and contacting electrolyte, into a subsurface structure, corrosion current 

 and attendant corrosion will be stopped if the external current flow counter- 

 acts the corrosion current at all parts of the substructure surface. In 

 this case the substructure becomes all cathode and is protected against 

 corrosion, i.e., if all areas on a corroding substructure are polarized (p r 

 made equal) to the same open circuit potential, corrosion will be impossible 

 because there will be no potential difference between anode and cathode 

 areas and no corrosion current can flow. In practice, the potential applied 

 to the surface, called polarization potential, must equal or exceed the 

 open circuit potential of the most anodic area in order to stop all corrosion. 



To determine when or if this condition exists, there must be some way 

 to measure the potential existing between the protected substructure and 

 its contacting electrolyte environment. Based on this concept, potentials 

 should be measured directly across the interface between the substructure 

 and its environment. This is relatively simple with marine substructures 

 in seawater but is seldom feasible when working with substructures in soil 

 such as pipelines or buried cylindrical storage tanks. Common practice 

 with buried substructures is to measure the potential between the substructure 

 and the soil at the surface directly above the substructure. The measured 

 potential includes polarization potential plus a potential, usually called 

 IR drop, caused by current flowing through a part of the resistance 

 between the structure and the external anode installation. 



(1) Potential Measurement Apparatus . As potential measurement is a 

 useful approach to determining if corrosion is present or absent, potential 

 measurement methods should be considered. The connection to the substruc- 

 ture usually is easily made by direct contact or by a suitable test wire 

 connection. The actual measured potential will vary (sometimes drastically) 

 with the method used to contact the environment. To get reproducible 

 results this contact must be made through some stable and reliable reference. 

 The method in common use for measurements of substructures in soil (and 

 frequently in water) environments is by means of a copper-copper-sulfate 

 half-cell reference electrode contacting the electrolyte. It may be referred 

 to as above or as "copper sulfate electrode," "copper sulfate reference," 

 "copper sulfate half cell," "CuSO^ electrode," or "CSE." The working parts 

 of a copper sulfate electrode are shown in Figure 97, along with an equiva- 

 lent circuit to illustrate the half-cell concept. 



A silver-silver-chloride electrode is similar. Silver metal is in 

 equilibrium with a 0.1 normal solution of silver chloride with a porous plug 

 contacting the electrolyte. 



There are a few precautions to be observed when using any reference 

 electrode. Care must be taken to prevent contamination of the fluid in the 

 electrode if its potential is to remain constant. Such contamination is 

 possible when making potential measurements in a fluid electrolyte such as 

 seawater. With a copper sulfate electrode, the observed potential will vary 

 slightly with temperature, showing a positive gradient of 0.9 millivolt per 



Celsius (0.5 millivolt per Fahrenheit) up to about 50 Celsius (120 

 Fahrenheit) where hydrated copper sulfate begins to change structure. One 

 authority advises correction of all copper sulfate electrode potential 

 readings to 25 Celsius (77 Fahrenheit) . Such precision usually is not 

 required for normal fieldwork. It is good practice, before measuring a 



353 



