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

Current through June 25, 2024
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.