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Exfoliation and related microstructures in 2024 aluminum body skins on aging aircraft
ELSEVIER
Exfoliation and Related Microstructures in 2024 Aluminum Body Skins on Aging Aircraft Maria Posada, L. E. Murr, C.-S. Niou, D. Roberson, D. Little, Roy Armwood, and Debra George Department of Metallurgical and Materials Engineering and FAST Center,* The University of Texas, El Paso, Texas Exfoliation,
a directional
attack along elongated
some detail in rolled 2024 aluminum samples
utilizing
grain boundaries,
has been examined
in
sheet and plate for KC-135 aging aircraft body skin
optical (light) metallography,
scanning
electron microscopy,
mission electron microscopy. A detailed analysis and comparison
of precipitates
and transwithin the
grains and in the grain boundaries were performed, as well as an examination of elemental depletion profiles across grain boundaries. These observations suggest that corrosion-related anodic sites play a far less significant role in the propagation of exfoliation than do the hard corrosion products creating wedging stresses within the elongated grain boundaries, which seem to demonstrate unique and unusual structural or energetic features or both. Science Inc., 1997
The directional attack along the elongated grain boundaries in these sensitized aluminum alloys results in a leafing action that seems to be promoted by corrosion products creating stresses or corrosion forces [lo] that further split the grains, much like water freezing in cracks in concrete. This creates a microscopic “pastry dough” or “bachlaveh” appearance in severely exfoliated sheets or plates, as illustrated schematically in Fig. 1. However, exfoliation is not accelerated by stress within the sheet or plate structure and does not lead to SCC. Exfoliation has been generally observed under specific environmental conditions [l, 21. These conditions tend to especially favor high-humidity, salt (NaCl)-containing environments [ll]. Although exfoliation can frequently be prevented or minimized by special tempering during production [6,12, 13], other protective measures must be considered for in-service structures such as aircraft wing and body skins-both mili-
INTRODUCTION The principal corrosion problems associated with high-strength aluminum alloys used in aircraft and aerospace vehicles have historically been connected with stress corrosion cracking (SCC) and exfoliation corrosion [l-4:]. Both are characterized primarily by intergranular corrosion. Exfoliation occurs predominantly in 2XxX, 5XxX, and 7XXX rolled sheet, plate, and related wrought alloys as a consequence, in part, of their elongated grains, which create a markedly directional microstructure, and their temper, which (apparently creates intergranular precipitates and related phenomena contributing to localized compositional variations. These compositional variations, which have been attributed to copper depletion as a consequence of grain boundary precipitation in the 2000 series alloys, for example [5-71, create preferential anodic sites that sensitize the material to corrosion [8,9].
*AFOSR
Center for Structural
0 Elsevier
Integrity of Aerospace Systems 259
MATERIALS CHARACTERIZATION 38:259-272 (1997) 0 Elsevier Science Inc., 1997 655 Avenue of the Americas, New York, NY 10010
1044-5803/97/$17.00 PU SlO44-5803(97)00083-l
260
M. Posada et al.
FIG. 1. Schematic sequence illustrating the evolution of exfoliation structures in 2024 aluminum sheet or plate: (a) plate corner showing elongated grain structure and precipitation; (b) corrosion onset within grain boundaries; (c) linking of corrosion sites and delamination (cracking) between grain layers and multilayers by corrosion product wedge action; (d) leafing and complete loss of sheet thickness integrity.
tary and commercial. On older or aging aircraft, exfoliation has been minimized by sprayed metal coatings consisting of aluminum, zinc, and magnesium [14], by paints, and through the use of elastomeric polysulfide sealants, particularly over fastener patterns and around fayed surfaces [15]. Because of the high cost of replacing aging aircraft in both the military and the commercial sectors, interest in efforts to extend the service life of existing and older aircraft has been renewed. However, in the context of these efforts, concern for general aircraft safety and maintenance policies has been mounting. Often, these issues appear to be completely at odds, and research
strategies to detect degradation and, especially, corrosion phenomena in aging aircraft metals, as well as their suppression, have intensified in the past several years. New and innovative strategies are of particular interest. In the present research, we have critically reexamined severe exfoliation in aluminum alloy skins (1.8mm-thick sheet samples) from military KC-135 aircraft by utilizing a wide range of materials characterization protocols in an effort to develop a broad, yet detailed, overview of both microstructural and macrostructural aspects of this corrosion phenomenon. Of particular interest in this investigation were the detailed
Exfoliation in 21024Aluminum
261
analysis and comparison of precipitation within the grains and in the gram boundaries, as well as the examination of elemental depletion profiles at grain boundaries by utilizing analytical transmission electron microscopy.
EXPERIMENTAL
DETAILS
Samples of KC-135 aircraft fuselage skin sections, which are similar to the 707 commercial aircraft body skin, were obtained with a nominal thickness of l.Blmm. A 25.4mm by 25.4mm sample was cut from the base material and sent out for laboratory analysis (Charles C. Kawin Company, Broadview, Illinois) to determine elemental composition of the thin plate. The thinplate skin had the following composition (wt.%): Cu 3.81, Mg 1.41, Mn 0.63, Fe 0.21, Si 0.09, Zn 0.05, Ti 0.02, Cr 0.01, and balance Al. To serve as a control or reference specimen a 12.7mm by 12.7mm sample was sectioned from the skin on an area in which corrosion was not present. This sample was mounted in plastic for easy handling. The top and thfe two sides perpendicular to each other were ground to a 1200 grit finish, polished with lprn alumina in suspension, 0.3km, alumina in suspension, and colloidal silica in suspension, respectively. All three surfaces were chemically etched with Keller’s reagent (2.5mL HN03, 1.5mL HCl, l.OmL HF, 95.0mL H20) to reveal the microstructure. Optical microscopy methods were u:sed to examine and develop a three-dimensional model of the microstructure by creating metallographic section views in the plane of the sheet and in each of the through-thickness sheet directions. Extensive collages of exfoliated regions also were developed by using light metallography. Scanning electron microscopy and energy dispersive spectroscopy methods were used to observe and analyze the corrosion products on a section cut from the exfoliated area of the skin section. Transmission electron microscopy and scanning transmission electron microscopy utilizing microdiffraction and energy-dispersive X-ray
spectrometry also were used to characterize the precipitates and to explore elemental concentration or depletion profiles along the grain boundaries. Transmission electron microscopic samples were prepared from small areas of noncorroded control sections. Grinding and polishing on both sides of the section was performed with 600-grit paper to bring the thickness of the 2024 aluminum sheet to 0.2mn-r. The thin sections were then punched into 3mm discs. The discs were jet polished, with the use of a Tenupol 3 dual-jet polisher, to allow transmission through the thinned area in the vicinity of the hole made by the etchant. The electrolyte used was composed of 20% HNO3 in methanol at a temperature of -20°C. Transmission electron microscopic samples were examined in a Hitachi H-8000 scanning transmission electron microscope operated at 200kV accelerating potential and employing a goniometer-tilt stage.
RESULTS
AND DISCUSSION
Figure 2 illustrates a typical three-dimensional sectional view of a representative 2024 aluminum sheet specimen from a wellpreserved and uncorroded part of a KC-135 aircraft body skin. This view illustrates the presence of second-phase precipitates and an elongated grain structure similar to the schematic view shown in Fig. l(a). The average aspect ratio (grain length/gram thickness) for the elongated grain structure in Fig. 2 was found to be about 8. The average grain size in the plane of the sheet, which corresponded approximately to the average, elongated grain length in the sheet (plate) thickness, was 175km. The grain structure in the plane of the sheet (Fig. 2) was generally equiaxed as a consequence of cross-rolling schedules during the plate manufacture, although this rolling created the elongations noted in the plate thickness, as evident in Fig. 2. The microstructure shown in Fig. 2 illustrates large, second-phase precipitates. However, these precipitates are the largest of a
262
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et al.
FIG. 2. Three-dimensional structure section for a representative 2024 aluminum body skin specimen from KC-135 aircraft. The arrow indicates the normal to the plane of the sheet.
range of different types of precipitates and inclusions. The larger particles (Fig. 2) are actually impurities created during early alloy solidification and development. Typical features of these large inclusions are illustrated in Fig. 3, which shows a lamellar or platelike (fault) nature with a periodicity of 8.038, and a doubling of this period to 16.06A [selected-area diffraction (SAD) pattern inset]. This precipitate and similar platelike precipitates, representing a regime of larger inclusions, are characteristic of (Al Cu, Fe)-type precipitates with the same periodicity even with variations in the Fe content and very low Cu. The calculated interplanar spacings of 23A and 7.6A do not match any International Center for Diffraction Data Powder Diffraction File. The closest match is tetragonal AlKuzFe
(c = 14.9A and II = b = 6.3A). These periodic structures may result from faulting or related shear phenomena associated with the sheet-forming operation because these large inclusions tend to consist of initial, solidification impurities. The more common precipitates, which resulted primarily from thermal treatment and which are more or less uniformly distributed throughout the grain volumes in the aircraft body skin samples, are illustrated in the comparative, tilt-contrast brightfield transmission electron micrographic images shown in Fig. 4. Figure 4(a) shows a strongly diffracting (g = [002] in SAD pattern inset) grain containing a very large inclusion (Fe Al,), at “i”, similar to that shown in Fig. 3, along with a very high density of dislocations that are entangled
Exfoliation in 2024 Aluminum
263
FIG. 3. Transmission electron micrographic bright-field image showing large inclusion layer structure possibly due to particle shear or faulting during continued sheet rolling. The SAD pattern inset shows the presence of regular domains perpendicular to the diffraction streaks.
with a homogeneous distribution of directionally oriented precipitates. These precipitates are more readily observed in the slightly tilted image in Fig. 4(b), where essentially all of the dislocations become invisible as a c’onsequence of a new operating reflection, g, where g.b = 0; and b is the dislocations’ Burgers vector [ 161. Although not all of the precipitates in Fig. 4(b) are strongly oriented, these precipitates tend to be aligned with the trace of the (ill) plane in the (112) oriented grain, along the [13Z]trace direction indicated by the arrow. Because the aircraft body skin samples seemed to have a strong [112] texture, this tendency was commonly observed, and this feature is reinforced in the transmission electron micrographic bright-field image shown in Fig. 5(a), which shows a grain
boundary separating a strongly diffracting grain (A) in terms of dislocation contrast from a grain (B) where dislocations are invisible and precipitate contrast dominates. Figure 5(b) shows a magnified view of the dislocation microstructures typical of the 2024 aluminum sheet [as in Fig. 4(a) and Fig. 5(a)]. Figure 5(a) also illustrates precipitates that are within or specifically associated with the grain boundary; this image will serve as a reference to illustrate the quantitative analysis performed in an attempt to differentiate the matrix precipitates (A) in Fig. 5(a) from the grain-boundary precipitates. Because the exfoliation illustrated in Fig. 1 is unique to the elongated grain boundaries in 2024 aluminum plate thickness, its origin has been historically associated with
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FIG. 4. Transmission electron micrographic bright-field, tilt-contrast sequence showing typical dislocation microstructure (a) and precipitation within (112) textured grain. The SAD pattern inset in (a) illustrates the (112) orientation and [lli] operating reflection. Part (b) is slightly tilted from the diffracting condition, [lli], in (a). Large inclusion is designated “i.” The arrow in (b) denotes the [132] direction.
Exfoliation in 2024 Aluminum
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FIG. 5. Transmission electron micrographic bright-field image of two (112 grains (A and B) separated by a grain boundary. Grain A is not diffracting, whereas grain B is strongly diffracting. The [132] direction is noted in grains A and B (arrows). The m&orientation angle between these common directions is about 4”. Note precipitation along the grain boundary. Part (b) is a magnified view of dislocation substructure in grain B of (a).
266
either grain-boundary precipitation or an anodic feature specific to grain-boundary depletion or other elemental concentration (or segregation). Copper depletion, as noted earlier, has been touted as the principal cause of anodic-behaving grain boundaries [5-71. However, in dozens of convergent electron beam scans across a variety of grain boundaries, as indicated along the dotted line in Fig. 5(a), and between precipitates spaced sufficiently within the grain boundary, there have been no indications of either elemental segregation or depletion of copper. Although the grain boundary in Fig. 5(a) is problematic because it has a rather steep inclination, several analytical profiles across grain boundaries perpendicular to the thin film surfaces also did not illustrate systematic variations in scanned elements Al, Cu, Si, Mn, Mg, and Fe. There were, as is apparent in Fig. 2, few examples of the large inclusions (Figs. 3 and 4) associated with the elongated grain boundaries, thereby providing anodic sites to initiate corrosion-assisted exfoliation. A systematic analysis of individual (and generally larger) precipitates in the grain interiors [as in Fig. 5(a) at A] and in the grain boundaries did reveal some subtle differences. These differences are illustrated by comparing representative energy-dispersive X-ray spectra for grain-boundary precipitates with precipitates in the grain interior (grain matrix) in Fig. 6. Figure 7 shows a more quantitative and statistically qualitative comparison of the incidence of elemental compositions for grain-boundary precipitates and matrix precipitates. In the elemental histogram in Fig. 7, some grainboundary precipitates contain Si, whereas some matrix precipitates contain less Si, and the difference is replaced by Mn. The propensity of precipitate compositions, as illustrated qualitatively in Fig. 7, tended to be Cu Al* and Fe Cu Al, with considerably lesser amounts of Si and Mn, (Cu Al) Si, Cu Al (Mn), Fe Cu Al, (Si), or (Fe Cu Al,) Mn. There were also numerous Al Fe, and (Al Fe,) Si precipitates in the matrix and occasionally in the boundaries. Because Si is more anodic than Mn [17], this difference
M. Posada et al.
may create a tendency for corrosion or oxidation initiation in the grain boundaries rather than in the matrix. On the other hand, Si combined in an intermetallic would not necessarily be anodic. In this context, it is of interest to note that, in recent studies by Gao et al. [18] and especially Wei et al. [19] in 2024-T3 aluminum, typical phases were identified as Cu A&, Cu Mg Ah, Fe4 Cu Alzs, and a (Fe, Cu, Mn, Si) Al phase, which are somewhat different from those identified in these 2024 aluminum aircraft skin samples. These workers also noted matrix dissolution around Fe- and Mn-containing particles such as Fed Cu Al= and (Fe, Cu, Mn, Si) Al, as well as Cu Alz particles. Cu Mg Ah particles also tended to exhibit matrix dissolution in an aerated 0.5 M NaCl solution, consistent with scanning electron microscopic pitting corrosion observations. We did not observe Mg-containing precipitates, and the Mg was observed to be somewhat uniformly distributed throughout the aircraft skin alloy. Because there are a number of heat-treatment (T2, T3, T4, etc.) schedules for 2024 aluminum alloy sheet, a wide range of compositions and morphologies of second-phase particles can arise. This is a complicating factor in examining aging aircraft skins and confounds efforts to elucidate specific corrosion mechanisms. There may also be some unusual and intrinsic grain-boundary properties or related structural features that are unique to such elongated grains (or gram boundaries) [20]. Although we have not yet conducted a large number of measurements, grain boundaries in this fuselage skin material [as illustrated generally in Fig. 5(a)] tend to be low angle and can therefore be expected to exhibit correspondingly low energy interfaces [20,21]. If this is in fact a feature of these interfaces, it does present an anomaly, because the general concept of an anodic interface requires high interfacial free energy. On the other hand, these interfaces may present some unique features. For example, there appear to be a preponderance of complex ledge structures at the grain boundaries [20]. These features could
267
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M. Posadaet al.
18 16
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for grain-boundary in Fig. 6).
play a significant role in the directional corrosion and delamination that characterize exfoliation. Grain-boundary structural anomalies could play a far more significant role than do precipitates in the boundary, for example. Figure 8 illustrates an example of typical exfoliation observed in a variety of locations on 2024 aluminum sheet KC-135 aircraft body skins. The exfoliated layers extend within the sheet thickness, as illustrated schematically in Fig. l(d). In many instances, the layers are one grain layer thick (Fig. 8). Examination of the exfoliated lay-
and grain-matrix
precipitate
(as determined
from
ers in the scanning electron microscope illustrates regions of thick precipitates or continuous corrosion products [Al (OH) or AlO,(O which appear to be hard and brittle as a consequence of extensive stress cracking observed in the two magnified areas in Fig. 9(a-c). Although our own observational evidence is not yet conclusive, strong evidence suggests that the formation of hard (and strong) corrosion products at the anodic precipitates or other specific sites within the grain boundaries provide a wedging force as they grow, and this force (stress) exacerbates cracking along the elon-
Exfoliation in 2024 Aluminum
269
FIG. 8. Light microscope composition showing extensive exfoliation behavior in 2024 aluminum body skin section: (a) sectional view showing extensive exfoliation; (b) magnified view showing extension of (a) to the right. “A” is the point of reference.
gated grain boundaries [lo] that, because of some unique structural or energetic features or both, enhances or facilitates the severe and extended grain-boundary delaminations evident in Figs. 8 and 9.
SUMMARY AND CONCLUSIONS The aircraft skin samples revealed the familiar, definitive features of exfoliation corrosion: numerous intergranular separations parallel to the plane of the sheet, penetrating long d:istances into the material along in-plane directions, with voluminous corrosion products apparently wedging between shelets of pancake-shaped grains. However, several of the observations are at
odds with conventional concepts of exfoliation mechanisms in 2XxX-series alloys. In particular, no copper-depleted zones were found in the boundaries, which were examined carefully by using state-of-the-art electron microscopic and microanalytical techniques. It is widely accepted that, in Al-Cu binary and 2XXX commercial alloys, grainboundary corrosion susceptibility is a consequence of the formation of copper-rich precipitates in the boundaries, creating copper-depleted zones adjacent to the boundaries and setting the stage for microgalvanic corrosion couples between the cathodic precipitates and the relatively anodic depleted zones [6,22,23]. The grain boundaries examined so far are those that are accessible to (edgewise) ex-
270
M. Posada ef al.
FIG. 9. Scanning electron micrograph of the exfoliated 2024 aluminum body skin section shown in Fig. 8. The arrow at the left illustrates the viewing direction for the metallographic view in Fig. 8. The right-hand part of (a) is near the edge of a rivet hole. Insets (b) and (c) show magnified views of specific areas exhibiting heavy corrosion products.
Exfoliation in 2024 Aluminum
amination in transmission electron microscopic foils, which are parallel to the plane of the rolled sheet; this limited area of observation may be important in regard to the phenomena noted. These aircraft skins are too thin for preparation of standard (3mm diameter) foil samples in orientations perpendicular to the sheet. Therefore, the boundaries available for detailed study are those that lie at high angles to the plane of the sheet. These boundaries are not the boundaries that are most susceptible in exfoliation. On the basis of currently available data, two statements can be made: Bounda:ries that lie at high angles to the plane of the sheet (inclined boundaries) are chemically different from those that are nearly parallel to this plane (parallel boundaries). The included boundaries lack the copper-depleted zones that have been viewed as being characteristic of Al-Cu and 2XXX alloys exhibiting intergranular attack. In this exfoliation scenario, intergranular cracks progress rapidly in the in-plane directions not just because of the simple contiguity of the available crack paths in the pancake grain structure, but also because the parallel boundaries are chemically more susceptible than the inclined boundaries. In strongly textured rolled sheet, grainboundary structures should depend on boundary orientation. The structural differences might well control exfoliation directly or indirectly by control of boundary precipitation and elemental depletion. The conventional picture of exfoliationsusceptible microstructures is erroneous or, at least, incomplete. If the parallel boundaries (which clearly are participating in classical exfoliation in these samples) al:so lack copper-depleted zones, then some other microstructural factor must be providing the opportunity for exfoliation attack. More than 30 years ago, Evlms [24] suggested that the nowconventional (CuA12 precipitate and impoverished layer) model for intergranular attack in aluminum-copper alloys is
271
oversimplified and that some other factor(s) must be involved. Magnesiumand zinc-depletion zones are often blamed in the 7XXX alloys [22]; but, in the present samples, not only Cu, but also Si, Mn, Mg, and Fe are not systematically depleted or enriched in boundary zones. It is hoped that further work now in progress will help to produce a sounder fundamental understanding of this corrosion phenomenon. We thank Captain Michael Church and Captain Dan Groner of the U.S. Air Force Wright Laboratories for providing the aircrajl skin specimens. This research was supported by AFOSRF49620-95-I-0518, administered through the FAST Center for Structural Integrity of Aerospace Systems at the University of Texas at El Paso, and through a Defense Acquisition Scholarship to Maria Posada, administered through the Northeast Consortium for Engineering Education.
References 1. F. Bollenrath and W. Bungardt: Stratified corrosion on aluminum alloys. Z. Metall. 34(7):160-165 (1942). 2. A. E. G. Liddiard, J. A. Whittaker, and H. K. Farmery: The exfoliation corrosion of aluminum alloys. J. Inst. Metals 89:38-45 (1960-1961). 3. A. M. Malloy: BUWEPS’ fight against corrosion. Nav. Aviat. News 266 (1965). 4. J. S. Leak: Corrosion: a study of recent air force experience. M&r. Pmt. Perform. 10(1):17-20 (1971). 5. R. H. Brown and R. 8. Mears: Electrochemistry of corrosion. Trans. Electrochem. Sot. 74495-504 (1938). 6. B. W. Lifka and D. 0. Sprowls: Significance of intergranular corrosion in high strength aluminum alloy products. In Localized Corrosion: Cause of Metal Failures. ASTM STP 516, p. 120 (1972). 7. W. F. Fink and L. A. Wiley: Quenching of 755 aluminum alloy. Metals Technol. 14(8):13-21 (1947). 8. D. A. Jones: Principles and Prevention of Corrosion. Macmillan, New York, pp. 306-307 (1992). 9. M. Habashi, E. Bonte, J. Galland, and J. J. Bondu: Quantitative measurements of the degree of exfoliation on aluminum alloys. Corms. Sci. 35(1-4): 169-183 (1993). 10. D. J. Kelly and M. J. Robinson: Influence of heat treatment and grain shape on exfoliation corrosion of Al-Li alloy 8090. Corrosion 49(10):787-794 (1993).
272 11. E. Tankins, J. Kozal, and E. W. Lee: The shipboard exposure testing of aircraft materials. 1. M&Z. 9: 40-43 (1995).
M. Posada
et al.
12. S. Ketcham and I. S. Shaffer: Exfoliation corrosion of aluminum alloys. In Localized Corrosion: Cause of Metal Failures. ASTM STP 516, p. 3 (1972).
19. R. I’. Wei, M. Gao, and C.-M. Liao: TEM studies of particle-induced corrosion in 2024-T3 and 7075-T6 aluminum alloys. Paper presented at 1997 TMS Annual Meeting (GRP Microstructure/Property Relationships: Corrosion and Deformation 147), February 9-13,1997, Orlando, FL.
13. W. A. Bell and H. S. Campb,ell: Prevention of exfoliation corrosion of an aluminum alloy (HEIs) by extended ageing. J, Inst. Metals 89:464-471 (1961).
20. L. E. MUX: lnferfacial Phenomena in Metals and AZZoys. Addison-Wesley, Reading, MA (1975); reprinted by Tech Books, Fairfax, VA (1990).
14. V. E. Carter and H. S. Campbell: The prevention of exfoliation corrosion of aluminum alloys by sprayed metal coatings. 1. Inst. MetaZs 89:472475 (1961). 15. R. N. Miller: Aircraft exfoliation corrosion methods for prevention in fastener holes. Mater. Prof. 6(2):55-58 (1967).
21. M. Posada, L. E. Murr, R. M. Arrowood, C.-S. Niou, D. Little, D. Roberson, and D. George: Exfoliation and related microstructures in 2024 aluminum body skins on aging aircraft. Microstruct. Sci. 25 (1998), to be published; paper presented at IMS Technical Meeting, Seattle, WA, July 1997.
16. L. E. Murr: Electron and Zen Microscopy and Microanalysis: Principles and Applications. 2nd ed., Marcel Dekker, New York (1991).
22. 8. W. Liflca: Corrosion of aluminum alloys. In Corrosion Engineering Handbook. P. A. Schweitzer, ed., Marcel Dekker, New York, pp. 129-133 (1996).
17. ASM Handbook: Vol. 13: Corrosion. ASM International, Materials Park, OH, p. 584 (1987).
23. J. R. Galvele and S. M. de DeMicheli: Mechanism of intergranular corrosion of aluminum-copper alloys. Corros. Sci. 10:795-807 (1970).
18. M. Gao, R. I’. Wei, and J. Feng: Identification of constituent particles in 2024-T3 and 7075-T6 aluminum alloys. Paper presented at 1997 TMS Annual Meeting (Microstructure/Property Relationships: Corrosion and Deformation l), February 9-13,1997, Orlando, FL.
24. U. R. Evans: The Corrosion and Oxidation of Metals: Scientific Principles and Practical Applications. Edward Arnold, London, pp. 668-675 (1960). Received Februa y 1997; accepted May 1997.
Exfoliation and Related Microstructures in 2024 Aluminum Body Skins on Aging Aircraft Maria Posada, L. E. Murr, C.-S. Niou, D. Roberson, D. Little, Roy Armwood, and Debra George Department of Metallurgical and Materials Engineering and FAST Center,* The University of Texas, El Paso, Texas Exfoliation,
a directional
attack along elongated
some detail in rolled 2024 aluminum samples
utilizing
grain boundaries,
has been examined
in
sheet and plate for KC-135 aging aircraft body skin
optical (light) metallography,
scanning
electron microscopy,
mission electron microscopy. A detailed analysis and comparison
of precipitates
and transwithin the
grains and in the grain boundaries were performed, as well as an examination of elemental depletion profiles across grain boundaries. These observations suggest that corrosion-related anodic sites play a far less significant role in the propagation of exfoliation than do the hard corrosion products creating wedging stresses within the elongated grain boundaries, which seem to demonstrate unique and unusual structural or energetic features or both. Science Inc., 1997
The directional attack along the elongated grain boundaries in these sensitized aluminum alloys results in a leafing action that seems to be promoted by corrosion products creating stresses or corrosion forces [lo] that further split the grains, much like water freezing in cracks in concrete. This creates a microscopic “pastry dough” or “bachlaveh” appearance in severely exfoliated sheets or plates, as illustrated schematically in Fig. 1. However, exfoliation is not accelerated by stress within the sheet or plate structure and does not lead to SCC. Exfoliation has been generally observed under specific environmental conditions [l, 21. These conditions tend to especially favor high-humidity, salt (NaCl)-containing environments [ll]. Although exfoliation can frequently be prevented or minimized by special tempering during production [6,12, 13], other protective measures must be considered for in-service structures such as aircraft wing and body skins-both mili-
INTRODUCTION The principal corrosion problems associated with high-strength aluminum alloys used in aircraft and aerospace vehicles have historically been connected with stress corrosion cracking (SCC) and exfoliation corrosion [l-4:]. Both are characterized primarily by intergranular corrosion. Exfoliation occurs predominantly in 2XxX, 5XxX, and 7XXX rolled sheet, plate, and related wrought alloys as a consequence, in part, of their elongated grains, which create a markedly directional microstructure, and their temper, which (apparently creates intergranular precipitates and related phenomena contributing to localized compositional variations. These compositional variations, which have been attributed to copper depletion as a consequence of grain boundary precipitation in the 2000 series alloys, for example [5-71, create preferential anodic sites that sensitize the material to corrosion [8,9].
*AFOSR
Center for Structural
0 Elsevier
Integrity of Aerospace Systems 259
MATERIALS CHARACTERIZATION 38:259-272 (1997) 0 Elsevier Science Inc., 1997 655 Avenue of the Americas, New York, NY 10010
1044-5803/97/$17.00 PU SlO44-5803(97)00083-l
260
M. Posada et al.
FIG. 1. Schematic sequence illustrating the evolution of exfoliation structures in 2024 aluminum sheet or plate: (a) plate corner showing elongated grain structure and precipitation; (b) corrosion onset within grain boundaries; (c) linking of corrosion sites and delamination (cracking) between grain layers and multilayers by corrosion product wedge action; (d) leafing and complete loss of sheet thickness integrity.
tary and commercial. On older or aging aircraft, exfoliation has been minimized by sprayed metal coatings consisting of aluminum, zinc, and magnesium [14], by paints, and through the use of elastomeric polysulfide sealants, particularly over fastener patterns and around fayed surfaces [15]. Because of the high cost of replacing aging aircraft in both the military and the commercial sectors, interest in efforts to extend the service life of existing and older aircraft has been renewed. However, in the context of these efforts, concern for general aircraft safety and maintenance policies has been mounting. Often, these issues appear to be completely at odds, and research
strategies to detect degradation and, especially, corrosion phenomena in aging aircraft metals, as well as their suppression, have intensified in the past several years. New and innovative strategies are of particular interest. In the present research, we have critically reexamined severe exfoliation in aluminum alloy skins (1.8mm-thick sheet samples) from military KC-135 aircraft by utilizing a wide range of materials characterization protocols in an effort to develop a broad, yet detailed, overview of both microstructural and macrostructural aspects of this corrosion phenomenon. Of particular interest in this investigation were the detailed
Exfoliation in 21024Aluminum
261
analysis and comparison of precipitation within the grains and in the gram boundaries, as well as the examination of elemental depletion profiles at grain boundaries by utilizing analytical transmission electron microscopy.
EXPERIMENTAL
DETAILS
Samples of KC-135 aircraft fuselage skin sections, which are similar to the 707 commercial aircraft body skin, were obtained with a nominal thickness of l.Blmm. A 25.4mm by 25.4mm sample was cut from the base material and sent out for laboratory analysis (Charles C. Kawin Company, Broadview, Illinois) to determine elemental composition of the thin plate. The thinplate skin had the following composition (wt.%): Cu 3.81, Mg 1.41, Mn 0.63, Fe 0.21, Si 0.09, Zn 0.05, Ti 0.02, Cr 0.01, and balance Al. To serve as a control or reference specimen a 12.7mm by 12.7mm sample was sectioned from the skin on an area in which corrosion was not present. This sample was mounted in plastic for easy handling. The top and thfe two sides perpendicular to each other were ground to a 1200 grit finish, polished with lprn alumina in suspension, 0.3km, alumina in suspension, and colloidal silica in suspension, respectively. All three surfaces were chemically etched with Keller’s reagent (2.5mL HN03, 1.5mL HCl, l.OmL HF, 95.0mL H20) to reveal the microstructure. Optical microscopy methods were u:sed to examine and develop a three-dimensional model of the microstructure by creating metallographic section views in the plane of the sheet and in each of the through-thickness sheet directions. Extensive collages of exfoliated regions also were developed by using light metallography. Scanning electron microscopy and energy dispersive spectroscopy methods were used to observe and analyze the corrosion products on a section cut from the exfoliated area of the skin section. Transmission electron microscopy and scanning transmission electron microscopy utilizing microdiffraction and energy-dispersive X-ray
spectrometry also were used to characterize the precipitates and to explore elemental concentration or depletion profiles along the grain boundaries. Transmission electron microscopic samples were prepared from small areas of noncorroded control sections. Grinding and polishing on both sides of the section was performed with 600-grit paper to bring the thickness of the 2024 aluminum sheet to 0.2mn-r. The thin sections were then punched into 3mm discs. The discs were jet polished, with the use of a Tenupol 3 dual-jet polisher, to allow transmission through the thinned area in the vicinity of the hole made by the etchant. The electrolyte used was composed of 20% HNO3 in methanol at a temperature of -20°C. Transmission electron microscopic samples were examined in a Hitachi H-8000 scanning transmission electron microscope operated at 200kV accelerating potential and employing a goniometer-tilt stage.
RESULTS
AND DISCUSSION
Figure 2 illustrates a typical three-dimensional sectional view of a representative 2024 aluminum sheet specimen from a wellpreserved and uncorroded part of a KC-135 aircraft body skin. This view illustrates the presence of second-phase precipitates and an elongated grain structure similar to the schematic view shown in Fig. l(a). The average aspect ratio (grain length/gram thickness) for the elongated grain structure in Fig. 2 was found to be about 8. The average grain size in the plane of the sheet, which corresponded approximately to the average, elongated grain length in the sheet (plate) thickness, was 175km. The grain structure in the plane of the sheet (Fig. 2) was generally equiaxed as a consequence of cross-rolling schedules during the plate manufacture, although this rolling created the elongations noted in the plate thickness, as evident in Fig. 2. The microstructure shown in Fig. 2 illustrates large, second-phase precipitates. However, these precipitates are the largest of a
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FIG. 2. Three-dimensional structure section for a representative 2024 aluminum body skin specimen from KC-135 aircraft. The arrow indicates the normal to the plane of the sheet.
range of different types of precipitates and inclusions. The larger particles (Fig. 2) are actually impurities created during early alloy solidification and development. Typical features of these large inclusions are illustrated in Fig. 3, which shows a lamellar or platelike (fault) nature with a periodicity of 8.038, and a doubling of this period to 16.06A [selected-area diffraction (SAD) pattern inset]. This precipitate and similar platelike precipitates, representing a regime of larger inclusions, are characteristic of (Al Cu, Fe)-type precipitates with the same periodicity even with variations in the Fe content and very low Cu. The calculated interplanar spacings of 23A and 7.6A do not match any International Center for Diffraction Data Powder Diffraction File. The closest match is tetragonal AlKuzFe
(c = 14.9A and II = b = 6.3A). These periodic structures may result from faulting or related shear phenomena associated with the sheet-forming operation because these large inclusions tend to consist of initial, solidification impurities. The more common precipitates, which resulted primarily from thermal treatment and which are more or less uniformly distributed throughout the grain volumes in the aircraft body skin samples, are illustrated in the comparative, tilt-contrast brightfield transmission electron micrographic images shown in Fig. 4. Figure 4(a) shows a strongly diffracting (g = [002] in SAD pattern inset) grain containing a very large inclusion (Fe Al,), at “i”, similar to that shown in Fig. 3, along with a very high density of dislocations that are entangled
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FIG. 3. Transmission electron micrographic bright-field image showing large inclusion layer structure possibly due to particle shear or faulting during continued sheet rolling. The SAD pattern inset shows the presence of regular domains perpendicular to the diffraction streaks.
with a homogeneous distribution of directionally oriented precipitates. These precipitates are more readily observed in the slightly tilted image in Fig. 4(b), where essentially all of the dislocations become invisible as a c’onsequence of a new operating reflection, g, where g.b = 0; and b is the dislocations’ Burgers vector [ 161. Although not all of the precipitates in Fig. 4(b) are strongly oriented, these precipitates tend to be aligned with the trace of the (ill) plane in the (112) oriented grain, along the [13Z]trace direction indicated by the arrow. Because the aircraft body skin samples seemed to have a strong [112] texture, this tendency was commonly observed, and this feature is reinforced in the transmission electron micrographic bright-field image shown in Fig. 5(a), which shows a grain
boundary separating a strongly diffracting grain (A) in terms of dislocation contrast from a grain (B) where dislocations are invisible and precipitate contrast dominates. Figure 5(b) shows a magnified view of the dislocation microstructures typical of the 2024 aluminum sheet [as in Fig. 4(a) and Fig. 5(a)]. Figure 5(a) also illustrates precipitates that are within or specifically associated with the grain boundary; this image will serve as a reference to illustrate the quantitative analysis performed in an attempt to differentiate the matrix precipitates (A) in Fig. 5(a) from the grain-boundary precipitates. Because the exfoliation illustrated in Fig. 1 is unique to the elongated grain boundaries in 2024 aluminum plate thickness, its origin has been historically associated with
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FIG. 4. Transmission electron micrographic bright-field, tilt-contrast sequence showing typical dislocation microstructure (a) and precipitation within (112) textured grain. The SAD pattern inset in (a) illustrates the (112) orientation and [lli] operating reflection. Part (b) is slightly tilted from the diffracting condition, [lli], in (a). Large inclusion is designated “i.” The arrow in (b) denotes the [132] direction.
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FIG. 5. Transmission electron micrographic bright-field image of two (112 grains (A and B) separated by a grain boundary. Grain A is not diffracting, whereas grain B is strongly diffracting. The [132] direction is noted in grains A and B (arrows). The m&orientation angle between these common directions is about 4”. Note precipitation along the grain boundary. Part (b) is a magnified view of dislocation substructure in grain B of (a).
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either grain-boundary precipitation or an anodic feature specific to grain-boundary depletion or other elemental concentration (or segregation). Copper depletion, as noted earlier, has been touted as the principal cause of anodic-behaving grain boundaries [5-71. However, in dozens of convergent electron beam scans across a variety of grain boundaries, as indicated along the dotted line in Fig. 5(a), and between precipitates spaced sufficiently within the grain boundary, there have been no indications of either elemental segregation or depletion of copper. Although the grain boundary in Fig. 5(a) is problematic because it has a rather steep inclination, several analytical profiles across grain boundaries perpendicular to the thin film surfaces also did not illustrate systematic variations in scanned elements Al, Cu, Si, Mn, Mg, and Fe. There were, as is apparent in Fig. 2, few examples of the large inclusions (Figs. 3 and 4) associated with the elongated grain boundaries, thereby providing anodic sites to initiate corrosion-assisted exfoliation. A systematic analysis of individual (and generally larger) precipitates in the grain interiors [as in Fig. 5(a) at A] and in the grain boundaries did reveal some subtle differences. These differences are illustrated by comparing representative energy-dispersive X-ray spectra for grain-boundary precipitates with precipitates in the grain interior (grain matrix) in Fig. 6. Figure 7 shows a more quantitative and statistically qualitative comparison of the incidence of elemental compositions for grain-boundary precipitates and matrix precipitates. In the elemental histogram in Fig. 7, some grainboundary precipitates contain Si, whereas some matrix precipitates contain less Si, and the difference is replaced by Mn. The propensity of precipitate compositions, as illustrated qualitatively in Fig. 7, tended to be Cu Al* and Fe Cu Al, with considerably lesser amounts of Si and Mn, (Cu Al) Si, Cu Al (Mn), Fe Cu Al, (Si), or (Fe Cu Al,) Mn. There were also numerous Al Fe, and (Al Fe,) Si precipitates in the matrix and occasionally in the boundaries. Because Si is more anodic than Mn [17], this difference
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may create a tendency for corrosion or oxidation initiation in the grain boundaries rather than in the matrix. On the other hand, Si combined in an intermetallic would not necessarily be anodic. In this context, it is of interest to note that, in recent studies by Gao et al. [18] and especially Wei et al. [19] in 2024-T3 aluminum, typical phases were identified as Cu A&, Cu Mg Ah, Fe4 Cu Alzs, and a (Fe, Cu, Mn, Si) Al phase, which are somewhat different from those identified in these 2024 aluminum aircraft skin samples. These workers also noted matrix dissolution around Fe- and Mn-containing particles such as Fed Cu Al= and (Fe, Cu, Mn, Si) Al, as well as Cu Alz particles. Cu Mg Ah particles also tended to exhibit matrix dissolution in an aerated 0.5 M NaCl solution, consistent with scanning electron microscopic pitting corrosion observations. We did not observe Mg-containing precipitates, and the Mg was observed to be somewhat uniformly distributed throughout the aircraft skin alloy. Because there are a number of heat-treatment (T2, T3, T4, etc.) schedules for 2024 aluminum alloy sheet, a wide range of compositions and morphologies of second-phase particles can arise. This is a complicating factor in examining aging aircraft skins and confounds efforts to elucidate specific corrosion mechanisms. There may also be some unusual and intrinsic grain-boundary properties or related structural features that are unique to such elongated grains (or gram boundaries) [20]. Although we have not yet conducted a large number of measurements, grain boundaries in this fuselage skin material [as illustrated generally in Fig. 5(a)] tend to be low angle and can therefore be expected to exhibit correspondingly low energy interfaces [20,21]. If this is in fact a feature of these interfaces, it does present an anomaly, because the general concept of an anodic interface requires high interfacial free energy. On the other hand, these interfaces may present some unique features. For example, there appear to be a preponderance of complex ledge structures at the grain boundaries [20]. These features could
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play a significant role in the directional corrosion and delamination that characterize exfoliation. Grain-boundary structural anomalies could play a far more significant role than do precipitates in the boundary, for example. Figure 8 illustrates an example of typical exfoliation observed in a variety of locations on 2024 aluminum sheet KC-135 aircraft body skins. The exfoliated layers extend within the sheet thickness, as illustrated schematically in Fig. l(d). In many instances, the layers are one grain layer thick (Fig. 8). Examination of the exfoliated lay-
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ers in the scanning electron microscope illustrates regions of thick precipitates or continuous corrosion products [Al (OH) or AlO,(O which appear to be hard and brittle as a consequence of extensive stress cracking observed in the two magnified areas in Fig. 9(a-c). Although our own observational evidence is not yet conclusive, strong evidence suggests that the formation of hard (and strong) corrosion products at the anodic precipitates or other specific sites within the grain boundaries provide a wedging force as they grow, and this force (stress) exacerbates cracking along the elon-
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FIG. 8. Light microscope composition showing extensive exfoliation behavior in 2024 aluminum body skin section: (a) sectional view showing extensive exfoliation; (b) magnified view showing extension of (a) to the right. “A” is the point of reference.
gated grain boundaries [lo] that, because of some unique structural or energetic features or both, enhances or facilitates the severe and extended grain-boundary delaminations evident in Figs. 8 and 9.
SUMMARY AND CONCLUSIONS The aircraft skin samples revealed the familiar, definitive features of exfoliation corrosion: numerous intergranular separations parallel to the plane of the sheet, penetrating long d:istances into the material along in-plane directions, with voluminous corrosion products apparently wedging between shelets of pancake-shaped grains. However, several of the observations are at
odds with conventional concepts of exfoliation mechanisms in 2XxX-series alloys. In particular, no copper-depleted zones were found in the boundaries, which were examined carefully by using state-of-the-art electron microscopic and microanalytical techniques. It is widely accepted that, in Al-Cu binary and 2XXX commercial alloys, grainboundary corrosion susceptibility is a consequence of the formation of copper-rich precipitates in the boundaries, creating copper-depleted zones adjacent to the boundaries and setting the stage for microgalvanic corrosion couples between the cathodic precipitates and the relatively anodic depleted zones [6,22,23]. The grain boundaries examined so far are those that are accessible to (edgewise) ex-
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FIG. 9. Scanning electron micrograph of the exfoliated 2024 aluminum body skin section shown in Fig. 8. The arrow at the left illustrates the viewing direction for the metallographic view in Fig. 8. The right-hand part of (a) is near the edge of a rivet hole. Insets (b) and (c) show magnified views of specific areas exhibiting heavy corrosion products.
Exfoliation in 2024 Aluminum
amination in transmission electron microscopic foils, which are parallel to the plane of the rolled sheet; this limited area of observation may be important in regard to the phenomena noted. These aircraft skins are too thin for preparation of standard (3mm diameter) foil samples in orientations perpendicular to the sheet. Therefore, the boundaries available for detailed study are those that lie at high angles to the plane of the sheet. These boundaries are not the boundaries that are most susceptible in exfoliation. On the basis of currently available data, two statements can be made: Bounda:ries that lie at high angles to the plane of the sheet (inclined boundaries) are chemically different from those that are nearly parallel to this plane (parallel boundaries). The included boundaries lack the copper-depleted zones that have been viewed as being characteristic of Al-Cu and 2XXX alloys exhibiting intergranular attack. In this exfoliation scenario, intergranular cracks progress rapidly in the in-plane directions not just because of the simple contiguity of the available crack paths in the pancake grain structure, but also because the parallel boundaries are chemically more susceptible than the inclined boundaries. In strongly textured rolled sheet, grainboundary structures should depend on boundary orientation. The structural differences might well control exfoliation directly or indirectly by control of boundary precipitation and elemental depletion. The conventional picture of exfoliationsusceptible microstructures is erroneous or, at least, incomplete. If the parallel boundaries (which clearly are participating in classical exfoliation in these samples) al:so lack copper-depleted zones, then some other microstructural factor must be providing the opportunity for exfoliation attack. More than 30 years ago, Evlms [24] suggested that the nowconventional (CuA12 precipitate and impoverished layer) model for intergranular attack in aluminum-copper alloys is
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oversimplified and that some other factor(s) must be involved. Magnesiumand zinc-depletion zones are often blamed in the 7XXX alloys [22]; but, in the present samples, not only Cu, but also Si, Mn, Mg, and Fe are not systematically depleted or enriched in boundary zones. It is hoped that further work now in progress will help to produce a sounder fundamental understanding of this corrosion phenomenon. We thank Captain Michael Church and Captain Dan Groner of the U.S. Air Force Wright Laboratories for providing the aircrajl skin specimens. This research was supported by AFOSRF49620-95-I-0518, administered through the FAST Center for Structural Integrity of Aerospace Systems at the University of Texas at El Paso, and through a Defense Acquisition Scholarship to Maria Posada, administered through the Northeast Consortium for Engineering Education.
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