You could make a study of the different ways in which a tractor can rust. The tin can over the exhaust pipe prevents the interior of the engine from filling with water. The pocked formation of the rust is from this tractor being exposed to the sun and snow away from the shelter of the surrounding trees. Each winter moisture saturates the metal, coating the textured pock marks with water that will expand when it freezes. over decades the pockmarks will become flakey cancerous blights that hold more water and tear the metal up further with each winter's freeze.