Boiler tubes in the penthouse portions are meant to extract the heat from the flue gases. The tube material and coatings on the boiler tubes are generally selected to mitigate the high-temperature oxidation and other metallurgical damage mechanisms. Tubes are not designed to experience aqueous corrosion from the external environment. For these reasons, boilers are designed and laid out in a fashion to limit the ingress of moisture and corrosive ions from external sources such as wind, cooling tower drift, etc. This paper addresses the case study on the failure of primer coating on the boiler tubes from the upset condition that allowed the flooding inside the penthouse leading to aggressive corrosion on the tube skin. This paper also discusses the multiple corrosion modes observed and their associated mechanisms followed by recommending corrective measures. Finally, the paper addresses the contribution of insulation material in heterogeneous corrosion morphologies.