Sacrificial Anode Cathodic Polarization of Steel in Seawater: Part 1 — A Novel Experimental and Analysis Methodology

API-2H, grade 42 steel (UNS K12037) specimens were cathodically polarized in natural seawater by galvanic coupling to an aluminum anode through an external resistor. The interdependence of the decay in potential (ϕ) vs current density (i) conformed analytically to a straight line, the slope of which was the product of the total circuit resistance and cathode surface area and the vertical intercept of which was the anode corrosion potential. From experiments with resistor sizes ranging from 75 Ω to 5,750 Ω, a sigmoidal shape for the curve defining the relationship between long-term ϕ and i values for cathodically polarized steel in seawater was identified, with –1.00 V_SCE being the potential of minimum steady-state (maintenance) current density (i_m). Evaluation of data from instrumented, newly deployed offshore structures and of survey data from older structures indicated that the ϕ-vs-i trend for these structures conformed to the same linear relationship as the laboratory specimens. A procedure was developed whereby polarization data from systems of vastly different geometries can be interrelated quantitatively.