Electrical currents flowing from a metallic structure, such as a steel tank, into the ground causes corrosion. Cathodic protection is a process that redirects this flow to prevent corrosion. Sacrificial anodes are installed at each end of a tank, (a sacrificial anode is a softer metal than the tank, either zinc or magnesium), this allows electrolysis of the soil to degrade the sacrificial anode as opposed to the tank.
Testing is performed with a special type of voltmeter, which measures in millivolts to see if the system is performing within factory standards. This test may be necessary to comply with local regulations, or certain tank warranty requirements. Test results are limited to the conditions existing at the time of the inspection only and does not exclude the possibility of a past, present, or future problem.
A structure to soil test voltage reading of at least minus 0.85 volts measured between the structure and a copper-copper sulfate electrode must be maintained for proper system operations. Many new and recently installed underground steel tanks such as the STIp3 come with cathodic protection.
Please reference the National Association of Corrosion Engineers' Recommended Practice of Corrosion Control for Underground Storage Tank Systems, RP-02-85, for additional information and cathodic protection criteria. If the test reading level is more negative than -1.800 volts, you should contact the Steel Tank Institute or your local STI-P3® tank supplier.