I have a 50 gal condensate return tank and my water feed is, when the system is on, hot condensate, which is distilled water, thus lower ph and without minerals, other than the rust it picks up from the 100 year old pipes. Do check your water quality as someone suggested. My boiler did have a section fail after 6 years, with lot of make-up water that was being introduced without us noticing. This was fixed, but now I use distilled (carbon filter plus an RO filter) for make up, and use Steamaster tablets to raise ph in the boiler. Our water was 170-180 tds, and chloride were in the 60s, which is way too high for a steam system. If you are blowing down thst boiler weekly and a lot of water is introduced, and is poor quality, this can add to the issues on the boiler itself. I'm quite convinced, however, that the nipple deterioration comes from turbulence. Kind of how copper water line can erode on the inside when the soldier joint leaks inside the pipe and causes the turbulence. Also, those hot tanks can also pump impurities (rust and other particles) into the boiler and over time scrape enough metal off the pipe. Also, remember, when you introduce new water, even hot one, to a bottom of otherwise boiling water, as is the case with pumped returns into the equalizer, you get steam collapsing but also new water flashing, and with all the crud at the bottom of the makeup tank rushing into the boiler, all sorts of weird things can happen.
Thanks for the follow up. Keep us posted on how the new nipple holds up after throttling down the pump.