Creeping on Crevice Corrosion

January 15, 2016

Crevice corrosion is a type of localized corrosion that occurs on metal surfaces at, or within, gaps between two surfaces, and occurs due to a difference in concentration of compounds in a fluid system, which creates an electrochemical cell.

Crevice corrosion is a type of localized corrosion that occurs on metal surfaces at, or within, gaps between two surfaces. The crevice is generally narrow and offers no circulation. The corrosive fluid is free to flow in, but prevented from flowing out, creating an environment where the corrosive fluid remains stagnant.

Crevice corrosion is similar to galvanic corrosion, which we have discussed previously, but there are some differences. In galvanic corrosion, the corrosion cell is made up of two metals in a single environment. However, the corrosion cell in crevice corrosion is made up of a single metal component being exposed to multiple environments. Crevice corrosion occurs due to a difference in concentration of compounds in a fluid system, which creates an electrochemical cell.

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Crevice Corrosion in Water

In systems where the corrosive fluid is water, the oxygen is consumed and split into OH- and H+ ions. The low oxygen, low pH environment inside the crack creates an anodic environment, which lowers the pH further and thus speeds up corrosion. The corrosion is fastest at the crack tip, where the ions have a greater propensity to concentrate. At the crack tip, the stress concentration is greatest, leading to catastrophic failure due to stress corrosion cracking, where cracks are formed at the base of the crevice.

Corrosion Resistant Alloys

Crevice corrosion is mostly a concern in Corrosion Resistant Alloys (CRAs) and stainless steels. The corrosion works at the protective film on the surface, damaging it, and speeding up the corrosion process. In downhole tooling, crevice corrosion is prevalent and dramatically reduces the strength of the material.

Prevention

Crevice corrosion can be prevented in a number of ways. The simplest is to ensure that no stagnant conditions are created, either by using joints that prevent crevices from forming or by ensuring that all cracks have appropriate circulation. The use of higher alloys and their higher resistance to crevice corrosion is another option.

When applicable, using composites in lieu of metallic materials is also a highly effective prevention method. This is because plastic composite materials are not susceptible to corrosion like metals.

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