ON, native and ON/OFF potential surveys
We survey the structure-to-soil potential at each test point to confirm that cathodic protection reaches the whole structure and meets the criterion.
What it is
The ON potential is taken with the cathodic protection system running. The OFF potential is taken an instant after interrupting the current, without the ohmic (IR) drop that distorts the ON reading. The native potential is that of the structure without protection, and it is the baseline for the 100 mV polarization criterion. All are taken with respect to Cu/CuSO4.
When it is needed
- Periodic surveys within an integrity program.
- Before energizing a new system, to record native potentials as a baseline.
- After commissioning, a rectifier adjustment or nearby works.
How we do it
We review the route and the test points, such as test stations and markers, with drawings and previous surveys.
We install GPS-synchronized interrupters on the rectifiers, anodes or current injection points that influence the section.
At each point we record ON and OFF with a Cu/CuSO4 electrode and the GPS location.
We document infrastructure findings with photos: test stations without covers, loose cables, damaged markers, rectifiers out of service.
We process the data and build the potential profile for each section.
Native potentials are taken before energizing the system, or with the system off and the structure depolarized.
Equipment
- High-impedance voltmeter or data logger
- Cu/CuSO4 reference electrodes
- GPS-synchronized current interrupters
- Smartphones with a survey app
Deliverables
- Daily report with a table of ON and OFF by point and a potential profile by section.
- Final report in PDF.
- Photographic record of findings, with criticality and replacement recommendation.
- Findings spreadsheet in Excel and georeferenced KMZ file.
- Instrument calibration certificates and personnel certifications.
- Assessment against the criterion (−850 mV OFF with respect to Cu/CuSO4 or 100 mV of polarization), when included in the scope.
Standards
NACE SP0169 – NACE TM0497 – ISO 15589-1 – NAG-100
Where we apply it
Pipelines – Tanks – Industrial plants – Service stations and fuel depots