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3- Crevice Corrosion |
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This form of
attack is generally associated with the presence of small volumes
of stagnant solution in occluded interstices, beneath deposits
and seals, or in crevices, e.g. at nuts and rivet heads. Deposits
of sand, dust, scale and corrosion products can all create zones
where the liquid can only be renewed with great difficulty. This
is also the case for flexible, porous or fibrous seals (wood, plastic,
rubber, cements, asbestos, cloth, etc.).
Crevice corrosion is encountered particularly in metals and alloys
which owe their resistance to the stability of a passive film,
since these films are unstable in the presence of high concentrations
of Cl- and H+ ions.
The basic mechanism underlying crevice corrosion in passivatable
alloys exposed to aerated chloride-rich media is gradual acidification
of the solution inside the crevice, leading to the appearance of
highly aggressive local conditions that destroy the passivity.
in an interstice, convection in the liquid is strongly impeded
and the dissolved oxygen is locally rapidly exhausted. A few seconds
are sufficient to create a "differential aeration cell" between
the small deaerated interstice and the aerated remainder of the
surface. However, "galvanic" corrosion between these
two zones remains inactive.
As dissolution of the metal M continues, an excess of Mn+ ions
is created in the crevice, which can only be compensated by electromigrationof
the Cl- ions (more numerous in a chloride-rich medium and more
mobile than OH- ions). Most metallic chlorides hydrolyse, and this
is particularly true for the elements in stainless steels and aluminium
alloys. The acidity in the crevice increases (pH 1-3) as well as
the Cl- ion concentration (up to several times the mean value in
the solution). The dissolution reaction in the crevice is then
promoted and the oxygen reduction reaction becomes localized on
the external surfaces close to the crevice. This "autocatalytic" process
accelerates rapidly, even if several days or weeks were necessary
to get it under way.
Means of preventing or limiting crevice corrosion : Use welds rather
than bolted or riveted joints, design installations to enable complete
draining (no corners or stagnant zones), hydrofuge any interstices
that cannot be eliminated, and in particular, grease all seals
and seal planes, use only solid, non-porous seals, etc.
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