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Corrosion Engineering Principles and Practice2

CHAPTER9Atmospheric.Corrosion.9.1Introduction.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 corrosionEconomic 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 corrosionThese constitute direct losses from corrosionAtmospheric 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 atmosphereSince 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. 9.1Thus, 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.329.Copyright © 2008 by The McGraw-Hill Companies, Inc. Click here for terms of use330Chapter 9FIGURE 9.1 Rusting galvanized roof with regular bands of rust.9.2 Types of Corrosive Atmospheres.While atmospheres have been traditionally classified into four basic.types, most environments are in fact mixed and present no clear.demarcation. Furthermore, the type of atmosphere may vary with the.wind pattern, particularly where corrosive pollutants are present, or.with local conditions (Fig. 9.2) [2].9.2.1IndustrialAn industrial atmosphere is characterized by pollution composed.mainly of sulfur compounds such as sulfur dioxide (SO2), a precursor to.acid rain, and nitrogen oxides (NOx), the backbone of smog in modern.cities. Sulfur dioxide from burning coal or other fossil fuels is picked up.by moisture on dust particles as sulfurous acid. This is oxidized by some.catalytic process on the dust particles to sulfuric acid, which settles in.microscopic droplets and fall as acid rain on exposed surfaces. The.result is that contaminants in an industrial atmosphere, plus dew or fog,.produce a highly corrosive, wet, acid film on exposed surfacesIn addition to the normal industrial atmosphere in or near.chemical plants, other corrosive pollutants may be present. These are.usually various forms of chloride which may be much more corrosive.than the acid sulfates. The reactivity of acid chlorides with most.metals is more pronounced than the reactivity of other pollutants.such as phosphates and nitratesAtmospheric CorrosionFIGURE 9.2 Bird damage to a 1931 bronze statue commemorating the 21st.Battalion battles in WW I. (Courtesy of Kingston Technical Software)9.2.2MarineA marine atmosphere is laden with fine particles of sea mist carried.by the wind to settle on exposed surfaces as salt crystals. The quantity.of salt deposited may vary greatly with wind velocity and it may, in.extreme weather conditions, even form a very corrosive sal