U.S. patent application number 12/172104 was filed with the patent office on 2009-06-25 for method for silver plating.
This patent application is currently assigned to DAIWA FINE CHEMICALS CO., LTD.. Invention is credited to Toshifumi Kadokawa, Shingo Kitamura, Seiji Omori, Masakazu Yoshimoto.
Application Number | 20090159453 12/172104 |
Document ID | / |
Family ID | 40787313 |
Filed Date | 2009-06-25 |
United States Patent
Application |
20090159453 |
Kind Code |
A1 |
Yoshimoto; Masakazu ; et
al. |
June 25, 2009 |
METHOD FOR SILVER PLATING
Abstract
The aim of the present invention is to provide a method for
silver plating that does not comprise the step of forming the
nickel-underlayer and that can form a silver-plated layer having
sufficient adherence directly on the difficult-to-plate substrates
with the use of a halide-free plating bath under a good working
environment. The present invention provides a silver plating method
onto a substrate on which an oxide layer inhibiting adherence of a
plated layer is prone to form, comprising at least the following
steps of; (A) degreasing the substrate, (B) removing the oxide
layer with a strongly acidic solution from the substrate, and (C)
silver plating onto the substrate, without a step of nickel or
nickel alloy strike plating in advance, utilizing a
phosphines-containing acidic silver plating bath which essentially
does not contain halide ion or cyanide ion.
Inventors: |
Yoshimoto; Masakazu;
(Akashi-shi, JP) ; Kitamura; Shingo; (Akashi-shi,
JP) ; Omori; Seiji; (Akashi-shi, JP) ;
Kadokawa; Toshifumi; (Akashi-shi, JP) |
Correspondence
Address: |
MARSHALL, GERSTEIN & BORUN LLP
233 SOUTH WACKER DRIVE, 6300 SEARS TOWER
CHICAGO
IL
60606-6357
US
|
Assignee: |
DAIWA FINE CHEMICALS CO.,
LTD.
Akasi-shi
JP
|
Family ID: |
40787313 |
Appl. No.: |
12/172104 |
Filed: |
July 11, 2008 |
Current U.S.
Class: |
205/263 |
Current CPC
Class: |
C25D 5/34 20130101; C25D
3/46 20130101 |
Class at
Publication: |
205/263 |
International
Class: |
C25D 3/46 20060101
C25D003/46 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 19, 2007 |
JP |
2007-341848 |
Claims
1. A silver plating method onto a substrate on which an oxide layer
that inhibits adherence of a plated layer is prone to form,
comprising at least; (A) degreasing the substrate, (B) removing the
oxide layer with a strongly acidic solution from the substrate, and
(C) silver plating onto the substrate, without a step of nickel or
nickel alloy strike plating in advance, utilizing a
phosphines-containing acidic silver plating bath which essentially
does not contain halide ion or cyanide ion.
2. The silver plating method according to claim 1, wherein the
method further comprises (D) silver plating onto the substrate
utilizing a sulfonic acid-containing acidic silver plating bath
preceded by step (C).
3. The silver plating method according to claim 1, wherein the
phosphines-containing acidic silver plating bath in (C) further
contains sulfonate ion.
4. The silver plating method according to claim 2, wherein the
silver plating baths in (C) and (D) both have a pH value of less
than or equal to 3.
5. The silver plating method according to claim 1, wherein the
silver plating bath in (C) contains one or more of the phosphines
represented by the general formula (1): ##STR00005## wherein
X.sub.1, X.sub.2 and X3, which may be the same or different, each
representing a hydrogen atom, a substituted or unsubstituted
C1.about.C10 alkyl group, or a substituted or unsubstituted benzene
ring, the substituents for the substituted alkyl group or the
substituted benzene ring being one or more selected from the group
consisting of a hydroxyl group, a carboxyl group, a sulfonic group
and an amino group, provided that not all of X.sub.1, X.sub.2 and
X.sub.3 being hydrogen atoms simultaneously.
6. The silver plating method according to claim 5 wherein the
phosphines are lower alkylphosphine represented by the general
formula (2): ##STR00006## wherein Y.sub.1, Y.sub.2 and Y.sub.3,
which may be the same or different, each representing an
unsubstituted C1.about.C3 alkyl group or a C1.about.C3 alkyl group
substituted with one or more substituents selected from the group
consisting of a hydroxyl group, a carboxyl group, a sulfonic group
and an amino group.
7. The silver plating method according to claim 1, wherein the
strongly acidic solution in (B) contains more than or equal to 10
wt % of acid.
8. The silver plating method according to claim 1, wherein the
strongly acidic solution in step (B) has a pH value of less than or
equal to 2.
9. The silver plating method according to claim 1, wherein the
strongly acidic solution in (B) essentially does not contain halide
ion.
10. The silver plating method according to claim 1, wherein the
strongly acidic solution in (B) contains sulfonic acid.
11. The silver plating method according to claim 1, wherein the
above mentioned substrate is made of magnesium, aluminum, titanium,
chromium, nickel cobalt, zinc and tin, or an alloy which at least
contains one or more metals selected from the group consisting of
these metals.
12. The silver plating method according to claim 11, wherein the
substrate is made of a chromium-containing alloy.
13. The silver plating method according to claim 12, wherein the
substrate is made of a stainless steel.
14. The silver plating method according to claim 13, wherein the
substrate is made of an austenitic stainless steel.
15. The silver plating method according to claim 11, wherein the
substrate is made of titanium.
16. The silver plating method according to claim 11, wherein the
substrate is made of tin or tin-alloy.
17. The silver plating method according to claim 11, wherein the
above mentioned substrate is an alloy of tin and at least one or
more metals selected from copper, zinc, silver, indium, gold, lead
and bismuth.
18. The silver plating method according to claim 1 wherein an
insoluble anode is used in at least one of (C) and (D).
19. The silver plating method according to claim 18 wherein the
insoluble anode is selected from the group consisting of a carbon
anode, a platinum anode, a platinum-coated titanium anode, an
oxidized ruthenium-coated anode and an oxidized iridium-coated
anode.
20. The silver plating method according to claim 18 wherein the
uppermost layer of the above mentioned insoluble anode comprises an
uppermost layer made of one or more material(s) selected from the
group consisting of spinel, garnet, glass, and perovskite.
21. The silver plating method according to claim 2 wherein an
insoluble anode is used in at least one of (C) and (D).
22. The silver plating method according to claim 21 wherein the
insoluble anode is selected from the group consisting of a carbon
anode, a platinum anode, a platinum-coated titanium anode, an
oxidized ruthenium-coated anode and an oxidized iridium-coated
anode.
23. The silver plating method according to claim 21 wherein the
uppermost layer of the above mentioned insoluble anode comprises an
uppermost layer made of one or more material(s) selected from the
group consisting of spinel, garnet, glass, and perovskite.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method of non-cyanide
silver electroplating and in particular, to a silver plating method
which provides good adherence onto the so-called difficult to plate
metals by utilizing an acidic plating bath.
BACKGROUND OF THE INVENTION
[0002] Silver has industrially applicable properties such as good
electroconductivity along with good solderability, and is thus
widely utilized for manufacturing of electrical parts. Furthermore,
its aesthetic appeal has been widely appreciated for decorative
purposes. Moreover, owing to its ductility, it can be utilized to
prevent seizure of screws or to improve lubricity and wear/abrasion
resistance of sliding parts.
[0003] To prevent seizure of screws, or to improve lubricity and
abrasion/wear resistance of sliding parts, silver plating is often
required onto so called difficult-to-plate metallic substrates.
Such substrates are prone to form an oxide layer on their surfaces,
or to cause displacement deposition, or the combination of the
both. As these factors leads to poor adhesion, they are referred to
as difficult-to-plate metallic substrates. Such substrates include
aluminum (alloys), magnesium (alloys), tin (alloys),
stainless-steel, titanium (alloys) and die cast zinc alloys.
[0004] As a method for plating onto the difficult-to-plate metallic
substrates with good adherence, nickel strike plating is commonly
utilized. The method comprises, immediately after removing an oxide
layer from a substrate with an acid, forming a very thin nickel
layer on the substrate using a nickel plating solution which
contains a large amount of chloride, and then forming a desired
plated layer thereon.
[0005] The method utilizing nickel strike plating has been widely
practiced as general procedure with some modifications over the
years.
[0006] For instance, Japanese unexamined patent publication No.
2005-133169 discloses a method for producing a silver-plated
stainless steel strip for moving contacts comprising forming a
nickel underlayer onto a stainless steel base metal surface, and
then forming a silver- or silver alloy-plated layer thereon,
[0007] Furthermore, Japanese unexamined patent publication No.
2002-237312 discloses a method for producing a silver or
silver-alloy plated metal separator for solid electrolyte batteries
comprising silver- or silver alloy-plating onto a stainless steel
having a nickel plated layer as an underlayer.
[0008] However, elimination of nickel-strike plating step affords
reduction in manufacturing cost. Furthermore, for optimization of
the favorable properties of a silver plated layer, the presence of
nickel underlayer is sometimes not preferable. For that purpose,
Japanese unexamined patent publication No. 2002-121693 discloses a
method and bath for silver plating onto stainless steel without
nickel strike plating, the method utilizing a plating bath
containing halide ion with a pH value of -1.0.about.2.0.
SUMMARY OF THE INVENTION
[0009] It has been known that fair adherence can be achieved
without a presence of nickel underlayer by utilizing a silver
plating bath which contains high concentration of halide ion, but
industrial application of such bath entails some problems. As an
iodide bath is commonly utilized as the halide bath, the problems
will be hereafter explained specifically in connection with the
iodide bath.
[0010] (1) With high concentration of halide ion, silver forms
complex which dissolves easily whereas with low halogen
concentration, it exhibits poor solubility and such property of
silver halide is utilized for detection of residual halogen. Since
precipitation of silver is generated in a low-concentration iodide
bath, use of a high-concentration iodide bath is preferable but
doing so may create problems such as corrosion of equipment or
wastewater management.
[0011] (2) Moreover, in a rinse process, because the plating
solution attached on the surface of the object to be plated is
diluted with rinse water and the concentration of iodide ion
becomes low, precipitation of silver iodide, which is very
difficult to remove, is formed on the surface.
[0012] (3) Silver-plated layer deposited from an iodide bath
exhibits low ductility and it is prone to cracking.
[0013] (4) In an iodide bath, iodine is generated through the
oxidization of iodide ion on the anode and that has, even if
slightly generated, negative effect on the plated layer which
causes poor adherence and coarsening. Furthermore, when significant
amount of iodine is generated, iodine may be released as a toxic
gas. In particular, since the generation of toxic gas becomes
significant when an insoluble anode is used, use of the insoluble
anode becomes unacceptable.
[0014] To optimize the properties of silver-plated layer, there has
been a demand for a method of silver plating that does not comprise
the step of forming a nickel plated layer between the
difficult-to-plate substrate and the silver plated layer. However,
there is much to be improved to meet industrial requirements.
[0015] Thus the object of the present invention is to provide a
method for silver plating that does not comprise the step of
forming the nickel-underlayer and that can form a silver-plated
layer having sufficient adherence directly on the
difficult-to-plate substrates with the use of a halide-free plating
bath under a good working environment.
[0016] The present inventors have found that a fine silver-plated
layer with good adherence can be formed onto the difficult-to-plate
substrate by utilizing a silver plating method that can prevent
formation of an oxide layer, the method comprising silver plating
onto the substrate using an acidic silver plating bath which
contains phosphines as a complexing agent and substantially does
not contain halide ion, preceded by degreasing the substrate and
then removing the oxide layer formed on the substrate surface from
the substrate with a strongly acidic solution.
[0017] Accordingly, in one aspect, the present invention is a
silver plating method onto a substrate on which an oxide layer that
inhibits adherence of a plated layer is prone to form, comprising
at least the following steps of;
[0018] (A) degreasing the substrate,
[0019] (B) removing the oxide layer with a strongly acidic solution
from the substrate, and(
[0020] (C) silver plating onto the substrate, without a step of
nickel or nickel alloy strike plating in advance, utilizing a
phosphines-containing acidic silver plating bath which essentially
does not contain halide ion or cyanide ion.
[0021] In one embodiment of the present invention, the method
further comprises (D) silver plating onto the substrate utilizing a
sulfonic acid-containing acidic silver plating bath preceded by
step (C).
[0022] In another embodiment of the present invention, the
phosphines-containing acidic silver plating bath in step (C)
further contains sulfonate ion.
[0023] In yet another embodiment of the invention, the silver
plating baths in steps (C) and (D) both have a pH value of less
than or equal to 3.
[0024] In yet another embodiment of the present invention, the
silver plating bath in step (C) contains one or more of the
phosphines represented by the general formula (1):
##STR00001##
wherein X1, X2 and X3, which may be the same or different, each
representing a hydrogen atom, a substituted or unsubstituted
C1.about.C10 alkyl group, or a substituted or unsubstituted benzene
ring, the substituents for the substituted alkyl group or the
substituted benzene ring being one or more selected from the group
consisting of a hydroxyl group, a carboxyl group, a sulfonic group
and an amino group, provided that not all of X1, X2 and X3 being
hydrogen atoms simultaneously.
[0025] In yet another embodiment of the invention, the phosphines
are lower alkylphosphine represented by the general formula
(2).
##STR00002##
wherein Y1, Y2 and Y3, which may be the same or different, each
representing an unsubstituted C1.about.C3 alkyl group or a
C1.about.C3 alkyl group substituted with one or more substituents
selected from the group consisting of a hydroxyl group, a carboxyl
group, a sulfonic group and an amino group.
[0026] In yet another embodiment of the invention, the strongly
acidic solution in step (B) contains more than or equal to 10 wt %
of acid.
[0027] In yet another embodiment of the invention, the strongly
acidic solution in step (B) has a pH value of less than or equal to
2.
[0028] In yet another embodiment of the invention, the strongly
acidic solution in step (B) essentially does not contain halide
ion.
[0029] In yet another embodiment of the invention, the strongly
acidic solution in step (B) contains sulfonic acid.
[0030] In yet another embodiment of the invention, the above
mentioned substrate is made of magnesium, aluminum, titanium,
chromium, nickel cobalt, zinc and tin, or an alloy which at least
contains one or more metals selected from the group consisting of
these metals.
[0031] In yet another embodiment of the invention, the substrate is
made of a chromium-containing alloy.
[0032] In yet another embodiment of the invention, the substrate is
made of a stainless steel.
[0033] In yet another embodiment of the invention, the substrate is
made of an austenitic stainless steel.
[0034] In yet another embodiment of the invention, the substrate is
made of titanium.
[0035] In yet another embodiment of the invention, the substrate is
made of tin or tin-alloy.
[0036] In yet another embodiment of the invention, the above
mentioned substrate is an alloy of tin and at least one or more
metals selected from copper, zinc, silver, indium, gold, lead and
bismuth.
[0037] In yet another embodiment of the invention, an insoluble
anode is used in the silver plating step (C) and/or (D).
[0038] In yet another embodiment of the invention, the insoluble
anode is selected from the group consisting of a carbon anode, a
platinum anode, a platinum-coated titanium anode, an oxidized
ruthenium-coated anode and an oxidized iridium-coated anode.
[0039] In yet another embodiment of the invention, the above
mentioned insoluble anode comprises an uppermost layer made of one
or more material(s) selected from the group consisting of spinel,
garnet, glass, and perovskite.
[0040] According to the cyanide-free silver plating method provided
by the present invention, oxidation of the surface of the
difficult-to-plate substrate can be prevented and direct silver
plating onto such substrate with good adherence can be achieved, by
forming a silver-plated layer using the acidic silver plating
solution containing phosphines as a reducing compound immediately
after removing the oxide layer from the substrate with an acidic
solution.
[0041] Furthermore, problems associated with silver plating baths
such as (1) corrosion of plating equipments and wastewater
management, (2) generation of silver compound on the substrate
surface during water-rinsing process, (3) cracking-prone plated
layer, (4) deterioration of plating bath caused by the generation
of iodine and (5) incompatibility with insoluble anodes, can all be
resolved.
[0042] Moreover, by utilizing the solution which contains sulfonic
acid as a strongly acidic solution for removing the oxide layer on
the difficult-to-plate substrate, it also becomes possible to
eliminate halide ion from the removal process and as a result,
elimination of halide ion from whole process can be essentially
achieved.
PREFERRED EMBODIMENTS OF THE INVENTION
[0043] Following is the detailed descriptions of the silver plating
method provided by the present invention.
[0044] The present invention is a silver plating method onto a
substrate on which an oxide layer that inhibits adherence of a
plating that inhibits adherence of a plated layer is prone to form,
comprising at least the following steps of;
[0045] (A) degreasing the substrate,
[0046] (B) removing the oxide layer with a strongly acidic solution
from the substrate, and
[0047] (C) silver plating onto the substrate, without a step of
nickel or nickel alloy strike plating in advance, utilizing a
phosphines-containing acidic silver plating bath which essentially
does not contain halide ion or cyanide ion.
[0048] Therefore one of the advantages provided by the present
invention is that it affords the elimination of nickel or
nickel-alloy strike plating step carried out in a case of the usual
difficult-to-plate substrates, thereby silver plating can be
applied directly following step (B). It is common that both the
strongly acidic solution as well as the silver plating bath being
provided in the form of aqueous solution.
[0049] By removing the oxide layer in step (B) and then plating
with the acidic silver plating bath which contains phosphines,
which are compounds with a strong reducing ability, the reformation
of the oxide layer is prevented and thus plated layers having good
adherence can be obtained even onto the substrate surface which is
prone to the formation of the oxide layer.
[0050] It is preferable to conduct step (C) immediately after step
(B), to prevent the reformation of the oxide layer on the substrate
surface. For instance, time spent between lifting the object to be
plated from the strongly acidic solution and immersing the object
into the silver plating bath should be 5.about.120 seconds, more
preferably 5.about.15 seconds including time for rinsing.
[0051] For the degreasing step (A), degreasing method can be
selected from any methods known to those skilled in the art and
include, but not limited to, acid degreasing, alkaline degreasing,
solvent degreasing, emulsion degreasing, electrolytic degreasing,
and machine degreasing. However, among them, acid degreasing is the
most preferable for the purpose of minimizing pH fluctuation of the
substrate surface, thereby improving adhesion.
[0052] According to the present invention, silver plating step can
be carried out with the acidic silver plating bath which contains
the above mentioned phosphines without any other plating bath.
However, silver plating may be carried out by a flash silver
plating with the phosphines-containing bath, followed by (D) a
silver plating with the sulfonic acid-containing acidic bath
exhibiting a higher deposition efficiency.
[0053] The requisite of the present invention is the utilization of
an acidic plating solution as the silver plating baths for the
above mentioned step (C) and (D). For the both steps, the acidic
bath with a pH value of less than or equal to 3 is preferable and
less than or equal to 2 is more preferable. As the acid to control
the pH of the silver plating bath, halide free-acids such as
organic sulfonic acids, sulfuric acid, phosphoric acid, fluoboric
acid are preferable. Among them, organic sulfonic acids, in
particular, methane sulfonic acid, alkanol sulfonic acid and
phenolsulfonic acid are preferable from the viewpoint of solubility
of silver ion and stability of the bath.
[0054] In the present invention, the silver plating bath for step
(C) contains at least one or more of aliphatic or aromatic
phosphines represented by the general formula (1):
##STR00003##
wherein X1, X2 and X3, which may be the same or different, each
representing a hydrogen atom, a substituted or unsubstituted
C1.about.C10 alkyl group, or a substituted or unsubstituted benzene
ring, the substituents for the substituted alkyl group or the
substituted benzene ring being one or more selected from the group
consisting of a hydroxyl group, a carboxyl group, a sulfonic group
and an amino group, provided that not all of X1, X2 and X3 being
hydrogen atoms simultaneously.
[0055] In addition, the lower alkyl phosphines represented by the
general formula (2) can be more preferably used.
##STR00004##
wherein Y1, Y2 and Y3, which may be the same or different, each
representing an unsubstituted C1.about.C3 alkyl group or a
C1.about.C3 alkyl group substituted with one or more substituents
selected from the group consisting of a hydroxyl group, a carboxyl
group, a sulfonic group.
[0056] The preferable phosphines include, for example,
unsubstituted alkyl phosphines in which each alkyl group is a
methyl group, an ethyl group or a propyl group; and substituted
alkyl phosphines in which each alkyl group is substituted by one or
more substituents selected from the group consisting of a hydroxyl
group, a carboxyl group, a sulfonic group, and an amino group. The
substituted alkyl phosphines include hydroxy lower alkyl phosphines
having a hydroxymethyl group, a hydroxyethyl group or a
hydroxypropyl group; carboxy lower alkyl phosphines having a
carboxymethyl group, a carboxyethyl group or a carboxypropyl group;
sulfo lower alkyl phosphines having a sulfomethyl group, a
sulfoethyl group or a sulfopropyl group; and amino lower alkyl
phosphines having an aminomethyl group, an aminoethyl group or an
aminopropyl group.
[0057] Among phosphines described above, tris(hydroxy lower
alkyl)phosphines, in which one hydrogen atom on each lower alkyl
group is substituted by a hydroxyl group to form each hydroxy lower
alkyl group selected from the group consisting of a hydroxymethyl
group, a hydroxyethyl group and a hydroxypropyl group can be more
preferably used in terms of cost and stability.
Tris(3-hydroxypropyl)phosphine can be most preferably used.
[0058] The phosphines-containing acidic silver plating bath for
step (C) may also contain sulfonate ion. Moreover, a sulfonic acid
bath can be preferably used as the bath for step (D) in light of
plating bath stability, appearance of electrodeposited layer and
electrical properties such as surface resistance.
[0059] Although any of the aliphatic or aromatic sulfonic acids may
be preferably used, aliphatic sulfonic acids are more preferably
used.
[0060] Among aliphatic sulphonic acids, aliphatic acids such as
alkane sulfonic acids or alkanol sulfonic acids can be preferably
used. As the above mentioned alkane sulfonic acids, for instance,
methanesulfonic acid, ethanesulfonic acid, 1-propanesulfonic acid,
2-propanesulfonic acid, 1-buthanesulfonic acid, 2-buthanesulfonic
acid and pentanesulfonic acid can be listed. As the above mentioned
alkanolsulfonic acids, for instance, 2-hydroxyethane-1-sulfonic
acid (isethionic acid), 2-hydroxypropane-1-sulfonic acid
(2-propanolsulfonic acid), 2-hydroxybuthane-1-sulfonic acid,
2-hydroxypentane-1-sulfonic acid as well as
1-hydroxypropane-2-sulfonic acid, 3-hydroxypropane-1-sulfonic acid,
4-hydroxybuthane-1-sulfonic acid and 2-hydroxyhexane-1-sulfonic
acid can be listed.
[0061] In the present invention, the solution containing more than
or equal to 10 wt % of acid is preferably used for step (B) to
remove the oxide layer. However, as corrosion of the equipments or
adverse effect to working environment are expected when the
concentration of the acid is excessively high, it is preferable
that wt % of the acid kept less than or equal to 50%. Typically,
the acid concentration is kept at 10.about.50 wt % and more
typically, it is kept at 10.about.20 wt %.
[0062] Moreover, pH of the corresponding solution is kept
preferably less than or equal to 2, and more preferably less than
or equal to 1 considering removing ability of the oxide layer.
Depending on the type of the substrate and the condition of the
oxide layer, the type of acid can be selected from the known acid
in the plain or mixed form. Nitric acid, sulfuric acid, phosphoric
acid, hydrochloric acid, hydrofluoric acid as well as the above
mentioned various sulfonic acids can be preferably used. However,
to prevent corrosion of the equipments and drag-in to the silver
plating solution in the following silver plating step, it is
preferable not to use the acid which contains halide ion such as
hydrochloric acid or hydrofluoric acid and thus the acidic solution
which contains sulfonic acid as its main component is preferably
used for the removal of the oxide layer.
[0063] The substrates on which an oxide layer that inhibits
adherence of a plated layer is prone to form and to which the
present invention can be suitably applied include magnesium,
aluminum, titanium, chromium, nickel cobalt, zinc, tin and silicon,
and an alloy which at least contains one or more metals selected
from the group consisting of these metals.
[0064] Those expressed as the substrate here include not only the
bulk metals (or alloys) but also plated layers.
[0065] Examples of the substrate include magnesium cast alloys
containing aluminum or zinc, aluminum bronze cast alloys, silzin
bronze cast alloys, aluminum cast alloys for bearings, stainless
steels, nickel-phosphorous plating films, aluminum or
copper-containing zinc die cast alloys, zinc alloy-plated layer
such as zinc-iron or zinc-nickel, tin or tin alloy plated layer
which contains copper, zinc, silver, indium, gold, lead or
bismuth.
[0066] Among these substrates, the direct silver plating according
to the present invention can be applied more preferably to
stainless steels and most preferably to austenitic stainless
steels.
[0067] According to the present invention, silver plating can be
carried out by separating cathode and anode compartments with the
ion-exchange membrane in either or both of (C) phosphine-containing
acidic silver plating bath substantially containing neither halide
ion nor cyanide ion and (D) sulfonic acid-containing acidic silver
plating bath.
[0068] Although the ion-exchange membrane method can be preferably
used for both baths (C) and (D), it can be most suitably used for
bath (C).
[0069] Although either a cationic ion-exchange membrane or a
anionic ion-exchange membrane can be used as the ion-exchange
membrane, the anionic ion-exchange membrane can be more suitably
used. By separating the cathode and anode compartments with the
anionic ion-exchange membrane, decomposition of the additives such
as a complexing agent, a leveler or a brightener which may be added
to the plating bath, and the negative effect on the plated layer by
the compound generated as the result of consumption or
decomposition of the additives, can be prevented. Moreover,
increase in silver concentration in the bath due to the use of
silver anode can be prevented and control of metal concentration in
the bath can be done easily.
[0070] Furthermore, according to the silver plating method of the
invention, an insoluble anode can be used as an anode. The anode
made of known materials such as carbon, platinum, platinum-coated
titanium, ruthenium oxide-coated titanium or carbon, or iridium
oxide-coated titanium or carbon may be utilized. Preferably, the
insoluble anode may comprise an uppermost layer made of one or more
selected from the group consisting of spinel, garnet, glass, and
perovskite.
[0071] Thus, the soluble silver anode, the above mentioned
insoluble anode or the combination of both, can be used as the
anode.
[0072] The silver plating method of the present invention utilizing
the acidic bath comprises, although not limited to, the steps of
(A) degreasing the substrate, (B) removing the oxide layer with a
strongly acidic solution from the substrate, and (C) silver plating
onto the substrate utilizing a phosphines-containing acidic silver
plating bath which essentially does not contain halide ion or
cyanide ion. Following the silver plating step (C), step (D) of
silver plating with the sulfonic acid-containing acidic silver
plating bath may be applied. Normally, water-rinsing is conducted
between each step.
[0073] The conditions for step (C) is that bath temperature should
be generally kept at 10.about.50 degrees C., more preferably at
20.about.35 degrees C.
[0074] Preferable current density is 0.5.about.5 A/dm2, and more
preferably 2-3 A/dm2. Plating time should be 10.about.300 seconds
and more preferably 20.about.100 seconds.
[0075] The conditions for step (D) is that bath temperature should
be generally kept at 10.about.50 degrees C., and more preferably at
15.about.40 degrees C. Preferable current density is 0.1.about.10
A/dm2, and more preferably 0.5.about.5 A/dm2. Plating time can be
set depending on the plating thickness required.
EXAMPLES
[0076] Following is the detailed descriptions of the present
invention with regard to the actual examples. These examples are
meant to be the illustrative of the invention and not limiting the
application of the invention.
[0077] Each plating process was evaluated from the aspect of
adhesiveness of the plated layer. Plating adherence was evaluated
through the bending test. The bending test (90 degrees.times.3
times) was conducted according to JIS-H8504 standard and the extent
of peeling was observed.
[0078] Daiwa Fine Chemical general purpose acidic cleaner AC-100
was used for degreasing.
Example 1
[0079] Stainless steel (SUS304) was selected as a substrate. (A)
acid degreasing, .fwdarw.(B) removal of an oxide layer with a
strongly acidic solution.fwdarw.(C) acidic silver plating drying
was carried out in this order. Water-rinsing step was conducted
between each step.
[0080] The composition of the solution used in each step is shown
below. Time spent from when the object to be plated was taken out
of the strongly acidic solution until it was immersed into the
silver plating bath was 15 seconds, including 5 seconds for
immersing the object into the rinse water.
TABLE-US-00001 strongly acidic solution methanesulfonic acid 150
g/L (15 wt %) temperature 25 degrees C. silver plating bath
tris(3-hydroxypropyl)phosphine 20 g/L sulfuric acid 40 g/L silver
sulfate (as silver) 3 g/L pH -0.4 type of anode carbon type of
ion-exchange membrane anion-exchange membrane temperature 25
degrees C. current density 3A/dm3 plating time 90 seconds
[0081] The silver-plated material thus obtained was subjected to
the bending test and no sign of cracking or peeling was observed,
thereby exhibiting excellent ductility and adherence.
Example 2.about.20
[0082] Unless otherwise stated in Table 1, silver plating for each
example was conducted according to the conditions stated in Example
1. The results of the bending tests are also provided in table
1.
[0083] If no cracking or peeling was observed after bending three
consecutive times and thus good ductility as well as adherence were
exhibited, such result was evaluated as grade "excellent". If
slight sign of cracking or peeling was observed after bending three
consecutive times, grade "good" if slight sign of cracking or
peeling was observed after bending twice, such result was evaluated
as grade "not bad". If obvious sign of cracking or peeling was
observed before the completion of the second bending, such result
was evaluated as "bad".
TABLE-US-00002 TABLE 1 No Modification from Example 1 Result Ex: 2
An aluminum-containing magnesium alloy was used Good as the
substrate. Ex: 3 A zinc-containing magnesium alloy was used as the
Good substrate. Ex: 4 An aluminum-bronze was used as the substrate.
Excellent Ex: 5 A silzin-bronze was used as the substrate.
Excellent Ex: 6 A stainless steel (SUS 316 ) was used as the
Excellent substrate. Ex: 7 The substrate (in example 1) plated with
a zinc- Good nickel alloy layer was tested. Ex: 8 The substrate (in
example 1) plated with a copper- Excellent tin alloy layer was
tested. Ex: 9 A silver plating bath with the following composition
Excellent was used: tris(3-hydroxypropyl)phosphine 30 g/L
methanesulfonic acid 50 g/L sulfuric acid 10 g/L silver
methanesulfonate (as silver) 2 g/L pH -0.1 Ex: 10 A silver plating
bath with the following composition Excellent was used
tris(hydroxymethyl)phosphine 30 g/L phosphoric acid 70 g/L silver
oxide (as silver) 1 g/L pH -0.1 Ex: 11 A silver plating bath with
the following composition Excellent was used
tris(2-hydroxyethyl)phosphine 60 g/L mehanesulfonic acid 30 g/L
silver methanesulfonate (as silver) 5 g/L pH -0.5 Ex: 12 A silver
plating bath with the following composition Excellent was used
tris(3-aminopropyl)phosphine 20 g/L sulfuric acid 50 g/L silver
sulfate (as silver) 10 g/L pH -0.3 Ex: 13 A strongly acidic
solution with the following Excellent composition was used sulfuric
acid 300 g/L (30 wt %) pH 0.8 Ex: 14 A strongly acidic solution
with the following Not bad composition was used methanesulfonic
acid 1 g/L (0.1 wt %) sodium hydrate 0.3 g/L pH 5 Ex: 15 The time
spent before the immersion into the silver Not bad plating bath was
100 seconds in total, including 20 seconds for water-rinsing. Ex:
16 following step (C), the extra silver plated-layer was Good
formed with the following silver plating bath methanesulfonic acid
50 g/L silver methanesulfonate (as silver) 20 g/L pH
.quadrature.0.3 bath temperature 25 degrees C. Ex: 17 A titanium
anode covered with platinum uppermost Good layer was used as the
anode. Ex: 18 An aluminum-containing magnesium alloy was used
Excellent as the substrate. Ex: 19 A stainless steel (SUS410) was
used as the Good substrate. Ex: 20 A carbon electrode covered with
spinel uppermost Excellent layer was used as an anode
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