Cathodic Protection
Cathodic protection (CP) makes a buried or immersed structure the cathode of a deliberate electrochemical cell, so that its anodic dissolution is suppressed. Two methods dominate: galvanic (sacrificial anodes of zinc, aluminum, or magnesium that are more active than the protected steel) and impressed current (an external DC source driving inert anodes such as mixed metal oxide or high-silicon cast iron).
Know the criteria. For steel in soil or water, a common criterion is a polarized potential of -850 mV versus a copper/copper sulfate reference electrode (CSE), measured with the IR drop removed, or a 100 mV polarization shift. Understand why IR drop matters: voltmeter readings include resistance through soil and coating, and a structure that reads -850 mV with IR included may not actually be protected.
Traps to expect: confusing current requirement with current output, forgetting that coatings and CP are complementary (better coating, lower current demand), and misreading reference electrode polarity. Also remember stray-current corrosion and interference: a new CP system can accelerate corrosion on a neighboring bare pipeline, which is why interference testing and drainage bonds exist.
Worked example: A coated steel pipeline has a current requirement of 2 mA/m² over 5,000 m² of bare-equivalent area. Total current = 0.002 A/m² × 5,000 m² = 10 A. If each anode discharges 5 A, you need at least 2 anodes, but design practice adds spare capacity and accounts for anode consumption over design life. Rule of thumb: always compute demand from area and current density, then size supply with margin.
Understand structure-to-soil potential surveys, close-interval surveys, and the difference between native and polarized potentials.