NACE SP0207 & SP0169 Pipe-to-Soil Potential Analysis
mV Cu/CuSO4 (negative values, e.g., -920, -850, -780)
If provided, used for IR-free potential and IR drop calculation.
Typical bare steel: -500 to -600 mV Cu/CuSO4
More negative than -1200 mV risks coating disbondment and hydrogen embrittlement.
Understand cathodic protection principles, survey techniques, IR drop correction methods, and NACE criteria interpretation
The -850 mV Cu/CuSO4 criterion (NACE SP0169 Section 6.2.2.1) is the most widely used cathodic protection criterion for buried steel pipelines. A pipe-to-soil potential of -850 mV or more negative (with cathodic protection applied) indicates adequate protection. This potential must be measured with IR drop compensation to be meaningful.
IR drop is the voltage error in pipe-to-soil potential measurements caused by CP current flowing through the soil resistance between the pipe and reference electrode. It makes readings appear more negative than the true polarized potential. IR drop correction methods include instant-off measurements, coupon measurements, and close-interval surveys with current interruption.
A close-interval survey (CIS) per NACE SP0207 measures pipe-to-soil potentials at closely spaced intervals (typically 2.5 to 5 feet) along the pipeline. Both ON and instant-OFF potentials are recorded. CIS identifies areas of inadequate CP protection, coating damage, shielding, and interference. It is the most comprehensive tool for evaluating external corrosion control.