Cathodic Protection
Cathodic protection (CP) forces a buried or immersed metal to act as the cathode of an electrochemical cell, suppressing anodic dissolution. Two methods dominate: galvanic (sacrificial anodes such as magnesium, zinc, and aluminum alloys) and impressed current (a rectifier driving current through inert anodes like mixed metal oxide or graphite). Candidates must understand the structure-to-electrolyte potential as the core measurement, the role of a copper/copper sulfate reference electrode (CSE) for soils, and the criteria used to judge protection.
A frequent trap is confusing the potential criterion with the shift criterion. The widely used -850 mV criterion refers to a potential measured with the current applied and with IR drop compensated; the native or depolarized potential is not the benchmark. Another trap: the 100 mV polarization shift criterion is measured as a change from the native potential, not a shift from -850 mV. Questions also test current distribution, anode bed placement relative to coated pipe, and why coating quality changes current demand dramatically.
Worked example
A pipeline reads -0.62 V (CSE) at native potential and -0.78 V with CP on, measured without IR compensation. The polarization shift is 0.16 V (160 mV), which satisfies the 100 mV shift criterion even though the pipe is not at -850 mV. A candidate who simply compares -0.78 V to -850 mV would wrongly call it unprotected. Always identify which criterion the question is invoking before answering.
Stray current corrosion, telluric effects, and interference from foreign structures are common conceptual questions. Understand that CP protects by supplying electrons; it does not stop all corrosion where shielding, disbonded coatings, or high-resistivity backfill prevent current from reaching the metal.