For such a reactive metal the longevity of iron is amazing. On a dig last summer we were excavating on a manor house abandoned over 300 years ago and we were pulling all sorts of iron out of the ground; broadhead arrowheads where the barbs and socket were still intact; door hinges; over a hundred nails; I found a stick-tang knife in a few pieces in what was once a drain, and later someone found another, just missing the tip, just where I'd been standing a few minutes earlier. Probably the most amazing find was this. And all these finds were in a fairly acidic soil; no wood survived, just building materials like stone, plaster, mortar etc., metal, some shells (only a fraction) and some bones (most quite delicate). A fair amount of coarcoal though.