Page 68 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

46 INTRODUCTION AND FORMS OF CORROSION

crevice corrosion and hydraulic splitting, supplies kinetic energy to drive abrasion
mechanism, pressurizes mill water to cause splitting, cavitation, and jet erosion of
metal and protective oxidized material, pressurizes mill water and gases to produce
unknown temperatures, phase changes, and decomposition on reaction products from
ore and water constituents, heats ball metal, and fluids to increase corrosive effects.

1.7.4 Corrosion Effects

Pits that can induce microcracking are produced. Microcracks at pits invite hydraulic
splitting because of impact. Roughens surface and reduces energy required to abrade
metal. May also produce hydrogen followed by absorption and cracking in steel.
Grain boundaries may be selectively attacked followed by weakening adjacent steel.
Wear damage mechanisms: Wear is the surface damage or removal of material from
one or both of two solid surfaces in a sliding, rolling, or impact motion relative to
one another. Wear damage precedes actual loss of material and it may also occur
independently.

1.7.5 Wear Damage Mechanisms
Wear is the surface damage or removal of material from one or both of two solid
surfaces in a sliding, rolling, or impact motion relative to one another.
Wear damage precedes actual loss of material and may also occur independently.
Wear as in the context of friction is not an inherent material property. It depends on
the operating conditions and surface conditions. Wear rate does not relate to friction.
Wear occurs by mechanical and/or chemical means and is generally accelerated by
frictional heating.
The principal wear mechanisms are: (i) adhesive; (ii) abrasive; (iii) fatigue; (iv)
impact by erosion and percussion; (v) chemical; and (vi) electrical arc-induced.
There are other mechanisms such as fretting, fretting corrosion, and fretting CF,
which is a combination of adhesive, corrosive, and abrasive forms of wear. Wear by
all mechanisms except fatigue mechanism, occurs by gradual removal of material.
One or more of these mechanisms may be operating in a particular machine. In many
cases, wear may be initiated by one mechanism but may proceed by other mechanisms
and thereby complicate failure analysis (60).

1.7.6 Adhesive Wear
Adhesive wear occurs because of adhesion at asperity contacts at the interface. These
contacts are sheared by sliding, which may result in the detachment of a fragment
from one surface to another surface. Some of the surfaces are fractured by a fatigue
process during repeated loading and unloading resulting in the formation of loose
particles. During sliding, surface asperities undergo plastic deformation and/or frac-
ture. The subsurface up to several micrometers in thickness also undergoes plastic
deformation and strain hardening with microhardness, by as much as a factor of two
or higher than the bulk hardness.