Page 79 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

MECHANICALLY ASSISTED CORROSION 57

and the process repeats itself. Friction appears to be the driving force of oxidation
wear (69).
A more plausible approach could be a combination of the two approaches cited
under wear-oxidation and oxidation wear with relative importance of one or the other
depending on the particular system and therefore a function of medium, surface finish
and the nature of the materials in contact. It is useful to note that oxygen accelerates
corrosion by fretting, in particular, in ferrous alloys (17).
Fretting is more severe in air than in an inert atmosphere (2). The damage in a
humid atmosphere is less than in dry air as humidity has a lubricant action, and the
hydrated oxides are less abrasive than dry oxides (2, 69, 71). Surfaces subjected to
fretting wear have red–brown patches on ferrous metals and adjacent areas. There
is no critical measurable amplitude below which fretting does not occur. When the
deflection is elastic, fretting damage is not likely to occur. The wear rate increases
with slip amplitude over a certain amplitude range. The fretting wear rate is directly
proportional to the normal load for a given slip amplitude. The frequency of oscilla-
tion has little effect in total slip situation.
In a partial slip situation, the frequency of oscillation has little effect on the wear
rate per unit distance in the low-frequency range, while the increase in strain rate at
high frequencies leads to increased fatigue damage and increased corrosion because
of an increase in temperature (60). The effect of temperature on fretting depends on
the oxidation characteristics of the metals. An increase in temperature might result in
the growth of a protective oxide layer that prevents metal–metal contact and hence a
lower fretting rate.
Wear rate increases with slip amplitude over a range of amplitude. Wear debris can
be plate-shaped, ribbon-shaped, spherical, and also irregularly shaped, on the basis
of morphology.
Wear of material depends on the mating material, surface preparation, and oper-
ating conditions. Clean metals and alloys exhibit high adhesion and, as a result, high
friction and wear. Any contamination prevents contact and any chemically produced
films reduce friction and wear. In dry sliding, identical metals such as iron on iron
exhibit high friction and wear and they must be avoided. Soft metals such as In, Pb,
and Sn exhibit high friction and wear. Metals such as Co, Mg, Mo, and Cz exhibit
low friction and wear. Lead-based white metals (babbits), brass, bronze, and gray
cast iron exhibit low friction and wear and are used in dry and lubricated bearing
and sea applications. In high-temperature applications, cobalt-based alloys that have
good galling resistance are used (60).

1.7.24 Modeling Fretting Corrosion
An equation has been proposed for steel to evaluate loss of weight W caused by fret-
ting corrosion on the basis of a model that combines the chemical and mechanical
effect of corrosion by fretting. The chemical factor concerns the adsorption of oxy-
gen resulting in oxidation of the metal to form the oxide, and the mechanical factor