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3. 4-12-12 with heat generation of 105300 Btu/ton; 4 160000 Btu/h i105300 Btu/ton = 39.5 tons/h optimum production rate. Experience: granulation good at 40 tons/h. Too much load on screens for efficient removal of fines. Judgement: this formula would be satisfactory at 40 tons/h if more screening surface were provided. Several plants made this grade at 30-33 tons/h, using steam as a granulation control. In the development of the ammoniator heat parameter, the authors have disregarded effects on granulation attributable to formula water, added water, temperature or particle size of the dry materials, soluble salts contained in the formulas, etc. These items all have a significant influence on bed conditions and must be considered in all actual situations. It is felt, however, that the heat parameter concept can be employed very usefully in selecting ammoniator dimensions, as in the design of a new plant, when the operating formulas are known, and any limiting effects of other equipment can be eliminated in the engineering stage. This concludes the discussion on the fundamentals of granulation and also the rotary drum ammoniator, which
is the most universally used device for achieving granulation. It was not the intention of this paper to go into elaborate detail on all aspects of granulation. The paper was intended to be introductory to the category entitled “Granulation” and for the other papers which will follow. Registry No. Ammonia, 7664-41-7, Literature Cited Bagnati, 0.; Burlani, L.; Innomoratl, I.; Maresca, A.; Monaidi, R.;Vitellaro, A. “Granulation Methods for Straight and Compound Fertilizers”, The International Superphosphate Manufacturers Association, Limited, Publication of Procedures of Joint Technical/AgrlcuRurai Conference, Stresa, Italy, 1967: pp IX-7 through IX-10. “Pilot-Plant Demonstrations of the Production of Granular Fertilizers”, Tennessee Valley Authority, Muscle Shoals, AL, 1959, pp 8-19. Yates, L. D.; Nielsson, F. T.: Hicks, G. C. Farm Chem. 1954, 777(7), 38-48; (8) 34-41.
Receiued for review September 24, 1981 Reuised manuscript receiued November 4 , 1982 Accepted November 29, 1982 Presented a t the 182nd National Meeting of the American Chemical Society, New York, NY, Aug 1981, Division of Fertilizer and Soil Chemistry.
Corrosion of Aluminum-Matrix Composites. Status Report M. Metzger” University of Illinois at Urbana-Champaign, Urbana, Illinois 6 180 1
S. G. Flshman Naval Surface Weapons Center, White Oak, Maryland 209 10
Light-weight aluminum-matrix composites, some of which may find applications in military or other vehicles, have not been examined extensively with regard to corrosion behavior. This paper collects the corrosion data that are available to summarize the observations made, identify questions that need attention, and assess the potential corrosion characteristics of these materials. The problems that may be encountered include selective attack at fiber-matrix or other interfaces (B/Al, G/Al), galvanic coupling with a continuous cathodic fiber (G/AI), and segregated matrix microstructures resulting from solidification processing (AI,03/AI, SiC/AI). Additional testing is required. Attractive candidates for further development efforts would be those composites for which the corrosion problems turn out to be relatively minor ones and which also have favorable estimated production costs.
Aluminum-matrix composites have high potential for providing high-strength and -stiffness light-weight materials. Research and development over some years was initially stimulated by the needs of the aerospace industry, but more recently there has been awareness that economical light-weight composites with less than the highest strength performance would have a large field of application in transportation and military vehicles. The mechanical properties of the product are sensitive to fabrication technique and have been used to monitor and guide process development. Some information is available on composite microstructure and its relation to processing and to mechanical properties, especially for the older composites. Several general reviews of composites are available (Lynch and Kershaw, 1972; Wright and Levitt, 1974; Kendall, 1974; Kreider and Prewo, 1974; Christian and 0196-4321/83/1222-0296$01.50/0
Adsit, 1977; Miller and Robertson, 1977). Although considerable progress and understanding have been achieved in the processing area, substantial problems remain. Because of the preoccupation with other problems, corrosion has received relatively little attention despite its importance for the viability of the product. In this report, observations on the corrosion behavior of aluminum-matrix composites have been collected from various sources for insight into what problems have been identified, what questions need attention, and what degree of corrosion resistance might be expected eventually for a given composite system. Composite Fabrication Composite fabrication techniques vary from conventional powder metallurgy to specially adapted CVD coating 62 1983 American
Chemical Society
Ind. Eng. Chem. Prod. Res, Dev., W. 22, NO. 2, 1983 297 Table I. Fabrication Techniques for Some Al-Matrix Composites composite fabrication technique ~~~
B/A1 G/Al
A1 0, /A1 SiC/Al Al/Al,Ca (model systems) Al/Al,Fe
foil-filament hot pressing to make B/Al tapes, which are diffusionbonded into plates, etc. graphite tows are CVD-coated and infiltrated with liquid aluminum alloy; diffusion bonding is used to consolidate the composite wires into plates, etc. liquid metal infiltration to near-net shape discontinuous-vacuum hot pressing of fiber-powder blend continuous-liquid metal infiltration unidirectional solidification of eutectic alloy
followed by liquid-metal infiltration. Those processes involving continuous filaments, most of which are fragile, are concerned with incorporating the fibers in such a manner that there is good matrix-fiber bonding as well as a minimum of fiber damage or degradation. These composites have the highest mechanical properties but, because of the complicated processing required, have remained expensive to fabricate. In the fabrication of some of the more recently developed composites which use discontinuous filaments or particulate reinforcement, considerably less effort is expended in preventing fiber damage. Consequently, the mechanical properties are more modest but so, too, is the price. The simultaneous application of heat and pressure known as diffusion bonding, which has been used to consolidate wires or tapes of B/A1, Borsic/Al (Borsic, a trademark of the United Aircraft Company, is Sic-coated boron fiber), G/A1 (G = graphite), SiC/Ti, Borsic/Ti, and W/superalloys, is the most widely used composite fabrication technique. In some cases, simple foil-filament arrays are laid up, with or without fugitive binders, and pressed together. The surface foils may be of a different alloy than the interior ones. In other cases, matrix-coated filaments obtained by liquid or vapor deposition are used to make up the “preform” arrays. During the fabrication and subsequent heat treatment of these materials, intermetallic reaction layers form through which the matrix and the filaments are bonded. The thickness of the interfacial product depends on the temperature and time of the high-temperature process as well as on the reactivity between the matrix and the filament. In some cases voids, delaminations, or other imperfections are associated with this layer. This tends to occur, for example, with the formation of aluminum boride in B/Al. The imperfections are related to such factors as the extent of chemical reaction and differential thermal contraction. The presence of such imperfections or the formation of an interfacial phase more reactive than the
matrix in aqueous environments can produce a continuous path for corrosion. Composites of polycrystalline A1203(DuPont FP fiber) in aluminum matrixes are produced by infiltration and direct casting to near-net shape in permanent molds. In this case, Li additions to the A1 lead to formation of Li2O.5Al2o3at the interface, which promotes wetting and yields strong fiber-matrix bonding. Similar castings of CVD-coated graphite cloths in aluminum matrixes have also been produced. In this case metallic coatings of Ta, Ni, Ti-B or Na-Sn-Mg in the neighborhood of 0.1-0.3pm thick have been employed to promote wetting and provide a diffusion barrier against reaction to form the Al& phase, which suffers rapid corrosion. Modified powder metallurgical procedures are used to fabricate discontinuous whisker or particulate composites. Although early efforts in such systems were involved with A1203and Si3N4,the difficulty of promoting bonding in the former and the expense of the latter have discouraged efforts to develop commercially viable systems incorporating them. More recently, the development of rice-hull Sic whiskers, which are potentially low in cost and easy to incorporate into A1 and Mg matrixes, has resulted in renewed interest in discontinuously reinforced metal-matrix composites. These materials can be hot-worked by rolling, forging, or extrusion. The bonding layer in these composites is extremely thin and has not yet been identified. Similar fabrication technology is used to produce composites reinforced with particulate Sic. These fabrication techniques are summarized in Table I. Cost will be a substantial factor in the scale of the effort mounted for further development of a given composite. Some estimates are given in Table 11. Considerable reductions in the price per pound are foreseen for quantity production as compared with research or demonstration quantities. However, it is anticipated that the cost will remain high for composites based on boron, Borsic, or graphite, these fibers being expensive to produce or to coat. With their superior mechanical properties, these composites would be expected to continue as candidates for critical aerospace or military applications. Composites based on discontinuous silicon carbide or FP alumina are potentially much cheaper. The extent of their applications, e.g., in the vehicular area, would also depend on how far they can be developed in regard to mechanical properties. Corrosion of Aluminum Alloys and Composites. General The corrosion behavior of aluminum alloys may for present purposes be summarized broadly as follows. In the usual service environment, i.e., atmospheric or marine exposure, the alloys are passive and corrosion resistant unless there is some form of pitting corrosion associated with the presence of chloride. The passive film on the aluminum alloy is a poor electronic conductor, and cathode reactions occur on the micron-size particles of impurity constituents or smaller precipitate particles. These de-
Table 11. Estimated Cost of Al-Matrix Composites Reinforced with Various Fibers dollars/lb 1981 price-research or or demonstration quantities manufacturers’ estimated for large quantity production
Ba 450 100 (or more)
Gb
Sicc cont.
Sic b,d discont.
1000
800
100
lOOf
60-25
10-15
FP A1.0. ‘ > e 600 10
a R. E. Fisher, AMERCOM, Inc., Chatsworth, CA 91311. J. F. Dolowy, DWA Composite Specialties, Inc., Chatsworth, CA 91311. J. A. Cornie, AVCO Specialty Materials Division, Lowell, MA 01851. J. L. Cook, ARC0 Metals Division, Greer, SC 29651. e H. S. Hartmann, E. I. du Pont de Nemours & Co., Inc., Wilmington, DE 19898. Fiber from pitch precursor.
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termine whether the alloy will self-polarize to its pitting potential and control the rate of pitting. Different alloys vary greatly in their susceptibility to damaging pitting corrosion. Some may suffer only temporary superficial pitting in the initial stages of exposure as cathodic phases at the surface are removed by corrosion or blanketed by precipitated aluminum hydroxide. Wrought alloys in the 3000 series (AI-Mn), 5000 series (Al-Mg) and 6000 series (Al-Mg-Si) generally perform well. Damaging forms of corrosion occur when pitting is not well distributed and nearly continuous preferential paths for corrosion exist. Such a situation occurs in 2000 series alloys (Al-Cu or Al-Cu-Mg) when improper heat treatment results in precipitation at grain boundaries. This produces, in the adjacent matrix, solute-depleted zones having a lower pitting potential than the body of the matrix. The corrosion problems which may be introduced or exacerbated in composites may be classified as follows. 1. In galvanic coupling, the reinforcing phase or the interface reaction layer may be an effective cathode. If it is continuous or of substantial length, it will provide increasing cathode 'area with corrosion of the matrix. 2. Selective corrosion in the interface region could result from an interfacial phase with a high corrosion rate or from crevice corrosion when there are some gaps or fissures in the interface. Possible sources of these gaps are unbonded patches, or local decohesion or cracking of a brittle interfacial layer resulting from loading. Selective corrosion along the interface would be damaging even with the loss of only a small volume of material because it would prevent load transfer from matrix to fiber. 3. Matrix defects can provide continuous corrosion paths. In hot-pressed composites, there may be residual voids at foil-foil interfaces. In composites fabricated by infiltration or other solidification techniques, shrinkage porosity and regions of alloying element or impurity segregation may occur in the vicinity of fibers. In the production of wrought alloys from ingots, the defects present as-cast are greatly reduced through extensive hot working by rolling or extrusion. With composites, the freedom of the fabricator to employ deformation processes is severely restricted by the need to avoid excessive fiber breakage. Corrosion Studies of Aluminum-Matrix Composites Since many of these studies have been made on composites at relatively early stages of development and not yet offered as engineeringmaterials, much of the corrosion work has been exploratory, often short-term, and based on simple criteria such as weight loss or strength reduction. More detailed electrochemical-metallographic studies are now being made. However, many questions remain concerning the corrosion processes and their relation to microstructure. There is usually insufficient information for a reliable estimate of the degree of corrosion resistance which may be achieved through improved design and processing.
1. B/AI Composites. The fibers, about 0.1 mm in diameter, are produced by deposition of boron on a tungsten wire. They are sometimes coated with Sic (Borsic)to diminish the reaction between AI and B, which produces a brittle aluminum boride interface layer. An early study by Porter and Wolff (1967),mainly with hot-pressed composites with a matrix formed from 99.95% A1 powder, did not show corrosion rates of composites grossly different from those of the matrix. However, in the solutions employed, most corrosion rates were so high as to make the results inapplicable for present purposes. In another early study, Evans and Braddick (1971) noted,
in 02-saturated 3% NaCl, severe attack at the interface in B/A1 composites. As is often the case, details of processing and composition were not given. Dardi and Kreider (1974) studied 50 vol % Sic-coated B fibers in 6061 or 2024 matrixes plasma-sprayed to form monolayer tapes which were subsequently diffusion bonded. Stressed specimens were exposed to synthetic sea-salt spray for up to 1month at 95 O F . After precipitation heat treatment (T6 temper), neither the depth of attack nor the flexural stength of the composite was greatly different from that of a wrought mill product of the matrix alloy. The severe degradation they observed for the composite with a 2024 matrix in the as-fabricated condition probably originated in improper thermal history for the matrix itself and was not related to the presence of the B fibers. Sedriks et al. (1971) examined composites made by diffusion bonding foils of the 2024 alloy with B fibers. Testing was done in aerated 53 g/L NaCl solution, sometimes with additions of H202to accelerate the test. The composites were inferior to the matrix alloy as measured both by strength loss in the long transverse direction after exposure unstressed and by time to failure on exposure at 90% of the 0.1% offset yield strength. The composites suffered severe attack at the fiber-matrix interface and along the diffusion bonds between foils. Polarization measurements showed anodic currents increased with fiber fraction but cathodic currents were unchanged. The preferential attack was attributed to the presence of "anodic sites" at the interfaces possibly due to imperfect bonding and the preexistence of fissures yielding crevice corrosion. It should be noted that there was a distracting factor in these experiments. The thermal history of the composite was such that the matrix was susceptible to intergranular corrosion. Pohlman (1978) examined B/2024 A1 and B/6061 A1 composites prepared by hot pressing with immersion, alternate immersion, or spray tests in NaCl solutions. He found the fiber-matrix interface region preferentially attacked in all cases. Fibers were extracted from the matrix and found to supply significant cathode current at the pitting potential of the alloy while virgin fibers did not. Consequently, significant galvanic currents flowed in couples of extracted fibers with the matrix alloy (up to 9 wA/cm2 at the equivalent of 46 vol % fibers). Pohlman concluded that an additional factor in the preferential attack was the presence of cathodes on the fiber surface (and therefore within the crevice) attributed to an aluminum boride compound formed by reaction during processing. That Sedriks et al. did not see evidence of cathodes on their fibers may have been due to differences in details of fabrication. The microstructural features responsible for the preferential attack and the mechanism by which this attack is influenced by cathodes on the fiber surface remain to be established. 2. G/Al Composites. Considerable effort has been expended in the development of G/Al composites since the material was first developed about 1970. Because graphite provides a good cathode and has a free corrosion potential something like 1V higher than that of aluminum, there was concern from the start about galvanic coupling effects in these composites. The composites studied were all manufactured by the same technique, Le., Ti-B CVD coating of the graphite fiber, infiltration of fiber bundles with liquid aluminum to yield composite wires, and hot pressing of these wires into plates. The questions on which information was provided fall into several categories: (1)
lnd. Eng. Chem. prod. Res.
Figure 1. Examples of preferential corrosion along wire-foil, w i r e wire, and foil-foil interfaces in a section of G/201-1100 AI c o m p i t e after 5 weeks alternate immersion in seawater (Snyder and Payer, 1976) (lWX).
effects of fiber type and matrix alloy, (2) corrosion paths, (3) effects of structural defects associated with the consolidation process, and (4) effectiveness of cladding and protective coatings. Composites with Beveral graphite fiher typea in 6061 and 5054 aluminum matrixes were reported by Dull et al. (1977). The fibers were T50 (Union Carbide Corporation, New York, NY,rayon precursor), HM 3OOO (Hercules, hc., Wilmmgton, DE, polyacrylonitrile precursor), and VSA-11 (Union Carbide Corporation, New York, NY, pitch precursor). Polarization tests were conducted in 3.5% NaCl solutions containing oxygen, and corrosion rates were measured in 10-day simple immersion tests. They found that the introduction of the fibers had a small influence on corrosion in oxygenated solutions. The rates for the composites were no more than 4 times those of the matrix alloy and generally less. In deaerated solutions, there were small differences as a function of fiber type, with composites containing T50, HM 3000, and VSA-11 corroding a t increasing raks. Several investigations have reported on the distribution of corrosion in G/Al composites. Kendall(1974) and Dull et al. (1975) have investigated 30 vol % T50 G/6061-T6 AI in distilled water and 3.5% NaC1. Davis and Sullivan (1973) examined composites of T50 fibers in 201 or 6061 A1 matrixes by salt-spray testing a t 95 to 98% relative humidity at 95 "F (utilizingASTM test B117-73 simulating marine exposure). Payer and Pfeifer (1975) and Pfeifer (1977a) exposed 30 vol % T50 G/201-1100 AI (Alloy 201 in the wires, Alloy 1100 used for the interlayer foils) and T50 G/202-1100 AI to the marine atmosphere at Daytona Beach, FL. Alternate immersion in seawater was also employed and other matrix and foil alloys were examined; see Pfeifer (197713). The general conclusion was that corrosion proceeded preferentially a t foil-foil, wire-foil, and wire-wire interfaces. The proceea is initiated by a pit penetrating the surface foil, such as shown in Figure 1,or at the end of a plate. Once initiated, the selective corrosion spreads rapidly, and exfoliation and disintegration of the composite can result from the wedging action of the corrosion product formed in the crevices. These effects have
MV.,
vd. 22.
NO. 2, 1983
ma
Figure 2. Corrosion within a wire in a G/ZOZ-llOO AI composite after 11-week marine exposure at Daytona Beach, FL (Payer and Pfeifer. 1975) (400X).
their origins in imperfect consolidation of the composite. In some cases, corrosion proceeded through the fiber bundles along certain paths of higher fiber density, Figure 2. This can probably be attributed to inadequate fiber wetting during infiltration of the fiber bundle since areas of fiber-fiber contact could be observed. Well-bonded composites appeared to exhibit good corrosion resistance. In one case (Payer and Pfeifer, 1975), a plate with a partially drilled hole exposing graphite fibers showed no evidence of attack at the bole. We may note that caution should be exercised in concluding from this observation that exposed fibers are not damaging in a well-bonded composite. Galvanic coupling to the graphite could affect pit growth in the aluminum a t some distance from the cathodes in the high conductivity salt solution. Another reason for caution in interpreting the corrosion results is related to the presence of two aluminum alloys in many of these composites. The pitting potential is raised significantly by copper in solid solution, so that with a copper-bearing alloy like 201 (4.6% Cu) coupled to 1100 AI (0.12% Cu) the couple would tend to run at the (lower) pitting potential of the latter. This would produce a preferential corrosion effect added to or combined with that due to imperfectly bonded interfaces. Even if an A1-Cu alloy were used in both wires and interlayer foils, there could be differences in pitting potential due to differences in the extent of precipitation of copper from solid solution. The corrosion of clad and coated G/A1 composites was studied by Snyder and Payer (1976) and Payer and Sullivan (1976). Composites with 6061 or 2024 claddings were exposed to alternate immersion in seawater or marine exposure. The behavior was similar to that of the bulk alloy, the cladding having not been breached. Paints (polyurethane, chlorinated rubber, and epoxy) were effective in protecting against corrosion, complete coverage having been achieved. Electroplated nickel also proved to be quite successful, one sample showed no evidence of corrosion after 8.5 months of marine exposure. Once the nickel coating is breached however, the matrix becomes
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a sacrificial anode and corrosion is rapid. In a stress-corrosion cracking investigation on composites with Thorns150 fibers infiltrated with AI 201 alloy and diffusion-bonded with Al2024 interlayer foils, Phillips (1977) studied compact tension specimens precracked in fatigue parallel to the fibers and dead-weight loaded perpendicular to them. There was a small decrease in applied stress-intensity factor for loo0 h life in 3.5% NaCl solution as compared with air. Large corrosion pits were present, but Phillips concluded that the failures in these composites, as with the B/AI composites he also studied, were mainly attributable to rmm temperature creep. Davis et aL (1982) conducted corrosion-fatigue tests on G/6061 Al composites. Fibers were VSB-32 (Union Carbide Corporation, pitch precursor) with either the standard Ti-B coating or coated with SiO,. When tested in seawater the composites suffered lower percentage reduction of the laboratory-air properties than did 6061-T6 alloy over most of the range of lives. With SiOTcoated fibers, the fatigue strength of the composite fell drastically beyond 5 X lo6 cycles. This was attributed to corrosion damage, which was evident visually, suffered a t the longer lives. Some stress-corrosion tests on the standard composite led to the analogous conclusion that the lm of strength at the longer lives was dominated by corrosion rather than stress. This is the reverse of Phillips’ (1977) conclusion for a different composite. It is difficult to foresee at this point how far advances in design and processing will improve the corrosion resistance of G/Al composites. In a perfectly bonded composite, the distribution of attack should he much less damaging than is currently observed. However, it appears unavoidable that in an unprotected composite galvanic coupling to the graphite would lead to accelerating and eventually severe pitting of the matrix. It is therefore expected that protection and maintenance thereof will remain necessary. 3. AI,O,/AI Composites. Modem efforts are based on the use of DuPont F P alumina produced aa nominally 2O-pm diameter continuous filaments of polycrystalline a-Al,O,, of approximately 98% density with a high tensile strength. There are wetting and bonding difficulties with a pure aluminum matrix and the approach has been to alloy the matrix with a component which can reduce alumina and form a bonding compound on the fiber surface. DuPont has developed composites prepared by vacuum infiltration of an FP fiber bundle with liquid AI-2% Li alloy. A thin silica coating on the fiber aids initial wetting but is removed during infiltration, and a Li20.5AI,0, layer typically 2-3 pm thick is formed. Champion et al. (1978) exposed these composites for up to 90 days to eight accelerated corrosion tests commonly used for testing wrought aluminum alloys (mainly immersion, alternate immersion, or spray tests in solutions containing NaCI). No evidence of “delamination or splitting” (Le., severe attack at the interface) was detected. Weight losses in most cases were only modestly higher than those of 6061-T6, a wrought alloy with good corrosion resistance. Comparisons made with 2219-T87 alloy in a few cases showed the composite to be superior. Six-month or 1-year stresxorrosion tests at 90% or 56% of the ultimate tensile strength showed no failures with alternate immersion in 3.5% NaCl or synthetic seawater. Experimental composites with FP fibers were prepared hy Levi et al. (1979) by introducing chopped fibers into a stirred 50% solids slurry of AI-Mg alloy. Bonding is achieved through formation of a MgAlzO, layer, which may be much less than 1pm in thickness. Composites of this
Figure 3. Pitting adjacent to fiber in experimental AI2O3/AI-Mg composite after 1 day in NaCI-H202accelerated test solution. The surface film is still preaent over the pit (Yang and Metzger, 1981). Scanning electron micrograph. 3wOX.
type with a 2% Mg matrix were examined by Yang and Metzger (1981) in an NaC1-H202 accelerated immersion test plus polarization studies to check questions of corrosion vs. microstructure. Although a matrix alloy similarly processed by itself suffered little corrosion, in the composite there could be severe selective attack in the matrix adjacent to the fibers, Figure 3. With evidence from electron microprobe studies, this was attributed to solidfication segregation of magnesium near the fiber resulting in some local precipitation of the Mg5AI8phase, which suffers rapid corrosion even below the pitting potential. In addition, the higher local concentration of magnesium in solid solution would lower the pitting potential slightly below that of the body of the matrix. There was also cathodic damage from concurrent segregation of iron-rich constituents, which were exposed by corrosion along the anodic paths. This work dealt with composites at an early stage of development, and its significance is not in evaluation of a particular composite but in identification of possible detrimental structures associated with solidification processing. Agarwala (1981). in A1,03/6061 AI composites, also noted preferential corrosion in the matrix adjacent to the fibers. In W o n t AI-Li matrix composites, Yang and Metzger (1980) found that segregation was not severe ai itself could not be determined). The distribution of attack was not specially damaging although there was some preferential pit nucleation (but not preferential growth) at the margin of the reaction layer, Figure 4. Thus, to date no serious corrosion problems with the AI,O,/AI-Li composites have been revealed. 4. SiC/AI Composites. Aluminum composites reinforced with Sic, especially those with discontinuous whiskers or particles, are currently the objects of considerable development efforts (Divecha et al., 1981). These are fabricated by blending with aluminum alloy powders and pressing in the partially molten state. Near-theoretical densities are achieved. There is evidence that the mode of fabrication may produce some special microstructural state-composites with 2024 AI matrixes required unusually long solution heat treatments to develop full strength after aging (Skibo, 1981), perhaps because some of the copper was tied up in coarse particles. The nature of the bond between Sic and matrix is not known. Some de-
Ind. Eng. Chem. Prod. Res. Dav.. Val. 22. No. 2. 1983 so1 30
I 1111111
I
I
,
I
I
o ZQ24
2M4-Sic
I
10
CURRENT DENSITY (mA/cmz)
Figure 6. Anodic polarization of SiCl2024 AI composite and of wmught 2024-T4AI in aerated 0.1 m NaCl (McCaffertyand hkoma, 1981).
Figure 4. Distribution of pitting in DuPont AI,O,/AI-Li composite after 1 day in NaCI-H,02 solution (Yang and Metzger, 1980). Scanning electron micrograph, 300x.
1 ii
% Sic. With 5456 A1 and 6061 Al matrixes, the pitting potential was approximately that of the wrought matrix alloy. However, with a 2024 Al matrix, the composite had a lower pitting potential, Figure 6. We point out that this last result may reflect a matrix microstructure which is different from that of the wrought matrix alloy. The pitting potential of the 2024 A1 matrix decreases with decreasing concentration of copperin solid solution, as noted earlier. The matrix of the composite would have leas than normal copper in solid solution if a substantial fraction of the copper were tied up as coarse intermetallic particles, 88 suggested by the sluggish response to solution heat treatment mentioned above. The cathodes provided by these particles could then be responsible for the higher corrosion potential of this composite as compared with the wrought matrix alloy, Figure 6.
!
1 Figure 5. Pit nucleation at exposed S i c whiskers in SiC/6061 AI composite (Lore and Wolf, 1981). Scanning electron micrograph, 1mx.
velopment of composites with continuous Sic is under way (Cornie, 1981), but corrosion studies are not yet included. The corrosion studies being made of composites with discontinuous SiC/Al have not reached the publication stage, and the following is a summary of work in progress. DeJarnette and Crowe (1981) compared 20 vol % SiC/2024 A1 with commercial extruded 2024-T4 in 3.5% NaCl and found corrosion rates to be the same in deaerated solutions; in aerated solutions the composites were corroding about 40% faster in a four-week exposure. Lore and Wolf (1981), for SiC/6061 A1 with 0 to 30 vol % SIC, found little difference in rates of weight loss in 3.5% NaC1. Both sets of investigators noted that pits initiated at Sic particles, Figure 5. McCafferty and Trzaskoma (1981) made polarization studies in 0.1 M NaCl of several composites with 20 vol
Preferential pit nucleation adjacent to the Sic whiskers, Figure 5, is not regarded as substantially detrimental so long as there is no preferential growth of the pit along the whisker margins. However, the work done so far on the corrosion of t h e e composites is too limited to judge whether or not they may have serious corrosion problems. The microstructure and ita relation to the corrosion also need examination. Other Composites Bicelli et al. (1979) prepared unidirectionally solidified Al-Al,Ca eutectic alloys and did polarization studies in 0.5 M NaC1. They compared the results with those done for the extruded eutectic alloy, commercially pure aluminum and an AlXu alloy. There was preferential attack of the A1,Ca reinforcing phase. Only the briefest mention was made of corrosion rates, and the prospects for this composite are not clear. The electrochemical behavior of the directionally solidified eutectic composite was ‘substantially equal” to that of the extruded alloy. Berke (1980) and Berke and Metzger (1980) prepared several A1-A13Fe eutectic composites of varied microstructure. This eutectic contains only 3.5 vol % A13Fe, but it is useful as a model system because the A1,Fe phase provides low overpotential cathodes and may be made to remain either unattacked or preferentially attacked at the corrosion potential. In 2.4 M H2S04, the A1,Fe was preferentially attacked. As a result, in directionally solidifed samples with continuous A13Fe phase, the cathodes persisted while in discontinuous eutectics the surface particles were removed rapidly and cathode currents (therefore corrosion rates) fell to much lower values. In 5 M NaCl at pH 1, the A1,Fe was unattacked and there was accelerating corrosion as additional cathode area was exposed in the pits. In discontinuous structures, a steady
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state is predicted when the surface is covered with pits and existing cathode particles are removed as fast as new ones are exposed. For continuous eutectics, there was some slackening of the acceleration because of breakage of the protruding A13Fe plates. For various experimental reasons, the observations themselves were limited in the extent of corrosion and the range of eutectic spacing covered. However, with the assumptions of random pit nucleation, nonpreferential pit growth, and no fiber breakage, the pitting corrosion current of the matrix was calculated for a continuous eutectic as
i = CkA,,ekSvt where k is the cathode c.d. on the fibers at the pitting potential, A , is the initial area fraction of fibers, S, is the surface area of fibers per unit volume of composite (obtainable through quantitative metallography), and C is a geometric constant. This predicts an acceleration which is higher and is detectable earlier the finer the microstructure (for which S, in eq 1 is larger). Even when the assumptions underlying eq 1 are not strictly applicable, it appears unavoidable that composites with continuous fibers which are effective cathodes and are not attacked will eventually be subject to rapidly accelerating corrosion and are not good prospects in regard to corrosion resistance. This conclusion would apply to G/Al composites. The problem would be less severe with lower volume fractions and coarser fibers (S, is then smaller and the corrosion remains longer in the region of kS,t
