If a tank previously leaked corrosive fluid through the bottom, accumulation of the fluid under the tank can cause external corrosion of the tank bottom. External corrosion also occurs when external insulation picks up groundwater, or when damaged or improperly sealed openings around nozzles and attachments allow water ingress. External surfaces of the tank, roof tank auxiliary equipment will corrode more rapidly if they are not protected with  protective coatings or with cathodic protection where surfaces are in contact with moisture. Continuous water contact due to localized corrosion. Areas susceptible to this should be coated with coatings designed to withstand immersion. The type of tank and the construction details used can affect the location and extent of external corrosion.