Page 360 - Corrosion Engineering Principles and Practice

CHAPTER 9

Atmospheric

Corrosion

9.1 Introduction
Atmospheric corrosion is surely the most visible of all corrosion
processes, for example, rusty bridges, flag poles, buildings, and outdoor
monuments. The large segment of the paint industry committed to the
manufacture and application of products for the protection of metals,
as well as the large-scale operations of the galvanizing industry attest
to the importance of controlling atmospheric corrosion.
Economic losses caused by atmospheric corrosion are tremendous
and therefore account for the disappearance of a significant portion
of metal produced. Consider, for instance, agricultural machinery,
steel structures, fences, exposed metals on buildings, automobile
mufflers or bodies, and the myriad of other metal items that are sent
to the scrap yard when they become unusable as a result of corrosion.
These constitute direct losses from corrosion.
Atmospheric corrosion has been reported to account for more
failures in terms of cost and tonnage than any other type of material
degradation processes. This particular type of material degradation
has recently received more attention, particularly by the aircraft
industry, since the Aloha incident in 1988, when a Boeing 737 lost a
major portion of the upper fuselage in full flight at 7300 m [1].
All of the general types of corrosion attack occur in the atmosphere.
Since the corroding metal is not bathed in large quantities of electrolyte,
most atmospheric corrosion operates in highly localized corrosion cells,
sometimes producing patterns difficult to explain as in the example of
the rusting galvanized roof shown in Fig. 9.1.
Thus, calculation of the electrode potentials on the basis of ion
concentration, the determination of polarization characteristics, and
other electrochemical operations are not as simple in atmospheric
corrosion as they are in liquid immersion corrosion. However, all of
the electrochemical factors which are significant in corrosion processes
do operate in the atmosphere.
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