The surface pretreatment of aluminium-lithium alloys for structural bonding

Thesurfacepretreatment of aluminium-lithium alloys for structuralbonding J.A. Bishopp*,

D. Joblingf

and G.E. Thompson+

(*Bonded Structures, Ciba-Geigy Plastics, Cambridge, UW+Corrosion Centre, UMIST, UK)

and Protection

Transmission electron microscopy examination of ultramicrotomed sections taken through a bonded joint allows both the effect of standard chemical pretreatments and an initial adhesive/adherend interfacial analysis to be made for structural joints prepared using BA BO9OGT3 aluminium-lithium alloy substrates. These observations, when compared with earlier work using Alclad 2024-T3 adherends, show that broadly similar films are formed on the aluminium-lithium surface; a rougher topography has, though, been noted on some specimens. In general, these films appear to be wetted and, in some cases, penetrated by the adhesive used in the same manner as the Alclad controls. Initial measurement of bond strengths shows that the pretreated aluminiumlithium is amenable to adhesive bonding. However, although examination of sections through the environmentally exposed joints shows no apparent interphasial damage, pickled aluminium-lithium substrates perform better than those anodized in phosphoric acid; this is contrary to expectations based on prior work using the more conventional aluminium alloys.

Key words: aluminium-lithium

alloys; surface pretreatments;

For today’s aircraft designer, one of the most important criteria when developing a new aeroplane is weight. A reduction of 5 tonnes in the manufacturer’s empty weight, in an aircraft initially designed to have a maximum certificated take-off weight (MTOW)of 200 tonnes, can reduce the final MTOWby nearly 9 tonnes and the engine thrust required by about 4.5%‘. This, amongst other savings, can lead to a significant reduction in fuel consumption. Little’ lists several possible approaches to enable this weight saving to be made in an entirely new aircraft: (a) advances in aerodynamics leading to lighter wing structures, (b) improvement in power plants leading, directly, to fuel saving, (c) new system concepts, eg fly-by-wire, reducing structure weight and (d) the use of novel, lighter, metallic or non-metallic structural materials. Considering the last of these options, the move from conventional aluminium alloys to carbon tibre or aramid-reinforced composite materials is now an established route to weight saving; carbon Iibrereinforced epoxy composites have been successfully used in the manufacture of control surfaces and engine cowls and are now used on primary structures such as 0143-7496/90/030153-08

peel strength

vertical and horizontal stabilizers, helicopter rotor blades and the wing of the ATR 72. Much work has also taken place in developing lighter metallic structures: steel can be replaced by titanium and the more conventional aluminium alloys by the newer, higher strength versions (for example, the 7000 series alloys). In the early 198Os,alloys of aluminium and lithium were developed which possessed lower densities (an Sg of 2.54 as opposed to 2.78) and higher stiffness than the current aluminium alloys of comparable strength. Aluminium-lithium alloys must, therefore, today be considered as yet a further potential material for use in the construction of aerospace structures. Although much work has already been done to determine the feasibility of using aluminium-lithium alloys in aircraft manufacture, it is still too early to say whether such materials will make a significant impact in this area. However, if they are to be used in such applications, individual components will have to be joined to produce the final structures. One possible method of joining is the use of structural adhesives. It is, therefore, already important that not only 0 1990 Butterworth-Heinemann Ltd

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should the suitability of adhesive bonding for such substrates be assessed but this should also include an evaluation of the possible chemical surface pretreatments available and their effect on the bonded joint and its long-term durability.

Table 1. Percentage composition of aluminium and aluminium-lithium alloys Alclad 2 0 2 4 - T 3 Element

BA 8 0 9 0 C - T 3 Core

Copper Lithium Magnesium Manganese Zirconium Iron Silicon Chromium Zinc Titanium Others (each) Aluminium

0.86-1.36 2.28-2.58 0.89-0.90

Cladding

Method of characterization and analysis Since the early 1940s, considerable knowledge has been gained on joining aluminium components, for structural aerospace applications, b 7 means of synthetic, high strength adhesives2-~; consequently it is now possible to use, for example, a modified epoxy, structural adhesive on a conventional aluminium alloy adherend as a bench mark when characterizing other substrates or adhesives. The use of this approach to evaluate structural adherends, such as fibre-reinforced composites and titanium, has shown the importance of using the correct pretreatment to obtain optimum adhesive properties on the novel substrates. This has proved of particular importance with carbon fibre-reinforced epoxy resin composites 5 where, even using the optimum method of pretreatment (a carefully controlled abrasion technique), some reduction in strength levels has to be accepted. Any evaluation as to the suitability of joining aluminium-lithium substrates by adhesive bonding must, therefore, commence with the characterization of the effects of surface pretreatment and an analysis of the adhesive/adherend interfaces produced on bonding, both before and after exposure to a 'hot/wet' environment. The method used, both to characterize the effects of pretreatment as well as to analyse the interfaces, is one which has already proved invaluable in gaining similar insights into bonded joints using the more conventional alloy - Alclad 2024-T36-s. This is the use of transmission electron microscopy to examine ultramicrotomed sections taken through the joint, perpendicular to the plane of the bond (Fig. 1). The use of ultramicrotomy, to generate electron transparent sections of aluminium-lithium alloys, had been established by Malisg; in this study the lithium and zirconium distribution in the 8090 alloy were characterized.

Materials Substrale: B A 8090C-T3 a l u m i n i u m - l i t h i u m alloy ex British Alcan; sheet thicknesses 0.55 mm and 1.6 mm.

Adhesive primer Carrier ~

i~

...............

jlt

Aluminium oxide Adhesive

I

Fig. 1 Schematic representation of a longitudinal section through a bonded joint

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0.13 0.13-0.17 0.04-0.06

Balance

3.80-4.90

0.10 max

1.20-1.80 0.30-0.90

0.05 max 0.05 max

0.50 max 0.50 max 0.10 max 0.25 max 0.15 max 0.05 max Balance

t

0.70 max 0.10 max 0.03 max 0.03 max Balance

The typical composition, as determined by Colvin and Starke t° is given in Table 1. Comparison substrate: The substrate used for comparison purposes was Alclad 2024-T311. The composition of this alloy (core and cladding) is given in Table 1. Adhesive: A toughened epoxy, structural film adhesive curing at 120°C. Adhesive support: A knitted nylon cloth (Nylon 6). Primer: Earlier work 7"8 has shown that the comparison of primed and unprimed substrates is not essential in characterizing the interfaces/interphases developed, and hence, none has been used in the work presented here.

Experimen2al Standard lap-shear 12 and floating roller peel panels 13 were produced for each of the following chemical pretreatments given to the aluminium-lithium substrates: • Potassium dichromate/sulphuric acid pickle (CSA) in accordance with DTD 915b (ii) 14. • Potassium dichromate/sulphuric acid pickle followed by chromic acid anodizing (CAA) to DEF STAN 03-24/115 • Potassium dichromate/sulphuric acid pickle followed by phosphoric acid anodizing (PAA) tO BAC 5555 rr. The test coupons were bonded using a standard cure cycle of 1 h at 120°C under a pressure of 275 kPa. 'Control' specimens were cut out and loaded to failure. The remainder of the peel joints were then exposed, at 70°C, for 30, 60 and 90 days at a relative humidity of at least 85%. At the end of the designated exposure time the joints were tested as before. Selected areas of the peel specimens were then mounted and prepared for ultramicrotomy in the usual manner tT. 5-45 nm thick specimens were cut, using a Du Pont Sorval or Reichert Ultracut ultramicrotome, and examined by transmission electron microscopy (Philips EM 301 and EM 400 microscopes). In the work reported here, the specimens were

nearly always taken well in advance of the crack tip (i.e, in the unruptured, essentially unstressed area of the joint) as early work 7"8 had shown this to be ideal both for characterizing the effects of the surface pretreatment as well as enabling some degree of interfacial analysis to be achieved - both before and after exposure to 'hot/wet' environments.

q5.0 D. ~E 30.0

m

B

lad 2024-T3 B B A 8090C-T3

15.0

/ I

Results and interpretation

,iil 2.0

The lap-shear and unexposed (0 day) peel strengths recorded are shown in Table 2, which also gives a comparison with results obtained previously, with the same adhesive, on Alclad 2024-T3 substrates. Table 3 shows the effect of environmental exposure on the peel strengths and, again, gives a comparison with bonded Alclad 2024-T3 specimens which have been given the same treatment. The transmission electron micrographs of the ultramicrotomed sections through the control (0 day) peel specimens are shown in Figs 3(a) - 5(a), and Figs 6 - 9; direct comparison with Alclad 2024-T3 can be seen in Figs 3(b) - - 5(b). Typical micrographs for the exposed specimens can be seen in Figs 10 - 14. Arrowsmith et a118 reported no difficulty in anodizing aluminium-lithium alloys, a finding essentially supported by this work. No 'burning' effect was seen on any of the anodized panels although a significant, uniform colour change to dark grey was noted with the chromic acid-anodized specimens.

,,, 4.0

I , ~ ~ I , , , I ~, , I ~, , I , , ~ I 6.0 8.0 10.0 12.0 14.0 16.0 Percent strain

(~,)

Fig. 2 Pseudo stress/strain curves for bonded joints showing the increased ductility of aluminium-lithium systems. (Note: curve A has a baseline offset of approximately 1%)

=3

Table 2. Comparison of mechanical strengths for aluminium-lithium bonded joints Pretreatment

Typical adhesive strengths on Alclad 2024-T3

CSA pickle CA anodize PA anodize

BA 8090C-T3

Lap-shear at 22°C

F-R* peel

Lap-shear at 22°C

F-R* peel

40 MPa 37 MPa 41 MPa

280 N 200 N 290 N

35 MPa 32 MPa 35 MPa

300 N 235 N 315 N

*F-R = Floating-roller

Table 3. 90-day humidity ageing floating-roller peel specimens Pretreatment

Peel strengths - N / 2 5 mm Alclad 2024-T3

BA 8090C-T3

Fig. 3 Transmission electron micro<3raphs of ultramicrotomed sections through unexposed csA pickled joints: (a) BA 8090C-T3; (b) Alclad 2024-T3

Humidity* ageing at 70°C (days)

Lap-shear specimens 0 CSA pickle CA anodize PAanodize

30

60

90

0

30

60

90

280 190 180 135 300 240 220 205 200 85 45 65 235 125 115 65 290 250 225 230 315 275 205 165

*85% Relative humidity (minimum)

Arrowsmith t8 examined the lap-shear performance of toughened acrylic and toughened epoxy adhesives on aluminium and aluminium-lithium substrates and found that strength levels appeared to be essentially independent of substrate, pretreatment and, to a certain extent, adhesive. Table 2 shows this not to be the case here. The

INT.J.ADHESION AND ADHESIVES JULY 1990

155

0 . 5 IJm

-4

L

~IB;-.

.

",-

~ .

Fig, 5 Transmission electron micrographs of ultramicrotomed sections through unexposed phosphoric acid anodized joints: (a) BA 8090C-T3; (b) Alclad 2024-'1"3

stresses are generated, which lead to an apparently lower shear value for the adhesive. This work shows the critical strength level, before onset of ductile behaviour, to be about 30 MPa: the m a x i m u m loads recorded in the Arrowsmith work were below this level and hence this effect would not have been seen. Peel specimens - 0 day controls

Fig. 4 Transmission electron micrographs of ultramicrotomed sections through unexposed chromic acid anodized joints: (a) BA 8090C-T3; (b) Alclad 2024-T3

marked difference between the lap-shear strengths on Alclad and aluminium-lithium can be explained by examination of the load-extension graph (Fig. 2) generated during testing. On Alclad substrates the total strain at failure is 2-3%; with the aluminium-lithium joints, however, this rises to 10-13%. Under tensile loading, therefore, the aluminium-lithium alloy appears to be far more ductile and thus, during test, deforms more easily. Hence, more significant peeling

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Table 2 shows that, whilst the move from standard Alclad to aluminium-lithium substrates appears to result in about a 10-15% improvement in peel strengths, the same order of ranking is apparent for both adherends; namely, pAA is a better pretreatment, as far as peel strengths are concerned, than CSA. which is better than CAA. Examination of transmission electron micrographs of ultramicrotomed sections, through these specimens, permits the film grown, during pretreatment, to be characterized and some analysis of the interfacial/ interphasial structures to be made. Comparison can also be made with the more conventional joints using Alclad 2024-T3.

Fig. 8 Transmission electron micrograph of an ultramicrotomed section through a phosphoric acid anodized aluminium-lithium joint. It shows the distribution of copper-rich intermetallics at the boundary layer

Fig. 6 Transmission electron micrograph of a 5 nm thick section through an unexposed phosphoric acid anodized joint (Alclad 2024-3"3) showing adhesive penetration into the oxide pores. (Note: specimen thickness, in this case, is about 30-50% of that used for the characterization work)

Fig. 9 Transmission electron micrograph of an ultramicrotomed section through a phosphoric acid anodized aluminium-lithium joint. It shows the surface pitting due to possible dissolution of surface lithium

Fig. 7 Transmission electron micrograph of a transverse section through the oxide film, formed by phosphoric acid anodizing, in an unexposed Alclad 2024-T3 bonded joint. Adhesive concentrations at the bottom of the pores can be seen

csA-pickled substrates (Figs 3(a) and 3(bJ) The film structure seen for aluminium-lithium is similar to that generated on Alclad 2024-T3. Both exhibit finely spaced, 25-50 nm high whiskers which, from the evidence of the micrographs, appear to be extensively penetrated by the adhesive.

CAA-pretreated substrates (Figs 4(a) and 4(b)) Producing sections from the aluminium-lithium specimens has proved difficult. This could indicate

either a more open film is produced on this material or the tougher substrate introduces significant mechanical damage on sectioning. Nevertheless, from the evidence which is available in these micrographs, it is possible to conclude that both adherends generate somewhat similar film morphologies following chromic acid anodizing. A porous surface film is present which, in the case of Alclad 2024-T3 is about 2-4 pm thick but is rarely thicker than 2.5/~m in the case of aluminium-lithium. Further, there appears to be more "texture" to the film grown on the aluminium-lithium alloy. In both cases, however, the characteristic planar interface is evident i.e. the grown film is well wetted but almost certainly not significantly penetrated by the adhesive in bulk.

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0.25 ~m

)

Fig. 10 Transmission electron micrograph of an ultramicrotomed section through a chromic acid anodized aluminium-lithium joint after exposure to a prolonged 'hot/wet' conditioning

I

Fig. 12 Transmission electron micrograph of an ultramicrotomed section through a phosphoric acid anodized aluminium-lithium joint after exposure to a prolonged 'hot/wet" conditioning

Fig. 13 Transmission electron micrograph of an ultramicrotomed section through a phosphoric acid anodized Alclad 2024-T3 joint after exposure to a prolonged "hot/wet' conditioning

Fig. 11 Transmission electron micrograph of an ultramicrotomed section through a chromic acid anodized Alcled 2024-T3 joint after exposure to a prolonged "hot/wet' conditioning

PAA-pretreated substrates (Figs 5(a) and 5(b)) Comparison of the two morphologies again shows similar structures are present; in both cases a porous surface film is grown. The film on the Alclad is generally about 0.5-1.0 p m thick whilst that on aluminium-lithium is only about 0.35-0.55 pm. Also, for both substrates, any original surface roughness is enhanced due to film material collapse during anodizing (caused by progressive thinning of

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the cell material adjacent to the pore wall). Careful examination of the micrographs shows that the adhesive not only wets the surface and revealed cavities well but also penetrates into the depths of the anodic film. As in the case of the CAA adherends, the anodic film grown on the aluminium-lithium appears to exhibit more 'texture' than for the comparable Alclad film. The above shows, in general, that similar film morphologies are developed on aluminium-lithium surfaces as are formed on the more conventional alloys. The differences in anodic film thickness can be explained by the lower current efficiencies when anodizing alloys (with their associated second phase materials) as opposed to clad materials where the cladding is 99.3% pure aluminium; the 'texture' seen on

the films grown on aluminium-lithium is, probably, a result of this lower anodizing efficiency. Although the use of transmission electron microscopy to examine both longitudinal and transverse ultramicrotomed sections through the joint has already yielded, and will continue to yield, important information as to the joint's interfacial structure, it is now clear that this approach needs to be augmented with other analytical techniques to obtain a more detailed insight into the structure?chemistry' present in the interphasial regions. This aspect of the work is very much in its early stages, but the following techniques do appear to offer the potential of generating the data required: electron energy loss spectroscopy (EELS),Auger electron spectroscopy (ms), X-ray photoelectron spectroscopy (x],s) and Fourier transform infra-red spectroscopy (v-r-m). Where important, new or confirmatory data has been generated, the relevant results of this new approach are referred to below. The use of EELS,although needing some further refinement, is confirming the extensive penetration of the film grown in phosphoric acid by the adhesive already evident, visually, in Figs 6 and 7. Interestingly, the initial analysis of the films produced by chromic acid anodizing, does show what appears to be some penetration by the adhesive (carbon 'fingerprinting') although these adhesive inclusions seem to exist as small, infrequent, isolated areas rather than the continuous, extensive penetration seen with the other two methods of pretreatment. The degree of penetration is certainly not significant enough to affect the environmental resistance of bonded, chromic acid anodized joints (see below). With so many points of similarity between the two substrates there are, however, some significant differences which appear to be independent of pretreatment but unique to the aluminium-lithium substrates. Copper- and copper/lithium-based intermetallics (confirmed by both energy dispersive X-ray spectroscopy (EOX)and EELS)have been found at the interface (Fig. 8) at a higher frequency than would be expected for normal distribution throughout the alloy. This phenomenon is, so far, unexplained. Current thinking favours the possibility that such inclusions are removed by the knife blade during ultramicrotomy and are then swept to the interface where they appear as simple artefacts. Further work is expected to clarify the situation. Localized areas of much rougher surface topography (Fig. 9) have also been identified - a degree of surface roughness not experienced with Alclad adherends. It is believed that alloying elements, particularly lithium, could relocate to the surface during heat treatment of the alloy. These could then etch away during the CSA treatment leaving a pitted surface on which it would still be possible to grow the sort of surface film seen in Fig. 9.

Environmentally exposed specimens Table 3 shows that, after 60 days' exposure, a significant difference in performance between the two alloys has occurred. With the joints prepared using the aluminium-lithium substrates, the rate of deterioration

of peel strength appears to be faster on PAA-pretreated adherends than it does on pickled. The reverse occurs when Alclad 2024-T3 substrates are used; in both cases the CAA-pretreated adherends impart the least environmental resistance. Transmission electron microscopy examination of the exposed bondlines has revealed no obvious damage to the surface film (on the aluminium-lithium), the interface or the interphasial layer on the adhesive side of the interface which could account for this difference in behaviour. Indeed, no damage has been seen for any of the pretreatments used - a finding in agreement with previous work on Alclad 2024-T3 substrates 8. Figures 10-13 are typical of the lack of readily evident change in interphasial structure, after prolonged exposure, for both Alclad and aluminium-lithium joints (cf., Figs 4 and 5 for the corresponding '0 day' specimens). The mechanism for bond degradation during the 'hot/wet' exposure of aluminium-lithium joints must, as has been argued for Alclad 2024-T3 joints s, be very subtle. In the work with Alclad adherends, it was argued that a more diffuse, and hence a longer, interface was important as far as environmental resistance was concerned. This means that for good environmental performance, significant penetration of the surface film, by the bulk adhesive, is essential. However, as can be seen in Figs 3 and 5, the films grown by CSA and by PAA pretreatment, on both Alclad and aluminiumlithium substrates are, indeed, significantly penetrated by the adhesive. Thus a difference in performance would not have been expected, especially as the PAA film grown on the aluminium-lithium contains, as does that grown on the Alclad, bound phosphorus which 'poisons' sites that would otherwise be susceptible to hydration. Obviously much more work needs to be carded out to ascertain the reasons for the change in performance and to this end both xes and EELSanalyses are already yielding useful information as to possible subtle changes in the interphasial chemistry. Hopefully, such an analysis will yield more precise data on the influence, if any, of the composition of aluminium alloys in general and their 'oxide' films not only on the environmental resistance of the joint but on the chemistry of the adhesive itself when exposed to 'hot/ wet' conditions. However, one possible explanation can be postulated, for this specific case, from the elemental composition of the two alloys (Table 1). The Alclad cladding contains a maximum 0.7% of iron, whereas the content in the BA 8090C-T3 is less than 0.2%. Fig. 14 shows some interfacial damage, in a CSA-pretreated Alclad joint, in the vicinity of an iron-containing intermetallic particle. No such damage has been found, as yet, with similarly pretreated aluminium-lithium specimens. The absence of this very limited interfacial attack might be sufficient to improve the performance on CSA-pretreated adherends and to change the ranking, for aluminium-lithium bonded joints, to CSA pretreatment being better than PAA,which is better than CA& -

Conclusions Although this work is very much in its early stages, with considerable investigations still to be carded out,

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• xvs and EELS techniques are beginning to give

interesting information concerning changes in chemistry to interphasial regions in the joint, following exposure to 'hot/wet' conditions.

Acknowledgement The authors gratefully acknowledge the help of Short Brothers, Belfast, in particular Dr W. Mcgarel and R. Hanna for supplying anodizing facilities at very short notice, and Miss J.A. Underwood and P.J. Hirst of Bonded Structures for preparing the test specimens and generating the mechanical strength data. This paper is reprinted from World Aerospace Technology 1990 with permission from Sterling Publications International Ltd. Fig. 14 Transmission electron micrograph of an ultramicrotomed section through a csA pickled Alclad 2024-T3 joint after exposure to a prolonged 'hot/wet' conditioning. Interfacial damage in the intermediate vicinity of the Fe/AI 3 intermetallic can be seen

it is already possible to draw some conclusions and make some speculations from the above. • No major difficulties have arisen in using any of the standard pickling or anodizing pretreatments to prepare aluminium-lithium surfaces for bonding. • The film morphologies formed after pretreatment are, in essence, similar to those already wellcharacterized on Alclad 2024-T3. The thinner and more highly textured anodic films would probably be expected owing to the known, lower current efficiency when anodizing aluminium alloys rather than the 99.3% pure aluminium coating on the 2024-T3. • Unlike Alclad substrates, some surface cavity formation is noted after pretreatment. This is possibly due to loss of lithium-containing second phase particles, at or near the surface, during pretreatment. • Chemically pretreated aluminium-lithium surfaces are very amenable to being joined by adhesive bonding. However, if simple bonded structures are to be designed to withstand >30 MPa tensile loads then the induced peel loads, occasioned by the higher ductility of the aluminium-lithium over the more conventional alloys, must be taken into account. • The wetting/penetration of the grown film by the adhesive appears to be similar to that for the Alclad adherends. Electron energy loss spectroscopy supplies an indication that the anodic films grown in chromic acid are slightly penetrated by the adhesive - not comparable in degree, however, to the penetration of the other 'oxide" films. • Unlike Alclad 2024-T3, CSA-pickled aluminiumlithium surfaces appear to be less susceptible to exposure to laboratory 'hot/wet' conditions than those anodized in phosphoric acid. • The 'durability ranking' for aluminium-lithium pretreatments is CSA better than PAA which is better than CAb, as opposed to PAA better than CSA better than c ~ for Alclad 2024-T3.

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References 1

Little, D. Overview, in: Proc Third Int Aluminium-Lithium Conf Vol I (The Institute of Metals, London. 1986) pp 15-21

2

de Bruyne, N.A. Bonded aircraft structures (Bonded Structures Ltd, Cambridge, 1957)

3

Patrick, R.L. Treatise on Adhesion and Adhesives Vol 4 (Marcel Dekker Inc, New York, 1976)

4

Kinloch, A.J. Durability of Structural Adhesives (Applied Science Publishers. London, 1983)

5

Parker, B.M. and Weghorne, R.M. Composites 13 3 (1982) p 280

6

Bishopp, J.A. Int J Adhesion Adhesives 4 4 (1984) p 153

7

Bishopp, J.A., Sire, E.K., Smith, T.V., Thompson, G.E. and Wood, G.C. The use of electron microscopy for the analysis of the adhesive-adherend interface in the aluminium-aluminium bonded joint, in: Adhesion t Z (ed. K.W. Allen) (Elsevier Applied Science, London, 1988) pp 248-264

8

Bishopp, J.A. etalJAdhesion 26 (1988) p 2 3 7

9

Malls, T. Characterisation of lithium distribution in aluminium alloys, in Proc Third Int Aluminium-Lithium Conf. Vol 2 (The Institute of Metals, London, 1986) pp 347-354

10

Colvin, G.N. and Starke, E.A. Sarape Quarterly 19 4 (1988) p 10

11

Federal Specification QQ-A-250/5F, Aluminium Alloy Alclad 2024 Plate andSheet (1971: amended 1974 and 1983)

12

Federal Specification, MMM-A. 132A ( 198~ : amended 1982 and 1984)

13

AE.C,M.A Standard, pr EN 2243-2 (1980: Draft - Issue 1 )

14

Aircraft Process Specification DTD 915b (Ministry of Supply, 1956)

15

Defence Specification, DEF STAN 03-24/I, Chromic Acid Anodising of Aluminium and Aluminium Alloys (Ministry of Defence, 1984)

16

Boeing Process Specification, BAC 5555, Phosphoric Acid Anodising of Aluminium for Structural Bonding (Boeing, 1974)

17

Furneeux, R.C., Thompson, G.E. and Wood, G.C. Corros Sci 18

18

Arrowsmith, D.J., Clifford, A.W., Moth, D,A. end Davies, R.J. Adhesive bonding of aluminium-lithium alloys, in: Proc Third/nt A]uminium-Lithium Conf Vol 1 (The InStitute of Metals, London 1986) pp 148-151

(1978) p 8 5 3

Authors J.A. Bishopp is with Bonded Structures, Ciba-Geigy Plastics, Duxford, Cambridge, UK. D. Jobling and G.E. Thompson are at the Corrosion and Protection Centre, University of Manchester Institute of Science and Technology, UK.