Cathodic Protection of Offshore Structures
What you must understand
CP shifts the steel potential to a protected range by supplying electrons. Galvanic (sacrificial anode) systems using aluminum or zinc anodes are standard offshore; impressed current systems (ICCP) are used where current demand is high or weight matters. Candidates must know protection criteria: a polarized potential of −800 mV vs Ag/AgCl (or −850 mV with IR considerations) for steel in seawater, and the more negative criteria for high-strength or SCC-susceptible steels. Understand current density requirements by zone, coating condition, and water velocity, and how to size anodes (net mass, consumption rate, current capacity, resistance to electrolyte).
Know the difference between structure-to-electrolyte potential measurements, IR drop, and polarization. Understand anode consumption, overprotection risks (hydrogen embrittlement, coating disbondment, wasted anode life), and the role of retrofits and monitoring (reference electrodes, anode current measurement).
Traps in questions
A frequent trap is applying an absolute potential without specifying the reference electrode. Another is confusing current density (A/m²) with total current, or assuming a coated structure needs the same anode mass as a bare one. Overprotection is often overlooked as a risk.
Worked example / rule of thumb
A new offshore structure typically needs higher initial current density to polarize the steel (e.g., on the order of 100–150 mA/m² for bare steel in seawater), dropping to a lower maintenance value once polarized. Rule of thumb: initial > mean > final current density; size anodes on total charge demand over design life, not just initial current.