Page 72 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

50 INTRODUCTION AND FORMS OF CORROSION

At low ambient temperatures, oxidation occurs at asperity contacts because of
frictional heating. At higher ambient temperatures, general oxidation of the entire
surface occurs and affects wear. In the case of steel, the predominant oxide present
in the debris depends on the sliding conditions. The predominant oxide at low speeds
and ambient temperatures is -Fe O , at intermediate conditions it is Fe O , and at
2 3 3 4
high speeds and temperatures the oxide is FeO. Oxidation of iron and many metals
follows a parabolic law with the oxide

h = Ct 1∕2

film thickness increasing with the square root of time “t,” h is the thickness, and C is
the parabolic rate constant at elevated temperatures (60).
As diffusion is thermally activated, the growth rate in oxide film thickness during
sliding as a function of temperature follows an Arrhenius equation

K = Ae (−Q∕RT)

where K is the parabolic rate constant for the growth of the oxide film, A is the
4
2
Arrhenius constant (kg /m s), Q is the parabolic activation energy associated with
the oxide (KJ/mol), R gas constant, and T the temperature. It has been observed that
the Arrhenius constant for sliding is several orders of magnitude larger than that for
static conditions. The oxidation rate during sliding may result from increased diffu-
sion rates of ions through a growing oxide film, which has high defect density because
of mechanical perturbations (60).

1.7.12 Electric-Arc-Induced Wear
When a high potential is present over a thin air film in a sliding process, a dielectric
breakdown occurs, resulting in arcing. During arcing, a relatively high power den-
sity occurs over a very short period. The HAZ is usually shallow (∼50 μm), and the
heating results in melting and resolidification, corrosion, hardness changes, and other
phase changes. Arcing might also cause ablation of material and craters. Any sliding
or oscillation after an arc may either shear or fracture the lips, leading to three-body
abrasion, corrosion, surface fatigue, and fretting (60, 63).

1.7.13 Erosion–Corrosion
All types of corrosive media such as aqueous solutions’ organic media, gases, and
liquid metals can cause erosion–corrosion. The corrodents can be a bulk fluid, a
film, droplets, or an adsorbed substance. Hot gases may oxidize a metal at high
velocity and abrade the protective scale. Slurries probably cause most damage in
erosion–corrosion (9, 17). All types of equipment such as pipelines (curves, elbows,
and T squares) floodgates, pumps, centrifugal fans, helixes, wheels of turbine, tubes
of intersections of heat exchangers, and measuring devices are subject to this form
of attack. Most metals and alloys are subject to erosion–corrosion, and the resulting