Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/04—Anodisation of aluminium or alloys based thereon
  • C25D11/06—Anodisation of aluminium or alloys based thereon characterised by the electrolytes used
  • C25D11/08—Anodisation of aluminium or alloys based thereon characterised by the electrolytes used containing inorganic acids

Definitions

• This invention relates to a method of fabricating structures from aluminium sheet and structures comprising aluminium components.
• aluminium as used here and throughout the specification is intended to include aluminium alloys.
• 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.
• aluminium structures have also been proposed for aluminium structures to be "weld-bonded", that is bonded with adhesive but also spot-welded.
• a method of fabricating structures from aluminium sheet which comprises the steps of subjecting the aluminium sheet to an anodising process in an acidic electrolyte for a period of 2 minutes or less to form an anodic oxide layer thereon; producing components of a desired shape from the aluminium sheet; and securing two components together by adhesive and by localised mechanical fastening means.
• a structure comprising aluminium components which are secured together by adhesive and by localised mechanical fastening means and in which the components have an anodic oxide layer formed thereon by being subjected to an anodising process in an acidic electrolyte for a period of 2 minutes or less.
• the invention enables structures to be fabricated from aluminium components which have undergone a rapid pre-treatment but which 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 that the anodising 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.
• 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 8djacent 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.
• the aluminium sheet is in the form of a coil for the advantage of rapid anodising and for convenience of storage and transport.
• 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.
• Existing aluminium treatment plants generally have a line speed of at least 50 m/min, and often of 150-250 m/min.
• 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 anodising times.
• anodic oxide formation on the aluminium sheet During the anodising 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 anodising current density used.
• Anodic oxide growth per unit time is substantially proportional to anodising current 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/m 2 .
• the total anodising input will usually be in the range 2 ⁇ 10 4 to 12x10 , particularly 3 ⁇ 10 4 to 6 ⁇ 10 4 , C/m 2 .
• Film attack is essentially controlled by the nature, concentration, and temperature of the electrolyte, with temperature being the nost important factor.
• 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 anodising electric current is preferably AC so that the aluminium sheet is alternately anodically polarised (during which time oxide growth predominates) and cathodically polarised (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.
• 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, eg at 35°C, the voltage is about 40V for 600 amps/meter 2 . The voltage is reduced as the temperature goes up.
• the temperature of the electrolyte is preferably at least 25 0 C for short anodising 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 electrolyte temperature is too high, then chemical dissolution may outpace oxide growth to the extent that all oxide is redissolved 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 acid is found preferable in order to achieve rapid anodising.
• the oxide layer formed on the aluminium sheet by the anodising 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 otherwise 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 anodising process can be carried out in a number of different electolytes 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.
• a preferred electrolyte comprises from 5 to 15% by weight of phosphoric acid.
• Phosphoric acid is capable of strongly attacking the anodic oxide layer so it is difficult to achieve a balance between oxide formation and oxide dissolution during the anodising process particularly when short anodising times are needed to be compatible with existing process lines.
• the current density used should preferably be at least 250 A/m 2 and may be as high as can be achieved with the equipment used, eg up to 3000 A/m 2 .
• a preferred current density range is 300 to 1500 A/m 2 .
• a further advantage of using a phosphoric acid electrolyte is that the anodic oxide layer formed incorporates significant 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.
• the oxide is readily attacked by the hot phosphoric acid electrolyte, rapid rinsing of the oxide layer surface is required after anodising, and this is readily achieved in a continuous coil process.
• the result of the phosphoric acid anodising 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 produce 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 the electrolyte is generally more concentrated and at a higher temperature than that used with phosphoric acid in order to maintain a sufficient dissolution rate of the oxide.
• the anodising process is preferably effected at a temperature of at least 50°C.
• the optimum electrolyte temperature is in the range 70° to 95°C.
• the oxide layer formed With a current density of at least 250 A/m 2 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.
• press lubricant which is selected to be compatible with the anodising 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.
• 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 anodising 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 after the application of the press lubricant.
• the press lubricant is preferably applied by machine, eg 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.
• 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 localised 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 anodised 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 considerable time between the anodising process and the application of adhesive.
• it is essential that the anodised 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 anodising of the aluminium sheet at one site, eg at an aluminium mill, and the forming of the aluminium sheet into components of desired shape at another site, eg in a vehicle production line.
• the storage stability of the anodised aluminium sheet is, of course, enhanced if the press lubricant is applied before storage.
• the aluminium sheet may be degreased before the anodising process but one advantage of AC anodising is that it renders the surface of the sheet cyclically anodic and cathodic with evolution of hydrogen at the surface. This tends to separate any grease or other contamination from the surface of the sheet so the contamination is lifted off the surface. Air agitation can also be used to assist in the removal of contamination.
• the electrolyte may also be passed through the bath in a direction opposite to that of advance of the aluminium sheet so that any contamination 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 oil, grease or water based.
• the removal of an appropriately selected press lubricant remaining on the formed components prior to application of the adhesive is not necessary. Indeed, the complete removal of lubricant prior to application of the adhesive would be impracticable in a mass production line.
• the press lubricant may be pushed aside by the subsequently applied adhesive but may also become dispersed within the adhesive.
• the press lubricant should therefore be compatible with the anodised aluminium and with the adhesive so that it does not unduly affect bond durability and strength.
• the press lubricant should also be capable of ready removal prior to any painting operation even after being subjected to any elevated temperature at which adhesive has been cured.
• 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 whilst this is still in paste form using 5 mm truncated cone electrodes.
• An electrode pressure of 500 pounds (2.2 KN) is held for a time equal to 10 cycles of the electrical 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.
• spot-welds 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 throught the adhesive before it has set.
• 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 localised mechanical fasteners such as those which involve localised 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 Association Incorporated Register.
• the optimum anodising 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.
• magnesium rich alloys 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.
• MgO magnesium oxide
• Panels of 5251 alloy were AC anodised in a 10% by weight phosphoric acid electrolyte at a temperature of 45°C and a current density of 600 A/m 2 for a period of 10 seconds. The panels were rinsed immediately after the anodising 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 43°C 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 anodised at 12V in 10% by weight phosphoric acid solution for 30 minutes.
• aluminium sheet 5251 alloy is degreased using trichloroethylene vapour.
• the sheet is then subjected to alkaline cleaning using a 10% by volume aqueous solution of Oakite NST (Trade Mark) at 50°C.
• the sheet is immersed in this solution for a period of 5 minutes and then rinsed in running water for a period of 5 minutes. This treatment resulted in a water break free surface.
• the surface is then deoxidised using a solution comprising 25 9/ 1 of potassium dichromate, 50g/1 of sulphuric acid with small additions of fluoride, ammonium, aluminium, calcium and phosphate ions.
• a suitable solution is Deoxodiser No 1 (Trade Mark) produced by ICI plc.
• the sheet is immersed in this deoxodising solution for a period of 3 minutes and then rinsed in running water for a period of 10 minutes. This removes the pre-existing air-formed oxide layer.
• the sheet is subjected to an AC anodising process for a period of 1 minute in a 10% by weight aqueous solution of orthophosphoric acid at 20°C with a current density of 80 A/m 2 .
• the sheet is finally rinsed in running water for a period of five minutes.
• this Example is applicable to a relatively slow moving line.
• Example 3 is similar to Example 2 but the sheet was subjected to an AC anodising process for a period of 10 seconds in a 10% by weight aqueous solution of sulphuric acid at 90°C with a current density of 1200 A/m 2 .
• test strips were pre-treated and then bonded together using Permabond ESP105.
• a first set (D) of test strips was subjected to an AC anodising 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 anodising 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 deoxidised in a solution comprising sodium dichromate and sulphuric acid in accordance with the Boeing 5555 specification. This involved a DC anodising process in a 12% by weight orthophsphoric 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 bonds was tested after the test strips had been exposed to salt spray at '43°C 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.
• 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 anodising 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.
• aluminium sheet of 5251 alloy 0.7 to 2.0 mm thick is AC anodised in an electrolyte comprising 15% by weight sulphuric acid at a temperature of 80°C.
• the charge input at the surface of the sheet is arranged to be 12000 coulombs/M 2 which is achieved using a current density of 1200A/M2 for a time of 10 seconds.
• This anodising -process forms a porous oxide layer about 0.15 microns thick on the surface of the aluminium sheet.
• Table 3 shows that the AC anodised 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 anodising (not illustrated).
• each of the strips was anodised for a period of 2 mintues.
• the temperature of the electrolyte was 20°C but in the first test with a current density of 4000A/m 2 the temperature rose to 40°C. This test was therefore repeated with the electrolyte maintained at a temperature of 10°C. It will be appreciated that as this test was conducted to test the weldability of anodised strips, the conditions used have been selected to maximise 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 anodised in accordance with Example 3 was also tested and could be spot-welded satisfactorily.
• the anodising process described above are believed to remove the air-formed oxide layer from the aluminium sheet and replace this by a new anodic oxide layer.
• the new anodic oxide layer comprises a-non-porous barrier layer portion and a porous structure above this barrier layer which 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 applied adhesive can be securely bonded. An increase in the surface area of the oxide layer thus tends to improve the bond which can be made to the subsequently applied adhesive.
• the porous structure formed by the anodic process is attacked by the acidic electrolyte so the initial pore structure is enlarged.

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• Chemical & Material Sciences (AREA)
• Metallurgy (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Inorganic Chemistry (AREA)
• Organic Chemistry (AREA)
• Laminated Bodies (AREA)
• Adhesives Or Adhesive Processes (AREA)
• Resistance Welding (AREA)
• Motor Or Generator Frames (AREA)
• Casings For Electric Apparatus (AREA)
• Pressure Welding/Diffusion-Bonding (AREA)

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• 1984
  • 1984-11-05 GB GB8427981A patent/GB2167443B/en not_active Expired
• 1985
  • 1985-10-30 IN IN869/MAS/85A patent/IN166522B/en unknown
  • 1985-11-01 AU AU49265/85A patent/AU583315B2/en not_active Ceased
  • 1985-11-01 EP EP85307939A patent/EP0181168B1/de not_active Expired - Lifetime
  • 1985-11-01 DE DE8585307939T patent/DE3576710D1/de not_active Expired - Lifetime
  • 1985-11-04 CA CA000494491A patent/CA1279842C/en not_active Expired - Lifetime
  • 1985-11-04 BR BR8505513A patent/BR8505513A/pt not_active IP Right Cessation
  • 1985-11-04 KR KR1019850008188A patent/KR930000470B1/ko not_active IP Right Cessation
  • 1985-11-04 ES ES548506A patent/ES8701858A1/es not_active Expired
  • 1985-11-05 JP JP60246441A patent/JPS61179897A/ja active Pending
• 1987
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• 1994
  • 1994-07-11 JP JP6180468A patent/JP2727299B2/ja not_active Expired - Lifetime

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