Table 55, 



Potential readings for various 

 reference electrodes 



Type of Reference 

 Electrode 



Structure-to-Electrode 

 Reading Equivalent to 

 -0.85 Volt to Copper 

 Sulfate Electrodes 

 (Volts) 



To Correct Structure-to- 

 Electrode Reading to 

 Equivalent Reading to 

 Copper Sulfate Electrodes 

 (Volts) . 



Calomel 

 (Saturated) 



Silver-Silver 



Chloride 



(0.1 N Kel solution) 



Pure Zinc (Special 

 High Grade, 99.99 

 pet pure) 



-0.778 

 -0.84Q 



+ 0.25 1 



Add -0.072 

 Add -0.010 



Add -1.10 1 



1 



1 Based on zinc open circuit potential of -1.10 volts to copper 

 sulfate electrode. 



(2) Measurement Techniques . NACE Standard RP-01-69 (1976 Revi- 

 sion), "Control of External Corrosion on Underground or Submerged Metallic 

 Piping Systems," applies equally to any buried or immersed structure. The 

 following information and criteria for cathodic protection are quoted from 

 the above publication: 



"6.2.3 The. criteria in Section 6.3 have been developed through 

 laboratory experiment or empirically determined by evaluating data 

 obtained from successfully operated cathodic protection systems. It 

 is not intended that the corrosion engineer be limited to these 

 criteria if it can be demonstrated by other means that the control of 

 corrosion has been achieved. 



"6.2.4 Voltage measurements on pipelines are to be made 

 with the reference electrode located on the surface as 

 close as practicable to the pipeline. Such measurements 

 on all other structures are to be made with the reference 

 electrode positioned as close as feasible to the structure 

 surface being investigated. The corrosion engineer shall 

 consider voltage (IR) drops other than those across the 

 structure-electrolyte boundary, the presence of dissimilar 

 metals, and the influence of other structures for valid 

 interpretation of his voltage measurements. 



"6.2.5 No one criterion for evaluating the effectiveness 



of cathodic protection has proven to be satisfactory for 



all conditions. Often a combination of criteria is needed 

 for a single structure. 



356 



