A quick search on Wikipedia turns this up.
Sodium hypochlorite is a chemical compound with the formula NaClO. Sodium hypochlorite solution, commonly known as bleach, is frequently used as a disinfectant or a bleaching agent.
Household bleach sold for use in laundering clothes is a 3-6% solution of sodium hypochlorite at the time of manufacture. Strength varies from one formulation to another and gradually decreases with long storage.
Household bleach is, in general, a solution containing 4-6% sodium hypochlorite and 0.01-0.05% sodium hydroxide; the sodium hydroxide is used to delay the breakdown of sodium hypochlorite into sodium chloride and sodium chlorate.[4]
I would say if you clean them before using them as storage... knowing that using an ammonia type cleaner on these barrels could possibly KILL you if there is any Bleach residue on the barrels.