There is a great deal of industry discussion concerning the appropriateness of the use of the "100 mV Polarization" cathodic protection criterion in facilities where multiple metallic compositions (mixed metals) are installed underground and are mechanically/electrically bonded. The purpose of this paper is to present technical support for the use of this criterion, in this application, and to recommend field practices to compensate for its' inherent weakness and improve confidence in its' use.
When constructability, operations, and maintenance are all considered, cathodic protection (CP) of complex facilities is usually best accomplished following either a very limited or a total protection philosophy. The "limited" style is when the Owner/Operator is only concerned about protecting a couple of piping systems (maybe fuel oil and/or fuel gas piping) and an above ground storage tank (AST) bottom or two. The "total" style is when the Owner/Operator has decided to protect all underground metallic components - generally on the theory that once the initial capital investment has been made, corrosion mitigation in the form of CP is cost effective - given the extended economic life of such facilities, the current industry initiative to reduce repair costs and down time, and a new regulatory focus on mechanical integrity. The Author engineers and designs CP systems for a broad range of complex facilities (terminals, petrochemicals, power generation, etc.). When a total CP philosophy is requested and when otherwise appropriate, the Author will commonly recommend the use of semi-remote/deep, area, impressed current CP systems and the use of the "100 mV polarization" criterion to verify the adequacy of the performance of the installed CP system(s). Because of the mass of buried metal structures (piping, conduit, structural steel, grounding, fencing, AST bottoms, etc.,), the recent NEC (2005 Revision) requirement to bond steel reinforcement of concrete foundations to facility grounding, and the need to bond all metallic structures together for personnel electrical safety, protecting an entire facility may require five to ten times the total CP current needed to protect targeted structures in the "limited" CP fashion. Combining the "total" CP philosophy with the "850 mV, polarized" criterion can lead to the need for even greater CP current capacity. Reducing the current capacity demand by the use of the "100 mV polarization" criterion, when appropriate, can and will yield benefits in the form of reduced probability of stray currents, operating power costs, and capital costs. There is a great deal of industry discussion concerning the appropriateness of the use of the "100 mV Polarization" cathodic protection criterion in facilities where multiple metallic compositions (mixed metal) are installed underground and are mechanically/electrically bonded. The balance of this paper is intended to present technical support for the use of this criterion, in this application, and to recommend field practices to compensate for its' inherent weakness and improve confidence in its' use
DEFINITIONS PER NACE International
The electrode of an electrochemical cell at which oxidation occurs. Electrons flow away from the anode in the external circuit. Corrosion usually occurs and metal ions enter solution at the anode.
