Soils, corrosion underground tanks

Cathodic Protection This electrochemical method of corrosion control has found wide application in the protection of carbon steel underground structures such as pipe lines and tanks from external soil corrosion. It is also widely used in water systems to protect ship hulls, offshore structures, and water-storage tanks. 

Today s underground tanks must last thirty or more years without undue maintenance. To meet these criteria they must be able to maintain their structural integrity and resist the corrosive effects of soil and gasoline, including gasoline that has been contaminated with moisture and soil. 

The second system is based on the application of impressed current that is forced through anodes to the protected structure such as the tank by a current source of sufficient potential. Properly designed CP systems that are well maintained and operate at the correct current density are a proven method of protecting tanks from the corrosive effects of contact with corrosive soils. In addition to protection of underground tanks, CP is also useful for aboveground double-bottom tanks and for internal corrosion protection. 

Standards require that today s underground tanks must last thirty or more years without undue maintenance. To meet these criteria, they must be able to maintain structural integrity and resist the corrosive effects of soil and gasoline, including gasoline that has been contaminated by moisture and soil. The tank just mentioned that was removed in 1991 met these requirements, but two steel tanks unearthed from the same site at that time failed to meet them. One was dusted with white metal oxide and the other showed signs of corrosion at the weld line. Rust had weakened this joint so much that it could be scraped away with a pocketknife. Tests and evaluations were conducted on the RP tank that had been in the ground for 25 years tests were also conducted on similarly constructed tanks unearthed at 51 and 71 years that showed the RP tanks could more than meet the service requirements. Table 6.3 provides factual, useftil data from these tests. 

Isophthalic polyesters provide corrosion resistance in a wide variety of end uses. Recent excavations of 25-year-old FRP underground gasoline storage tanks offer evidence to the resin s resistance to internal chemical attack or external attack by water in the siuroimding soil. Isopolyester pipes and containment vaults also provide protection against failme from corrosion by acid media. Reference [2] provides information on FRP piping systems. 

This is one of the oldest techniques of corrosion protection of steel in a corrosive environment, such as water or soils. Cathodic protection has been applied very successfully to the protection of pipelines, storage tanks and underground structures in petroleum and oil industries. Cathodic protection has been widely applied for the protection of concrete structures. In principle, a small negative voltage is applied to the steel in concrete which makes all the steel surface the cathode and eliminates the anode areas on the steel surface. This technique has been discussed in detail in Chapter 5. The cathodic reaction at the surface leads to increased alkalinity which passivates steel. 

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