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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.enter image description here

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    $\begingroup$ Which direction is gravity acting in this photo? Maybe water settles in the troughs? Also, is the pure Fe or is it an alloy or plated? The bends will have more cracks and therefore more susceptible to corrosion. $\endgroup$ – Tunk Dec 5 '19 at 6:26
  • $\begingroup$ That is an aluminum roofing sheet and the photo was taken on its bottom part not the sun facing part. $\endgroup$ – user616424 Dec 5 '19 at 9:13
  • $\begingroup$ Sorry for the mistake. It's not aluminum $\endgroup$ – user616424 Dec 5 '19 at 14:21
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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).

https://hardide.com/technical/astm-b117-corrosion-resistance/ rust

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