With the continued offshore exploration and recovery of minerals and petroleum, there is need to protect assets used at depths in different oceanic environments. Corrosion prevention mechanisms such as cathodic protection (CP) may be applied to extend asset lifespan or prevent premature, costly failures.
However, there has been a lack of data on how deep water affects CP requirements, according to the French Corrosion Institute (Brest, France). Additionally, the available laboratory results have differed from observations and research in deep water environments, even when conditions are reproduced.
The institute has been conducting a series of studies in Azores in the Atlantic Ocean, including research on the influence of biofilms on cathodic activity and the crevice corrosion of stainless steel (SS) and nickel-based alloys. To investigates these effects, its recent study* exposed CS (ASTM A572 grade 50), as well as corrosion and environmental sensors, for 11 months at depths of 1,020 m (1,115.5 yd) and 2,020 m (2,209.1 yd).
Every 2 h during this time, sensors collected information about the seawater flow velocity, temperature, dissolved oxygen (DO), conductivity, and depth. At the end of the study, the impact of these conditions was studied alongside the corrosion product formed and corrosion rate.
Effects of biofilms on cathodic activity
• The environmental parameters stayed relatively consistent. There were small changes in the current speeds at both depths. The shallower depth had higher temperature and slightly higher flow velocity, but lower DO saturation. The corrosion rate between the two depths was similar, with the 1,020 m depth having a slightly higher rate. The amount of oxygen diffusion may be the reason for this difference—the authors postulate that the higher the temperature, the higher the corrosion rate at a given DO level.
• The current densities (CDs) decreased across time and were slightly lower in the deeper water. This is assumed to be due to the formation of a protective calcareous deposit and the level of oxygen diffusion. The growth of the calcareous deposit, which acted as a physical barrier over the cathode surfaces, was expected to limit oxygen diffusion and current demand over time.
• The makeup of the calcareous deposits was analyzed, and it was found that in the 1,020 m depth, the main compounds were (in decreasing order) magnesium calcite, calcite, and brucite. In the 2,020 m depth, the main compounds were calcite, magnesium calcite, monohydrocalcite, and brucite.
Results from this study were compared to results from other studies that took place in the Arabian Sea and the Indian Ocean. These previous studies used mild steel at different depths and exposure temperatures, finding that corrosion rate was higher in deeper waters. However, the institute instead found a slightly lower corrosion rate in deeper water. Factors such as flow velocity, calcareous deposit formation, and different limiting parameters involved in the electrochemical processes were attributed as the reasons for this difference.
The analysis of the environmental conditions and their effect on corrosion rate and CDs will help to inform future design of appropriate corrosion protection methods used in the Atlantic Ocean.
* Carbon Steel Corrosion and Cathodic Protection Data in Deep North Atlantic Ocean by E. Diler, N. Larché, D. Thierry, Corrosion Journal 76, 11 (2020).
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Source: Originally appeared in AMPP’s Maritime News, authored by Sammy Miles.