Why tanks corrode? Underground steel tanks corrode due to an electrochemical reaction between the tank and the surrounding soil. The process of corrosion occurs due to small voltage differences on the steel surface that result in the flow of DC current from one location to another. Where current flows from the tank into the soil, corrosion occurs. This location is called the anode in a corrosion circuit. Where current flows from the soil to the tank, no corrosion occurs. This location is called the cathode. The progress of corrosion is determined by the anode/cathode remain constant over time. Corrosion rates are generally higher in wet soil environments since the conductivity of the soil promotes the flow of DC current in the corrosion circuit.
Corrosion generally exhibits itself on underground tanks in either a general overall rusting or more commonly, a pitting attack. Pit locations may result from metallurgical conditions of the steel surface or soil variations such as rocks, salts, fertilizer, moisture concentration, oxygen concentration, etc.
Protecting underground tanks from corrosion is easily achieved by the use of two commonly applied protection methods: external coating and cathodic protection. These two methods are complementary and should be used in conjunction with the other. An effective external protective coating insulates the steel from the soil environment, thus preventing the flow of corrosion current from the anode to the cathode. An effective external coating can protect over 99% of the tank surface area. However, no coating is perfect. Damage from construction or soil stresses create tiny defects, which may result in accelerated corrosion at the defect.
Cathodic protection prevents corrosion at those defects by applying DC current from an external source, forcing the tank to become a cathode. Application of sufficient DC current to the tank will prevent any corrosion from occurring. The two general types of cathodic protection systems are sacrificial and impressed current. Sacrificial systems are used when the amount of current required for protection is small, such as in underground propane tanks. Impressed current systems are more commonly used for large structures such as large diameter pipelines.
Electrical isolation of the tank from metallic piping systems and electrical grounds is critical or the cathodic protection system's effectiveness.