Hi Bill,
Excuse me if this answer is a bit muddled, I have a horrible cold and a fuzzy head
In the future we may be able to carbon date iron, I know Oxford has been working on this with varing amounts of success. Ideally a small uncorroded sample can tell us information such as if there is slag inclusions. The type of slag could in some circumstances suggest a relative date. Elemental analysis can also sometimes suggest whether or not it is new or old iron. We can also tell about the iron/carbon content and heat treatments. Testing rust will only tell you it is iron. Due to segretation and leaching in and out of elements during corrosion, as far as I know you can not get any useful information out of rust. I would be very scptical about any one who says they can learn alot from corrosion. I have found relic structures in corrosion products. It is hard to say what can be learnt about it without seeing it. If you want, send it to me and I can take a look at it and suggest possible aspects for research.
Ann