|MadSci Network: Chemistry|
To calculate the rate of corrosion without disturbing the rust on the nail is very difficult, unless you use very specialized laboratory equipment in a research laboratory.
By the way, what are you exposing the nails to to make them corrode - Salt water? or something else?
The usual procedure is to weigh the test specimens [e.g. the nails] at the beginning of the test, then periodically remove a number of the test specimens and determine the rust at that time. Remove all of the rust [try a steel wire brush] and weigh the specimen again. The weight lost will be the amount of iron that has rusted.
Usually the test specimens are a number of pieces, or "coupons," of sheet metal, several square inches in size. Measure the test specimens and calculate their surface area [on both sides!]. The coupons are thoroughly cleaned; any oil etc. is removed with detergent, etc.
If you are studying steel, be sure it is not "Galvanized" or Cadmium plated.
Most corrosion tests take many days to complete. If you can, don't use just one nail - use about a dozen. After 1 week's exposure, remove 3 or 4 nails, clean them of rust and re-weigh. After, say, 2 weeks exposure, remove 3 or 4 more nails, clean these and reweigh, and so on to the end of the test.
Corrosion testing is a little tricky, unforeseen things go wrong. This is why it is always best to use not just one, but 3 or 4 test specimens for each stage of the test.
Try the links in the MadSci Library for more information on Chemistry.