Page 210 - Corrosion Engineering Principles and Practice

184 C h a p t e r 6 R e c o g n i z i n g t h e F o r m s o f C o r r o s i o n 185

6.4 Velocity Induced Corrosion
Velocity induced corrosion refers to problems caused or accelerated
by the relative motion of the environment and the metal surface. It is
characterized by surface features with a directional pattern which are
a direct result of the flowing media.
With the exception of cavitation, flow-induced corrosion problems
are generally termed erosion–corrosion, encompassing flow enhanced
dissolution and impingement attack. The fluid can be aqueous or
gaseous, single or multiphase [23]. There are several mechanisms
described by the conjoint action of flow and corrosion that result in
flow-accelerated corrosion (FAC) [24;25]:
Mass transport–control: Mass transport–controlled corrosion
implies that the rate of corrosion is dependent on the convective
mass-transfer processes at the metal/fluid interface. When steel
is exposed to oxygenated water, the initial corrosion rate will be
closely related to the convective flux of dissolved oxygen toward
the surface, and later by the oxygen diffusion through the iron
oxide layer. Corrosion by mass transport will often be streamlined
and smooth.
Phase transport–control: Phase transport–controlled corrosion
refers to conditions when wetting of the metal surface by a cor-
rosive phase is flow-dependent. This may occur because one
liquid phase separates from another or because a second phase
forms from a liquid. An example of the second mechanism is the
formation of discrete bubbles or a vapor phase from boiler water
in horizontal or inclined tubes in high heat-flux areas under low
flow conditions. The corroded sites will frequently display rough,
irregular surfaces and be coated with or contain thick, porous
corrosion deposits.
Erosion–corrosion: Erosion–corrosion has been associated with
mechanical removal of the protective surface film resulting in a
subsequent corrosion rate increase via either electrochemical or
chemical processes. It is often accepted that a critical fluid velocity
must be exceeded for a given material. The mechanical damage by
the impacting fluid imposes disruptive shear stresses or pressure
variations on the material surface and/or the protective surface
film. Erosion–corrosion may be enhanced by particles (solids or
gas bubbles) and impacted by multi-phase flows. The morphol-
ogy of surfaces affected by erosion–corrosion and FAC may be in
the form of shallow pits or horseshoes or other local phenomena
related to the flow direction (Fig. 6.37).
Cavitation: Cavitation sometimes is caused by the formation
and collapse of vapor bubbles in a liquid near a metal surface.
Cavitation removes protective surface scales by the implosion of