Soil Side Bottom (SSB) corrosion of aboveground storage tanks (ASTs) is a major cause of tank bottom failures that are costly to repair and potentially hazardous to the environment. This specific type of corrosion is called soil side pitting, and is typically caused by the ingress of external contaminants and moisture through gaps between the metal tank bottom and the soil or concrete slab it rests on. The continual filling and emptying of the stored product within the tank causes its metal bottom plates to flex, creating a bellows effect that draws in moisture and contaminants.
Traditional cathodic protection (CP) methods have been employed to prevent soil side bottom corrosion, but a cathodic protection system cannot function when vapor space gaps disrupt the electro-chemical circuits formed by direct contact between the soil and the tank bottoms. Paint on bottom plates can be destroyed by the heat of welding. Bitumen layers tend to dry and crack at the edges allowing moisture to enter. The concrete in foundations or double bottom fills is porous, and over time allows entry of moisture and contaminants. These methods do not work, hence the global prevalence of tank SSB leaks.
Tank bottom plate corrosion leads to costly repairs and lost revenue!
- Tank operators have to suspend operations in order to complete patchwork repairs, or replace the entire bottoms altogether.
- Storage tank leaks caused by bottom plate corrosion are hazardous to the environment.
- The opportunity costs when storage tanks are out of operation can be crippling.
NTIC has developed an innovative solution, Zerust ReCAST-SSB, which works as a three-part system combining cathodic protection, volatile corrosion inhibitors, and soluble corrosion inhibitors, providing efficient protection at all times. NTIC monitors the effectiveness of the inhibitors without causing any unscheduled stoppages. This three-part system allows for inhibitor concentrations to be adjusted through an inhibitor delivery system with a delivery station that is installed outside the tank base.
Pitting corrosion of soil-side bottoms of Aboveground Storage Tanks (ASTs) causes the formation of holes that allows product leakage into the environment. This specific type of corrosion forms in the gaps or “vapor spaces” between the metal tank bottom and the soil/concrete slab it rests on. These “vapor spaces” are formed when the metal bottom flexes up and down when product is filled and removed from the AST. In addition, when the AST is constructed using bituminous beds below the tank bottoms (a common practice), it leads to attack from the acids in the surrounding soil, which seep through these same beds. Because of the leaks caused by corrosion, tank operators have to periodically stop tank operations for repairs or to replacement of the entire bottoms. The stoppage time can come within the first few years of a tank’s life (2 to 4 years), or at scheduled inspection intervals (6 to 10 years depending on relevant API 650 guidelines).