Page 78 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

56 INTRODUCTION AND FORMS OF CORROSION

material easily lost from between the contacting surfaces is an example of fretting cor-
rosion in ferrous alloys. Fretting corrosion is encountered in shrink-fits, bolted parts,
and splines. The contacts between hubs, shrink- and press-fits, and bearing housings
on loaded rotating shafts or axles and many parts of vibrating machinery are prone to
fretting corrosion. Fretting wear damage also occurs in flexible couplings and splines
particularly where they form a connection between two shafts and are designed to
accommodate some misalignment. Fretting corrosion is frequently observed between
the crown of a ball bearing and its axle, or the head of a screw and the metallic sur-
face, and in jewel bearings, elements of machines in movement, suspension springs,
electric relay contacts, and kingpins of auto steering mechanisms (8, 69).
Fretting corrosion appears as discoloration and takes the form of local surface dis-
locations and deep pits. Fatigue cracks nucleate at these pits. The pits and cracks
occur in regions where slight movements have occurred between mating and highly
leaded surfaces (9). In the course of time, fretting corrosion can result in tarnished
appearance of the metallic surface and variable piece sizes. Products of fretting cor-
rosion may also cause blockages in machines in movement. Some examples (69) are:
NiO + Ni; Cu O, CuO, Cu; Fe O , Fe, Al O + Al.
2 2 3 2 3

1.7.23 Mechanism of Fretting Corrosion
Fretting is a form of adhesive or abrasive wear where the normal load causes adhe-
sion between asperities, and oscillatory movement causes ruptures, resulting in wear
debris. Fretting is generally associated with corrosion. In the case of steel parti-
cles, the freshly nascent surface oxidizes (corrodes) to Fe O and the characteristic
3
2
reddish-brown powder known as “cocoa” is produced. These oxide particles are abra-
sive. Because of the close fit of the surfaces and the oscillatory small amplitude
motion (about a few tenths of micrometers) the surfaces are never out of contact and
hence there is no opportunity for the products of action to escape. Oscillatory motion
causes abrasive wear and oxidation. Thus the extent of wear per unit sliding distance
because of fretting may be larger than that from adhesive and abrasive wear (60).
There are two approaches depending on the phenomenon that initiates and prop-
agates the damage: wear-oxidation and oxidation wear. In wear-oxidation, two sur-
faces are in contact and the surfaces are imperfect. The surfaces are in contact through
their asperities and the relative displacements of the two surfaces involve the wear of
the crests (70). This phenomenon is similar to cold welding or fusion at the inter-
face of metal surfaces under pressure. During displacements, the points of contact
break and pieces of metal are produced. The small pieces oxidize following the heat
generated because of friction. This process is repetitive and leads to accumulation
of residue. Further oxidation of the damaged material has a secondary effect. Wear
without debris occurs in the case of noble metals, mica, glass, and so on (17).
In oxidation wear, most of the metallic surfaces are initially protected from atmo-
spheric oxidation by a thin adherent oxide film. When metals are in contact (under
load) and subjected to repetitive weak movements, the oxide layer is broken at the
level of asperities and it removes some of the oxide and the resulting metal is oxidized