Cathodic protection is the electrochemical technique that stops buried and submerged steel pipelines from corroding by turning the whole pipe into the cathode of a controlled circuit. This guide explains how the two types - sacrificial anode and impressed current - work, how protection is verified, and why CP is mandatory on transmission pipelines.
Cathodic Protection in one line: Cathodic protection (CP) prevents external corrosion of buried or submerged steel by making the pipe the cathode of an electrochemical cell, so metal loss shifts to a separate anode instead of the pipe wall. In a sacrificial (galvanic) system, a more active metal such as magnesium or zinc corrodes preferentially and drives protective current into the pipe. In an impressed current system, a rectifier forces DC through inert anodes to protect longer, larger lines. CP works alongside a protective coating, cleaning up corrosion only at coating defects.
Steel corrodes because tiny anodic and cathodic sites form on its surface and drive a corrosion current, dissolving metal at the anodes. Cathodic protection floods the pipe with electrons from an external source so that the entire pipe surface becomes cathodic; corrosion current can no longer leave the steel, so metal loss stops. The corrosion is deliberately relocated to the sacrificial or inert anode instead of the pipe.
A sacrificial anode system connects a block of magnesium, zinc, or aluminum to the pipe; because that metal is more electronegative, it corrodes and supplies protective current with no external power. An impressed current cathodic protection (ICCP) system uses a DC rectifier connected to a groundbed of inert anodes; the rectifier pushes a controllable, much larger current, suiting long transmission lines, poorly coated pipe, or low-resistivity soils where sacrificial anodes cannot deliver enough current.
Protection is confirmed by measuring the pipe-to-soil potential against a copper/copper-sulfate reference electrode. The widely used criterion is a potential of at least -850 mV (with the CP current applied, or as an instant-off polarized value to remove voltage-drop error). Technicians run close-interval potential surveys along the route and monitor rectifier output; test stations along the line give access points for these readings.
CP always pairs with coating. A good coating covers most of the steel, and CP concentrates its current at the few holidays (coating defects) where bare metal is exposed - a division of labor that keeps current demand and cost manageable. In oil and gas, CP is standard on buried gathering and transmission pipelines, well casings, storage tank bottoms, and offshore structures, and it is a core part of regulatory pipeline integrity programs. Rectifier output and test-station readings are increasingly telemetered so operators can watch CP health remotely rather than only during field surveys.
A sacrificial (galvanic) system uses a more active metal like magnesium or zinc that corrodes to supply protective current with no external power, suiting small or well-coated lines. An impressed current system uses a rectifier to force DC through inert anodes, delivering the large, controllable current that long transmission lines need.
It is a common acceptance criterion for CP: the pipe-to-soil potential, measured against a copper/copper-sulfate reference electrode, should be at least -850 mV (often as an instant-off polarized value). Meeting it indicates the steel is polarized enough to effectively stop external corrosion.
No - they work together. Coating covers most of the steel, and cathodic protection supplies current to the small coating defects where bare metal is exposed. Relying on CP alone would demand impractically high current, so a good coating and CP are used in combination.
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