This work deals with the corrosion evaluation of steel rebars in an underground water-storage tank using visual inspection, half-cell potential, and electrochemical impedance spectroscopy (EIS). The studied case has a service life of 65 years. From the inside, red stains were observed on the roof of the tank, and some cracks, as well as spalling of the concrete’s cover, raised suspicion of corrosion damage. Twenty-seven points on the tank’s roof were selected to drill out cores for further investigation. Measurements of the chloride penetration in the roof showed that the bottom part of the roof that was exposed to greater humidity and chloride-laden vapor was at risk of heavy corrosion. EIS studies of the steel rebars at different depths confirmed that those located in the upper part of the roof still have appropriate resistance to corrosion, but the lower ones exhibited a high corrosion rate. As a result, a repair of at least 12 cm in depth of the bottom of the roof and the installation of a proper ventilation system inside the tank were recommended.

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