matters.
There are a ton of popular uses for neodymium magnets where the nickel-copper-nickel plating provides more than adequate corrosion protection for the life of the product. Magnets in your hard drive aren’t rusting. Magnets in the speakers of your AirPods – also not rusting.
If you’re going to have a magnet outside, exposed to the elements, consider some means of sealing the magnet away from moisture. Maybe some potting compound holds it in place and completely covers it. Maybe a plastic cover is ultrasonically welded over it. Whatever the solution, keep the wetness and high humidity away from magnets.
We do offer a number of plastic coated magnets that are sealed in this way. They’re great, but do come with some drawbacks. The injection molding is fairly thick, so you get less strength from the magnet.
Are you designing a medical or dental product? Will it need to be run through an autoclave for sterilization? Exposed neodymium magnets are not a good idea. Consider encasing magnets in thin stainless steel. Also consider high temperatures when choosing your magnet shape and grade. See our Temperature and Neodymium Magnets article for more details.