This is quote from OxyChem document on Hypochlorites:
Steel tanks in caustic soda service become passivated with a gray-black film forming on the metal. Thereafter, caustic soda at less than 130° F will dissolve very little iron from the tank. The protective film should be preserved whenever practical. Therefore, when sediment is rinsed from a tank, only the bottom should be washed. Caustic Soda solution should then be put into the tank as soon as the washing is finished. Otherwise a soft rust will form and contaminate any future storage.
I cannot understand what can be this gray-black film chemical formula. Plus is he referring soft rust to common Iron (II) Oxide when it comes to O2 rich atmosphere. May be thats why he instructs to refill cleansed container as soon as possible?