So, I made some standard preserved lemons and the mason jar mlid had this corrosion after a batch of lemons. I think it's still safe to use. Thoughts? I don't can at home and would like to avoid buying a box of lids for one jar.
I'm a big fan of canning so I'd say you should try it at least if you haven't already.
Anyway, there are usually two parts, the lid and the ring. For some reason, the rings get corroded, rusty or just don't look nice after awhile. Happens to mine all the time. But if the lid is fine, then there's no reason to replace it. When you can, the ring is simply to lock the lid in place during the water bath. After the can has sealed, you can remove the ring. Some people remove it, some don't. You can't reseal with a used lid but I re use the rings all the time and not all in perfect condition. Your lid doesn't look that bad.
I've been keeping my preserved lemons in an old ceramic crock. I added a little vinegar to top them off and they seem to be doing fine. ( I just checked them.)
If you are apprehensive about using these rings, place a layer of plastic wrap over the lid portion before you apply the ring portion. That should eliminate any problem the corrosion might cause to the food contained in the jar.
It is probably fine, but if you are going to keep the lemons for a while and / or make more in the future, you may want some plastic lids. We can a good bit, and whenever we open anything that will be kept in the refrigerator for a while, we switch over to a plastic lid to prevent corrosion of the rings.
If you don't want to buy a box of plastic lids, keep a couple from mayo or regular size peanut butter jars. Most brands will fit a standard mason jar. The bigger jars of PB will do a wide mouth.