CREVICE CORROSIONCREVICE CORROSION

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Crevice Crevice CorrosionCorrosion• Crevice corrosion is a localized form of corrosion usually

associated with a stagnant solution .

• Such stagnant tend to occur in crevices (shielded areas) such as those formed under gaskets, washers, insulation material, fastener heads, surface deposits, disbonded coatings, threads, lap joints and clamps.

• Occurs under rivets and bolts, between valve disks and seats.

• Well-known examples of such geometries including flanges, gaskets, disbonded linings/coatings, fasteners, lap joints and surface deposits.

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Mechanism: Crevice corrosion is initiated by a difference in

concentration of some chemical constituents, usually oxygen, which set up an electrochemical concentration cell . Outside of the crevice (the cathode), the oxygen content and the pH are higher - but chlorides are lower. Chlorides concentrate inside the crevice(the anode), worsening the situation. Ferrous ions form ferric chloride and attack the stainless steel rapidly. The pH and the oxygen content are lower in the crevice than in the bulk water solution. The pH inside the crevice may be as low as 2 in a neutral solution. Once a crevice has formed, the propagation mechanism for crevice continues

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Factors affecting crevice corrosion: The major factors influencing crevice corrosion are:crevice type: metal-to-metal, metal-to-non-metalcrevice geometry: gap size, depth, surface

roughnessmaterial: alloy composition (e.g. Cr, Mo), structureenvironment: pH, temperature, halide ions, oxygen

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Examples:

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in this figure a stainless tube suffer crevice corrosion due to gap between the tube and tube sheet

Fig. Pack rust is a form a localized corrosion typical of steel components that develop a crevice into an open atmospheric environment. This expression is often used in relation to bridge inspection to describe built-up members of steel bridges which are showing signs of rust packing between steel plates.

Zebra mussels- an example of marine environment

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Underside of panel where severe corrosion was found

Close-up picture showing the severity of corrosion6

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Significance:The susceptibility to crevice corrosion varies widely

from one material-environment system to another. In general, crevice corrosion is of greatest concern for materials which are normally passive metals, like stainless steel or aluminum.

Crevice corrosion tends to be of greatest significance to components built of highly corrosion-resistant superalloys and operating with the purest-available water chemistry. For example, steam generators in nuclear power plants degrade largely by crevice corrosion.

Crevice corrosion is extremely dangerous because it is localized and can lead to component failure while the overall material loss is minimal. The initiation and progress of crevice corrosion can be difficult to detect.

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How to prevent Crevice Corrosion:•Use welded butt joints instead of riveted or bolted joints in new equipment •Eliminate crevices in existing lap joints by continuous welding or soldering•Avoid creating stagnant conditions and ensure complete drainage in vessels•Use solid, non-absorbent gaskets such as Teflon. •Use higher alloys (ASTM G48) for increased resistance to crevice corrosion.

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