Is there a way to remove rust from cogs, or better to purchase a new cassette?

Bike is only a year old, but was left out in the rain a handful of times.

That would be very light rust.

Assuming you have the DIY tools, you need to remove all the individual cogs and soak them in pure CLR.

And since you already have all the cogs removed, you can just decide to throw on a replacement set, new or used.

Functionally, there’s very little need to.
For peace of mind, first hit with wire steel brush. Then ride.
For aesthetics, I’d pull the cassette off, then soak in the de-ruster of your choice. Polish until satisfied or bored. Reassemble and ride.

I agree with the above, it if shifts well the just keep lubed *and* out of the constant rain. no need to replace.

A soft wire brush is probably the most I'd do for a freewheel. If it was a cassette, I might take it off and do a little better job with the brush..... might.

With either, I'd for sure just put some lube on it and go ride.

One could make the argument that the rust will wear out your chain, but probably not much quicker than it already is going to wear out.

If it has to look pretty, then get a new one. I generally keep my eyes to myself and don't look at others cassette's or freewheel's. <grin>

It's just surface rust, nothing to worry about. ..Lube it with some oil and keep riding.

Forgot to mention it was a 7-speed freewheel. Seems to be shifting fine. I might take it off and mess around with it to add another DIY repair skill to my limited repertoire.

Thanks for the guidance, much appreciated.

While cassettes are clearly easier to disassemble, it’s not that difficult to remove the cogs from a freewheel body. You’ll need at least one chain whip (two makes it much easier) for the task. If you don’t have one you should have one anyway and they’re not that expensive. Google the process. Once the rusty cogs are separated from the body, a 24-48 hour soak in EvapoRust should do the trick, especially if it’s just surface rust. Before reassembly treat them with some sort of rust preventing lube. Avoid heavy oil or grease as that will just attract dirt and/or other gritty grime. As mentioned, protect it from the elements as much as possible. Clean and reapply the rust preventive lube as needed.

A wire brush dipped in kerosene or diesel is the quick way to get rid of surface rust. It would take a few years for rust to get bad enough to require the replacement of the cassette.

Rusty cogs don't really pose any structural problems but soaking them in white vinegar solution overnight then a light scrub with some steel wool can give the cogs a good shine.

Wire brush worked perfectly. Thanks!