Page 70 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

48 INTRODUCTION AND FORMS OF CORROSION

with the size of the abrasive particles gives a measure of the kinetic energy of the
impinging particles, which is proportional to the square of the velocity. The ductile
materials undergo wear by plastic deformation in which the material is removed by
displacing or the cutting action of the eroded particle. Some brittle materials undergo
wear predominantly either by flow or fracture, depending on the impact conditions.
Wear rate dependence on the impact angle for ductile and brittle materials is different
(60, 61).
Slug flow is the dominant flow regime in multiphase systems. Flow visualization
has shown that bubbles distort and elongate in the vicinity of a pipe wall in a manner
similar to collapsing bubbles. The corrosion rate increases because of a thinning of
the mass transfer and corrosion product layers, as well as because of localized damage
of the corrosion product film.
Multiphase environments are present in several industries. Changes in pressure
and temperature in process equipment and the mixing of various streams can force
the mixture into two or three phase environments. The corrosion because of the mul-
tiphase environment is of concern in nuclear and thermal power plants, chemical
process industries, and in waste management systems. Slug flow occurs more readily
in smaller diameter pipes, where the coalescence of bubbles can result in change of
bubble flow into slug flow. Slug flow affects corrosion in two ways, namely: (i) a dra-
matic increase in turbulent intensity that can increase the mass transport of corrosive
species by 1000 times; (ii) extensive damage of the corrosion product layer leading
to extensive attack.
Percussion is a repetitive solid body impact, such as that encountered in print ham-
mers in high-speed electromechanical applications and high asperities of the surfaces
in a gas bearing. Repeated impacts lead to loss of solid material. Percussive wear
occurs by hybrid wear mechanisms consisting of a combination of adhesive, abrasive
surface fatigue, fracture, and tribochemical wear (60).

1.7.10 Chemical or Corrosive Wear

Corrosive wear occurs when sliding takes place in a corrosive environment. The most
dominant corrosive agent in air is oxygen. Thus, the chemical wear in air is known
as oxidative wear. In the absence of sliding, the oxides formed because of corro-
sion would slow down or even arrest the corrosion process. The sliding action can
remove the oxide film and expose the bare metal to oxygen and formation of metal
oxide. Thus, the chemical wear requires both sliding (rubbing) and oxygen. Corro-
sion occurs in a highly corrosive environment and in high-temperature and highly
humid environments. The chemical wear requires a corrosive as well as a rubbing
motion. Corrosive fluids provide a conductive medium for the electrochemical reac-
tion to occur on sliding surfaces. In an aqueous medium, the corrosive agents are
oxygen and carbon dioxide, which are responsible for corrosion (60).
Chemical wear is important in many industries, such as mining, mineral pro-
cessing, chemical processing, and slurry handling. An example of a corroded roller
subsequent to running in a bearing is shown in Figure 1.15. The corrosion caused
a multitude of dark-bottomed pits, with surroundings polished by running. It is also