Do magnets rust or react with water in any other unpleasant way?

no, don’t think so.

I suppose it depends what the magnet is made of but yes some magnets can rust.

Is there any way to find out what it’s made of? The package doesn’t say. It just says “heavy duty magnets”.

Some do rust.
I would test one in water overnight.

Some magnets are in a plastic base, other’s feel a bit more ‘stone’ like because they’ve been moulded in a ceramic. Unless there are some sort of ‘iron filings’ on the surface that could rust, I think you’d be OK.

I would paint the magnets with something, like a clear lacquer, to minimise the risk. You could make them quite pretty too.

^^ I agree, coating them should do the trick.

Magnets, made of steel, will rust. You can expect this, especially in the use around or in water. I would think that coating a magnet would make its magnification less. Nothing to prove this, just that a coating would lessen the adhesion of the magnet to a metal source.

@john65pennington
I’ve tried looking it up and I still don’t really understand what a magnetic field is “made of”, so I don’t know what it takes to block it.
But magnetism can attract stuff right through paper, cardboard, wood… I don’t think a coating of lacquer would get in its way.
@augustlan, if you want to be sure, coat a sheet of paper in the same lacquer, then see if the magnet’s attraction is significantly weaker through that sheet than through another sheet with no coating on it.

Rust / tarnish / corrosion is a chemical reaction between metal atoms at the surface and oxygen atoms present in dry air, humidity, or liquid water. Generally chemical reactions are not affected by magnetic fields in any fundamental way, so what happens to the magnet in a harsh environment depends on the material it’s made of—not its magnetization per se.

Your shower curtain magnets might well be of the newer rare earth neodmium type (smaller yet stronger than earlier alnico or iron—surface fields approaching one tesla). Quoting K&J Magnetics:

Neodymium magnets are a composition of mostly Neodymium, Iron and Boron. If left exposed to the elements, the iron in the magnet will rust. To protect the magnet from corrosion and to strengthen the brittle magnet material, it is usually preferable for the magnet to be coated. There are a variety of options for coatings, but nickel is the most common…

I’ve seen nickel-plated neodymium magnets: they have a silvery sheen and are probably fine in the shower. But if your magnets have a flat black finish, they might be uncoated—just paint them & check contact points periodically for corrosion. Better yet, find some kind of thin elastic coating that goes on as a liquid then turns rubbery & waterproof. Hope that helps.

Paint will scratch and chip off. Also over time and exposure to water rust will bleed through it. I wonder if a wax coating would last longer.

A normal, uncoated magnet will rust due to the fact that it is made of metal, including iron. If the magnets look like the have a non-coated, black metal surface, they probably are. To prevent rust, you need to coat them with a waterproof substance. I think that a coating of normal paint is not entirely waterproof, and so you should use something else. Since you most likely don’t have the equipment to coat it with non-rusting metal such as nickel, you need to find some sort of rubber or enamel coating to apply. One product that I have found on the internet that is designed to be waterproof is Resene Waterborne Enamel.

These are definitely uncoated, so it looks like I have some work ahead of me. I wonder if Rustoleum spray paint (the kind people use for metal surfaces that are kept outdoors) would do the trick?

I misspelled neodymium (Nd #60) earlier.
Rust-oleum paint should be fine. If money’s no object then use a primer first, after cleaning with steel wool (but creates magnetized dust that sticks) or a wire brush. Also check out plasti dip, with or without first painting.

@augustlan I think that Rustoleum will work. It sounds like it is designed to protect metal from rusting in rain and protect it from the elements, so it should work.