Deepwater Cathodic Protection: Part 1—Laboratory Simulation Experiments

Experiments were performed where steel specimens were cathodically polarized in natural seawater by galvanic coupling through an external resistor to an aluminum anode. Temperature was either ambient or 5°C, and pressure was atmospheric or 8.96 ± 0.14 MPa (1,300 ± 20 psi), which is equivalent to a water depth of 899 m (2,950 ft). For some experiments, dissolved oxygen concentration was controlled at 5.5 ± 0.2 mg/L and pH was 7.8. These corresponded to values that were measured at the indicated depth for a specific Gulf of Mexico site. The apparent steady-state potential (ϕ_c) and current density (i_c) for the different experiments were compared with previously reported ambient temperature and pressure data. Calcareous deposits that formed on specimens from each of the test categories were viewed and analyzed. The long-term ϕ_c-i_c trend for the different tests was the same at the two pressures and for 5.5 mg/L compared to 9 mg/L O₂. Also, i_c was independent of ϕ_c over the potential range investigated (~0.80 V vs saturated calomel electrode [SCE] to -1.10 V_SCE), despite differences in the calcareous deposit structure and composition. The results are discussed in terms of, first, design criteria for deepwater cathodic protection (CP) and, second, experimental testing to develop such criteria.