Crevice Corrosion
Intensive localized corrosion frequently occurs within crevices and other shielded areas on
metal surfaces exposed to corrosives. The attack associated with small volumes of stagnant
solution caused by holes, gasket surfaces, lap joints, surface deposits, and crevices under bolt and
rivet heads.
Methods and procedures for combating crevice corrosion are as follows:
- Use welded butt joints instead of bolted joints in new equipment if system permits for maintainability
- Close crevices in existing lap joints by continuous welding, caulking or soldering.Example:Pipe in SORF
flange joint should be fillet welded from inside of flange bore and from out side at flange socket end to
avoid crevice
- Design vessels for complete drainage; avoid sharp corners and stagnant areas.
- Inspect equipment and remove deposits frequently.
- Remove solids in suspension early in the process or plant flow sheet, if possible.
- Remove wet packing materials during long shutdowns.
- Provide uniform environments, if possible, as in the case of back-filling a pipeline trench.
- Use "solid"Spiral wound metallic gaskets with inner ring of MOC compatible with corrosive media or other
suitable metallic gasket where ever possible so as to avoid formation of crevice.
- Seal weld instead of pure rolling joint in tubes, in tube sheets of heat exchangers.
Pitting Corrosion
Pitting is a form of extremely localized attack. That results in holes in the metal. It is a cavity
or hole with the surface diameter about the same as or less than the depth.
Pitting is one of the most destructive form of corrosion, it causes equipment to fail because of
perforation with only a small percent weight loss of the entire structure. Pitting may be considered
as the intermediate stage between general overall corrosion and complete corrosion resistance.
Oxidizing metal ions with chlorides are aggressive pitters. Cupric, ferric and mercuric
halides are extremely aggressive even our most corrosion resistant alloys can be pitted by CuCl2
and FeCl3.
Prevention
The methods suggested for combating crevice corrosion generally apply also for pitting. Materials
that show tendencies to pit, shall not be used to build the plant under consideration. For example the addition of 2% molybdenum to 18-8S (Type 304) to produce 18-8S
Mo (Type 316) results a very large increase in resistance to pitting.
Intensive localized corrosion frequently occurs within crevices and other shielded areas on
metal surfaces exposed to corrosives. The attack associated with small volumes of stagnant
solution caused by holes, gasket surfaces, lap joints, surface deposits, and crevices under bolt and
rivet heads.
Methods and procedures for combating crevice corrosion are as follows:
- Use welded butt joints instead of bolted joints in new equipment if system permits for maintainability
- Close crevices in existing lap joints by continuous welding, caulking or soldering.Example:Pipe in SORF
flange joint should be fillet welded from inside of flange bore and from out side at flange socket end to
avoid crevice
- Design vessels for complete drainage; avoid sharp corners and stagnant areas.
- Inspect equipment and remove deposits frequently.
- Remove solids in suspension early in the process or plant flow sheet, if possible.
- Remove wet packing materials during long shutdowns.
- Provide uniform environments, if possible, as in the case of back-filling a pipeline trench.
- Use "solid"Spiral wound metallic gaskets with inner ring of MOC compatible with corrosive media or other
suitable metallic gasket where ever possible so as to avoid formation of crevice.
- Seal weld instead of pure rolling joint in tubes, in tube sheets of heat exchangers.
Pitting Corrosion
Pitting is a form of extremely localized attack. That results in holes in the metal. It is a cavity
or hole with the surface diameter about the same as or less than the depth.
Pitting is one of the most destructive form of corrosion, it causes equipment to fail because of
perforation with only a small percent weight loss of the entire structure. Pitting may be considered
as the intermediate stage between general overall corrosion and complete corrosion resistance.
Oxidizing metal ions with chlorides are aggressive pitters. Cupric, ferric and mercuric
halides are extremely aggressive even our most corrosion resistant alloys can be pitted by CuCl2
and FeCl3.
Prevention
The methods suggested for combating crevice corrosion generally apply also for pitting. Materials
that show tendencies to pit, shall not be used to build the plant under consideration. For example the addition of 2% molybdenum to 18-8S (Type 304) to produce 18-8S
Mo (Type 316) results a very large increase in resistance to pitting.
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