IIRC leather promotes rust in two ways:
1. being absorbent, its damp surface against steel will get the rust going
2. some of the tanning ingredients can be pretty corrosive
Info from http://oror.essortment.com/leathertanning_rdcu.htm
VEGETABLE TANNING
Hides which have been tanned with a vegetable tanning agent solution produce flexible, but stiff leathers, such as those used in luggage, furniture, leashes, belts, hats, and harnesses.
Vegetable tanning consists of stringing hides on large frames, located inside large vats, and exposing them to tannin, a natural product found in the bark, wood, leaves and fruits of chestnut, oak and hemlock trees. Hides are transferred to many different bins during this step, each containing a stronger solution of tannin. Vegetable tanning prevents the skin from decay and shrinkage.
MINERAL TANNING
Mineral or chrome tanning is performed on skins which will be used for softer, stretchier leathers, such as those found in purses, bags, briefcases, shoes, gloves, boots, jackets, pants, and sandals.
Hides which are tanned with minerals are pickled first in an acid and salt mixture. From there, hides are soaked into a chromium-sulfate solution. This process is much faster than vegetable tanning, and is usually a 1-day project.
quits pouring out. Then they are worn dry for a custom fit.