ABSTRACT The -0.850 volt copper sulfate electrode (CSE), cathodic protection(CP) criterion for typical buried coated steel pipelines was determined by Robert J. Kuhn during his employment at New Orleans Public Service, Inc. from 1913 to 1933, 1,2 the year he published it in Reference #2 and as A. W. Peabody 3 notes in his book, Control of Pipeline Corrosion on Page 35, Experience has proved the adequacy of this criterion for practical use.
Kuhn?s Contributions to the early understanding of the application of CP were so notable that a medal was struck in Germany in 1970 to commemorate him as the Father of CP. 4
Indeed it would be easy to conclude that Kuhn?s postulation in 1933 that the protective potential is probably in the neighborhood of -0.850 V is as accurate today as it was then and furthermore, may not change substantially over the next sixty (60) years in terms of its practical application. 4
This article offers information confirming that the criterion is scientifically sound.
INTRODUCTION The following is copied direct from Reference #2, Kuhn?s 1933 article.
An electrically continuous insulated and coated pipeline therefore assumes some potential when placed in the earth and backfilled, and this potential is an average of the electrical potentials or voltages with respect to the soil of various exposed metal parts of the line depending on the metal used in the line, the nature of the soil, the amount of mill scale and rusted surface, etc. This potential is determined with reference to a standard non-polarized copper-sulfate electrode placed in the earth several feet from the pipe line, using a McCollum earth current meter as a high resistance voltmeter (1,000,000 ohms per volt). Values commonly encountered range around -0.600 volt with the steel pipe negative to the copper-sulfate electrode.
On lines which were covered with mill scale and rust before coating, the values will probably be less negative, ranging possibly around -0.500 volt, a less favorable potential than -0.600 volt.
The potential to a copper-sulfate electrode, to which a pipe must be lowered in order to stop corrosion, is probably in the neighborhood of -0.850 volt. This means that a pipe line free of mill scale must be lowered about -0.250 volt, while one covered with mill scale under the coating by -0.350 volt or the difference between the initial and the final values. 2
It is important to note the following facts from the above.
1. The -0.850 volt, CSE, CP criterion was determined for buried coated steel pipelines and Kuhn made no reference to its use on buried bare steel pipelines which is economically impractical in most cases and is virtually impossible to achieve in many cases. Also, a bare pipeline polarized to this potential with the CP current applied would be grossly over protected in essentially all cases. By 1933 the 300 mV shift, E log I and earth current technique criteria for buried bare steel pipelines had been widely accepted for several years.
2. The placement for the CSE was in the earth several feet from the pipeline.
3. The potential was measured with the CP current applied.
4. The CSE was considered to be non-polarized when the potential was measured with a 1,000,000 ohms per volt voltmeter. Since the maximum current flow through the CSE at a measurement of 1 volt would be 1 volt divided by 1,000,000 ohms or 1 microampere, this is acceptable, especially when the measurement was completed without lingering long enough to allow the current flow to partially polarized the CSE.
5. The coated pipeline was insulated (electrically isolated) from all other structures such as pump stations, compressor statio