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
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.
