Page 71 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

MECHANICALLY ASSISTED CORROSION 49

Figure 1.15 Quenched and tempered roller bearing after corrosive wear factor. (Reproduced
by permission, John Wiley and Sons (60).)

found to create extensive, surface-originated spallings from a multitude of initiated
points (60).
Friction modifies the kinetics of chemical reactions of sliding bodies with each
other, and with gaseous or liquid environment, to the extent that reactions that occur
at high temperatures occur at moderate, even ambient temperatures during sliding.
Chemistry dealing with modification of chemical reactions by friction or mechanical
energy is known as tribochemistry and the resulting wear as tribochemical wear (62).
The mechanisms by which friction increases the rate of chemical reaction are fric-
tional heat, removal of product scale to expose fresh surface, accelerated diffusion,
and direct mechanochemical excitation of surface bonds. The tribochemical reactions
result in oxidative wear of metals and tribochemical wear of ceramics (60).

1.7.11 Oxidative Wear

Interface temperatures produced at the asperity contacts during sliding of metallic
pairs under nominally unlubricated conditions result in thermal oxidation, which
produces oxide films several micrometers thick. This is beneficial as oxide film gives
protection from further corrosion. The thick oxide film reduces the shear strength of
the interface, which suppresses the wear as a result of plastic deformation. In many
cases, tribological oxidation can reduce the wear rate of metallic pairs by as much as
two orders of magnitude, compared with the same pair in an inert atmosphere. Tribo-
logical oxidation can also occur under conditions of boundary lubrication when the
oil film thickness is less than the combined surface roughness of the interface. The
oxidation can prevent severe wear. In oxidation wear, debris is generated from
the oxide film (60).