Cathodic protection is considered to be effective when active corrosion is transferred from the metal surface to the installed anode. The effectiveness of the transference can be determined by electrical measurements. Industry accepted criteria for effective protection using these measurements are fully described in various NACE International publications including the Standard Recommended Practice "RP0169-96 Control of External Corrosion on Underground or Submerged Metallic Piping Systems".

Of the available techniques, the primary measurement used by industry to determine effectiveness of protection is known as the -0.850 volt, pipe-to-soil potential criteria. This technique measures the voltage difference between the protected structure and a copper-copper sulfate electrode placed in or on the electrolyte near the structure. If the voltage difference is more negative than -0.850 volts, then the structure is considered protected. If the value is more positive than -0.850 volts, the structure is either unprotected or only partially protected. Normal values of unprotected steel in soil typically range from -0.500 to -0.700 volts with respect to copper sulfate. The interpretation of pipe-to-soil potentials requires consideration of the effects of measurement errors such as IR drop in the soil between the pipe and the electrode. The most common consideration of this effect is through an "instant off" measurement obtained by interrupting current sources. Other techniques include potential measurement at permanent "coupon type test stations".

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