Empty ballast tanks will corrode faster than areas fully immersed due to the thin - and electo conducting - moisture film covering them.
[4] Many maritime accidents have been caused by corrosion, and this has led to stringent regulations concerning protective coatings for ballast tanks.
Therefore ongoing research attempts to find water treatment systems that kill invasive species, while not having a destructive effect on the ballast tank coatings.
[6] As double-hulled tankers were introduced it meant that there was more ballast tank area had to be coated and therefore a greater capital investment for ship owners.
With the onset of the OPA 90 and later the amendments to MARPOL annex 1, single hull tankers (without alternative method) have basically phased out.
Empty tanks act as insulation from the cold sea and allow the warm cargo areas to retain their heat longer.