Any geometry that can trap water can accelerate corrosion drastically. moisture will act as an electrolyte in reaction cells between the air's oxygen and surface electrons of the metal, leading to rust.
The C type in your figure encourages the shedding of the water which is an essential component of rusting.
Water, for example is made of an oxygen atom with ...
A is worse because the item holds water. Holding water resuults in corrosion, certainly near the surface. C sheds water so it will normally be dry.
Accumulation of impurities seems a needlessly complicated explanation.