[blam]

" The film will only form on the surface exposed to moisture and oxygen, which is only (but everywhere) on the surface of the entire pillar from top to bottom."

And, will continue to grow at a decreasing rate as less iron is exposed to the ambient oxygen.

[Wonder Warthog]

"And, will continue to grow at a decreasing rate as less iron is exposed to the ambient oxygen."

Probably. That depends on the permeability of the film to oxygen. Some films are very impermeable (oxides of aluminum and titanium). Some are less impermeable (other iron oxides, copper oxide, and the like).

From the situation, I'd guess that this particular film is one of the highly impermeable ones.