Corrosion protection of subsea equipment using alternative cathodic potentials

The efficacy of alternative potentials in cathodically protecting carbon steel in natural seawater was investigated. The corrosion rates and corrosion morphology of A36 carbon steel were evaluated considering the use of alternative cathodic potentials equal to -700 mV Ag/AgCl and - 750 mV Ag/AgCl , in comparison to the conventional cathodic protection of - 800 mV Ag/AgCl and open circuit potential (OCP). Laboratory tests were conducted to simulate the seawater depths of 0 m, 500 m, and 1,000 m. To this end, the temperature and pressure of the tests were adjusted accordingly, while the dissolved oxygen content and flow velocity were controlled. Low uniform corrosion rates, comparable to those achieved with the conventional potential of –800 mV Ag/AgCl, were obtained using the alternative potentials. Moreover, similar corrosion morphologies were found for alternative and conventional potentials, with no evidence of localized corrosion events under cathodic protection. In contrast, localized corrosion events were evident on the A36 samples tested at OCP. These results demonstrate that cathodic protection using alternative potentials effectively mitigated the corrosion of A36 carbon steel in natural seawater. The results indicate that, in this environment, the effectiveness of cathodic protection is primarily governed by kinetic factors rather than equilibrium thermodynamic concepts, or in other words, immunity. • Cathodic protection in seawater is primally governed by kinetic factors. • Low corrosion rates were measured with the use of alternative cathodic potentials. • No localized corrosion was observed using alternative potentials. • Alternative cathodic potentials were effective in controlling corrosion.