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Carbon Steel and Stainless Steel Cathodic Protection Design Data in Deepsea Water – Influence of the Environment on the Biofilm Cathodic Activity

In natural seawater many parameters might influence the cathodic protection current demand such as potential temperature dissolved oxygen content biofilm and fouling activity and calcareous deposit formation. The actual deepsea environment cannot be easily reproduced at laboratory scale at the levels measured in deep sea.In this study the influence of the depth on the cathodic protection criteria of carbon steel and stainless steel was investigated in intermediary (1020 m depth) and deep water (2020 m) at the same location. For this purpose at set of corrosion and environmental sensors as well as metallic coupons were exposed during 327 days in Azores in the Atlantic Ocean.On stainless steel characteristic cathodic depolarizations due to biofilm activity were observed in intermediary and deep water. However a strong cathodic activity induced by the biofilm was only measured in deep water. Thus a potential ennoblement on stainless steel in presence of biofilm is not necessary associated to a strong cathodic activity (current demand). The obtained results show that the increase in the current density induced by the biofilm depends on the limiting parameter of the reaction which can be under anodic or cathodic control. Thus as a function of the environment and the experimental setup a “strongly-active” or “weakly-active” biofilm can be formed on stainless steel. Under cathodic control high current densities were measured after several months in deep water. When the above conditions are favorable to a “strongly-active” apparition a strong CP depolarization might appear.In this study new autonomous biofilm sensors exposed for the first time in open and deep water appear relevant to discriminate cathodically “strongly-active” and “weakly-active” biofilm.For carbon steel the cathodic protection data collected in situ show that the initial and mean (after 11 months) current densities are higher than those recommended by the DNVGL RP B401 standard. Although mean current densities are expected to continue to decrease slowly with the exposure time so tending to the standard recommendations the DNVGL RP B401 standard might not be conservative in terms of current densities criteria for studied environments.

Product Number: 51319-13124-SG
Author: Erwan Diler
Publication Date: 2019
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