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 11-5-2-280.1-6.5 - Dissimilar Metals/Bimetallic Couples

Current through June 25, 2024
The effect bimetallic couples may have must also be
						considered whenever structure-to-soil potentials are obtained during the survey
						of a cathodic protection system. The concept of dissimilar metals/bimetallic
						couples and the impact they can have on the proper evaluation of cathodic
						protection systems is a difficult and controversial subject and a full
						discussion is beyond the scope of this document. However, you should be aware
						that bimetallic couples may substantially influence the structure-to-soil
						potentials of a tank system to the extent that the 100 mV polarization
						criterion is not applicable. Because the validity of the 100 mV criterion may
						be suspect, consideration should be given to only utilizing the -850 mV instant
						off criterion when evaluating impressed current systems. A brief discussion
						follows.
Caution must be exercised when evaluating steel UST
						systems that have metals of lower electrochemical potential electrically
						connected to them. Typically, bimetallic couples are only of concern on
						impressed current systems since those steel components protected by galvanic
						systems are electrically isolated from other metallic structures. Copper is the
						metal of lower potential that is commonly of concern. Sources of copper at UST
						facilities include the water service lines and the grounding system of the
						electrical power grid. Since the AC power supply to the submersible turbine
						pump should be continuous with the electrical service grounding system, which
						may in turn be continuous with the water lines, a significant amount of copper
						may be coupled to the steel UST system.
The effect this type of bimetallic couple has on the
						impressed current system can sometimes be clearly seen on those UST systems
						that store fuel for emergency power generators. Commonly these generator tank
						systems are installed with copper supply and return lines. When these tanks
						were retrofitted with an impressed current system, the copper lines were bonded
						into the cathodic protection system. In these instances, it is not uncommon to
						observe native structure-to-soil potentials on the UST system of-450 mV or more
						positive.
If the native structure-to-soil potential of the UST
						system is substantially lower than what you would normally expect, it is likely
						that a significant amount of copper is electrically bonded to the UST system.
						Typically, the expected native potential of a steel UST system should not be
						more positive than -500 mV.
To illustrate the effect of the copper-steel couple,
						consider the following example: A steel UST system that is coupled to copper
						has a native structure-to-soil potential of -300 mV with the bimetallic couple
						intact. If the copper couple is broken the UST system native potential is -600
						mV. With the copper couple intact, the polarized (off) potential of the UST
						system -450 mV. Although the voltage shift satisfies the 100 mV polarization
						criterion (from -300 mV to -450 mV), it is likely that the steel UST system is
						not adequately protected. This is because the UST system is not polarized at
						least 100 mV beyond the native potential of the steel. Since the true native
						potential of the steel UST system in this example is -600 mV, you would need to
						reach a polarized (instant off) potential of-700 mV or more negative.
Because the unaffected native potential of steel UST
						systems is generally not known, the application of the 100 mV polarization
						criterion would be inappropriate when there is a significant amount of copper
						(or other more noble metal) electrically continuous. For this reason, it is
						always desirable to demonstrate that the UST system satisfies the 850 off
						criterion when evaluating a cathodic protection system.