U.S. patent application number 12/766312 was filed with the patent office on 2010-09-23 for silver ternary alloy.
This patent application is currently assigned to Argentium International Limited. Invention is credited to Peter Gamon Johns.
Application Number | 20100239454 12/766312 |
Document ID | / |
Family ID | 42751803 |
Filed Date | 2010-09-23 |
United States Patent
Application |
20100239454 |
Kind Code |
A1 |
Johns; Peter Gamon |
September 23, 2010 |
SILVER TERNARY ALLOY
Abstract
A firestain and tarnish-resistant ternary alloy of silver,
copper and germanium contains from more than 93.5 wt % to 95.5 wt %
Ag, from 0.5 to 3 wt % Ge, optionally 0.5 wt % Zn and the
remainder, apart from incidental ingredients (if any), impurities
and grain refiner, copper. In order to further protect an article
made from the alloy, it may be surface treated with an alkanethiol,
alkyl thioglycollate, dialkyl sulphide or dialkyl disulphide.
Embodiments of the above alloy exhibit relatively low elution of
copper when subjected to a simulated sweat test.
Inventors: |
Johns; Peter Gamon;
(Hertfordshire, GB) |
Correspondence
Address: |
KELLEY DRYE & WARREN LLP;STEVEN J. MOORE
400 ALTLANTIC STREET , 13TH FLOOR
STAMFORD
CT
06901
US
|
Assignee: |
Argentium International
Limited
Surrey
GB
|
Family ID: |
42751803 |
Appl. No.: |
12/766312 |
Filed: |
April 23, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10559092 |
Aug 24, 2006 |
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PCT/GB2004/002317 |
Jun 1, 2004 |
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12766312 |
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Current U.S.
Class: |
420/504 ;
420/502 |
Current CPC
Class: |
C23F 11/161 20130101;
C22C 5/06 20130101; C22C 5/08 20130101; C23F 11/16 20130101; C23C
22/02 20130101 |
Class at
Publication: |
420/504 ;
420/502 |
International
Class: |
C22C 5/08 20060101
C22C005/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 3, 2003 |
GB |
03 12693.5 |
Claims
1. An alloy, comprising: from 93.5 wt % to 95.5 wt % of silver;
from 0.5 to 3 wt % of germanium; 1-40 parts per million of boron;
and the remainder, apart from impurities, being copper; the weight
ratio of copper to germanium being from 4:1 to 3:1; and the alloy
being resistant to the development of porosity and brittleness, the
development of hot short cracking defects when investment cast, the
development of cracks or shattering on annealing and quenching and
the development of cracks and sagging when heated for joining or
torch annealing.
2. The alloy of claim 1, wherein the weight ratio of copper to
germanium is about 3.5:1.
3. The alloy of claim 1, which comprises from 1.0 wt % to 1.5 wt %
of germanium.
4. The alloy of claim 1, which comprises about 94.5 wt % of silver,
about 4.3 wt % of copper and about 1.2 wt % of germanium.
5. The alloy of claim 1, which comprises 5-20 ppm of boron.
6. The alloy of claim 1, wherein said alloy is a finished or
semi-finished shaped article.
7. The alloy of claim 6, wherein said finished or semi-finished
shaped article is casting.
8. The alloy of claim 6, wherein said finished or semi-finished
shaped article is produced or partially produced from a sheet or a
strip.
9. The alloy of claim 1, which comprises more than 93.5 wt % of
silver.
10. The alloy of claim 1, which further comprises 0.5 wt % of
zinc.
11. An alloy, comprising: from 93.7 wt % to 95.5 wt % silver; from
1.0 to 1.5 wt % germanium; 1-40 parts per million of boron; and the
remainder, apart from impurities, being copper; the weight ratio of
copper to germanium being from 4.5:1 to 3:1; and the alloy being
resistant to the development of porosity and brittleness, the
development of hot short cracking defects when investment cast, the
development of cracks or shattering on annealing and quenching and
the development of cracks and sagging when heated for joining or
torch annealing.
12. The alloy of claim 11, which further comprises 0.5 wt % of
zinc.
13. The alloy of claim 11, comprising about 93.7 wt % of silver Ag,
about 5.1 wt % Gu of copper and 1.2 wt % of germanium.
14. A ternary alloy of silver, copper and germanium, comprising:
from more than 93.5 wt % to 95.5 wt % of silver; from 0.5 wt % to 3
wt % of germanium; and a remainder comprising, apart from
incidental ingredients in amounts that are not detrimental to
mechanical strength and tarnish resistance of said ternary alloy,
impurities and grain refiner and copper, said ternary alloy being
resistant to developing of hot short cracking defects when
investment cast and resistant to developing of cracks and sagging
when heated for joining or torch annealing.
Description
REFERENCE TO PRIOR APPLICATIONS
[0001] This application is a continuation-in-part application of
U.S. patent application Ser. No. 10/559,092 filed Nov. 22, 2005
which claims priority from International patent application
PCT/GB2004/002317 filed Jun. 1, 2004 (Publication No. WO
2004/106567) which claims priority from UK Patent Application 03
12693.5 filed Jun. 3, 2003 and issued as a patent GB 2402399, which
disclosures are herein incorporated by reference in their
entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to a ternary alloy of silver,
copper and germanium, to finished or semi-finished shaped articles
made from the alloy, and to the use for the surface treatment of
the alloy with alkanethiol, alkyl thioglycollate, dialkyl sulfide
or dialkyl disulfide.
BACKGROUND TO THE INVENTION
[0003] Since ancient times it has been appreciated that unalloyed
`fine` silver is too soft to withstand normal use, and it has been
the practice to add a proportion of a base metal to increase
hardness and strength. In the UK, legislation that has existed
since the fourteenth century specifies a minimum silver content of
articles for sale at 92.5% (the Sterling standard) or 96% (the
Britannia standard), but does not specify the base metal
constituents. Experience convinced early silversmiths that copper
was the most suitable of the metals available to them. Modern
silver-sheet manufacturers generally adhere to this composition,
although sometimes a proportion of copper is replaced by cadmium to
attain even greater ductility. Sterling with a 2.5% cadmium content
is permitted to be used in the UK for spinning and stamping,
although the use of cadmium is becoming less widespread because
when the alloy is in the molten state fumes of cadmium are given
off and are toxic. For that reason, a specific alloy composition is
produced in continental Europe that contains 93.5 wt % silver. It
is sold as a sterling grade alloy with enhanced forming properties
for deep drawing or spinning operations. The high silver content
reduces the hardness of the alloy, but not to a level where a
finished item would be too soft and subject to excessive damage
from handling. It therefore can provide a cadmium-free spinning
grade, sold to perform similar forming operations to the
cadmium-containing silver grades available in the UK.
[0004] The relatively high value assigned to silver compared to the
other alloying constituents in the different grades has meant that
manufacturers have aimed to produce their alloys as closely as
possible to the minimum legal silver content. This has also
resulted in a system in the UK of ensuring that the minimum silver
content is assured by means of independent external
verification--the Assay Office System.
[0005] In all but the largest manufacturing companies, most of the
annealing and soldering required to assemble finished or
semi-finished articles is carried out with the flame of an air-gas
blowtorch. The oxidising or reducing nature of the flame and the
temperature of the articles are controlled only by the skill of the
silversmith. Pure silver allows oxygen to pass easily through it,
particularly when the silver is heated to above red heat. Silver
does not oxidise in air, but the copper in a silver/copper alloy is
oxidised to cuprous or cupric oxide. Pickling of the oxidised
surface of the article in hot dilute sulphuric acid removes the
superficial but not the deeper seated copper oxide so that the
surface consists of fine or unalloyed silver covering a layer of
silver/copper oxide mixture. The pure silver is easily permeated
during further heating, allowing copper located deeper below the
surface to become oxidised. Successive annealing, cold working and
pickling produces a surface that exhibits the pure lustre of silver
when lightly polished but with heavier polishing reveals dark and
disfiguring stains known as `fire-stain` or `fire`. Soldering
operations are much more productive of deep fire-stain because of
the higher temperatures involved. When the depth of the fire-stain
exceeds about 0.025 mm (0.010 inches) the alloy is additionally
prone to cracking and difficult to solder because an oxide surface
is not wetted by solder so that a proper metallurgical bond is not
formed.
[0006] Secondly, it is a well-known fact that with exposure to
everyday atmospheric conditions, silver and silver alloys develop a
lustre-destroying dark film known as tarnish.
[0007] The addition of germanium to silver alloys was found to
offer a solution to both these problems. Patent GB-B-2255348
(Rateau, Albert and Johns; Metaleurop Recherche) disclosed a novel
silver alloy that maintained the properties of hardness and lustre
inherent in Ag--Cu alloys while reducing problems resulting from
the tendency of the copper content to oxidise. The alloys were
ternary Ag--Cu--Ge alloys containing at least 92.5 wt % Ag, 0.5-3
wt % Ge and the balance, apart from impurities, copper. The alloys
were stated to be stainless in ambient air during conventional
production, transformation and finishing operations, to be easily
deformable when cold, to be easily brazed and not to give rise to
significant shrinkage on casting. They were also stated to exhibit
superior ductility and tensile strength and to be annealable to a
required hardness. Germanium was stated to exert a protective
function that was responsible for the advantageous combination of
properties exhibited by the new alloys, and was in solid solution
in both the silver and the copper phases. The microstructure of the
alloy was said to be constituted by two phases, a solid solution of
germanium and copper in silver surrounded by a filamentous solid
solution of germanium and silver and copper. The germanium in the
copper-rich phase was said to inhibit surface oxidation of that
phase by forming a thin GeO or GeO.sub.2 protective coating which
prevented the appearance of fire-stain during brazing and flame
annealing which results from the oxidation of copper at high
temperatures. Furthermore the development of tarnish was
appreciably delayed by the addition of germanium, the surface
turned slightly yellow rather than black and tarnish products were
easily removed by ordinary tap water. The alloy was said to be
useful inter alia in jewelery. However, the alloy disclosed in the
above patent suffers limitations insofar as it can exhibit large
grain size, leading to poor deformation properties and formation of
large pools from low-melting eutectics resulting in localised
surface melting when the alloy is subject to the heat of an air
torch.
[0008] Furthermore, U.S. Pat. No. 6,168,071 and EP-B-0729398
(Johns) disclosed a silver/germanium alloy which comprised a silver
content of at least 77 wt % and a germanium content of between 0.4
and 7%, the remainder principally being copper apart from any
impurities, which alloy contained elemental boron as a grain
refiner at a concentration of greater than 0 ppm and less than 20
ppm. The boron content of the alloy could be achieved by providing
the boron in a master copper/boron alloy having 2 wt % elemental
boron. It was reported that such low concentrations of boron
surprisingly provided excellent grain refining in a
silver/germanium alloy, imparting greater strength and ductility to
the alloy compared with a silver/germanium alloy without boron. The
boron in the alloy inhibited grain growth even at temperatures used
in the jewellery trade for soldering, and samples of the alloy were
reported to have resisted pitting even upon heating repeatedly to
temperatures where in the previously suggested alloys a briefly
mentioned copper/germanium eutectic in the alloy would melt. Strong
and aesthetically pleasing joints between separate elements of the
alloy could be obtained without using a filler material between the
free surfaces of the two elements and a butt or lap joint could be
formed by a diffusion process or resistance or laser welding
techniques. Compared to a weld in Sterling silver, a weld in the
above-described alloy had a much smaller average grain size that
improved the formability and ductility of the welds, and an 830
alloy had been welded by laser welding and polished without the
need for grinding. It may be noted with regard to the above
mentioned eutectic that although its adverse effects are reduced by
the reduction in grain size, the ability of the eutectic to form
and become molten on post-formation thermal treatment is retained
because that is governed by the chemical composition of the alloy
rather than its crystal structure.
[0009] Silver alloy according to the teaching of GB-B-2255348 and
EP-B-0729398 is now commercially available in Europe and in the USA
under the trade name Argentium, and the word "Argentium" as used
herein refers to these alloys. The 925 grade Argentium alloy
comprises 92.5 wt % (minimum) Ag, 1.1-1.3 wt % Ge, 6 ppm B, the
balance being copper and impurities. The alloy shows excellent
resistance to tarnishing even under very arduous conditions. A
passive layer is formed by the germanium, which significantly slows
the formation of silver sulphide, the main cause of tarnishing on
conventional silver alloys. Even in a hydrogen sulphide atmosphere
the degree and depth of tarnish is significantly less compared to a
conventional silver alloy or a silver plated item. The same
mechanism that creates the tarnish resistance also results in the
formation of a passive layer which significantly reduces the depth
of `fire-staining` or the `fire layer` that is produced in this
alloy when torch annealing in air. Trials have shown that the depth
of the `fire-staining` to be up to three times greater in
conventional silver alloys compared to the Argentium silver alloys.
This reduces the amount of polishing that the alloy requires and
can result in other considerable cost savings in manufacturing.
[0010] Despite the advantages of existing Argentium alloy grades,
there is a need for further improvement of the alloy with respect
to its stability under thermal processing and in particular to its
resistance to pitting and/or sagging when heated for the purposes
of annealing or joining. There is also a need for alloys that
combine these favourable properties with hardness and resistance to
tarnishing. There is a further need for alloys that on investment
casting has a reduced propensity to formation of "hot short"
(cracking) defects.
[0011] The need for improvement of alloys of silver, copper and
germanium created by a low melting eutectic phase was recognised in
GB-A-2355990. That specification explained that over a germanium
content range of 0.1% to 3%, it was found that the addition of tin,
antimony, silicon or indium in small quantities, less than 2% by
weight, brought about improvement in the mechanical properties of
the resultant silver/copper/germanium alloy. For example, heat
distortion such as sagging of the silver/copper/germanium alloy at
high solder temperatures was reduced, the alloy could be more
readily rolled further from the as cast state and the alloy did not
work harden so quickly during other cold work processes. In one
example, a silver/copper/germanium alloy had a germanium content of
1% and included a tin content of between 0.01 and 1%. The
silver/copper/germanium alloy also had an elemental boron content
of 100 ppm or less. The addition of tin positively influenced the
distribution of the low melting point eutectics and caused a
reduction in coarse dendritic formations and secondary dendritic
arms. Thus, excessively high concentrations of low melting point
phases which predisposed the alloy to heat distortion were avoided
and the resultant alloy was less susceptible to heat distortion at
high soldering temperatures (typically >700.degree. C.) than the
same silver/copper/germanium alloy would have been without the tin
content. The effects of using antimony, silicon or indium in place
of tin as an additive were comparable. Additionally, beneficial
effects were achievable with an additive comprising a combination
of any or all of tin, antimony, silicon or indium. There was no
suggestion that the alleged advantages could be obtained in the
absence of antimony, silicon or indium, and in practice the alleged
advantages were insufficiently obtained.
[0012] As a further matter, so far as long-term tarnish resistance
is concerned, various proposals have been made for cleaning or
protecting Sterling silver and other known grades of silver to
remove tarnish and/or to inhibit the formation of tarnish.
GB-A-1130540 is concerned with the protection of a finished surface
of Sterling or Britannia silver as a step in a production run, and
discloses a process that comprises the steps of:
[0013] wetting a clean silver surface of an article with a solution
comprising 99 parts by weight of a volatile organic solvent, for
example trichloroethylene or 1,1,1-trichloroethane and from 0.1-1.8
parts by weight of an organic solute containing a --SH group and
capable of forming a transparent colourless protective layer on the
silver surface, for example stearyl and cetyl mercaptan or
thioglycollate;
[0014] allowing the solution to react with the surface to form such
a layer and allowing the solvent to evaporate; and
[0015] washing the surface with a detergent solution, rinsing the
surface with hot water and allowing it to dry. The above process is
stated to provide a "long-term finish" intended to last the
intended shelf-life until the article reaches the user.
[0016] Treatments of the above kind are believed to result in the
formation of a self-assembled coating derived from the thiol
compounds in which the sulphur atoms are bound onto the metal
surface and the alkyl tails are directed away from the metal
surface, see U.S. Pat. No. 6,183,815 (Enick). Yousong Kim et al,
http://www.electrochem.org/meetings/past/200/abstracts/symposia/h1/1026.p-
df reported that the adsorption of thiols onto silver proceeds
through an anodic oxidation reaction that produces a shift of the
open circuit potential of the substrate metal in the negative
direction or if the potential is fixed an anodic current peak:
RSH+M(0).fwdarw.RS-M(I)+H++e-(M) (M=Au or Ag) [0017] Kwan Kim,
Adsorption and Reaction of Thiols and Sulfides on Noble Metals,
Raman SRS-2000, 14-17 Aug. 2000, Xaimen, Fujian, China,
http://pcoss.org/icorsxm/paper/kuankim.pdf, also discloses the
formation of self-assembled monolayers and discloses that
alkanethiols, dialkyl sulfides and dialkyl disulfides self-assemble
on silver surfaces with aliphatic dithiols forming dithoiolates by
forming two Ag--S bonds. In contrast, the literature on formation
of alkylthiols of germanium is relatively sparse. The reaction of
alkanethiols with Ge to form a high quality monolayer has been
reported in the context of semiconductor and nanotechnology by Han
et al, J. Am. Chem. Soc., 123, 2422 (2001). In the experiment
described, a Ge(111) wafer is sonicated in acetone to dissolve
organic contaminants and immersed in concentrated HF to remove
residual oxide and produce a hydrogen-terminated surface, after
which the wafer is immersed in an alknaethiol solution in
isopropanol, sonicated in propanol and dried.
SUMMARY OF THE INVENTION
[0018] Although GB-B-2255348 discloses a range of silver content
from Sterling to Britannia grades, as previously explained a
skilled person would not contemplate using a silver content above
the legal minimum for the intended grade because of the cost of the
silver. He is not given by the disclosure of that patent or that of
the subsequent patents relating to the ternary alloys any incentive
to adopt intermediate silver contents. However, it has now been
found that silver-copper-germanium ternary alloys having silver
contents between those of the Sterling and Britannia grades possess
valuable properties that facilitate both casting, welding and other
thermal treatments used in the manufacture of silver articles.
[0019] In particular, the applicants have become aware of the
desirability of reducing or avoiding the formation and/or melting
of the above mentioned binary copper-germanium eutectic which melts
at 554.degree. C. During the production of e.g. 925 Argentium
silver alloys, the formation of this phase can be avoided by
careful control of the casting conditions since under equilibrium
cooling conditions the crystallisation is complete at below
640.degree. C. However, this binary phase can create problems
during subsequent thermal treatment of the alloys, e.g. using
brazing alloys which typically have melting points in the range
680-750.degree. and torch annealing which typically involves
heating a workpiece to a dull red heat at 700-750.degree. C. On
heating the workpiece to or beyond these temperatures incipient
melting occurs with a small amount of material corresponding to
this binary phase becoming molten while the bulk remains stable.
When the workpiece returns to ambient temperature, porosity
develops where the alloy has liquefied. This contributes
brittleness and e.g. as noted in GB-B-2255348 there is a tendency
for the alloy to sag when heated for joining or annealing
operations. Although the use of the boron grain refiner of U.S.
Pat. No. 6,168,071 and EP-B-0729398 significantly reduces the
pitting and sagging consequent on formation and melting of the
binary eutectic, the formation and melting of that eutectic is, as
previously mentioned, not eliminated and there is still scope for
the further development of the ternary alloy to improve its pitting
and sagging properties. By increasing the silver content above the
level for Sterling but less than that for Britannia it is possible
to produce an alloy in which the above binary eutectic either does
not form or gives rise to reduced problems in subsequent heat
treatment. This provides alloys with a much greater inherent
stability under thermal processing. The germanium addition prevents
the reduction in hardness that would be seen in a silver-copper
alloy of this composition. The alloy also shows resistance to
tarnishing, even under very arduous test conditions.
[0020] The invention therefore provides a ternary alloy of silver,
copper and germanium containing from more than 93.5 wt % to 95.5 wt
% Ag, from 0.5 to 3 wt % Ge, 1-40 ppm of B, optionally 0.5 wt % of
any of Zn, Cd and Sn, optionally 0.1-1 wt % Si, and the remainder,
apart from impurities copper, wherein the weight ratio of Cu to Ge
is from 4:1 to 3:1.
[0021] An embodiment of the above mentioned alloys has Ag 93.5-94
wt % e.g. 93.7 wt %. Another typical alloy that has been found to
be suitable contains about 94.5 wt % Ag, about 4.3 wt % Cu and
about 1.2 wt % Ge. In the above alloy the weight ratio of Cu to Ge
is about 3.6:1 whereas in the existing 925 grade Argentium the
ratio can be from 5.8:1 (1.1 wt % Ge) to 4.8:1 (1.3 wt % Ge).
[0022] The applicants consider that it is the reduction in the
Cu:Ge weight ratio to values <4.8:1 e.g. <4.7:1 e.g.
<4.5:1 that is responsible for the reduced thermal processing
problems, the CuGe eutectic either not forming or forming in a
significantly reduced amount during post-melt thermal processing.
In particular the ratio is advantageously from 4.5:1 to 3:1 e.g.
4:1 to 3:1, preferably about 3.5:1. Above 4:1 the alloy is more
likely to exhibit firestain, whereas below 3:1 the high germanium
content gives rise to formability problems. The applicants are
aware of a .zeta.-phase which is of hexagonal close packed
Cu.sub.5Ge and which forms the eutectic and a less harmful face
centred cubic phase Cu.sub.3Ge. Controlling the compositional ratio
of the copper and germanium to around 3:1 promotes formation of the
Cu.sub.3Ge phase c. A slight excess of copper above its 3:1 Cu:Ge
ratio allows the formation of a silver-rich a solid solution of
silver and copper, typical of the binary+silver-copper alloy.
However, the optimum level of germanium is 1.2-1.5%, so to enable
this ratio between copper and germanium to be achieved the silver
content of the alloy must be raised above the level for sterling
silver (92.5%) By complying with these limitations it is possible
to avoid the formation of the eutectic composition and the
.zeta.-phase with its hcp crystals structure which is well
documented to be a poor crystal structure for formability.
[0023] The increase in the silver level within the alloy to control
crystal structure, rather than controlling the cooling rate of the
molten alloy, is a novel solution to the management of the crystal
structure of this range of silver alloys, and hence resultant
physical properties of the alloy. It is contrary to the
conventional best practice within the silver manufacturing
industry, where silver content is very tightly controlled to reduce
costs.
[0024] In an experiment, strips of three different
germanium-containing silver alloys of identical size and each of
thickness 0.9 mm were placed on top of steel cotter pins on a
heatproof tile, and heated by means of a natural gas/compressed air
torch to a temperature in the range 745-778.degree. C. sufficient
to melt a "hard" silver solder. In each sample boron was present in
an amount effective for grain refinement but <10 ppm. The
samples were evaluated visually as to whether they had remained
straight or had sagged. Results are shown below, amounts being wt
%:
TABLE-US-00001 Ag Cu Ge Zn Cu:Ge Sagging 93.0 5.8 1.2 n/a 4.83 yes
93.5 5.3 1.2 n/a 4.41 no 93.5 4.8 1.2 0.5 4.0 no
[0025] The above results show that between a silver content of 93.0
and 93.5 and a Cu:Ge ratio of 4.8 to 4.4 the thermal stability
properties of the alloy change from poor to good, and that thermal
stability is compatible with addition of 0.5 wt % zinc.
[0026] In embodiments of the above alloy, preferred Ag contents
range from about 94.0 to about 95.5 wt %, lower values being
preferred for reducing the expense of the silver used. It has been
found, surprisingly, that if the Ag content is increased to 96 wt %
it is difficult to avoid firestain even at high Ge contents. As
regards Ge, contents of from 1.0 to 2.0 wt % are preferred. Below
1.0 wt % Ge, consistent resistance to firestain and tarnish may not
be obtained, whereas above 2 wt % Ge there is an increasing risk of
embrittlement of the alloy. Furthermore, Ge is expensive and its
expense makes it desirable to reduce its content to a minimum. The
applicants have found that consistent resistance to firestain and
tarnish are obtained at Ge contents of from 1.1 to 1.3 wt %. The
alloy will preferably further comprise boron in an amount effective
for grain refinement, typically 1-40 ppm and preferably 5-10 ppm.
Excessive amounts of boron may give rise to boron hard spots, but
in the case of alloys supplied for casting it will often be
desirable to incorporate relatively large amounts of boron to
compensate for losses on re-melting.
[0027] The alloy may contain one or more incidental ingredients
known per se in the production of silver alloys in amounts that are
not detrimental to the mechanical strength, tarnish resistance and
other properties of the material. For example, zinc may be added
e.g. in an amount of about 0.5 wt % to reduce the melting point of
the alloy, to add whiteness, to act as a copper substitute, as a
deoxidant and to improve the fluidity of the alloy. Cadmium may
also be added in similar amounts although its use is presently not
preferred. Tin may be added, typically in an amount of 0.5 wt %.
Indium may be added in small quantities e.g. as a grain refiner and
to improve the wettability of the alloy. Silicon may also be added
in e.g. amounts of 0.1 to 1 wt %.
[0028] Embodiments of the above alloys exhibit low copper elution.
Experiments have been carried out using specimens with a range of
different silver alloys with silver concentrations between 93.5%
and 97.3%, in each case with germanium at 1.3% and with boron at
about 4 ppm. These alloys were then subjected to a copper elution
sweat test based on BS EN 1911:1999 in which the samples are
exposed to synthetic sweat for one week. The elution rate of copper
is then quantified by spectrophotometric analysis of the liquid.
The results were as follows, the elution rate being measured in
.mu.g of copper/cm.sup.2/week and weight % Ag being as made up by
the experimentalist. Made up weights are preferred in the context
of precious metals since the practice in the industry which was
followed in this case is to weigh out constituents to at lease four
significant figures: assay figures are also available but silver
content appears to have been systematically overstated.
Corresponding rates are also given for conventional sterling
silver.
TABLE-US-00002 wt % Ag Elution rate 93 13.1 94 4.8 95 4.49 96 4.24
97 0.96 Sterling 5.84
[0029] The samples at 94, 95 and 96 wt % Ag were made up without
added boron. Since boron as grain refiner improves the
microstructure of the alloy, it is likely that even lower elution
rates would have been observed in boron-containing alloys. It will
be noted that there is a dramatic change in elution rate between
93.5 wt % Ag and 94.1 wt % Ag, where the elution rate falls from a
value more than twice that for sterling silver to a value less than
that for sterling silver.
[0030] A reason why it is feasible to reduce the copper content of
the alloy to improve physical properties and reduce copper elution
compared to standard Argentium alloys is because of the unique
hardening properties of the AgCuGe system. Hardening can occur
either by slow cooling alone or by low temperature baking which is
advantageous because quenching any red hot silver alloy into cold
water will always lead to cracking and solder joint failure. We
have observed a surprising difference in properties between
conventional sterling silver alloys and other silver alloys of the
Ag--Cu family on the one hand and silver alloys of the Ag--Cu--Ge
family on the other hand. Gradual cooling of e.g. the binary
Sterling-type alloys results in coarse precipitates and little
precipitation hardening, whereas gradual cooling of Ag--Cu--Ge
alloys (including those containing the further additives and
incidental ingredients set out above) results in One precipitates
and useful precipitation hardening, especially in those embodiments
where the silver alloy contains an effective amount of grain
refiner e.g. boron.
[0031] Experimental evidence has shown that Ag--Cu--Ge alloys of Ag
content 93.5 wt % and above become precipitation hardened following
cooling from a melting or annealing temperature by baking at e.g.
200.degree. C.-400.degree. C. and that baking the alloy can achieve
a hardness of 65 or above, preferably 70 HV or above and still more
preferably 75 DV or above which is equal to or above the hardness
of standard sterling silver used to make jewelery and other
silverware. These advantageous properties are believed to be the
result of the combination of Cu and Ge in the silver alloy and are
independent of the presence and amounts of Zn or other incidental
alloying ingredients.
[0032] Addition of germanium to sterling silver changes the thermal
conductivity of the alloy compared to standard sterling silver. The
International Annealed Copper Scale (IACS) is a measure of
conductivity in metals. On this scale the value of copper is 100%,
pure silver is 106%, and standard sterling silver 96%, while a
sterling alloy containing 1.1% germanium has a conductivity of 56%.
The significance is that the Argentium sterling and other
germanium-containing silver alloys do not dissipate heat as quickly
as standard sterling silver or their non-germanium-containing
equivalents, a piece will take longer to cool, and precipitation
hardening to a commercially useful level (e.g. to about Vickers
hardness 70 or above, preferably to Vickers hardness 110 or above,
more preferably to 115 or above) can take place during natural air
cooling or during slow controlled air cooling. Silver alloy of Ag
973 parts per thousand and containing about 1.0 wt % Ge, balance
copper, has been successfully precipitation hardened by gradual air
cooling from an annealing temperature, and it is believed that
Ag--Cu--Ge alloys with silver content above this level are also
precipitation hardenable
[0033] The benefit of not having to quench to achieve the hardening
affect is a major advantage of the present silver alloys. There are
very few times in practical production that a silversmith can
safely quench a piece of nearly finished work. The risk of
distortion and damage to soldered joints when quenching from a high
temperature would make the process not commercially viable. In fact
standard sterling can also be precipitation hardened but only with
quenching from the annealing temperature and this is one reason why
precipitation hardening is not used for sterling silver.
[0034] In order to distinguish the operations of annealing and
precipitation hardening (which are regarded as distinct by
silversmiths) annealing temperatures may be defined to be
temperatures above 500.degree. C., whereas precipitation hardening
temperatures may be defined to be in the range 150.degree.
C.-400.degree. C., the lower value of 150.degree. C. permitting
embodiments of the alloys of the invention to be precipitation
hardened in a domestic oven.
[0035] The alloy may be produced by continuous casting. The initial
casting conditions may be as for an equivalent silver-copper grade
except that the germanium addition will give solidus and liquidus
temperatures approximately 15.degree. C. lower than the equivalent
silver-copper alloys. The germanium content also alters the
emissivity of the alloy. This will affect the rate at which heat
can be removed from the die (if continuous casting) or standing
time (if static casting). It may also be desirable to re-calibrate
optical (infra-red) pyrometers when casting the present tertiary
alloys. This is because the germanium content gives the alloys a
different emissivity. Typically the sensors would give a much lower
reading than the actual temperature if they were not adjusted.
During billet production from continuously cast slabs it is a
requirement to remove the surface layer, which has been in contact
with the die. This is the layer that acted as the starting point
for solidification of the metal and it contains the most
impurities. To remove this layer a minimum of 0.01 mm should be
removed from each side of the cast slab by either a mechanical or
an abrasive technique.
[0036] To produce a sheet with an internal structure suitable for
further use by the silversmith a rolling regime which contains a
minimum of two 50% reductions and two anneals is recommended. This
will remove the `as-cast` grain structure and prevent any orange
peel effects when the sheet is being formed by the silversmith. As
an example, to produce sheet at 2.5 mm thick the following would be
the minimum rolling requirements:
TABLE-US-00003 Starting size 10 mm thick Roll to 5.0 mm thick (50%
reduction) Anneal Roll to 2.5 mm thick (50% reduction) Anneal
[0037] When cross-rolling to increase the width of sheets, at the
end of the cross-rolling regime the sheet should be annealed prior
to the commencement of the normal rolling schedule.
[0038] When drawing the alloy into wire, the required drawing
sequence depends on the internal grain structure of the starting
material. This is because the wire could be from two possible
sources, either a cold or warm working operation (e.g. extrusion)
or from an `as-cast` wire size (e.g. a `mini` casting system). For
material that has come from a previous cold worked source the only
constraint is that the material has a minimum of 25% cold work
prior to each anneal. This will prevent excessive grain growth. A
maximum of 60% cold work between each anneal is recommended. For
example, the following work procedures would apply to wire from a
previously cold worked source:
TABLE-US-00004 Starting size 6 mm diameter Draw to 5.2 mm diameter
(25% reduction) Or Starting size 6 mm diameter Draw to 3.8 mm
diameter (60% reduction) Anneal Draw to 2.4 mm diameter (60%
reduction) Anneal etc.
[0039] For material that is from a cast source then a drawing
sequence involving two reductions of a minimum of 50% and two
anneals recommended to give a grain size suitable for further work
by the silversmith. Work procedures would be similar to those given
above.
[0040] When annealing the alloy, it is important that the furnace
gas, although protective, does not deplete the surface layer of
germanium, as this will reduce the tarnish resistance of the alloy
and its resistance to "fire stain". It is the ability of the
surface layer of germanium to form a germanium oxide which then
acts as a barrier preventing any further penetration of the oxide
layer or build up of tarnishing products. For this reason a furnace
atmosphere based on cracked ammonia is not recommended. To prevent
depletion of the germanium from the surface of the alloy the
presence of a small amount of oxygen or a slightly "wet" furnace
atmosphere is beneficial. Typically the furnace atmosphere should
contain approximately 0.1-0.5% oxygen and have a dew point of
20-40.degree. C. The exact balance of these values depends on the
type of furnace being used. It is important that the balance is not
set to far in the opposite direction as this may result in
oxidation of the copper content of the alloy. The annealing
temperature may be within the range 620-650.degree. C. and should
preferably not exceed a maximum temperature of 650.degree. C. The
annealing time for this temperature range is 30 to 45 minutes.
[0041] As regards cleaning procedures, although GB-A-1130540 was
alleged to provide a long-term finish, in the inventor's experience
this type of treatment does not fully solve the difficulties
created by tarnish in the period between manufacture and supply to
the ultimate purchaser or user and suffers from a number of
shortcomings. Although a silver product might arrive at the
retailer in an untarnished state, it was largely the result of the
wrapping applied by the manufacturer, which protected the article
from air. Once the wrapping was removed and the article was
displayed in a retail environment such as a display case in a hotel
where it was subject to ambient air and the heat of artificial
lighting, an article of conventional Sterling silver would require
re-polishing after one week and after two weeks would normally be
so tarnished as to be un-saleable. At an exhibition, the life of an
article on display before significant tarnish sets in may be as
short as 3-4 days. Re-polishing produces wear and fine handling
scratches, so that unless the article can be sold quickly it looses
its pristine appearance. The need to polish display silver at
frequent intervals adds to the labour cost of a jeweler or other
retail establishment, whose management take the view that its staff
should be employed to sell products and not to clean stock. Tarnish
at point of sale is therefore a serious problem that reduces the
willingness of those in the distribution chain to stock and display
silver products, and which has not yet been adequately solved.
[0042] When the product reaches the ultimate purchaser, it is of
course desirable that the task of tarnish removal should be made as
infrequent and undemanding as possible.
[0043] It has now been found that an alkanethiol, alkyl
thioglycollate, dialkyl sulphide or dialkyl disulphide can be used
for the surface treatment of the above described alloys, preferably
so as to reduce or further reduce tarnishing of the alloy such that
a sample can be subjected to hydrogen sulphide gas above a 20%
solution of ammonium polysulphide for at least 30 minutes and
typically 45-60 minutes while retaining a generally untarnished
appearance.
[0044] The invention also therefore includes the use of an organic
compound containing --SH or --S--S-- bonds e.g. a C.sub.12-C.sub.24
alkanethiol, alkyl thioglycollate, dialkyl sulphide or dialkyl
disulphide in the preparation of a tarnish inhibitor for an article
of a the silver-copper-germanium alloy described above.
[0045] The invention further provides an alloy of silver as
described above, or a shaped article formed of said alloy, that has
been treated with a C.sub.12-C.sub.24 alkanethiol, alkyl
thioglycollate, dialkyl sulphide or dialkyl disulphide.
[0046] In an accelerated tarnish test in which the article is
subject to hydrogen sulphide gas from the ammonium polysulphide
solution above which it is suspended at a height of e.g. 30 mm
corresponds to a period of a year or more in a retail environment
where an article is on display and exposed to ambient atmosphere
and may be subject to elevated temperatures. It is the combination
of the protective function of the germanium content of the alloy
with the further protection from the organo-sulphur compound that
is believed to be responsible for the observed increase in tarnish
resistance. The period during which the article retains its
untarnished appearance under these severe conditions may be three
or more times the corresponding period for an article that has not
been treated with an organo-sulphur compound, which is unexpected
because the same accelerated tarnish test carried out under the
same conditions on a conventional Sterling silver article not
containing protective germanium does not reveal a significant
increase in untarnished lifetime between its untreated and
organo-sulfur treated states.
[0047] Accelerated tarnishing tests with Argentium Sterling using
ammonium polysulphide have been reported by the Society of American
Silversmiths, see [0048]
http://www.silversmithing.com/largentium4.htm
[0049] and in a comparative test the Argentium Sterling remained
untarnished after one hour whereas conventional Sterling became
tarnished after less than 15 minutes. However, in this test 0.5 ml
of 20% ammonium polysulfide solution is mixed with 200 ml of
distilled water, so that the test is greatly less severe than when
samples are exposed to the 20% solution itself. In WO 02/095082,
samples were suspended above 20% ammonium polysulphide, but the
exposure times were relatively short, and onset of yellowing was
reported for Ag--Cu--Ge alloys after 3-5 minutes exposure. Other
tests reported in that specification involve placing samples in a
desiccator containing flowers of sulphur and calcium nitrate and
are less severe than the ammonium polysulphide test.
[0050] As protective agent there may be used a compound containing
a long chain alkyl group and a --SH or --S--S-- group, e.g. an
alkanethiol, dialkyl sulfide or dialkyl disulfides in which the
chain is preferably at least 10 carbon atoms long and may be
C.sub.1-12--C.sub.24. The --SH or --S--S-- compounds that many be
used include straight chain saturated aliphatic compounds
containing 16-24 carbon atoms in the chain, for example stearyl
mercaptan, cetyl mercaptan (octadecyl mercaptan) and stearyl and
cetyl thioglycollates whose formulae appear below.
##STR00001##
[0051] Stearyl mercaptan is a white to pale yellow waxy solid that
is insoluble in water. The protective agent may be used in solution
in a solvent e.g. a non-polar organic solvent such as an alcohol
e.g. methyl or ethyl alcohol, a ketone e.g. acetone or methyl ethyl
ketone, an ether e.g. diethyl ether, an ester e.g. n-butyl acetate,
a hydrocarbon, a halocarbon e.g. methylene chloride,
1,1,1-trichloroethane, trichloroethylene, perchloroethylene or HCFC
141b. The protective agent may comprise 0.1-1 wt % of the solvent.
Solvents based on n-propyl bromide are presently preferred on the
ground of the short atmospheric life of that compound, its
relatively low toxicity compared to other halocarbons, its
favourable chemical and physical properties and its boiling point,
specific heat and latent heat of vaporization.
[0052] U.S. Pat. No. 5,616,549 discloses a solvent mixture
comprising: 90 percent to about 96.5 percent n-propyl bromide; 0
percent to about 6.5 percent of a mixture of terpenes, the terpene
mixture comprising 35 percent to about 50 percent cis-pinane and 35
percent to about 50 percent trans-pinane; and 3.5 percent to about
5 percent of a mixture of low boiling solvents, the low boiling
solvent mixture comprising 0.5 percent to 1 percent nitromethane,
0.5 percent to 1 percent 1,2-butylene oxide and 2.5 percent to 3
percent 1,3-dioxolane. The solvent mixture has the following
advantages:
[0053] (i) it is properly stabilized against any free acid that
might result from oxidation of the mixture in the presence of air,
from hydrolysis of the mixture in the presence of water, and from
pyrolysis of the mixture under the influence of high
temperatures;
[0054] (ii) it is non-flammable and non-corrosive;
[0055] (iii) the various components of the solvent mixture are not
regulated by the U.S. Clean Air Act; and
[0056] (iv) none of the various components of the solvent mixture
are known cancer causing agents (i.e., the various components are
not listed by N.T.I., I.A.R.C. and California Proposition 65, nor
are they regulated by OSHA). Moreover, the solvent mixture has a
high solvency with a kauri-butanol value above 120 and, more
preferably, above 125. In addition, the solvent mixture has an
evaporation rate of at least 0.96 where 1,1,1-Trichloroethane=1.
Upon evaporation, the solvent mixture leaves a non-volatile residue
(NVR) of less than 2.5 mg and, more preferably, no residue.
Solvents made in accordance with the above patent are available
from Enviro-Tech International, Inc of Melrose Park, Ill., USA
under the trade name EnSolv.
[0057] The surface treatment may be carried out after the
manufacturing stages for a shaped article made of the alloy have
been completed. The article may be of flatware, hollowware or
jewellery. Fabrication steps may include spinning, pressing,
forging, casting, chasing, hammering from sheet, planishing,
joining by soldering brazing or welding, annealing and polishing
using buffs/mops and aluminium oxide or rouge. An article to be
treated may be de-greased ultrasonically in a treatment bath,
dipped into a bath containing the treatment agent e.g. 1 wt %
stearyl mercaptan in solvent e.g. EnSolv, rinsed in one or more
baths of the solvent and allowed to dry by evaporation. The solvent
should leave no or substantially no residue, so that subsequent
washing with water or aqueous solvents should be unnecessary and
the article can be allowed to dry. The article may then be packed
for delivery into the distribution chain. This may include wrapping
the article in one or more protective sheets, placing it in a
presentation box, and wrapping the presentation box in a protective
wrapping e.g. of heat-shrunk plastics film. Articles which have
been treated with an organic compound containing --SH or --S--S--
groups as aforesaid and packaged should not only reach their point
of sale in good condition but should if displayed e.g. on a shelf
or in a cabinet for an extended period, expected to be at least 6
months and possibly 12 months or more, remain without development
of significant tarnish.
[0058] For many purposes, e.g. light industrial applications, it
may be preferred to carry out the anti-tarnish treatment using a
predominantly aqueous solvent system. For this purpose, the
protective agent may be dissolved in a water-immiscible organic
solvent, for example a solvent based on n-propyl bromide, the
resulting solution may be mixed with a relatively concentrated
water-based soap or detergent composition which acts as a
"carrier", after which water is added to the resulting mixture to
provide an aqueous treatment dip or combined degreasing and
treatment solution. Thus an aqueous dip has the advantages that a
solvent degreasing system is not necessary, the dip is easily made
and may be used cold, all areas of immersed articles can come into
contact with the stearyl mercaptan or other treatment agent, the
present alloy only requires 2 minutes in the dip, rinsing and
drying of articles are made easy as water droplets are repelled
from the surface of the polished alloy, and the dip can be easily
used in a manufacturing environment before articles are sent to
retailers.
[0059] Preferred water-based detergents may be based on anionic,
alkoxylated non-ionic or water-soluble cationic surface active
agents or mixtures of them and preferably have a pH at or close to
7. Anionic surfactants may be based on alkyl sulphates and alkyl
benzene sulphonates, whose harshness on prolonged skin exposure may
be reduced by the co-presence or use of alkyl ethoxy sulphates
(U.S. Pat. No. 3,793,233, Rose et al.; U.S. Pat. No. 4,024,078
Gilbert; U.S. Pat. No. 4,316,824 Pancherni). Other known
surfactants e.g. betaines may also be present, see e.g. U.S. Pat.
No. 4,555,360 (Bissett). A suitable formulation containing 5-15 wt
% non-ionic surfactants and 15-30 wt % anionic surfactants is
available commercially in the UK under the trade name Fairy Liquid
(Proctor & Gamble).
[0060] An aqueous liquid may also be made by dissolving the
treatment agent in a non-organic solvent and adding a relatively
concentrated aqueous detergent liquid, for example undiluted Fairy
Liquid. This provides a detergent liquid that has a number of
advantages: the soapy liquid is easily made, the liquid is easily
applied to the articles of the present alloy with a damp
sponge/cotton wool/cloth etc, the liquid and lather enables the
stearyl mercaptan or other treatment agent to get into those
awkward areas on an article where a cloth may not be able to reach,
rinsing and drying of articles are made easy as water droplets are
repelled from the surface of the polished silver, the process can
be easily used in a manufacturing environment before articles are
sent to retailers and can also be easily used in a retail or
domestic environment.
[0061] The articles may alternatively simply be polished with a
polish containing 1-5 wt % of the organo-sulphur compound e.g.
stearyl mercaptan together surfactants and a cleaning agent e.g.
diatomaceous earth in a solvent. As a further alternative, they may
be simply polished with a cloth impregnated with the sogano-sulphur
compound e.g. stearyl mercaptan. The advantages of a cleaning cloth
are that it is easily manufactured, can be easily used in a retail
or domestic environment and is good for general upkeep of articles
of the present alloy (if required).
[0062] The invention will now be further described, by way of
illustration only, with reference to the following examples.
Throughout the examples, the term "enhanced tarnish resistance" of
samples treated with stearyl mercaptan refers to the comparison
with samples of Argentium Silver which have not had any treatment
except for polishing and cleaning in EnSolv 765.
EXAMPLE 1
Production and Properties of Continuously Cast Alloy
[0063] A ternary silver-copper-germanium alloy (Ag=94.5 wt %,
Ge=1.2 wt %, Cu=4.1 wt %, B=0.0008 wt % (8 ppm) is prepared by
melting silver, copper and germanium together at 1050.degree. C.
under an atmosphere of nitrogen, and adding the boron as a
copper-boron master alloy at the last possible moment. The molten
mixture is then continuously cast into strip of width 50 mm and of
thickness 10 mm, after which an impurity-rich surface layer of at
least 0.1 mm is scalped off the surface of the cast strip by a
metal planer. The cast strip is then cold rolled to 5 mm thickness,
annealed at 620-650.degree. C. for 30-45 minutes in a slightly wet
protective gas atmosphere containing 0.1-0.5% oxygen and having a
dew point of 20-40.degree. C., these conditions being selected to
promote formation of GeO.sub.2 without oxidizing copper to copper
oxides, then subjected to a second rolling to a thickness of 2.5 mm
and a second annealing, after which the "as cast" grain structure
has been substantially removed and the sheet can be formed by a
silversmith without exhibiting objectionable orange-peel
effects.
[0064] The rolled annealed sheet has a measured hardness of 64 HV
which is comparable to values for Sterling silver. A sample of the
rolled sheet can be annealed by heating to about 600.degree. C.
followed by quenching, and the procedure can be repeated six times,
after which the sample is in good condition apart from slight edge
cracking and exhibits no fire-stain. This behaviour differs from
that of normal Sterling silver which tends to crack if quenched
from red heat, and also from that of 925 Argentium whose
low-melting (554.degree. C.) ternary silver-copper-germanium
eutectic would be liquid at the quenching temperature and would
cause the alloy to shatter when quenched.
EXAMPLE 2
Production and Properties of Investment Cast Strip
[0065] The molten alloy of Example 1 is formed into strip by
investment casting. The resulting strip is substantially free of
"hot short" defects and brittleness, and has a hardness of 63.5
HV.
EXAMPLE 3
Solvent Dip Cleaning
Solvent Degreased Samples
[0066] Solutions are made up containing stearyl mercaptan (0.1, 0.5
and 1.0 gram) in EnSolv 765 (100 ml). Samples of the rolled
annealed ternary alloy sheet of Example 1 which have been polished
and ultrasonically degreased in EnSolv 765 for 2 minutes are each
immersed in one of the stearyl mercaptan solutions for periods of 2
minutes, 5 minutes and 15 minutes. The samples are then buffed with
clean cotton wool. [0067] In order to evaluate tarnish resistance,
the alloy samples are supported on a glass slide in a fume cupboard
about 25 mm above the surface of 20% ammonium polysulphide solution
so as to be exposed to the hydrogen sulphide that arises from that
solution. All of the samples demonstrate good tarnish resistance
during a one-hour test, with very slight yellowing after 45 minutes
exposure to the hydrogen sulphide. The light film on the samples is
easily removed with a cleaning cloth impregnated with stearyl
mercaptan.
[0068] By way of comparison, a standard Sterling silver sample
starts to discolour as soon as it is subjected to the above test
and after one hour had forms a heavy black tarnish which can not be
removed with a cleaning cloth impregnated with stearyl
mercaptan.
EXAMPLE 4
Effect of Post-Treatment Solvent Cleaning
[0069] Example 3 is repeated for the ternary alloy samples except
that instead of buffing with cotton wool after the mercaptan
treatment, the samples are ultrasonically degreased in EnSolv 765
for 2 minutes. The samples are then tarnish tested as described in
Example 3 and all show enhanced tarnish resistance. The ability of
the protective effect of the stearyl mercaptan treatment to survive
ultrasonic cleaning in EnSolv suggests that the tarnish resistance
is achieved by a surface reaction involving the stearyl mercaptan
and possibly the germanium in the present alloy, and not by
formation of a grease or oil layer on the surface of the present
alloy.
EXAMPLE 5
Aqueous Dip Application
Solvent Degreased Samples
[0070] An anti-tarnish treatment solution is prepared using the
following ingredients:
TABLE-US-00005 Stearyl mercaptan 1 g EnSolv 765 5 ml Detergent
(Fairy Liquid) 40 ml De-ionised water 100 ml
[0071] The Stearyl Mercaptan is dissolved into the EnSolv 765 after
which the resulting solution is mixed with detergent (Fairy Liquid)
and diluted with water to provide an aqueous dip. Samples of the
ternary alloy of Example 1 are polished and ultrasonically
degreased in EnSolv 765 for 2 minutes, immersed into the above
aqueous dip for 2 minutes at ambient temperatures and then rinsed
under running tap water It may be noted the water is immediately
repelled from the polished surface, which leaves the samples dry.
Samples may be tarnish tested as described in Example 3 and all
show enhanced tarnish resistance.
EXAMPLE 6
[0072] Direct "Sponge" Application--Neat Detergent Solutions
(solvent degreased/aqueous degreased samples)
[0073] The following solutions are prepared: [0074] 1 gram stearyl
mercaptan [0075] 5 ml EnSolv 765 [0076] 40 ml detergent (Fairy
Liquid) (Preferred quantities) [0077] 1 gram stearyl mercaptan
[0078] 5 ml EnSolv 765 [0079] 40 ml soap (liquid hand soap)
[0080] The stearyl mercaptan is initially dissolved into the
EnSolv. The detergent is then mixed into the solutions. Samples of
the rolled annealed alloy of Example 1 are polished and
ultrasonically degreased in EnSolv 765 for 2 minutes. The stearyl
mercaptan/EnSolv/detergent solutions are then directly applied to
the surface of the samples using damp cotton wool and massaged into
lather. The samples are then rinsed under running tap water. In
each case, it is noted that water is repelled off of the polished
surface, leaving the samples dry. Samples are tarnish tested as in
Example 3 by being exposed to neat ammonium polysulphide solution
over a period of 1 hour. They all show enhanced tarnish resistance.
The above direct "sponging" method for applying the Stearyl
Mercaptan is tested on ternary alloy strip samples degreased in a
2% Fairy Liquid aqueous solution. Enhanced tarnish resistance is
again achieved.
* * * * *
References