Beware of the insidious effects of localised corrosion damage in crevices and below gaskets
Crevice corrosion can have highly detrimental consequences due to the localised nature of attack that often goes undetected in-service until final failure, associated with leakage or localised stress concentrations, occurs.
When metal corrodes in an aqueous solution, the metal anode is oxidised to form metal ions (typically M+), releasing electrons in the process. These electrons are consumed in the cathodic (reduction) reaction, during which oxygenated water is reduced to hydroxyl ions (4OH-). In the case of a bolted connection immersed in aerated seawater, such as a flanged interface on an offshore structure, both of these reactions initially occur uniformly over the entire submerged surface. However, the reduction reaction occurring in any region where fluid flow is limited, such as a crevice, will soon cease due to oxygen depletion of the stagnant fluid within the crevice. Continual dissolution of the metal anode within the oxygen-depleted zones results in the development of excess positive charges (M+) 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 increase to levels of 2-3, resulting in the rapid dissolution of the metal. The process eventually becomes autocatalytic and exacerbates itself. Associated reduction reactions taking place on material regions adjacent to the crevice inadvertently provides a degree of cathodic protection, effectively eliminating corrosion of the areas adjacent to the crevice.
Materials that rely on surface oxide layers for corrosion protection, such as stainless steel and aluminium, are particularly susceptible to crevice corrosion, as chlorides attack the protective surface oxides and the oxygen required to replenish the protective surface oxide layer is deficient within the crevice.
A major concern with crevice corrosion is its inherent formation at material interfaces, such as gasket surfaces, lap joints, tube to tube sheet interfaces within industrial boilers, areas of contact in plate coolers and even the contact surfaces of bolts and nuts. This inevitably allows the process to remain undetected, often only being identified after the component has failed or been damaged beyond repair. Although crevice corrosion typically occurs at material interfaces, crevice corrosion can also occur under process material, deposits or contaminants on the surfaces of the component which can provide suitable localized regions of stagnation and lead to crevice corrosion. The protection afforded by the crevice or buildup allows the process to continue virtually unhindered by external environmental effects, such as varying flow rates or periodic cleaning.
Clearly, preventing areas in which fluid can stagnate is of utmost importance in combating crevice corrosion. Every effort must be made to seal crevices formed at material interfaces, eliminate the use of permeable coatings and prevent the in-service buildup of process material or contaminants on surfaces of the component through regular cleaning, or through suitable water treatment that prevents the formation/precipitation of deposits on the surface of the material.
Published in Technical Tips by Origen Engineering Solutions on 1 November 2017