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4- Pitting corrosion |
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Pitting corrosion
is particularly insidious. The attack is in the form of highly
localized holes that can penetrate inwards extremely rapidly, while
the rest of the surface remains intact. A component can be perforated
in a few days with no appreciable loss in weight on the structure
as a whole.
Pitting corrosion is most aggressive in solutions containing chloride,
bromide or hypochlorite ions. Iodides and fluorides are much less
harmful. The presence of sulfides and H2S enhances pitting corrosion,
and systematically impairs the resistance criteria for this type
of attack. The thiosulphate species plays a similar role, since
its electrochemical reduction causes "sulphidation" of
the exposed metallic surfaces.
The presence of an oxydizing cation (Fe+3, Cu+2, Hg+2, etc.) enables
the formation of pits even in the absence of oxygen. However, in
the presence of oxygen, all chlorides become dangerous, and this
is also true in the presence of hydrogen peroxide.
The stainless steels are particularly sensitive to pitting corrosion,
but other metals, such as passive iron, chromium, cobalt, aluminium,
copper and their alloys are also prone to this form of damage.
Very often, in non-passivatable metals, a "tubercular" surface
morphology is observed, beneath which pits develop.
Contrary to crevice corrosion, the cause of pitting is not always
completely local in nature. Thus, although alterations or intrinsic
defects at the metal-solution interface (e.g. inclusions emerging
through the passive film in stainless steels) often represent nuclei
for local dissolution, all such potential nuclei are not attacked.
The stabilization and development of these nuclei always show a random
nature. Galvanic coupling is then established between the discontinuous
zones, which form small anodes where metal dissolution occurs, and
the remainder of the surface where the cathodic reaction takes place.
Means of reducing or preventing pitting corrosion : Choose the material
most appropriate for the service conditions, avoid stagnant zones
and deposits, Reduce the aggressivity of the medium, use cathodic
protection.
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