Castrol Rustilo™ Aqua 21 (previously called Aquasafe™ 21) is a medium viscosity, mineral oil based product containing film-forming additives and coupling agents, which is supplied as a concentrate for dilution with water prior to use. After the evaporation of the water phase, the product leaves an ultra-thin, oily film on ferrous components. Rustilo Aqua 21 is an inter-operational, short to medium term corrosion preventive which is used as an emulsion. It is particularly recommended for protecting metal components from corrosion after other aqueous treatments.