Abstract: The macro inspection and ultrasonic wave inspection to the outside shell of insulated tanks are made in Qilu Petrochemical Corporation. The inspection results indicate that corrosion pits distribute over the surface of tank body. The corrosion mechanism of A3 steel for the corrosion occurred under the insulating layer is analyzed. It is pointed out that rain and water in the air permeate into the insulating layer, which makes the insulating layer bubbling, the water acts with chlorion produced in the insulating layer in the tank shell, thus, a galvanic corrosion occurs. Some measures to deal with the shell corrosion are put forward based on the selection of coating materials and corrosion control method of tank base.