A leak developed on the water wall for several boiler tubes within five months of the commissioning of a plant boiler. In this case, the boiler tubes were partially covered with a castable refractory layer near the water drum at the bottom and at the top of the steam drum. Severe corrosion was noticed on the external surface of the boiler tubes, particularly at the lower section near the water drum. A detailed analysis and microstructural characterization of the corroded tubes and refractory samples were conducted to understand the root cause for the corrosion. Caustic gouging was determined as the predominate cause for the corrosion of the boiler tubes’ external surface. A review of the plant’s history noted that the boiler had been idled for a long time before commissioning. Wet refractory had promoted migration of alkalis to the tube surfaces, thus subsequently initiating the corrosion process.