I've been doing this off and on for 8 years now, and soldering a bit longer than that. In the past, I would replace solder tips when they wouldn't stay tinned (or before I used brass to clean them, when they started to look rusty).
In the past year, working with lead-free solder, I've started having the tips just straight up fall apart on me (The tinned part starts to get thin, bend, and then eventually break off completely). Apparently this is a known issue with lead-free solder.
Has anyone else run into this, and if so, is there anything one can do to cut down on the number of tips they're going through.
I've heard that you need to get certain tips for soldering with lead-free. Otherwise they get messed up like you're experiencing.
Molten tin rapidly eats iron, and to a somewhat slower degree, it eats copper. I believe there are tin-plated tips that will last longer, but I'm no expert....
My tips do that with lead solder, but it takes a long time. I can get a good six months out of one with regular use.
Just last week, I had to replace the tip on my WESD51 solder station......
..... that I bought in 2008 :o 8)
Lead-free solder is harder on tips for a couple of reasons. They are usually used at higher temps and the tin content is higher.
This is an interesting read:
https://www.elexp.com/Images/Weller_Coping_with_Lead_Free.pdf
Although I use leaded solder, I have run into a similar problem with tips going bad.
I have an ancient Weller iron and a regular sponge to clean the tip.
Recently, I tried using a brass cleaning thing and it ate up my tips fast. At first,
I thought my iron was going bad, but when I switched back to the sponge, my tips stopped getting eaten up.