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 11-5-2-280.1-6.4 - Stray Current

Current through June 25, 2024
An unintended current that is affecting the structure you
						are trying to protect is referred to as a stray current. Stray currents can
						cause rapid corrosion failure of a buried metallic structure and are caused by
						an electric current flowing through the earth in an unintended path. If the
						metallic object you are trying to cathodically protect is buried near the path
						of the stray current, the current may "jump-on" the protected structure because
						it offers a lower resistance path for the current to flow. The affected
						structure will be cathodic where the stray current enters but will be highly
						anodic where the stray current returns to the earth. At the point where the
						current discharges, rapid corrosion of the structure intended to be protected
						will occur.
Although stray currents are relatively rare on UST
						systems, common sources include: a) Railroad crossing signals (powered by
						batteries); b) Traffic signals that have induction type sensors buried in the
						pavement; c) Portable or fixed emergency power generators; d) Electrical
						railway systems such as streetcars or subways in urban areas; e) DC welding
						operations and other types of industrial machinery or processes that utilize DC
						power.
If unsteady readings are observed on the protected
						structure and you have determined that it is not because of a bad electrical
						connection, you should suspect that stray current is affecting the protected
						structure. In some cases, a pattern can be seen in the potential whereby it
						alternates between two relatively stable readings. These patterns can sometimes
						help to identify the source of the stray current. If you suspect that stray
						current may be affecting the UST system, a thorough investigation must be
						conducted as soon as possible by a qualified corrosion expert since stray
						current can cause a rapid failure of the affected structure.
Cathodic Interference - When the impressed current
						cathodic protection system operating on the structure you are trying to protect
						causes an unintended current on some other nearby structure, this type of stray
						current is referred to as "cathodic interference". Cathodic interference can
						cause a rapid failure of the water lines and other buried metallic structures
						at the facility where the cathodic protection system is operating. If you
						observe what you believe to be an abnormally high (more negative) potential on
						a buried metallic structure, you should suspect that the impressed current
						system operating on the UST system is causing cathodic interference. Instances
						where cathodic interference may be present include: a) copper water lines that
						are not bonded to the impressed current system and have a polarized potential
						of greater than -200 mV; b) metallic flex connectors associated with fiberglass
						reinforced plastic piping that have abnormally high (more negative) potentials
						and are not bonded to the impressed current system; c) sti-P3 tanks are buried
						at a facility where there is an impressed current system operating and are not
						bonded to the negative circuit. When the sti-P3 tanks have zinc anodes and a
						potential more negative than -1100 mV (more negative than -1600 mV in the case
						of magnesium anodes) is observed, it is likely that cathodic interference is
						occurring. Because of the potential for stray current to impact sti-P3 tanks,
						it is normally necessary to bond them into the impressed current system. A
						corrosion expert must be consulted whenever cathodic interference is suspected
						in order to properly investigate and make any repairs/modifications that may be
						necessary.