U.S. patent application number 10/207431 was filed with the patent office on 2004-02-05 for electroplating solution.
Invention is credited to Baldwin, Kevin Richard.
Application Number | 20040020567 10/207431 |
Document ID | / |
Family ID | 31186691 |
Filed Date | 2004-02-05 |
United States Patent
Application |
20040020567 |
Kind Code |
A1 |
Baldwin, Kevin Richard |
February 5, 2004 |
Electroplating solution
Abstract
A conversion coating solution comprises at least one precursor
agent, at least one activator agent, and water solvent, wherein on
provision to a solution suitable for the deposition of a
substantially metallic coating, provides an in-situ conversion
coating to the surface of the substantially metallic coating.
Inventors: |
Baldwin, Kevin Richard;
(Farnborough, GB) |
Correspondence
Address: |
DR. KEVIN RICHARD BALDWIN
P.O. BOX 310
FARNBOROUGH HAMPSHIRE
GU14 7WG
GB
|
Family ID: |
31186691 |
Appl. No.: |
10/207431 |
Filed: |
July 30, 2002 |
Current U.S.
Class: |
148/273 |
Current CPC
Class: |
C25D 3/02 20130101; C25D
5/48 20130101 |
Class at
Publication: |
148/273 |
International
Class: |
C23C 022/48 |
Claims
1. A conversion coating solution comprises at least one precursor
agent, at least one activator agent, and water solvent, wherein on
provision to a solution suitable for the deposition of a
substantially metallic coating, provides an in-situ conversion
coating to the surface of the substantially metallic coating.
2. A solution according to claim 1, wherein the solution further
comprises at least one modifying agent.
3. A solution according to claim 1 or claim 2, wherein the at least
one precursor agent is a non-fully reducible metal ion. Preferably
it is chosen from Y.sup.3+, Rb.sup.+, Li.sup.+, La.sup.3+,
Ce.sup.3+, Ce.sup.4+, Pr.sup.3+, Nd.sup.3+, Sm.sup.3+, Eu.sup.3+,
Gd.sup.3+, Td.sup.3+, Dy.sup.3+, Ho.sup.3+, Er.sup.3+, Tm.sup.3+,
Yb.sup.3+, Lu.sup.3+.
4. A solution according to claim 3 wherein the at least one
precursor agent is chosen from Y.sup.3+, La.sup.3+, Ce.sup.3+,
Ce.sup.4+, Pr.sup.3+, Nd.sup.3+.
5. A solution according to claim 1 or claim 2, wherein the at least
one activator agent comprises a peroxide, peroxonium salt, a
hydroperoxide, ozone analogue of peroxide, or nitrate.
6. A solution according to claim 1 or claim 2, wherein the
activator agent is peroxide.
7. A solution according to claim 1 or claim 2, wherein the solution
suitable for the deposition of a substantially metallic coating
comprises at least one of the following: Zn.sup.2+, Fe.sup.2+,
Fe.sup.3+, Ni.sup.2+, Co.sup.2+, Ag.sup.+, Sn.sup.2+, Sn.sup.3+,
Cu.sup.+, Cu.sup.2+, Mn.sup.2+, Mn.sup.3+, Mn.sup.4+,
Mn.sup.6+.
8. A solution according to claim 7, wherein the solution suitable
for the deposition of a substantially metallic coating comprises at
least one of the following: Zn.sup.2+, Fe.sup.2+, Fe.sup.3+,
Ni.sup.2+, C.sup.2+.
9. A solution according to claim 7, wherein the solution suitable
for the electrodeposition of a substantially metallic coating is an
electroplating solution.
10. A solution according to claim 7, wherein the solution suitable
for the electrodeposition of a substantially metallic coating is a
proprietary electroplating solution.
11. A solution according to any preceding claim further comprises a
conversion coating modifying agent, comprising a non-metallic
anion.
12. A solution according to claim 11, wherein the non-metallic
anion comprises one of silicate, oxalate, iodate, formate,
phosphate, or carbonate.
13. A solution according to any preceding claim, wherein the
solvent is acid or alkaline.
14. A solution according to any preceding claim, wherein the
solvent is water.
15. An article treated using a solution according to any previous
claim.
16. A process for coating an article, wherein a conversion coating
solution comprises at least one precursor agent, at least one
activator agent, and water solvent, wherein on provision to a
solution suitable for the deposition of substantially metallic
coatings, imparts a conversion coating layer to the substantially
metallic coating.
17. A process for coating an article wherein a conversion coating
solution comprises at least one precursor agent, and water solvent,
wherein use is provided to an aqueous solution suitable for the
deposition of substantially metallic coatings, wherein an article
is brought into contact with the solution, wherein use a
substantially metallic coating is formed on the article, the at
least one activator agent is provided to the solution, such that a
conversion coating layer is imparted to the substantially metallic
coating.
18. A process for coating an article wherein a conversion coating
solution comprises at least one precursor agent, and water solvent,
wherein use is provided to an aqueous solution suitable for the
electrodeposition of substantially metallic coatings, an article is
brought into contact with the solution, electrical current is
passed to the article such that a substantially metallic coating is
deposited onto the article, the current is substantially reduced or
halted, the at least one activator agent is provided to the
solution, such that a conversion coating layer is imparted to the
substantially metallic coating.
19. A process for coating an article wherein a conversion coating
solution comprises at least one activator agent, and water solvent,
wherein use is provided to an aqueous solution suitable for the
electrodeposition of substantially metallic coatings, an article is
brought into contact with the solution, electrical current is
passed to the article such that a substantially metallic coating is
deposited onto the article, the current is substantially reduced or
halted, the at least one precursor agent is provided to the
solution, such that a conversion coating layer is imparted to the
substantially metallic coating.
20. A process according to any of claims 15 to 19, wherein the
activator agent comprises a peroxide or nitrate.
21. A process according to any of claims 15 to 20, wherein the
precursor agent comprises Y.sup.3+, La.sup.3+, Ce.sup.3+,
Ce.sup.4+, Pr.sup.3+, Nd.sup.3+, Sm.sup.3+, Eu.sup.3+, Gd.sup.3+,
Tb.sup.3+, Dy.sup.3+, Ho.sup.3+, Er.sup.3+, Tm.sup.3+, Yb.sup.3+,
Lu.sup.3+.
22. A process according to claim 21 wherein the at least one
precursor agent is chosen from Y.sup.3+, La.sup.3+, Ce.sup.3+,
Ce.sup.4+, Pr.sup.3+, Nd.sup.3+.
23. A process according to any of claims 15 to 22, wherein the
solution suitable for the deposition of a substantially metallic
coating comprises at least one of the following: Zn.sup.2+,
Fe.sup.2+, Fe.sup.3+, Ni.sup.2+, Co.sup.2+, Ag.sup.+, Sn.sup.2+,
Sn.sup.3+, Cu.sup.+, Cu.sup.2+, Mn.sup.2+, Mn.sup.3+, Mn.sup.4+,
Mn.sup.6+.
24. A process according to claim 23, wherein the suitable for the
deposition of a substantially metallic coating comprises at least
one of the following: Zn.sup.2+, Fe.sup.2+, Fe.sup.3+, Ni.sup.2+,
Co.sup.2+.
25. A process according to any of claims 15 to 24, wherein the
solution suitable for the electrodeposition of a substantially
metallic coating is an electroplating solution.
26. A process according to claim 25, wherein the solution suitable
for the electrodeposition of a substantially metallic coating is a
proprietary electroplating solution.
27. A process according to any of claims 15 to 26, wherein the
solution further comprises a modifying agent.
28. A process according to claim 27, wherein the modifying agent is
a non-metallic anion comprising at least one of silicate, oxalate,
iodate, formate, phosphate, or carbonate.
29. A process according to any of claims 15 to 28, wherein the
solvent in acid or alkaline.
30. A process according to any of claims 15 to 29, wherein the
solvent is water.
31. A process according to any of claims 15 to 30, wherein the
activator is provided to the solution prior to
electrodeposition.
32. A process according to any of claims 15 to 30, wherein the
activator is provided to the solution during electrodeposition.
33. A process according to any of claims 15 to 30, wherein the
activator is provided to the solution following
electrodeposition.
34. A process according to any of claims 15 to 33, wherein the
electrical current provided to reduce metal ions in the plating
bath to their metallic state is direct current.
35. A process according to any of the claims 15 to 34, wherein the
electrical current provided to reduce metal ions in the plating
bath to their metallic state is pulsed current.
36. A process according to any claims 15 to 35, wherein the
electrical current provided to the plating bath is direct current
superimposed with alternating current.
37. A process according to any claims 15 to 36, wherein the
substrate is a metal.
38. A process according to claim 37, wherein the substrate is
steel.
39. A process according to any of claims 15 to 38, where the
substrate is a non-metal.
40. A process according to claim 39, wherein the substrate is a
plastic or carbon fibre.
41. An article treated using a process according to any of claims
15 to 40.
Description
BACKGROUND
[0001] This invention related to a conversion coating solution, a
process that provides conversion coatings to substantially metallic
coatings during or following electroplating, and articles treated
using the solution and process.
[0002] Coatings that consist substantially of metallic elements are
provided to substrate materials such as steels on an industrial
scale. The most usual method is through electrodeposition from
aqueous solutions, known in the art as electroplating. A further
method for the deposition of substantially metallic coatings is
electroless plating, also familiar to those practiced in the art.
Metallic coatings are often not deposited as a pure metal, due to
side reactions such as the deposition of hydrogen, the uptake of
precipitated substances, or the intentional inclusion of elements,
and it is more accurate to employ the term `substantially
metallic`.
[0003] Substantially metallic coatings are used to obtain
properties that cannot be afforded by the substrate material alone.
These properties include appearance, corrosion protection, and
engineering or physical properties such as improved wear
resistance. Chromium is used for decorative effects, and is usually
deposited over an undercoat of copper or nickel. Others include
gold, silver, platinum, nickel, copper, and rhodium. For
applications where improved engineering or physical properties are
required, chromium also finds extensive use, as do nickel, tin,
lead, and silver. In applications where sacrificial corrosion
resistance is required, zinc, and to a lesser extent, cadmium, are
the preferred choices. In the case of zinc, it can be
electrodeposited either alone, or in combination with other metals
to form alloy coatings, for example, zinc-cobalt, zinc-iron, and
zinc-nickel. In some cases, particles are incorporated into
metallic deposits to improve properties such as wear resistance,
known in the art as composite coatings. There is a vast range of
applications for all of these coatings.
[0004] In particular, the electrodeposition of substantially
metallic coatings takes placed on a global scale. It is known to
those practised in the art that the electrodeposition of
substantially metallic coatings requires the following: an external
circuit, consisting of a source of electrical current, means of
conveying current to the plating cell, and associated instruments
such as ammeters, and means of regulating the voltage and current;
the negative electrodes or cathodes, which are the articles to be
coated, along with a means of positioning the article in the
plating solution so that contact is made with the current source;
the plating solution itself, almost always aqueous, and contained
within a tank; and the positive electrodes, the anodes, usually of
the metal being plated.
[0005] The electrolyte most often comprises an aqueous solution
containing ions of the metal to be deposited. Sometimes the
solution also contains ions of a different metal, for example, when
an alloy deposit is desired. On the passage of electrical current
through the cell, the metal ions are attracted to the article,
where they are reduced to form a coating of that metal. The current
flow is terminated when the required thickness of coating has been
achieved. The coated article is then most usually rinsed several
times in water, to remove surplus solution, brought into contact
with a neutralising solution to counteract remaining traces of the
electrolyte, and again rinsed in water. If the coated article is to
be subject to conversion coating treatment, it is most often then
brought into contact with an acid rinse to activate the surface.
This basic sequence of rinses and treatments may be modified
according to the nature of the metallic coating.
[0006] The various treatment solutions, including rinse water, are
most often contained within steel tanks often lined with plastic
materials. The electrodeposition of steel strip involves
continuously moving the strip through the various solutions. For
small items such as fasteners, jigging is avoided through the use
of rotating barrels that contain electrolyte solution. The various
treatment solutions must be maintained at a constant temperature,
and heating is usually provided through the use of electrical
elements. Large volumes of clean water are required particularly
for the numerous rinse stages. The treatment solutions,
particularly those used to deposit the metallic coating, require
regular chemical analysis to ensure that the formulation is within
specification. If the solution is outside specification, the
appropriate chemicals are added in the required amounts to the
solution, thus returning the solution to within specification.
Other solutions, such as the acid rinse, gradually become
contaminated and must be drained from the tank and replenished at
regular intervals.
[0007] The multiple treatment tanks also produce `drag out`. This
refers to small amounts of water or solution that adheres to the
article surface following removal from the tank. This becomes `drag
in` when the article is then brought into contact with the next
solution in the sequence. This solution is thus contaminated with
the `drag in`. A great deal of effort is expended in minimising the
effects of `drag out`, which depletes solution, and `drag in`,
which can lead to serious contamination problems. One approach is
to use continuous filtration of the most important solutions to
remove contaminating ions.
[0008] Before the article is subject to the application of coating
treatments, it is understood by those practiced in the art, that
appropriate cleaning procedures must be carried out. For steel
articles, solvent degreasing may be used to remove oils and
greases, followed by contact with solutions that render the article
surface in a chemically clean condition, such that if a water film
were applied to the surface, no break in that water film would be
observed.
[0009] Solutions that are suitable for use in coating deposition
are known widely in the art. For electrodeposition, proprietary
solutions are available for the full range of substantially
metallic coatings. For other purposes, the compositions of suitable
solutions are widely published in the open literature, and are well
known by those practiced in the art. The solutions all contain
metal ions of the metals that are to be deposited onto the article.
For acid solutions these are most often salts of the metals that
are sufficiently soluble in water. For alkaline solutions, lower
concentrations of the metal salts are added to concentrated
solutions of hydroxide salts such as sodium hydroxide.
[0010] The remaining additions to the electroplating solution
wholly depend on the qualities that are desired of the
substantially metallic coating, although some general points can be
made. Wetting agents are widely used to reduce the surface tension
of the solution, which has the effect of minimising the formation
of pores in the deposit. A common example of a wetting agent is
sodium lauryl sulfate. The addition to the solution of complexing
agents is sometimes required. These form complexes with the metal
ions in order to facilitate the deposition of the metal, without
which their deposition might not be possible. The addition of
complexing agents is of particular importance with alkaline
solutions. The throwing power of acid solutions is often improved
through the addition of salts that increase conductivity, for
example, salts of alkali metals or ammonium compounds. For acid
solutions, it is often necessary to stabilise the pH through the
addition of buffers such as boric acid or acetic acid. For all
solutions, the physical form of the deposit can be modified or
regulated through the addition of levelling agents, which assist in
the formation of uniform deposits, or brightening agents, which
promote the deposition of bright coatings. Other chemical additions
may be required to aid in the dissolving of anodes, and to modify
other properties, either of the solution or of the deposit,
depending on the specific case.
[0011] Many substantially metallic coatings consisting
substantially of chromium, nickel, rhodium, platinum and gold,
possess inherent tarnish resistance, and can be put straight into
the use for which they are intended. For others, the coating
deposit formed on the article requires further treatment to impart
the range of properties demanded by the end user. These include
zinc, cadmium, silver, tin, and copper, and alloys such as
zinc-cobalt, zinc-nickel, and zinc-iron. In the case of those
coatings consisting substantially of zinc, the surfaces are often
dull and susceptible to staining, and possess inadequate corrosion
resistance. Moreover, their tendency to oxidise in the atmosphere
renders them unfit for use, and the formation of loosely adherent
corrosion products prevents the successful application of
paints.
[0012] For coatings containing substantial quantities of metals
such as zinc, cadmium, silver, tin, and copper, the further
treatment consists of the application of a conversion coating. This
is carried out in a series of treatment tanks separate from those
involved in electrodeposition of the coating onto substrate
materials such as steels. A conversion coating is known in the art
to consist of a coating produced by chemical or electrochemical
treatment on a substantially metallic surface, which gives a
superficial layer containing compounds of the metals present.
[0013] To those practiced in the art, a conversion coating is known
not to be necessarily identical to the films provided to
substantially metallic coatings known as passivation treatments.
Conversion coatings may act as a barrier layer, in which case the
underlying metallic surface is not rendered to a passive state.
Other conversion coatings, for example those containing appreciable
amounts of chromate, however, are known to change the surface of
some metallic coatings from an active state to a more passive
state, in which case they are sometimes referred to as passivation
films.
[0014] The most widely used conversion coating treatments are those
based on chromates or phosphates, or combinations of chromates and
phosphates. These are widely available as proprietary solutions
that are formulated for specific substantially metallic coatings.
For those coatings containing appreciable amounts of cadmium,
copper, and zinc, as well as other metals, treatments containing
chromates are the preferred option. Phosphates are also widely
used, for example, on zinc-iron alloys, where they improve paint
adhesion. There are different types of phosphate treatment
available, including iron-phosphates and zinc-phosphates as
treatments before painting, and manganese phosphates, which provide
additional protection against wear.
[0015] Chromate conversion coatings are most often imparted to the
surface of substantially metallic coatings through immersion or
spraying of aqueous solutions of hexavalent chromate salts. The
solutions are most often strongly acidic and the contact time with
the coating is usually no more than 30s. This brief contact is
sufficient to polish and smooth the otherwise dull coating surface
to a brilliant and specular finish. The chromate solutions also
remove haze or other surface films that would otherwise interfere
with specular reflection.
[0016] In most countries, there are many proprietary chromate
treatment solutions available. They have been used routinely since
the early years of the twentieth century. They are formulated to
impart characteristic coloured finishes to articles coated with
substantially metallic deposits, and are often called up in
specifications. For maximum corrosion resistance, the yellow
chromates are the preferred option, since these conversion coatings
contain the highest concentrations of chromate. These conversion
coatings are used on coated steel articles from fasteners on
aircraft, to automobile bodies, through to large steel structures.
Olive drab is used on military equipment. Iridescent chromates are
less corrosion resistance but impart a distinctive decorate finish.
There are other chromates available including black, which are
often used on automobile parts. Chromate finishes can also be dyed
for colour coding purposes. Chromate conversion coatings are also
applied onto metallic coatings as a base for paints and other
organic finishes. The chromate conversion coating provides `anchor
points` to which the paint film can adhere; it also retards the
spread of corrosion if the paint is scratched or damaged.
[0017] The colour and lustre of chromate conversion coatings vary
according to the nature of the substantially metallic coating, the
composition of the treatment solution, method of application, and
the treatment conditions such as temperature, time and acidity. It
is essential that the conversion coating treatments impart the
required colour and reflective brilliance, lest the treated article
be rejected.
[0018] Chromate conversion coatings are formed by a chemical
reaction between the substantially metallic coating surface and the
hexavalent chromium in solution. The metals present in the surface
of the metallic coating are `converted` to their oxides, and the
hexavalent chromium is reduced to trivalent chromium. The
conditions at the surface become more alkaline and this causes the
trivalent chromium to precipitate in the form of a gel, which
entraps some of the hexavalent chromium solution, thus generating
the conversion coating. Chromate conversion coatings are in essence
amorphous gels and are considered to `harden` over time. This
arises from gradual dehydration of the conversion coating, leading
to the formation of micro cracks. Under normal circumstances, this
is not considered to be detrimental, although excessive heat can
dehydrate the gel and damage corrosion resistance.
[0019] The deposition of substantially metallic coatings followed
by the application of a conversion coating, has formed established
practice for many years. There are two drawbacks inherent with the
established practice. Firstly, the separate provision of the
conversion coating requires the use of numerous rinsing and other
treatment stages following electrodeposition. Secondly, the use of
hexavalent chromates is extremely undesirable because they are
known to be carcinogenic and harmful to the environment. The
chromate solutions, including the treatment solution, and rinse
water used to remove surplus solution, requires extensive treatment
before it can be recovered for subsequent reuse, or disposed of.
Those containing phosphates are substantially less harmful,
however, the treatment is of a similarly multi-stage nature.
[0020] The industry has attempted to replace chromate solutions
with less toxic alternatives, but again as a separate conversion
coating treatment stage, with the associated rinsing and treatment
solutions. This approach has fallen down on two counts, firstly,
the alternative solutions do not match the overall performance
provided by chromates, and, secondly, they are invariably more
expensive than chromates.
[0021] The use of chromates or phosphates as conversion coatings
treatments currently requires the use of many treatment stages, and
extensive ancillary equipment such as filtration plant and chemical
analysis facilities. Separate plant is required for the treatment
of water contaminated with chromates. Any solid chromate waste must
be stored or taken to licensed landfill sites. Others employ ion
exchange columns to separate out the chromates, allowing the
cleaned water to be reused. These multiple treatments were once
acceptable when the raw materials required to service the many
treatment stages were relatively inexpensive, and the treatment of
chromate waste less regulated by governments. This picture has
changed in most developed countries, and water, electricity,
equipment, and factory space are going up in cost. This is
exacerbated by the continued use of chromates, and there is
currently no answer to the threat posed by national and
international legislation that is likely to prohibit their use.
BRIEF SUMMARY OF THE INVENTION
[0022] The invention enables electroplating and chemical conversion
coating, currently separate processes with numerous intermediate
stages, to be conducted from a single solution. No chromates or
other toxic substances are employed.
DETAILED DESCRIPTION
[0023] In accordance with a first aspect of the present invention,
a conversion coating solution comprises at least one precursor
agent, at least one activator agent, and water solvent.
[0024] Preferably, the conversion coating solution further
comprises at least one modifying agent.
[0025] In accordance with a second aspect of the present invention,
a process for coating an article, wherein a conversion coating
solution comprises at least one precursor agent, at least one
activator agent, and water solvent, wherein on provision to a
solution suitable for the deposition of substantially metallic
coatings, imparts a conversion coating layer to the substantially
metallic coating.
[0026] In accordance with a third aspect of the present invention,
a process for coating an article wherein a conversion coating
solution comprises at least one precursor agent, and water solvent,
wherein use is provided to an aqueous solution suitable for the
deposition of substantially metallic coatings, wherein an article
is brought into contact with the solution, wherein use a
substantially metallic coating is formed on the article, the at
least one activator agent is provided to the solution, such that a
conversion coating layer is imparted to the substantially metallic
coating.
[0027] In accordance with a fourth aspect of the present invention,
a process for coating an article wherein a conversion coating
solution comprises at least one precursor agent, and water solvent,
wherein use is provided to an aqueous solution suitable for the
electrodeposition of substantially metallic coatings, an article is
brought into contact with the solution, electrical current is
passed to the article such that a substantially metallic coating is
deposited onto the article, the current is substantially reduced or
halted, the at least one activator agent is provided to the
solution, such that a conversion coating layer is imparted to the
substantially metallic coating.
[0028] In accordance with a fifth aspect of the present invention,
a process for coating an article wherein a conversion coating
solution comprises at least one activator agent, and water solvent,
wherein use is provided to an aqueous solution suitable for the
electrodeposition of substantially metallic coatings, an article is
brought into contact with the solution, electrical current is
passed to the article such that a substantially metallic coating is
deposited onto the article, the current is substantially reduced or
halted, the at least one precursor agent is provided to the
solution, such that a conversion coating layer is imparted to the
substantially metallic coating.
[0029] The solution suitable for the deposition of a substantially
metallic coating is employed to provide the metallic coating on an
article, to the surface of which can be provided the conversion
coating.
[0030] To those skilled in the art, a solution suitable for the
deposition of a substantially metallic coating will be recognised
as any that wherein use provides a substantially metallic coating
to an article. These solutions are widely available either as
laboratory formulations or on a proprietary basis from any supply
house.
[0031] Preferably, the solution suitable for the deposition of
substantially metallic coatings is an electroplating solution.
[0032] Preferably, the electroplating solution is suitable for the
deposition of substantially metallic coatings comprising the
following metals: Zn, Fe, Ni, Co, Ag, Sn, Cu, Mn.
[0033] Preferably, the electroplating solution is suitable for the
deposition of substantially metallic coatings comprising the
following metals: Zn, Fe, Ni, Co.
[0034] Preferably the electroplating solution is acid, alkaline, or
substantially neutral. Particularly suitable are acidic solutions
containing counter ions of sulphates, chlorides, or combinations of
sulphates and chlorides.
[0035] The provision of the conversion coating is mostly
independent from the provision of the metallic coating. The
requirement is that the conversion coating solution and process of
the present invention does not act to interfere with the deposition
of the substantially metallic coating.
[0036] The conversion coating solution comprises at least two
parts, the at least one precursor agent and the at least one
activator agent.
[0037] The at least one precursor agent is a chemical species that
on contact with the at least one activator agent, reacts to form a
solid precipitate.
[0038] Preferably, the at least one precursor agent is a
non-reducible metal ion. Preferably it is chosen from a salt of the
following: Y.sup.3+, La.sup.3+, Ce.sup.3+, Ce.sup.4+, Pr.sup.3+,
Nd.sup.3+, Sm.sup.3+, Eu.sup.3+, Gd.sup.3+, Tb.sup.3+, Dy.sup.3+,
Ho.sup.3+, Er.sup.3+, Tm.sup.3+, Yb.sup.3+, Lu.sup.3+. Suitable
salts are chlorides or sulphates, or mixtures of the two.
[0039] More preferably, the at least one precursor agent is
selected from Y.sup.3+, La.sup.3+, Ce.sup.3+, Ce.sup.4+, Pr.sup.3+,
Nd.sup.3+.
[0040] Preferably, the at least one activator agent is peroxide
(O.sub.2.sup.2-), peroxonium salts (H2OOH).sup.+, hydroperoxide
(OOH).sup.-, or ozone analogues of peroxide (HOOOH), or nitrate
salts.
[0041] More preferably, the at least one activator agent is
hydrogen peroxide.
[0042] The modifying agent is a chemical species that is entrapped
during the formation of the solid precipitate, arising from the
reaction between the precursor and activator agents. This serves to
impart additional properties on the conversion coating, for
example, those relating to appearance such as colour or reflection,
and properties such as corrosion resistance.
[0043] Preferably, the at least one modifying agent is an anion of
non-metals selected from: silicate, oxalate, iodate, formate,
phosphate, or carbonate.
[0044] More preferably the at least one modifying agent is silicate
or phosphate.
[0045] Preferably, the at least one precursor agent, the at least
one activator agent, and the at least one modifying agent are
dissolved in a solvent prior to being provided to the plating
solution.
[0046] Preferably, the solvent used to dissolve the at least one
precursor agent, the at least one activator agent, and the at least
one modifying agent is water.
[0047] Preferably, the concentration of the precursor agent is in
the range 0.001 g/L to saturation.
[0048] Preferably, the concentration of the activator agent is in
the range 0.001 g/L to saturation.
[0049] The concentration of hydrogen peroxide requires particularly
careful control lest the rate of conversion coating formation is
excessive, interfering with the initiation and growth of the
metallic coating, and causing the growth of macroscopic
dendrites.
[0050] Preferably, the concentration of the modifying agent is in
the range 0.001 g/L to saturation.
[0051] Preferably, the precursor, activator, and modifying agents
are provided separately to the solution suitable for the formation
of metallic deposits.
[0052] More preferably, the precursor agent and modifying agents
are provided to the solution prior to the provision of the
activator agent.
[0053] The current invention allows the provision of conversion
coatings from the same solution that provides the metallic coating
onto an article. This substantially reduces the number of treatment
stages required in order to provide a conversion coating to a
metallic coating. This brings about major advantages over existing
practices, including reduced water consumption, reduced energy
consumption, and the need for much fewer treatment tanks. All of
the costs and risks currently associated with chromates are
eliminated.
[0054] The plating solution once provided with the at least one
precursor agent, the at least one activator agent, and the at least
one modifying agent, can be re-used until the aforementioned
additions are exhausted, after which they may be replenished so
that their concentrations are sufficient to permit conversion
coating formation.
[0055] The present invention provides conversion coatings that are
colourless, iridescent, or coloured. The formulations and processes
can be chosen to provide conversion coatings that are identical in
appearance as those provided by chromates. The process of the
present invention may be used to treat any article substrate that
can be treated using traditional chromate or phosphate conversion
coating processes.
[0056] For the provision of coloured conversion coatings that are
identical in appearance to those provided by chromates, purely for
illustrative purposes, the precursor agent might comprise
Ce.sup.3+, the activator agent, H.sub.2O.sub.2. The precursor agent
is provided to a zinc sulfate plating bath, and the article brought
into contact with the bath. Electrical current is passed through
the solution such that zinc coating is provided to the article, and
the current substantially reduced or switched off. The peroxide is
provided to the solution. This reacts with Ce.sup.3+ to form a
sparingly soluble precipitate of hydrated cerium oxide on the
surface of the metallic deposit, which is gelatinous in substance
and yellow in colour. The precipitate forms a continuous coating
layer of approximately 2 .mu.m in thickness that is visually
indistinguishable from those provided by chromates.
[0057] The activator agent may be provided to the solution before,
during, or after, the formation of the substantially metallic
coating.
[0058] The precursor agent, the activator agent, and modifying
agent, require periodic replenishment. Where the activator
constitutes peroxide, this substance decomposes naturally, and must
be replenished frequently.
[0059] Preferably, the electrical current to reduce the metal ions
in the plating bath, thereby providing a substantially metallic
coating to the article, is direct current. Also suitable is pulsed
current, or current with AC superimposed.
[0060] Preferably, the conversion coating once imparted is rinsed
in cold water to remove surplus solution.
[0061] Preferably, the rinsed conversion coating is further rinsed
in water in the temperature range 20-80.degree. C., but more
preferably in the temperature range 30-55.degree. C.
[0062] Preferably, the hot rinse solution contains the at least one
precursor agent used in the prior provision of the conversion
coating, in the concentration range 0.001g/l to saturation.
[0063] Preferably, to the rinsed conversion coating is imparted a
sealing agent.
[0064] Preferably, the sealing agent is an organic substance.
[0065] More preferably, the sealing agent is a silane. Suitable
silanes are those of the general formula R--Si(OX).sub.3, where R
is a functional group and X is an alkoxy group or other
organo-functional group, which are hydrolysed to silanols of the
general formula R--Si(OH).sub.3. Other suitable sealing agents are
metallo-siloxanes, silicates, silicate esters, polyesters,
polyoxyethylenes, titanates, thioglycollates, trithioglycollates,
and acrylics.
[0066] Preferably, the article material is a metal such as those
consisting substantial quantities of iron, titanium, or
aluminium.
[0067] Preferably, the article material is non-metallic, such as a
plastic or carbon fibre, onto which a thin substantially metallic
coating has been added.
[0068] More preferably, the article material is steel.
[0069] The invention may be described by way of example only with
reference to the following drawings.
[0070] FIG. 1 shows a schematic diagram of a current state of the
art conventional electroplating process.
[0071] FIG. 2 shows a schematic diagram of a surface treatment
process according to the present invention.
[0072] FIG. 3 shows a schematic diagram of a plating cell suitable
for the provision of the conversion coating solution of the present
invention.
[0073] FIG. 4 shows a profile of the operation of a surface
treatment process according to one aspect of the present
invention.
[0074] FIG. 1 shows a conventional electroplating process. Each box
represents a separate treatment stage, through which the cleaned
article is moved in a sequential manner. The electrodeposition
stage provides the substantially metallic coating to the article.
Two rinsing stages remove surplus solution, remaining solution is
neutralised, and neutralising solution removed by water rinsing. An
acid rinse removes surface films. The coated article is immersed in
a bath to provide the chromate conversion coating. Two rinsing
stages remove surplus solution. The treated article is rinsed in
hot water and then dried in warm air. The treatment cycle is
completed and the treated article is inspected and stored in
readiness for dispatch.
[0075] FIG. 1 shows the flow of clean water to the rinse stages for
the removal of surplus solution following electrodeposition,
annotated (1) on the diagram. The water becomes contaminated with
metals from the electrodeposition stage through the effects of
drag-in. The rinse water runs into a general waste stream (2). The
water may be sent to a treatment plant, where it is cleaned or
disposed. For disposal, the water may be piped to open tanks where
the water is allowed to evaporate. The preferred option is to
recover the water through removing contamination, using techniques
such as reverse osmosis or ion exchange. It is then suitable for
reuse and returned to the clean water stream (3).
[0076] FIG. 1 shows the flow of clean water to the rinse stages for
the removal of surplus solution following chromate treatment (4).
Once again the water becomes contaminated but this time with
hexavalent chromate, and is run into a separate waste stream (5).
This requires special treatment due to the extreme toxicity of
chromate. One approach is to run the water into evaporation tanks,
reduce the hexavalent chromate to trivalent chromium through
chemical additions, which then precipitates out. The precipitate is
allowed to settle in the tanks to form sludge. The sludge is taken
to landfill whilst the water is then treated further to remove all
traces of chromate, using techniques such as ion exchange. The
treated water is then returned to the water stream (6).
[0077] FIG. 2 illustrates a process of the present invention to
allow comparison with traditional electroplating practices as
outlined above. The conversion coating solutions are held in a
series of automatic titrators (7) that provide the solutions to the
electroplating solution when required. This enables the conversion
coating to be formed in-situ, alongside the electrodeposition
stage. The water stream (8) is required only for a cold-water rinse
to remove surplus solution, and a hot rinse. For maximum corrosion
resistance the optional no-rinse sealing solution is employed. The
sealing solution provides a thin transparent film to the coated
article. The coated article is dried in a stream of air. The
wastewater stream (9) contains metal ions from the drag-in, and
requires no special treatment. The metal ions may be removed from
the waste stream through the use of recovery techniques such as ion
exchange. The treated water is then returned to the water stream
(10). The metal ions can be separated, or precipitated, and reused.
The present invention can be arranged such that it forms a closed
loop. The electrolyte may also be passed through an external column
containing activated carbon or similarly surface active materials
to facilitate cleaning of the electrolyte.
[0078] FIG. 3 illustrates a cell suitable for the operation of one
aspect of the present invention. The article to be coated (12) is
held on a conducting bar (13) and immersed in plating solution
(14). An electrical heater (15) maintains the plating solution
temperature at a constant value. An anode material (16) is present
in the plating solution, and connected to a power supply. Air is
introduced into the cell through a frit (17), creating small
bubbles and vigorous agitation of the solution. The solutions
comprising the conversion coating solution are held in reservoirs
above automatic titration devices (18). These possess valves that
release pre-set quantities of the solutions into the plating cell.
The titrators are connected to ion selective electrodes (19) or
other detection apparatus that determine the concentrations of the
different species in solution, and, if they fall below the required
limits, activate the titrators to release additional solution.
[0079] FIG. 4 shows the profile of the process according to one
aspect of the present invention. This shows four consecutive
treatment cycles. In cycle A, the voltage is increased sufficiently
to permit the electrodeposition of the substantially metallic
coating. This is followed by a soak period (20) where the
conversion coating is formed. Cycles B and C are identical. In
Cycle D, additions of the conversion coating formulation are made
(21) in order to maintain their concentration in solution, since
some of the solution would have been consumed in Cycles A, B and
C.
EXAMPLE 1
[0080] According to the present invention, a solution comprises
cerium sulphate, Ce.sub.2(SO.sub.4).sub.3, 1 g/L, hydrogen
peroxide, H.sub.2O.sub.2 (30 vol), 0.5 ml/L, and zinc sulphate
heptahydrate, ZnSO.sub.4.7H.sub.2O, 220 g/L, in distilled
water.
[0081] The solution was contained in a plastic vessel into which a
platinised titanium anode was placed. The plastic vessel was heated
by an electrical element to a temperature of 25.degree. C. The
solution was aspirated with air. A cleaned steel article was
connected to the negative terminal of a direct current power
supply, whilst the anode was connected to the positive terminal.
The steel article was immersed in the solution thus forming the
cathode in the cell, and current passed through the cell to provide
a cathodic current density of 2 A/dm.sup.2. A zinc coating was
formed on the surface of the steel article that was light grey in
appearance, and after 20 mins, the thickness achieved was 10 .mu.m.
The current was switched off and the conversion coating formation
took place, through reaction of the cerium salt and peroxide,
forming an adherent and continuous layer on the coated article.
After 2 mins the article was removed from the solution, rinsed in
distilled water at ambient temperature, rinsed in water at
60.degree. C., and then dried in a stream of air.
[0082] The conversion coating was light yellow with a brilliant
lustre. The conversion coating was visually identical to
traditional yellow chromate.
[0083] The adherence of the conversion coating was determined using
a tape test. Adhesive tape was pressed onto the surface of the
conversion coating and pulled off in one rapid movement. The tape
was examined and there were no signs of the conversion coating,
indicating no loss of adhesion.
EXAMPLE 2
[0084] The electroplating-conversion coating solution employed in
Example 1 was reused after 30 mins. The sequence of treatment
stages detailed in Example 1 was repeated. The conversion coating
obtained was identical to that obtained in Example 1.
EXAMPLE 3
[0085] After a period of 24 hrs the solution employed in Example 2
was reused. The sequence of treatment stages detailed in Example 1
was repeated, however, an additional aliquot of 0.5 ml/L 30 vol
H.sub.2O.sub.2 was provided to the solution immediately following
the electrodeposition stage. The conversion coating obtained was
identical to that obtained in Example 1.
EXAMPLE 4
[0086] Solutions were prepared identical to those in Example 1,
except that H.sub.2O.sub.2 was excluded. The same sequence of
treatment stages was employed, except that immediately following
the electrodeposition stage, 0.2 ml/L of H.sub.2O.sub.2 was added
to the solution. The conversion coating was formed on the surface
of the zinc coating. After 5 mins, the coated article was removed,
and subject to rinsing identical to that in Example 1. The
conversion coating obtained was identical to that obtained in
Example 1.
EXAMPLE 5
[0087] Solutions were prepared identical to that in Example 1, and
the same treatments used, except that following the
electrodeposition stage, the current was reduced such that the
cathode voltage increased to a value of -0.8V, from -1.2V during
electrodeposition. The conversion coating was formed and after 4
mins, the current flow was switched off. The conversion coating
thus obtained was identical to that obtained in Example 1.
EXAMPLE 6
[0088] According to the present invention, a solution comprises
cerium sulphate, Ce.sub.2(SO.sub.4).sub.3, 1 g/L, and hydrogen
peroxide, H.sub.2O.sub.2, 30 vol, 1 ml/L, dissolved in distilled
water. This was added to a solution comprising zinc sulphate
heptahydrate, ZnSO.sub.4.7H.sub.2O, 220 g/L and nickel sulphate
hexahydrate, NiSO.sub.4.6H.sub.2O, 90 g/L.
[0089] The solution was contained in a plastic vessel into which a
platinised titanium anode was placed. The plastic vessel was heated
by an electrical element to a temperature of 25.degree. C. The
solution was aspirated with air. A cleaned steel article was
connected to the negative terminal of a direct current power
supply, whilst the anode was connected to the positive terminal.
The steel article was immersed in the solution thus forming the
cathode in the cell, and current passed through the cell to provide
a cathodic current density of 2 A/dm.sup.2. A zinc alloy coating,
containing 10 weight per cent nickel, was formed on the surface of
the steel article that was light grey in appearance, and after 20
mins, the thickness achieved was 10 .mu.m. The current was switched
off and the conversion coating formation took place, forming an
adherent and continuous layer on the coated article. After 2 mins
the article was removed from the solution, rinsed in distilled
water at ambient temperature, and then dried in a stream of
air.
[0090] The conversion coating was light yellow with a brilliant
lustre. The conversion coating was visually identical to
traditional yellow chromate.
EXAMPLE 7
[0091] A solution and process identical to that described in
Example 6 was prepared and followed, except that cobalt chloride,
CoCl.sub.2, 50 g/L, was substituted for NiSO.sub.4.6H.sub.2O. The
metallic coating obtained was a zinc alloy containing 0.8 weight
per cent cobalt. The conversion coating was light yellow with a
brilliant lustre. The conversion coating was visually identical to
traditional yellow chromate.
EXAMPLE 8
[0092] A solution and process was employed identical to that in
Example 1. Following the formation of the conversion coating and
rinse stages, the coated article was immersed in a proprietary
silane sealing solution to which was added
Ce.sub.2(SO.sub.4).sub.3, 1 g/L. The solution was maintained at
30.degree. C. and the contact time was 2 mins.
[0093] The sealing solution provided a thin transparent film to the
surface of the yellow conversion coating.
[0094] The corrosion resistance of the sealed conversion coating
was determined through the used of a standard neutral salt spray
test. Mild steel panels treated using the method described in this
example were placed in the test chamber with chromated zinc
coatings of the same thickness. The time to the formation of white
corrosion products, namely the corrosion products of zinc, was
estimated to determine the corrosion resistance of the conversion
coatings. The time to the formation of red corrosion products,
namely the corrosion products of zinc, was estimated to determine
the corrosion resistance of the overall coating treatment in its
ability to protect steel.
[0095] The table provides the corrosion data obtained in hours
1 Conversion Coating White rust Red rust Yellow chromate on Zinc
168 502 Solution and Process of Example 8 168 670
[0096] The corrosion test demonstrates that the conversion coating
of the present invention affords the same degree of corrosion
protection to the zinc coating as do traditional chromate finishes,
and are more protective to the steel substrate.
EXAMPLE 9
[0097] According to the present invention, a solution comprises
cerium sulphate, Ce.sub.2(SO.sub.4).sub.3, 1 g/L, sodium silicate,
Na.sub.2SiO.sub.4, 1 g/l, and hydrogen peroxide, H.sub.2O.sub.2, 30
vol, 1 ml/L, dissolved in distilled water. This was added to zinc
sulphate heptahydrate, ZnSO.sub.4.7H.sub.2O, 220 g/L,
[0098] The solution was contained in a plastic vessel into which a
steel anode was placed. The plastic vessel was heated by an
electrical element to a temperature of 25.degree. C. The solution
was aspirated with air. A cleaned steel article was connected to
the negative terminal of a direct current power supply, whilst the
anode was connected to the positive terminal. The steel article was
immersed in the solution thus forming the cathode in the cell, and
current passed through the cell to provide a cathodic current
density of 2A/dm.sup.2. A zinc coating was formed on the surface of
the steel article that was light grey in appearance, and after 20
mins, the thickness achieved was 10 .mu.m. The current was switched
off and the conversion coating formation took place, forming an
adherent and continuous layer on the coated article. After 2 mins
the article was removed from the solution, rinsed in distilled
water at ambient temperature, rinsed in water at 60.degree. C., and
then dried in a stream of air.
EXAMPLE 10
[0099] According to the present invention, a solution comprises
zinc sulphate heptahydrate, ZnSO.sub.4.7H.sub.2O, 220 g/L, yttrium
chloride, YCl.sub.3, 1 g/L, NdCl.sub.3 1 g/L and hydrogen peroxide,
H.sub.2O.sub.2, 30 vol, 1.5 ml/L, dissolved in distilled water.
[0100] The solution was contained in a plastic vessel into which a
steel anode was placed. The plastic vessel was heated by an
electrical element to a temperature of 25.degree. C. The solution
was aspirated with air. A cleaned steel article was connected to
the negative terminal of a direct current power supply, whilst the
anode was connected to the positive terminal. The steel article was
immersed in the solution thus forming the cathode in the cell, and
current passed through the cell to provide a cathodic current
density of 5A/dm.sup.2. A zinc coating was formed on the surface of
the steel article that was light grey in appearance, and after 20
mins, the thickness achieved was 10 .mu.m. The current was switched
off and the conversion coating formation took place, forming an
adherent and continuous layer on the coated article. After 2 mins
the article was removed from the solution, rinsed in distilled
water at ambient temperature, rinsed in water at 60.degree. C., and
then dried in a stream of air.
[0101] The conversion coating was iridescent with a brilliant
lustre. The conversion coating was visually identical to
traditional iridescent chromate.
EXAMPLE 11
[0102] According to the present invention, a solution comprises
zinc sulphate heptahydrate, ZnSO.sub.4.7H.sub.2O, 220 g/L,
praseodymium sulphate, Pr.sub.2(SO.sub.4).sub.3, 2 g/L, and
hydrogen peroxide, H.sub.2O.sub.2, 30 vol, 0.3 ml/L, dissolved in
distilled water.
[0103] The solution was contained in a plastic vessel into which a
platinised titanium anode was placed. The plastic vessel was heated
by an electrical element to a temperature of 25.degree. C. The
solution was aspirated with air. A cleaned steel article was
connected to the negative terminal of a direct current power
supply, whilst the anode was connected to the positive terminal.
The steel article was immersed in the solution thus forming the
cathode in the cell, and current passed through the cell to provide
a cathodic current density of 2 A/dm.sup.2. A zinc coating was
formed on the surface of the steel article that was light grey in
appearance, and after 20 mins, the thickness achieved was 10 .mu.m.
The current was switched off and the conversion coating formation
took place, forming an adherent and continuous layer on the coated
article. After 2 mins the article was removed from the solution,
rinsed in distilled water at ambient temperature, rinsed in water
at 60.degree. C., and then dried in a stream of air.
[0104] The conversion coating was light yellow with a brilliant
lustre. The conversion coating was visually identical to
traditional yellow chromate.
EXAMPLE 12
[0105] According to the present invention, a solution comprises
zinc sulphate heptahydrate, ZnSO.sub.4.7H.sub.2O, 220 g/L, cerium
sulphate, Ce(SO.sub.4).sub.2, 2.5 g/L, lanthanum chloride,
LaCl.sub.3, 1 g/L, and hydrogen peroxide, H.sub.2O.sub.2, 30 vol, 1
ml/L, dissolved in distilled water.
[0106] The solution was contained in a plastic vessel into which a
platinised anode was placed. The plastic vessel was heated by an
electrical element to a temperature of 25.degree. C. The solution
was aspirated with air. A cleaned steel article was connected to
the negative terminal of a direct current power supply, whilst the
anode was connected to the positive terminal. The steel article was
immersed in the solution thus forming the cathode in the cell, and
current passed through the cell to provide a cathodic current
density of 2 A/dm.sup.2. A zinc coating was formed on the surface
of the steel article that was light grey in appearance, and after
20 mins, the thickness achieved was 10 .mu.m. The current was
switched off and the conversion coating formation took place,
forming an adherent and continuous layer on the coated article.
After 2 mins the article was removed from the solution, rinsed in
distilled water at ambient temperature, rinsed in water at
60.degree. C., and then dried in a stream of air.
[0107] The conversion coating was a golden yellow with a brilliant
lustre.
EXAMPLE 13
[0108] According to the present invention, a solution comprises
zinc sulphate heptahydrate, ZnSO.sub.4.7H.sub.2O, 220 g/L,
neodymium chloride, NdCl.sub.3, 1 g/L, and sodium nitrate,
NaNO.sub.3, 1 g/L, dissolved in distilled water.
[0109] The solution was contained in a plastic vessel into which a
platinised titanium anode was placed. The plastic vessel was heated
by an electrical element to a temperature of 25.degree. C. The
solution was aspirated with air. A cleaned steel article was
connected to the negative terminal of a direct current power
supply, whilst the anode was connected to the positive terminal.
The steel article was immersed in the solution thus forming the
cathode in the cell, and current passed through the cell to provide
a cathodic current density of 2 A/dm.sup.2. A zinc coating was
formed on the surface of the steel article that was light grey in
appearance, and after 20 mins, the thickness achieved was 10 .mu.m.
The current was switched off and the conversion coating formation
took place, forming an adherent and continuous layer on the coated
article. After 2 mins the article was removed from the solution,
rinsed in distilled water at ambient temperature, rinsed in water
at 60.degree. C., and then dried in a stream of air.
[0110] The conversion coating was light blue with a brilliant
lustre.
* * * * *