I've got an antique cast iron pot that my mother had. I know that it has to date at least back to the 1930's and frankly it could easily be older than that.
Some years ago my wife decided that the air in our house was way too dry in the winter. Instead of doing something like get a humidifier she decided that the easiest solution was to put a pot of water on the stove at a medium heat and let the water evaporate into the air that way. My antique cast iron pot was elected for the job.
What was not taken into account is that the water here is VERY hard. You could break cement blocks with it.
I'm not joking. By the time I saw what was happening to it the pot was effectively ruined. It had a quarter of an inch of white mineral buildup on the inside.
One wit suggested that I give it a thorough hydrogen peroxide cleaning by soaking it in the stuff for a week. Since then I've tried about every product on the market and nothing has worked. I honestly think that the only cure at this point would be to have it sandblasted.