Note: Descriptions are shown in the official language in which they were submitted.
CA 02415556 2003-O1-09
- 1 -
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.
CA 02415556 2003-O1-09
- 2 -
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
CA 02415556 2003-O1-09
- 3 -
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
CA 02415556 2003-O1-09
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
CA 02415556 2003-O1-09
- 5
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.
CA 02415556 2003-O1-09
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,
CA 02415556 2003-O1-09
7
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.
CA 02415556 2003-O1-09
_ 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.
CA 02415556 2003-O1-09
10
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.
35
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
- 10 -
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
4
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
CA 02415556 2003-O1-09
. 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
CA 02415556 2003-O1-09
- 12 -
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;
CA 02415556 2003-O1-09
- Z3 -
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.
CA 02415556 2003-O1-09
- 14 -
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
