The spot is not the typical red/brown orange rust and simple scaling off layers, rather it is a formation of innumerable mm to cm sized depressed pits with flat black bottom. The rust also has smoother appearance. The bottom of black pit is likely to be magnetite. The ridges are quite smooth.
The sample is of a oil pipeline that was never installed but was left in open field under sky, in a suburban atmosphere far (~400 km) from sea, so no chance for acid rain.
One can attribute smoothing action to raindrop impact, but why the pits are of this shape which has no reason for macroscopic segregation on chemical inhomogeneity as seen from such scale? Even if initial raindrops initiate the pit, then why the pattern from a raindrop on steel surface persists while steel strength is almost always above 200 MPa? And is my guess correct that the bottom is more rust-resistant magnetite? (I do not have access to the sample, only the image). Picture width approx.15 cm.
Sumit Bhowmick
The steel pipe surface morphology seen attached to your query reflects the two - phase microstructure, leading to sets up a galvanic cells responsible for accelerating the corrosion process. In this case, two corrosion rates obtained, the higher rate is comes from the anode area. Now, think about the rain impact on the stability of the as formed oxide layer which surely be washed out continuously leaving the anode area continuously active, looking as pits or spots as you described.
Best regards
I can suppose that this picture has resulted from the raindrops retained on the steel surface. Then, drops are prone to remain in the same places due to original waviness or some quantities of lubrication on the surface. This assumption is justified by the elongated patterns on the side of the pipe where water flows down.
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