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
  • C25D11/14—Producing integrally coloured layers

Definitions

• the present invention relates to the production of anodic oxide films on aluminium, including aluminium alloys and in particular it relates to a method of producing coloured anodised films.
• aluminium alloys are often protected by a hard, low porosity, anodic oxide film to prevent corrosion.
• oxalic acid as the electrolyte in an anodising process has already been described.
• oxalic acid anodising using direct current, a range of attractive colours can be developed in the anodic oxide film on certain aluminium alloys, but these are not of the same order of intensity (except on certain silicon-containing alloys) as is obtained with the sulphonated organic acids referred to above, operating under the same conditions of current density on the same alloys.
• anodic oxide films depends to a large degree on the temperature at which the anodising process is performed.
• Aluminium, bearing an anodic oxide film which has become coloured during the anodising process, is mainly used for architectural purposes, such as window frames and shop front fittings produced from extruded aluminium sections. For these purposes it is important that the anodic oxide film should be hard and have low porosity, so that it may provide prolonged protection for the metal.
• High current densities are employed in anodising baths and thus much heat is developed in the electrolyte.
• the temperature, to which the electrolyte can be held depends upon the associated equipment for cooling the electrolyte and in practice, the cooling capacity of such equipment controls the total current capacity of an anodising bath of a given size. Beyond a certain size the capital cost of the associated equipment for cooling the electrolyte becomes prohibitive and thus the output of an anodising bath in terms of area of anodised aluminium is a function of the current density.
• the current density is also limited by the fact that under D.C. anodising conditions the heat produced in the pores must be dissipated to avoid burning of the film.
• significant darkening of the colours developed in anodic oxide films may be achieved if the direct current applied to an anodising bath containing oxalic acid is pulsed without change of the average current density, so that during each pulse the maximum current density is appreciably increased as compared with operation under steady direct current conditions.
• the heat developed in the anodising bath is not much greater than if a steady direct current were passed, but the depth of the colour developed is increased on the aluminium-magnesium-silicon alloys conventionally employed in the production of extrusions for external use.
• the anodising bath may contain mineral acid, salts of mineral acids or monocarboxylic acids to improve conductivity, oxalic acid must form the major proportion of the solute.
• One of the principal advantages of this method is that it permits a whole range of colours to be developed in an anodic oxide film of a given thickness by variation of the mark/ space ratio of the pulses i.e. the ratio of the period during which the current flows to that Where it is zero or substantially zero, without changing the average current density.
• the thickness of the anodic oxide film is a function of the product of the average current density and the treatment time.
• the darkening of the colour developed in the anodic oxide film is not particularly significant unless the pulse frequency is less than a maximum value and the mark/ space ratio of the pulses is below a maximum value. Thus it is believed that these conditions are required so that the increased current density at the metal surface is maintained during each pulse for a sufficient time interval to enable the local lighter as the peak current density was decreased.
• the time required to produce an anodic film of desired thickness varied a little from alloy to alloy, but the above time figure is typical.
• Alloys HE9 and HEZO are aluminium alloys conventemperature at the bottom of the pores to increase sig- 5 tionally used for production of extrusions for architecturntificgntly abotvleD Ehe tergperature 1n th1s region during ?lfi1sage and have a nominal composition respectively as s ea y curren ano ismg. o ows:
• the maximum frequency of the p ls HE9Si, 0.6%; Mg. 0.45,- balance Al and impurities. is of the order of 50 pulses per second and preferably 10 HE204i, 05%; 10%; 25%; Cr, 25%; the pulse frequency is much lower than this, very satisance A1 and impurities t e y results being obtained when the Pulse frequency Alloy HEZO cannot be extruded at the same high rate l ri iirliii tii iii Zii$$$$$$ as to have as if E iiiwere i uce ex ruszons in is a oy.
• the peak current density employed can be as high as at lihemarl/slpace rgtrofills about 50% g pft l l ge t 500 amps/sq. foot without burning the anodic oxide sta ens; at; that: antenna, :55: as; We If a smoo current were app ie t e y h expressed alternatively y seylhg that the No difiiculties were experienced keeping the electrolyte mlnllhllm Yatlo Of the P e l h dehslty il a l?
• the minimum mean anode current density is about 10 physicga? examination of the anodic oxide films amps/ square foot, with a maximum mean current density pared by the use of Pulsed DO curmm with an oxalic of about amPS /Square foot- Above that value the acid anodising bath, operated under the conditions set t t g g fi t g fi e h gg i forth above, shows no substantial difference, except colon r, $2 1; :ich ie v d Altl' rorf li t iie u e (if hi hei' current be-tveen E filn-ls and thosehpgxglced by z g e e aci ano 1s1n usm a smoot current.
• the mean current density results can be obtained by overageing, that is to say by applied and the temperature of the electrolyte carrying out the normal artificial ageing heat treat- It is preferred that the temperature of the oxalic acid ment f a longer i d/ at a hi h temperature electrolyte should be in the range of l5-3() C.
• Mgzsitype it is desirable Mgzsitype to apply a reduced voltage at the beginning of the anodisa- 1, tion process to avoid uneven colour distribution; applicaa i i z; for i 2 i heat; tion of full process voltage at the beginning of the treatmg at fi y slgfn f H ar F i o ment leads to greater intensity of colour around the edges 60 the rfsultant ozude 1S obtanted If t e a CY 15 0V6! of the piece than in the middle of the Surface aged for a sultable tlme at a higher temperature, when In a series of tests carried out using a saturated oxalic helhg Pulsed euflzehtsatlstaetofy results are acid anodising bath at 20 C.
• overaged material allows the process to be operated with a less expensive source of pulsed D.C.
• Oxalic acid may have employed in conjunction with it a small proportion of another acid, either an inorganic acid, such as sulphuric acid, or an organic acid, such as formic acid, to improve the conductivity of the electrolyte.
• an inorganic acid such as sulphuric acid
• an organic acid such as formic acid
• a metal salt such as ferrous sulphate
• Such addition in the case of sulphuric acid or ferrous sulphate is not in excess of 1% and preferably much less so as not to interfere with the type of film formed by anodising in oxalic acid.
• the same HE9 alloy was used for all the experiments and the anodising conditions used were kept constant at a mean current density of 24 a./sq. ft. and a time of 40 55 mins. A peak/mean current density ratio of 6:1 was used for the pulsed conditions.
• a process for producing a coloured anodic oxide coating on aluminium comprising suspending the material to be coated as an anode in an aqueous electrolyte bath containing a solute of which a major proportion is oxalic acid and passing through said bath a pulsed direct current, in which the pulse frequency is 1 /z-20 pulses per second and the ratio of anode peak current density to anode mean current density is at least 3 1.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electroplating And Plating Baths Therefor (AREA)
• Printing Plates And Materials Therefor (AREA)
• Electroplating Methods And Accessories (AREA)
• Electrochemical Coating By Surface Reaction (AREA)

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Citations (3)

• 1965
  • 1965-07-14 GB GB29940/65A patent/GB1150882A/en not_active Expired
• 1966
  • 1966-07-08 US US563691A patent/US3468772A/en not_active Expired - Lifetime
  • 1966-07-11 DE DE1496718A patent/DE1496718C3/de not_active Expired
  • 1966-07-13 SE SE09579/66A patent/SE338220B/xx unknown
  • 1966-07-14 JP JP41045723A patent/JPS5012379B1/ja active Pending

Patent Citations (3)

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