Degradation of materials in the atmosphere Common materials are vulnerable to atmospheric attack
F 5 Heavy coats of paint protect the Golden Gate Bridgefrompollution-induced corrosion
T.E. Graedel R. McCill AT&TBeN Laboratories Murray Hiii, N.J. 07974 Gold is highly prized not only because of its relative scarcity but because it
generally retains its luster after extended exposure to the atmosphere. This property is, however, the excep tion rather than the norm. Examples of more usual susceptibilityto degradation include the corrosion of iron, the tarnishing of silverware, and the formation of nonconducting surface films on electronic components.
M)13.93BWBg109~1MM$Ol.W0 Q 1986 American C h e m i d Society
The interaction of materials with the atmosphere has recently received increased attention as a result of concerns regarding the effects of acid deposition. The deposition of acidic trace species on surfaces, both during precipitation and by turbulent gas transport and adsorption processes in the absence of precipitation, inextricably links materiEnviron. SCi.Technol.,MI. 20. No. 11, 1988 1093
of the material to particular species. consequence of biological activity, and This approach is used for two reasons. its presence in the atmosphere reflects First, in only a few cases have l a b human activity only indirectly. The ratory studies been extensive enough to N02-N03- pair, on the other hand, is specify quantitatively the susceptibility directly related to human activity. of materials to atmospheridy realistic The next three corrodent entries are concentrations of potentially degrading sulfur-containing species. Hydrogen species. Second, in most cases a quali- sulfide (HzS) and carbonyl sulfide tative assessment is suffcient to meet (COS) have both natural and a n t h r o p the needs of potential users of the infor- genic sources, and they degrade most mation. Although temperature and ex- materials by forming dark-colored sulposure to light are involved in at least fides. The sulfur dioxide-sulfate pair some degradation processes, the data (SO, [sas p h a ~ - S O , ~ -[in precipitaare few and for clarity there is no at- tion or on aerosol particles]) reacts to tempt to treat those processes in this form sulfates on many materials; the reaction products often are friable or article. Water, the first of the species listed in less rugged in other ways than the origTable 1, degrades many metals even in inal materials are. the apparent absence of other atmosHydrogen chloride (gasphase)-chlopheric trace constituents. Next in the ride ion (liquid phase) degradation octable is carbon dioxide, which after wa- cum for many materials. Particularly ter, nitrogen, oxygen, and the rare noticeable in marine environments gases is the most abundant species in where sea-salt aerosols are common, the atmosphere. The concentrations of the corrosion products are highly colwater and CO, are controllableonly in- ored, often soluble, and lead to the disdoors. Of more interest are the other solution of the attacked material. species listed, their presence indicates The only organic entries in the list emissions into (and often chemistry are formaldehyde and organic acids within) the atmosphere. (principally formic and acetic acids). The fmt two of these. species are the Many different sources emit these comnitrogen-containingcompounds ammo- pounds into the atmosphere. In addinia (",) and nitrogen dioxide OJOZ) tion, they are readily formed by gasin the gas phase and ammonium phase chemistry and perhaps by Assessing degradation potential and nitrate (NO;) ions in the liquid-phase chemistry within the Few quantitative assessments of the liquid phase. (Where appropriate in the atmosphere. They have recently been degradation of materials by specific table, molecules and ions are paired in found to be common constituents of atmospheric species have been per- their common gaseous and aqueous precipitation; formaldehyde has long formed. In principle, most materials forms.)TheNH,-m-pairislargelya been known to be common in the suffer some degradation upon exposure to the atmosphere, but the available data limit our presentation (Table 1) to 19 materials, divided into six groups. TABLE 1 The fmt group, the electrical and deco Sensitivities to potentially damaging atmospheric specie* rative metals, includes silver, copper, bronze, and brass. The ferrous metals Maerlal imd SteaP Tinb Lead' Soided SiheP Copper B m n d Bras# are represented by iron and carbon Water L L L L H H L L steel. Solder and its components, lead Carbon dioxide and carbonate L L L M M M and tin..~~~~ constitute a third erouo. A fourth metals group incluies 'zinc, I Ammonia nickel, and aluminum, three metals that and ammonium M L M M L I are degraded upon certain atmospheric N exposures. Galvanized steel (steel Hydmgen N M coated with zinc) behaves essentially as sulfide H H does zinc in the atmosphere, so it is not Carbonvl sulfide H H listed separately ( I ) . Stone in its variSulfur dioxide and sulfate L L ous forms constitutes a fifth group. The Hydyyn sixth group includes organic materials: chlori e polymers and rubber, paint, paper, phw L and chloride tographic materials, textiles, and Formaldehyde leather. The assessments apply to unOrganic acids protected materials in their normally Ozone manufactured state. Hydrogen peroxide L The 19 materials form the abscissa of Table 1. On the ordinate are 12 atmos-
als effects with the gas- and liquidphase chemistry of the atmosphere. The detailed mechanisms of degradation, however, are poorly understood, partly because of the interconnectedness of the problems and partly because the gas-liquid-solid systems are diffcult to treat theoretically. Despite these scientificuncertainties, it is useful to provide at least a qualitative perspective on materials degradation problems. The first step is to review and present the degree of sensitivity of a number of materials to potentially degrading atmospheric species. We next examine the chemical composition of several different atmospheric regimes near the ground (in the gas phase, the atmospheric aerosol, cloud droplets, fog droplets, dew, raindrops, snowflakes, and indmr air). Correlating this compositional information with the materials' sensitivities helps us to predict the degradation of materials in different atmospheric regimes, along with the probable causative agents. We also attempt to indicate qualitative, long-term trends in the atmospheric species concentrations and to predict the long-term trends in degradation potential for materials exposed to the atmosphere.
(m+)
I I
~~
~
~~
iiformation in the.Iiterature to make a qualitative assessment of the sensitivity 1094 Envimn. Sci. Technol., MI. 20.No. 11, 1988
Zinc"
L
M M
atmospheric gas phase. As Table 1 shows, many materials are degraded by these organic compounds. The final two species are the strong oxidizers, ozone (4)and hydrogen peroxide (H2Q).How these molecules interact with materials is imperfectly understood, but it is h o w n to include both the oxidation process (2-57) and the enhancement of other corrosion reactions (58). At fmt glance, Table 1 and the previous discussion seem to imply that much of the information needed to assess atmospheric damage to materials is at hand. To some degree, that inference is valid. However, it is worth noting that data exist for only 98 of the 209 p s i b l e assessments (excluding water) in the table. In some cases contradictory data exist. For example, it is clearly stated that H2Shas no noticeable effect on iron (59), a conclusion that seems to contradict the experience of the oil and natural gas industry (w). Another point is that most degradation is enhanced in the presence of water; our assessments assume that sufficient water is present for degradation to OCCUI, but this may not be the case in some arid regions. Less obvious but of qual concern is the dificulty of choosing between such qualitative terms as moderate sensitivity and low sensitivity. Each assessment in Table 1 has been reviewed by experts familiar with atmospheric e m on specific materials. Nonetheless, the entries are more
umww Water
accurately regarded as informed assessments than as defhtive judgments.
Degradation by atmospheric gases The degradation potential of a particular phase of the atmosphere (the atmospheric gas, for example) for a particular material is a function of both the concentration of a specific degrading species within that phase and the sensitivity of the material to that species. The latter property has been summarized in Table 1 for materials and species of interest. The atmospheric chemical literature can be used to derive the gas concentrations (61, 62). Figure 1 presents the materials degradation potential for gaseous degrading species in a typical urban area. This is the fmt of several similar diagrams, designed for the efficient display of substantial amounts of information. The assessments for sensitivity and concentration (actually, concentration relative to that needed to cause the degradation of materials) are indicated by a two-dimensional color code (63-67). This approach was chosen because the number of different colors that an observer can readily distinguish is quite small (6fl and because codes in which color variation is not highly constrained have demonstrated poor success (65). In Figures 1-6, each row is characterized hy a single hue (red, red-brown, yellow, or p n ) weU removed chromatically from its adjacent row or rows. The division of the row into col-
Polymer8 NkkeY AhmlnurnmStone' and rubber. Palnip Fnpen Pholor
N
L
L
N
N
H"
L
mnilw Leather' N M
a r b o n dioxide
L andcarbonate Ammonia andammonium N N m n dioxide and nitrate M Hydrogen sulfide N Carbond sulfide Sulfur dioxide and suitale
h
;and k%? chloride
I
Formaldehyde Organic acids Dzone
I
Hydrvn
pemxlde
1
L
'References 22,s 46. .52'Referencer 14. 15, 18,23,36.47, 51. *References50,51,'References 6.9, 14. 35.38,48,51, %eIerencea 14.20,34,40.41.51: mRelerenc8810. 13, 35.37.45.51,"Relerences 2. 11, 30. 44. %3elerence812. W DReIerenm 30.43.55.WRefersnoss 5.56. 'References31. 5 6 'References 12. 30. 58: 'Refaences 39.56.'L For gaPphase H a
umns is accomplished by varying the intensity of the hue. The codes of particular interest are the bright red code @igh sensitivityhigh concentration), red-brown (bigh sensitivity-moderate concentration), and bright yellow (moderate sensitivity-high Concentration). The bright red can be thought of as indicating liiely materials degradation; red-brown and bright yellow indicate a reasonable probability of degradation. Four bright red regions appear in Figure 1. In thm cases, these refer to direct attack by water on iron, steel, and paper. The fourth is that for the degradation of zinc by formaldehyde. Red-brown and bright yellow regions are numerous. Most of the materials shown have at least moderate potential for damage as a result of atmospheric gas interactions; those with NO, and SQ are the most widespread, but HC1, H,S, O,, and formic acid (HWOH) also are significant. It also is of interest to examine the combinations of materials and potential degrading species that do not appear likely to demonstrate degradation in the atmospheric gas regime. Aluminum, brass, and tin show little degradation potential, for example. The effects of hydrogen peroxide likewise are not thought to he of concern, but assessments of the sensitivities of a number of materials to this and other gases must still be performed. Finally, the plus and minus signs overprinted in the figures indicate the long-term trends of the corrodents in question. These trends are derived from a variety of sources of air quality information (6-67). Carbon dioxide is definitely increasing in concentration, sulfur dioxide shows a decreasing trend in most regions around the world, and nitrogen dioxide exhibits a slight increasing trend in many areas. Atmospheric measurements are, in general, too sparse to permit trend estimates for the remaining corrodents, although it has k e n suggested that a long-term increase is indicated for &. The .implicationsof these trends are obvious. SO2 concentrations, now deemed to be moderate in urban areas, will slowly decline. Thus, the redbrown areas in FiguTe 1 attributable to SO, exposure eventually will shift to dull red, and materials degradation from gaseous SQ will decrease. Conversely, urban areas that now have moderate C02, N Q , and O3concentrations may eventually experience somewhat increased degradationof materials that are sensitive to CQ, NQ, and 03. It is notable that specific geographic areas may have different trends and that for many atmospheric species the trends have yet to be established. Environ. Sci. Technol., Vol. 20,NO.11, 1986 1095
Degradation by rain and snow Many corrodents are present in ionic form in rain and in other aqueous phases in the atmosphere: NH4+rather than NH3,N(3- rather than NO, (or HN03), rather than SO2, CIrather than HCI. Organic acids are partially ionized and are thus present in ionic and molecular forms. Although rainwater, particularly standing rainwater, can degrade materials, its washing action on exposed surfaces may sometimes prove beneficial. Figure 2 shows the predicted effect on various materials of these aqueousphase corrcdents in rain. The areas of most concern in this figure (the bright red regions) are those rows for the organic species formaldehyde and formic acid. These species are present in rain at relatively high concentrations-on the order of 10 pmollL-I (6s). A number of materials are sensitive to them. Formaldehyde and formic acid are related through atmospheric chemical processes to a variety of anthropogenic and natural hydrocarbon precursors, but the dominant sources are uncertain, as are the long-term trends. Among the materials of concern here are bronze, copper, iron, lead, and zinc. Iron and steel are highly sensitive to chloride ions in solution. Chloride ions are common in rain, especially in marine environments. A number of materials, such as iron, steel, solder, zinc, stone, marble, and leather, are highly sensitive to dissolved SO,, which is present in rainwater in moderate concentrations. Atmospheric SO, concentrations are decreasing on a global basis, and the corresponding potential for material damage can be expected to decrease as well. The degradation potential of snow is not shown separately becaw it appears to be very similar to that of rain. It is worth noting that the most severe effect of snow on materials may occur not during its presence in frozen form, but during run off following melting when the snow constituents are more mobile and reactive. Degradation by airborne particles Airborne particles have atmospheric lifetimes of hours to days (69).At relative humidities above 4056,the aerosol particles are customarily enveloped by a water shell (70). As a consequence of the water shell and the long lifetimes, particles provide convenient sites for the concentration of gases with moderate to high aqueous solubilities. The accumulation of trace species on particles throughout the day is readily demonstrated by sequences of particle collection and analysis (71). Figure 3 shows the materials degradation ptential of airborne particles. 1096 Environ. Sci Technol.. Val. 20, NO.11. 1986
1
FIOURE 1
Material degradation potentlal of atmospheric gases in an urban are@
I I ozone
I
HYd,WB"
Concentration
Low Medium High
Tmnd
Higt Sensitivity
Increasing
Medim
Stable
LO!
Decreasing
FiGUREP
Typical materials degradation potential of rain in the northeastern Unlted State@
Ammonia dioxide
Ozone peroxide C0"C
Trend Hig
Sensitivily
Mediui LOW
Increasing
stable Decreasing
The bgradalbn palsnlial 01 m is Iimilsr m that 01 min. The laofnofsm Fipm 1 mxplamm the gr8pnlW mfstim.
FiGURE 3
Typical materials degradation potential of atmospheric particles in the central United States'
The only material-corrodent pair of serious concern is that of zinc and formaldehyde. Lesser but significant potential effects are produced on a number of materials by ammonium ion and sulfate ion and, to a lesser degree, by formaldehyde, formic acid, and hydrogen chloride. The effects of particles may be amplified by extended surface contact after deposition. Indeed, the enhancement of corrosion of metals at or near deposited particles indicates the important role that can be played by corrodents concentrated on particles (72, 73). In addition to the chemical c o w quences of deposited particles, soiling and optical degradation of materials can OCCUI.
chloride Formaldehyde
m
Organic acids
Ozone Hydroge. peroxide Trend
U
Increasing Stable OBcreasing
FiGURE 4
Typical materials degradation potential of dew in DetroiP
water
Carbon
Hydrogen FormaiOrganic
wraxide Concentration High Sensitivity
Medium
Law Medium High
Increasing Stable Decreasing
Degradation by dew and fog Dew is present on horizontal groundlevel surfaces 1&20% of the time (74 76). Most metals corrode more rapidly when thin fiims of water are present (76,77). To date, chemical analyses of dew are rare. In particular, concentrations of dissolved H202 or 4 would be of interest. The general finding thus far is that dew is somewhat less acidic than precipitation is, probably because of the influence of basic aerosol particles deposited on the surfaces upon which the dew condenses. The concentrations of some cations and anions in dew are higher than those in rain or snow. Figure 4 shows the materials degradation potential of dew. 'Rvo compo nents seem most likely to cause detericration: formaldehyde and dissolved sulfur dioxide. Materials of concern are zinc (on galvanized fencing, for exam ple), copper (roofing and statuary), iron and steel (automobile bodies, structural members), and masonry (building stone and statuary). Fog is generally less frequent and less long-lived than dew, but in some locations it can occur more than 80 days each year (78). The pH value of fog can be lower than 2.0, and the concentrations of corrodents can be quite high (75'). Figure 5 shows the degradation potential for fog, which is substantial. Dissolved sulfur dioxide and formaldehyde have significant effects on materials, more so than in dew because the concentrations are higher. High concentrations of ammonium ion are also a cause for concern. Most materials, including steel, masonry, and deccrative metals are subject to deterioration by fog. Indoor degradation of materials The indoor environment differs markedly from that outdoors. For the most part, concentrationsof corrodents having outdoor sources are significantly lower. If indoor sources of corEnviron %I. Technoi.. MI. 20, NO.11. 1986 1097
rodents are present, however, concentrations of corrcdents can be much higher than those outdoors. Among the corrcdents for which this enhancement is known to occur on occasion are nitrogen dioxide (from natural-gas heating systems, stoves, and clothes dryers), formaldehyde (from building materials), sulfur dioxide (from heating system), and ozone (from ultraviolet light sources) (so). Figure 6 shows the materials degradation potential of indoor air. Formaldehyde is of concern for zinc and iron. Nitrogen dioxide degrades solder, nickel, and textiles. Ozone is a potential accelerator of degradation of several materials, including polymer materials and paint. Also of concern are sulfur dioxide and formic acid, each of which is known to interact with a number of materials. Synergistic effects Table 1 presents the sensitivities of materials to potential degrading species, but it leaves out one important fact: Atmosphericspecies are presented to materials in mixtures rather than one by one. Because degradation is a chemical process, it is subject to the usual chemical complexities of catalysis, inhibition, favored reaction routes, the presence of light, and the differences in the behavior of competing reactions at different temperatures. Studies involving mixtures of species are very few, but those that have been conducted clearly reveal the potential for synergism. Among the examples of synergistic behavior are the effects of sulfur dioxide with nitrogen dioxide on carbon steel (81, 82). of ozone with hydrogen sulfide on copper (33,and of a photochemical smog mixture with sulfur dioxide on galvanized steel (83). In each case, materials effects were not equal to the sum of the effects due to the individual corrcdents. Studying this problem is similar to assessing the effect of chlorofluorocarbons on the stratosphere; it has become clear that concentrations of stratospheric ozone are sensitive to the concentrations of oxides of nitrogen, methane, and perhaps brominated and sulfonated species, as well as the chlorofluorocarbons. A comprehensive assessment of the synergistic materials degradation p tential of atmospheric constituents is beyond the current state of the art. Until new methods are developed, it will be necessary to assume that synergistic effects do not cause major changes in the sensitivitiesshown in Figure 1. This assumption is likely to be true more often than not, but in the complicated chemical environment that the atmosphere provides, some synergistic ef1098 Environ. SCi.Technol., Vol. 20, NO.11. 1986
FIGURE 5
Typical materials degradation potential of fog in Los AngeleP
Hydrogen
dioxide Chloride dehyde
Hydrqen Trend
Senritivily
E
Medium LOW
Increasing Stable Decreasing
The loofnole to Figure 1 explains m e graphicl notaton
FIGURE 6
Typical materials degradation potential of air inside a mobile home
Water Ammonia
Sulfur
Hydrogen Formal-
omne
I
Medium Hig
C0"W
High Sensitivity
Medium LOW
fects of importance will certainly be present. It is imperative, therefore, that the fundamental chemical processes in materials degradation become well understood so that potential synergistic effects are assessed properly.
Specific responses This article has so far presented a discussion of the materials degradation potential of different atmospheric regimes, It is worth changing that focus here to discuss the materials themselves, together with the uses and environments in which their response will be salutary, average, or poor. Electronic and decorative metals. Silver generally is not used outdoors because it tarnishes when it is exposed to reduced sulfur gases or ammonia. This tendency limits its indoor use as well. Copper is degraded by the same species and by formaldehyde, but the greenish-blue patina formed on copper is both protective and attractive; copper is therefore oherr exposed to the environment in decorative (but not electronic) uses. Bronze behaves similarly to copper, although there are some suggestions that cast bronze is less environmentally rugged than are wrought bronze and copper (84). Brass, which is used a great deal both indoors and out, is much more resistant to deterioration by sulfur gases than either copper or bronze, although it suffers loss of zinc on exposure. Ferrous metals. Iron and the many different steels, when unprotected, have a range of responses to atmospheric exposure. The simple carbon steels, however, are by far the most widely used because of their low cost and good working properties (85).Iron and the carbon steels are highly susceptible to moisture, particularly when combined with chloride, sulfate, ammonium, formate, or acetate ions or formaldehyde. These species are common in airborne particles, dew, fog, and rain. Solder and components. Solder is sensitive to sulfates, which puts it at hazard in fog, and to NO2, which may be a problem in some urban atmospheres and indoors. Tin has excellent resistance to atmospheric degradation. Lead is affected principally by organic acids, rendering it susceptible to rain, snow, and deposited particles. Other metals. Zinc is sensitive to formaldehyde, an extremely common atmospheric species. It seems likely that pure zinc and galvanized steel are degraded primarily by formaldehyde, with contributions from SO2,NH3, and HCOOH.Nickel has good degradation resistance, being attacked only by gaseous NO2 and Os; thus the outdoor and indoor air may be of concern. Alumi-
num has excellent resistance to degradation; of the common atmospheric corrodents, it is susceptible only to chloride ion. Stone, The varieties of stone most often used for buildings and sculpture are marble, limestone, sandstone, granite, and slate. The first two are almost entirely calcium carbonate. Sandstone consists of grains of quartz (SiOz) cemented together with calcite or silicatebased minerals to form a solid rock. Granite and slate are silicate materials. Silicate rocks are far more resistant to degradation than are carbonate rocks (89. Nearly all studies of stone deterioration concern carbonate rocks. Carbonate rocks are quite sensitive to SO2, apparently in both gaseous and dissolved form. Significant degradation is thus anticipated in high-SO2 errvironments, such as fog, and perhaps gas, rain, dew, and deposited particles. Some synergistic sensitivity of carbonate rocks to ozone also has been suggested but has not been confirmed. Organic materials. Some paints degrade on exposure to atmospheric corrodents: lead-based paint to H2S, oilbased paint to SO2, and certain artists' pigments to 03.These materials have problems in atmospheric gas, fog, and indoor air, respectively. Acid-sensitive paper is susceptible to indoor SO2, which must be taken into account when storage facilities for books and other printed materials are designed. Photographic materials are moderately sensitive to H2S and SO2 and slightly sensitive to several other corrodents, including NO2 and 0,; poor indoor air quality is certainly of concern for these materials. Textiles are sensitive to NO2 and SOz; again, indoor air is of most concern, as it is with leather, which is markedly degraded by SO2. The information assembled and discussed here provides perspective on the potential for materials degradation as a conseqv :e of atmospheric exposure. Ferrous metals, masohry, zinc, copper, and perhaps some paints, appear most likely to be degraded. The regimes of greatest concern vary with different materials, but they include dew, fog, airborne particles, and indoor air. The results, however, rest on a rather sparse data base and take no account of synergistic deterioration effects of corrodents; thus, our presentation should be considered a starting point for discussion and experimentation. Much interesting and valuable research on the degradation of materials exposed to the atmosphere awaits the atmospheric and materials science communities.
Acknowledgment The authors dedicate this article to the memory of John Spedding, who died in
1984. A professor of chemistry at the University of Auckland, New Zealand, Spedding was among the first modern scientists to study the interactions of materials and atmospheric compounds. The authors thank N. S. Baer, J. F? Franey, V. Kucera, and S. I. Sherwood. Before publication, this article was reviewed for suitability as an ES&T feature by Glen R. Cass of California Institute of Technology, Pasadena, Calif. 91125; and W. Thomas Chase of the Smithsonian Institution, Washington, D.C. 20560.
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