Carbon Steel Corrosion and Cathodic Protection Data in Deep North Atlantic Ocean

Abstract

Many parameters may influence the corrosion and the cathodic protection current demand in natural seawater. These are potential, temperature, dissolved oxygen content, biofilm and fouling activity, hydrostatic pressure, and calcareous deposit formation. In this study, the influence of the exposure depth on the corrosion, cathodic protection current demand, and nature of the calcareous deposit formed on carbon steel was investigated at 1,020 m and 2,020 m depth. For this purpose, a set of coupons, cathodic protection, and environmental sensors were exposed in Azores in the Atlantic Ocean for 11 months. The higher corrosion rate and current density observed at 1,020 m can be explained by the higher temperature and oxygen diffusion. The cathodic current demand decrease with time can be attributed to the calcareous deposit formation. The current densities after 11 months are in agreement with the literature with 143 mA/m2 and 124 mA/m2 at 1,020 m and 2,020 m depth. Calcareous deposits formed, characterized by Raman spectroscopy, x-ray diffraction, and scanning electron microscopy/electron dispersive x-ray spectroscopy, highlight (i) the favored formation of calcite and hydrocalcite at the expense of aragonite in deep and cold water, (ii) the presence of a thin deposit after 11 months, (iii) the decrease of the Ca/Mg ratio with immersion depth, (iv) the presence of CaMgCO3 compounds, and (v) a higher decrease of the current demand with time in deep water, suggesting the formation of a more protective deposit. The capacity for aluminum-gallium and aluminum-indium galvanic anode were in agreement with the literature for long-term exposures.