Page 80 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

58 INTRODUCTION AND FORMS OF CORROSION

involves the loss of particles, at the asperities on the opposite surface
1 C
( )
W = K L 2 − K L + K ILC
0 1 2
f
Chemical factor Mechanical factor
where L is the charge between surfaces, C is number of cycles, f is frequency of
movement, I is slip, K , K , and K are constants.
0 1 2
The chemical contribution decreases with increasing frequency of movement as
there is little time for the chemical reaction. The mechanical factor is a function of
the slip and the load. In the presence of nitrogen, the wear is a function of mechanical
factor and is independent of the frequency (8).

1.7.25 Fretting CF
Fretting CF failures are encountered in aircraft engine parts, such as connecting rods,
knuckle pins, splined shafts, clamped and bolted flanges, couplings, and other parts.
Failures because of fretting CF also occur in railway axle shafts at the wheel seats
and in automobile axle shafts, suspension springs, steering knuckles, and others
(2, 69, 72). The oscillatory movement is usually the result of external vibration,
but in many cases it is the consequence of one of the members of contact being
subjected to cyclic stress (fatigue), which results in initiation of fatigue cracks
leading to fretting fatigue. Fretting can also cause rupture of adhesive ties because
of the strengths of oscillations that can generate fine cracks that can propagate to a
major fracture of the sample.
It has been found that in some instances the quantity of metal lost by fretting is
directly related to the reduction of the resistance to fatigue. The frequent oscillations
cause formation of pits that initiate cracks of fatigue leading to increased suscepti-
bility to fatigue fractures. Fretting corrosion causes loss of material in highly loaded
bolted equipment. Bolts and studs are loosened and become more prone to fatigue
failure. This is a serious problem when the bolt or stud is very short as in aircraft
engine cylinder hold-down studs (72). The initiation of the fretting fatigue crack is
located at the boundary of the fretting scar at the fretted zone. The fatigue crack then
propagates into the surface at an angle to the surface.

1.7.26 Prevention of Fretting Wear
Various design changes minimize fretting wear. The machinery should be designed to
reduce oscillatory movements, reduce stresses, and eliminate two-piece design. Some
steps involved in the prevention of fretting wear are: (i) lubrication of fraying surfaces
with low-viscosity, high-tenacity oils and grease to exclude direct contact with air
(17). Phosphate conversion coatings in conjunction with lubricants, which are porous
and provide oil reservoirs may be used (17); (ii) use of gaskets to absorb vibration and
limit oxygen at bearing surfaces; (iii) restricting the degree of movement such as shot
peening, which induces residual surface compression stress to retard corrosion and