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 › Appendix 11-5-2-280.1-C

Current through June 25, 2024
GENERALIZED INTERPRETATION OF
							 STRUCTURE-TO-SOIL POTENTIAL MEASUREMENTS (VOLTAGES) OBTAINED ON GALVANIC
							 CATHODIC PROTECTION SYSTEMS
Listed in this table are some generalized
							 observations that can be applied to the interpretation of structure-to-soil
							 potentials. Depending on the specific site conditions and other factors,
							 differing interpretations are possible.
VOLTAGE (mV) "ON"
GENERALIZED INTERPRETATION
POSITIVE
Test leads are reversed (negative should be connected
							 to the reference electrode and the positive should contact the structure you
							 are testing in order to observe negative voltages). Could indicate that stray
							 current is affecting the structure (consult with a corrosion expert).
0 to -100
Usually occurs when you are attempting to measure a
							 structure that has a test lead that is not continuous with the tank. Because
							 you are measuring the potential of a copper wire with reference to the
							 copper-copper sulfate half-cell, the potential is zero or very near it.
							 Disregard test lead and make direct contact with the protected structure.
-101 to -399
Try again A reading in this range is not normally seen
							 on an underground steel structure. Could indicate that steel structure is
							 electrically connected to a significant amount of a more noble metal (e.g.
							 copper). Very corroded low carbon steel may also be indicated.
-400 to -599
Steel structure does not meet regulatory requirements.
							 Usually means that the steel structure has no cathodic protection. Existing
							 sacrificial anodes could be completely "burned-out" or were never there to
							 begin with.
-600 to -849
Steel structure does not meet regulatory requirements.
							 Usually means that the steel structure has anodes but for whatever reason,
							 something is causing a low reading that may indicate adequate cathodic
							 protection has not been provided. The anodes may be trying to protect a
							 structure that requires more current than they can produce. The protected steel
							 structure may not be electrically isolated from all other metallic structures
							 (conduct continuity testing). The environmental conditions may not be favorable
							 at the time you are attempting to obtain the reading. Retest during the next 90
							 days to see if an acceptable reading can be obtained.
-850 to -1100
Steel structure protected by zinc anodes meets
							 regulatory requirements and cathodic protection is judged to be adequate.
							 Readings in this range are what you would expect on most
							 sti-P3 tanks that have not been modified and are reading
							 "good" since nearly all come from the manufacturer with zinc anodes.
-850 to -1600
Steel structure protected by magnesium anodes meets
							 regulatory requirements and cathodic protection is judged to be adequate.
							 Readings in this range are what you would typically expect on steel piping that
							 is reading "good" since magnesium anodes are generally installed on piping. You
							 may also find readings up to -1600 mV on a sti-P3 tank
							 that has been retrofitted or was supplied at the factory with magnesium anodes.
MORE NEGATIVE THAN -1100 WITH ZINC ANODES
							 ONLY
Voltages more negative than -1100 mV are theoretically
							 not possible if there are only zinc anodes installed. If you have a reading
							 more negative than -1100 mV and you are sure magnesium anodes are not present,
							 you should suspect that stray current may be affecting the cathodically
							 protected structure. A corrosion expert should be contacted immediately since
							 stray current can cause a corrosion failure in a relatively short period of
							 time.
MORE NEGATIVE THAN -1600
Voltages more negative than -1600 mV are theoretically
							 not possible with any sacrificial anode cathodic protection system. If you have
							 a reading more negative than -1600 mV on any galvanic cathodic protection
							 system, you should suspect that stray current may be affecting the cathodically
							 protected structure. A corrosion expert should be contacted immediately since
							 stray current can cause a corrosion failure in a relatively short period of
							 time.
VARIABLE
If the voltmeter readings vary you should suspect that
							 stray current may be affecting the cathodically protected structure. Sometimes,
							 the stray current can cause a pattern to develop that is recognizable. An
							 example would be the on/off pattern of a nearby DC powered welding operation. A
							 corrosion expert should be contacted immediately since stray current can cause
							 a corrosion failure in a relatively short period of time.
RAPIDLY FLUCTUATING
If the voltmeter will not stabilize, it usually means
							 that there is a high electrical resistance somewhere. Check all lead wires and
							 connections and make sure that you are making a solid and clean metal to metal
							 connection. Soil where the reference electrode is placed could be too dry. Add
							 water to the soil or wait until a heavy rain occurs and try again. Petroleum
							 contaminated soils may cause a high contact resistance. The tip of the
							 reference electrode may need to be cleaned or replaced.