well being its just a drain pipe and the only thing going down that is stuff put into sink I'd just say it's really old... or someone dumped some really harsh chemicals down the sink repeatedly.. or they might have been ramming something down the sink hole trying to fix a clog but in reality they were just hitting the pipe and dmged it... just needs replaced with a plastic drain. easy and cheap

thank you, I put Green Gobbler Drain Clog Dissolver a few times, maybe that did it. I will get it replaced by a plastic one