as depth increases:
temperature decreases,
o2 concentration decreases...
and since o2 is the necessary factor for the normal corrosion to occur, and high temperature speed the corrsion process, the lack of o2 and low temp predict minimal corrosion should occur, if any...
it leads nicely to the desulfurvibrio bacteria, which reduces sulphate ions and at the same time oxides Fe to cause an unexpected corrosion...