Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The invention relates to an improved device for anodic oxida-
tion by pad electrolysis, of the type in which the electrolyte is
retained within an absorbent mass or pad contacting a cathode which
comprises an :lnner cooling circuit. The cathode is connected to the
S ne~atLve poLe of an electric power supply and supported on an electrically
LnsuLated gr:Lpp:Lng means.
Ln the most sophisticated known deviees of the above type,
which are used in particular for anodic oxidation of aluminum and alumi-
num alloys, the cathode is a graphite block with an inner chamber through
which cooling water Elows. The cathode is also provided with a tapped
hole opening into this chamber and intended to accommodate a threaded
rod oE a gripping means which consists of a conduetive mass eovered with
an insulating shroud and eonneeted to the positive pole of an eleetrie
power supply. The eathode inner ehamber has a water inlet eonduit and
lS a wa~er outlet eonduit, both of whieh are integral with the gripping
means and whieh allow water to eireulate in the eham~er.
Unfortunately, as graphite is a porous material, it is neeessary
to earry out the time-eonsuming step of sealing the walls of the eathode
inner ehamber with water-tight resins. In addition, sinee such a
sealing step cannot be carried out in the are~ of the tapped hole, there
may be leakage of the cooling water at that level, which greatly li~its
the pressure which can be applied to this water, thus affecting the
water flow rate and consequently the cooling effectiveness. Moreover, the
graphite cathode has poor ability, after a certain period of service, to
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withstand the quite severe temperature, acidity and electric voltage
and intensity conditions to which it is subjected during the anodic
oxlclatLon process. As a result, frequent changes of the cathode are
reclllLrecl. FLnaLly, the Lnner chamber may be difficult to make in the
grapllLte, especLally when the cathocle ls of smal:L siæe or compl:lcated
shape.
The purpose of the invention is to cope with the above-men-
tioned drawbacks by providing a device which is characterized in that
the cathode consists of a stainless steel tube which has at least two
legs or branches, at least one of which is locally held by the grip-
ping means, the free end of one or more of the legs being connected to
a cooling fluid supply and the free end of the remaining leg or legs
being connected to a cooling fluid outlet.
Stainless steel not only withstands particularly well the
operating conditions required for anodic oxidation, but it can also
be shaped into a tube which can be cooled by internal circulation of
a coollng fluid in a highly efficient manner. In anodic oxidation
by pad electrolysis, it is essential to work at the lowest possible
temperatures to achieve a protective coating of high grade and suffi-
cient thickness. Thus, it is essential to remove the heat producedby the very nigh applied current intensities from the cathode area.
As the stainless steel tube does not pose any sealing
problem, the cooling fluid can be circulated through it under very
high pressure and at a very high flow rate, which allows for increased
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cathode cooling.
In a partlcular embod:Lment, the connectlon between the cathode
tube and the negatLve pole ls provided by an electrlcally conductive
mnsfJ contaLned Ln the gr:LppLng means, at Least one oE the tube legs
belng Ln contact wLth thls conductlve mass.
The cathode will preferably be substantially U-shaped, both
legs of the U being secured in the gripping means close to their free
end.
The resulting device is thus compact and easy to handle and
allows, by selection of appropriate leg lengths for the U-shaped tube,
treatment of areas to which access is difficult or even impossible with
prior art devices.
It should be noted that the cooling fluid may be the electro-
lyte ltself. In such a case, the cathode is provided wlth small oriflces
for electrolyte flow adJacent to the part of the cathode ln contact with
the absorbent mass. In this way, it is possible to ensure the cathode
cooling and simultaneously maintain the absorbent mass impregnated
constantly with electrolyte. This avoids using, as is the case in
known devices, an outer electrolyte feed inlet independent of the
anodizing device proper, which is cumbersome and complicates the anodi-
zation steps.
However, the device, in accordance with the invention, will
preferably include an electrolyte feed inlet adjacent to the cathode
and the absorbent mass so that the latter will then be able to absorb
the electrolyte coming from the feed inlet, which is locally held by
the grLpping means. In such a case, the cathode will be continuously
coo:led by the coolin~ Eluld Elow:lng through it, and the electrolyte
wlll come Erom n ~ource other thnn the cathode. This source, whenever
Lt coll~JIst~ oE a staLnleYs stee:l tube closed at one end, provicled with
sma:ll or:LElces allowLng electrolyte passage and held in the gripping
means adjacent to its free end, is intimately incorporated into the
anodLzing device and forms a compact unit therewith. Such an arrange-
ment is more advantageous than the arrangement described above in
that it permits adjustment as desired of the amount of electrolyte
to be poured onto the absorbent mass without, however, adversely
affecting the cathode cooling or increasing the space occupied by
the anodizing device, nor complicating the structure and, thereby,
the handling.
The electrolyte feed tube is advantageously arranged sub-
stantially in the longitudinal plane of symmetry oE the U-shaped tube.
Thus, when this tube ls positioned between the two legs of the U-shaped
tube, the space occupied by the cathode-electrolyte feed tube assembly
is mlnimum. By properly selecting the orifice locations, it is possible
to maintain the absorbent mass, thoroughly impregnated with electrolyte,
in the working area of the cathode. Moreover, when the electrolyte
feed tube is arranged slightly above the plane defined by both legs of
the U-shaped tube, it becomes possible, again by selecting the locations
of the orifices provided in the tube, to ensure that the electrolyte
flowing from the orifices will spill over either one or both of the
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cathode legs, thus helping to cool the cathode.
AlternatLvely, the electrolyte feed tube may contact tlle
ele(:trLcally concluctLve mass and Ln such a case :Lt wLll also act as n
cnthode nncl can llelp the anocllc oxidat:Lon process.
S AclvantageollsLy, the cathode leg or legs contacting the
conductive mass and, :Ln some cases, the electrolyte feed tube, are
respect:Lvely firmly secured in bores provided in this conductive mass.
Such an assembly not only allows a good electrical contact but also
gives a good stiffness to the device.
In a preferred embodiment, the absorbent mass is in the form
of a sleeve closed at one end, which fits over all or part of that
cathode portion and, where appropriate, of the electrolyte feed tube,
not held ~n the gripping means.
The cathode-electrolyte feed tube assembly is thus surrounded
by the absorbent mass, which prevents the electrolyte from dripping
out. It consequently allows use of the anodizing device in any
position.
The absorbent masses known at this time in the pad electroly-
sis field consist essentially of nylon-cotton blends. However, such
materials withstand the conditions required for anodic oxidation very
poorly.
Thus, the invention is also directed to a new type of absor-
bent mass, particularly useful in the anodizing device described above
and below. More specifically, this absorbent mass consists of a polyester
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wadding wool enclosed in a sheath of woven polypropylene or polyester
fibers.
In partLcular, this polyester in the Eorm oE a wadding or
woven ELbers may result Erom copolymerizatlon oE var:Lous glyco:ls and
'j nroma~Lc dLcarboxyl.Lc acids, preEerab:Ly from copolymer:L~ation of glycol
and ~ercphtila:LLc ac:Ld.
'~he polypropylene used to make the sheath will preferably
be an isotactic polymer of the MERAKLON type. Meraklon is a trade mark.
Finally~ the invention is further directed towards new
electrolytes for use in anodic oxidation by pad electrolysis, which
may be used especiaLly in the device according to the present invention.
More specifically, these electrolytes comprise an aqueous
solut:Lon of concentrated sulfuric acid, chromium trioxide and sulfamic
acid. They are particularly well-suited to anodic oxidation of aluminum.
The electrolytes may also contain magnesium sulfate, in
which case they are particularly suitable for anodic oxidation of
lightweight aluminum alloys.
These novel electrolytes make it possible to achieve oxide
layers of higher grade than the layers obtained when known electrolytes
are used, in particular in respect to chemical resistance, marine or
sea corrosion resistance, hardness, abrasion resistance and thickness.
An embodiment of the device according to the invention is
shown as an example in the accompanying drawings, in which:
Figure 1 is a cross-sectional elevation view of an embodiment
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of the device in accordance with the invention, and
Figure 2 is a cross sectional view along the line II-II of
the device as shown in Figure 1.
The device as shown in the drawings comprises a block 1 of
plastic material, having substantially the shape of a truncated cone,
provided with a handle 2, which is secured to block 1, for instance,
by simply screwing a threaded rod (not shown), supported on the handle,
into a tapped hole (not shown), provided in the block. The smaller
base ~ of the tapered block 1 has an opening 4 which opens into an inner
chamber in which a cylindrical conductive mass 5 is force-fitted,
which is preferably made of stainless steel, the central axis of which
preferably coincides with the axis of tapered block 1. This mass 5
is provided with three bores (6, 7, 8), aligned respectively with
bores of the same cross-sectional area (6a, 7a, 8a) which are provided
in block 1.
In the illustrated embodiment, two out of three registering
palrs of bores (6, 6a; 7, 7a) are substantial:ly located in the hori-
~ontal plane of symmetry of block 1, on each side and at the same
distance from the vertical plane of symmetry of block 1, whereas the
third pair of registering bores (8, 8a) is substantially located in
the v0rtical plane of symmetry, below the other two pairs of bores
(6, 6a; 7, 7a). Although the above~disclosed position of the various
bores in block 1 is presently considered as the most suitable, the
bores could be positioned quite differently without the gist of the
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lnvention being affected in any way. As an example, pairs of bores
(6, 61; 7, 7a) could be positioned in a plane other than the horizon-
tal plane oE ffymmetry and/or ln two separate planes. Llkewise, the
palr oE bores t8, 8a) could be pos:ltioned outside the vertical
pl~me oE ffymmetry and/or above the pairs oE bores (6, 6a; 7, 7a).
Moreover, the area of the portion disposed above the hori-
zonal plane of symmetry oE block 1, mass 5 extends, through a projec-
tion 9, towards the larger base 10 of tapered block 1, without, however,
reaching the base mentioned. The projection 9 is provided with a tapped
hole 11 which is orthogonal to base 10 and in the alignment of a bore
12 of the same cross-sectional area, that is provided in block 1 and
extends from said base 10. Che tapped hole 11 accommodates a threaded
shank 13 of an electric connection pin 14 which can be connected to
the negative po]e of an electric power supply (not shown). Of course,
projection 9 could have quite another position and pin 14 could be
secured to projection 9 in quite another manner without the operation
of the anodizing device being affected.
In pairs of bores (6, 6a; 7, 7a) are respectively force-fitted
so as to completely pass through block 1, the two legs (15, 16) of a
U-shaped tube 17 of stainless steel. Both legs have counterbent portions
in front of base 3, which are slightly inclined towards each other and
respectively extend into two horizontal sections (20, 21), which merge
into the U bottom and are portioned in the same plane, which plane, in
turn, is positioned below block 1.
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The free end 15 of the tube which extends above base 10 can
be connected to an inlet ~not shown) for a cooling fluid, e.g~ water,
whLle the Eree end 16 of the tube, which also extends above base 10,
c:an he connect.ed to a cooLlng f:luLd outLet (not shown).
S Ln the palr oE bores (8, 8a) Ls also force-E:Ltted, so as to
cc)lllpl~ely pa~s through block 1, a tube 22 of sta:Lnless steel which
a:Lso has a counterbent portion 23 extending into a horizontal section
24 which is closed at its remote end and positioned immediately above
the plane defined by sections (20, 21), the section 24 being provided
with two rows of small orifices t25, 26) respectively registering
with these sections (20, 21). The free end of tube 22, i.e., the
tube end which extends above base 10, can be connected to an electrolyte
feed tube.
Finally, a substantially cylindrical sleeve 27 which is closed
at one end is inserted over sections (20, 21, 24) which are thus
completely surrounded by the sleeve. The latter is made of polyester
wadding 28 enclosed within a sheath 29 made from woven fibers of
MER~KLON (a polypropylene polymer). It should be understood that the
shape of the sleeve need not be cylindrical but in fact depends upon
the cathode shape. In the disclosed embodiment, the cathode is U-shaped,
but it could have another shape suited to geometry of the surface to be
anodized.
The device described above is operated as follows: the ends
of legs (15, 16) are respectively connected to an inlet and an outlet
for the cooling fluid, which may be circulated, e.g., by a pump. Like-
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wise, the end of tube 22 is connected to an electrolyte feed inlet,
which is in turn connected to an electrolyte feed pump, the flow rate of
wh:Lch can be controlled. Finally, pin 1~ is connected to the minus pole
oE a suLtable electric generator, whlle ~he workpiece to be anodized is
connectecl to the posltlve pole oE the generator.
The eLectrLc generator can be of any type, e.g., a DC genera-
tor or a pulse generator.
Once the above connections have been made, and sleeve 27 has
been thoroughly impregnated with electrolyte, the anodizing is carried
out by moving the sleeve over the surface to be treated, while moni-
toring the electrolyte feed so as to compensate for any evaporation
and to ensure that the sleeve is continuously and sufficiently fed with
electrolyte.
S:Lnce the lower the anodization temperature, the higher the
grade of ~he resulting oxide layer, the electrode will be cooled as
much as possible and the employed electrolyte will be at the lowest
possible temperature, preEerably at a temperature of 20C or lower.
The operating voltage of the device in accordance with the
invention, will generally be in the range of 25 to 50 V, at which
voltage the anodizing eEficiency will be substantially constant,
whereas the current density may vary from 15 to 250 A/dm2.
The workpiece to be anodized can be made of any metal
capable of being subjected to anodic oxidation, such as, for example,
aluminum and lightweight aluminum alloys, as well as titanium and
titanium alloys. The type of electrolytes to be used in the device in
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accordance with the invention will depend, of course, on the nature
o:E the workpiece metal.
I:E the workp:Lece metal :Ls aluminum, the e:Lectrolyte wll:L
pre.Ee:rnb:ly consLst o:E an aqueous so:lut.Lon contalnlng from 30 to 100 g/
:L:Lte:r, preEerab:Ly about 35 g/l:Lter of sulfam:Lc acld; Erorn lO to 60
g/l:lter, preEerably about 17 g/llter of chromlum trloxlde; and
from 9 to 55 g/llter, preferably about 13 g/liter of concentrated
sulfuric acid.
Finally, lf the workpiece metal is a lightweight aluminum
alloy, such as an A-U4G alloy, the electrolyte will preferably consist
of an aqueous solution containing from 30 to 100 g/liter, preferably
about 100 g/liter of sulfamic acid; from 10 to 60 g/liter, preferably
about 48 g/liter of chromium trioxide; from 9 to 55 g/liter, preferably
about 37 g/liter of concentrated sulfuric acid; and about lO0 g/liter
of magnesium sulfate heptahydrate.
