US3468772A - Anodising treatment for aluminium - Google Patents

Description

United States Patent 3,468,772 AN ODISING TREATMENT FOR ALUMINIUM Peter Geoffrey Sheasby, Banbury, Edward Percival Short, North Aston, and Roman Dominik Guminski, Banbury, England, assiguors to Aluminium Laboratories Limited, Montreal, Quebec, Canada No Drawing. Filed July 8, 1966, Ser. No. 563,691 Claims priority, application Great Britain, July 14, 1965, 29,940/ 65 Int. Cl. C23b 9/02 U.S. Cl. 204-58 6 Claims ABSTRACT OF THE DISCLOSURE In procedure for producing coloured coatings on aluminium by anodising the aluminium in an oxalic acid solution, significantly deeper colours are obtained while providing an efiective anodic oxide coating, by performing the anodic treatment with a pulsed direct current advantageously having stated characteristics, including defined values of the relative peak and means current density and of the pulse frequency, the pulses preferably being of approximately square wave form.

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.

It is well known that for external uses, such as window frames and curtain walling panels for buildings, aluminium alloys are often protected by a hard, low porosity, anodic oxide film to prevent corrosion.

It is also well known that colours can be developed in such films during the anodising process when certain acids are used in the anodising bath, the intensity and shade of the colour being dependent upon a number of factors, including the actual constituents of the aluminium alloy and the composition of the anodising bath. Thus it is known that very dark films are obtained with aluminium alloys having a high silicon content, and that bronze shades are obtained with many other aluminium alloys when the anodising is performed in a bath containing certain sulphonated organic acids, such as sulphophthalic acid and sulphosalicylic acid. Although the processes using these acids produce strongly coloured anodic oxide films, nevertheless they are expensive to operate because of the high cost of the electrolyte and because certain expensive auxiliary equipment, such as ion-exchanger columns, are found to be necessary to avoid high electrolyte loss.

The use of oxalic acid as the electrolyte in an anodising process has already been described. With 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.

It is well known that the hardness of 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.

To ensure the production of anodic oxide films of this character it is found necessary to control the temperature ICC of the acid anodising bath to a temperature of 30 C. at most; 20 C. being the temperature commonly used.

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. Furthermore, 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.

According to the present invention, as compared with normal direct current anodising, 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.

Although 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.

It is found very desirable to avoid any negative component in the pulsed current, because this leads to soft anodic films.

From studies which we have carried out, we have concluded that the colours developed in anodic oxide films formed in organic electrolytes are mainly due to optical interference effects arising from the, presence of fine particles dispersed in the anodic aluminium oxide. We have postulated that the colours developed in anodic films produced in baths containing oxalic acid are due, at least in part, to carbon particles produced by decomposition of the oxalic acid component of the anodising bath. We have further postulated that such decomposition is due to the development of localised high temperatures, resulting from high resistances in the oxide film at the bottom of the individual pores.

On the basis of these hypotheses it seems probable that deepening of the colour developed in the anodic oxide film is produced if such localised high temperatures are momentarily increased by pulsing the current, because during the pulses the current density at the surface of the aluminium is increased. Although it might be thought that the same result could be obtained by increasing the current density in a standard constant D.C. anodising process the greater amount of heat generated at the surface of the metal results in the metal itself becoming overheated and the occurrence of local burning of the anodic aluminium oxide film.

It is however found that 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:

It is found that 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 iiii Zii$$$$ as to have as if E iiiwere i uce ex ruszons in is a oy. square a Wave form as possible and the maximum value 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? at the desired operating temperature, using refrigeration t0 the e h e dehslty 1S Preferably t e that and temperature controls normally available in a DC. P'i t In e range l 1 3;? lhet t g oxal1c ac1d anodising tank for the production of hard, z el g l t a r 13 iieo f g yco ege i e ive l; corrosion resistant anodic oxide films for external use. 1e e e m SPaee T 1 0 0 h e No abnormal signs of electrolyte deterioration were dehettel results, the cost of the heeessery eqhlpmeht to P tected and the electrolyte has substantially the same service 2 g y gg gz g gg s g 1S Probably so great life as the oxalic acid electrolyte in the DC. oxalic acid I r 1 u e anodisin rocess.

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 duced 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. cce rate density leads development of darker Polours corrosion test s hav e shown no difference in corfosion z gz q f g gi 22 352212 21 ggfigig range 15 resistance between anodic oxide films obtained using In one series of tests the DC. current-pulses were $15: Dc and Smooth other vanable bemg the generated by using thyristors in a three-phase half-wave rectifier system, the firing of the thyristors being coni f 6 2 5 i by R is trolled by means of a variable mark/space ratio pulser, 40 i ar m mate at t e co ours ave Sans which controls the time interval during which the thyris toryhg asmess' tor remains conductive, the pulser also having a variable Accordmg to a further dftvelopment of the mlfentlon repetition period. it has been found possible in some cases to obta n very It was found that a considerable rang: of colours dark colours in anodic films produced in an oxalic acid could be obtained on aluminium and aluminium alloys, t 15mg 3 relatlvely small Peak current/mean current when anodised in oxalic acid solution, by variation of the l'atlo of Order P 611 y treatment the metal t mark/space ratio and repetition period (pulse fredevelop the formation of fine particles which survive 1n quency). In addition to those factors, the colour obtained the anodic xide film. with an anodic film of a given thickness, depends on the In the case of the alloy HE9, referred to above, good alloy selected for treatment, 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. and that tha n 1s required for the development of the optimum the aqueous electrolyte should contaln at least 8%doxa1l1c physical propel.tigcs The Overageing heat treatment is f and Shou d prefefab y Commute? e f lieved to increase the development of particles of the tion. In accordance with known pract1ce, 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. and a mean current density tamed 1f g g 15 carried ut at 220i5 C. for 10 of 24 amps/sq. ft., the following process conditions hours. Although this somewhat decreases the physical were used: properties of alloy extrusions, it is not to such extent Mark/ Peak Peak Voltage space current Pulse frequency ratio, density Alloy Alloy Time lfilm (cycles/second) percent (a./sq.ft.) HE9 H1320 (ruins) (microns) The first conditions given produced the darkest colour that the extrusions become unacceptable on strength on all alloys tested. The colours became progressively 75 grounds. These treatment conditions have been selected .5 since they are foundto produce reproducible colours, in spite of minor operational changes of the treatment conditions.

The use of overaged" material allows the process to be operated with a less expensive source of pulsed D.C.

current, capable of a maximum peak'current/rnean cur- 5 rent ratio of 6: 1.

Using such apparatus and anodising to a film thickness of 25-35 microns, generally considered necessary for external application of aluminium in the English climate we have produced the following range of colours on HE9 10 alloy overaged at 220 C. for 10 hours.

Mean Approxi- Peak to current Anodicfilm Anodising Electrolyte at Sample mean curdensityin thickness time in temperature Peak mean ak No. rentratio amps/sq. it. inmicrons min. 111C. voltage vo tage Colour 6:1 24 35 60 20 115-160 137 Black. 6:1 25 55 105-145 125 Brown-bronze. 3:1 15 55 20 90-130 110 Bronze. 6:1 24 25 105-155 130 Brown-bronze. 6:1 15 25 25 100-135 117 Bronze. 3:1 15 25 55 25 -120 Light bronze. 6:1 24 35 50 30 100-125 112 Brown. 6:1 15 25 55 30 90-120 Gold-brown. 3:1 15 25 55 30 Gold-bronze.

The following table shows the mechanical properties of HE9 alloy averaged under varying conditions.

We have so far exemplified the process with reference to anodising baths containing oxalic acid alone.

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. Alternatively a metal salt, such as ferrous sulphate, may be used for the same purpose. 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 table below summarises the results Obtained using the pulse current anodising technique in various mixed electrolytes containing 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.

All percentages for the electrolyte components are on a Weight to volume basis.

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 wave form of each current pulse is approximately square and the ratio of anode peak current density/anode mean current density is 36:1.

2. A process for producing a coloured anodic oxide coating on aluminium (including aluminium alloys) 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.

3. A process according to claim 2, in which the wave form of each current pulse is approximately square.

4. A process according to claim 3, in which the mean anode current density is 10-36 .amperes per square foot.

'5. A process according to claim 3, in which the bath is a substantially saturated aqueous solution of oxalic acid.

6. A process according to claim 3, in which the material to be anodised is an aluminium alloy which consists essentially of about 0.6% Si" and about 0.45% Mg, balance aluminium and impurities, and for which a normal ageing treatment is heating at C. for 8 hours, said process further comprising, as a step preceding the aforesaid anodic treatment, subjecting the aforesaid alloy to a heat ageing treatment which in at least one, of the respects very patchy film.

It will be seen from these results that a darkening of the film is obtained by the use of the pulsed current technique only in those cases where the character of the an- 75 of temperature and time exceeds the aforesaid normal ageing treatment.

(References on following page) 7 8 References Cited FOREIGN PATENTS UNITED STATES PATENTS 716,554 10/1954 Great Britain.

1,735,286 11/1929 Kujirai et a1 20458 JOHN MACK, Primary Examiner 2,920,018 1/ 1960 Miller 204-56 5 R. L. ANDREWS, Assistant Examiner UNITED STATES PATENT OFFICE CILRTLFICATE OF CORRECTIUN Patent 3,468, 772 Dated September 23 1969 PETER GEOFFREY SlIEASBYf EDWARD PERCIVAL SHORT and Inventor(s) DQMIAII'K GHMINSKI it is certified that error appears in the above-identified PGLGUC and that said Letters Patent are hereby corrected as shown below:

Column 4, line 10, "Cu. 25%; Cr, 25%" should read --Cu 0.25%; Cr 0 2 S L slum AND SEALED -Em a orr mmmx.-. m. L, 1m Oomissiom of Patents-