CA2415556A1 - Method for treating the surfaces of aluminium or aluminium alloys by means of formulations containing alkane sulfonic acid - Google Patents

Abstract

The invention relates to a method for treating the surfaces of aluminium or aluminium alloys by anodically oxidising the aluminium or aluminium alloys in an electrolyte, the electrolyte containing 3 to 30 wt. % of an alkane sulfonic acid. The invention also relates to the use of aluminium or aluminium alloy workpieces produced according to said method, in the building trade, in car or aeroplane construction and in packaging. The invention further relates to an electrolyte composition for the anodic oxidation of aluminium or aluminium alloys, wherein the electrolyte contains 3 to 30 wt. % of an alkane sulfonic acid, as well as the use of an alkane sulfonic acid in a method for the anodic oxidation of aluminium or aluminium alloys in order to increase the speed of the anodic oxidation and to reduce the required energy consumption for the process.

Description

METHOD FOR TREATING THE SURFACES OF ALUMINIUM OR
ALUMINIUM ALLOYS BY MEANS OF FORMULATIONS
CONTAINING ALKANE SULFONIC ACID
The im~ention relates to a prdcess for the surface breaboaant of ah~minum or aluminum alloys by anodic oxidation of the alunainuzu oz aluminum alloy (anodization) and to the use of an. alkanesul~o~oac acid in a process for the anodic oxidation of aluminum or alumiauxu alloys, au electrolyte composition for the eaodic oxidation of aluminum or aluminum alloys and the use of workQieces based on ahuninum or aluminum alloys and produced by ~e process of the present invention.
In. air, bare alwuoz~aww very quickly becomes covered with a very thin oxide skis which gives it a higher corrosion resistance than would be expected, on the basis of its staadac~d potential of -1.69 'V. The corrosion resistance can be increased further by thickening the natural oxide skin. by chemical or electrochtaoaical methods. The thickened oxide skin is absorbent, so that it can be colored using water soluble dyes or dye precursors.
Furthermore, the oxide surfaces offer an excellent base for adhesion of paints and the abrasion resistance of workpieces is increased by anodic surface oxidation The surface oxidation of the alumi~amoa surface or the surfacx of ale alloys can be carried out by electrocbepodcal ~oaeans by dipping the workpieces into solutions of slightly aggressive age~ats oz by chromating and phosphating.
However, anodic oxidation by electrochennical. means (anodization, eloxal process) is generally more advantageous, since thicker oxide coatings can be obtained in this way than by chemical treatment.
The most frequently used processes employ sul~~uric acid (S), oxalic acid (X~
or chromic acid solutions as electrolyte. Exclusively direct curncnt is used in the chromic acid process;
while the sulfuric acid and oxalic acid processes are car~.ied out using either direct entreat (DS or DX process) and using alternating current (AS or AX process). Xt is also possible to usa a mixture of sulfuric acid and oxalic acid (DSO pmc~ss). It is therefore of some relevance that the mixtmte can be used at higher bath temperattaes (22-24°C) than can an electrolyte based on puze sulfuric acid (18_22°C). In these processes, the thickness of the oxide layer is from about I O to 30 Vim.

At low temperatures (up to about +IO°C, preferably from 2 to 3°C), high current densities (up to 2.5 A/dmz) and generally low sulfu~ci~c acid concentrations (up to about IO% stcengtb;
by weight), if desired in ~e with phosphoric acid, very bard, abrasion msistaat oxide layers are obtained (hard anodizing). Here, a thiclmess of the oxide layer of >50 hem can be achieved. These workpieces obtained by hard anodization are used, in particular, for aluminum, pressure castings, e.g. for engine construction. There is a maximum achievable layer thickness, which in the case of the DS pxooess, for example, is about 45 Vim. At this maximum layer thielcaess, the dissolution rate of the alurni.num oxide is equal to its fozmation rate.
rn. addition, there are fa~,er specific anodic oxidation processes, e.g.
aluminum coil coating (for can manufacture) which is generally carried out by passing an altmamum ship through a sulfuric acid electrolyte. Here, layer thicknesses of from 2 to 3 Eraz are desired.
It is an object of the present invention to provide an anodizatiozt process for aluminum or aluminum alloys which is faster than the classical processes of the prier art and also gives a beroter current yield, i.e. sugars from lower energy losses due to cooling.
This process should be suitable both for aaodi~,tion by dipping and for continuous anodization, e,g. of strip or wire by means of an electrolytic pull-through process. Furthermore, the process 2 0 should, in hard anodi~aon, make it possi.'b1e to achieve a greater maximum layer thickness than is possible using the processes of the prior art, e.g. the DS process.
We leave found that this object is achieved by a process for the sur~fa~ce treabmerrt of aluminum or aluminum alloys by anodic oxidation of the aluminum or th,e aluminum alloys (anodization) in an electrolyte containing from 3 to 30% by weight of an alkanesuJ.fonic acid.
The electrolyte preferably contains from 10 to 30% by weight, particularly preferably From 10 to 2S% by weight, of as atkaaesulfonic acid. In addition, the electrolyte may farther 3 0 comprise other acids, in particular acids selected from among sulfinic acid, phosphoric acid and oxalic acid. ht a preferred embodiment, the electrolyte comprises sulfuric acid in addition to an allcaucsuLfonic acid Zn a further preferred embodiment, an elecixolyte based exclusively on an alkauesulfonic acid is used.
3 5 The use of alkanesuTfonic acids in the surface treatment of aluminum ox aluminum alloys is already known, from the prior art. Howe~xe~, these lmown processes concern essentially the use of alkanesul~onic acids in the electrolytic ~ooe~tal salt coloring of aluminum, where an alkanesulfonic acid is used as additive or basis of an acid electrolyte solution, 2nd not the use of atlraaesulfoazc acid in anodie oxidation (anodization) of aluminum or an aluminum alloy.
Thos, US 4,128,460 relates to a process for colox~ng aluxuinum or aluminum alloys by electrolysis, comprising the aaodization of alumimtm or the aluminum alloys by customary methods and subsequent electrolysis in a bath comprising ~. aliphatic sulfonic acid and a metal salt, is particular a tin, copper, lead or silver salt, of the sulfonie acid. According to US 4,22$,460, the stability of the electzolysis bath is increased by an increased oxidation stability of the metal salts used and a uniforra coloration of the surface of the aluminum or the aluminum alloys achieved.
The Brazilian patent applications BR 91001174, SR 9501255-9 and BR 9501280-0 also relate to processes for coloring the eloxi~ized aluminum by electrodippipg, using electrolytes and metal salts which are mainly composed of pure m~ethanesulfonic acid, 7.5 u~ethanesulfonates of tin or copper or methanesulfonates of nickel, Iead or other salts.
According to these paxent applications, an increase in the specific eleeixical conductivity of the solution, a reduction in the time for coloring in a simple manner and with reliable control, reproducibility of the color shade and low operating costs are achieved in this way.
2 0 dnly BR 9501255-9 discloses specific reaction conditions for anodizatiton of the surface of aluminum, wvith the use of metbaacsuifonic acid as additive in an electrolyte based on sulfuric acid being mentioned. In this electrolyte, n~sl~~an~.esulfonic acid is used in an amount of 10 parts by weight based on sulfuric acid, i,e. less than 2% by weight of the electrolyte. No further indication of the use of alkanesulfonic acids in the anodization step 2 5 or advantages of such a use are disclosed in BR 9501255-9.
According to the present invention, it has been found that use of alkanesulfonic acids as basis of the electrolytes used in the anodi2aiion step leads to more rapid azzodization than when using the methods of the prior art. This is also of critical importance in xespect of 3 0 subsequent electrolytic colouca~on o~the anodized surface, since the aaodization is the raxe determining step in such a two-stage process comprising anodization and subsequent coloa~ation of the anodized surface. The anodization step is, depending on the color of the surface, from 5 to 50 times slower than the subsequent coloration step.
Increasing the rate of the anodization step thus xnal~es the process more economical since higher throughputs 3 5 per unit time eau be achieved.
The electrolysis time for achievi~qg an almainxma oxide layer thickness optimum for a subsequent coloration step, which is generally from 10 to 30 ~Cm, preferably from 15 to t 25 Vim, is gcnerally from. 5 to 40 minutes, preferably from 10 to 30 minutes, with the precisc time bcing dependent, inter alia, on the current density.
Furthermore, alkanesulfonic acids ba.~re a significantly Lower cozzosive action on the aluminum oxide layer foamed in the anodization than does, for example, the sulfuric acid customarily employed. The process of the present invention thus makes it possible, particularly in hard anodization, to achieve greater layer thicknesses in a shorter time than when using the processes of the prior art.
A further great advantage of the process of the present i~avention is the sign~cautly Lower energy consumption during anodization, since a sig~nifcantly lower voltage compared to the pare st~l~ric acid eleclmlyte is established at the same cuxrez~t. ,A,s a consequence, the energy required for cooling the anodization bath is significantly Lower.
'fhe process of the present invention is suitable both, for anodizaxion of a3.u~ninum or aluminum alloys by the electtodipping process and for continuous anodization, for example of strip, pipe or wire, by means of an electrolytic pull-through.
process, e.g. for producing aluminum sheets for can manufacture.
2 0 'The process of the present invention can be operated either using direct current or using alternating current; the process is preferably carried out using direct content.
In. addition to the allcanesulforitC acid, the electrolyte can furtlies comprise other acids, for examplc sulfiaic acid, phospho~c acid or oxalic acid. Ix~ a preferred embodiment of the process of the present iave~oa, the electrolyte comprises either an alksaesuLfonic acid or a mixhue of sulfuric acid end alkanesulfonic acid as only acid. The elettrolytc preferably comprises from 20 to 10U parts by weight of an alkanesulfonic acid and from 80 to 0 parts by weight of a further acid selected from among sulfuric said, phosphoric acid and oxalic acid, where the sum of alkaaesulfonic acid and sulfuric acid, phosphoric acid or oxalic acid is I00 parts by wcight and makes up from 3 to 30°/ by weight of the electrolyte. The electrolyte particularly preferably comprises from ~0 to 90 parts by weight of an atkanesuifonic acid and firm 80 to 10 parts by weight of sulfiunic acid. The use of atkanesulfonic acid as sole acid in the electrolyte is, however, likerwise possible.
3 5 For the purposes of the present invention, allcanesulfonic acids are aliphatic sulfonic acids.
The aliphatic radical of these may, if deszred, be substituted by functional groups or heteroatoms, e_g. hydroxy groups. Preference is given to using alkanesulfo~anc acids of the formulae R S03H or TAO-R'-S03H.
Here, R is a hydrocarbon radical which may be branched or unbranched and has from I to 12 carbon atoms, preferably 1 to 6 carbon atoms, particularly preferably an unbranched , hydrocarbon radical ha~cring from 1 to 3 carbon atoms, very particularly preferably 1 carbon atom, i.e. mcttianesulfonic acid.
R' is a hydrocarbon radical wbach may be braoo~chcd or unbranched and has from 2 to 12 carbon atoms, preferably 2 to 6 carbon atoms, particularly preferably an unbranched hydrocarbon radical having from 2 to 4 carbon atoms, where the hydroxy group and the sulfonic acid group can be bound to any carbon atoms, with the restriction that they are not bound to the same carbon atom. ' ' According to the present invention, particular preference is given to using metbanesulfonic amid as allcanesulfonic acid.
Alu~anniauu~ and aluminum alloys can be anodically oxidized by the process of the present invention. Particularly sui~ble alunninum alloys are alloys of aluminumwith silicon, manganese, zinc, copper and/or magnesium. In thcsc, silicon, manganese, zinc, copper 2 0 and/or magnesium can be present in the alloy in a proportion of 15% by weight (Si), 4% by 'weight (Mn), 5% by weight (2n), S% by weight (Cu) and 5% by weight (M~, with casting alloys also being included.
in the case of son0.e aluminum materials, a tendency for pit concosion to occur is found 2 5 when using electrolytes comprising alkanesulfonic acids. In. such cases, a brief prcanodiza~tion step in su~.e acid electrolytes is advantageous. In the subsequent anodiaatiox~ in an alkanesulfonic acid electrolyte, the aluaminuzn oxide skin which has already been formed protects the warkpicce from corrosive attack. This preanodizaxion step is generally carried out for a period of from 3 s to 5 min, preferably for tom I to 3 0 3 minutes.
The present invention accordingly also provides a process in which the anodic oxidatio~a is carried out in two stages, comprisiz~g:
3 5 - preanodizaxion of the aluminmn or the aluminum alloy in an electrolyte comprising sulfuric acid as sole acid or a mi~t~ue of suLfvric acid and oxalic acid;
- o~idaxioz~ in an electrolyte according to the present invention comprising an atkanesulfonic acid.

The process conditions of the preanodization preferably correspond to the conditions of the classical DS (direct current sulfuric acid) or DSX (direct current sulRnric acid-oxalic acid) electrolysis lmown from the prior art.
The anodic oxidation (anodi~on) is pzeferably carried out at from 0 to 30°C, If excessively high temperatures are employed, irregular deposition of the oxide layer occurs, which is undesirable.
rn general, hard anodization in which thick oxide layers having a low porosity and thus a high hardness and high proteetyon of the aluminum surface are sought is carried out at low temperatures of generally fro»a 0 to S°C, preferably from 0 to 3°C. swing to the fact that allcanesulfonic acids are less corrosive toward aluminum oxide than is pure sulfiuic acid, high thiclmesses of the oxitde layer of >30 pm, preferably from 40 to 100 Esm, particularly preferably from 50 to 80 Vim, are possible by means of the process of the present invention in shorter times than when using pure sulfuric acid as basis of the electrolyte. ~'hese aluminum oxide surfaces obtained by hard anodization are generally not used for a subsequent step to color the surface.
The anodization according to the present invention. for obtaining a porous aluxninum oxide 2 0 sWace which is particularly well-suited for subsequent coloration of the surface is generally carried out at from 17 to 30°C, preferably from 18 to 28°C. The process of the present invention differs firnm processes of the prior art in that it can be earned out at a higher temperature than the processes of the prior art. Usually, temperatures above about 2A°C give unusable, nonuuiform o7wide layers, while the process of the present invention 2 5 allows the anodi~atiou to be carried out at up to 30°C. The ability of the process to be carried out at higher temperatures saves energy costs. In general, cooling of the electrolyte solution during anodization is necessary, since the anodization is exothe~tmic. This embodiment of the pxocess of the present invention at generally from 17 to 30°C gives, depending on tb,e ctnient density and the electrolysis lime, layer thiclmesses of from 5 to 30 40 um, preferably from 10 to 30 prn.
The process of the present iaven'tnon leads to aluminuxn oxide surfaces wbzch are optimally suited to subsequent coloration, so that uniformly colored aluminum oxide layers can be obtained The process of the present invention is generally carrzed out at a current density of from 0.5 to S A/dmZ, preferably from 0.5 to 3 A/dm2, particularly preferably from 1 to 2.5 A/dm2. The voltage is generally from 1 to 30 V, preferably from 2 to 20 V, Apart from the allcanesulfonic acid or mi~h~re of alkanesulfonic acid and sulfuric acid used according to the present invention, the electrolyte generally further comprises water and, if necessary, further additives such as aluminum sulfate.
Apparatuses suitable for carrying out 'the process of the present invention are generally all ;
lmowa apparatuses which are suitable for electrodipping or for continuous anodxc , oxidation of aluminum or aluminum alloys, e.g. by means of au electrolytic pull-through process. Particular pzeference is given to using appazatuses made of metals which are .
resistant to alkauesutfonic acids or apparatuses which are lined with plastic, e.g. .
polyethylene or poiyproylene.
'fhe present invention fuxther provides a process for the surface treatment of aluminum or .
aluminum alloys, comprising the follorovzng steps:
a) pretreatment of the aluminum or the aluminum alloy;
1 S b) anodic oxidation by the process of the present invention (anodizabion);
c) if desired, coloration of the oxidized surface of the aluminuux or the , aluminum alloys;
d) ai~er-treatment of the workpiece obtained after steps a), b) and, if employed, c);
2 0 e) if desired, recovery of the allcanesulfonic acid used and/or its salts, where ;
step e) can follow or be carried out in parallel with any step in which an , alkar,es,~.fonic aczd can be used, in partic-.~law the steps b) and/or, if employed, c).
2 5 Step a) ?he pretreatment of the ahmainum or the aluminum alloys is a critical step since it determines the optical quality of the end product. Since the oxide layer produced in aaodization is transparent and this transparency is retained during the coloration process in step c), every surface defect on the metallic workpiece remains visible on the finished part.
The pretreatment is generally carried. out by customary methods such as mechanical polishing or electropolishing, dewva~.ng using ~aeutrat surfactants or organic solvents, brightening or pickling. This is genexally followed by rinsing with water.
3 5 rn a preferred embodiment of the present invention, solutions comprising all~aaes~,ilfonic acids are preferably also used in step a) (e.g. in tb~e case of brightening and elcctropolisbing). Prcfared alkan~esnlfonic acids have already been mentioned above for use in the anodizing step (step b)). Particular preference is given to using metbanesulfonic acid.

_ g _ Step b) Step b) is the anodization process according to the present invention which follows the pretreatment of the ahmoamnn oz the aluminum alloy, This process according to the present invention has been described in detail above.
Step c) Xf the anodized aluminum. or the anodized aluminum alloy is not to be used directly without coloration of the aluminum oxide Iayex, which is generally the case for, for ZO example, hard anodization, in which case dense, thitck layezs are obtained, the aluminum oxide layer obtained in step b) can, be colored.
Coloration of the aluminum oxide layer occuzs by uptake of organic or inorganic dyes into the eapi.Itary-shaped pores of the oxide layer obtained by anodizati.on in step b).
For the proposes of the present invention, it is generally possible to use all processes known from the prior art for coloring anodized alumuinum in step c). A
distinction is usually made betv~een chemical and electrolytic colozation_ 2 0 In chemical coloration, anodized aluminum or aluminum alloy is colored in the aqueous phase by means of suitable organic or inorganic compounds nn the absence of an electric current. Organic dyes (eloxal dyes, e.g. dyes from. the inn series or indigo dyes) ofren have the disadvantage of being insufficiently lightfast. Inorganic dyes can, in a chemical coloration step, be deposited in the poxes by precipitation reactions or by hydrolysis of 2 5 heavy metal salts. However, the processes which occur here are difficult to control and there are frequently reproducibility problems, i,e. problems in obtaining constant color shades. For this reason, electrolytic processes for coloring aluminum oxide layers have become increasingly established for some time_ 3 0 Step c) of the process of the present invention is therefore preferably carried out by art electrolytic method in an electrolyte comprising metal salts.
The aluminum oxide Layers obtained after step b) of the process of the present invention are colored in an electrolyte comprising metal salts by means of diucect ar alter~avatzag 35 current, preferably by means of alternating curt. Here, metal is deposited izn the bottom of the pores of the oxide layer from the metal salt solution_ The use of salts of various metals and various operating conditions give different colors. The colors obtained are very lightFast.

Suitable metal salts are generally salts selected ~rom among tin, copper, silver, cobalt, nicl~el, bismuth, chromium, palladium and Lead and mzxtures of two or xuore of these metal salts- Preference is given to using tin, copper or silver salts or mixtures tlxereof m the process of the present invention.
In geuerat, the sulfates of the abovementioued metals are used, and electrolyte solutions based on sulfuric acid are used.. Additives can be additionally added to the electrolyte to improve the scatter cad reduce oxidation of the metal ions used, e,g. the oxidation of tin(1>]
to the insoluble tia(I~.
In a particularly preferred embodiment of the process of the present invention, the electrolyte comprises from 20 to 100 parts by weight of an alkanesulfonic acid and from 80 to 0 parts by weight of sulfuric acid, rovhere the sum of afoaic acid and sulfuric acid is 100 parts by weight and makes up from 0.1 to 20% by weight, preferably from 0.1 to I5% by weight, of the electrolyte. The electrolyte very particularly preferably comprises 100 parts by weight of an alkanesulfonic acid.
Alkancsulfonic acids suitable for step c) o~ the process have been disclosed above for use is the aaodi~tiou (step b)). Particular preference is given to metbanesulfonic acid.
Compared to purely sulfuric acid electrolytes, electrolytes based ova alkanesulfonic acids have a higher electrical conductivity, bring about more rapid coloration and display a reduced oxidation action, as a result of which the precipitation o~ for example, tin(I~
salts from electrolytes comprising tin(I>] salts is prevented and the addition of additives 2 5 such as environmentally harmful phenolsulfonic or tolueaesulfonic acid is not necessary.

The metal salts are generally used in a concentration of from 0.1 to 50 g/l, preferably from 0.5 to 20 g/l, particularly preferably from 0~ to IO g/1, based on the metal used, in the electrolyte. .
In addition to the appropriate acid, preferably sulfuric acid or an alkanesulfonie acid or a mixture of the two acids, and the mefial salt used or a mixture of a plurality of metal salts, the electrolyte generally further comprises water and, if necessary, further additives such as scattering improvers. However, pardicularly when using electrolytes connprising alkanesulfoaic acids, the addition of additives zs generally not necessary.
The electrolysis time in step c) is generally from 0.1 to 10 minutes, preferably from 0.5 to 8 minutes, particularly preferably from 0.5 to 5 minutes, with the electrolysis time depending on the xnetat salts used and the desired depth of color.

.. CA 02415556 2003-O1-09 The electrolytic coloratiion in step c) is usually carded out using sltemat~ag current The current density is generally from 0.1 to 2 A/dm2, preferably from O.Z to 1 A/dm2. The voltage is generally from 3 to 30 V, preferably from 5 to 20 V.
All apparatuses suitable for the electrolytic coloration of aluminum oxide layers can. be Suitable eloctrodes are the electrodes which are uxually suitable in a process for the electrolytic coloration of aluminum oxide layers, for example stainless steel or graphite electrodes. It is alsa possible to use one electrode made of the metal to be deposited, e.g.
tin, silver or copper.
In a particularly preferred embodiment of the process of the pxesent invention, a gold color of the oxidized surface of the aluminum or the aluminum alloys is achieved in au electrolyte comprising silver salts, if desired in admixfiire with tin salts and/or copper salts.
Such gold-colored alumunum workpieces are of particular intereest for producing decorative objects, since the demand for gold-colored aluminum objects is great.
2 0 These gold-colored aluminum oxide surfaces are preferably obtained by carrying out the coloxation process in step c) at a concentration of an all~anesulfonate of sicker, calculated as Ag+, of from 2 to 50 g/1., prefex2bly from 3 to 20 g/1, and a product of current density and voltage of from 0.5 to 10 AV/dm2, preferably from 1 to 5 AV/dmZ, for a period of generally from 0.05 to 4 minutes, prefer~.bly from 0.3 to 3 minutes. A precise description 2 5 of the production of gold-colored alunninum oxide layers may be found in the patent application bE-A ... having the title "Production of gold-colored surfaces of aluminum or aluminum alloys by means of silver-containing formulations", which was filed at the same tune.
3 0 Step a~
The after-treatment of the workpiece obtained after step b) or, if employed, c) may be divided into two steps:
dl) Rinsing 3 5 To remove residues of the bath from the pores of the oxide Iayer, the worlcpieces are gcnezally rinsed with water, in particular with nmning water. This rinsing step follows both step b) and step c) if this is carrzed out.
d2) Sealing . 11 Subsequent to step b), if step c) is not carried out, or subsequent to step c) if this is carried out, the pores of the oxide layer produced are generally sealed to provide good corrosion protection. This sealing can be achieved by dipping the wozkpieces onto bowling distilled water for from about 30 to 60 minutes. '1"his causes swelling of the oxide layer, as a result of vcrhich the pores axe closed. The water c,~n also contain additives. Tn a particular embodiment, the workpieces are after-txeated in pressuzized steam of from 4 to 6 bar instead of in boili.~ag water.
Further methods of sealing are possible, for example by dipping the wozkpieces into a solution of readily hydrolyzable salts, as a result of which the pores are blocked by sparingly soluble metal salts, or into chromate solutions, which is predominanxly employed for alloys rich in silicon and/or heavy metals. Treatment ix~ dilute water glass solutions also leads to sealing of the pores if the silica is precipitated by subsequent dipping into sodium acetate solution. Furrhermoze, the pores can be sealed by means of iuasoluble metal silicates 1.5 or organic, water-repellent substances such as waztes, resins, oils, paraffin's, varnishes and plastics.
However, sealing is p~eeferably carried out by means of watex or steam.
2 0 e) Recovery of the alkanesulfox~ic acid used a~dlor its salts To save costs and for ecological reasons, the alkanesulfonic acid used and/or its salts can be recovered. This recovery caw follow or be carried out in parallel with any step in which an alkanesulfonic said can be used. Recovery can be carried out, for example, in combination with the rinsing step (dl) following step b) and, if it is carried out, step c).
2 5 Such a recovery can be carried out, for example, by means of electrolytic membrane cells, by cascade rinsing, or by simple concentration, for example, of the ri~asing solutions.
The present invention fiuther provides for the use of au atkanesulfonic acid in, a process for the anodic oxidation of aluminum. or aluminum alloys (anodization) fox increasing the raze 3 0 of the anodic oxidation. This makes it possible to achieve more rapid aluminum oxide deposition than when using the processes of the prior art. Furthermore, in bard anodization, thicker layers can be obtained in a shorter time when usiung alkanesulfonic acids as basis of the electrolyte than when using pure sulfuric acid as electrolyte basis. In addition, the energy consumption is significantly lower since a lower voltage is established and less 3 5 cooling has to be employed.
Furthermore, an electrolyte composition containuag from 3 to 30% by wei~t of an alkaaesuIfonic acid for the anodic oxidation of aluminum or aluminunn alloys is claimed.
Preference is given to an eleciznlyte composition comprising from 20 to 100 parts by weight of an alkanesulfonic acid and from 80 to 0 parts by weight of sulfuric acid, where the sum of alkanesulfonic acid and sulfuric acid is I00 parts by weight and makes up from 3 to 30% by weight of the electrolyte. Suitable alkanesulfonic acids have akeady been mentioned above. The alkanesuLfonic acid used is particularly preferably methanesulfonzc acid. These electrolyte compositions are very suitable for use in a process for the anodic oxidation of aluminuzn or aluminum alloys and lead to more rapid aluminum o~dde deposition than the processes of the prior art and to a thicker aluminum oxide layer in a shorter time, which is of particular interest in hard anodization, and to a reduced energy consumption.
The workpieces based on aluminum or aluminum alloys produced according to the present invention can be used, for example, in. building and construction, in particular for producing window profiles or e7rterior wall components, in automobile or aircraft co~nstruc-tion, both for producing body parts and fox producing aluminum pressure castings, e.g. in engine construction, and in the paclcagang industry, in particular for producing cans, for example by a continuous electrolytic pull-through process, e.g. continuous coil anodiza~tion.
The following examples illustrate the invention.
Examples Example 1 2 5 .Anodixation electrolytes compxzsing, in each case, 1$°/ by weight of an acid or an acid mixt<n~e and 8 g/1 of aluminum, were used. The electrolytes were used, for the anodixation of pure aluminum sheets which had in each case been preanodized for 2 minutes by the classical DS method_ Anodization was in each case caxricd out at a cuzrent density of 1.2 A/dmZ for 30 minutes, The anodization bath rxras in each case therraostated at 20'C.
3 0 The thickness of the aluminum oxide layer, the porosity or micros-fracture of the surface and the znticrohardness were determined on the anodized workpieces. Table I
below shows the thicknesses of the oxide layer obtained as a function of the electrolyte used and the anodszab.on voltage and any cooling necessary;

Table I
r. ~~ , , ' ' N ' 1 t ~yi~.,~y i ~y ;~; ~ ~' v _i , . ~, ~ ~ ~ , ~ Yf ~ j '~. .
~~"! .' '1.~ .~.~. ~
'"r' llii "~' .
~~~1 '~ J
~i ice, G
~
.
1' 'S~
.
~e,4 ~~
~''~e.,~' 1j~
.
S
A
v i ~
y!C'.' "W
t.
d I1~ SO4 12 ca.l2~ Stro ' 2.~~ HZSO,a/oxalic 11 ca. I I Strong ' acid 90: I0 3. MSAz~ 16 ca.2.5 Sli t 4. MSAlHzSO 50:50 14 ' ca 2.5 Sli t .

I) Comparative experiment 2) MSA: methaaesulfonic acid Example 2 S This was carried out using a method analogous to Example 1, but electrolysis was carried out at 2°C fox 40 minutes.
The layers all displayed a significantly lower porosity and an increased.
hardness compared to Example I. The aluminum sheefis anodized in MSA (xnethanesulfonic acid) had a 20~/°
greater thicltness anal an about 10% greater hardness than the aluminum sheets anodized in HzSa4.
Example 3 7.5 This was carried out by a method analogous to Example I, but electrolysis was carried out at 28°C.
The layers all displayed a sig~oificantly increased porosity aad a reduced hardncss; the porosity of the aluminum sheets 3 and 4 (accordi»~g to the present iuventrou, the acid in the 2 0 electrolyte corresponds to the compositions indicated in Table 1 under No.
3 and. 4, respectively) is lower than that of the others.
Coloring experiments in an electrolyte compx~sing silver metbanesuIfonate were cawied out on all aluminum sheets. Only in the case of aluminum sheets 3 and 4 (experizx~cnts 2 s accozding to the present fnven~on) were high~~it5, gold colors achieved.
In the case of aluminum sheet 2, relatively good gold colors were still achieved.

Coloration A colo~ting electrolyte was made up from 19 g/I of silvez' methan.esulfonate (I O g/1 of Ag''~ ' and S7 g/1 of methanesulfonic acid. At a current density of 0.2 .A/dm2 and a voltage of 6 about 8 V, the aluminum sheets anodized as indicated for No. 3 and 4 in.
'fable I were colored for different periods of time. Fox both alu~ainum. .sheets, the colors indicated in _ Table 2 below were obtained: ' Table 2 1 ~
' ~ ~ ~~
' ~:~ i V
' aa ,~ ' 'ay ~f' t 1 v ~ f v ~
Jv s , a 15 Pale old 30 L' old ' t 60 Gold i2o Geld ' 180 D old

Claims (14)

We claim:

1. A process for the surface treatment of aluminum or aluminum alloys by anodic oxidation of the aluminum or the aluminum alloys (anodization) using direct current in an electrolyte containing from 3 to 30% by weight of an alkanesulfonic acid.

2. A process as claimed in claim 1, wherein the electrolyte comprises from 20 to 100 parts by weight of an alkanesulfonic acid and from 80 to 0 parts by weight of a further acid selected from among sulfuric acid, phosphoric acid aid oxalic acid, where the sum of alkanesulfonic acid and the further acid is 100 parts by weight and the coaceatration of the alkanesulfonic acid is from 3 to 30% by weight of the electrolyte.

3. A process as claimed in claim 1 or 2, wherein the alkanesulfonic acid is methanesulfonic acid.

4. A process as claimed in any of claims 1 to 3, wherein the anodic oxidation is carried. out at from 0 to 30°C.

5. A process as claimed in any of claims 1 to 4, wherein the anodic oxidation is carried out in two stages, comprising:

- preanodization of the aluminum or the aluminum alloy in an electrolyte comprising sulfuric acid as sole acid or a mixture of sulfuric acid and oxalic acid;
- oxidation in an electrolyte comprising an alkanesulfonic acid as claimed in any of claims 1 to 3.

6. A process for the surface treatment of aluminum or aluminum alloys, comprising the following steps:

a) pretreatment of the aluminum or the aluminum alloy;
b) anodic oxidation by a process as claimed in claims 1 to 5 (anodization);
c) if desired, coloration of the oxidized surface of the aluminum or the aluminum alloys;

d) after-treatment of the workpiece obtained after steps a), b) and, if employed, c);
e) if desired, recovery of the alkanesulfonic acid used and/or its salts, where step e) can follow or be carried out in parallel with any step in which an alkanesulfonic acid can be used, in particular the steps b) and/or, if employed, c).

7. A process as claimed in claim 6, wherein solutions comprising alkaniesulfonic acids are also used in the pretreatment of the aluminum or the aluminum alloys in step a).

8. A process as claimed in claim 6 or 7, wherein the coloration of the oxidised surface of the aluminum or the aluminum alloys in step c) is carried out by an electrolytic process in an electrolyte comprising metal salts.

9. A process as claimed in claim 8, wherein a gold color of the oxidised surface of the aluminum. or the aluminum alloys is achieved in an electrolyte comprising silver salts, if desired in admixture with tin salts and/or copper salts.

10, A process as claimed in claim 8 or 9, wherein the electrolyte comprising metal salts comprises from 20 to 100 parts by weight of an alkanesulfonic acid and from 80 to 0 parts by weight of sulfuric acid, where the sum of alkanesulfonic acid and sulfuric acid is 100% by weight and males up from 0.1 to 20% by weight of the electrolyte.

11. The use of an alkanesulfonic acid in a process for the anodic oxidation of aluminum or aluminum alloys (anodization) using direct current to increase the rate of anodic oxidation and to reduce the energy consumption.

12. An electrolyte composition consisting of from 20 to 100 parts by weight of as alkanesulfonic acid and from 80 to 0 parts by weight of a further acid selected from among sulfonic acid, phosphoric acid and oxalic acid, where the sum of alkanesulfonic acid and further acid is 100 parts by weight and the concentration of the alkanesulfonic acid is from 3 to 30% by weight of the electrolyte, water and optionally further additives such as aluminium sulfate.

13. An. electrolyte composition as claimed in claim 12, wherein the alkanesulfonic acid is methanesulfonic acid.

14. The use of workpieces with a surface based on aluminum or aluminum alloys, wherein the surface was treated by a process as claimed in any of claims 1 to 10, in building and construction, in particular for producing window profiles or components of exterior walls, in automobile or aircraft construction and in the packaging industry, in particular for producing cans.