Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR ELECTROLESS DEPOSITION ON MAGNESIUM USING A NICKEL
HYDRATE PLATING BATH
SCOPE OF THE INVENTION
The present invention provides a method and process of electroless coating of
magnesium
and magnesium alloy substrates with metal coatings and more preferably,
coatings of nickel
phosphorus (Ni-P) and/or its alloys, including zinc (Zn) in the form of (Ni-Zn-
P) and (Ni-p-Zn)
and/or Tungsten (W) in the form of (Ni-W-P), as for example part of a method
of preventing the
galvanic corrosion of magnesium.
BACKGROUND OF THE INVENTION
Magnesium is the eighth most abundant metal on earth. Magnesium is two-thirds
the
weight of aluminum; whilst having physical and mechanical properties
approaching that of steel;
and is easy to form and machine. Heretofore, the use of magnesium in
commercial metal
fabrication applications has been limited due to its extreme susceptibility to
galvanic corrosion,
particularly when it is placed in contact with metals other than aluminum and
zinc, in the
presence of electrolytes.
The electroless coating of metals is a widely used process that has been in
existence for
some time. Conventional electroless coating studies have generally found that
the deposition of
coating metals in electroless coating processes falls off dramatically as the
pH of the coating
solution approaches 10 or higher. In particular, it has therefore been
generally accepted that
electroless coating is so excessively slow, that it cannot be commercially
achieved at pH levels
of 11 or higher. In addition, in electroless coating processes, to obtain a
good intermetallic bond
between the coating material and the parent substrate, it is necessary that
the substrate be clean;
free of all foreign material; and free of any oxidation. Oxidized metal
surfaces are generally
understood as unsuitable bonding sites for any metal, including metals
deposited by electroless
coating processes. These factors are even more pronounced when attempting to
coat magnesium
using electroless processes.
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When magnesium is coated using conventional electroless coating baths, the
bath
chemistry has been found to promote the oxidation and/or corrosion of the
surface of the
magnesium prior to any coating metal deposition. This in turn may result in
the formation of a
poor finish, poor adhesion around oxidized and corroded areas, and/or
intermittent coverage of
the coating layer over the magnesium surface. Further, with conventional
plating baths where
magnesium oxide remains present on the substrate surface, little or no
deposition frequently
occurs in oxidized locations.
One earlier process developed by the inventors provided for the deposition of
a copper
intermediate layer onto magnesium substrate using a high pH (i.e. a pH of
about 14) electroless
coating solution. The copper provides an intermediate base layer for
subsequent metal
deposition through electroless or electroplating process onto the finished
part. As a multi-step
process, however, the requirement of pre-coating with copper has yet to
receive widespread
commercial acceptance by magnesium parts manufacturers, other than for use as
decorative
magnesium parts requiring chrome finishes.
SUMMARY OF THE INVENTION
To overcome at least some of the disadvantages associated with conventional
electroless
plating processes, the present invention provides a plating process for the
deposition of metal
coatings directly on magnesium and magnesium alloys using a nickel hydrate
plating bath.
Preferably, the plating bath includes a nickel hydrate compound, and more
preferably one
or more of a nickel acetate tetrahydrate, a nickel sulfate hexahydrate and/or
a nickel sulfamate
tetrahydrate.
In one preferred mode, nickel phosphorous or nickel phosphorus alloys are
deposited as a
metal coating layer on magnesium and/or magnesium substrates, either with or
without first
removing all or part of any magnesium oxides therefrom. Such alloys include,
without
restriction, nickel phosphorous-zinc alloys and nickel phosphorous tungsten
alloys.
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In one possible method, a plating bath is prepared as a single component
solution which
includes a suitable hydroxide in an amount selected to provide the bath with
pH of at least 9,
preferably 10.5 to 14, and most preferably between about 11 to 14. The
solution is heated to a
temperature of greater than about 35 C, preferably at least about 50 C and
most preferably
between about 65 C to 80 C. The process plating bath solution preferably is
provided with
between about 5 to 50 g/L, and preferably 13 to 38 g/L of one or more
hydroxides, such as
sodium hydroxide and/or ammonium hydroxide, with ammonium hydroxide being more
preferred. The hydroxides are provided at a level selected to maintain the
solution pH in the
alkaline region.
Preferably, the solution also includes 5 to 20 g/L and more preferably 6 to 13
g/L of one
or more of nickel hydrate compound such as nickel acetate, nickel acetate
tetrahydrate, nickel
sulfamate tetrahydrate and/or nickel sulfate hexahydrate.
to 30 g/L and preferably 17 to 24 g/L of a combination of compounds of sodium
citrate tribasic dihydrate and sodium hypophosphite hydrate are also provided
maintaining a
sodium citrate tribasic dihydrate level, to maintain the Ni in solution.
Optimally, the solution
may include upto 15 g/L of a zinc hydrate compound, such as zinc sulfate
heptahydrate.
Most preferably, the solution is substantially free of chloride salts in any
form.
Accordingly, in one aspect, the present invention resides in a process for
electroless plating a
plating metal on a substrate, and preferably a magnesium or magnesium alloy
substrate, where
the method comprises preparing a plating bath solution comprising: 6 to 13 g/L
of a nickel
hydrate compound; 15 to 25 g/L sodium hydrate compound, for stability and
plating; 10 to 50
g/L of ammonium hydroxide to maintain an alkaline pH; optionally sodium
hydroxide in an
amount upto 3 g/L; heating the bath solution to a temperature of at least 50
C; and immersing the
substrate to be plated in the heated solution.
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In another aspect, the present invention relates to a method for the
electroless plating of
nickel or a nickel alloy on a magnesium or magnesium alloy substrate, said
method comprising,
preparing a plating bath solution comprising: 7 to 15 g/L of a nickel hydrate
compound, where
the nickel hydrate compound being selected from the group consisting of nickel
acetate
tetrahydrate, nickel sulfate hexahydrate and nickel sulfamate tetrahydrate; 15
to 25 g/L sodium
hydrate compound, the sodium hydrate compound comprising at least one of
sodium citrate
tribasic dihydrate and sodium hypophosphite hydrate; 5 to 50 g/L of ammonium
hydroxide;
sodium hydroxide in an amount upto 3 g/L; heating the bath solution to a
temperature of at least
50 C; and immersing the substrate in the heated solution.
Accordingly, in one aspect, the invention provides a plating process for the
depositing
nickel phosphorous and nickel phosphorous alloy coatings directly on magnesium
and
magnesium alloys using a plating bath comprising a nickel hydrate compound and
one or more
of nickel acetate tetrahydrate, a nickel sulfate hexahydrate and/or a nickel
sulfamate tetrahydrate.
The plating bath is prepared as a single component solution having a pH of 10
or more and
heated to a temperature of at least 35 C.
In another aspect, the invention provides a process for electroless plating a
plating metal
on a substrate comprising, preparing a plating bath solution having a pH of at
least 9, the bath
solution comprising: 7 to 15 g/L of a nickel hydrate compound; 15 to 25 g/L
sodium hydrate
compound; 10 to 50 g/L of ammonium hydroxide; and optionally sodium hydroxide
in an
amount upto 3 g/L; heating the bath solution to a temperature of at least 50
C; and immersing the
substrate to be plated in the heated solution.
In yet a further aspect, the invention provides a method for the electroless
plating of
nickel or a nickel alloy on a magnesium or magnesium alloy substrate, said
method comprising,
preparing a plating bath solution having a pH of between 10.5 and 14, the bath
solution being
substantially free of chloride salts and comprising: 6 to 13 g/L of a nickel
hydrate compound,
the nickel hydrate compound being selected from one or more of the group
consisting of nickel
acetate tetrahydrate, nickel sulfate hexahydrate and nickel sulfamate
tetrahydrate; 17 to 24 g/L
sodium hydrate compound, the sodium hydrate compound comprising at least one
of sodium
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citrate tribasic dihydrate and sodium hypophosphite hydrate; 13 to 38 g/L of
ammonium
hydroxide and/or sodium hydroxide; heating the bath solution to a temperature
of at least 50 C;
and immersing the substrate in the heated solution.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides for a process for the metal coating or plating
of
magnesium and magnesium alloy substrates or component parts. More preferably,
the invention
provides a bath solution for the electroless deposition of a nickel
phosphorous and/or nickel
phosphorous alloy coating layers on a magnesium or magnesium alloy substrate
either with or
without requiring a first step of magnesium oxide layer removal prior to
immersion of the
substrate in the plating bath.
The applicant has appreciated that suitable phosphorous alloys to be used in
the plating
process includes without restriction, nickel phosphorus (Ni-P), nickel
phosphorus zinc (Ni-P-Zn)
nickel zinc phosphorus (Ni-Zn-P) and nickel tungsten phosphorus (Ni-W-P). In
particular, the
nickel/nickel alloys may be successfully deposited or magnesium at good rates
of deposition
using an electroless deposition process with the plating bath having pH of at
least 9, and most
preferably between from a pH of 11 to pH 14 at room temperature.
The plating bath is preferably substantially free of chloride salts and
provided as a single
solution component which includes one or more nickel hydrate compounds
selected to provide
nickel ions in solution, one or more sodium hydrate compounds selected to
stabilize the bath
and reduce the nickel ions to nickel metal deposits at the magnesium/magnesium
alloy part
surface, and one or more suitable hydroxides, in an amount selected to provide
the bath with the
desired alkalinity.
The applicant has appreciated that the bath chemistry of the plating bath
allows for
reaction not only with unoxidized magnesium, but also with magnesium oxide
surfaces, and
allow the formation of good nickel and nickel alloys coatings with
substantially direct
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uninterrupted surface contact across the substrate including these surfaces
compromised by
magnesium oxide.
In a preferred process, the temperature of the plating bath is elevated to a
temperature
greater than about 50 C and most preferably to between about 68 to 74 C, and
with the pH level
decreasing slightly. The applicant has appreciated that the use of an alkaline
solution reduces the
rate of oxide formation on any oxide-free magnesium surfaces of the substrate.
This allows the
enhanced deposition of nickel and/or nickel alloys to form an intermetallic
bond with clean
magnesium surfaces whilst effecting the deposition of the coating over any
existing oxidized
surfaces. As such, the electroless deposition bath may advantageously be used
to form a
continuous uninterrupted nickel or nickel alloy coating over the entire
surface of the finished
magnesium part.
Since the bath solution used in the current process is highly alkaline, the
plating bath
solution further has a limited corrosive effect on the magnesium surface
during the coating
process. As a result, the surface of the substrate or part is dimensionally
unchanged under the
nickel and nickel alloy coating. The nickel and/or nickel alloy coating which
is formed provides
a continuous uniform, uninterrupted surface over the entire surface of the
part. Further, in one
preferred process the coating may be formed so as to substantially fully
encapsulate the part, thus
isolating the magnesium or magnesium alloy from direct contact with other
metals in the
presence of any potential electrolyte and thereby inhibiting or preventing any
potential galvanic
corrosion. At the same time the coated magnesium substrate maintains all of
its electrical and
thermal properties in a multi-metal structure.
In one process of manufacture, it is envisioned that the electroless
deposition of a nickel
coating may be provided as a final treatment on a finished magnesium component
or part, prior
to its assembly into a final structure. Since the nickel and nickel alloy
electroless coating process
is tolerant to oxidized magnesium surfaces, a chemical cleaning bath of
tartaric acid or sulfuric
acid may optionally be provided as a pre-treatment step for use in removing
magnesium oxide
from the substrate. In the alternative, or in addition, one or more mechanical
processes as for
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oxide removal from the substrate such as abrasion, grinding or the like, may
also be used on
exterior and/or easy to reach contact surfaces of the substrate.
SAMPLE BATHS
Sample bath solutions Si to S7 were prepared in accordance with the following
Table 2
for the electroless nickel phosphorous deposition on a test magnesium alloy
substrate
composition AZ91D. In the sample plating process, the electroless plating
solution was based on
a single solution that is stable until it is heated to deposition temperature.
The applicant has
appreciated that deposition rates with nickel phosphate (Ni-P) coatings have
proven
commercially acceptable where deposition occurs comparatively fast, with good
surface
deposition completed in under 1 minute. As alloy substrates or parts are
added, the coating
deposition rate will tend to decrease, requiring minutes to complete and with
most commercially
acceptable coatings achieved in under 5 minutes. In the test samples, bath
solution S7 zinc
deposition was however shown to achieve acceptable coatings at a slower rate
closer to 10
minutes.
In the test solution a magnesium alloy substrate was selected as per the
following Table 1:
Alloy Al Zn Mn Ni Cu Si Fe Magnesium
AZ91D 8.3-9.7 0.35-1.0 0.15-0.5 <0.002 <0.03 <0.10 <0.005 Balance
Table 1: The composition of AZ91D magnesium alloys (in wt.%)
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Sample deposition baths for forming nickel/nickel plating test magnesium
substrates were
prepared according to test bath formulations Si to S7 shown in Table 2:
Chemical Chemical Bath Composition (g/L)
Name Formula Bath S1 Bath S2 Bath S3 Bath S4 Bath S5
Bath S6 Bath S7
Nickel acetate Ni(C711102)2
9.940 9.940
tetrahydrate 414/0
Nickel sulfate
NiSO4 = 6f-120 10.499 10.499
6.299
hexahydrate
Nickel
Ni(H2NS01)2
s LI lfamate 12.899 12.899
= 4H)0
tetrahyd rate
Zinc sulfate ZnSO4
4.594
heptahydrate 7H20
Sodium citrate
Na3C6F1507
tribasic 23.500 23.500 23.500 23.500 23.500 23.500
23.500
2H20
di hydrate
Sodium
NaPH202
hypophosphite 17.500 17.500 17.500 17.500 17.500 17.500
17.500
14/0
hydrate
Sodium
NaOH 1.250 1.250 1.250
hydroxide
Ammonium
N1-140H 12.5 12.5 12.5 37.5 37.5 37.5
40.0
hydroxide
Average pH before use (20 C) 11.81 11.98 11.85 11.86 11.92
11.93 11.69
Table 2: Electroless nickel phosphorous [Ni-13] thin film deposition bath
formulations.
Deposition temperature tested: 68-74 C.
From test samples, it was observed that the inclusion of ammonium hydroxide
advantageously facilitated deposition on the magnesium substrate. The addition
of ammonium
hydroxide further advantageously resulted in an increase in the alkaline level
of the bath, and
acted to offset any acidic components of the bath solution. More preferably
the bath solution is
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kept substantially free of chloride (Cl) ions which could result in rapid
decomposition of the
bath. In particular, free chloride ions are frequently derived from Nickel
Chloride (Ni-C12), and
which are generally believed unsuitable for use in solutions for the
electroless deposition of
coatings on magnesium.
In the test samples, Bath S7 was the only plating bath containing zinc. The
applicant has
appreciated that as the bath pH is increased, so does the amount of zinc that
can be carried within
the bath (see Table 3 below). Preferably, however, the zinc sulfate
heptahydrate is maintained at
a level below 60% (cut) of the Nickel present in the plating bath. In the
resulting test sample, the
coated surface was shown to have a consistent 20% +/- zinc content.
pH Nickel Zinc Phosphorous
6.3 70 4 26
7.4 70 10 20
9.5 71 19 10
Table 3: Constituent atomic percentage concentration of the elements in the
films as a function
of metalizing bath pH value (bath temperature kept constant at 85 C).
As an optional process step, it is envisioned that prior to immersion in the
plating bath
solution, the magnesium/magnesium alloy substrate or part to be coated is pre-
treated in an acid
cleaning bath to first remove magnesium oxide. Suitable cleaning bath
solutions for such
magnesium and/or magnesium alloys include tartaric and/or sulfuric acid baths
as follows:
Tartaric Acid Bath
Chemical Formula Concentration
Tartaric Acid C4H606 30 to 60 g/L (approx.
53.0 g/L preferred)
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Sulfuric Acid Bath
Chemical Formula Concentration
15 to 30 mL/L
Sulfuric Acid H2SO4
(approx. 20 mL/L
preferred)
COMMERCIAL PROCESS
The applicant has appreciated that in one preferred process of manufacture,
magnesium and
magnesium alloy parts or component blanks may undergo the electroless plating
of nickel and
nickel alloys by immersion, whereby:
1. An alkaline plating bath is provided with a pH level of 11 to 14 at
elevated temperatures.
2. The coating process conducted in a plating bath at temperature between 60
to 85 C.
3. Preferred plating bath formulations are as per Table 2, and most preferably
Bath
formulations Si, S4, S5, S6 and S7.
4. The plating bath is provided as a stable single formulation, as contrasted
with a two
component system.
5. The inclusion of ammonium sulphate in the plating bath may result in rapid
bath
deteriorating. As such ammonia used in the bath is most preferably derived
through the
use of ammonium hydroxide.
6. A tartaric acid or sulfuric acid bath formulation may optionally be used as
part of a pre-
plating de-oxidation process to remove the magnesium oxide from the magnesium
part.
7. The plating solution is most preferably chloride free, with nickel chloride
salts being
avoided in the process.
In preferred commercial process for the production of nickel coated magnesium
parts and
components, finished magnesium blanks ready for assembly are first cleaned of
foreign
materials.
The cleaned blanks are next dipped into a tartaric or sulfuric acid cleaning
bath described,
above, and which is provided at room temperature for less than about 45
seconds to remove any
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formed magnesium oxide. Preferably, whilst so immersed, the blank is
manipulated and/or
agitated to better effect the acid bath contact with all part surface areas.
Following oxide removal, blank is thereafter transferred to the electroless
plating bath
made up of one of formulations S 1 -S7, and which has been heated to a
temperature between
50 C and 85 C and preferably at about at 68' to 74 C. The blank is placed in
the plating bath
such that all surface areas to be nickel coated are provided in contact with
the solution.
Optionally the part may be manipulated and/or agitated in the plating
solution, and/or a volume
of the plating solution may be pumped on, into or through the blank. Depending
on the solution
formulation and the desired plating thickness, the blank is left in the
plating solution for a period
of time of from between about 1 to 10 minutes, until the desired surface
nickel coating build-up
is achieved.
Following plating, the part is then removed from the electroless solution,
rinsed in de-
ionized water at room temperature; and dried, after which it is ready for
use/installation.
While the detailed description describes and illustrates various preferred
embodiments, the
invention is not limited strictly to the precise embodiments which are
disclosed. Modifications
and variations will now occur to persons skilled in the art. For a definition
of the invention,
reference may be had to the appended claims.
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