Rust has formed on the bolts in your tank as a result of years of condensation and water exposure. This rust is caused by oxidation, which occurs when iron, steel, or another metal comes into contact with water and oxygen over time. Metal toilets generally contain iron parts (bolts, springs, etc.) that are exposed to moisture in the air and liquid from accidents or normal use. These components will oxidize and turn rusty if they are not protected. To prevent this corrosion from happening, you need to avoid exposing metal parts of your toilet to water. If an accident does cause water to contact these metals, then you should try to clean it up immediately. Use a non-acidic cleaner to remove any residue that may have formed due to the accident. You can also apply a protective coat every few years if you want to extend the life of your toilet. If you leave your toilet bolt loose enough to rotate, then water will continue to leak out even after you've fixed the hole. This allows more time for water to contact the metal inside the bowl and increase the risk of corrosion. When replacing toilet bolts, make sure you get ones with lock washers and no rust on them. The locks serve to keep the bolts in place while still allowing them to be turned.