U.S. patent application number 11/320488 was filed with the patent office on 2006-07-20 for method of electroplating and pre-treating aluminium workpieces.
This patent application is currently assigned to Corus Aluminium Walzprodukte GmbH. Invention is credited to Jacques Hubert Olga Joseph Wijenberg, Adrianus Jacobus Wittebrood.
Application Number | 20060157352 11/320488 |
Document ID | / |
Family ID | 36682746 |
Filed Date | 2006-07-20 |
United States Patent
Application |
20060157352 |
Kind Code |
A1 |
Wijenberg; Jacques Hubert Olga
Joseph ; et al. |
July 20, 2006 |
Method of electroplating and pre-treating aluminium workpieces
Abstract
Disclosed is a method of applying a metal layer onto at least
one surface of an aluminium or aluminium alloy workpiece, including
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.
The metal layer is selected from the group consisting of nickel,
iron, cobalt, and alloys thereof.
Inventors: |
Wijenberg; Jacques Hubert Olga
Joseph; (Amsterdam, NL) ; Wittebrood; Adrianus
Jacobus; (Velserbroek, NL) |
Correspondence
Address: |
STEVENS DAVIS MILLER & MOSHER, LLP
1615 L STREET, NW
SUITE 850
WASHINGTON
DC
20036
US
|
Assignee: |
Corus Aluminium Walzprodukte
GmbH
Koblenz
DE
|
Family ID: |
36682746 |
Appl. No.: |
11/320488 |
Filed: |
December 29, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60644530 |
Jan 19, 2005 |
|
|
|
Current U.S.
Class: |
205/213 |
Current CPC
Class: |
C25D 5/44 20130101; C25D
5/12 20130101; C25D 3/12 20130101; C25D 3/20 20130101 |
Class at
Publication: |
205/213 |
International
Class: |
C25D 5/44 20060101
C25D005/44; C25D 11/16 20060101 C25D011/16 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 19, 2005 |
EP |
05075082.7 |
Claims
1. A method of applying a metal layer onto at least one surface of
an aluminium or aluminium alloy workpiece, comprising 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,
cobalt, and alloys thereof.
2. The method according to claim 1, wherein the pre-treatment bath
comprises 15 to 200 g/l of NiSO.sub.4.H.sub.2O and 50 to 350 g/l of
H.sub.2SO.sub.4.
3. The method according to claim 1, wherein the pre-treatment bath
comprises 80 to 150 g/l of NiSO.sub.4.H.sub.2O and 150 to 250 g/l
of H.sub.2SO.sub.4.
4. The method according to claim 1, wherein the pre-treatment bath
further comprises boric acid (H.sub.3BO.sub.3).
5. The method according to claim 1, wherein the pre-treatment bath
further comprises 1 to 50 g/l boric acid (H.sub.3BO.sub.3).
6. The method according to claim 1, wherein the pre-treatment bath
further comprises 20 to 40 g/l boric acid (H.sub.3BO.sub.3).
7. The method according to claim 1, wherein the plating bath
comprises a Watts bath.
8. The method according to any one of claims 1, wherein the plating
bath comprises a Watts bath comprising a mixture of nickel sulphate
(NiSO.sub.4), nickel chloride (NiCl.sub.2) and boric acid
(H.sub.3BO.sub.3).
9. The method according to claim 1, wherein the plating bath
comprises a citrate-gluconate bath comprising a mixture of
NiSO.sub.4, and (NH.sub.4).sub.2SO4 and sodium citrate and sodium
gluconate.
10. The method according to claim 1, wherein the plating bath
comprises a citrate-gluconate bath comprising a mixture of
NiSO.sub.4, nickel chloride, (NH.sub.4).sub.2SO.sub.4, bismuth
concentrate, sodium citrate and sodium gluconate.
11. The method according to claim 1, wherein the plating bath
comprises a citrate-gluconate bath comprising a mixture of 100 to
180 g/l of NiSO.sub.4.6H.sub.2O, 10 to 50 g/l of
NiCl.sub.2.6H.sub.2O, 1 to 10 ml/l of a bismuth concentrate
containing 100 g/l of Bi, 10 to 50 g/l of (NH.sub.4).sub.2SO.sub.4,
100 to 180 g/l of sodium citrate.2H.sub.2O, and 10 to 50 g/l of
sodium gluconate.
12. The method according to claim 1, wherein the plating bath
comprises a citrate-gluconate bath comprising a mixture of
NiSO.sub.4, nickel chloride, (NH.sub.4).sub.2SO.sub.4, sodium
citrate and sodium gluconate.
13. The method according to claim 1, wherein the plating bath
comprises a citrate-gluconate bath comprising a mixture of 100 to
180 g/l of NiSO.sub.4.6H.sub.2O, 10 to 50 g/l of
NiCl.sub.2.6H.sub.2O, 10 to 50 g/l of (NH.sub.4).sub.2SO.sub.4, 100
to 180 g/l of sodium citrate.2H.sub.2O, and 10 to 50 g/l of sodium
gluconate.
14. The method according to claim 1, wherein the pre-treatment bath
is devoid of any fluoride containing components.
15. The method according to claim 1, wherein the temperature of the
pre-treatment bath is maintained at an elevated temperature in a
range of above room temperature to 95.degree. C.
16. The method according to claim 1, wherein the temperature of the
pre-treatment bath is maintained at a temperature in a range of
55.degree. C. to 80.degree. C.
17. The method according to claim 1, wherein the temperature of the
pre-treatment bath is maintained at a temperature in a range of
between about 60.degree. C. and 70.degree. C.
18. The method according to claim 1, wherein the cathodic
activation applies an activation current in a range of -200 to
-2000 A/m.sup.2,
19. The method according to claim 1, wherein the cathodic
activation applies an activation current in a range of -500 to
-1400 A/m.sup.2.
20. The method according to claim 1, wherein the time spent by the
product in the pre-treatment bath is in the range of 1 to 50
sec.
21. The method according to claim 1, wherein the time spent by the
product in the pre-treatment bath is in the range of 5 to 15
sec.
22. The method according to claim 1, wherein the applied metal
layer has an average thickness of less than 2 .mu.m.
23. The method according to claim 1, wherein the applied metal
layer has an average thickness of less than 1.0 .mu.m.
24. The method according to claim 1, wherein the applied metal
layer has an average thickness of 0.2 to 1.0 .mu.m.
25. The method according to claim 1, further comprising applying a
second metal layer on top of the layer of Ni, Fe, Co, or alloys
thereof.
26. The method according to claim 1, wherein the layer of Ni, Fe,
Co, or alloys thereof is a nickel-containing layer, further
comprising applying a tin-containing layer on top of the
nickel-containing layer.
27. The method according to claim 1, wherein the method is carried
out as a continuous plating operation.
28. The method according to claim 1, wherein 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, and whereby the metal
layer is applied on the clad layer.
29. The method according to claim 28, wherein the clad layer
further comprises as alloying element a wetting agent in a range of
up to 1 wt. %.
30. The method according to claim 29, wherein the wetting agent is
selected from the group consisting of lead, bismuth, lithium,
antimony, tin, silver, thallium and any mixture thereof.
31. The method according to claim 1, wherein the aluminium
workpiece is an aluminium conductor, and preferably made of an
alloy selected from the group consisting of AA1370, AA1110 and
AA6101.
32. An aluminium alloy product, electrolytically plated with a
metal layer selected from the group consisting of nickel, iron,
cobalt and alloys thereof by using the method of claim 1.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This claims the benefit of U.S. provisional patent
application No. 60/644,530, filed Jan. 19, 2005 and European patent
application number EP-05075082.7 filed Jan. 19, 2005, both
incorporated herein by reference in their entirety.
FIELD OF THE INVENTION
[0002] 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.
[0003] 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
[0004] 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.
[0005] 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.
[0006] In the past, fluxless brazing techniques have therefore been
developed and employed as described for example in
US2003/0098338-A1, incorporated herein by reference in its
entirety. In one such technique, a braze-promoting 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.
[0007] 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
[0008] 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.
[0009] 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.
[0010] The present invention solves one or more of these objects
through a method of applying a metal coating and an aluminium alloy
product.
[0011] 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, 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
[0012] FIG. 1 shows an angle-on-coupon for brazing tests.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] 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.
[0014] 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.
[0015] The pre-treatment bath preferably contains about 15 to 200
g/l, and preferably 80 to 150 g/l, of NiSO.sub.4.H.sub.2O, and
about 50 to 350 g/l, preferably about 150 to 250 g/l of
H.sub.2SO.sub.4. In a preferred embodiment, the pre-treatment bath
also contains boric acid as a buffer, e.g. in a range of 1 to 50
g/l, and preferably 20 to 40 g/l.
[0016] The preferred bath for pure nickel plating is a Watts bath
containing nickel sulphate, nickel chloride and boric acid.
[0017] A preferred bath for nickel-bismuth plating is a
citrate-gluconate bath containing nickel sulphate, nickel chloride,
(NH.sub.4).sub.2SO.sub.4, bismuth concentrate, sodium citrate and
sodium gluconate. Preferred concentration ranges of these
substances are 100 to 180 g/l of NiSO.sub.4.6H.sub.2O, 10 to 50 g/l
of NiCl.sub.2.6H.sub.2O, 1 to 10 ml/l of a bismuth concentrate
containing 100 g/l of Bi, 10 to 50 g/l of (NH.sub.4).sub.2SO.sub.4,
100 to 180 g/l of sodium citrate.2H.sub.2O, and 10 to 50 g/l of
sodium gluconate. This bath may be used for pure nickel plating as
well, in which case the bismuth concentrate is omitted.
[0018] It has been found that the pre-treatment is already
effective at elevated temperatures of from above room temperature
to less than, or up to, 95.degree. C., and preferably in the range
of 55.degree. C. to 80.degree. 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
temperatures below 70.degree. C., thereby increasing the lifetime
of the activation bath. Hence, the pre-treatment bath is preferably
maintained at temperatures between 55.degree. C. and 80.degree. C.,
and most preferred between about 60.degree. C. and 70.degree.
C.
[0019] 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/Nm.sup.2,
and more preferably in a range of -500 to -1400 A/m.sup.2. 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.
[0020] The average thickness of the applied metal layer of Ni, Co,
Fe or alloys of each of these metals, is preferably less than 2
.mu.m, more preferably less than 1.0 .mu.m, and even more
preferably in a range of 0.2 to 1.0 .mu.m.
[0021] The method is preferably carried out as a continuous plating
operation, which allows the continuous treatment of an infinite
strip of metal.
[0022] 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.
[0023] 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.
[0024] 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, 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] The following non-limiting examples illustrate the
invention.
EXAMPLES
[0031] 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 .mu.m. The
aluminium brazing sheets used consisted of an AA3003-series
aluminium core alloy conventionally clad on both sides with an AlSi
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.
[0032] In producing nickel plated brazing sheet products the
following procedure has been used:
[0033] Cleaning for 180 sec. at 50.degree. C. using 35 g/l ChemTec
30014 (a commercial available bath), followed by rinsing;
[0034] Activation using a current density of -1000A/m.sup.2,
followed by rinsing;
[0035] Ni or Ni-Bi plating using a current density of
-1000A/m.sup.2, followed by rinsing.
[0036] 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/m.sup.2, and which is disclosed in U.S. Pat. No. 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.
[0037] 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).
[0038] 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.
[0039] On a laboratory scale of testing the brazing tests were
carried out in a small quartz furnace. Small coupons of 25
mm.times.25 mm were cut out of the nickel-bismuth-plated sheets. A
small strip of a bare AA3003 alloy measuring 30 mm.times.7
mm.times.1 mm was bent in the centre to an angle of 45.degree. 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.degree. C., dwell time at 580.degree. C. for 1 minute,
cooling from 580.degree. 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.
[0040] The results of the experiments carried out and the adhesion
and brazing performance for the various samples are summarized in
Table 5. TABLE-US-00001 TABLE 1 Composition of the cathodic
activation bath NiSO.sub.4.6H.sub.2O 100 g/l 96% H.sub.2SO.sub.4
(S.G. 1.84 kg l.sup.-1) 200 g/l 113.2 ml/l H.sub.3BO.sub.3 30
g/l
[0041] TABLE-US-00002 TABLE 2 Composition of the hydrofluoric acid
bath NiCl.sub.2.6H.sub.2O 125 g/l 40% HF (S.G. 1.13 kg l.sup.-1)
2.7 g/l 6 ml/l H.sub.3BO.sub.3 12.5 g/l
[0042] TABLE-US-00003 TABLE 3 Composition of the Watts bath
NiSO.sub.4.6H.sub.2O 270 g/l NiCl.sub.2.6H.sub.2O 50 g/l
H.sub.3BO.sub.3 30 g/l
[0043] TABLE-US-00004 TABLE 4 Composition of the citrate-gluconate
bath NiSO.sub.4.6H.sub.2O 142 g/l NiCl.sub.2.6H.sub.2O 30 g/l
(NH.sub.4).sub.2SO.sub.4 34 g/l Na-citrate.2H.sub.2O 140 g/l
Na-gluconate 30 g/l bismuth concentrate (100 g/l Bi) 5 ml/l
[0044] TABLE-US-00005 TABLE 5 Summary of the experiments carried
and the results on adhesion and brazeability Activation Plating
Clad Activation Temperature Ni/ Braze- Sample layer bath (.degree.
C.) Ni--Bi Adhesion ability 1 B H.sub.2SO.sub.4 70 Ni--Bi 5 .+-. 2
B H.sub.2SO.sub.4 93 Ni--Bi 4 .+-. 3 A H.sub.2SO.sub.4 50 Ni 6 + 4*
A HF 50 Ni 9 + 5 A none -- Ni 1 - 6 A H.sub.2SO.sub.4 45 Ni 4 + 7 A
H.sub.2SO.sub.4 60 Ni 9 + 8 A H.sub.2SO.sub.4 70 Ni 10 + 9 A
H.sub.2SO.sub.4 93 Ni 9 + 10* A H.sub.2SO.sub.4 70 Ni 9 - 11* A
H.sub.2SO.sub.4 93 Ni 10 - *Samples 4, 10 and 11 involved anodic
activation instead of cathodic activation as per Samples 1 to 3 and
6 to 9.
[0045] 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.
[0046] 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.
[0047] 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.degree. C. However, if the temperature
is lowered further to below 50.degree. C. (Sample 6), the adhesion
level becomes unacceptable. From Samples 1 and 2 and 8 and 9 it can
be seen that neither adhesion nor brazeability suffers when the
temperature is lowered from 93.degree. C. to 70.degree. 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.degree. C. or lower,
thereby increasing the lifetime of the activation bath.
[0048] The addition of a wetting agent such as Bi is favourable for
the brazeability performance of the resultant brazing sheet
product. From the Samples 1 and 8 it can be seen that the wetting
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.
[0049] 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 .degree. C. The process can be operated in
a reliable manner over a wide range of current densities and
treatments times.
[0050] 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.
[0051] 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.
* * * * *