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Chemical Process Technology

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Saturday, January 17, 2009

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Crevice is location /area / space where normal fluid has less contact and access with it. Typical example of crevice are gaps between parts, space between gaskets and bolt, inside seals, inside cracks due to external impact or scratches, spaces filled with deposits, plastic paper lay on the metal plate, etc. In crevice, the environment is different than area expose to normal fluid. For example, gasket with bolt. Bolt surface expose to atmosphere is oxygen rich while wet air trapped between bolt and gasket is stagnant and with limited oxygen. Localized corrosion occur in the crevice is called crevice corrosion.

Crevice Corrosion Example
Following is an example of crevice corrosion at pipe support.

Crevice corrosion is pretty similar to pitting corrosion as discussed in "Pitting Corrosion - Mechanism & Prevention".

A metal surface with gasket at shown above will potentially experience crevice corrosion. Oxygen rich fluid enters crevice between gasket and metal surface.

- Genenal Oxidation Corroion
Normal corrosion (general oxidation corrosion) will occur through the metal surface outside and inside the crevice. As oxygen in trapped fluid consumed oxygen, environment within crevice is deoxygenated (low in oxygen level) increases the potential difference between crevice environment and oxygen rich environment.

Metal (E.g. FE) surface (expose to atmosphere) is oxygen rich will becomes the cathode whilst the metal surface in the crevice (gasket contacted area) is low in oxygen level will becomes anode. This form a complete circuit where metal at the crevice (FE) will be ionized to release electron (e) and form ion Ferum (FE2+), this electron will travel to the metal surface expose to atmosphere to react with Oxygen (O2) and water (H2O) to form ion hydroxides (OH-). Ion Ferum (FE2+) will react with ion hydroxides (OH-) to form Ferum Oxide (Fe2O3) which typically a brown rust.

- Increases Acidity in Crevice Environment
The ions FE2+ formed potentially hydrolyze water (H2O) in tapped fluid and produced positive ion (i.e. H+) and FE (corrosion product). The corrosion product will further block the movement of trapped fluid and increase the corrosion potential. The H+ will further increase the acidity of the trapped fluid and this severely increases corrosivity of trapped fluid.

- Other Corrosion i.e. CSCC
Production of ion positive (H+) will also attract negative ions i.e Chlorides, Sulfates, etc outside crevice travel into the trapped fluid in crevice, accumulation of these negative ions will potentially results Chloride and Sulfate associated corrosion such Chloride stress corrosion cracking (CSCC).

Preventive measures
There are several preventive measures to minimize crevice corrosion.

i) Avoid / minimize crevices during design stage i.e. keep junction points as wide open as possible.
ii) Avoid / Minimise crevices during fabrication i.e. smooth weld
iii) Avoid / minimize solution get into crevice i.e. greasing bolt / nut
iv) Use high resistance material (high PRE material)
v) Avoid / Minimise crevices during operation. Scale settled on metal surface will form "crevice" and trapped fluid. Routine cleaning to remove scale is one of the effective way to minimise crevices.
vi) Avoid/ Minimize objects i.e plastic bag put on metal surface.
vii) External coating

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posted by Webworm, 3:06 PM


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