CA1291962C - Anodizing and press-lubricating aluminum sheet - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Disclosed are aluminium sheets suitable for forming a structure of press-formed components secured together by adhesive and a method for producing such aluminium sheet.
The sheet has an anodic oxide layer formed thereon by having been subjected to an anodizing process in an acidic electro-lyte for a period of 2 minutes or less and an adhesive compat-ible press-lubricant on the anodic oxide layer. The invention enables structures to be fabricated from aluminium components which have undergone a rapid pre-treatment but nevertheless gives rise to bonds of strength and durability compatible to those achieved using the Boeing process.

Description

This is a divisional application of Serial No. 494,491 filed November 4, 1985.
This divisional application relates to novel aluminium sheet useful for fabricating structures of aluminium com-ponents and a method for producing such aluminium sheet. The parent application relates to a method of fabricating structures from aluminium sheet and structures comprising aluminium com-ponents.
It should be noted that the expression "this invention"
in this specification may include the subject matters of the parent app],icationas well as those of this divisional appli-cation.
The term "aluminium" as used here and throughout the specification is intended to include aluminium alloys.
It is well known for aluminium structures to be fabrica-ted by bonding components together after pre-treatment of the surfaces of the components. One such pre-treatment is DC
phosphoric acid anodizing as used in the aircraft industry, particularly by Boeing (British Patent 1,555,940), and this form of pre-treatment is considered to be one of the best available for long-term durability in structural applications.
This durability is thought to depend on the structure of the oxide layer produced by DC phosphoric acid anodizing under the Boeing conditions described and many papers have been written on this subject (e.g. J. D. Venables, et al., Appl.
Surface Science 3, 1979, 88-98). However, the Boeing process requires an anodizing time of 5-60 minutes in a phosphoric acid electolyte at a temperature of 10-30C. In practice an anodizing time of 20-30 minutes is usually used, and clearly this is only suitable for batch treatment of components rather than as a continuous treatment for aluminium coil. Although oxide layer thicknesses are not reported in the patent examples, in practice a minimum thickness of 300-400 nm appears necessary to achieve the desired properties.
Oxide layers produced by the Boeing process have excellent properties as adhesive substrates, to the extent that they constitute a standard to which the rest of the industry aspires.
It has also been proposed for aluminium structures to be "weld-bonded~, that is bonded with adhesive but also spot-welded.
According to one aspect of the parent application, there is provided a method of fabricating structures from aluminium sheet which comprises the steps of subjecting the aluminium sheet to an anodizing process in an acidic electrolyte for a period of 2 minutes or less to form an anodic oxide layer thereon; forming the pre-treated sheet to produce components of a desired shape; and applying adhesive to the components and securing two components together by means of the adhesive.
According to another aspect of the parent application, there is provided a structure comprising aluminium components which are secured together by adhesive and in which the components have an anodic oxide layer formed thereon by being subjected to an anodizing process in an acidic electrolyte _ 3 _ 20388-1571D
l~l9t:2 for a period of 2 minutes or less, and have thereafter been subjected to a press-forming operation.
According to one aspect of this divisional application, there is provided an aluminium sheet suitable for forming a structure of press-formed components secured together by adhesive, which sheet has an anodic oxide layer formed thereon by being subjected to an anodizing process in an acidic electro-lyte for a period of 2 minutes or less, and an adhesive-compatible press-lubricant on the anodic oxide layer.
According to another aspect of this divisional application, there is provided a process for producing an aluminium sheet suitable for forming a structure of press-formed components secured together by adhesive, which process comprises:
subjecting a sheet of aluminium which may be an alloy thereof to an anodizing process in an acidic electrolyte for a period of 2 minutes or less, thereby forming an anodic oxide layer formed on the sheet, and applying a press-lubricant onto the anodic oxide layer, the press-lubricant being compatible with the adhesive.
Preferred features of this invention will be apparent from the subsidiary claims of the specification.
The invention enables structures to be fabricated from aluminium components which have undergone a rapid pre-treatment but nevertheless gives rise to bonds of strength and durability comparable to those achieved using the Boeing process.
Preferred forms of the invention also have the advantage 4 _ 20388-1571D

t,2 that the anodizing process can be carried out on aluminium .sheet in coil form and can thus be effected continuously.
Preferred forms of the invention also have the advantage of being spot-weldable.
Factors affecting features of this invention and their influence on preferred forms of the invention will now be discussed merely by way of illustration.
Equipment for continuous anodizing of aluminium strip is well known, and is described for example in "Automation in Anodizing" by W. E. Cooke (Aluminium Association, Aluminium Finishing Symposium, Chicago, March 1973). Suitable equipment includes an elongated bath with inlet and outlet ports for electrolyte and with opposed end faces having seals if necessary through which the continuous aluminium strip passes, the arrangement being such that the electrolyte preferably flows countercurrent to the strip. Two or more electrodes are positioned adjacent or indeed surrounding the moving strip, the electrodes being spaced in the direction of travel of the strip. Current leakage through the electrolyte is low because the electrolyte has a much lower conductivity than the metal.
In a preferred form of the invention the aluminium sheet is in the form of a coil for the advantage of rapid anodizing and for convenience of storage and transport. In order for the anodizing process to be carried out continuously, the tail of one coil may be joined to the head of the next so that the sheet can be continuously passed through a bath of electrolyte.

_ 5 _ 20388-1571D
~ t Existing aluminium treatment plants generally have a line speed of at least 50 m/min, and often of 150-250 m/min. Thus, in order to avoid the need for very long electrolyte baths, the anodizing process should take place rapidly. A contact time of 15 seconds between the sheet and the electrolyte is the longest that is likely to be practicable on existing plant.
Electrolyte contact times of 1 to 6 seconds, and preferably 2 to 3 seconds, are likely to be convenient, and times as short as 0.5 seconds are possible. The electrolyte contact time at any particular line speed may be regarded as a fixed feature of the plant, and one about which the other process variables are adjusted. On certain types of plant much slower lines can be used and hence longer anodizing times.
During the anodizing process a satisfactory balance must be achieved between anodic oxide formation on the aluminium sheet and dissolution of the oxide in the acidic electrolyte.
Sufficient anodic oxide must be grown to give adequate structural strength to the oxide layer and to provide an adequate surface area to provide a good substrate for adhesive.
Equally, dissolution of the anodic oxide layer takes place so that the surface area is effectively enlarged by attack of the pore wall structure. However, this attack must not be sufficient to cause breakdown and powdering of the oxide layer.
Anodic oxide formation is essentially controlled by the anodizing current density used. Anodic oxide growth per unit time is substantially proportional to anodizing current 1~91~iZ

density. With the short contact times available, the current density needs to be high to achieve a sufficiently thick oxide layer. The current density is preferably at least 250 A/m2.
It is convenient to relate current density with electro-lyte contact time in order to achieve a desired oxide thickness.
This may be expressed by saying that the total anodizing input will usually be in the range 2x104 to 12x104, particularly 3x104 to 6x104, C/m2.
Film attack is essentially controlled by the nature, concentration, and temperature of the electrolyte, with temperature being the most important factor. In considering the nature of this attack, it needs to be borne in mind that an anodic oxide film is created at the metal/oxide interface, ie.at the inner surface of the oxide layer remote from the electrolyte. Chemical dissolution occurs at the outer surface of the oxide layer, and it is thus the oldest remaining oxide that is subject to attack.
The anodizing electric current is preferably AC so that the aluminium sheet is alternately anodically polarized (during which time oxide growth predominates) and cathodically polarized (during which time chemical dissolution of the oxide predominates). Biased AC wave forms may be employed with advantage to achieve the desired balance between anodic growth of the oxide layer and chemical dissolution. The AC
frequency may be greater or (more likely) less than the standard 50 c/s. Alternatively DC may be employed, either continuously or as a pulsed current to increase the extent of chemical dissolution (between the pulses) relative to growth of the oxide layer.
The voltage is determined by the value of current density at which one has chosen to operate. Hence it finds its own level according to the current density and temperature (it is quite markedly effected by temperature at constant current density). For example at the lower end of the temperature range, e~,at 35C, the voltage is about 40V for 600 amps/meter2.
The voltage is reduced as the temperature goes up.
The temperature of the electrolyte is preferably at least 25C for short anodizing times. If the electrolyte temperature is too low, then no significant chemical dissolution of the oxide takes place during the (limited) electrolyte contact time and the surface area thereof is not increased. If the electro-lyte temperature is too high, then chemical dissolution may outpace oxide growth to the extent that all oxide is redis-solved as fast as it is formed. The preferred temperature range depends on the acids used in the electrolyte. Generally, with an acid that readily attacks aluminium oxide a lower temperature is needed than with an acid that attacks the oxide less readily.
Electrolyte concentration has a much less marked effect on the rate of chemical dissolution of the oxide film than temperature. The dissolution rate increases with electrolyte concentration and a concentration of at least 5% by weight of t;~

acid is found preferable in order to achieve rapid anodizing.
The oxide layer formed on the aluminium sheet by the anodizing is preferably relatively thin compared to that produced in the Boeing process. If the components are to be spot-welded (described below in more detail) the thickness of the oxide layer is preferably kept to 500nm or less other-wise the resistance of the layer may be too great to enable satisfactory spot-welds to be easily formed. The thickness of the oxide layer is also preferably at least 15nm as below this level controlled chemical dissolution of the oxide is difficult to achieve.
The anodizing process can be carried out in a number of different electrolytes based on acids such as phosphoric acid and sulphuric acid or other acids in which porous aluminium oxide layers are formed, such as chromic acid or oxalic acid.
The electrolyte may also comprise a mixture of such acids.

` - -1~919~Z
20388-1571~
g _ A preferred electrolyte comprlses from 5 to 15% by weight of phosphoric acid. Phosphoric acid ls capable of strongly attacking the anodic oxlde layer so it is difficult to achieve a balance between oxlde formation and oxide dissolution during the anodlsing process particularly when short anodlsing times are needed to be compatible with existing process lines. With an anodlsing time of 15 seconds or less, the current density used is preferably at least 250 A/m2 and may be as high as can be achieved wlth the equlpment used, eg. up to 3000 A/m2. A preferred current denslty range 18 300 to 1500 A/m2.
As phosphorlc acld attacks alumlnium oxide so readily, lt ls dlfflcult to achieve sufficlent oxlde growth at high temperatures. It has not proved possible to generate an anodic oxlde layer under AC conditions ln a phosphoric acid electrolyte at 90C even wlth a current density of 1250 A/m2. When AC
anodlslng 18 employed, the optlmum electrolyte temperature i8 llkely to be ln the range 30 to 70C. With DC anodlsing, somewhat hlgher temperatures up to 80C may be useful.
Wlth the optlmum conditions described, anodising times a~ ~hort a8 0.5 seconds may be achieved.

1~91~i2 A further advantage of using a phosphoric acid electro-lyte is that the anodic oxide layer formed incorporates signifi-cant amounts of phosphate. Phosphate is known to be a hydration inhibitor with oxide surfaces, and as deterioration of the pre-treated surface often occurs through hydration of the oxide, at least at its surface, the presence of a hydration inhibitor at this point is beneficial.
Because the oxide is readily attacked by the hot phos-phoric acid electrolyte, rapid rinsing of the oxide layer surface is required after anodizing, and this is readily achieved in a continuous coil proce~s.
The result of the phosphoric acid anodizing process is an aluminium sheet carrying a porous anodic oxide layer which contains phosphate ions, the pores of which are enlarged so that the effective surface area of the oxide layer is increased. The oxide layer is generally 15 to 200 nm thick. With an electrolyte contact time of no more than 15 seconds, it is difficult to pro-duce an oxide layer more than 200 nm thick.
Another preferred electrolyte comprises 10 to 30% by weight of sulphuric acid. Sulphuric acid attacks aluminium oxide less readily than phosphoric acid so that such electrolyte will generally need to be more concentrated and at a higher temperature than that used with phosphoric acid in order to maintain a suffi-cient dissolution rate of the oxide. With a sulphuric acid elec-trolyte the anodizing process is preferably effected at a tempera-ture of at least 50C. The optimum electrolyte X

~ 20388-1571D
i2 temperature is in the range 70 to 95C.
With a current density of at least 250 A/m2 and an electrolyte contact time of between 0.5 and 15 seconds, the oxide layer formed generally has a thickness of 300 nm or less.
Conditions similar to those described in UK Patent Specification No. 1235661 which discloses a method of anodizing aluminium sheet in a sulphuric acid electrolyte in preparation for the application of lacquer may also be suitable.
After the anodizing process, press lubricant which is selected to be compatible with the anodizing process used and the adhesive subsequently applied, for instance Houghtodraw 7002 (Trademark) made by Edgar Vaughan Limited, is applied onto the oxide layer. The aluminium sheet is then cut into discrete lengths or is recoiled for ease of storage. Alterna-tively, the aluminium sheet can be cut into discrete lengths before the press lubricant is applied. It is also possible to recoil the aluminium sheet after the anodizing process for storage and to apply the press lubricant after it has been uncoiled again, the sheet being cut into discrete lengths either before or af~er the application of the press lubricant.
The press lubricant is preferably applied by machine, e~g.by spraying or roller coating, to ensure that a uniform coating is formed. As little press lubricant as is necessary for satisfactory forming is used - usually less than 20 grammes/
square metre, and preferably less than 5 grammes/square metre.
A light oil or separating agent such as dioctyl sebacate as used prior to coiling aluminium strip, or lacquer as used in the canning industry, are not suitable as press lubricants.
In some forming operations no press lubricant is required.
Some structure can also be produced without any forming opera-tion.
Having produced discrete lengths of aluminium sheet carrying the oxide layer and press lubricant by any of the routes described above, these are then formed into components of desired shapes. In the case of components for a motor vehicle body, this may involve pressing the sheet between dies and the punching of any holes required. An epoxide adhesive, for instance that produced by Permabond Inc. under the Trade Mark ESP105, is then applied to the components which are assembled together in a jig. The components are then secured together by localized mechanical fastening means, for instance spot-welds, while the adhesive is still fluid. The structure can thus be removed from the jig before the adhesive has cured.
The adhesive is cured for 10-30 minutes at a temperature of 150 to 180 or such other times and temperatures as are suitable for the particular adhesive used. Phenolic or acrylic adhesives can be used in place of the epoxide adhesive.
The anodized aluminium coil or cut sheets can be stored for up to 6 months in many typical storage conditions without any significant deterioration in the oxide layer. The oxide layer is thus capable of providing a sound base for a strong and durable adhesive bond even if the sheet is stored for a 1'~91~ti'~

considerable time between the anodizing process and the application of adhesive. In practice, it is essential that the anodized sheet is storage stable as there is often a delay of at least 48 hours (2 days) and usually more than 168 hours (7 days) between the anodizing of the aluminium sheet at one site, e~g.at an aluminium mill, and the forming of the aluminium sheet into components of desired shape at another site, e.g.in a vehicle production line. The storage stability of the anodized aluminium sheet is, of course, enhanced if the press lubricant is applied before storage.
The aluminium sheet may be degreased before the anodizing process but one advantage of AC anodizing is that it renders 1~91~t~2 the surface of the sheet cyclically anodic and cathodic with evolution of hydrogen at the ~urface. This tends to separate any grea~e or other contamlnatlon from the surface of the sheet so that the contamlnatlon 18 llfted off the surface. Alr agitatlon can also be used to assist in the removal of contamlnation. As mentloned above, the electrolyte may also be passed through the bath in a dlrection opposlte to that of advance of the aluminlum sheet so that any contamlnation in the bath is swept away from the area of the bath where the sheet emerges from the electrolyte.
The press lubricant applied to the oxide layer may be oll, grease or water based. The removal of an approprlately selected press lubricant remaining on the formed components prior to appllcation of the adhesive is not necessary. Indeed, the complete removal of lubricant prlor to applicatlon of the adheslve would be impracticable in a mass productlon llne. The press lubricant may be pushed aside by the subsequently applied adhesive but may also become dispersed withln the adhesive. The press lubrlcant ~hould therefore be compatible wlth the anodl~ed alumlnlum and wlth the adhesive 80 that lt does not unduly affect bond durablllty and strength. The press lubrlcant should also be capable of ready removal prlor to any palntlng operatlon even after belng sub~ected to any elevated temperature at whlch adheslve has been cured.

t;Z

The adhesive used in the joints should be capable of retaining its strength under a wide variety of conditions such as temperature and humidity. The adhesive should wet the surface it is applied to but preferably be such that it does not sag or drip when applied to a vertical surface. Thixotropic materials may thus be preferred. The adhesive may be applied by any suitable method and may be applied to form a layer from about 0.1 to 3.0 mm thick in the final joint depending on joint geometry. The adhesive is preferably sufficiently fluid to be squeezed out of the way at locations in the joint where pressure is applied by a spot-welding tool. It is also possible to use adhesive in a powdered or tape form. The adhesive is not usually applied over the entire surface of the components although this may be convenient when a powdered adhesive is used.
Resistance spot-welding is carried out through the adhesive wllilst this is still in paste form using 5 mm trun-cated cone electrodes. An electrode pressure of 500 pounds t2.2 KN) is held for a time equal to 10 cycles of the electri-cal welding power prior to a 3 cycle weld at 23,000 amps, and is followed by a holding time of 10 cycles to allow the molten slug of aluminium produced to solidify. Adjacent welds are spaced about 6 inches to 1 foot (15 to 30 cms) apart.
It has been found that a structure formed in the manner described above is strong enough to be load bearing and has durable bonds which substantially retain their strength with ti2 time. It will be appreciated that besides holding the structure together when it is removed from the jig, the spot-welds or other localized mechanical fasteners also increase the strength of the joint between the bonded components and in particular increase the peel strength of the joint.
Although it is possible for the spot-welds to be carried out at locations where there is no adhesive, it will generally be found desirable to spot-weld at a position where there is adhesive, the spot-welding being carried out through the adhesive before it has set.
As an alternative to spot-welding, the components may be rivetted together preferably using rivets which do not pierce both of the components so that the seal between the components is not broken. Other forms of localized mechanical fasteners such as those which involve localized mechanical distortion of the components to secure them together, eg.
Tog-L-Lok (Trade Mark) of the BTM Corporation, may also be used.
The aluminium sheet may be an aluminium alloy such as the 2000, 3000, 5000 or 6000 Series of the Aluminium Associa-tion Incorporated Register. The optimum anodizing conditions will generally differ for each alloy and tighter control of the conditions may be required with the 2000 Series than with the others to ensure that a satisfactory oxide layer is produced. It should also be noted that magnesium rich alloys - 17 - 2o388-l57lD
1~19~iZ

of the 5000 Series form an oxide layer containing magnesium oxide (MgO) which is more soluble in acidic electrolytes so a lower temperature may need to be used with such alloys.
Examples of conditions used in the anodizing process will now be given merely by way of illustration.
Figure 1 is a graph showing the failure strength of test strips with respect to time.

lZ919~Z

Panels of 5251 alloy were AC anodized in a 10% by weight phosphoric acid electrolyte at a temperature of 45C and a current density of 600 A/m2 for a period of 10 seconds. The panels were rinsed immediately after the anodizing process.
The panels were then bonded in a perforated lap-shear joint configuration using a toughened epoxy adhesive ESP105 (Trade Mark) produced by Permabond Inc. The initial bond strength was measured and the perforated joints were exposed to a neutral salt spray at 43C for periods of 2, 4, and 8 weeks.
At these intervals, samples were taken and the retention of initial bond strength monitored. As a control, material prepared as in British Patent specification 1555940 was also bonded and tested. This was 5251 alloy, DC anodized at 12V
in 10% by weight phosphoric acid solution for 30 minutes.
Initial bond strengths were identical; after the elapse of 8 week~ the retention of bond strength of the material prepared as described in Example 1 was 71.9% as compared to 70.1% for the DC prepared material. This demonstrates the potential performance of surfaces prepared by anodizing using extremely short pre-treatment times.

- 1~91~

20388-1571~

In the second example, alumlnlum sheet 5251 alloy ls degreased uslng trlchloroethylene vapour. The sheet 18 then subjected to alkallne cleaning using a 10% by volume aqueous solutlon of Oakite NST (Trade Mark) at 50C. The sheet is immersed in this solution for a perlod of 5 mlnutes and then rinsed ln runnlng water for a perlod of 5 minutes. Thls treatment resulted ln a water break free surface. The surface ls then deoxldised uslng a solution comprlslng 25 g/l of pota~slum dlchromate, 50 g/l of sulphuric acld wlth small addltlons of fluorlde, ammonlum, alumlnium, calcium and phosphate ions. A suitable solution ls Deoxodiser No 1 (Trade ~ark) produced by ICI plc. The sheet 18 lmmersed ln thls deoxldislng solutlon for a perlod of 3 mlnutes and then rlnsed in running water for a perlod of 10 mlnutes. Thls removes the pre-exlstlng alr-formed oxlde layer. Next, the sheet ls sub~ected to an AC anodislng process for a period of 1 mlnute ln a 10% by welght aqueous solutlon of orthophosphorlc acld at 20C wlth a current denslty of 80 A/m2. The sheet ls flnally rlnsed ln runnlng water for a perlod of flve mlnutes.
With an anodising time of 1 mlnute this Example 18 appllcable to a relatlvely slow movlng llne.

1~'31~t~

- 20 - 203a8-157]D

Example 3 is similar to Example 2 but the sheet was subjected to an AC anodizing process for a period of 10 seconds in a 10% by weight aqueous solution of sulphuric acid at 90C with a current density of 1200 A/m2.
In order to assess the durability of the adhesive bonds formed in Examples 2 and 3, test strips were pre-treated and then bonded together using Permabond ESP105. A first set (D) of test strips was subjected to an AC anodizing pre-treatment in phosphoric acid as described in Example 2 for 1 minute and a second set (E) to a similar pre-treatment for 2 minutes. A third set (F) was subjected to an AC anodizing pre-treatment in sulphuric acid as described in Example 3 for 10 seconds. A set (C) of control strips were also tested.
The control strips were vapour degreased and alkaline cleaned as described in Example 2 and then deoxidized in a solution comprising sodium dichromate and sulphuric acid in accordance with the Boeing 5555 specification. This involved a DC
anodizing process in a 12% by weight orthophosphoric acid electrolyte at a temperature of 20 to 25~C and at 10 volts for a period of 20 minutes.

Test strips for each of the sets C, D, E, and F were bonded together by the adhesive ESP105 (Trade Mark) as described above. The lap shear strength of these un-perforated t~onds was tcstcd aftcr the tcst striL>s had b~cn cxposcd to salt 1~91~

spray at 43C for 2, 4, 8, 14, 27 and 48 weeks. The results obtained are illustrated by the accompanying graph which shows the failure strength of the un-perforated joints with respect to time for each of the sets. The strength retention after 48 weeks is also shown in Table 1.

1~:91~

INITIAL FAILURE STRENGTH
PRE-TREATMENT FAIL~RE STRESS AFTER RETENTION
STRESS (MPa) 48 WEEKS (MPa) C 21.1 - 0.3 14.6 - 0.7 69.2 . +
D 20.9 - 0 4 12.5 - 0.7 59.8 _ 21.0 - 0.6 13.2 - 0.9 62.9 F 21.0 - 0.5 15.2 - 1.3 72.4 Strength Retention after 48 Weeks Exposure to Salt Spray As will be seen, the strips pre-treated in sulphuric acid compare very favourably with those pre-treated by the BAC 5555 process. The strips pre-treated in phosphoric acid also show strength retention after 48 weeks only slightly lower than that achieved with the Boeing 5555 process. This should be contrasted to strips which have not been pre-treated at all and which would lose all strength within a few weeks. The great advantage of the AC anodizing process as compared to the Boeing 5555 process is that it forms an oxide layer which gives good strength retention and which can be spot-welded through. It would not be feasible to spot-weld through the oxide layer produced by the Boeing 5555 pre-treatment.

?t-.2 Example 4 In Example 4, aluminium sheet of 5251 alloy 0.7 to 2.0 mm thick is AC anodized in an electrolyte comprising 15% by weight sulphuric acid at a temperature of 80C. The charge input at the surface of the sheet is arranged to be 12000 coulombs/M2 which is achieved using a current density of 1200A/M2 for a time of 10 seconds. This anodizing process forms a porous oxide layer about 0.15 microns thick on the surface of the aluminium sheet.

In order to assess bond durability of joints formed on sheet anodized in this way and to make comparisons with other processes a number of tests were carried out.
In the first test, strips cut from 5251 aluminium sheet anodized in the manner described in Example 4 were bonded together and exposed to neutral salt spray for a range of times and the shear strength of the bonds then measured by the perforated lap shear method. Various thicknesses of oxide layer were used and the adhesive ESP105 mentioned above as well as that produced by the 3M Company under the Trade Mark EC2214 were used. Similar test strips were prepared from aluminium sheet which had undergone a conventional Boeing phosphoric acid anodizing pre-treatment process (PAA) for 30 minutes and these were tested in the same manner for comparison.
The results of these testsare shown in Table 2. As will be seen, the ACanodizing pre-treatment gives results similar to those using DC phosphoric acid anodizing according to the Boeing process.

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Tests strips which had been pre-treated by the anodizing process described above and then stored in various conditions before being bonded together were also tested. Three different storage conditions were used:

OD - Office Conditions (dry and relatively warm) CW - Exposure in a deep shelter (cold and relatively humid) HW - Humidity Cabinet (hot and relatively humid) The results of these tests are shown in Table 3. Table 3 shows that the AC anodized pre-treated surface withstands storage in reasonable conditions for at least 6 months without affecting subsequent bond strength but rapidly deteriorates under hot wet conditions. This is similar to results found using conventional phosphoric acid anodizing (not illustrated).
Finally, tests were carried out with bonded test strips being stressed and exposed to a humid atmosphere. These tests were also carried out on strips which had undergone pre-treatment by the Boeing phosphoric acid anodizing process (PAA).
The results of these tests are shown in Table 4. The results for strips with 0.05 microns thick oxide layer produced by AC anodizing and those with a 0.15 and 0.3 microns thick layer are similar to those with the Boeing phosphoric acid anodizing.

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- 27 - ~ ~91~Z

TA~3LE 4 IllMIDITY/STr~ESS TI~TI~ (ADI~SIVE ESP 105) _ _ Time to Initial Sustained Failure Strenath Stress (Days) Pre-treat.~ent (MN/~2) Level . 1st 2nd3rd PAA/30 ,0.1 _ 0.9 5 68 74 78 ~0~.05uAC ) ~ ~ 20.2 + 1.8 5 ~78 96 135 Hbt AC ) ._ _ 0.15u ) ~ 19.8 ' 2.1 5 52 63 78 ) ~ ._ . _ l~ot ~C
0.3u ) ~. 20.~ ' 1.5 55 ~7 68 - 28 - 2038~-1571D
1~1962 Tests have also been carried out on strips whic}l have ~een subjccted to AC ~ osphoric anodizing using a variety of conditions to dctcrminc wheth~r th~y could hc s~ot-woldcd througll satisfactorily. An electrolyte comprising 10% by weight of phosphoric acid was used with a range of current dcnsitics. I'ach of the strii~s was anodizcd for a i)eriod of 2 minutes. The temperature of the electrolyte was 20C but in the first test with a current density of 4000A/m the temperature rose to 40C. This test was thcreforc rcpeated with the electrolyte maintained at a temperature of 10C.
It will be appreciated that as this test was conducted to test thc woldability of anodized strip.s, thc conditions u.sed havc been selected to maximize the thickness of the oxide layer formed and do not necessarily represent preferred conditions for producing strong durable joints. The conditions used and results achieved are summarized in Table 5. As will be seen, all the test strips could be satisfactorily spot-welded together.
A strip anodized in accordance with Example 3 was also tested and could be spot-welded satisfactorily.
The anodizin~ processes described above are believed to remove the air-formed oxide layer from the aluminium sheet and replace this by a new anodic oxide layer.

1~91~i2 Table 5 S~ot-Weldina Strips Anodised in 10% bv Weiq!ht PhosDhoric Acid Current Density Ancdisina Ti~e ¦ Temperature SDct-welded?
~inutes) (oc) (YesjNo) 2 20 Yes 250 2 20 'fes 500 2 20 ~ Yes 4000 2 20 ~ 40 I Yes 00 2 10 ¦ Yes 1~91~

The new anodic layer comprises a non-porous barrier layer portion and a porous ~tructure above thls barrler layer whlch together may have a total thickness of at least 30nm. Different conditions in the anodising process produce differences in the structure and proportions of these two components. The porous nature of the new oxide layer may provide a key to which the subsequently applled adheslve can be securely bonded. An increase in the surface area of the oxide layer thus tends to improve the bond to the subsequently applied adhesive. The porous structure formed by the anodic process is attacked by the acldic electrolyte so the initlal pore structure is enlarged. This again increases the effective surface area of the oxide layer and permits better penetration of the sub6equently applled adhesive into the pores.
It has been found that structures formed in the manner described above can be strong enough to form the structural, load bearing parts of a motor vehicle body. Accelerated tests also indicate that ~uch structures are capable of retainlng adequate strength under the wlde variety of conditions that a motor vehicle generally encounters for a time at least equal to the useful ~ervice life of the vehicle. The anodising processes described may be carrled out much more quickly than many of the pre-treatments u~ed in the prlor art. The anodl~ed sheet can be cut and formed without causlng substantlal damage to the porous oxlde layer, even when the forming of the sheet lnvolves presslng lt between - 31 - 2o388-l57lD

dies, so the oxide layer is still able to provide a base for strong and durable adhesive bonds. As the anodizing process may also be effected before the sheet is cut into discrete lengths it can also be carried out continuously and can be carefully controlled.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Aluminium sheet suitable for forming a structure of press-formed components secured together by adhesive, which sheet has a porous anodic oxide layer formed thereon by having been subjected to an anodising process in an acidic electrolyte for a period of 2 minutes or less, and an adhesive-compatible press-lubricant on the anodic oxide layer.

2. Aluminium sheet as claimed in claim 1 wherein the sheet is in coil form.

3. Aluminium sheet as claimed in claim 1 or 2 wherein the press-lubricant is compatible with epoxy adhesive.

4. Aluminium sheet as claimed in claim 1 or 2 wherein the sheet is from 0.7 mm to 2 mm thick.

5. A process for producing an aluminium sheet suitable for forming a structure of press-formed components secured together by adhesive, which process comprises:

subjecting a sheet of aluminium which may be an alloy thereof to an anodising process in an acidic electrolyte for a period of 2 minutes or less, thereby forming a porous anodic oxide layer formed on the sheet, and applying a press-lubricant onto the anodic oxide layer, the press-lubricant being compatible with the adhesive.

6. A process as claimed in claim 5, wherein the first step comprises:
subjecting the aluminum sheet to a continuous anodising process by passing the aluminum sheet through an acidic electro-lyte bath such that the sheet is in contact with the electrolyte for a period of 0.5 to 15 seconds under appropriate anodising con-ditions to achieve a balance between anodic oxide formation on the aluminum sheet and dissolution of the oxide in the acidic electro-lyte, thereby forming an anodic oxide layer on the aluminum sheet.

7. A process as claimed in claim 6 in which the anodising process is effected by means of AC in an electrolyte comprising phosphoric acid in the range 5 to 15% by weight, at a temperature in the range 30 to 70°C and with a current density in the range 250 to 3000 A/m2.

8. A process as claimed in claim 6 in which the anodising process is effected by means of AC in an electrolyte comprising sulphuric acid in the range 10 to 30% by weight and at a tempera-ture in the range 70 to 95°C.