From the EPA...
Cathodic protection is one option for protecting an underground storage tank (UST) from corrosion. There are two types of systems for cathodic protection:
Sacrificial anodes can be attached to a coated1 steel UST for corrosion protection (see below, left and center). Sacrificial anodes are pieces of metal more electrically active than the steel UST. Because these anodes are more active, the corrosive current will exit from them rather than the UST. Thus, the UST is protected while the attached anode is sacrificed. Depleted anodes must be replaced for continued corrosion protection of the UST.
An impressed current system uses a rectifier to convert alternating current to direct current (see below, right). This current is sent through an insulated wire to the anodes, which are special metal bars buried in the soil near the UST. The current then flows through the soil to the UST system and returns to the rectifier through an insulated wire attached to the UST. The UST system is protected because the current going to the UST system overcomes the corrosion-causing current normally flowing away from it.
Federal regulations require that the cathodic protection systems installed at UST sites (field-installed) be designed by a corrosion expert.
The system must be tested by a qualified cathodic protection tester within six months of installation and at least every three years thereafter. In addition, cathodic protection systems must be tested within six months of any repair to any cathodically protected UST system.
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