During the plant start-up in 1984, about 70% of more than fifty new field erected ground storage tanks were showing signs of aggressive corrosion on the bottom underside. This was found to be due to deficiencies in the cathodic protection system, use of limestone aggregate in the foundation pads and aggressive soil conditions,
In subsequent years, an intensive tank bottom inspection program was implemented, especially for tanks containing hazardous products. Several tank bottoms were either replaced with foundation pad modifications or repaired with fiberglass lining. The cathodic protection system was also upgraded by adding more ground beds and rectifier units. Inspection results of several tanks, 10 years after the repairs show no significant signs of subsequent corrosion.
Storage tanks, used for product storage in chemical plants when installed in severe environments can be subject to bottom underside corrosion. A good corrosion control program should be considered the first line of defense against product losses and ground contamination. It is our responsibility to protect our environment as well as to avoid production losses. Therefore, having a good design and inspection program for all tanks protects the environment, keeps the plant running and saves money in the long run.
The intent of this paper is to explain our company?s experience with tank bottom underside corrosion, which occurred, on majority of the storage tanks prior to and during the plant start-up. It also explains the inspection and repair procedures that were used to redress this problem.
The company has about 180-ground storage tanks of different sizes built to API-650 and 620 standard. They are located in different areas in the plant. These tanks are fabricated from carbon steel A283 grade-c, with various types of lining systems used in some of the storage tanks. They contain products such as sodium hydroxide (Caustic), styrene, benzene, crude industrial ethanol (CIE) and ethylene dichloride (EDC) in addition to water and brine.
About 80% of the tanks are erected on aggregate pads. These pads prove to be the source of corrosion cells under the tank bottoms that a proper cathodic protection (CP) system may have minimized. However, the CP system was not energized during the tank construction. This allowed the corrosion process to continue, resulting in severe bottom underside corrosion in a short span of time.
BACKGROUND AND DISCUSSION
The construction of the tank farm started in late 198 1. In November 1984 during a pre-start up inspection of the installed lining system in a 50% caustic storage tank, the lining inspector discovered random indications of blistering to the completed lining film on the floor plate. The lining film blisters showed signs of rust leaching circumferentially. The inspector lifted one of the blisters and found corrosion deposits, which filled the hole, which penetrated right through the bottom plate. Figure (1) shows the hole through the plate under the paint blister. Further scrutiny of this storage tank floor led to the discovery of an irregular pattern of plate holes throughout the floor. This was determined to be the result of bottom underside corrosion. It was a very surprising discovery as the storage tank botto