Can the enamel of the bathtub become black and rusty really badly if it was exposed to sewer gas? Or it probably was treated by chemicals or paint?
Sewer gas should never ever reach a bathtub there is a p-trap below the tub full of water which prevents gas from coming in, it is definitely a cleaning product or it could also be your water is consider hard which will all so rust a tub out quicker.
I agree with the other Mark. Sometimes the metal drain becomes corroded and starts the rusting process, or a scratch or chip in the enamel can also start the process. Once it starts it just keeps going. The cause is usually the water but may also include chemicals used in cleaning agents. There isn't really a quick fix. You should consider replacement and if so, have everything checked out for possible mould or rotted wood under and around the tub. SDorry, it's one of those unfortunate issues.
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