JANUARY 31 2003
of this voltage measurement. Consideration is understood to mean the application of
sound engineering practice in determining the significance of voltage drops by methods
Measuring or calculating the voltage drop(s).
Reviewing the historical performance of the cathodic protection system.
Evaluating the physical and electrical characteristics of the pipe and its
Determining whether or not there is physical evidence of corrosion.
All the errors listed in paragraph 7-2 must be evaluated. Interruption of the
CP current does not fall under this criterion, since that would pertain to the -0.85 instant-
OFF or the 100 mV polarization criterion. Measuring or calculating the voltage drop(s)
includes measuring all the factors that affect the magnitude of the voltage errors present
in the ON reading. These measurements include the anode output, rectifier current
output, structure coating efficiency, location of the reference cell in relation to the
anodes and the structure, electrolyte resistivity, comparison to previous potentials
(native, ON, and/or instant OFF) and other factors which may contribute to the corrosion
rate (presence of stray current, interference, bi-metallic connections, pH, temperature,
homogeneity of the soil, amount of oxygen, presence of bacteria, and presence of other
ions or contaminants which may affect the corrosion rate). Implementation of this
criterion is only possible when these factors can be quantitatively verified by
measurement these factors, or historical evidence that these factors have been
Factors that decrease magnitude of the voltage drop errors or otherwise slow
or stop the corrosion rate include:
High dielectric strength coating. A 99 percent to 99.7 percent effective
coating drastically lowers the amount of current to obtain adequate
cathodic protection; consequently IR error is also drastically lowered.
Low electrolyte resistivity. As resistivity is lowered, the IR drop error is
lowered. Also for impressed current systems, the circuit resistance is
lower, resulting in a lower voltage at the anode (to obtain the same
current), lowering any anode gradient errors.
High pH (7 to 13). A high pH in the electrolyte near the protected structure
indicates cathodic protection is present, but amphoteric materials could be
damaged by the high alkalinity created by the cathodic protection.