Barnacles and Crevice Corrosion

Corrosion, at the most basic level, may be considered as the dissolution of a metal in the presence of an aqueous solution (such as water). When metal corrodes in an aqueous solution such as oxygenated water, the metal anode is oxidised to form metal ions (typically M+), releasing electrons in the process. The electrons are in turn consumed in a complementary cathodic (reduction) reaction, which results in the formation of hydroxyl ions (4OH-). This is the nominal behaviour in free, open access situations, such as on a marine structure, and both of these reactions initially occur uniformly over the entire submerged surface. However, in the case of flanged interfaces, lapped joints (of a heat exchanger, say), gaskets or bolted systems, the reduction reaction occurring in any region where fluid flow is limited, such as a crevice between two adjacent components, will soon cease due to oxygen depletion of the stagnant fluid within the crevice.

Continual dissolution of the metal anode within the oxygen-depleted zone results in the development of excess positive charges (Mz+) in the crevice, which accelerates the migration of chlorides, sulfates and hydroxide ions into the crevice (opposite polarities attract).  Metal chlorides are particularly problematic, as they hydrolyse in water to form metal hydroxides and free acid which can cause the pH of a crevice in a neutral solution to decrease to acidic levels of pH 2-3, resulting in the rapid dissolution of the metal and the process eventually becomes autocatalytic.  Materials that rely on surface oxide layers for corrosion protection, such as stainless steels, are particularly susceptible to crevice corrosion, as chlorides attack the protective surface oxide layers and the oxygen required to replenish the protective surface oxide layer is deficient within the crevice. 

Crevice corrosion can also occur under foreign material, deposits or contaminants on the surfaces of the component, as these inhibit the migration of oxygen to the surface which can provide suitable localized regions of stagnation and lead to crevice corrosion.  

The formation and growth of marine organisms, such as barnacles, provide just such ideal environments for crevice corrosion, with the shielded region becoming anodic.  Generally the process results in minimal corrosion growth while the animal is alive, but severe attack is often evident on stainless steel surfaces, below the shells of barnacles that have died and decay on the surface (and were not eaten by fish).  It has been postulated that during the decay by sulphate reducing bacterial and thiobacilli the pH in the shell is lowered.  The acidic product perforates the shell at the centre and then diffuses across the base of the shell.  Differential aeration and differences in pH between the centre and the crevice at the edge of the shell, causes corrosion that begins at the edge of the shell with the deepest pitting occurring where the shell adhered most firmly.  Creation of a deep pit at the centre that can occur in some instances, is thought to occur due to the development of an aerobic environment within the shell. 

This mix of crevice corrosion and MIC is particularly insidious and is often only identified after the structure has been damaged severely.  Although barnacles often get the blame for the damage, it is essentially not the barnacles themselves but the sulphate reducing bacteria that are the underlying cause of the damage.