Cathodic protection (CP) reference for marine and industrial steel structures. Compares Impressed Current Cathodic Protection (ICCP) against sacrificial (galvanic) anode systems. Covers alloy selection (zinc, aluminium, magnesium), current density design per DNV-RP-B401, reference cell types (Ag/AgCl, Cu/CuSO₄, Zn), and typical anode consumption rates.
| Method | Working principle | Typical use |
|---|---|---|
| Sacrificial anode (galvanic) | A metal more electronegative than the structure (Zn, Al, Mg) is bonded to the structure. It corrodes preferentially, protecting the structure. Self-regulating; no external power. | Ballast tanks, small hulls, buried pipe with modest current demand, indoor water tanks |
| Impressed Current CP (ICCP) | A rectifier drives DC current from an inert anode (mixed-metal-oxide titanium or platinized niobium) to the structure. Current is regulated by a feedback loop from a reference cell. | Large ship hulls, offshore platforms, buried pipelines, large tank interiors |
| Alloy | Nominal open-circuit potential (V vs Ag/AgCl) | Capacity (A·h/kg) | Use |
|---|---|---|---|
| Zinc (MIL-A-18001K) | -1.05 | 780 | Seawater, mild bilge; not below +40 °C |
| Aluminium-Indium (Al-Zn-In) | -1.10 | 2,500 | Seawater, ballast tanks, offshore |
| Aluminium (Galvalum III) | -1.10 | 2,600 | Seawater, ballast tanks (most common today) |
| Magnesium (H-1 alloy) | -1.55 | 1,230 | Fresh / brackish water, buried pipe soil, indoor tanks |
DNV-RP-B401 assigns a design current density (mA/m²) for each combination of surface type, environment and coating condition. Initial design current density is used to polarise the structure quickly; average design current density sets the anode mass; final design current density verifies that current is still delivered at end-of-life. For a coated ballast tank in North Sea seawater with 3% coating breakdown at end of life: initial 6 mA/m², average 1.5 mA/m², final 3 mA/m². For a bare steel jacket in the same environment: initial 200 mA/m², average 90 mA/m², final 100 mA/m².
| Reference cell | Standard potential (V) | Typical use |
|---|---|---|
| Ag / AgCl (seawater) | +0.25 vs SHE | Marine — hulls, ballast tanks, offshore |
| Cu / CuSO₄ (buried) | +0.318 vs SHE | Buried pipelines, onshore tanks |
| Zn (pure, seawater) | -0.80 vs SHE | Long-term marine monitoring, low-drift |
The industry consensus criterion is that a bare-steel or coated-steel structure is fully protected when its potential vs Ag/AgCl reference cell reads more negative than -0.80 V (seawater) or -0.85 V vs Cu/CuSO₄ (soil). More negative than -1.10 V vs Ag/AgCl risks hydrogen embrittlement in high-strength steel and coating disbondment on soft coatings — this is the "over-protection" limit. Measurements are taken at the anode's local zone of influence, not at random points.
Anode consumption over service life = (average current density × wetted area × design life × utilisation factor) / anode alloy capacity. Utilisation factor accounts for the last 15% of anode mass being physically detached before it delivers current (typical 0.85). A 30 kg Al-Zn-In anode delivering 0.3 A average lasts about 25 years (2,600 A·h/kg × 30 kg × 0.85 utilisation / 0.3 A / 8760 h/yr).