[GordMay]

I understand that you intend to perform a “prophylactic” cleansing on your Aluminum Water Tank. Subject to the following cautions, a Chlorine Bleach soaking should suffice to disinfect your tanks. The chlorine will kill any mold, and most bacteria - but only a manual scrubbing will remove the scum (and dead mold carcases). Hope you''ve got an access panel...

Long-term exposure to Chlorine is not recommended for any metal, Aluminum & Stainless Steel included.

Notwithstanding, it should be OK to shock-treat your tanks with Chlorine, provided you rinse very well after.

Despite being high in the Reactivity series, aluminum is resistant to corrosion because of the thin layer of aluminum oxide (Al2O3) which forms on its surface in air.
The oxide layer protects the metal beneath and renders it inert to any further reaction.

The thin oxide layer will react with an acid or alkali to form the corresponding aluminum salt + water. The exposed aluminum metal will then further react with acid or alkali to form the aluminum salt + hydrogen.

For this reason, aluminum cookware, water tanks (etc.) should not be allowed to come into prolonged contact with citrus fruits or vinegar (acids), or alkali cleaners such as caustic soda (sodium hydroxide).

As with the other elements, aluminum is absorbed and accumulated in the body, and has been linked to serious illnesses including osteoporosis, extreme nervousness, anemia, headache, decreased liver and kidney function, forgetfulness, speech disturbances and memory loss (Alzheimer’s).

Aluminum and chlorine do not react rapidly at room temperature, but if the aluminum is heated, the reaction is vigorous. The product is aluminum chloride, a white powder that fumes in air, reacting with water to form HCl gas and aluminum oxide.
Chlorine gas is poisonous and a very strong irritant. Inhalation of chlorine should be avoided. The reaction is sufficiently exothermic that some chlorine and aluminum chloride dust may be formed in the tank, so proper ventilation is necessary. Anhydrous aluminum chloride reacts violently and exothermically with water, forming hydrogen chloride, which is an irritant. Inhalation of aluminum chloride dust can irritate or burn mucous membranes.

HTH
Gord