Cathodic Protection: Principles and Practice
What you must know
Cathodic protection (CP) makes the structure the cathode of a corrosion cell, polarizing it to a potential where anodic dissolution is suppressed. Know the two methods: galvanic (sacrificial anodes such as zinc, aluminum, magnesium) and impressed current (rectifier, inert anodes). Understand the criteria: the -850 mV (Cu/CuSO4) polarized potential criterion, the 100 mV polarization criterion, and the 300 mV shift criterion for coated/unknown structures. Know that these are criteria, not guarantees, and that IR drop must be considered.
Understand structure-to-electrolyte potential measurements, reference electrodes, and the difference between "on" and "instant-off" potentials. Know the components of an impressed current system: transformer-rectifier, anode bed, cabling, and test stations. Understand coating and CP as complementary: coatings reduce current demand; CP protects holidays and damage.
Know interference and stray current: telluric currents, DC transit systems, and foreign structure interference. Understand that overprotection can cause coating disbondment and hydrogen embrittlement on susceptible steels.
Traps
Questions often mix up the criteria or apply -850 mV to the wrong reference. Another trap: assuming a reading taken with the current on reflects true polarization; IR drop can make a protected structure read less negative than it is. Also, a structure that meets -850 mV may still be underprotected if the reading is taken at the wrong location.
Rule of thumb
Measure instant-off to remove IR drop; if the instant-off potential is more negative than -850 mV (Cu/CuSO4), the structure is generally protected.