Environmental Engineering Reference
In-Depth Information
Therefore, uniform corrosion is minimized combining appropriate materi-
als (nickel alloys and stainless steels), surface fi nish, passivating treatments
and alkaline pH.
Flow-accelerated corrosion or fl ow-assisted corrosion (FAC) is a mecha-
nism in which the passive layer dissolves in fast fl owing water, without any
mechanical erosion. As a consequence, the underlying metal continuously
corrodes to recreate the protective oxide. FAC rate decreases when the fl ow
velocity decreases and when the pH increases. FAC stops as soon as oxygen
is dissolved in water. FAC affects carbon steel piping of the secondary cir-
cuit where water or wet steam circulates.
Pitting is a localized corrosion forming holes at the surface of the metal,
induced by the local depassivation of an area, which becomes anodic while
a large area becomes cathodic. The acidity inside the pit is sustained by the
spatial separation of the cathodic and anodic half-reactions, which creates
a potential gradient and the transport of anions into the pit. The presence
of surface defects, such as scratches and local changes in chemical compo-
sition promote pitting which causes little loss of material but it may lead
to deep corrosion in a component. Pitting mainly affects materials such as
austenitic stainless steels exhibiting a good resistance to uniform corro-
sion thanks to their good passivation. However, the presence of chlorides
and oxygen at relatively low temperature (typically 80°C) may weaken
the passive layers and enhance pitting via an autocatalytic process: Cl
ions start to concentrate in the pits for charge neutrality and promote the
reaction of positive metal ions with water to form a hydroxide corrosion
product and H + ions. The increasing acidity within the pits accelerates the
process.
Stress corrosion cracking (SCC) is a progressive failure affecting metals
subjected to a tensile stress (residual or applied) while they are exposed
to a corrosive environment. SCC occurs in specifi c and limited conditions
in terms of water chemistry, material and loading. SCC usually involves a
long incubation period prior to initiation, followed by a slow crack exten-
sion stage and transition in a fast crack propagation stage leading to failure.
Stress concentrations, cold work and irradiation promote SCC. The mate-
rial most susceptible to SCC is Alloy 600 (in both primary and secondary
waters). However, stainless steel becomes susceptible to SCC in primary
water under specifi c conditions: polluted environments (oxygen plus chlo-
rides), high level of cold work and irradiation. The susceptibility of nickel
alloys to SCC strongly decreases when the chromium content of the mate-
rial increases, especially above 20%. Basically, SCC results in the oxide
ingress, usually at grain boundaries, which locally weaken the material. The
oxide penetration is enhanced by the presence of strain and is affected by
precipitation. If local stresses are suffi cient to fail weakened grain boundar-
ies, a crack extension occurs.
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