If the object is made of one metal only, then it won't corrode as quickly due to a lack of galvanic reaction. In addition, if it can be covered by a thin oxide

If the object is made of one metal only, then it won't corrode as quickly due to a lack of galvanic reaction. In addition, if it can be covered by a thin oxide layer early on then it can be preserved longer. 900 year old iron in seawater isn't unthinkable. In addition, due to the inclusion of carbon into the iron by the smelting process, it is very easy to radiocarbon date up to 50,000 year old objects and verify their true age.