Corrosion is one of the main causes of the failure of the storage tanks. In any plant, a number of tanks are used to store and deliver a variety of liquids and gases. The biggest problem is the corrosion of the bottom plate.
The complete network of plant consists of more than 100 tanks. These are used for storing and distributing potable water, petrol, diesel and H.F.O. About 70 numbers of Large Above Ground Storage Tanks (AST) are used for the storage of Heavy Fuel Oil and water. During internal inspection of one of the tanks, bottom plates shown signs of pitting corrosion of varying degrees and roof had numerous holes of varying sizes. But they mainly centered on the locations where roof was touching rafters. Magnetic flux leakage test (MFL) test indicated underside corrosion of bottom plates in number of tanks.
The paper presents and discusses the aspects involved with the corrosion of tank roof and the bottom plates. An attempt has been made to discuss the inspection methodology, the causes for corrosion, and methods of repair and refurbishment of the tanks. It also describes the suitable Cathodic Protection system for the underside corrosion protection of tank bottom plates.
Scope of this study is to analyze the corrosion behavior of the tanks and to find out various causes promoting corrosion of the tank material and to recommend a suitable remedial action.
CONSTRUCTION DETAIL OF THE TANK: (available in full paper)
Tank is installed over a concrete ring wall foundation. Bottom plate is resting on fine grade asphalt concrete 50 mm thick followed by compact sand, then lean concrete followed by oil and sea water proof foil (membrane). In theory all these layers form a perfect insulating material which in normal condition doesn't allow CP current to pass in order to protect the exposed bottom surface. Likewise it doesn't allow corrosion current to leave the tank bottom to enter soil and corrode the bottom plate ref fig #1.
CORROSION PROTECTION PROVIDED IN THE DESIGN:
Tank internal surface areas are painted and no other corrosion prevention method is applied, the tank is not insulated. Tank external surface areas are painted for corrosion protection. But bottom plate soil side has no painting.
From a corrosion engineer's point of view Bottom plate and Roof are the main areas of concern. Among many, Cathodic Protection (CP) is a proven method for controlling corrosion. However cathodic protection is not provided for these H.F.O. tanks.
INSPECTION & OBSERVATION:
Complete tank was drained. About 300 mm thick sludge and slurry were cleaned. The tank was hydro cleaned and gas freed for internal inspection. Visual inspection before sand blasting revealed numerous water filled blisters in the existing paint. After sand blasting the floor was inspected again and a number of through holes were observed with corrosion pitting up to a depth of one mm ref fig # 2. But no generalized corrosion was noticed.