I have noticed that the rusting of iron starts from the squeezed or bent parts of iron piece. I want to know what is the peculiar reason for the observation that rusting starts from bent or irregular surface than smooth part of a given iron piece and it spreads to everywhere.
I think this question is same as wondering as to why a certain cancer patient has a cancer in the kidney but not on the skin? Nobody knows. In the same way, rusting is a random process as well. It can start anywhere. This iron sheet is not ultrapure iron sheet of homogeneous nature. One can see a lot of spots in troughs and crests of this sheet. Secondly you do not know the history of this sheet. Is this galvanized or not galvanized? This rusted spot may have a local impurity which can initiate a local electrochemical cell.
In formal atmospheric corrosion studies, my colleague uses flat sheets of test material and rusting pattern is totally random. They also do a scratch test by drawing a cross X on the flat coated sheet, in case they wish to test a coating. The purpose is to expose the "fresh" metallic surface. The rusting then starts from the X pattern. This image is an example of flat sheet used in ASTM coating. The rusting pattern is nothing but random design on the uncoated sheet (left most).