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Crevice corrosion - Wikipedia, the free encyclopedia
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Crevice corrosion
From Wikipedia, the free encyclopedia
Crevice corrosion refers to corrosion occurring in confined spaces to which the access of the working
fluid from the environment is limited. These spaces are generally called crevices. Examples of crevices
are gaps and contact areas between parts, under gaskets or seals, inside cracks and seams, spaces filled
with deposits
and under sludge piles.
This photo shows that corrosion occurred in the crevice between
the tube and tube sheet (both made of type 316 stainless steel) of
a heat exchanger in a sea water desalination plant. [1]
Contents
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1 Mechanism
2 Mode of attack
3 Stress s corrosion cracking
4 Significance
5 References
6 External links
Crevice corrosion of type 316
stainless steel
Mechanism
The corrosion resistance of a stainless steel is dependent on the presence of an ultra-thin protective oxide
film (passive film) on its surface, but it is possible under certain conditions for this oxide film to break
down, for example in halide solutions or reducing acids. Areas where the oxide film can break down can
also sometimes be the result of the way components are designed, for example under gaskets, in sharp re
-entrant corners or associated with incomplete weld penetration or overlapping surfaces. These can all
form crevices which can promote corrosion. To function as a corrosion site, a crevice has to be of
sufficient width to permit entry of the corrodent, but narrow enough to ensure that the corrodent remains
stagnant. Accordingly crevice corrosion usually occurs in gaps a few micrometres wide, and is not found
in grooves or slots in which circulation of the corrodent is possible. This problem can often be overcome
by paying attention to the design of the component, in particular to avoiding formation of crevices or at
least keeping them as open as possible. Crevice corrosion is a very similar mechanism to pitting
corrosion; alloys resistant to one are generally resistant to both. Crevice corrosion can be viewed as a
less severe form of localized corrosion when compared with pitting. The depth of penetration and the
rate of propagation in pitting corrosion are significanatly greater than in crevice corrosion.
Crevices can develop a local chemistry which is very different from that of the bulk fluid. For example,
in boilers, concentration of non-volatile impurities may occur in crevices near heat-transfer surfaces
because of the continuous water vaporization. "Concentration factors" of many millions are not
uncommon for common water impurities like sodium, sulfate or chloride. The concentration process is
often referred to as "hideout" (HO), whereas the opposite process, whereby the concentrations tend to

http://en.wikipedia.org/wiki/Crevice corrosion
Crevice corrosion - Wikipedia, the free encyclopedia
7/17/2012
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even out (e.g., during shutdown) is called "hideout return" (HOR). In a neutral pH solution, the pH
inside the crevice can drop to 2, a highly acidic condition that accelerates the corrosion of most metals
and alloys.
For a given crevice type, two factors are important in the initiation of crevice corrosion: the chemical
composition of the electrolyte in the crevice and the potential drop into the crevice. Researchers had
previously claimed that either one or the other of the two factors was responsible for initiating crevice
corrosion, but recently it has been shown that it is a combination of the two that causes active crevice
[2]
corrosion.
Both the potential drop and the change in composition of the crevice electrolyte are caused
by deoxygenation of the crevice and a separation of electroactive areas, with net anodic reactions
occurring within the crevice and net cathodic reactions occurring exterior to the crevice (on the bold
surface). The ratio of the surface areas between the cathodic and anodic region is significant.
Some of the phenomena occurring within the crevice may be somewhat reminiscent of galvanic
corrosion:
galvanic corrosion
two connected metals + single environment
crevice corrosion
one metal part + two connected environments
However, The mechanism there are of crevice sufficient corrosion differences can be to warrant
(but is not a separate
always) treatment.
similar to For
that example,
of pitting in corrosion.
crevice
corrosion, one has to consider the geometry of the crevice and the nature of the concentration process
leading to the development of the differential local chemistry. The extreme and often unexpected local
chemistry conditions inside the crevice need to be considered. Galvanic effects can play a role in crevice
degradation.
Mode of attack
Depending on the environment developed in the crevice and the nature of the metal, the crevice
corrosion can take a form of:
■ pitting (i.e., formation of pits),
■
filiform corrosion (this type of crevice corrosion that may occur on a metallic surface underneath
an organic coating),
■
intergrannular attack, or
■
stress corrosion cracking.
Stress corrosion cracking
A common form of crevice failure occurs due to stress corrosion
cracking, where a crack or cracks develop from the base of the
crevice where the stress concentration is greatest. This was the
root cause of the fall of the Silver Bridge in 1967 in West
Virginia, where a single critical crack only about 3 mm long
suddenly grew and fractured a tie bar joint. The rest of the bridge
fell in less than a minute. The eyebars in the Silver Bridge were

http://en.wikipedia.org/wiki/Crevice corrosion
Crevice corrosion - Wikipedia, the free encyclopedia
7/17/2012
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not redundant, as links were composed of only two bars each, of
high strength steel (more than twice as strong as common mild
steel), rather than a thick stack of thinner bars of modest material
strength "combed" together as is usual for redundancy. ncy. With only
two bars, the failure of one could impose excessive loading on
the second, causing total failure—unlikely if more bars are used.
While a low-redundancy chain can be engineered to the design
requirements, the safety is completely dependent upon correct,
high quality manufacturing and assembly.
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.
The Silver Bridge upon completion in
1928
The collapsed Silver Bridge, as seen
from the Ohio side
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.
References
1. ^ Crevice Corrosion (http://www.corrosionclinic.com/types_of_corrosion/c
.com/types_of_corrosion/crevice_corrosion.htm)
revice_corrosion.htm)
2. ^ Kennell, G.F., K.L. Heppner, R.W. Evitts. (2008) A Critical Crevice Solution and iR Drop Crevice
Corrosion Model (http://dx.doi.org/doi:10.1016/j.corsci.2008.02.020).02.020) . Corrosion Science ce 50: 1716.
External links
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Crevice Corrosion at Pipe Supports: Causes and Solutions (paper)
(http://www.stoprust.com/6pipesupports.htm)
Crevice Corrosion of Stainless Steels
(http://www.corrosionist.com/Pitting_Crevice_Corrosion.htm)
Retrieved from "http://en.wikipedia.org/w/index.php?title=Crevice_corrosion&oldid=499781866"
Categori
ries: Corrosion
Fouling
■ This page was last modified on 28 June 2012 at 17:02.
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