Never heard of the use of chalk....but I guess it could work as a drier.  If you were to use it I would first put it in the oven at 200 degrees for a few hours to bake out any moisture.  If the chalk has been sitting in the open air for any length of time it will have absorbed moisture from the air.

If you live in a dry climate and the tank will not be subject to any temperature or humidity swings you can probably just tape over the openings and seal the tank in a plastic bag.

For storage in damp climates with humidity changes I would dump a bit of an oil and diesel fuel mix and slosh it around to coat things before I taped over the openings and stored it in the plastic bag.

When it is time to bring the tank back to life clean it with some Simple Green or a similar degreaser, then fix the surface rust with phosporic acid or a vineger treatment....then flush and dry.