Page 69 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

MECHANICALLY ASSISTED CORROSION 47

1.7.7 Abrasive Wear
This type of wear occurs when asperities of a rough hard surface or hard particles
slide on a softer surface and damage the interface by plastic deformation or fracture
in the case of ductile and brittle materials, respectively. In most cases, the wear mech-
anism at the start is adhesive, which generates wear particles that get trapped at the
surface, leading to three-body abrasive wear. In most of the abrasive wear situations,
scratching is observed with a series of grooves parallel to the sliding direction (60).

1.7.8 Fatigue Wear

Subsurface and surface fatigue are observed during repeated rolling (negligible fric-
tion) and sliding with coefficient of ≥0.3, respectively. The repeated loading and
unloading cycles to which the materials are exposed may induce the formation of sub-
surface and surface cracks, which, eventually after a critical number of cycles, lead
to the breakup of the surface with the formation of large fragments leaving large pits
on the surface. In this mode, negligible wear occurs, as wear does not require direct
physical contact between two surfaces. Mating surfaces experience large stresses,
transmitted through the lubricating film during the rolling motion, such as in the case
of well-designed rolling element bearings. The failure time in wear fatigue is statis-
tical in nature. Chemically induced crack growth such as in ceramics is referred to
static fatigue. In the presence of tensile stresses and water vapor at the crack tip in
ceramics, a chemically induced rupture of the crack tip bonds occurs rapidly, which
increases the crack velocity. Chemically enhanced deformation and fracture result in
increased wear of surface layers in static and dynamic conditions such as in rolling
and sliding operations.
Prevention of wear fatigue corrosion involves the inherent physical properties of
the alloy. For example, a gear must be tough and fatigue resistant and also have a
wear-resistant surface. In the case of applications requiring a moderate degree of
impact strength, fatigue resistance, and wear resistance, higher carbon through hard-
ening steel may suffice. For more severe conditions, surface-hardened steel may be
used (9).

1.7.9 Impact Wear
This wear consists of: (i) erosive wear and (ii) percussive wear. Erosion can occur
by jets, liquid droplets, and implosion of bubbles formed in the fluid and streams of
solid particles. Solid particle erosion occurs by the impingement when discrete solid
particles strike the surface and the contact stress arises from the kinetic energy of the
particles flowing in an air or liquid stream as it encounters the surface. Wear debris
formed in erosion occurs as a result of repeated impacts (60). Neighboring particles
may exert contact forces, and flowing fluid when present will cause drag. Under some
conditions, gravitational force may be important.
The erosive wear mechanism, as in the case of abrasion, involves both plastic
deformation and brittle fracture. The particle velocity and impact angle combined