Note: Descriptions are shown in the official language in which they were submitted.
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Method of electroplating and pre-treating aluminium workpieces
FIELD OF THE INVENTION
The present invention relates to a method of applying a metal layer onto
at least one surface of an aluminium or aluminium alloy workpiece or article,
comprising the steps of a simple pre-treating step of cleaning and activating
the
surface and yet producing good adhesion of the subsequently applied metal
layer. The invention also relates to an aluminium alloy product plated on at
least one surface with a metal layer. More specifically, the invention relates
to a
method of applying a metal layer of a braze-promoting metal onto the clad
layer
of an aluminium alloy brazing sheet product to be used in a fluxless brazing
operation.
As will be appreciated herein below, except as otherwise indicated, all
alloy designations and temper designations refer to the Aluminium Association
designations in Aluminium Standards and Data and the Registration Records,
as published by the Aluminium Association.
BACKGROUND OF THE INVENTION
Nickel plating of aluminium products is widely used because nickel
provides a bright, shiny appearance, is long lasting and can conduct
electricity.
Another, more particular use of nickel plating is made in the manufacture of
brazing sheet products. Aluminium alloy brazing sheets comprise an aluminium
alloy core and a clad layer of filler alloy on one or both sides. Aluminium
brazing sheets are widely used, e.g., in the production of heat exchangers.
However, the use of aluminium-silicon alloys as filler material is
problematic because the aluminium oxide layer has to be disrupted during
brazing. This may be effected by applying a chemical flux onto the workpiece
before brazing. Fluxes for use in brazing aluminium alloys usually consist of
mixtures of alkali and alkaline earth chlorides and fluorides or cryolite. The
flux
operates at the brazing temperature to disrupt, spread and dissolve the oxide
film. However, applying the chemical flux onto the workpiece is a rather
laborious and therefore expensive process.
In the past, fluxless brazing techniques have therefore been developed
and employed as described for example in US-2003/0098338-A1, incorporated
herein by reference in its entirety. In one such technique, a braze-promoting
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metal of cobalt, iron, or more preferably nickel, is coated on a part to be
brazed.
During brazing, the nickel reacts exothermically with the underlying aluminium
alloy, thereby disrupting the aluminium oxide layer and permitting the
underlying molten aluminium clad metal to flow together and join. As this
method does not require a fluoride flux, it is also suitable for utilization
with
magnesium-enriched aluminium alloys, such as are beneficially used in heat-
exchanger constructions.
In addition to the nickel, iron or cobalt coating, a wetting agent may also
be added in order to improve the wettability of the clad alloy during the
brazing
process. However, nickel plating requires extensive pre-treatment of the
metal surface such as cleaning, etching, desmutting, etc. This is again due to
the presence of the tenacious oxide layer. If the aluminium alloy surface has
not been properly pre-treated, the nickel coating will either have poor
adhesion,
or will be contaminated and thereby impede the brazeability of the product.
Therefore, nickel plating with all necessary pre-treatment steps is an
expensive
and environmentally unfriendly process.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method for applying by
electroplating a metal layer onto an aluminium alloy product, which method
requires as few steps as possible, and does not require the use of fluoride
containing components.
It is furthermore an object of the present invention to obtain a metal
coated aluminium alloy product, wherein the applied metal coating adheres well
and may serve to break up the oxide layer during a subsequent brazing
operation.
r
The present invention solves one or more of these objects through the
method of applying a metal coating according to claim 1 and the aluminium
alloy product according to claim 14.
The method of applying a metal layer onto at least one surface of an
aluminium or aluminium alloy workpiece, comprises the steps of pre-treating
the surface by cathodic activation in a pre-treatment bath containing
sulphuric
acid and metal-ions selected from the group consisting of nickel, iron and
cobalt, and applying a metal layer by electroplating the pre-treated
workpiece,
and wherein the metal layer is selected from the group consisting of nickel,
iron,
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cobalt, and alloys thereof. It will be immediately clear to the skilled person
that
when the applied metal layer contains nickel or an a nickel-alloy that the pre-
treatment bath should contain nickel-ions, and where iron or cobalt or alloys
thereof are being applied that the pre-treatment bath contains iron-ions and
cobalt-ions respectively.
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 shows an angle-on-coupon for brazing tests.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention is based on the finding that direct metal, for example nickel,
plating of aluminium alloy products is possible after cathodic activation in a
simple sulphuric acid solution to which only nickel ions, e.g. in the form of
nickel
sulphate, has been added. No fluoride components are needed in this
activation process. Because the activation bath contains the same ingredients
as a Watts bath which is preferably used as nickel plating bath, cross-
contamination is excluded. Also, no problems in effluent treatment are
expected.
It is believed that, during cathodic activation in the sulphuric acid solution
containing nickel ions, nickel nuclei may be created through the thin
aluminium
oxide film on the surface, thereby forming anchor spots for the nickel layer
which is applied in a subsequent plating step. Thus, the cathodic activation
step
has the same effect as the creation of a thin bonding layer between the
aluminium surface and the nickel coating. The same applies mutatis mutandis
for the situation where iron or cobalt is being used.
The pre-treatment bath preferably contains about 15 to 200 g/1, and
preferably 80 to 150 g/l, of NiSO4-H2O, and about 50 to 350 g/I, preferably
about 150 to 250 g/I of H2SO4. In a preferred embodiment, the pre-treatment
bath also contains boric acid as a buffer, e.g. in a range of I to 50 g/l, and
preferably 20 to 40 g/l.
The preferred bath for pure nickel plating is a Watts bath containing nickel
sulphate, nickel chloride and boric acid.
A preferred bath for nickel-bismuth plating is a citrate-gluconate bath
containing nickel sulphate, nickel chloride, (NH4)2SO4, bismuth concentrate,
sodium citrate and sodium gluconate. Preferred concentration ranges of these
substances are 100 to 180 g/I of NiSOa.-6H2O, 10 to 50 g/I of NiC12=6H2O, I to
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mI/I of a bismuth concentrate containing 100 g/I of Bi, 10 to 50 g/l of
(NH4)2SO4, 100 to 180 g/I of sodium citrate=2H2O, and 10 to 50 g/I of sodium
gluconate. This bath may be used for pure nickel plating as well, in which
case
the bismuth concentrate is omitted.
5 It has been found that the pre-treatment is already effective at elevated
temperatures of less than 95 C, and preferably in the range of 55 C and 80 C.
This is a great advantage, since working at lower temperatures makes the
introduction into a strip plating line much easier, because evaporation losses
will be limited. Furthermore, aluminium dissolution is much lower at
10 temperatures below 70 C, thereby increasing the lifetime of the activation
bath.
Hence, the pre-treatment bath is preferably maintained at temperatures
between 55 C and 80 C, and most preferred between about 60 C and 70 C.
The activation current is cathodic. As demonstrated by the examples, the
current density is not critical to the quality of the final product. The same
applies
to activation time of the product in the pre-treatment bath. The activation
current of the cathodic activation is preferably in a range of -200 to -2000
A/m2, and more preferably in a range of -500 to -1400 A/m2. The time spent by
the product in the pre-treatment bath is typically in the range of 1 to 50
sec.,
and preferably in the range of 5 to 15 sec.
The average thickness of the applied metal layer of Ni, Co, Fe or alloys of
each of these metals, is preferably less than 2 pm, more preferably less than
1.0 pm, and even more preferably in a range of 0.2 to 1.0 pm.
The method is preferably carried out as a continuous plating operation,
which allows the continuous treatment of an infinite strip of metal.
In an optional additional step a further metal layer may be applied on top
of the layer of Ni, Fe, Co, or alloys thereof, in order to improve for example
the
corrosion resistance of the final product. For example a thin layer of tin can
be
applied onto the nickel-layer on a brazing sheet product, which results in a
significant improvement of the post-braze corrosion resistance.
The method according to this invention may include the additional step of
degreasing of the surface prior to the cathodic activation and/or the
electroplating step in order the clean the surface.
To avoid work hardening of soft annealed coils while processing them in a
(vertical) plating line, it is advantageous to plate full hard material.
Moreover,
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full hard material is easier to slit than soft annealed material. Thus, it is
preferred to plate wide coils in full hard condition and split them afterwards
into
multiple coils of desired width, thereby reducing conversion costs. The coils
may be soft annealed afterwards.
5 In an embodiment of the method according to the invention the aluminium
workpiece is a brazing sheet product, the brazing sheet product including a
core layer and a clad layer formed of a brazing alloy including aluminium and
2-
18 wt.% silicon, preferably in the range of 7 to 14%, (such as AA4343 and 4045
alloys), and whereby the metal layer is applied on the clad layer. The metal
layer of nickel, iron, cobalt or alloys of each of these metals act as a braze-
promoting element during brazing.
In a preferred embodiment of the brazing sheet product the clad layer
further comprises a wetting agent as alloying element in a range of up to 1
wt.%
in order to improve the wettability of the clad alloy during the brazing
process.
And preferably the wetting agent is selected from the group consisting of
lead,
bismuth, lithium, antimony, tin, silver, thallium and any mixture thereof.
In another embodiment of the method according to the invention the
aluminium workpiece is an aluminium conductor, and preferably made of an
alloy selected from the group consisting of AA1370, AA1110 and AA6101. The
aluminium conductor can be in the form of an aluminium strip or aluminium wire
or aluminium tube. For the embodiment the applied metal layer is preferably
consisting of nickel in order to improve the electrical contact properties.
The
aluminium conductors can be used for the transmission of electrical and/or
thermal energy. These conductors are usually in the form of bars, wire or
cables when used as electrical conductors, and in the form of strips, bars or
tubes when used as thermal conductors.
In a further aspect of the invention there is provided an aluminium alloy
product, preferably a brazing sheet product, electroplated with a metal layer
selected from the group consisting of nickel, iron, cobalt and alloys thereof
manufactured with the method of the invention as set out in the present
specification and claims. Such a brazing sheet product can be applied
successfully in a Controlled Atmosphere Brazing ("CAB") process in the
absence of a brazing flux.
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As shown by the following examples, the aluminium alloy product
according to the invention has an excellently adhering nickel or nickel-
bismuth
coating. In a particularly preferred embodiment, the product is an aluminium
alloy brazing sheet comprising a core, a clad layer and a nickel-containing
layer
plated on top of the clad layer. This brazing sheet will have good
brazeability
and low manufacturing costs. It may either contain a wetting agent like Bi in
the
clad alloy, or in the nickel-containing layer.
The following non-limiting examples illustrate the invention.
EXAMPLE
Two different types aluminium brazing sheets products of 0.4 mm
thickness have been used for plating with a nickel or nickel-alloy layer
having
an average thickness of 0.5 pm. The aluminium brazing sheets used consisted
of an AA3003-series aluminium core alloy conventionally clad on both sides
with an AISi brazing alloy, whereby clad layer A contained, in wt.%, 10% Si,
1.5% Mg and 0.08% Bi, whereas clad layer B contained, in wt.%, 12%Si and no
Mg or Bi.
In producing nickel plated brazing sheet products the following procedure
has been used:
- Cleaning for 180 sec. at 50 C using 35 g/I ChemTec 30014 (a
commercial available bath), followed by rinsing;
- Activation using a current density of -1000A/m2, followed by rinsing;
- Ni or Ni-Bi plating using a current density of -1000A/m2, followed by
rinsing.
The cathodic activation bath in accordance with the invention was
prepared on basis of sulphuric acid (see Table 1). Nickel sulphate was
selected
to supply nickel-ions to the solution, and preferably boric acid was added as
buffer. As an alternative a fluoride based activation bath was used (see Table
2) and consisting of anodic activation at a current density of +1000A/m2, and
which is disclosed in US-6,780,303-B2, incorporated herein by reference. The
cathodic activation was carried at various temperatures. Two Samples 10 and
11 have been carried out using the same activation bath but whereby the
current was reversed such that anodic activation occurred.
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After activation, either a nickel layer was plated from a Watts bath (see
Table 3) or a nickel-bismuth alloy layer from a citrate-gluconate bath (see
Table
4).
The quality of the resulting plated substrates were evaluated using an
adhesion test and a brazeability test. The adhesion tests consisted on the
Erichsen dome test (cup height of 5 mm), whereafter adhesive tape (Scotch
Tape 3M No. 610) is applied to the deformed area and pulled off in one move.
Adhesion is quantified by classifying the amount of nickel on the tape. An
overall adhesion assessment was rated from 1(poor) to 10 (excellent), wherein
a level of 6 was considered acceptable as it was comparable to existing
commercially available brazing sheet with a Ni-Pb layer.
On a laboratory scale of testing the brazing tests were carried out in a
small quartz furnace. Small coupons of 25 mm x 25 mm were cut out of the
nickel-bismuth-plated sheets. A small strip of a bare AA3003 alloy measuring
30 mm x 7 mm x 1 mm was bent in the centre to an angle of 45 and laid on the
coupons (see Fig. 1). The angle-on-coupon samples were heated under flowing
nitrogen, with heating from room temperature to 580 C, dwell time at 580 C for
1 minute, cooling from 580 C to room temperature. The brazing process was
judged on possible formation of wrinkles, capillary depression and fillet
formation. An overall assessment was given where: (-) = poor brazeability, ( )
= fair brazeability, and (+) = good brazeability.
The results of the experiments carried out and the adhesion and brazing
performance for the various samples are summarized in Table 5.
Table 1. Composition of the cathodic activation bath.
NiSO4=6H2O 100 g/I
96 % H2SO4 (S.G. 1.84 kg 1") 200 g/l 113.2 mi/I
H3BO3 30 g/l
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Table 2. Composition of the hydrofluoric acid bath.
NiCI2=6H20 125 g/I
40 /a HF (S.G. 1.13 kg 1-1) 2.7 g/I 6 ml/I
H3BO3 12.5 g/I
Table 3. Composition of the Watts bath.
NiSO4=6H20 270 g/I
NiCI2=6H20 50 g/I
H3B03 30 g/I
Table 4. Composition of the citrate-gluconate bath.
NiSO4=6H20 142 g/I
NiCI2=6H2O 30 g/I
(NH4)2SO4 34 g/I
Na-citrate=2H20 140 g/I
Na-gluconate 30 g/I
bismuth concentrate (100 g/I Bi) 5 mI/I
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Table 5. Summary of the experiments carried and the results on adhesion and
brazeability.
Sample Clad Activation Plating Adhesion Brazeability
layer Ni / Ni-
Bi
Activation Temperature
bath [ C ]
1 B H2SO4 70 Ni-Bi 5
2 B H2SO4 93 Ni-Bi 4
3 A H2SO4 50 Ni 6 +
4* A HF 50 Ni 9 +
A none - Ni 1 -
6 A H2SO4 45 Ni 4 +
7 A H2SO4 60 Ni 9 +
8 A H2SO4 70 Ni 10 +
9 A H2SO4 93 Ni 9 +
10* A H2SO4 70 Ni 9 -
11 * A H2SO4 93 Ni 10 -
(*) Samples 4, 10 and 11 involved anodic activation instead of cathodic
5 activation as per Sample I to 3 and 6 to 9.
From the comparison of Samples 3, 4 and 5 it can be seen that if no
activation is used both the adhesion and the brazeability is poor. Whereas a
hydrofluoric acid bath obtains good adhesion and good brazeability,
comparable to a sulphuric acid bath. However, a hydrofluoric acid bath
contains
fluoride and is therefore not the environmentally preferred method.
From examples 10 and 11 it can be seen that anodic activation provides
excellent adhesion, but brazeability is seriously undermined possibly due to
the
formation of an oxide film. In accordance with the invention it has been found
that both good adhesion and brazeability were obtained by cathodic activation.
However, for those applications were brazeability is not required such as with
aluminium conductors it might be a valuable pre-treatment method.
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The temperature of the cathodic activation bath appeared to have a
strong influence on the adhesion of the plated Ni layer. Sample 7 shows that
adhesion and brazeability are still excellent at temperatures of about 60 C.
However, if the temperature is lowered further to below 50 C (Sample 6), the
5 adhesion level becomes unacceptable. From Samples I and 2 and 8 and 9 it
can be seen that neither adhesion nor brazeability suffers when the
temperature is lowered from 93 C to 70 C. This makes introduction into a
continuous strip plating line much easier, because evaporation losses will be
limited. Furthermore, aluminium dissolution is much less at 70 C or lower,
10 thereby increasing the lifetime of the activation bath.
The addition of a wetting agent such as Bi is favourable for the
brazeability performance of the resultant brazing sheet product. From the
Samples I and 8 it can be seen that the wetfiing agent might be added either
to
the Ni layer or to the brazing clad layer without affecting the adhesion or
the
brazeability. Adding the wetting agent to both the clad layer and the nickel
layer
has no adverse effect on the brazeability.
Thus, it has been demonstrated that direct nickel plating of a brazing
sheet product is possible after cathodic activation in a simple sulphuric acid
solution to which only nickel sulphate is added. No fluoride is needed in this
activation process. Because the activation bath contains the same ingredients
as a Watts bath, cross-contamination is excluded. Also, no problems in
effluent
treatment are expected. Satisfying results are obtained at bath temperatures
of
about 60 C. The process can be operated in a reliable manner over a wide
range of current densities and treatments times.
It is believed that similar results will be obtained where iron or cobalt
instead of nickel is used as braze-promoting metal on the brazing sheet
product.
Having now fully described the invention, it will be apparent to one of
ordinary skill in the art that many changes and modifications can be made
without departing from the spirit or scope of the invention as herein
described.
