Mississippi Administrative Code
Title 11 - Mississippi Department of Environmental Quality
Part 5 - Underground Storage Tank Regulations
Chapter 2 - Mississippi Commission on Environmental Quality Underground Storage Tanks Regulations Technical Standards and Corrective Action Requirements for Owners and Operators of Underground Storage Tanks (UST) (Adopted March 22, 1989; Amended August 25, 2011, Last Amended August 23, 2018)
Appendix 11-5-2-280.1 - GUIDELINES FOR THE EVALUATION OF UNDERGROUND STORAGE TANK CATHODIC PROTECTION SYSTEMS
Section 11-5-2-280.1-6 - CATHODIC PROTECTION TESTING
Section 11-5-2-280.1-6.2 - Test Criteria

There are three test criteria that can be utilized to indicate if adequate cathodic protection is being provided to the structure being evaluated:

850 On - A structure-to-soil potential of-850 mV or more negative with the protective current applied. This is commonly referred to as "850 on" or the "on potential". This criterion is normally the only one available for galvanic systems since the protective current usually cannot be interrupted.

Voltage drops (see Section 6.3) other than those across the structure to electrolyte boundary must be taken into consideration whenever this criterion is applied. Voltage drops may have a significant impact on the potentials observed when testing impressed current systems with the protective current applied. Therefore, the 850 on criterion is not applicable to impressed current systems.

850 Off- A structure-to-soil potential of-850 mV or more negative with the protective current temporarily interrupted. This is referred to variously as "850 off', "polarized potential" or "instant off potential". This criterion is applicable to impressed current and galvanic systems where the protective current can be interrupted. Caution must be exercised when testing impressed current systems to ensure that no active sacrificial anodes are also installed near the protected structure. If there are active anodes influencing the observed potential, the 850 off criterion is not applicable.

The instant off potential is the 2nd value that is observed on a digital voltmeter the instant the power is interrupted. The first number that appears immediately after power interruption must be disregarded. After the second number appears, a rapid decay (depolarization) of the structure will normally occur. In order to obtain instant off potentials, a current interrupter or a 2nd person is necessary. If a current interrupter is not available, have the second person throw the power switch at the rectifier off for 3 seconds and then back on for 15 seconds. Repeat this procedure until you are sure an accurate instant off reading has been obtained.

This criterion is considered by most to be the best indicator that adequate cathodic protection has been provided. Therefore, consideration should be given to adjusting the rectifier output upward until the 850 off criterion has been met if this is feasible.

100 mV Polarization - A polarization voltage shift of at least 100 mV. Commonly referred to as "100 mV polarization" or "100 mV shift". This criterion is applicable to galvanic and impressed current systems where the protective current can be temporarily interrupted. Either the formation or the decay of at least 100 mV polarization may be used to evaluate adequate cathodic protection.

The "true" polarized potential may take a considerable length of time to effectively form on a structure that has had cathodic protection newly applied. If the protective current is interrupted on a metallic structure that has been under cathodic protection, the polarization will begin to decay nearly instantaneously. For this reason, it is important that the protective current not be interrupted for any significant length of time. Generally, not more than 24 hours should be allowed for the 100 mV depolarization to occur. On a well-coated structure complete depolarization may take as long as 60-90 days. Complete depolarization of uncoated structures will usually occur within 48 hours although it could take as long as 30 days.

The base reading from which to begin the measurement of the voltage shift is the instant off potential. For example, a structure exhibits an on voltage of-835 mV. The instant off voltage is -720mV. In order to meet the 100 mV polarization criteria, the structure-to-soil potential must decay to at least -620 mV (final voltage).

The use of native potentials to demonstrate the formation of 100 mV polarization is generally only applicable when a system is initially energized or is re-energized after a complete depolarization has occurred. This is because it is necessary to leave the reference electrode undisturbed (or returned to the exact position) between the time the native and the final voltage are obtained.

It is only necessary to conduct a 100 mV polarization test on that component of the UST system where the lowest (most positive) instant off structure-to-soil potential exists in order to demonstrate that the UST system meets this criterion. If the criterion is met at the test point where the potential is most positive, it can be assumed that it will be met at all other test locations.