Accelerated low-water corrosion in harbours: MIC, ALWC and choice of cathodic protection

The corrosion of carbon steel structures in port environments incurs high risks for economic activities and safety, especially the localized accelerated corrosion that is difficult to include in longterm maintenance management plans.

In marine environments, corrosion rates in excess of standard values (i.e. 0.10 to 0.15 mm/side/year) can be induced by repeated removal of a protective corrosion product layer, dissimilar metal corrosion, stray electrical currents, or chemical pollutants. However, microorganisms often worsen the effects of accelerated corrosion – especially in the case of Accelerated Low- Water Corrosion (ALWC). ALWC is generally recognized as a form of Microbially Influenced Corrosion (MIC).

Low-Water Corrosion (LWC) is a phenomenon during which very important degradations appear, in particular on sheet piles. LWC is due to a differential aeration between the intertidal zone, which is a cathodic-like zone; and the low water zone, just under the tidal zone, which is an anodic-like zone. MIC
dramatically increases the consequences of LWC, as in Figure 1.

More often, ALWC is not differentiated from LWC. ALWC is very likely a continuous phenomenon with LWC, linked with electrochemical reactions, much like differential aeration cells. However, most ALWC cases are MIC in addition with other environmental parameters such as pollutants and dissolved oxygen.

Beatrice Kopczynski, Corrodys, Cherbourg, France
Edition: Edition 46

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