Note: Descriptions are shown in the official language in which they were submitted.
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METHODS FOR DEPOSITING FINISH COATINGS ON SUBSTRATES OF
ANODISABLE METALS AND THE P1~DUCTS THEREOF
Field of the Invention
This invention is concerned with improvements in or
relating to methods for depositing metal coatings on substrates
of anodisable metals, such as aluminum and its anodisable
alloys, and to the products of such methods.
Review of the Prior Art
The deposition of metals on a substrate, usually steel
or aluminum, is a well-developed art. Plating on less easily
oxidized metals such as steel is relatively routine, involving
for example the deposition of a layer of copper directly on the
steel substrate, followed in succession by a thick "semi-bright"
nickel layer, a thinner °bright° nickel layer, and an even
thinner finish layer of chromium; the chromium is
semi-transparent and the bright appearance is actually provided
by the bright nickel layer seen through the finish chromium
layer.
Plating on anodisable metals, such as aluminum and its
anodisable alloys, is considerably more difficult owing to their
relative ease of oxidation, and the consequent inevitable
presence of an oxide coating which must be removed if adequate
adhesion of the deposited layers to the underlying metal
substrate is to be obtained. The art currently is dominated by
two methods of preparing the substrate surface, namely zincate
and stannate immersion. In these processes the substrate
surface is immersed in a suitable zincate or stannate solution,
usually of the sodium salt, together with other additions that
have been found in practice to increase the appearance and
adhesion of the coatings. The zinc or tin atoms respectively
displace aluminum atoms at the surface, in the process removing
the oxide layer, to result in an adherent zinc or tin layer on
which other layers, for example copper followed by nickel and
chromium can be deposited. Both of these processes are
relatively expensive and are therefore mainly used on expensive
commodities. The stannate immersion processes are reported to
provide better anti-corrosion performance and adhesion of the
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resultant coatings, but are the more expensive of the two
because of the more expensive components and longer processing
times.
It has also been proposed to deposit adherent metal
coatings directly on aluminum or aluminium alloy substrates by
producing a porous anodized layer at the surface of the -
substrate onto which the subsequent metal layers are deposited;
this anodized layer incorporating the oxide layer that was
present on the substrate surface. In an article entitled
"Plating on Aluminum, a Review" by D.S. Lashmore, published in
the ,Tune 1985 issue of "Plating & Surface Finishing" (pp 36-39),
summarizing previous publications, it was reported that studies
have shown that there must be a minimum pore size in the
anodized coating, into which the subsequent metal coatings can
mechanically "lock" or "key", and that this limits the process
to the use of electrolytes that will produce fairly large pores
of the order of 0.07 micrometres (700 Angstroms). The report
goes on to state it has been found empirically that only
anodising solutions comprising phosphoric acid are sucessful, i
sulfuric or oxalic acid sometimes being mixed with the
phosphoric acid. The report further states that the adhesion of
the subsequent coatings is primarily mechanical, with the
cohesive strength of the porous oxide coatings to the metal
substrate being the limiting factor, so that improvements in the
anodic process should be directed towards increasing this
cohesive strength and the strength of the oxide layer itself.
Despite developments of these phosphoric acid anodizing/coating
processes for over 50 years they have not yet been widely
adopted commercially, apparently because of relatively poor
adhesion and brightness.
Definition of the Invention
It is therefore the principal object of the present
invention to provide new methods for plating metal layers on
substrates of aluminum and its alloys, and to provide substrates
of aluminum and its alloys having metal layers plated thereon by
the new methods.
In accordance with the present invention there is
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provided a new method of depositing metal on a surface of a
substrate of an anodisable metal, said method comprising:
a) anodising the substrate using phosphoric acid to
produce a porous anodised layer which is of pore size greater
than about 0.03 micrometre and of thickness from about 0.5 to
about 50 micrometres;
b) electrolytically depositing pore-filling metal into
the pores to adhere to the walls thereof and so as to fill each
pore from about 3$ to about 30~ of its volume; and
c) continuing the deposition of pore-filling metal by
electroless deposition on the electrolytically deposited metal
to fill each pore to the required extent.
Also in accordance with the invention there is provided
a metal plated product consisting of an anodisable metal
substrate acid anodised to have a porous anodised layer and
having metal deposited in the pores thereof wherein:
a) the anodised layer is phosphoric acid anodised to
have pores of pore size greater than about 0.03 micrometre and
to have a thickness of about 0.5 to about 50 micrometres;
b) the porous layer has pore-filling metal
electrolytically deposited in the pores thereof so as to adhere
to the walls thereof and fill each pore from about 3$ to about
30$ of its volume; and
c) the porous layer has pore-filling metal
electrolessly deposited in the pores on the electrolytically
deposited metal so as to fill each pore to the required extent.
Description of the Drawings
Methods of depositing various layers of metals on a
surface of an anodisable substrate, and the products of such
methods, constituting particular preferred embodiments of the
invention, will now be described by way of example with
reference to the accompanying drawings, wherein:-
".-M,
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Figure 1 is a plane cross-se~ion through the surface
and adjacent portion of an aluminum substrate, through the
porous anodised layer formed thereon by phosphoric acid
anodising, and through the various layers of metal that have
been deposited on the anodised layer; and
Figure 2 is a plane cross-section to a much enlarged
scale of the small portion 2 in Figure 1, showing the anodised
Iayer and the immediately adjacent metal layers.
Description of the Preferred embodiments
As indicated above, Figure 1 is a cross-section through
an aluminum substrate 10 at the upper surface of which there has
been formed by acid anodising a layer 12 of aluminum oxide, a
portion of which, together with the immedia~ely adjacent
portions of the substrate and deposited metal layers, are shown
to a larger scale in Figure 2. The anodising employs phosphoric
acid which produces elongated wide pores 14 (not seen in Figure
1), and in accordance with this invention at least the bottom
portions of the pore walls have had applied thereto by
electrolytic deposition a layer~of adherent pore-filling metal
16 (also not seen in Figure 1). The electrolytically deposited
metal is found initially to deposit principally at the bottoms
of the pores and the immediately adjacent parts of the side
walls, and the resultant coatings or layers then grow
progressively in thickness upwards in the pores as more metal is
deposited. It is also found initially that discontinuous
patches 17 of the metal are deposited on the side walls in what
appears at present to be a random manner. After sufficient
metal has been electrolytically deposited the filling of the
pores is continued using eleetroless deposited metal 18 until,
in this embodiment, they are completely filled and a continuous
support layer has been formed over the entire surface of the
anodised layer 12. In this embodiment cobalt is used as the
initial electrolytically deposited metal 16 and 17, while
electroless nickel is employed for the metal 18. The deposition
of metal layers is continued to provide a semi-bright nickel
layer 20, a bright nickel layer 22 and a tri-chrome finish layer
24.
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The invention thus employs the electrolytic deposition
of pore filling metal to apply an inital "seed" coating to the
bottom wall portion of each pore and to at least the lower
portion of the side wall of each pore. It is found that such an
electrolytically deposited metal coating adheres very well to
the anodised material, and the subsequently electroless
deposited metal adheres very well to the electrolytically
deposited meal, whereas metals deposited by electroless
processes directly on aluminum oxide do not adhere well and
result in lower strength metal coatings. Electroless coating
processes have the advantage that they are more efficient than
ele~rolytic processes in filling the pores and result in more
dense or compact coatings, and the processes of the invention
enable advantage to be taken of this property while overcoming
the potential problem of insufficient adherence of the
electroless deposited metal to the anodised layer.
The anodised layer is inherently porous in structure
because of the manner of its formation, and a typical structure
of a layer 12 obtained by phosphoric acid anodising of the
aluminum substrate 10 is illustrated by Figure 2. For
convenience in drawing the horizontal surfaces of the layers are
shown as flat, but in practice they will be seen to be highly
irregular even at quite low magnification.
Figure 1 illustrates an embodiment in which an anodised
layer 12 of aluminum oxide (A1203) has been produced of
about 2 micrometres (20,000 Angstroms) thickness, typically by
use of phosphoric acid at about 20°C and of about 109g/litre or
10% by weight concentration, employing an anodising voltage of
about 50-60 volts for 10 minutes. The porous structure obtained
is relatively uniform, although highly idealised as shown in
Figure 1 for convenience in drawing, and typically the pores 14
will be found to average 0.09 micrometze (900 Angstzoms) in
transverse dimension, spaced on average about 0.07 micrometre
(700 Angstroms) from one another. The pores have an average
length/width ratio of 20:1. The bottoms of the pores do not end
at the surface of the aluminum substrate, but instead they are
on average spaced about 0.07 micrometre (700 Angstroms) from
that surface to form a continuous non-porous barrier layer 26 of
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the relatively non-conductive aluminum oxide, the thickness of
this layer depending principally directly on the value of the
anodising voltage. It may be noted that references herein and
in the literature to pore sizes, etc. are usually made in
Angstroms, while references to tt~.icknesses are made in
micrometres, merely to avoid the need to refer to large numbers
or small fractions, 1 micrometre being equal to 10,000 Angstroms.
It has been found possible in previous commercial
practice wit. sulf uric acid anodising that produces long narrow
pores of average transverse dimension 0.015 micrometre (150
Angstroms) to deposit pore filling metal layers that are
sufficiently strong and stable of up to about 5 micrflmetres
thickness, but beyond this value.the hydrogen that is generated
in the long, narrow pores (i.e, length to width ratio of about
330:1) by the electrolytic deposition process tends to cause
spalling of the anodised coating, destroying its strength to the
extent that it is unsuitable to receive and retain the pore
filling metal. Another problem is that it is difficult to
deposit a sufficiently adherent coating of a pore-filling metal
into the long narrow pores employing conventional D.C. plating
methods. Thus, there is too great a tendency for the plating
step to cause physical disruption of the anodised layer, so that
the plated metal layer is poorly adherent.
Metal deposition processes may use either alternating
current or direct current, or a combination thereof. A.C.
deposition is usually much slower that the equivalent D.C.
'a~ current and D.C, is therefore preferred if speed is important.
However, D.C. has a greater tendency to cause disruption of the
coating especially with the narrow pores characteristic of
sulfuric acid anodising. It is therefore also know to use
modified A.C., preferably one in which a predetermind
negative-going D.C. has been superi~osed on the A.C. Such a
system avoids the disrcption that would be produced by a pure
D.C. current. A.C. produces metal deposition owing to the
rectification characteristic of the aluminum oxide, but as the
thicknesses of the coatings increase such unmodified A.C.
deposition gives poorer pore penetration and slower deposition
rates. The D.C. component is therefore increased to the maximum
lzvel that does not cause aisruption. This method of deposition
1 341 32 7
is disclosed for example in U,S. Patent No. 4,226,680, assigned
to Alcan Research and Development Limited; these processes have
now become known as the Alcan 'ANOLOK' (Trade Mark) processes.
Other modified A.C. systems are also possible; for example,
another system offsets the A.C. waveform in a manner that will
produce an effective negative bias, while a further way is to
increase the amplitude of the negative portion of the waveform
relative to that of the positive portion, which again has the
same ef f ect .
The processes of the invention are applicable generally
to anodisable metals, their alloys and composites; whether
rolled, pressed, cast or wrought. Cast metals are generally
less dense and more porous in stru~ure than the corresponding
rolled, pressed or wrought product. Attempts to use only
electroless deposition, or only ele~rolytic deposition,
directly on the anodised surface of a cast material have not
been as successful as the processes of the invention because of
this higher porosity and because of the usual higher silicon
content (e. g. 7-12%) of such metals. Thus, the more porous
metal structure results in anodised layers of lower strength and
quality, and electrolytic deposits are more adversely affected
by the anodic layer quality than are electroless layers, it is
believed because of entrapment of some of the silicon in the
barrier layer which interferes with the normal flow of electrons
in the deposition current. On the other hand, as explained
above, layers applied by direct electroless deposition are
generally poorly adherent to the already lower strength anodised
layer.
Suitable substrate metals, in addition to aluminum and
its anodisable alloys, are magnesium~and its anodisable alloys.
Metals suitable for the ele~rolytic deposition of the initial
'seed' layer are cobalt, nickel, zinc, copper, tin and
palladium. When the substrate is aluminum or an alloy thereof
cobalt has been found to be particularly suitable for
electrolytic deposition and nickel for electroless deposi~ion.
It is found that as the thickness of the electrolytic
layers within ~he pores increases a point may be reached a~
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which the adhesion to the aluminum begins to decrease, and this
then sets an upper limit at which the electrolytic deposition
should be stopped and replaced by the electroless deposition.
Of course the electrolytic deposition can be discontinued
earlier depending upon the other parameters of the particular
process. It is found that the thickness of the initial seed
coatings, as measured from the bottom of the pores, correlates
well with the apparent colour of the substrate surface as seen
by an observer, and the table below shows a specific correlation
that is obtained when the anodised layer that has been produced
by sulfuric acid anodising is 5 micrometres thick; the
electrodeposited metal is cobalt. The same principle applies
with phosphoric acid anodising but the colours obtained are
slightly different. The thickness of the electroiytically
deposited metal is most expeditiously expressed in the units
milligrams (mg) of metal per square metre (m2) of anodic
surface. It is found with this combination that the cut-off for
good adhesion is between about 550 and about 850 mg/m2.
Colour mg Co/ %Fill Adhesion
_m2
Champagne 70 3.5 good
Very Light Bronze 180 9.0 good
Light Bronze 340 17.0 good
Medium Bronze 550 27.5 good
Dark Bronze 850 42.5 . poor
Black 2000 100.0 poor
It will be seen that as little as about 3% will give-
3U good adhesion and this is the minimum value for satisfactory
results, while at the upper thickness limit of medium bronze for
pores of 5 micrometres depth it is estimated that each pore is
about 30% filled in volume with the electrolytically deposited
metal, leaving the remaining 70% to be ffilled with the
electroless deposited metal. Nickel is found to produce
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approximately the same colour correlation as cobalt. Copper
produces a range of different colours extending from pink
through light maroon and dark maroon to black. Tin requires a
thicker anodised coating of about 10 micrometres before black is
obtained.
The amount of electroless applied metal that is
deposited in the pores will of course depend upon the required
properties and intended use of the resultant product, and for
some applications the pores may not need to be completely
filled; from about 6% to about 60% of complete filling may be
all that is required. Thus, while a useful range for the
electrolytic deposition is about 3% to about 30%, a useful range
for the electroless deposition is also from about 3% to the
remainder rec3uired to f ill the pores to the required extent.
Once the initial electrolytic seed deposit and the
subsequent electroless deposit have been applied the subsequent
processing steps will also depend upon the commercial
application of the resultant product and the characteristics and
appearance that are required. For example, the electroless
deposition can simply be continued until a final layer (over the
anodised layer) of adequate thickness is obtained, the usual
range for such an application being from about 50 micrometres to
about 75 micrometres. More usually the electroless deposition
is continued until it forms a support layer of adequate
thickness over the entire surface of the anodised layer, as
illutrated by Figures 1 and 2, the usual values being from about
U.5 micrometre to about 3 micrometres, more preferably in the
. range 1-2 micrometres. Thereafter, a finish layer (for example
chromium) may be applied over the support layer, with or without
the provision of one or more intermediate layers between the
support and finish layers.
The use of phosphoric acid ~is found to be particularly
advantageous with cast materials, and it has been found for
example that the cast aluminum such as is used for automotive
wheels the adhesion of tze final coatings was inc:eased by at
least 50% upon use of phosphoric acid in place of sulfuric acid
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. for the anodising. A suitable test for adhesion is to cut the
finished part through the substrate and coatings and then to
attempt to lift or peel the coating away from the substrate by
use of a sharp knife edge; it was found possible with this test
to peel the coatings from sulfuric acid anodised substrates with
various degrees of difficulty depending on the processing
conditions, but not possible to peel it from phosphoric acid
anodised substrates.
The use of an anodised layer before plating introduces
the possibility, if desired, of a reduction in the thickness of
the subsequent plated layers with consequent cost savings. The
anodising processes described employing acid baths in the
temperature range of 20-35°C are usually characterised as
"conventional" anodising, but "hard anodising" processes can
also be employed for the invention, the usual bath temperature
being in the range 3-7°C; such hard anodised layers are usually
thicker than the conventional anodised layers. Further
reductions in the subsequent layers therefore are possible by
using a thicker and/or stronger anodic film such as that
produced using these lower temperature anodising processes.
Such hard layers also constitute an excellent basis for the
pore-filling metal deposition characteristic of the invention,
the electroless layer being the support layer for further
deposits, which can be thinner than those normally previously
used. It will be understood that this industry is particularly
cost conscious, especially with regard to the relatively
expensive corrosion-resistant metals that are employed in the.
intermediate and finish coatings, so that any saving that can be
achieved in their thickness for ah equivalent performance in
protection and/or appearance is commercially important.
In the processes of the invention the anodised layer 10
can be of a thickness in the range 0.5 - 50 micrometres, usually
in the range 1-10 micrometres, preferably in the range 2-6
micrometres, and more preferably 3-5 micrometres, with a
G:ic:cness of 5 micrometres being usually commercially suitable.
The electroless-deposited pore-filling material need not form a
1 34 ~ 32 7
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support coating of more than about 2 micrometres thickness and
excellent results can be obtained with the application of a
single thin finish coating of chromium over the support layer.
The preferred electroless deposited metal is nickel. Metals
other than nickel, such as cobalt, tin or copper, can also be
used. Because of the thin coatings that are employed it is
preferred in some processes to pre-treat the surface of the
anodisable metal to obtain a very smooth surface; this can be a
'macro" treatment by buffing and/or a "micro" trea'-,.ment of
chemical or electro-brightening. The finished chromium layer if
provided preferably is of thickness in the range of 0.2-0.3
micrometres.
The invention is further illustrated by the following
specific example:
Example 1
The process is employed to provide a bright finishing
procedure for articles such as cast aluminum automotive wheels,
giving a simulation of the appearance of bright chrome or
stainless steel, and includes the following steps.
1. An aluminum substrate consisting of cast alloy
A413, or forge grade material, is pre-treated by cleaning with
appropriate alkaline and/or acid solutions, or is pretreated by
mechanical buffing.
2. The pretreated substrate is then subjected to a
phosphoric acid anodising treatment using acid of 109g/L (10%)
concentration by weight at 21°C; the anodising is begun at 60VDC
for 30 minutes and is then tamped down to approximately 18 VDC
for about 0.5 minute.
3. The anodised substrate has the initial 'seed"
ele~rolytic coating of cobalt applied using a cobalt-based
'ANOLOR" (trade mark) electrolyte as disclosed in U.S. Patent
No. 3,616,309 at 21°C and 12.5 V.A.C., with or without up to 4
V.D.C. bias. The electolytic deposition proceeds for a period
of about one minute.
4. A pore-filling coating of nickel is then applied by
immersion in an electroless nickel solution (Harshaw "Alpha
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1038" - trade mark) for 20 minutes at pH4.7 and temperature
93°C, thus completely filling the pores and forming a support
coating of about 0.5 to about 3.0 micrometres thickness
5. The support layer is coated with an
electrolytically-deposited semi-bright layer of nickel of about
micrometres thickness using Harshaw "PERFLOw" (trade mark)
semi-bright solution at pH4.3; temperature 57°C; current
density 5 amps per square decimetre (A/dm2); and for a period
of thirty minutes.
10 6. The semi-bright layer is coated with a bright layer
of nickel of about 10 micrometres thickness using Harshaw
"SUPREME" (trade mark) bright solution at pH4.0; temperature
66°C; current density 4A/dm2 and period 10 minutes.
7. The example is completed by electrolytically
depositing a trichrome finish layer using Harshaw "TRI-C~ROh~
PLUS" (trade mark) solution at pH2.7; temperature 30°C; current
density 10 A/dm2 for 5 minutes.
During the process the substrate will be rinsed in
known manner which need not be detailed here. It may be noted
that in this and the other examples described a cyanide or
hexavalent chromium bath is not used, which is environmentally
desirable.
Example 2
In the process of Example 1 the cobalt-based
electrolyte employed to deposit the initial layer is replaced
with a copper-based electrolyte comprising for example 35g/1 of
CuS04.5H20; 20g/1 MgS04.7H20 and 5g/1 H2S04 at pH
1.3 and 21°C.
Example 3
In the process of Example 1 the cobalt-based
electrolyte employed to deposit the initial layer is replaced
with a tin-based electrolyte comprising for example lOg/1 of
SnS04 and 20g/1 of H2S04 at pH 1.3 and 21°C.
Example 4
In the process of Example 1 the cobalt-based
electrolyte employed to deposit the initial layer is replaced
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with a conventional Watt's nickel-based electrolyte comprising
for example 240g/1 of NiS04.6H20; 60g/1 of NiC12.6H20
and 45g/1 of H3803 at pH 4.5 and 21°C.
Example 5
In the process of Example 1 the cobalt-based
electrolyte employed to deposit the initial layer is replaced
with a palladium-based electrolyte comprising for examaple a 10
ml/litre 'PAr.~R~E' (trade mark) aqueous solution of Technic
Inc. This process is particularly suited for articles with a
simulated stainless look for exterior application.
Example 6
To obtain a bright black finish, especially for cast
aluminum automobile wheels, the process of any of of examples 1
through 5 is followed by the deposition of a black-chrome finish
layer using Harshaw 'Ci~tOMONYX" (trade mark) solution at
temperatures 21°C; current density 10-40 A/dm2 and period of 5
minutes.
Example 7
To obtain articles with a simulated appearance of
stainless steel and for interior applications a substrate cast
alloy is subjected to the processes of any one of examples 1
through 5 with the omission of the deposition of the semi-bright
nickel layer.
Example 8
,25 To obtain machine parts and articles suitable for other
engineering applications the anodising and electrolytic
deposition steps of any one of examples 1 through 5 are followed
by a nickel electroless deposition step in which the deposition
period is about 30 minutes to about 120 minutes, as required, to
give layers of about 10.0 micrometres to about 40.0 micrometres
thickness.
Example 9
To obtain black material particularly suitable for the
fabrication of solar panels a substrate is subjected to the
anodisi:.g, initial electrolytic plating and electroless plating
steps of any one of examples 1 through 5, followed by the
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deposition of semi-bright nickel for a reduced period of about
lU to about 20 minutes, and the deposition of black chrome by
the step in example 6.
Example 10
Bright aluminum plated composite articles are prepared
fran a substrate of composite material AA-6061 (incorporating
10$ by weight of aluminum oxide) using the plating procedure of
any one of examples 1 through 5.
