Page 77 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

MECHANICALLY ASSISTED CORROSION 55

impacts on the metal surface, it dislodges the protective surface films and thereby
deforms the metal locally.
The solid material absorbs the impact energy, leading to elastic or plastic defor-
mation or even fracture. This may cause localized deformation and/or erosion of the
solid surface. It has been suggested that the bubble grows and collapses, resulting
in high pressure for a few milliseconds (65). The local pressure observed may be
∘
about 4000 atmospheres with a temperature increase up to 800 C. These conditions
of high pressure and temperature accelerate the corrosion rate. The cycle of exposure
of fresh surfaces to corrosion, followed by reformation of protective films that leads
to cavitation repeats itself (9, 60).
Single-phase flow has a definite pattern, whereas several flow patterns exist for
multiphase flows. Multiphase flow may involve oil/water or oil/water/gas. Slug flow is
the dominant flow regime in multiphase systems. It involves unique flow mechanisms
with pulses of gas bubbles being released into a turbulent mixing zone because of a
mixing vortex behind the slug front. These bubbles impact and collapse on the pipe
wall, causing severe localized, cavitation-type corrosion, which can be up to 1000
times the values normally encountered in other flow regimes (64, 66).

1.7.21 Prevention

The following preventive measures may be used in addition to the measures listed
under erosion–corrosion: (i) proper design should be used to minimize hydrodynamic
pressure differences along with specifying a smooth finish on all critical metal sur-
faces. The pressure and temperature should be adjusted such that the formation of
damaging steam bubbles is reduced. Proper operation of pumps and equipment is
recommended. (ii) Cathodic protection may prove useful to avoid cavitation. Deaer-
ation may be useful as dissolved air or gases cause nucleation of cavitating bubbles
at low pressures; (iii) hard, tough metals, or elastomeric polymers may be useful in
resisting cavitation erosion. Polymers and rubber have resilience, which is the capac-
ity to dispose of the energy without absorption. The polymers and rubber are also
resistant to abrasion (2).

1.7.22 Fretting Corrosion

This phenomenon is a combination of wear and corrosion in which the material
is removed from the contacting surfaces when the motion of the surfaces consists
of small amplitude oscillations with the relative movement ranging from fractional
nanometers to fractional micrometers. Fretting occurs when low-amplitude oscilla-
tory motion in the tangential direction takes place between two contacting surfaces,
which are nominally at rest (67, 68). It is necessary that the load be sufficient to pro-
duce distortion of the surfaces. Fretting corrosion occurs in most machinery subject
to vibration both in transit and in operation.
The most common factor in fretting is oxidation. In oxidizing systems, fine metal
particles removed by adhesive wear are oxidized and trapped between the fretting sur-
faces. The oxides act like abrasives and increase the rate of material removal. The red