Method to Produce Golden Bronze by Diffusion of Tin Into Copper Under Controlled Conditions

FIELD OF THE INVENTION

The present invention relates to the production of bronze objects having a golden appearance. More particularly, the present invention provides a multi-layer plating method to produce a golden bronze alloy layer on substrates, comprising non-cyanide plating of successive metals.

BACKGROUND OF THE INVENTION

Bronze is commonly defined as an alloy of copper and tin. However, other metals can be used, defining different bronze alloy variations such as commercial bronze (copper, zinc), architectural bronze (copper, zinc, lead) or aluminum bronze (copper, aluminum, nickel). The color of the bronze depends on the exact composition of the different metals used in the fabrication of the alloy. For instance, a copper-rich bronze alloy may have a reddish appearance whereas a tin-rich bronze alloy may have a silvery-white appearance. The golden appearance of bronze is then the result of a specific metallic composition.

Bronze can be obtained as a solid alloy by pyrometallurgy or as a plated material. As a plated material, bronze made of copper and tin is traditionally obtained by using cyanide plating baths. Bronze can be deposited directly as an alloy if cyanide chemicals are used. The product is thus formed by co-depositing copper and tin as plating takes place. More particularly, cyanide plating solutions are used during the electroplating of coinage blanks to obtain a golden bronze alloy layer. In a first example, the American U.S. Pat. No. 4,579,761 (Ruscoe at al.) provides a method of making aureate colored coins, medallions and tokens and products so made. The product is electroplated with alkaline cyanide copper-tin plating bath and then introduced in an annealing furnace at a constant temperature. After further cleaning treatment, Ruscoe et al. obtain a product coated with a shiny gold colored bronze.

Almost all commercial bronze plating operations use cyanide based plating solutions because it is relatively simple, well known and thus used by every company in the art, to obtain a gold like color metallic finish. However, cyanide-based plating solutions are very toxic. This toxicity is long lasting and poses extremely high health and safety risks to humans, live animals and fish far downstream from the source of spills and leaks in case of accidents or negligence in handling the plating solutions. Disposal of the waste is expensive and dangerous since the chemicals used to destroy cyanides are also toxic by themselves. Minute amounts of cyanides, in the parts per million range can cause severe damages and can be lethal in some cases to people and animals.

Non-cyanide bronze by the plating process can be obtained by proprietary commercial formulations but the results are usually poor because the plated products come out in a reddish color, resembling very much copper, rather than goldish as one would expect when bronze is desired. The non-cyanide plating solutions tend to be unstable, expensive and difficult to control for consistent results and color. This is the reason why non-cyanide bronze plating is not popular and not used, particularly, when plating is done for large quantities of industrial products, such as coinage blanks. In order to reduce the cost of coinage, pure metals such as nickel, copper or aluminum, and solid alloys such as cupronickel, cartridge brass or aluminum bronze are being replaced gradually with coins made of a cheaper material such as steel for the core, plated over with nickel, copper and bronze in a single layer, double layer or triple layer as outer layers covering the steel core. The steel for the core is sometimes replaced with zinc, or copper, or a low cost alloy such as cartridge brass as variations of the process. The American U.S. Pat. No. 5,151,167 and U.S. Pat. No. 5,139,886 provide coins coated with nickel, copper and then nickel and a process for making such coins with the use of non-cyanide plating solutions. The resulting coins have a regular surface exempted of surface pinholes which is normally an inherent problem of successive metals electroplating followed by annealing diffusion. The use of non-cyanide plating solution has then been successful in the successive coating of nickel, copper and nickel. Brass is also made by plating copper, then plating zinc on top of copper with non-cyanide plating solutions. The successive deposits of copper and zinc followed by diffusion of zinc into copper at controlled high heat and temperature to obtain a brass alloy. This non-cyanide brass alloy is commercially done at the Royal Canadian Mint. However, unlike zinc in brass diffusion, tin does not easily diffuse into the copper matrix because of its low melting point. Thus, by following the same approach with the combination of copper and tin, golden bronze cannot be produced.

A general method for plating various alloys without the use of cyanide solutions is disclosed in the US patent application published under the number US 20060286400 (McDaniel et al.). McDaniel discloses a method which includes the steps of electroplating a layer of a first metal onto a substrate, electroplating a second layer of a second metal onto the first electroplated layer, and heating the combination of the substrate to produce an alloy finish including a bronze alloy. The teaching of McDaniel is very general in nature and vague, with no specific data and samples to show how it is done. It does not allow the production of commercially suitable golden bronze. Without being specific and precise, the teaching of that reference is inapplicable and unworkable for a person skilled in the art searching a solution for the production of non-cyanide golden bronze.

There is thus a need for a technology to produce golden bronze that overcomes at least some of the drawbacks of what is known in the field.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method to produce bronze which satisfies the above-mentioned need. More particularly, the present invention provides a multiply-electroplating method to produce golden bronze by diffusion of tin into copper under controlled conditions.

It should be understood that golden bronze includes any bronze having a yellow gold color resembling gold, in other words a golden tone or gold appearance.

In one aspect of the present invention, there is provided a multilayer plating method for producing a golden bronze comprising the following steps. First, the methods comprises providing a copper plated substrate comprising a core plated with at least one copper layer having a copper layer thickness. Then, the method comprises plating the copper plated substrate with a tin layer having a tin layer thickness representing 3.5 to 12% of the copper layer thickness. A multiple-layer substrate is thereby obtained. The method further comprises annealing the multiple-layer substrate at a gradually increasing annealing temperature. This gradual increase of temperature allows a complete diffusion of the tin layer into the at least one copper layer for producing an inter-diffused outer bronze layer on the multiple-layer substrate, the inter-diffused outer bronze layer having a golden appearance. Additionally, the method comprises burnishing the inter-diffused outer bronze layer to remove undesired oxide formations.

A person skilled in the art would understand that, cleaning/rinsing and/or drying steps may be inherent to the method and performed between each of the above-mentioned step of the method.

The progressive annealing temperature rise advantageously enables the tin layer to gradually diffuse into the copper layer to produce a dull yellow bronze layer and to prevent the tin from vaporising by sublimation or from forming puddles on the surface of the plated substrate. The burnishing enables to reveal a bright yellow gold colored bronze layer.

According to a preferred embodiment, the annealing step is performed in an annealing furnace comprising a plurality of heating zones to ensure the gradual increase of the annealing temperature from 425° C. to 815° C. The gradual increase of the annealing temperature is in accordance with an annealing residence time inside the furnace. More preferably, the last heating zone of the furnace has an annealing temperature ranging from 600° C. to 815° C. The annealing furnace may comprise at least five heating zones, the first heating zone ranging from 425° C. to 500° C., the second heating zone ranging from 500° C. to 650° C., the third heating zone ranging from 600° C. to 700° C., the fourth heating zone ranging from 700° C. to 775° C. and the fifth heating zone ranging from 775° C. to 815° C.

The annealing residence time in the furnace is chosen in function of the loading of the substrate which affects the heat transfer rate and thus, the formation of bronze alloys by tin diffusion.

According to another preferred embodiment, the method further may comprise etching the copper plated substrate with an acidic solution. The adhesion of the tin layer on the copper layer is optimized by this etching.

According to another preferred embodiment, the substrate may be a coinage blank. Furthermore, the plating of the substrate may be an electroplating using electroplating solutions comprising acidic, cyanide, non-cyanide, neutral or slightly basic electroplating solution. More preferably, the electroplating may be done by using non-cyanide electroplating solutions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the method steps for the formation of a bronze alloy layer on coinage blanks according to a preferred embodiment of the present invention.

FIG. 2 is the binary phase diagram of the Cu—Sn alloy.

FIG. 3 is a photography of a coinage blank with tin puddles.

FIG. 4 is a photography of a coinage blank with a gold-like color bronze surface produced by a method according to an embodiment of the present invention.

FIG. 5 is a schematic sectional view of three configurations of tin diffusion into copper.

FIG. 6 is a table representing different steps of tin diffusion into copper in corresponding heating zones of an annealing furnace according to an embodiment of the present invention.

FIG. 7 is a cross sectional view of a coinage blank plated with 1 μm of tin, annealed until 650° C. during 45 minutes according to an embodiment of the present invention.

FIG. 8 is a schematic view of a furnace suitable to perform the method according to an embodiment of the present invention.

While the invention will be described in conjunction with example embodiments, it will be understood that it is not intended to limit the scope of the invention to these embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included as defined by the appended claims.

DETAILED DESCRIPTION

The present invention provides a multilayer plating method to produce bronze with a golden appearance by using electroplating of copper and tin, followed by diffusion of tin into copper under controlled conditions. For the purpose of better describing the invention, the further examples will be based on the production of a bronze layer on coinage blanks. However, the present invention is not reduced to the only use of coinage blanks but concerns generally all metallic substrates that can be electroplated. The substrate may thus have a core comprising steel, zinc, copper or a low cost alloy such as cartridge brass.

Bronze is an alloy of copper and tin. A layer of bronze can be plated on substrates by electroplating. To perform the electroplating of a metal, an electrolytic cell is used. The electrolytic cell comprises electrodes composed of a cathode and an anode. The substrate to be plated is the cathode and the anode is made up with the metal to be plated on the substrate. The electrodes are immersed in an electroplating solution containing ions, cations and anions, and preferably corresponding cations of the metal to be plated. For instance, if copper is electroplated, the electroplating solution may be copper sulphate (CuSO₄) based, which contains Cu²⁺ cations and SO₄²⁻anions in solution. The cyanide electroplating solutions are well known and efficient. However, a cyanide electroplating solution contains cyanide anions CN⁻which are very toxic. The electrosolution has to conduct the current supplied by a power supply connected to the electrodes. The metal of the anode is oxidized and releases corresponding metallic cations which interact with the anions of the electroplating solution. These cations are then reduced at the cathode and form the desired metallic deposit thereon.

The present invention provides a multilayer plating method to produce a bronze with a golden appearance. The substrate is not limited to a mere copper plated substrate but generally includes any substrate plated with at least one copper layer. Preferably, the substrate may be plated with three layers comprising a nickel layer, a copper layer and a tin layer. The substrate may also be plated with four layers comprising a nickel layer, a copper layer, a tin layer and a zinc layer. Therefore, the present method may comprise a step of plating a copper layer or another metallic layer on the substrate.

In the case of a coinage blank with a steel core, disregarding the economical factor and the method to deposit copper, the method is valid when, besides acid copper and cyanide copper solutions, non-acidic, non-cyanide, neutral or slightly basic copper solution is used to deposit the copper needed for the bronze formation by the diffusion process, directly on the steel core. It should be noted that the present method enables to advantageously avoid the use of cyanide electroplating solutions which are commonly encountered despite their toxicity.

The metallic layers have to be successively plated; this is the reason why the method is defined as a multilayer plating method. To form bronze, at least copper and tin need to be present. FIG. 1 shows the steps of the method to successively deposit nickel, copper and tin and ultimately produce a bronze alloy layer on coinage blanks formed from metal coils. Steps 2 to 6 are performed to obtain cleaned blanks before proceeding to the electroplating of nickel in step 7. Copper and tin are respectively plated in step 9 and 12. After each plating step, blanks needs to be rinsed (steps 8, 10, 13). The nickel layer and the copper layer have to be etched (step 8 and 11) to promote and contribute to the adhesion of copper on nickel, and the adhesion of tin on copper during the electroplating of step 9 and 12. The multiple-layer blanks are then submitted to a heat treatment under an annealing temperature allowing diffusion of tin into copper so as to form an inter-diffused outer bronze layer on the blanks (step 15). The blanks are then burnished in step 16 and dried in step 17. The plated bronze obtained by the diffusion of step 15, after cleaning and burnishing, has a nice bright yellow gold color or a dull yellow. The succession of the method steps is insufficient to obtain the desired golden appearance of bronze. The present invention provides controlled conditions used in the above mentioned steps to force the copper-tin alloy equilibrium to take place.

The heat treatment is preferably performed in an annealing furnace. It should be understood that the annealing furnace includes any furnace allowing diffusion between metallic layers upon heat treatment.

The formation of a yellow gold-like color bronze by diffusion of tin into copper depends heavily on three factors. First, the thickness of the plated tin layer relatively to the thickness of the plated copper layer is crucial. Secondly, a particular care has to be given to the relationship between the annealing residence time in the furnace and the annealing temperature. An appropriate annealing residence time allows a complete diffusion of tin to take place under gradual increase of the annealing temperature (as in step 15), thereby forming an inter-diffused outer bronze layer on the multiple-layer substrate. Preferably, the annealing residence time ranges from 10 to 90 minutes and more preferably, from 20 to 30 minutes. It should be understood that the annealing residence time may be set or controlled with a precision of more or less 5 minutes.

Thirdly, the annealing atmosphere composition has to be controlled because it influences the transformation of excess tin to tin oxide or a combination of tin and tin oxide, which in turn makes the burnishing (as in step 16) of the final product, easy or difficult, effective or ineffective. The annealing furnace has therefore preferably a controlled atmosphere composition comprising atmosphere, nitrogen, or a mixture of nitrogen and hydrogen. This kind of composition results in a reducing (non-oxidizing) atmosphere.

Referring to FIG. 2, the phase diagram of bronze alloy shows that bronze can exist in an infinite composition combination depending on the temperature and the proportion of copper and tin. However, in order to obtain the gold-like color for bronze, the percentage of tin in the bronze alloy should be between 8% and 20%. There is a shift from yellow gold tone as the concentration of tin increases: the color shifts towards the light “whitish” metallic color of tin when the tin concentration is above 20%. The color is pinkish gold when the tin content in the alloy is 8% or less. To find these proportions in the bronze alloy after diffusion, the relative thickness of the copper and tin deposits must be controlled. Theoretically, any thickness of copper can be used. For economical reason, the copper plating thickness should be such that it is compatible with the tin layer to form a complete alloy between the two. The important consideration is that the copper layer must be thick enough to absorb all the available tin as the diffusion takes place. There is an optimum ratio between the copper and tin layer thicknesses. If there the thickness of the copper layer is insufficient or the thickness of the tin layer is in excess, the bronze is formed as an inner inter-diffused layer but the excess tin will form puddles on the outer surface of the coinage blank.

Referring to FIGS. 2 and 3, the right ordinate of the binary phase diagram shows that alloys with copper can be formed as the temperature is raised. However, when the annealing temperature increase above 231.9° C., which is the melting point of tin, there is a strong possibility that tin becomes liquid too fast and is not absorbed fast enough to alloy with copper. Tin then coalesces and forms droplets which can falls off the coinage blank surface or/and upon cooling, forms puddles. Instead of an outer inter-diffused layer of bronze, there are puddles of tin on the copper layer surface as the temperature is raised too fast. For example, if a multi-layer plated coinage blank is plated with 5 microns of nickel, 20 microns of copper and 10 microns of tin, it will look grey (tin finish look) at the end of the plating process. Indeed, as can be seen in FIG. 2, upon heating in an annealing furnace up to 600° C., and upon cooling, the blank may appear slightly yellow, but with a strong indication of tin grey still present on the surface; almost all tin has diffused into the copper and the composition on the surface would be about 86% copper and 14% tin, enough to give the yellow gold color (Point B in FIG. 2). However, the excess tin, which has not diffused, congregates on the surface in the form of tin rich alloy puddles (point D in FIG. 2). FIG. 3 shows the plated coinage blank with excess tin having formed tin-rich alloy large puddles on the surface of the blank, relating to the above-mentioned alloy behaviour at points B and D of FIG. 2.

Referring to FIG. 2, on the other hand, if the temperature is raised too slowly when the thickness of the tin layer is insufficient (too thin a layer of tin deposited on too thick a layer of copper), all the tin is absorbed to form an alloy whose tin content is insufficient to form a yellow gold color but a reddish bronze. This result has been obtained for a coinage blank plated with multiple layers including 5 microns of nickel, 20 microns of copper and 0.5 microns of tin, heated up to a temperature of 800° C. and then cooled down. As shown on the binary phase diagram in FIG. 2, the tin layer is completely diffuse into the copper layer to form an inter-diffused bronze alloy layer. However, there is not enough tin in the bronze alloy and the result is a blank surface plated with reddish bronze comprising about 2.5% of tin (Point E in FIG. 2).

Finally, referring to FIGS. 2 and 4, if a blank is plated with multiple layers including 5 microns of nickel, 20 microns of copper and 2 microns of tin, it will look grey (tin finish look) at the end of the plating process. Upon heating gradually in an annealing furnace up to 800° C., and upon cooling, the blank appears with a nice yellow gold like bronze color. The composition of the outer alloy may be about 88% copper and 12% tin (Point C in FIG. 2). FIG. 4 shows the blank appearing with a nice yellow gold like bronze color.

The present invention therefore contributes to a better control of the composition of the bronze alloy through the relative thickness of plated copper and tin.

The control of the relative thickness of the plated copper and tin layers has to be done in parallel with the control of the annealing residence time in the annealing furnace (step 15 in FIG. 1). FIG. 5 schematically shows substrate with a steel core and plated with nickel, copper and finally tin in different configurations A, B and C corresponding to different annealing residence time in the annealing furnace. When the residence time is appropriate and when there is enough copper and tin in the right proportion to give an alloy in the range of 8 to 14% tin, the inter-diffused outer bronze layer has a good golden color. A residual layer of copper may be present when the copper has not completely reacted with the tin (A in FIG. 5). When the residence time is relatively short and when there is not enough tin compared to copper, the alloy formed on the surface is slightly less yellow and some residual copper may not have been alloyed yet with the tin (C in FIG. 5). When the annealing residence time is correct, when the copper and the tin are in the right ratio and when the annealing temperature is correctly set or controlled, a single outer layer of bronze is obtained with varying alloying ratios of copper and tin (B in FIG. 5).

The combination of controlled operating conditions comprising the relative layer thicknesses and the annealing residence time is crucial but insufficient to guaranty a gold-like color bronze for a given loading of substrates such as coin blanks. Indeed, a right balance has to be achieved between annealing temperature, annealing residence time (related to the diffusion rate) and right combination of copper and tin layer thicknesses, to form a bronze alloy with the proper yellow gold color without creating pockets of tin rich puddles on the substrate surface.

More particularly, the substrate has to be plated with copper and tin in appropriate thicknesses, and then the multiple-plated substrate is submitted to an annealing treatment (heat treatment) during a sufficient annealing residence time. The annealing furnace used for the annealing treatment comprises a plurality of heating zones which are set with an increasing temperature gradient, in order to succeed in facilitating the diffusion of tin into copper to produce a gold like colored bronze. In this furnace, different controls are used to regulate the amount of energy available for heating, which results in very well defined heating zones. Instead of having a uniform one-temperature furnace, a belt furnace long enough to allow at least five heating zones is preferred. More preferably, the five heating zones have respectively a temperature of 425° C., 550° C., 675° C., 725° C., 800° C. According to the shade of gold-like appearance desired, the temperature of the last two heating zones may be the same.

In one embodiment, the annealing temperature may be set or controlled to increase incrementally from one heating zone to another heating zone. The annealing temperature therefore gradually increases over a short transition from the exit of one heating zone to the entrance of the following heating zone. For instance, during this transition, the plated blanks entering the fifth heating zone will first stay at the annealing temperature of the fourth and then gradually reach the annealing temperature of the fifth zone after a short period.

The annealing furnace may comprise a belt conveyor or a rotational screw conveyor and may also comprise a forced convection system in order to ensure even heat conduction and distribution. The annealing furnace may further comprise a belt conveyor or a screw conveyor with instant abrupt quenching at the end to control and stop the diffusion at the desired gold color shade. The belt conveyor or rotational screw conveyor may also be set or controlled at a conveying speed which may vary from a heating zone to another heating zone, or which may be set to a constant conveying speed. Preferably, the conveyor is set or controlled at a constant conveying speed.

Referring to FIG. 8, the annealing furnace 2 is provided with five temperature sensors 4 for each of the five heating zones. The annealing temperature of the heating zones may be controlled according to these sensors 4. An evacuation duct 6 is connected to each zone of the furnace 2 and directs an exhaust gas upward to a filtration unit for removing gaseous metallic impurities from the annealing atmosphere. The exhaust gas duct 6 reduces the condensation of metallic vapours in the furnace, thereby limiting time-consuming furnace shut down and hard work cleaning during operation. For instance, when using coinage blanks, the exhaust gas may contain zinc which has evaporated. The zinc vapours may further be condensed in an external condenser and easily removed with a metallic filter. The purified inert gas containing recycled hydrogen and/or nitrogen is returned to each heating zones of the furnace 2 through a recycling duct 8.

Referring to FIG. 6, as the plated substrate travels on the belt conveyor through the furnace, the pure tin gradually diffuses into the copper layer to form a inter-diffused layer of tin-copper alloy, which in turn can sustain higher temperature which allows more diffusion of tin into the copper-rich alloy. The tin can build up in the alloy to form the yellow gold color of bronze alloys. At the end, after the multiple-layer plated substrate of nickel, copper and tin has gone through the at least five zones of the furnace (section 5), the tin layer has completely disappeared and diffused into the copper layer so as to form a new inter-diffused outer bronze layer. The copper layer may or may not disappear completely depending on the inter-diffused layer thickness of the copper-tin alloy. A residual copper layer may subsist as seen on FIG. 5A and section 5 of FIG. 6. The inter-diffused outer bronze layer may also replace totally the tin and copper layers.

The present invention provides a method using an annealing furnace comprising a plurality of heating zones where three key parameters are set or controlled to allow the formation of golden bronze: the relative tin plated thickness to the copper plated thickness, the annealing temperature and the annealing residence time inside the furnace.

In the annealing furnace, the temperature is gradually raised from 425° C. to 815° C. along the different heating zones of the annealing furnace, thereby providing effective diffusion. A bronze alloy, with a higher melting point than the tin melting point, is gradually formed by diffusing tin into copper and preventing it from vaporising by sublimation. The tin layer thickness has to be in a proper ratio of 3.5% to 12% relatively to the copper layer thickness to obtain a final gold-like bronze according to the annealing residence time. For example, if the thickness of the copper layer is 14 microns, the tin layer to be deposited should have a thickness ranging from 0.49 to 1.68 microns. The annealing residence time in the furnace is a function of the tin-copper loading which affect the heat transfer rate and thus, the formation of bronze alloys by tin diffusion. To obtain a gold-like colored bronze, the annealing temperature should be ranging from 650° C. to 850° C. in the last heating zone of the furnace. The composition of the atmosphere in the diffusion annealing furnace may comprise atmosphere composition, nitrogen, or a mixture of nitrogen and hydrogen.

The method further comprises a step of burnishing the bronze formed by diffusion to remove oxide that may form during the annealing step. The presence of residual tin oxide or other metallic impurities oxides can cause problems during further minting of coin blanks for example. The burnishing step comprises polishing the outer bronze layer so as to reveal the bright yellow gold color of the bronze.

The importance of the above mentioned key parameters of the present method is demonstrated in the following example.

Example

A series of experiments have been performed to optimize key variables such as the relative thickness of copper and tin, the annealing temperature of the furnace, the annealing residence time and the composition of the atmosphere inside the furnace.

Experimental Conditions:
Blanks

The blanks that were used in the experiments were Canada loon dollars. The blanks had an edge thickness of 1.39 mm and a diameter of 26.1 mm. The weight of each blank was approximately 6.31 grams. The blanks have a steel core and are plated the same way with the same thickness of nickel and the same thickness of copper, but with different thicknesses of tin. The thickness of nickel and copper layers are approximately 8 μm and 14 μm respectively at the center of the blanks. Barrel plating was used for the experiments.

Tin Plating
Pre-Treatment and Plating Solutions

Three steps were used in order to plate tin on the blanks. The first pre-treatment step was an alkaline electroclean called Tec-1000. The bath (also referred herein as electroplating solution) composition was:

- Tec-1000: 75 grams/litre

The neutral anode used for the electroclean was made in stainless steel. The barrel was submerged in the solution and a current was passed through the barrel and stainless steel plates. The conditions of the cleaning bath were:

- Temperature: 65 to 75° C.
- Current density: 0.8 A/dm₂
- Time: 5 minutes

The next step was the acid pickling (also referred herein as etching) that activates the copper surface of the blanks. The solution used for the acid pickling is a solution of 5% sulphuric acid. The conditions are listed below.

- Temperature: 18 to 22° C.
- Time: 3 minutes

The final step is the tin plating. The plating solution that was used to plate tin on the blanks was Stannolume 160. This is a highly conductive acidic tin plating solution able to plate over a large range of current densities. The solution composition is:

- Stannous Sulphate: 30.0 grams/litre
- Sulphuric Acid: 10.0% by volume
- Stannolume 160 Carrier: 2.0% by volume
- Stannolume 160 Additive: 0.5% by volume

Pure tin anodes were used in the plating bath. The tin bath operating conditions are listed below.

- Temperature: 18 to 22° C.
- Current density: 0.3 A/dm₂

Plating the Blanks

Different thicknesses of tin were deposited on the blanks. The tin thickness that was deposited on the blanks ranged from 0.5 μm to 10 μm of tin. Either 125 or 250 blanks were used for each trial. The current density used to plate the blanks was 0.3 A/dm². The plating time was calculated based on the theoretical thickness, the number of blanks and the current density. See Appendix.

Annealing the Blanks

After plating, the blanks were annealed in a belt conveyor annealing furnace with controlled heating zones, ranging from room temperature to progressively hotter diffusion temperatures to induce the diffusion of tin into the copper without prematurely melting the tin and causing the tin to congregate into a puddle and falling off the faces of the blanks. Several factors were controlled during annealing. The first factor is the annealing temperature which ranged between 300° C. to 800° C. The second is the annealing time which varied between 10 to 90 minutes. The last factor is the atmosphere in the furnace. The atmosphere of the furnace can be changed between air, nitrogen or a combination of nitrogen and hydrogen gas.

Burnishing

The final step is the burnishing of the blanks. The blanks were first burnished in a 90 liter-tumbler with an acidic solution. The acidic solution used was C24 or L243. Then the blanks were rinsed and burnished in a basic solution. The basic solution used was L300 or C45. The blanks were rinsed for one final time and were then dried.

Results

TABLE 1a

Overview of the variation of operation conditions and resulting observations

Tin Plating
Annealing
Annealing
Annealing

Thickness
Temperature
Time
Atmosphere
Observations

1 μm
Zone 1 300° C.
40 to 90
Nitrogen and
Little to no diffusion

2 μm
Zone 2 350° C.
minutes
Hydrogen
For 5 and 10 μm of tin; tin melted

3 μm
Zone 3 400° C.

and formed clusters around the

5 μm
Zone 4 450° C.

edges

10 μm
Zone 5 500° C.

For 1 to 3 μm of tin, blanks were

still white

1 μm
Zone 1 300° C.
40 to 90
Nitrogen and
1 to 3 μm of tin started to show a

2 μm
Zone 2 400° C.
minutes
Hydrogen
very faint bronze colour but blanks

3 μm
Zone 3 500° C.

were still very white

5 μm
Zone 4 550° C.

2 to 3 μm of tin had small pin size

10 μm
Zone 5 600° C.

puddles of excess tin

5 and 10 μm of tin still had large

clusters of melted tin on edges of

blanks

TABLE 2b

Overview of the variation of operation conditions and resulting observations

Tin Plating
Annealing
Annealing
Annealing

Thickness
Temperature
Time
Atmosphere
Observations

0.5 μm
Zone 1 300° C.
15 to 45
Air
Outer layer of the blanks was

1.0 μm
Zone 2 450° C.
minutes

oxidized

1.5 μm
Zone 3 600° C.

Uniform colour of bronze was

2.0 μm
Zone 4 700° C.

found for tin thicknesses between

2.5 μm
Zone 5 750° C.

0.5 μm to 1.5 μm

2.0 μm to 2.5 μm of tin still have

pin size puddles of excess tin but

are smaller than at the lower

temperatures

Higher temperatures and longer

time in furnace helped reduce the

presence of the excess tin on the

blanks but formed a ternary alloy

underneath the bronze alloy of Cu

Ni Sn

Temperatures at 650° C. gave a

good bronze like colour and

showed 3 distinct layers of nickel,

copper and bronze for tin

thickness of 0.5 μm and 1 μm

Bronze layer for higher

temperatures was pinker.

0.5 μm
Zone 1 300° C.
15 to 45
Nitrogen
Blanks were all very pink and

1.0 μm
Zone 2 400° C.
minutes

similar in colour

1.5 μm
Zone 3 600° C.

Cross-section of blanks showed

2.0 μm
Zone 4 700° C.

large amounts of a ternary alloy

2.5 μm
Zone 5 800° C.

between nickle, copper and tin.

This layer increased with thicker

amounts of plated tin.

Blanks with 2.5 of μm still have

pin size puddles of excess tin

TABLE 2

Tin plating weight and efficiency

Actual

Analytical Sn Thickness
Analytical Weight
Weight
Plating Efficiency

1.5 μm
2.94 g
1.80 g
64.7%

3.0 μm
5.87 g
3.80
61.2%

From all the experiments, with a copper layer thickness of 14 μm, the optimum tin layer thickness is found to be between 0.5 μm to 1.5 μm. This range of layer thickness is the most successful to form the bronze alloys with a range of colours. The tin layer thickness should represent 3.5% to 12% of the copper layer thickness. It is observed that only a small amount of tin can diffuse into the copper while any of the excess tin will melt and form a puddle of tin. Puddles of tin tend to be very hard to remove during burnishing. In the range of 0.5-1.5 μm of tin layer thickness, a consistent bronze alloy with a composition of about 10% of tin is obtained. Any thicker tin plated thickness (greater or equal to 2 μm) will result in large amounts of excess tin on the surface of the blanks.

These experiments also show that the gaseous atmosphere used for the annealing process is not very critical in order to diffuse tin into the copper to form bronze. When the blanks were annealed under natural atmosphere or nitrogen and hydrogen, there was no visible difference on the diffusion of the tin into the copper to form the bronze alloy. There was not a lot of oxidation on the blanks after the annealing unlike the known and commonly used brass process. For the tin thickness ranging from 0.5 μm to 1.5 μm, the blanks came out yellow and a little dull. No black oxidation layer is seen on the blanks. The dull yellow surface can be then burnished to a bright, shiny yellow colour using a mild acid and mild soap. Furthermore, if the annealing atmosphere does not have an effect on the formation of bronze alloy, it does have some effect on the ability to obtain a bright bronze finish or a dull bronze finish after burnishing. The blanks that were annealed in air gave the most successful results. The oxide layer on the surface of the blanks was more easily removed during burnishing and the resulting blanks were very bright. The nitrogen atmosphere also gave good results but was not as good as the blanks that were annealed in air. The blanks that were hardest to burnish were the blanks that were annealed in a nitrogen and hydrogen atmosphere. The blanks were still dull after the burnishing. Finally, the set of chemicals that best cleaned the surfaces of the blanks was the combination of C24 and C45. These chemicals cleaned the surface of the blanks most easily and gave a bright finish to the burnished blanks. The combination of L243 and L300 didn't clean the blanks as well. The blanks were duller after using equal burnishing times. The bronze formed by diffusion should therefore be burnished to remove oxide formed in annealing and to reveal the yellow gold color of the bronze.

The table 2 shows the results of the tin plating. After the tin was plated on the copper, the tin layer was light gray and very dull. The bent test revealed that an excellent adhesion was obtained. The tin plating is ranging from 0.5 μm to 10 μm. Two trials were used to calculate the efficiency of the tin plating. The calculation for the efficiency is done as follows.

In order to calculate the theoretical weight difference, the following formula must be used:

$Theoretical Weight Difference = \frac{tIM}{2 F}$

Where t is the time of the plating in seconds

I is the current used to plate the blanks in amps

M is the molar mass in

$\frac{g}{mol}$

F is Faraday's constant

The efficiency of the plating can also be calculated once the theoretical thickness is found. The efficiency can be calculated using the theoretical thickness and the following equation:

$Efficiency of plating = \frac{Actual Weight Difference}{Theoretical Weight Difference} * 100 %$

Two experiments were run using a theoretical thickness of 1.5 and 3 μm. The following data was collected:

t_{3 μm}=1960 s I_{3 μm}=4.89 Amp

- WeightDifference_{3 μm}=3.91 g
  
  t_{1.5 μm}=980 s I_{1.5 μm}=4.89 Amp
- WeightDifference_{1.5 μm}=1.80 g

Given that the literature values of tin are:

$M = 118.1 \frac{g}{mol}$

$F = 96485 \frac{Coulomb}{1 mol e^{-}}$

$Theo . Weight Dif . for 3 µm Tin = \frac{tIM}{2 F} = \frac{1960 s * 4.89 Amp}{\frac{1 mol Sn}{118.1 g} * \frac{2 mol e^{-}}{1 mol Sn} * 96485 \frac{Coulomb}{1 mol e^{-}} * \frac{1 Joule}{1 Coulomb}} = 5.87 g$

$Efficiency of plating for 3 µm Tin = \frac{3.80 g}{5.87 g} * 100 % = 64.7 %$

$Theo . Weight Dif . for 1.5 µm Tin = \frac{tIM}{2 F} = \frac{980 s * 4.89 Amp}{\frac{1 mol Sn}{118.1 g} * \frac{2 mol e^{-}}{1 mol Sn} * 96485 \frac{Coulomb}{1 mol e^{-}} * \frac{1 Joule}{1 Coulomb}} = 2.94 g$

$Efficiency of plating for 1.5 µm Tin = \frac{1.80 g}{2.94 g} * 100 % = 61.2 %$

$\begin{matrix} Average efficiency = \frac{64.7 % + 61.2 %}{2} \\ = 63.0 % \end{matrix}$

The experiments showed that the color as well as the thickness of the bronze alloy is influenced by three factors which are the plated tin layer thickness, the annealing temperature and the annealing residence time. As the tin layer thickness is increased, more tin is available to diffuse into the copper which results in a more yellow bronze alloy. For example, a tin layer thickness of 0.5 μm and 1.5 μm will result in different colours. The blanks with 0.5 μm will be pinker than the blanks with 1.5 μm. The most important factors for controlling the colour of the bronze alloy are the annealing temperature and annealing residence time. At lower temperatures, around 600° C., or at fast annealing residence time, tin will not have enough time to diffuse completely into the copper layer which will result in the tin to be concentrated near the surface of the blanks. This will give a thinner layer of the bronze alloy but yellower and higher in tin composition. FIG. 4 shows the golden successful result on a coinage blank plated with 1 μm of tin and annealed at 650° C. for 45 minutes, a golden bronze blank with a high tin composition and no excess amounts of tin is obtained. The cross section of this same blank is shown on FIG. 7, the multiply layer is easily observed. This golden bronze blanks are obtained with an annealing temperature ranging from 650° C. to 700° C. and with a blank loading of 600 kg per hour on a 900 mm wide belt furnace, the optimum annealing residence time may be between 20 to 30 minutes in the whole heating zone of the furnace with a constant conveying speed. When the annealing temperature in the last heating zone was increased up to 750-800° C. or the annealing time was increased, the tin had more time to diffuse through the copper layer. The bronze layer is thus thicker and has a lower composition of tin which resulted in a more pinkish bronze color.

Method to Produce Golden Bronze by Diffusion of Tin Into Copper Under Controlled Conditions

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