"6.3.1.1 A negative (cathodic) voltage of at least Q. 85 

 volt as measured between the structure and a saturated 

 copper-copper sulfate reference electrode contacting the 

 electrolyte. Determination of this voltage is to be made 

 with the protective current applied. 



"6.3.1.2 A minimum negative (cathodicl voltage shift of 300 

 millivolts, produced by the application of protective 

 current. The voltage shift is measured between the 

 structure surface and a stable reference electrode 

 contacting the electrolyte. This criterion of voltage 

 shift applies to structures not in contact with dissimilar 

 metals. 



"6.3.1.3 A minimum negative (cathodicl voltage shift of 

 100 millivolts measured between the structure surface and 

 a stable reference electrode contacting the electrolyte. 

 This polarization voltage shift is to be determined by 

 interrupting the protective current and measuring the 

 polarization decay. When the current is initially interrupted, 

 an immediate voltage shift will occur. The voltage reading 

 after the immediate shift, shall be used as the base 

 reading from which to measure polarization decay." 



Paragraphs 6.3.1.4 and 6.3.1.5 are not quoted as they apply only to 

 specific situations not generally encountered. 



The U.S. Department of Transportation has issued "Regulations for the 

 Transmission of Natural and Other Gas by Pipeline, Part 192, Title 49," 

 provisions of which are now in effect. Subpart I contains requirements for 

 corrosion control. Criteria for protection are, in effect, identical with 

 those in the NACE publication quoted above. 



(a) Components of Potential Measurement . The voltage drop 

 between two points in a medium is equal to the current flowing between the 

 points, I, multiplied by the resistance of the medium R. For this reason, 

 voltage or potential drops caused by current flowing through resistive 

 electrolytes are called IR drops. 



The NACE Standard RP-Q1-69, 1976 Revision, quoted above states, in 

 paragraph 6.2.4, that IR drops are to be "considered." Unfortunately, there 

 are still many workers in the field who ignore the IR drop contribution to 

 potential measurements and continue to record structure-to-electrolyte 

 potentials with no consideration of IR drop. In such cases the recorded 

 potential will always be higher (more negative) than it actually is. If a 

 reading of -0.85 volt was recorded, indicating protection according to the 

 criterion given in Paragraph 6.3.1.1, the true potential, after deducting IR 

 drop, may well he substantially below protective levels. The amount of 

 error will depend primarily on electrolyte resistivity. Examples are shown 

 in Table 56. 



As can be seen in Table 56, the only time that IR drop can be ignored 

 safely is when potentials are measured in seawater or similar electrolyte 

 with a resistivity less than 50 ohm centimeters. In seawater the IR drop is 



357 



