U.S. patent application number 14/075454 was filed with the patent office on 2014-03-06 for system and method of plating metal alloys by using galvanic technology.
This patent application is currently assigned to CREATE NEW TECHNOLOGY S.R.L.. The applicant listed for this patent is LORENZO BATTISTI. Invention is credited to LORENZO BATTISTI.
Application Number | 20140061035 14/075454 |
Document ID | / |
Family ID | 40314127 |
Filed Date | 2014-03-06 |
United States Patent
Application |
20140061035 |
Kind Code |
A1 |
BATTISTI; LORENZO |
March 6, 2014 |
SYSTEM AND METHOD OF PLATING METAL ALLOYS BY USING GALVANIC
TECHNOLOGY
Abstract
The invention relates to a system and a method of plating metal
alloys, as well as to the structures thus obtained. The system for
plating metal alloys comprises an electrolytic cell containing an
electrolytic solution (3) in which an anode (4,4a,4b), a cathode
(5), and a plurality of metal components to be plated onto the
cathode are immersed, the anode (4,4a,4b) and the cathode (5) being
electrically connected to means (6) adapted to apply a potential
difference between said anode (4,4a,4b) and said cathode (5). The
invention is characterized in that the means (6) adapted to apply a
potential difference between said cathode (5) and said anode
(4,4a,4b) impose a potential difference value that changes over
time according to a predefined law.
Inventors: |
BATTISTI; LORENZO; (Trento,
IT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BATTISTI; LORENZO |
Trento |
|
IT |
|
|
Assignee: |
CREATE NEW TECHNOLOGY
S.R.L.
Caldonazzo
IT
|
Family ID: |
40314127 |
Appl. No.: |
14/075454 |
Filed: |
November 8, 2013 |
Current U.S.
Class: |
204/228.6 ;
204/229.5 |
Current CPC
Class: |
C25D 17/10 20130101;
C25D 5/18 20130101; C25D 21/12 20130101; Y10T 428/12493 20150115;
C25D 17/00 20130101 |
Class at
Publication: |
204/228.6 ;
204/229.5 |
International
Class: |
C25D 17/00 20060101
C25D017/00; C25D 21/12 20060101 C25D021/12 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 5, 2007 |
IT |
TO2007A000704 |
Oct 3, 2008 |
IB |
PCT/IB08/02612 |
Claims
1. A system of plating metal alloys, comprising: an electrolytic
cell containing an electrolytic solution (3) in which an anode
(4,4a,4b) and a cathode (5) are immersed, a plurality of metal
components of an alloy to be plated onto the cathode, the anode
(4,4a,4b) and the cathode (5) being electrically connected to means
(6,60) adapted to apply a potential difference between said anode
(4,4a,4b) and said cathode (5), characterized in that said means
(6,60) adapted to apply a potential difference between said cathode
(5) and said anode (4,4a,4b) impose a potential difference having a
value that changes over time according to a predefined law.
2. The system according to claim 1, wherein said predefined law
comprises at least one stage in which the potential difference
value changes over time.
3. The system according to claim 2, wherein said law is a periodic
one.
4. The system according to claim 3, wherein said predefined law
prescribes that the potential difference stays constant at a first
value for a first time interval t.sub.1 and then stays constant at
a second value for a second time interval t.sub.2
5. The system according to claim 4, wherein said second value is
greater than said first value.
6. The system according to any one of claims 1 to 5, wherein said
law prescribes that the potential difference between said cathode
(5) and said anode (4,4a,4b) is eliminated as soon as a stationary
condition is achieved wherein the concentration ratios of the ions
of said metal components in solution no longer change.
7. The system according to claim 1, wherein said predefined law
prescribes that the potential difference value stays constant for a
time depending on the variation of ions concentrations of said
metal components in solution.
8. The system according claim 1, wherein said predefined law
depends on one or more parameters selected from a group including:
distance between the cathode and the anode, agitation of the
solution (3), pH of the solution (3), temperature of the solution
(3), concentration in the solution (3) of the materials to be
deposited, charge transfer superpotentials at the interfaces
between the liquid of the electrolytic solution and the
cathode.
9. The system according to claim 1, wherein said metal components
comprise a plurality of metal elements or components which are
present in said solution in ionic form.
10. The system according to claim 1, wherein said anode (4)
comprises at least some of said metal components.
11. The system according to claim 10, wherein the anode (4) is a
soluble one.
12. The system according to claim 11, characterized in that the
anode (4) comprises all of the metal components to be deposited
onto the cathode.
13. The system according to claim 12, characterized in that the
anode (4) has the same in weight composition as the alloy to be
obtained on the cathode.
14. The system according claim 1, further comprising means for
agitating the electrolytic solution (3).
15. The system according to claim 1, wherein said electrolytic
solution (3) contains only acids and complexing agents.
16. The system according to claim 1, further comprising means for
purifying said solution (3), which are adapted to collect and
filter any impurities depositing in the electrolytic cell.
17. The system according to claim 1, additionally comprising at
least a second anode (4b) electrically connected in parallel to
said anode (4a).
18. The system according to claim 17, characterized in that said
anodes (4a,4b) comprise a plurality of soluble anodes made of any
single metals to be plated or alloys thereof.
19. The system according to claim 17 or 18, characterized in that
the second anode is a compensating anode for said system.
20. The system according to claim 19, characterized in that said
compensating anode has an electric resistance such that, when said
potential difference is applied between said cathode and said
compensating anode, the compensating anode is crossed by a preset
current which is equal to the current used for discharging H+ ions
being present in said solution onto the cathode.
21. The system according to claim 20, wherein said compensating
anode comprises graphite or coal.
22. The system according to claim 21, wherein said compensating
anode is made of graphite or coal.
23. The system according to claim 1, characterized in that said
cathode comprises a conductive matrix from which an electroformed
coating can be detached.
24. The system according to claim 1, wherein said materials to be
deposited comprise at least three different chemical elements.
25. The system according to claim 24, characterized in that said
metal components to be plated comprise chemical elements which are
suitable to form a Hastelloy alloy.
26.-57. (canceled)
Description
[0001] The present invention relates to a system of plating metal
alloys by using galvanic technology and to an associated plating
method, as well as to a structure plated by using said system and
method.
[0002] The application field of the invention is that of galvanic
technologies, in particular the plating of metal alloys onto the
cathode of an electrolytic cell. More in general, the invention
relates to the field of technologies for producing metal
alloys.
[0003] In the field of cathode-plating galvanic technologies,
several technologies of plating different binary alloys, such as
Ni--Cr or Fe--Ni alloys for magnetic applications or Pb--Sn alloys
for tribologic applications, have become widespread over time.
[0004] The literature also describes galvanic technologies of
plating metal alloys made up of three or four components, which
however have found no practical applications in the industry.
[0005] As a matter of fact, many problems arise when using galvanic
technology to obtaining a simultaneous and constant deposition of a
plurality of metal components onto the cathode while also
maintaining a certain composition in weight. It is in fact
necessary, but not sufficient, that all the various metals have
similar electrochemical potential values. The potential of each
component is also related to the respective superpotentials, to the
concentration of the saline solution in the galvanic bath, to
activity coefficients, to the presence of complexing agents in the
solution, and to the physical conditions at the boundaries of the
bath itself. The galvanic technologies known in the art are based
on the principle that the deposition of each metal component onto
the cathode is implemented by controlling the galvanic bath supply
current. The process is typically carried out by using
electromotive means adapted to apply an appropriate electromotive
force or potential difference between the cathode and the anode of
the electrolytic cell, and means for controlling the electric
features of the current supplied by said electromotive means, in
particular the intensity of said current. Such means typically
consist of an electric generator associated with a current
rectifier which adjusts the intensity of the current flowing in the
galvanic bath.
[0006] As known, in the case of a single metal element to be
deposited onto the cathode, the potential difference applied
between the anode and the cathode of an electrolytic cell is
related to the current applied thereto according to the following
simplified equation:
E.sub.cell=E.sub.0,cell+.eta..sub.A-.eta..sub.C+RI
where E.sub.cell is the potential difference applied to the cell,
E.sub.0,cell is the counterelectromotive force, .eta..sub.A and
.eta..sub.B are respectively the anodic and cathodic
superpotentials of the metal, R is the electric resistance of the
bath, and E.sub.0,cell is the current intensity. The
counterelectromotive force E.sub.0,cell is the potential difference
exerted by the pile made up of the anode-solution-cathode system,
which is function of the concentration of both the reducing and the
oxidizing components. In short, any concentration, current or
voltage variations in the galvanic bath can affect the system
balance and are related to one another by precise balance laws. In
the industrial practice, the plating process is adjusted by
maintaining a wanted saline concentration in the galvanic bath
through proper additions of metal salts during the plating process.
These additions require the galvanic bath be regularly and
constantly checked and adjusted.
[0007] The methods known in the art are based on the fact that, if
current is fixed and the ratios among the concentrations of the
metal components to be plated are kept at certain values, the
potential difference will stay almost constant and the cathode
plating process will take place in a sufficiently controlled and
regular manner. The main reason for a fixed current being applied
to the bath is that the current flowing through the bath can be
directly related to the thickness and quantity of the metal
depositing onto the cathode over time.
[0008] In the practical implementation of known galvanic
technologies, which as aforementioned are based on controlling the
bath supply current and saline concentration, it is very difficult
to control the plating deposition in case of more than two metal
components are to be plated, especially when anodes made of metal
alloys are used. To give an idea of such difficulties, let it
suffice to say that the addition of a single metal to a bath will
affect the solubility of the other metals; therefore, effects which
are thoroughly different from the expected ones may result by
adding a metal to a solution.
[0009] Actually, when operating under imposed current, changes take
place rover time in the potentials and concentrations of the
different metal components, which substantially cannot be kept at
fixed values. It follows that the cathode metal deposition is
characterized by layers having different compositions and a
different degree of uniformity. Moreover, the potential difference
variation occurring over time allows other electrochemical
reactions to take place in the bath, e.g. parasitic or dissipative
reactions, such as redox short circuits, which may put the system
totally out of control.
[0010] In conclusion, known technologies are only partially
effective when the plating process uses two metal elements, and
they turn out to be ineffective when using three or more metal
elements. In fact, by setting a density value for the current
flowing through the galvanic bath it is possible to control the
number of charges globally discharged onto the cathode, but not
their qualitative and quantitative distribution, i.e. the
respective proportions in weight which are necessary in order to
create the wanted alloy.
[0011] On the whole, several problems of a strong impact on such
known galvanic technologies arise, among which: [0012] solubility
of the single metal components in solution; [0013] polarization
phenomena, in particular anodic polarization; [0014] typology and
electric features of the bath supply current; [0015] presence of
metal elements with different oxidation numbers and electrochemical
potentials.
[0016] A direct current supplied to the galvanic bath, for example,
leads to the formation of column-like structures which will
exfoliate after just a few microns of deposition due to the high
internal tensions accumulated during the deposition process.
[0017] When we consider metal elements having different oxidation
numbers, such as Fe and Cr, such elements require, in order to be
plated, the presence of complexing agents, typically organic ones,
for maintaining in the solution the most appropriate oxidation
number for plating, generally the lowest one. In fact, if different
redox systems are simultaneously present with regard to a metal
element in solution, unwanted reactions may take place which
sometimes would make plating impossible. For example, the
simultaneous presence of Fe.sup.3+ and Fe.sup.2+ may cause current
dissipation, since it is possible that an atom is oxidized on the
anode and reduced on the cathode, thereby returning to its previous
state without any deposition taking place, while also heating up
the solution.
[0018] The present invention aims at overcoming the above-mentioned
limitations of the prior art by providing a system and a method of
plating metal alloys which will eliminate said limitations of the
prior art while minimizing or even completely cancelling the
effects of the above-listed problems.
[0019] It is an object of the present invention to perform a
cathode plating process with two or more metal components by
optimally controlling the percentages in weight of the obtained
alloy, in particular when an alloy made up of three, four or more
elements is to be obtained.
[0020] It is another object to carry out a plating process wherein
it is possible to control the cathode deposition process of the
metals in a simple and effective manner.
[0021] It is another object to carry out a plating process which,
once started, takes place in a substantially automatic manner, i.e.
without requiring any external control or adjustment, e.g. changes
to the saline bath chemical composition.
[0022] It is a further object to obtain metal structures on the
cathode which are characterized by low internal tension and
excellent mechanical characteristics, in particular consisting of
crystalline structures substantially void of impurities.
[0023] It is a further object to obtain structures on the cathode
which have particularly complex and/or irregular shapes and
excellent mechanical characteristics.
[0024] Said objects are achieved by the present invention by
providing a system and a method of plating metal alloys having the
features as set out in the appended claims, which are intended to
be an integral part of the present description.
[0025] The present invention is based on the fundamental concept
that the plating process is carried out under voltage control, in
particular by imposing between the anode and the cathode of the
electrolytic cell a potential difference having a value that
changes over time according to a predefined law. This solution
differs from all known plating processes, which control the
intensity of the current flowing through the bath.
[0026] The law that defines the potential difference value over
time depends on the alloy to be plated and on other parameters of
the galvanic bath, e.g. pH and temperature. This allows to select
the law which is most suited to the bath depending on the
conditions at the boundaries.
[0027] Also, said law may prescribe that either a constant or a
time-variable potential difference must be applied to the anode and
the cathode of the electrolytic cell, depending on plating
conditions and required performance.
[0028] Further objects and advantages of the present invention will
become apparent from the following description and from the annexed
drawings, wherein:
[0029] FIG. 1 shows a metal alloy plating system according to the
invention, in particular an electrolytic cell;
[0030] FIG. 2 shows a variant of the system of FIG. 1, in
particular an electrolytic cell fitted with a plurality of
anodes.
[0031] With reference to FIG. 1, the electrolytic cell 1 comprises
a tank 2 containing an electrolytic solution 3 which includes salts
and/or acids in the appropriate quantity and composition for the
plating to be obtained. A potential difference E.sub.cell is
applied to two electrodes immersed in the solution 3, i.e. an anode
4 and a cathode 5, through a direct voltage generator 6.
[0032] In a per se known manner, the generator 6 may consist of
electromotive means and a voltage rectifier. For the purposes of
the present invention, the generator 6 is preferably equipped with
a control logic capable of adjusting the potential difference
E.sub.Cell applied between the anode and the cathode. In
particular, means are provided which are adapted to change the
potential difference E.sub.Cell between the anode 4 and the cathode
5 over time, so that a potential difference that changes over time
according to a predefined law can be imposed between the anode and
the cathode. In other words, said means are operative during the
plating process to the purpose of imposing said predefined law.
[0033] The potential difference imposed between the cathode and the
anode is chosen, in particular, according to parameters, criteria
and operating modes such as, for example:
I) the imposed potential difference value is such that each metal
element of the wanted alloy can diffuse from the anode to the bath
and can deposit itself onto the cathode; II) the potential
difference value is such that the metal elements to be plated can
only diffuse into the bath when they are in the wanted oxidation
state, which is usually the state corresponding to the lowest
electrochemical potential; III) physical distance between the anode
and the cathode in the galvanic bath: the longer is this distance,
the greater the potential drop occurring between the anode and the
cathode, due to the resistance of the electrolytic solution of the
bath; IV) agitation of the electrolytic solution of the bath, i.e.
solution mass flows: the greater is the agitation, the wider is the
range of applicable potential differences leading to a successful
plating process; V) pH number of the electrolytic solution: a lower
number allows to keep more easily the metal ions in solution, so
avoiding any precipitates in the solution; however, this number
must not drop below a certain value in order to prevent the
liberation of gaseous hydrogen, which generates a reducing of the
cathode effeciency; VI) temperature of the galvanic bath: a higher
temperatures increases the velocity at which the metal ions diffuse
through the solution, while at the same time also increasing the
size of the metal grains; VII) concentration of the metals in
solution: the higher is the concentration, the higher the currents
and therefore the potential differences that can be applied to the
galvanic bath; VII) charge transfer superpotentials at the
interfaces between the liquid of the electrolytic solution and the
cathode, which depend on several factors, among which cathode
composition and formation, metal elements to be diffused and
transferred into the solution and their respective compositions in
weight, and composition of the electrolytic solution.
[0034] For the plating system and method according to the invention
being able to properly operate and control the process, it is
preferable that the anode employed is a soluble one, even though it
is nevertheless still possible to implement the process by using
insoluble anodes. In particular, the soluble anode may
advantageously be made of the same alloy as the one to be
deposited, i.e. it may contain all, and only, the elements to be
deposited, so that no unwanted metals can deposit onto the cathode
and no slag can precipitate into the solution.
[0035] Furthermore, the anode may advantageously have the very same
composition in weight as the metal alloy to be obtained onto the
cathode, as will be further explained below.
[0036] The electrolytic solution of the galvanic bath may consist
of a solution having an arbitrary composition of the elements to be
plated, with the sole limitation that it must contain an adequate
quantity of composition acids and complexing agents for the plating
process to be carried out, in order to sustain those concentration
ratios of the metal species to be plated which are necessary to
depositing the alloy onto the cathode in the wanted percentage in
weights and physical conditions. Its actual composition will be
specified later on in the description of some examples of invention
embodiment.
[0037] The cathode of the galvanic bath may consist of either a
matrix made of metal material, onto which the electroformed coating
of the plated metal alloy is deposited and to which said coating
adheres permanently, or a conductive material from which the
electroformed coating can be detached, thus obtaining a component
having any shape.
[0038] Since the method described herein allows for depositing a
few millimeters of material even in case of complex or irregular
shapes, it is possible to obtain structures having particularly
complex and/or irregular shapes and excellent strength
characteristics.
[0039] In particular, the method and system according to the
invention effectively and advantageously allow to coat a
micro-perforated matrix for obtaining micro-porous structures, e.g.
of the type described in patents GB2356684. U.S. Pat. No. 6,488,238
or U.S. Pat. No. 6,682,022, with a metal alloy having a wanted
composition in weight, and in particular which is especially
suitable for aeronautical applications, such as Hastelloy.
[0040] Means adapted to change the potential difference between the
anode and the cathode of the electrolytic cell over time are
adapted, in particular, to apply a potential difference that
follows a law having a pulsed nature, i.e. a potential difference
that follows, at least for a certain period of time, a pulse-like
or step-like law with respect to the time variable, as clearly
illustrated and exemplified below.
[0041] Advantageously, this causes a cathode deposition of
crystalline, in particular micro-granular metal structures, which
are free from internal stresses and offer excellent mechanical
characteristics.
[0042] Advantageously, the potential difference variation law
applied between the anode and the cathode may be of any kind, i.e.
either constant or variable within a certain period of time,
provided that it is previously established.
[0043] Said anode-cathode potential difference variation law may
advantageously be repeated cyclically for a time period T equal to
a fraction or to the entire length of the plating process.
[0044] According to a preferred embodiment, said law can be
expressed as follows:
{ E Cell = E Cell , b for n ( t 1 + t 2 ) < t < ( n + 1 ) t 1
+ n t 2 E Cell = E Cell , b + .DELTA. E Cell for ( n + 1 ) t 1 + n
t 2 < t < ( n + 1 ) ( t 1 + t 2 ) ( 1 ) ##EQU00001##
where t.sub.1 is the length of a time interval in which the
potential difference is kept at a lower level E.sub.Cell,b, t.sub.2
is the length of the time interval in which the potential
difference is kept at a higher level
E.sub.Cell,b+.DELTA.E.sub.Cell, and n is an integer between 0 and
(T/(t.sub.1+t.sub.2))-1.
[0045] In other words, (1) indicates that the potential difference
E.sub.Cell to be applied consists only of the basic potential
difference E.sub.Cell,b for a time t.sub.2, followed by a voltage
pulse .DELTA.E.sub.Cell having a duration t.sub.2.
[0046] Such a pulse-like trend is found in every time interval
t.sub.1+t.sub.2 that follows (n 0); therefore, it follows that for
a new time t.sub.1 the applied potential difference returns to the
value E.sub.Cell,b, and then, in the next time interval t.sub.2, it
goes back again to the value E.sub.Cell,b.times..DELTA.E.sub.Cell
and so on, for the entire duration of the period T. The values of
these times t.sub.1 and t.sub.2 are related to each other through a
time constant .tau.=t.sub.2/(t.sub.1+t.sub.2), which determines the
time ratio between the duration of each pulse and the overall
length of the period of the pulse-like law.
[0047] Tests have shown that the choice of the constant .tau. can
affect the successful outcome of the process, i.e. by obtaining a
plating having a crystalline grain with particularly good
mechanical characteristics, depending on the different alloys to be
plated onto the cathode.
[0048] The E.sub.Cell,b and .DELTA.E.sub.Cell factors may be
constant with respect to time, as in the following examples of
embodiment of the invention, or they may be any functions which are
dependant on the time variable.
[0049] The method according to the invention imposes a basic
potential difference value E.sub.Cell,b chosen according to any of
the above points I)-VIII).
[0050] According to the invention, the plating process is divided
into two stages, i.e. an initial stage, called "training stage",
and a plated structure production stage. The first training stage
is characterized by a chemical imbalance situation.
[0051] The imposition of a potential difference between the cathode
and the anode as defined by law (1) determines concentration and
activity values of the ionic species of the metals included in the
galvanic bath, which are variable over time with respect to the
initial conditions. In fact, as can be evicted from Fick's
diffusion laws, the galvanic bath has a dynamic behaviour because,
when the concentration of a generic metal ion in solution grows,
the speed of dissolution of that metal from the anode decrease,
while its speed of deposition onto the cathode will increase. In
this stage, the quantity of charges depositing onto the cathode for
each metal will depend on the instantaneous concentration
conditions of the respective metal ions in solution.
[0052] Preferably, this initial stage of the plating process is
conducted by using a cathode, called training cathode, onto which
the various ligands, i.e. the components of the deposited metal
alloy, deposit in ratios which are generally different from the
wanted ones and following compositions in weight changing over
time.
[0053] During this training stage, each cation in solution
progressively reaches a stationary flow condition, characterized in
that the ratios between the concentrations of the single elements
stay constant over time.
[0054] This implies that the speed of dissolution of the metal
cations, which are considered to be produced at the anode, equals
the speed of deposition of the anions onto the cathode. This
condition is true when there are no collateral reactions that
decrease the cathode deposition efficiency of the plating process,
such as, for example, the reaction that releases gaseous hydrogen.
In such a case, while it is still true that the metal will deposit
onto the cathode with the wanted composition in weight, the
concentration of each metal however tends to grow over time due to
the release of gaseous hydrogen.
[0055] The production of gaseous hydrogen causes a higher pH and a
solution composition variation, which requires to be checked and
corrected over time by adding water and acid in appropriate
proportions. If not corrected, this phenomenon actually leads the
solution to saline saturation, with unwanted metal salt
precipitation and the setting of time-stable concentration ratios
among the various metal species, characterized by ratios in weight
which are unsuitable to obtain the wanted plating.
[0056] This problem can be prevented by including electrolytic
solution agitation means, e.g. a centrifugal pump, in particular
having the outlet directed towards the cathode of the electrolytic
bath. Advantageously, a strong agitation of the electrolytic
solution allows to keep the global concentration of the metal ions
in solution within a certain range of appropriate values ensuring a
perfect cathode plating process.
[0057] An even more effective system to the purpose of preventing
the metal concentrations in solution from growing over time is the
artificial production of hydrogen ions in the same number as those
discharged onto the cathode and thereon released in gaseous form.
To this end, a suitable means consists of an auxiliary anode,
hereafter referred to as compensating anode, which may be either
soluble or insoluble depending on the bath chemism, and which is
connected in parallel to the bath anode. The function of said
compensating anode is to generate H.sup.+ ions in the same number
as those discharged onto the cathode and released in gaseous form,
by taking the necessary current, called compensation current, from
the anode in the manner described below. This allows to keep
constant the concentration of the H.sup.+ ions in solution, and
therefore also the pH number thereof. From a practical viewpoint,
the current that must flow through the compensating anode is
experimentally determined by measuring the cathode efficiency when
no current intensity flows to the compensating anode, i.e. with the
compensating anode being not inserted in the electrolytic solution.
Cathode efficiency is measured by monitoring the plating process
for a certain time interval, in particular by measuring the masses
of the anode and cathode in order to calculate the difference
between the bigger mass dissolved from the anode and the smaller
mass deposited on the cathode. This mass difference is directly
related to the electric current used in the solution for
discharging the H.sup.+ ions onto the cathode, which does not
translate into metal deposit. Once the value of this current has
been calculated, the compensating anode is dimensioned with an
electric resistance such that the exact compensation current will
be generated in the bath, i.e. the current that is used in the bath
for discharging the H.sup.+ ions and that will not anymore be used
for the dissolution of metals from the anode. Thus, once the
compensation anode has been dimensioned as described, the system
will be in conditions wherein the anodic metal dissolution current
is equal to the cathodic metal deposition current.
[0058] Electrodes made of graphite or coal may preferably be
employed as compensating anodes, which can advantageously be used
in any type of galvanic bath.
[0059] At the end of a training period, in the absence of any
parasitic reactions, the cathode deposition speeds of the single
metals is equal, in an absolute sense, to the anode dissolution
speeds, and the solution is balanced as well. When the process is
carried out in conditions of gaseous hydrogen release, the anodic
currents of the metals will be higher than their cathodic currents
according to a coefficient which is the same for all elements. The
deposition of the single metals will still take place according to
the same ratio in weight, but with hydrogen release. In any case,
the condition of balanced solution without hydrogen release is to
be preferred; in particular, this condition is accomplished by
adjusting the bath acidity to a value which is not too high, and
through a strong agitation of the solution and/or by using
compensating anodes.
[0060] The training stage ends as soon as a stationary situation is
achieved, wherein the concentration ratios of the metal ions to be
plated in solution no longer changes; the solution is now balanced
and the actual plating stage can be carried out.
[0061] The training cathode is then removed and replaced with the
one onto which the wanted alloy will have to deposit.
[0062] Subsequently, a potential difference also following a
predefined law, which is preferably identical to that used in the
training stage, is applied between the anode and the cathode.
[0063] Preferably, the plating method according to the invention is
implemented after the following preliminary steps have been
completed: [0064] the composition of the wanted alloy is analysed
in terms of quantity and quality of the metal elements or
components to be plated onto the cathode, in particular by noting
the standard electrochemical potentials of the single metal
elements; [0065] the basic potential difference E.sub.Cell,b at
which the galvanic bath must operate is determined; typically, this
corresponds to the most negative potential among the range of
electrochemical potentials of the elements to be plated (e.g. the
potential of chrome Cr in Hastelloy plating, with reference to the
following example 1) is taken as a reference and used as a minimum
basic potential difference at which the first attempt will be made.
If no current flows, then the value of the basic potential
difference will be increased gradually, in particular according to
preset increments, until it is ascertained, by using known methods,
that all the wanted anodic elements are present in the solution (to
this purpose, a solution called "blank solution" is used, which
includes all the elements of the bath except metals. By doing in
this way, it will be easy to verify the anode dissolution with
known means); [0066] it is checked whether any parallel reactions
occur in the bath in addition to the electrodeposition one, e.g.
reactions between Fe--Cr in the aforementioned example 1; [0067]
based on the above check, the galvanic bath composition is
determined and prepared, in terms of quantity and type of acids,
complexing agents and salts of metals to be plated, so that the pH
of the electrolytic solution is adjusted to a predefined value;
[0068] the galvanic bath tank is fitted out and prepared according
to known procedures; [0069] the anodic and cathodic treatment of
the bath is implemented by subjecting the anode and the cathodic
matrix, respectively, to pickling operations, in particular by
using the electrolyte in order to avoid any contamination; said
operations take place in separate baths for the anode and the
cathode; [0070] a training cathode is inserted into the tank.
[0071] The electrolytic cell with its galvanic bath is prepared in
this manner before starting the cathode plating process for the
wanted alloy, which is typically implemented by following the
method described above, which comprises the following steps:
a) applying a potential difference between the anode and the
cathode in the galvanic bath according to a predefined law, e.g.
the above-mentioned law (1), which includes periods wherein the
applied potential difference is only equal to the basic potential
difference and other periods wherein a pulse having a predefined
width is added to said basic potential difference, e.g. 50% of the
basic potential difference, as shown in example 1 below; b)
verifying the achievement of a condition in which the concentration
ratios of the metal ions to be plated in the solution do not change
over time, the condition being named as "balanced solution", i.e.
when it is possible to start plating the wanted metal composition;
c) extracting the training cathode from the galvanic bath and
inserting thereinto a cathodic matrix onto which the alloy is to
deposit, advantageously keeping the same bath potential difference
as in the previous steps; d) maintaining the predefined potential
difference law until the alloy has completely and/or as wanted
deposited onto the cathode.
[0072] In the present description, the term "cathodic matrix"
generally refers to any conductive or semiconductive structure or
element onto which the alloy to be obtained in the process must be
plated.
[0073] In the advantageous case wherein a compensating anode is
used, an additional step is also implemented for generating H.sup.+
ions in the bath electrolytic solution in the same number as those
discharged onto the cathode and released in gaseous form, taking
the necessary compensation current from the anode as explained
above.
[0074] With reference to step a), the potential difference between
the anode and the cathode is set according to the above-described
preliminary steps.
[0075] In their practical implementation, said preliminary steps
require that a potential difference be applied between the anode
and the cathode by starting from an initial potential difference
value chosen as described above, the value being increased until
current circulation and all the wanted elements dissolving from the
anode is verified. The attainment of such a condition determines
the value of the basic potential difference to be applied to the
galvanic bath. Also, the potential difference variation law over
time must be such to ensure optimum dissolution and deposition,
respectively from the anode and onto the cathode, of the metal
elements that make up the alloy to be deposited. Advantageously, in
general terms the law above described is excellent from this point
of view as well.
[0076] When, during step d), the applied potential difference stays
constant over time, the electrolytic solution will be saturated and
balanced, and a controlled and uniform deposition of the metals
will take place on the cathode, in particular in the very same
proportions in weight as those existing on the anode, if the anode
is a soluble one.
[0077] Preferably, step d) is implemented by applying a potential
difference value between the anode and the cathode which changes
over time according to the same law as the one used for the
potential difference applied during the training stage. However,
other laws may also be applied during step c), different from those
of step a).
[0078] If nevertheless one should want, during step c) of the
method, to carry out the plating process under constant-current
control (as taught per se by the prior art), e.g. by using a
current value which can be deduced by measuring the previously
imposed potential difference, one would run into considerable risks
in terms of plating results over time. In fact, since current is
related to concentrations and potential difference, it is apparent
that any incidental modification of any parameters affecting the
plating process would imply the risk of losing control over the
ratio in weight and deposition uniformity of the metal elements
depositing onto the cathodic matrix, just as it happens with known
technologies. This risk increases with deposition thickness, i.e.
as time passes during the plating process.
[0079] Due to the above reasons, it is apparent that it is
important, in order to implement the plating method according to
the invention, to impose a potential difference between the anode
and the cathode of the galvanic bath according to a predefined law,
i.e. to only control this electric feature, and no other bath
parameter.
[0080] In particular, the best result in terms of process
effectiveness is obtained when the control is accomplished through
a law that prescribes that a preset potential difference is to be
applied between the anode and the cathode for the entire plating
process, which would otherwise suffer from transients that would
make it difficult to control the plating and the bath phenomena
generating therefrom.
[0081] In summary, when the electrolytic solution is in balanced
conditions, the galvanic bath reaches a ratio among the
concentrations of the single cations of the metals to be plated
which is stable over time and which can be used for plating the
alloy until the anode is completely dissolved, the anode being a
soluble one.
[0082] It is also clear that the choice of the initial
concentrations of the metals in solution and of their reciprocal
ratios is a marginal factor for a successful implementation of the
method, since the initial solution may consist only of acids and
complexing agents at a certain pH value, i.e. with no metal salts
dissolved in ionic form. Advantageously, by using only acids and
suitable complexing agents it is possible to obtain a deposition
void of any of those impurities which are typical of metal salts;
also, it promotes metal solubility.
[0083] According to another important aspect of the present
invention, the control over the concentration of the metal ions in
solution during the plating process definitely turns out to be of
minor importance than in prior-art systems. In fact, the currents
generated in the galvanic bath follow the evolution of the various
concentrations which, being in constant reciprocal ratios, generate
current intensity ratios which are constant as well.
[0084] Therefore, the system and method according to the invention
prove to be self-consistent, i.e. the galvanic bath has
self-saturation properties in terms of absolute values of current
density of the single cations and of the ratios thereof, which are
mutually related through the mass percentages depositing onto the
cathode. In other words, the system electrochemically evolves
through a potential difference imposed between the anode and the
cathode until it reaches a thermodynamic and electrochemical
balance state which ensures equal anode dissolution and cathode
deposition speeds at any time for each metal involved. In
particular, when the anode advantageously provides the same
composition in weight as the alloy to be deposited, it is possible
to attain considerable plating thicknesses because the anode
completely dissolves in solution, thus providing the greatest mass
flow supply.
[0085] After a certain time from start-up, with the system
according to the invention in a stationary condition, it is no
longer necessary to correct the ionic concentration of the metals
to be plated, since the system has reached a balance among the
various ratios thereof which remain constant over time (balanced
solution condition), nor the plating process requires any other
adjustments.
[0086] The plating system and method according to the invention,
wherein a potential difference is imposed between the anode and the
cathode of the galvanic bath, advantageously allows to select the
cationic species to be deposited onto the cathode, because the
applied potential difference represents an actual energy barrier
which cannot be crossed by certain species. This advantageously
allows to prevent the formation of compounds having a high
oxidation number, which would otherwise interfere in several ways
with the galvanic bath and the plating process, e.g. like
chromates, manganates or Fe.sup.3+ based compounds. Furthermore,
any deposition of impurities onto the cathode is successfully
prevented, which might have unfavourable effects on the final
mechanic or electromagnetic properties of the plated alloy.
[0087] In accordance with the method of the invention, a metal
alloy can be plated onto the cathode even by using electrolytic
solutions comprising the wanted concentrations of the metals to be
plated and by using insoluble anodes, once the solutions have
proven to be balanced. However, the outcome will not be wholly
satisfactory over time, due to the progressive exhaustion of the
metal cations in solution, resulting in solution balance
variations. It follows that, with insoluble anodes, it is much more
difficult to plate thick alloy layers while at the same time
preserving the pure crystalline structure of the deposited
material.
[0088] In conclusion, the present invention is successful in
obtaining, on the cathode of the galvanic bath, a crystalline metal
structure particularly free from impurities and having excellent
mechanical characteristics, which are much superior to those of an
analogous structure obtained through a thermoforming process.
[0089] It also allows to obtain a large number of metal alloys
having many different compositions in weight, even those which
cannot be obtained by the means of thermoforming techniques. The
invention therefore opens the path to a new metallurgy, consisting
of metal alloys with percentages in weight never implemented
before.
[0090] Furthermore, the plating process takes place in a
substantially automatic manner after the training stage, i.e. with
no need of continuously monitoring the process in order to change
the bath parameters, unlike the galvanic methods known in the
art.
[0091] Further objects, features and advantages of the present
invention will become apparent from the following detailed
description of some preferred, but non-limiting, embodiment
examples.
EXAMPLE 1
[0092] A metal alloy for aeronautical applications, called
Hastelloy and containing the basic components listed in Table 1, is
to be obtained on the cathode of a galvanic bath.
TABLE-US-00001 Alloy element Min. % in weight Max. % in weight Cr
20.5 23.0 Co 0.5 2.5 Mo 8.0 10.0 Fe 17.0 20.0 W 0.2 1.0 Ni
Remaining % in weight
The initial electrolytic composition of the galvanic bath and its
electrical and physical parameters are those shown in the following
Table 2:
TABLE-US-00002 Galvanic bath composition g/l Ni (total sum of
compounds) 70 NiSO.sub.46H.sub.2O 242 NiCl.sub.2 6H.sub.2O 68 Boric
acid 30.0 FeCl.sub.26H.sub.2O 6 TEA (Triethanolamina) 60 HCit 6 HCl
at 33% to pH <0.5 Bath parameters measured value Temperature
20-50.degree. C. Basic potential difference E.sub.Cell,b 2.5-3 V
Width of pulse .DELTA.E.sub.Cell 50% of E.sub.Cell,b Pulse time
constant.sup..tau. 0.23 Anode/cathode surface ratio >2.5
The potential difference law imposed on the galvanic bath has a
pulsed nature and follows the time law (1) as described above,
i.e.:
{ E Cell = E Cell , b for n ( t 1 + t 2 ) < t < ( n + 1 ) t 1
+ n t 2 E Cell = E Cell , b + .DELTA. E Cell for ( n + 1 ) t 1 + n
t 2 < t < ( n + 1 ) ( t 1 + t 2 ) ##EQU00002##
[0093] Said law has been applied for a time T equal to the entire
duration of the plating process, including the solution training
period.
[0094] In this example, the galvanic bath employs an anodic
electrode to be dissolved, which is made of the same alloy as the
one to be deposited onto the cathode and in the exact percentages
in weight, in particular obtained by thermoforming or casting. As
can be seen, for the purpose of controlling the deposition, in
particular of chrome Cr and iron Fe, the process uses
Triethanolamina and HCit as respective complexing agents, boric
acid as a pH buffer, and hydrochloric acid as necessary to obtain a
pH value of the electrolytic solution lower than 0.5. The plating
process has been carried out by following the steps a)-d) of the
method as previously described, obtaining on a cathodic metal
matrix the deposition of Hastelloy having excellent purity and
mechanical strength behaviours.
[0095] By following the galvanic technology according to the
present invention, it has been possible to plate a metal alloy
having as many as six distinct metal components; this result has
never been achieved by using any known technology.
EXAMPLE 2
[0096] This example relates to a bronze alloy (Cu, Sn) for
tribologic applications, the exact composition of which has been
omitted for simplicity. The following Table 3 lists the components
of the galvanic bath and the values of the electric parameters
applied thereto:
TABLE-US-00003 g/l Galvanic bath composition Tin fluoborate (II)
150 Copper fluoborate (II) 40 TEA 100 Fluoboric acid 100 Boric acid
30 Hydrochloric acid to pH 1-0.5 Bath parameters Temperature
15-50.degree. C. Basic potential difference E.sub.Cell,b 0.5 V
Width of pulse .DELTA.E.sub.Cell 70% of E.sub.Cell,b Pulse time
constant.sup..tau. 0.23 Anode/cathode surface ratio >3
[0097] In the above-detailed bath, fluoboric acid and boric acid
are used in order to lower the pH of the solution as well as to act
as complexing agents of tin Sn and copper Cu. An anodic electrode
made of the same bronze alloy to be obtained is used.
[0098] The potential difference implementation law applied to the
bath is identical to the one illustrated for the preceding example,
and it is likewise applied for the entire duration T of the plating
method.
[0099] Among the peculiarities of this bath, the cathode needs to
be inserted into the bath under voltage, i.e. in the so-called
"live mode", in order to avoid a preferential, non-adhering
deposition of copper compared to tin.
[0100] It is clear that many changes may be made by technicians
skilled in the art to the metal alloy plating system and method
according to the invention as described in the appended claims; it
also clear that in the practical implementation of the invention
the illustrated, details may have different shapes or be replaced
with other technically equivalent elements.
[0101] For example, a metal alloy can advantageously be plated onto
the cathode of a galvanic bath by using a bath which comprises a
plurality of soluble anodes made of single metals to be plated, or
of alloys thereof, wherein the cations of the alloy to be deposited
onto the cathode are obtained from each anode dissolving
separately.
[0102] An example of such a variant will now be described with
reference to FIG. 2, which shows a cell 1 that comprises a tank 2
containing a bath 3 in which two anodes 4a, 4b and one cathode 5
are immersed. The anodes 4a, 4b are electrically connected in
parallel to an electric circuit 60 fitted with means 61 for
controlling the potential difference supply provided by suitable
electromotive means 62, so that the anodes 4a and 4b have the same
potential as the galvanic bath. This parallel electric connection
prevents an anode from behaving like a cathode towards the other
anode, which would result in unwanted deposits on the anodes
themselves.
[0103] Advantageously, this variant provides control over the
anodic dissolution process of every single metal in solution, since
it allows to obtain predetermined bath compositions and cathode
alloy plating compositions by changing, for example, the number of
anodes for each metal to be plated or the electric resistance of
the single anodes, thus generating the wanted electric currents for
each metal component of the alloy to be plated.
[0104] In addition, the solution using a plurality of anodes allows
to maximise the ratio between the anodic surface and the cathodic
surface of the bath, thereby improving the dissolution of the
anodes in solution, increasing the concentration in solution of the
respective salts and thus the respective diffusion towards the
cathode, and increasing the overall effectiveness of the entire
plating process.
[0105] A further variant of the plating system and method according
to the invention includes means for purifying the saline solution
which comprise, for example, pumping means, which may
advantageously be the same ones that participate in the agitation
of the electrolytic solution, having an inlet in fluid connection
with a wall on the electrolytic cell side, preferably the bottom
thereof, and selectively associated with filtering means.
Advantageously, said purification means are adapted to collect and
filter any impurities deposited on the bottom of the electrolytic
cell, thus eliminating any risk of contamination of the cathode
alloy deposition process.
* * * * *