ABSTRACT
Based on some articles 1,2,3,4,5,6,1,8 that have been published in Materials Performance (MP) and the NACE Courses 9 and Standards for Cathodic Protection (CP) 10,11,12,14 practitioners, this author has concluded that present day CP practitioners, in general, do not have an adequate understanding of the basic scientific principles associated with the measurement of the voltage (pipe-te-soil, PIS, potential) between a test lead on a buried steel pipeline and the copper rod in a copper sulfate electrode (CSE) with the CSE wood or ceramic bridge deep enough in the surface of the soil to be in contact with naturally moist soil directly over the pipeline at the test lead. This article presents basic scientific principles from textbooks, valid references and field measurement data in an effort to help promote a better understanding of the basics and to achieve improved CP practices to better serve industry and the public.
INTRODUCTION
In general CP practitioners have become overly concerned about perceived serious IR drop problems that are actually non-existent in typical PIS potential measurements at test lead locations on a buried steel pipeline. 1,2,3,4.5,6.7.8 It is often published in articles 1,2.3.45,6,7,8 in MP that all measurements of PIS potential include lR drop contributions. This necessarily implies that these IR drops are other than those across the PIS boundary. This is misleading because there are PIS potential measurements that do not include any lR drop, and there are PIS potential measurements that do not include any 'voltage (lR) drops other than those across the structure-lo-electrolyte boundary", Quoting 6.3.1.1 from Reference #1 S. RP0169: "A negative (cathodic) voltage of at least -0.85 volt as measured between the structure surface and a saturated copper-copper sulfate halfcell contacting the electrolyte. Determination of this voltage is to be made with the protective current applied. The Corrosion Engineer shall consider voltage (IR) drops other than those across the structure electrolyte boundary for valid interpretation of the voltage measurement."15 Quoting 6.2.2.L1 from Reference #10, RPO169-2002: "A negative (cathodic) potential of at least 850 mV with the cathodic protection applied. This potential is measured with respect to a saturated copperlcopper sulfate reference electrode contacting the electrolyte. Voltage drops other than those across the structure-to-electrolyte boundary must be considered for valid interpretation of this voltage measurement." 10 From the above quotations it is clear that it has been stipulated in aU issues of RPOl69 that voltage (IR) drops other than those across the PIS boundary require consideration and all voltage (IR) drops across the PIS boundary are deemed to be integral parts of the protective potential measurement and they require no consideration.
DISCUSSION
The use of the word "consider" in 6.3.1.1 in Reference #15, the original RP0169 IS and its subsequent revisions through RPOI69-83, 16 was outwardly criticized by some with limited knowledge of IR drops that can be associated with PIS potential measurements as the followingquotation from Reference #6 by Dale Miller reveals.
"IR Drop Controversy:
At the center of these controversial discussions is the importance of 'IR drop' readings in monitoring a cathodic protection system. The proposed changes to the criteria suggest that TR drops should be removed from the potential measurements rather than just considered ascurrently stated in RPOI69",6 (RP0169-83).16
