In Situ Evaluation of the Carbon Steel Corrosion Rate Under Low Cathodic Polarization in Intermediary and Deep Sea, in South Atlantic Ocean.

Abstract In offshore carbon steel infrastructure, corrosion mitigation relies on organic coatings and cathodic protection (CP). Galvanic anodes, designed to last decades, keep the structure’s potential below a recommended protection threshold (Ep). When anodes exhaust, potential drifts toward open-circuit potential (OCP), entering an under-polarized phase with measurable corrosion. For deepwater structures, replacing CP systems is inherently complex and expensive, prompting evaluation of acceptable corrosion allowances under low polarization. Limited field data exists on residual corrosion rates in such conditions, influenced by factors like low temperature, slow flow, and reduced oxygen. This study deployed two carbon steel mooring lines offshore Brazil at 475 and 1085 meters for one year, alongside detailed, controlled laboratory experiments in France. Environmental parameters, CP performance, and biofilms were monitored using sensors. Pseudo-potentiostatic conditions were achieved with resistors, diodes, and galvanic anodes. Results indicate deep, cold, low-flow environments can yield very low corrosion rates even above Ep.