CA2883815C

CA2883815C - Plating solution and plating process for multi-layer cyanide-free plating copper-tin alloy coating, and coins made by the process

Info

Publication number: CA2883815C
Application number: CA2883815A
Authority: CA
Inventors: Bo Zhang; Wei Xu; Zhuoxin WANG; Min Xu; Jinhua Song; Gaojun ZHANG; Yi Lu; Bin Wang; Yazhe CAO
Original assignee: SHANGHAI MINT CO Ltd; China Banknote Printing and Minting Corp
Current assignee: SHANGHAI MINT CO Ltd; China Banknote Printing and Minting Corp
Priority date: 2012-09-06
Filing date: 2012-11-14
Publication date: 2018-09-04
Anticipated expiration: 2032-11-14
Also published as: CN103668359A; WO2014036785A1; CA2883815A1; CN103668359B

Abstract

The invention relates to the technical field of coinage, and particularly relates to electroplating liquid of multi-layer cyanide-free electroplated copper-tin alloy plating, an electroplating technology and a coin produced by the technology. The pyrophosphate electroplating solution of multi-layer cyanide-free electroplated copper-tin alloy plating provided by the invention comprises a cyanide-free brass tin major brightening agent consisting of solutes brightener A and brightener B, wherein the concentration of the brightener A in the major brightening agent is 1-10g/L, and the concentration of the brightener B in the major brightening agent is 0.05-0.5g/L. By adopting the pyrophosphate electroplating solution and the electroplating technology of multi-layer cyanide-free electroplated copper-tin alloy plating, a coin product of which the plating thickness is over 20 microns and the plating is uniform and dense can be obtained. After high-temperature thermal treatment, the coil plating is of a single-layer structure; the content of tin in the single-layer plating is 11-14% by weight; the plating appearance is in uniform golden yellow color without chromatic aberration, and the problem acknowledged by the electroplating section at present that a single-layer cyanide-free electroplated alloy plating is relatively thin is solved.

Description

Title PLATING SOLUTION AND PLATING PROCESS FOR MULTI-LAYER
CYANIDE-FREE PLATING COPPER-TIN ALLOY COATING, AND
COINS MADE BY THE PROCESS
Background of the Present Invention Field of Invention The present invention relates to the technology field of coinage, and particularly to electroplating liquid of multi-layer cyanide-free electroplated copper-tin alloy plating, an electroplating technology and a coin produced by the technology.
Description of Related Arts The history of cyanide plating may be traced back to 1831, and the practical electroplating technology begins from the patent of cyanide silver plating granted to Elkington in 1840.
Cyanide zinc plating has been practically applied during the First World War, and then, cyanide plating technology is widely used in the electroplating of many single metals such as zinc, copper, cadmium, silver and gold or alloy plating. However, cyanides are highly toxic substances, and the lethal dose is only 5 mg, as a result, the requirements for management of toxic cyanide and wastewater treatment of cyanide electroplating liquid are relative high.
In 1970s, research on cyanide-free zinc plating technology first got a breakthrough, and until now, cyanide-free copper plating, cyanide-free gold and silver plating, and cyanide-free copper alloy plating and other technologies have been developed one after another, and have been applied in certain industrial fields. At present, electroplating materials used in the international coinage industry mainly include plated copper, plated nickel, plated copper alloy, and the like, wherein the electroplating copper-tin alloy technology still adopts a cyanide electroplating or adopts a heat treatment method to form an alloy layer for respective single metal plating.
Copper-tin alloy electroplating is a conventional plating technology that substitutes for nickel plating, and can be used for barreling and rack plating. Cyanide system copper-tin alloy plating is a relative mature nickel substitute plating technology. Due to the requirements for environmental protection and human health, in recent years, development and researches on new process of cyanide-free copper-tin alloy electroplating have attracted extensive attention.
The current reported solution systems for cyanide-free copper-tin alloy electroplating mainly include pyrophosphate, pyrophosphate-stannate, citrate-stannate and HEDP and the like, wherein the pyrophosphate solution system has the great potential for substituting the cyanide solution system. For the cyanide-free brass-tin plating process at the present stage, the electroplating time is very long, and thus problems of fogged and loose plating will occur. As a result, this process is mostly used in decorative platings, but has fewer breakthroughs in functional plating. Therefore, the object of the present invention is to solve the existing problems as brass-tin plating is used in functional plating, i.e., continuously thickening of the plating, uniformity and compactness of the plating, stability of the plating solution, and the like.
Summary of the Present Invention In view of the disadvantages in the existing technology that the cyanide-free electroplated brass-tin cannot be plated thicker and the plating is uniform, the object of the present invention is to provide a pyrophosphate electroplating liquid of multi-layer cyanide-free electroplated copper-tin alloy plating, an electroplating technology for the multi-layer

2 cyanide-free electroplated copper-tin alloy plating, and a coin product fabricated by such electroplating technology. By using the pyrophosphate electroplating solution and the electroplating technology for multi-layer cyanide-free electroplated copper-tin alloy plating, a coin product with a uniform and compact plating having a thickness of up to 20 m can be obtained.
In an aspect, there is provided a pyrophosphate electroplating solution of multi-layer cyanide-free electroplated copper-tin alloy plating, comprising: a cyanide-free brass-tin brightening agent consisting of (1) 1 to 10 g/L of MirapolTM WT and (2) 0.05 to 0.5 g/L of 2-mercapto benzimidazole, wherein the concentration of the cyanide-free brass-tin brightening agent in the pyrophosphate electroplating solution is 3 to 20 ml/L.
In another aspect, there is provided an electroplating method for multi-layer cyanide-free electroplated copper-tin alloy plating, comprising: sequentially electroplating 2 to 4 plating layers of copper-tin alloy on a coin substrate; and subjecting the plating layers on the coin substrate to a high-temperature treatment, to form a coin with multi-layer cyanide-free electroplated copper-tin alloy platting; wherein the even layer(s) of plating and the surface layer are electroplated by adopting the pyrophosphate electroplating solution as disclosed herein.
According to the electroplating technology for multi-layer cyanide-free electroplated copper-tin alloy plating and the coin product fabricated by adopting the electroplating technology of the present invention, its billet adopts a low-carbon steel coinage blank as a substrate, on which a first layer, a second layer, a third layer and a surface layer are electroplated in sequence.
The electroplating technology is a multi-layer electroplating technology using a pyrophosphate solution system, and the entire electroplating technology features an identical main salt system, so that the risk of cross contamination of electroplating liquid among different platings can be avoided, and the electroplating liquid can be rinsed with water after each layer is plated, thereby omitting the activation process. The problem that the monolayer cyanide-free electroplated copper-tin alloy plating is thinner than the cyanide electroplated copper-tin alloy plating is solved by adopting a multilayer electroplating. Also, the electroplating liquid for the plating of

3 each layer in the electroplating technology is a pyrophosphate solution system, which is a cyanide-free environment friendly system, thereby greatly reducing the management cost of the high toxic cyanide, improving the plating environment, decreasing the pressure of wastewater on environmental influence, and significantly improving the fabrication level of the coin plating cladding material as well.
In accordance with the present invention, the provided electroplating technology for multi-layer cyanide-free electroplated copper-tin alloy plating and the coin structure fabricated by the electroplating technology differ from the conventional cyanide plated copper-tin alloy coins and the current fabrication process and coin structure of the other electroplated copper-tin alloy coins at home and abroad. The conventional process for cyanide electroplated copper-tin alloy coin is directly electroplating a copper-tin alloy on steel-cored billet, and the coin has a monolayer structure. Other copper-tin alloy electroplating technologies provided at home and abroad include, for example, firstly plating a base layer on an discus, then performing single metal alternating electroplating, and performing heat treatment diffusion after the plating is completed, so as to obtain an alloy layer of a certain thickness.
In order to achieve the above object and other objects, the present invention adopts the following technical solutions:
A pyrophosphate electroplating solution of multi-layer cyanide-free electroplated copper-tin alloy plating comprises a cyanide-free brass-tin major brightening agent, and the solute of the cyanide-free brass-tin major brightening agent consists of a brightening agent A and a brightening agent B; wherein, the concentration of the brightening agent A
in the cyanide-free brass-tin major brightening agent is 1 to 10 g/L; and the concentration of the brightening agent B
in the cyanide-free brass-tin major brightening agent is 0.05 to 0.5 g/L.
Preferably, the concentration of the cyanide-free brass-tin major brightening agent in the pyrophosphate electroplating solution is 3 to 20 ml/L.
Preferably, the solute of the cyanide-free brass-tin major brightening agent consists of

4 the brightening agent A and the brightening agent B, and the solvent thereof is a mixture of water and organic solvent; wherein the optimum ratio of water and the organic solvent is such a value that the brightening agent A and the brightening agent B can be just dissolved. In the mixture of water and the organic solvent, the organic solvent is selected from a mixture of an organic solvent and water capable of dissolving the brightening agent A and the brightening agent B.
Preferably, the brightening agent A is the brightening agent MirapolTM WT
manufactured by Rhodia Inc., France; and the brightening agent B is 2-mercapto benzimidazole.
4a The addition of the Mirapol WT may significantly shorten the electroplating time, improve the uniformity and throwing power of plating, and improve the corrosion resistance of plating as well; also, the plating may have good salt and fog resistance and flexibility. In the present invention, both the brightening agent A and the brightening agent B
are added to the electroplating liquid, and thus by virtue of the synergism thereof, a uniform and compact brass-tin plating can be obtained in a wide range of current density.
Preferably, the pH value of the pyrophosphate electroplating solution of multi-layer cyanide-free electroplated copper-tin alloy plating is 8.0 to 10.0, and the density thereof is 1.30 to 1.45 g/cm3. The pH value of the pyrophosphate electroplating solution of the present invention can be adjusted to a desired pH value by using hydrophosphate and phosphoric acid.
Further, the pyrophosphate electroplating solution of multi-layer cyanide-free electroplated copper-tin alloy plating also contains the following components and concentrations:
pyrophosphate 350 to 450 g/L;
soluble copper salt 20 to 35 g/L;
soluble tin salt 1.8 to 3.0 g/L;
conductive salt 0 to 80 g/L; and cyanide-free brass-tin adjuvant 10 to 50 ml/L.
The solvent of the pyrophosphate electroplating solution is water.
Preferably, the pyrophosphate is one selected from potassium pyrophosphate and sodium pyrophosphate. Preferably, the pyrophosphate is potassium pyrophosphate.
Preferably, the soluble copper salt is one, two or more selected from copper pyrophosphate, copper sulfate, copper chloride, basic copper carbonate, copper methane sulfonate and copper sulfamate. Preferably, the soluble copper salt is copper pyrophosphate.

Preferably, the soluble tin salt is one, two or more selected from stannous pyrophosphate, stannous sulfate, stannous chloride, tin fluoborate and tin alkylsulfonate.
Preferably, the soluble tin salt is stannous pyrophosphate.
Preferably, the conductive salt is one, two or more selected from potassium chloride, sodium chloride, dipotassium hydrogen phosphate, ammonium chloride, potassium sulphate, sodium sulphate, potassium carbonate and sodium carbonate. Preferably, the conductive salt is dipotassium hydrogen phosphate.
Preferably, the solute of the cyanide-free brass-tin adjuvant consists of an auxiliary complexing agent A and an auxiliary complexing agent B; wherein the concentration of the auxiliary complexing agent A in the cyanide-free brass-tin adjuvant is 5 to 10 g/L, and the concentration of the auxiliary complexing agent B in the cyanide-free brass-tin adjuvant is 5 to g/L. The solvent of the cyanide-free brass-tin adjuvant is water.
More preferably, both the auxiliary complexing agent A and the auxiliary complexing agent B are one, two or more selected from glycolic acid, sodium gluconate, HEDP (hydroxy ethidene diphosphonic acid), citric acid, sodium citrate, ammonium citrate, potassium sodium tartrate, methanesulfonic acid, triethanolamine, oxalic acid and glycine;
while the auxiliary complexing agent A and the auxiliary complexing agent B will not select the same substance simultaneously. Preferably, the auxiliary complexing agent A is glycolic acid;
and the auxiliary complexing agent B is sodium gluconate.
In accordance with the present invention, the pyrophosphate electroplating solution of multi-layer cyanide-free electroplated copper-tin alloy plating may further include a stabilizer;
and the concentration of the stabilizer is 0.01 to 0.05 g/L.
Preferably, the stabilizer is one selected from hydroquinone, catechol, resorcinol, 13-naphthol, ascorbic acid and hydroxy benzenesulfonic acid.

In accordance with the present invention, the electroplating solution has a simple composition and is easy to maintain, and is applicable to a wide range of current density, and the plating thickness can be up to 20 j_im without the occurrence of embrittlement. The content of tin in the plating is 11% to 14% by weight; and the plating appearance is in uniform golden yellow color without chromatic aberration.
The present invention further provides a electroplating method for multi-layer cyanide-free electroplated copper-tin alloy plating, in which 2 to 4 plating layers of copper-tin alloy are sequentially electroplated on a coin substrate, and then, after performing high-temperature treatment, a coin with multi-layer cyanide-free electroplated copper-tin alloy platting is obtained; wherein, the even layer(s) of plating and the surface layer are electroplated by adopting the above pyrophosphate electroplating solution of multi-layer cyanide-free electroplated copper-tin alloy plating.
Preferably, the number of the layers of copper-tin alloy plating is 2 or 4.
Preferably, the temperature of the high-temperature treatment is 600 C to 800 C.
In accordance with the present invention, the electroplating method for multi-layer cyanide-free electroplated copper-tin alloy plating specifically comprises the following steps:
1. Electroplating a first layer: taking a coinage blank of low-carbon steel as a coin substrate, after removing oil, pickling and activating, the coinage blank is placed in a first electroplating liquid, to electroplate a first layer with a thickness of about 1 to 5 micrometers at a temperature of 20 C to 30 C, so as to obtain the first layer of copper-tin alloy with a tin content of less than 2%; and then wash with water.
Preferably, in Step 1, the current density for electroplating the first layer is 0.5 to 1.5 A/dm2; and the electroplating time is 30 to 60 min.
The adopted first electroplating liquid in Step 1 and Step 5 is electroplating liquid of cyanide-free low tin copper-tin alloy, which may adopt commonly used electroplating liquid of cyanide-free low tin copper-tin alloy in the prior art, such as electroplating liquid containing the following solute concentrations: potassium pyrophosphate of 250 to 370 g/L;
copper pyrophosphate of 20 to 30 g/L; stannous pyrophosphate of 0.2 to 0.5 g/L;
dipotassium hydrogen phosphate of 0 to 80 g/L; cyanide-free alkaline copper additive of 10 to 20 ml/L; wherein the density is 1.25 to 1.35; and the solvent is water.
Preferably, the water washing after electroplating the first layer is to place the first layer electroplated coinage blank in deionized water at room temperature for rinsing.
In accordance with the present invention, the total thickness of the plating of the blank or billet is no less than 20 micrometers; all the binding force, corrosion resistance, abrasion resistance, hardness and other indexes of the plating of the blank or billet meet the requirements of mint application.
2. Electroplating a second layer: the obtained water washed coinage blank in Step 1 is placed in the pyrophosphate electroplating solution of multi-layer cyanide-free electroplated copper-tin alloy plating of the present invention, to electroplate a second layer with a thickness of about 10 to 20 micrometers at a temperature of 25 C to 35 C, so as to obtain a second layer of copper-tin alloy with a tin content of 14% to 18%; and then wash with water.
Preferably, in Step 2, the current density for electroplating the second layer is 0.5 to 1.5 A/dm2; and the electroplating time is 200 to 550 min.
Preferably, the water washing after electroplating the second layer is to place the second layer electroplated coinage blank in deionized water at room temperature for rinsing.
3. Electroplating a third layer: the obtained water washed coinage blank in Step 2 is placed in the first electroplating liquid, to electroplate a third layer with a thickness of about 3 to micrometers at a temperature of 20 C to 30 C, so as to obtain a third layer of copper-tin alloy with a tin content of less than 2%; and then wash with water.
Preferably, in Step 3, the current density for electroplating the third layer is 0.5 to 1.5 A/dm2; and the electroplating time is 60 to 90 min.
Preferably, the water washing after electroplating the third layer is to place the third layer electroplated coinage blank in deionized water at room temperature for rinsing.
4. Electroplating a fourth layer (also called as a surface layer): the obtained rinsed coinage blank in Step 3 is placed in a pyrophosphate electroplating solution of multi-layer cyanide-free electroplated copper-tin alloy plating, to electroplate a fourth layer with a thickness of about 10 to 12 micrometers at a temperature of 20 C to 30 C, so as to obtain the fourth layer of copper-tin alloy with a tin content of 14% to 18%; and then wash with water.
Preferably, in Step 4, the current density for electroplating the fourth layer is 0.5 to 1.5 A/dm2; and the electroplating time is 200 to 270 min.
Preferably, the water washing after electroplating the fourth layer is to place the fourth layer electroplated coinage blank in deionized water at room temperature for rinsing.

5. The obtained water washed coinage blank with two layers of plating in Step 2 or the obtained water washed with four layers of plating in Step 4 is dried and subjected to high-temperature heat treatment in sequence, to obtain a coin of multi-layer cyanide-free electroplated copper-tin alloy plating, i.e., a mono-plating coin of copper-tin alloy.
Further, the oil removal step in Step 1 sequentially includes an alkaline oil removal step and an electrolytic oil removal step; the pickling and activating step in Step 1 is to perform pickling and activating on the coinage blank with hydrochloric acid solution.
Preferably, there is a water washing step after each of the alkaline oil removal step, electrolytic oil removal step and pickling and activating step. The water washing is preferably to perform rinsing with deionized water at room temperature. The alkaline oil removal step, electrolytic oil removal step and pickling and activating step of the present invention may adopt a conventional alkaline oil removal step, electrolytic oil removal step and pickling and activating step in the prior art.
The present invention further provides a coin product, which is a coin of multi-layer cyanide-free electroplated copper-tin alloy plating obtained by using the above electroplating method for multi-layer cyanide-free electroplated copper-tin alloy plating of the present invention; in the coin of monolayer copper-tin alloy plating formed after high-temperature heat treatment, the content of tin in the monolayer plating is 11% to 14% by weight; and the plating appearance is in uniform golden yellow without chromatic aberration.
Further, the obtained coin of multi-layer cyanide-free electroplated copper-tin alloy plating has a thickness of coin plating of 20 to 24 micrometers when adopting two layers of plating; and the obtained coin has a thickness of coin plating of 25 to 31 micrometers when adopting four layers of plating.
In accordance with the present invention, the adopted electroplating liquid for each of plating of the coin product is pyrophosphate solution system. By taking full advantage of the advancement and superiority of cyanide-free alloy electroplating, combining with the manner of multi-layer electroplating, and rationally considering the combination between the thickness of the multi-layer plating and the alloy composition, it is able to solve the current difficult problem in the field of electroplating that the monolayer cyanide-free electroplated alloy plating is thin.
In accordance with the present invention, by adopting the electroplating method of multi-layer cyanide-free electroplated copper-tin alloy plating, it enables to save the management cost of highly toxic cyanide, and significantly improve the electroplating conditions, which is conducive to the health of workers and environmental protection; the entire electroplating liquid system is pyrophosphate system, which prevents the risk of cross contamination among plating solutions, and makes the whole process more smooth and easier for controlling.
Brief Description of the Drawings FIG. 1 is a flow chart of an electroplating process of the present invention;
FIG. 2 shows the influence of a brightening agent A on the appearance of a Hull cell test piece;
FIG. 3 shows the influence of a brightening agent B on the appearance of a Hull cell test piece;
FIG. 4 shows the appearance of a Hull cell test piece under different current with the addition of a brightening agent A and a brightening agent B;
FIG. 5 shows the influence of the addition of an auxiliary complexing agent A
on the components of plating;
FIG. 6 shows the influence of the addition of an auxiliary complexing agent B
on the components of plating; and FIG. 7 shows the influence of the addition of an auxiliary complexing agent A
and an auxiliary complexing agent B on the components of plating.
Detailed Description of the Preferred Embodiments Hereinafter, the implementation manners of the present invention are illustrated with specific examples, so that persons of ordinary skill in the art can easily understand other advantages and efficacies of the present invention from the disclosure of the specification. The present invention can also be implemented or applied in other different specific implementation manners, and various modifications and alternations can be made on details in the specification based on different views and application without departing from the spirit of the present invention.
In accordance with the following Table 1 to Table 7 and Embodiments 1 to 6, the cyanide-free brass-tin major brightening agent consists of a brightening agent Mirapol WT (a brightening agent A) with concentration of 1 to 10 g/L and 2-mercapto benzimidazole (a brightening agent B) with concentration of 0.05 to 0.5 g/L.
In accordance with the following Table 1 to Table 7 and Embodiments 1 to 6, the cyanide-free brass-tin adjuvant consists of glycolic acid with concentration of 5 to 10 g/L and sodium gluconate with concentration of 5 to 10 g/L.
In accordance with the following Table 1 to Table 7 and Embodiments 1 to 6,the cyanide-free alkaline copper additive consists of glycolic acid with concentration of 50 to 100 g/L and 2-mercapto benzimidazole with concentration of 0.05 to 0.5 g/L.
In accordance with the following Table 1 to Table 7 and various embodiments, the process parameters of the electroplating method for multi-layer cyanide-free electroplated copper-tin alloy plating are preferably shown in Table 1 to Table 7.
Table 1 Process parameters of alkaline oil removal Alkaline oil removal Parameter range Oil remover 50 to 70 g/L
Temperature 55 C to 65 C
Table 2 Process parameters of electrolytic oil removal Electrolytic oil removal Parameter range Oil remover 60 to 80 g/L
Temperature 55 C to 65 C
Current density 0.5 to 1.3 A/dm2 Table 3 Process parameters of hydrochloric acid activation Hydrochloric acid activation Parameter range Concentrated hydrochloric acid (35%) 350 to 500 ml/L
Temperature 20 C to 30 C
Table 4 Process parameters for electroplating a first layer Process parameters of the electroplating Parameter range liquid Potassium pyrophosphate 250 to 370 g/L
Copper pyrophosphate 20 to 30 g/L
Stannous pyrophosphate 0.2 to 0.5 g/L
Dipotassium hydrogen phosphate 0 to 80 g/L
Cyanide-free alkaline copper additive 10 to 20 ml/L
pH value 8.0 to 10.0 Density 1.25 to 1.35 Temperature 20 C to 30 C
Current density 0.5 to 1.5 A/dm2 Anode material oxygen-free cathode copper Table 5 Process parameter for electroplating a second layer Process parameters of the electroplating Parameter range liquid Potassium pyrophosphate 350 to 450 g/L
Copper pyrophosphate 20 to 35 g/L
Starmous pyrophosphate 1.8 to 3.0 g/L
Dipotassium hydrogen phosphate 0 to 80 g/L
Cyanide-free brass-tin major brightening 3 to 20 ml/L
agent Cyanide-free brass-tin adjuvant 10 to 50 ml/L
pH value 8.0 to 10.0 Density 1.30 to 1.45 Temperature 25 C to 35 C
Current density 0.5 to 1.5 A/dm2 Anode material oxygen-free cathode copper Table 6 Process parameters for electroplating a third layer Process parameters of the electroplating Parameter range liquid Potassium pyrophosphate 250 to 370 g/L
Copper pyrophosphate 20 to 30 g/L
Stannous pyrophosphate 0.2 to 0.5 g/L

Dipotassium hydrogen phosphate 0 to 80 g/L
Cyanide-free alkaline copper additive 10 to 20 ml/L
pH value 8.0 to 10.0 Density 1.25 to 1.35 Temperature 20 C to 30 C
Current density 0.5 to 1.5 A/dm2 Anode material oxygen-free cathode copper Table 7 Process parameter for electroplating a surface layer Process parameters of the electroplating Parameter range liquid Potassium pyrophosphate 350 to 450 g/L
Copper pyrophosphate 20 to 35 g/L
Stannous pyrophosphate 1.8 to 3.0 g/L
Dipotassium hydrogen phosphate 0 to 80 g/L
Cyanide-free brass-tin major brightening 3 to 20 ml/L
agent Cyanide-free brass-tin adjuvant 10 to 50 ml/L
pH value 8.0 to 10.0 Density 1.30 to 1.45 Temperature 25 C to 35 C
Current density 0.5 to 1.5 A/dm2 Anode material oxygen-free cathode copper Embodiment 1:
Take a coinage blank of low-carbon steel as a substrate, thereon electroplate a first layer and a second layer in sequence, to obtain a product. The specific steps are as follows:
(1) Alkaline oil removal The coinage blank is placed in an alkaline oil remover with concentration of 50 g/L, and is cleaned for 20 min at a temperature of 55 C, and then rinsed with deionized water at 60 C.
(2) Electrolytic oil removal The alkaline washed coinage blank is placed in an electrolytic oil remover with concentration of 60 g/L, and is subjected to anode electrolytic cleaning for 20 min at a temperature of 55 C and with current density of 0.5 A/dm2, and then rinsed with deionized water at 60 C.
(3) Hydrochloric acid activation The coinage blank after electrolytic oil removal is placed in an HC1 solution with concentration of 350 ml/L, and is subjected to acid activation for 7 min at a temperature of 20 C, and then rinsed with deionized water at room temperature.
(4) Electroplating a first layer The activated coinage blank is placed in a first layer electroplating liquid with a pH
value of 8.0, and is electroplated a first layer at a temperature of 20 C, wherein the current density is 0.5 A/dm2, and the electroplating time is 60 min. The first layer electroplating liquid consists of the following components: potassium pyrophosphate, 250 g/L; copper pyrophosphate, 20 g/L; stannous pyrophosphate, 0.2 g/L; cyanide-free alkaline copper additive, ml/L; and the first layer has a thickness of about 2 to 4 micrometers.
(5) Water washing The first layer electroplated coinage blank is placed in deionized water, and rinsed with deionized water at room temperature.

(6) Electroplating a second layer After rinsing in deionized water, the coinage blank is placed in a second layer electroplating liquid with a pH value of 8.0, and is electroplated a second layer at a temperature of 20 C, wherein the current density is 0.5 A/dm2, and the electroplating time is 540 min. The second layer electroplating liquid consists of the following components:
potassium pyrophosphate, 350 g/L; copper pyrophosphate, 20 g/L; stannous pyrophosphate, 1.8 g/L;
cyanide-free brass-tin major brightening agent, 3 ml/L; cyanide-free brass-tin adjuvant, 10 ml/L;
and the second layer has a thickness of about 18 to 20 micrometers and a tin content of 14% to 18%.

(7) Water washing and drying The surface layer electroplated coinage blank is placed in deionized water, and rinsed with deionized water at room temperature, and then the coinage blank is dried.

(8) High-temperature heat treatment The dried coinage blank is placed in a high-temperature heat treatment furnace being flowed with reduced protective atmosphere, and is subjected to heat treatment for 7 min at 650 C and then for 7 min at 680 C. After the heat treatment, the plating of the product is diffused into one layer having a tin content of 11% to 14% and a plating thickness of 20 to 24 micrometers.
Embodiment 2:

Take a coinage blank of low-carbon steel as a substrate, thereon electroplate a first layer and a second layer in sequence, to obtain a product. The specific steps are as follows:
(1) Alkaline oil removal The coinage blank is placed in an alkaline oil remover with concentration of 60 g/L, and is cleaned for 20 min at a temperature of 60 C, and then rinsed with deionized water at 60 C.
(2) Electrolytic oil removal The alkaline washed coinage blank is placed in an electrolytic oil remover with concentration of 70 g/L, and is subjected to anode electrolytic cleaning for 20 min at a temperature of 60 C and with current density of 1.0 A/dm2, and then rinsed with deionized water at 60 C.
(3) Hydrochloric acid activation The coinage blank after electrolytic oil removal is placed in an HC1 solution with concentration of 480 ml/L, and is subjected to acid activation for 7 min at a temperature of 25 C, and then rinsed with deionized water at room temperature.
(4) Electrolating a first layer The activated coinage blank is placed in a first layer electroplating liquid with a pH
value of 9.0, and is electroplated a first layer at a temperature of 25 C, wherein the current density is 1.0 A/dm2, and the electroplating time is 60 min. The first layer electroplating liquid consists of the following components: potassium pyrophosphate, 300 g/L; copper pyrophosphate, 25 g/L; stannous pyrophosphate, 0.35 g/L; cyanide-free alkaline copper additive, 20 ml/L; and the first layer has a thickness of about 2 to 4 micrometers.
(5) Water washing The first layer electroplated coinage blank is placed in deionized water, and rinsed with deionized water at room temperature.
(6) Electroplating a second layer After rinsing in deionized water, the coinage blank is placed in a second layer electroplating liquid with a pH value of 9.0, and is electroplated a second layer at a temperature of 25 C, wherein the current density is 1.2 A/dm2, and the electroplating time is 540 min. The second layer electroplating liquid consists of the following components:
potassium pyrophosphate, 400 g/L; copper pyrophosphate, 25 g/L; stannous pyrophosphate, 2.2 g/L;
dipotassium hydrogen phosphate, 45 g/L; cyanide-free brass-tin major brightening agent, 20 ml/L; cyanide-free brass-tin adjuvant, 50 ml/L; and the second layer has a thickness of about 18 to 20 micrometers and a tin content of 14% to 18%.
(7) Water washing and drying The surface layer electroplated coinage blank is placed in deionized water, and rinsed with deionized water at room temperature, and then the coinage blank is dried.
(8) High-temperature heat treatment The dried coinage blank is placed in a high-temperature heat treatment furnace being flowed with reduced protective atmosphere, and is subjected to heat treatment for 7 min at 650 C and then for 7 mm at 680 C. After the heat treatment, the plating of the product is diffused into one layer having a tin content of 11% to 14% and a plating thickness of 20 to 24 micrometers.
Embodiment 3:
Take a coinage blank of low-carbon steel as a substrate, thereon electroplate a first layer and a second layer in sequence, to obtain a product. The specific steps are as follows:
(1) Alkaline oil removal The coinage blank is placed in an alkaline oil remover with concentration of 70 g/L, and is cleaned for 20 min at a temperature of 65 C, and then rinsed with deionized water at 60 C.
(2) Electrolytic oil removal The alkaline washed coinage blank is placed in an electrolytic oil remover with concentration of 80 g/L, and is subjected to anode electrolytic cleaning for 20 min at a temperature of 65 C and with current density of 1.2 A/dm2, and then rinsed with deionized water at 60 C.
(3) Hydrochloric acid activation The coinage blank after electrolytic oil removal is placed in an HC1 solution with concentration of 480 ml/L, and is subjected to acid activation for 7 min at a temperature of 29 C, and then rinsed with deionized water at room temperature.
(4) Electroplating a first layer The activated coinage blank is placed in a first layer electroplating liquid with a pH
value of 9.8, and is electroplated a first layer at a temperature of 28 C, wherein the current density is 1.4 A/dm2, and the electroplating time is 60 mm. The first layer electroplating liquid consists of the following components: potassium pyrophosphate, 360 g/L; copper pyrophosphate, 28 g/L; stannous pyrophosphate, 0.45 g/L; cyanide-free alkaline copper additive, 15 ml/L; and the first layer has a thickness of about 2 to 4 micrometers.
(5) Water washing The first layer electroplated coinage blank is placed in deionized water, and rinsed with deionized water at room temperature.
(6) Electroplating a second layer After rinsing in deionized water, the coinage blank is placed in a second layer electroplating liquid with a pH value of 9.8, and is electroplated a second layer at a temperature of 28 C, wherein the current density is 1.8 A/dm2, and the electroplating time is 540 min. The second layer electroplating liquid consists of the following components:
potassium pyrophosphate, 450 g/L; copper pyrophosphate, 32 g/L; stannous pyrophosphate.
2.8 g/L;
dipotassium hydrogen phosphate, 70 g/L; cyanide-free brass-tin major brightening agent, 10 ml/L; cyanide-free brass-tin adjuvant, 30 ml/L; and the second layer has a thickness of about 18 to 20 micrometers and a tin content of 14% to 18%.
(7) Water washing and drying The surface layer electroplated coinage blank is placed in deionized water, and rinsed with deionized water at room temperature, and then the coinage blank is dried.
(8) High-temperature heat treatment The dried coinage blank is placed in a high-temperature heat treatment furnace being flowed with reduced protective atmosphere, and is subjected to heat treatment for 7 min at 650 C and then for 7 min at 680 C. After the heat treatment, the plating of the product is diffused into one layer having a tin content of 11% to 14% and a plating thickness of 20 to 24 micrometers.
Embodiment 4:
Take a coinage blank of low-carbon steel as a substrate, thereon electroplate a first layer, a second layer, a third layer and a surface layer in sequence, to obtain a product. The specific steps are as follows:
(1) Alkaline oil removal The coinage blank is placed in an alkaline oil remover with concentration of 50 g/L, and is cleaned for 20 min at a temperature of 55 C, and then rinsed with deionized water at 60 C.
(2) Electrolytic oil removal The alkaline washed coinage blank is placed in an electrolytic oil remover with concentration of 60 g/L, and is subjected to anode electrolytic cleaning for 20 min at a temperature of 55 C and with current density of 0.5 A/dm2, and then rinsed with deionized water at 60 C.
(3) Hydrochloric acid activation The coinage blank after electrolytic oil removal is placed in an HC1 solution with concentration of 350 ml/L, and is subjected to acid activation for 7 mm at a temperature of 20 C, and then rinsed with deionized water at room temperature.
(4) Electroplating a first layer The activated coinage blank is placed in a first layer electroplating liquid with a pH
value of 8.0, and is electroplated a first layer at a temperature of 20 C, wherein the current density is 0.5 A/dm2, and the electroplating time is 30 min. The first layer electroplating liquid consists of the following components: potassium pyrophosphate, 250 g/L; copper pyrophosphate, 20 g/L; stannous pyrophosphate, 0.2 g/L; cyanide-free alkaline copper additive, ml/L; and the first layer has a thickness of about 1 to 2 micrometers.
(5) Water washing The first layer electroplated coinage blank is placed in deionized water, and rinsed with deionized water at room temperature.
(6) Plating a second layer After rinsing in deionized water, the coinage blank is placed in a second layer electroplating liquid with a pH value of 8.0, and is electroplated a second layer at a temperature of 20 C, wherein the current density is 0.5 A/din2, and the electroplating time is 270 min. The second layer electroplating liquid consists of the following components:
potassium pyrophosphate, 350 g/L; copper pyrophosphate, 20 g/L; stannous pyrophosphate, 1.8 g/L;
cyanide-free brass-tin major brightening agent, 3 ml/L; cyanide-free brass-tin adjuvant, 10 ml/L:
and the second layer has a thickness of about 10 to 12 micrometers and a tin content of 14% to 18%.
(7) Water washing and drying The second layer electroplated coinage blank is placed in deionized water, and rinsed with deionized water at room temperature, and then the coinage blank is dried.
(8) Electroplating a third layer The water washed coinage blank is placed in a third layer electroplating liquid with a pH value of 8.0, and is electroplated a third layer at a temperature of 20 C, wherein the current density is 0.5 A/dm2, and the electroplating time is 90 min. The third layer electroplating liquid consists of the following components: potassium pyrophosphate, 250 g/L; copper pyrophosphate, 20 g/L; stannous pyrophosphate, 0.2 g/L; cyanide-free alkaline copper additive, 15 ml/L; and the third layer has a thickness of about 3 to 5 micrometers.

(9) Water washing The third layer electroplated coinage blank is placed in deionized water, and rinsed with deionized water at room temperature.

(10) Electroplating a surface layer After rinsing in deionized water, the coinage blank is placed in a surface layer electroplating liquid with a pH value of 8.0, and is electroplated a surface layer at a temperature of 20 C, wherein the current density is 0.5 A/dm2, and the electroplating time is 270 min. The second layer electroplating liquid consists of the following components:
potassium pyrophosphate, 350 g/L; copper pyrophosphate, 20 g/L; stannous pyrophosphate, 1.8 g/L;
cyanide-free brass-tin major brightening agent, 10 ml/L; cyanide-free brass-tin adjuvant, 30 ml/L; and the surface layer has a thickness of about 10 to 12 micrometers and a tin content of 14% to 18%.

(11) Water washing and drying The surface layer electroplated coinage blank is placed in deionized water, and rinsed with deionized water at room temperature, and then the coinage blank is dried.

(12) High-temperature heat treatment The dried coinage blank is placed in a high-temperature heat treatment furnace being flowed with reduced protective atmosphere, and is subjected to heat treatment for 7 min at 650 C and then for 7 min at 680 C. After the heat treatment, the plating of the product is diffused into one layer having a tin content of 11% to 14% and a plating thickness of 25 to 31 micrometers.
Embodiment 5:
Take a coinage blank of low-carbon steel as a substrate, thereon electroplate a first layer, a second layer, a third layer and a surface layer in sequence, to obtain a product. The specific steps are as follows:
(1) Alkaline oil removal The coinage blank is placed in an alkaline oil remover with concentration of 60 g/L, and is cleaned for 20 min at a temperature of 60 C, and then rinsed with deionized water at 60 C.
(2) Electrolytic oil removal The alkaline washed coinage blank is placed in an electrolytic oil remover with concentration of 70 g/L, and is subjected to anode electrolytic cleaning for 20 min at a temperature of 60 C and with current density of 1.0 A/dm2, and then rinsed with deionized water at 60 C.
(3) Hydrochloric acid activation The coinage blank after electrolytic oil removal is placed in an HC1 solution with concentration of 400 ml/L, and is subjected to acid activation for 7 min at a temperature of 25 C, and then rinsed with deionized water at room temperature.
(4) Electroplating a first layer The activated coinage blank is placed in a first layer electroplating liquid with a pH
value of 9.0, and is electroplated a first layer at a temperature of 25 C, wherein the current density is 1.0 A/dm2, and the electroplating time is 30 min. The first layer electroplating liquid consists of the following components: potassium pyrophosphate, 300 g/L; copper pyrophosphate, 25 g/L; stannous pyrophosphate, 0.3 g/L; cyanide-free alkaline copper additive, 20 ml/L; and the first layer has a thickness of about 1 to 2 micrometers.
(5) Water washing The first layer electroplated coinage blank is placed in deionized water, and rinsed with deionized water at room temperature.
(6) Electroplating a second layer After rinsing in deionized water, the coinage blank is placed in a second layer electroplating liquid with a pH value of 9.0, and is electroplated a second layer at a temperature of 25 C, wherein the current density is 1.2 A/dm2, and the electroplating time is 270 min. The second layer electroplating liquid consists of the following components:
potassium pyrophosphate, 400 g/L; copper pyrophosphate, 25 g/L; stannous pyrophosphate, 2.2 g/L;
cyanide-free brass-tin major brightening agent, 20 ml/L; cyanide-free brass-tin adjuvant, 50 ml/L; and the second layer has a thickness of about 10 to 12 micrometers and a tin content of 14% to 18%.
(7) Water washing and drying The second layer electroplated coinage blank is placed in deionized water, and rinsed with deionized water at room temperature, and then the coinage blank is dried.
(8) Electroplating a third layer The water washed coinage blank is placed in a third layer electroplating liquid with a pH value of 9.0, and is electroplated a third layer at a temperature of 25 C, wherein the current density is 1.0 A/dm2, and the electroplating time is 90 mm. The third layer electroplating liquid consists of the following components: potassium pyrophosphate, 300 g/L; copper pyrophosphate, 25 g/L; stannous pyrophosphate, 0.3 g/L; cyanide-free alkaline copper additive, 18 ml/L; and the third layer has a thickness of about 3 to 5 micrometers.
(9) Water washing The third layer electroplated coinage blank is placed in deionized water, and rinsed with deionized water at room temperature.
(10) Electroplating a surface layer After rinsing in deionized water, the coinage blank is placed in a surface layer electroplating liquid with a pH value of 9.0, and is electroplated a surface layer at a temperature of 25 C, wherein the current density is 1.0 A/dm2, and the electroplating time is 270 min. The surface layer electroplating liquid consists of the following components:
potassium pyrophosphate, 400 g/L; copper pyrophosphate, 25 g/L; stannous pyrophosphate, 2.2 g/L;
cyanide-free brass-tin major brightening agent, 18 ml/L; cyanide-free brass-tin adjuvant, 40 ml/L; and the surface layer has a thickness of about 10 to 12 micrometers and a tin content of 14% to 18%.

(11) Water washing and drying The surface layer electroplated coinage blank is placed in deionized water, and rinsed with deionized water at room temperature, and then the coinage blank is dried.
(12) High-temperature heat treatment The dried coinage blank is placed in a high-temperature heat treatment furnace being flowed with reduced protective atmosphere, and is subjected to heat treatment for 7 min at 650 C and then for 7 min at 680 C. After the heat treatment, the plating of the product is diffused into one layer having a tin content of 11% to 14% and a plating thickness of 25 to 31 micrometers.
Embodiment 6:
Take a coinage blank of low-carbon steel as a substrate, thereon electroplate a first layer, a second layer, a third layer and a surface layer in sequence, to obtain a product. The specific steps are as follows:
(1) Alkaline oil removal The coinage blank is placed in an alkaline oil remover with concentration of 70 g/L, and is cleaned for 20 min at a temperature of 65 C, and then rinsed with deionized water at 60 C.
(2) Electrolytic oil removal The alkaline washed coinage blank is placed in an electrolytic oil remover with concentration of 80 g/L, and is subjected to anode electrolytic cleaning for 20 min at a temperature of 65 C and with current density of 1.3 A/dm2, and then rinsed with deionized water at 60 C.
(3) Hydrochloric acid activation The coinage blank after electrolytic oil removal is placed in an HC1 solution with concentration of 500 ml/L, and is subjected to acid activation for 7 min at a temperature of 30 C, and then rinsed with deionized water at room temperature.
(4) Electroplating a first layer The activated coinage blank is placed in a first layer electroplating liquid with a pH
value of 9.8, and is electroplated a first layer at a temperature of 30 C, wherein the current density is 1.5A/dm2, and the electroplating time is 30 min. The first layer electroplating liquid consists of the following components: potassium pyrophosphate, 370 g/L; copper pyrophosphate, 30 g/L; stannous pyrophosphate, 0.4 g/L; cyanide-free alkaline copper additive, 15 ml/L; and the first layer has a thickness of about 1 to 2 micrometers.
(5) Water washing The first layer electroplated coinage blank is placed in deionized water, and rinsed with deionized water at room temperature.
(6) Electroplating a second layer After rinsing in deionized water, the coinage blank is placed in a second layer electroplating liquid with a pH value of 10.0, and is electroplated a second layer at a temperature of 28 C, wherein the current density is 1.8 A/dm2, and the electroplating time is 270 min. The second layer electroplating liquid consists of the following components:
potassium pyrophosphate, 450 g/L; copper pyrophosphate, 35 g/L; stannous pyrophosphate, 3.0 g/L; cyanide-free brass-tin major brightening agent, 10 ml/L; cyanide-free brass-tin adjuvant, 30 ml/L; and the second layer has a thickness of about 10 to 12 micrometers and a tin content of 14% to 18%.
(7) Water washing and drying The second layer electroplated coinage blank is placed in deionized water, and rinsed with deionized water at room temperature, and then the coinage blank is dried.
(8) Electroplating a third layer The water washed coinage blank is placed in a third layer electroplating liquid with a pH value of 10.0, and is electroplated a third layer at a temperature of 28 C, wherein the current density is 1.5 A/dm2, and the electroplating time is 90 min. The third layer electroplating liquid consists of the following components: potassium pyrophosphate, 370 g/L; copper pyrophosphate, 30 g/L; stannous pyrophosphate, 0.5 g/L; cyanide-free alkaline copper additive, 12 ml/L; and the third layer has a thickness of about 3 to 5 micrometers.
(9) Water washing The third layer electroplated coinage blank is placed in deionized water, and rinsed with deionized water at room temperature.
(10) Electroplating a surface layer After rinsing in deionized water, the coinage blank is placed in a surface layer electroplating liquid with a pH value of 10.0, and is electroplated a surface layer at a temperature of 30 C, wherein the current density is 1.8 A/dm2, and the electroplating time is 270 min. The second layer electroplating liquid consists of the following components:
potassium pyrophosphate, 450 g/L; copper pyrophosphate, 32 g/L; stannous pyrophosphate, 2.8 g/L; cyanide-free brass-tin major brightening agent, 18 ml/L; cyanide-free brass-tin adjuvant, 40 ml/L; and the surface layer has a thickness of about 10 to 12 micrometers and a tin content of 14% to 18%.
(11) Water washing and drying The surface layer electroplated coinage blank is placed in deionized water, and rinsed with deionized water at room temperature, and then the coinage blank is dried.
(12) High-temperature heat treatment The dried coinage blank is placed in a high-temperature heat treatment furnace being flowed with reduced protective atmosphere, and is subjected to heat treatment for 7 min at 650 C and then for 7 min at 680 C. After the heat treatment, the plating of the product is diffused into one layer having a tin content of 11% to 14% and a plating thickness of 25 to 31 micrometers.
Influence of the Mirapol WT brightening agent A (additive A) and the brightening agent B (2-mercapto benzimidazole, i.e., additive B) in the cyanide-free brass-tin major brightening agent as well as the cyanide-free brass-tin adjuvant on the electrodeposition of copper-tin alloy:
(I) Tests of individual brightening agent A, individual brightening agent B, and synergistic effect of both the brightening agent A and the brightening agent B
with test results as shown below:
1. Brightening agent A (WT) The additive A may effectively improve the burning in the high-current density zone and increase the brightness of the plating, and due to the addition of A, the electrodeposition characteristic of the alloy electroplating liquid is changed from electrochemical step control into diffusion step control. Therefore, in the high-current density zone, better electrodeposition can be achieved merely by reducing the concentration polarization of metal ions. In the meanwhile, due to the diffusion and adsorption and inhibition of the additive A on the electrode surface, the cathode polarization is increased, so that the alloy plating crystal is bright and unifoim in appearance. As the plating liquid has a composition of Cu2P207.3H20, 25 g/L; Sn2P207, 3.0 g/L; 1(413207.3H20, 450 g/L; K2HPO4-3H20, 60 g/L, with a pH value of 8.5 and a temperature of 25 C, and as the brightening agent A is of 0.1 g/L, the appearance of a Hull cell under different current is as shown in FIG. 2, which is also shown to the influence of the additive A on the appearance of a Hull cell test piece.
It can be known from FIG. 2 that the addition of the brightening agent A can effectively improve the burning in the high-current density zone and has a certain effect on alloy co-deposition in the low-current density zone, so that the appearance of Hull cell test piece is brighter, and the golden yellow alloy co-deposition area is increased.
However, with the increase the current, the burning range in the high-current density zone gets bigger and bigger, indicating that the operational range of the plating current is narrow.
2. Brightening agent B (2-mercapto benzimidazole) The brightening agent B is taken as a grain refiner for the electrodeposition of copper ion, which enhances the cathode polarization of copper ions in the low-current density zone, so that the crystal of metal copper gets finer, and at the same time, the precipitation of copper is inhibited, thereby increasing the tin content in the plating of the low-current density zone. As the plating liquid has a composition of: Cu2P207=3H20, 25 g/L; Sn2P207, 3.0 g/L; K4P207=3H20, 450 g/L;
K2HPO4.3H20, 60 g/L; with a pH value of 8.5, a temperature of 25 C, and the brightening agent B of 0.0015 g/L, the appearance of a Hull cell under different currents is as shown in FIG
3, which is also shown to the influence of the brightening agent B on the appearance of a Hull cell test piece.
It can be known from FIG. 3 that the addition of the brightening agent B can effectively improve the co-deposition in low-current density zone, so that the appearance of the Hull cell in the low-current density zone is changed from pink into golden yellow. With the increase of the current, the range of the pink plating in the low-current density zone gets smaller and smaller. However, with the increase of the current, the burning range in the high-current density zone gets bigger and bigger, indicating that the operational range of the plating current is narrow.
3. Synergistic effect of the brightening agent A and the brightening agent B:
As the plating liquid has a composition of Cu2P207=3H20, 25 g/L; Sn2P207, 3.0 g/L;
K4P207.3H20, 450 g/L; K2HPO4.3H20, 60 g/L, with a pH value of 8.5, and with the brightening agent A being 0.1 g/L and the brightening agent B being 0.0015 g/L, at a temperature of 25 C, the appearance of a Hull cell under different currents is as shown in FIG. 4, which is also shown to the appearance of a Hull cell test piece under different currents after the addition of the additives.
It can be known from FIG. 4 that in the plating liquid having the brightening agent A
and the brightening agent B simultaneously, as the current is 0.3 A, no burning phenomenon occurs in the high-current density zone, and the range of pink plating in the low-current density zone is very narrow. As the current is 0.5 A, the entire Hull cell test piece is golden yellow. With the increase of the current, the burning in the high-current density zone occurs, while the range is narrow. Therefore, As the brightening agent A and the brightening agent B
simultaneously exist in the plating liquid, synergistic effect is generated, so that not only the burning phenomenon in the high-current density zone is effective solved, but also the occurrence of pink plating in the low-current density zone is eliminated.
(II) Tests of individual auxiliary complexing agent A, individual auxiliary complexing agent B, and synergistic effect of the auxiliary complexing agent A and the auxiliary complexing agent B with test results being shown below:
1. Auxiliary complexing agent A (glycolic acid) An auxiliary complexing agent for tin ion can enhance the complexing of tin ion, eliminate the generation of monovalent copper through reaction of free bivalent tin and copper ions, and at the same time, can effectively inhibit the oxidation of tin ion. As the plating liquid has a composition of Cu2P207.3H20, 25 g/L; Sn2P207, 3.0 g/L; K4P207.3H20, 450 g/L; K2HPO4.3H20, 60 g/L, with a pH
value of 8.5, and a temperature of 25 C; and the auxiliary complexing agent A being 0.3 g/L, the plating components under different current densities are as shown in FIG. 5, which is also shown to the influence of the auxiliary complexing agent A on the plating components.
In FIG. 5, the horizontal axis represents the current density, and the vertical axis represents the mass percentage of tin in the plating. The curve B shows the plating components of plating liquids under different current densities without the addition of auxiliary complexing agent A, and the curve C shows the plating components of plating liquids under different current densities with the addition the auxiliary complexing agent A. It can be known from FIG
that, with the increase of the current density, the tin content in the plating is gradually increased; as the current density is 0.1 A/dm2, the tin content in the plating is 12.45%; and as the current density is 2 A/dm2, the tin content in the plating can be up to 15.67%. With the addition of the auxiliary complexing agent A, it has a small influence on the tin content in the plating in the low-current density zone; while in the high-current density zone, the precipitation of tin is effectively inhibited, and as the current density is 2 A/dm2, the tin content in the plating is 14.73%, which indicates a drop of 0.94%. It can be seen that, the addition of the auxiliary complexing agent A can effectively inhibit the difference in tin content in the plating under different current densities, thereby increasing the uniformity of the plating.
2. Auxiliary complexing, agent B (sodium gluconate) An auxiliary complexing agent for copper ion in an alkaline solution system can enhance the complexing of copper ion, and have a synergistic effect with glycolic acid, thereby greatly improving the stability of the plating liquid. As the plating liquid has a composition of Cu2P207.3H20, 25 g/L; Sn2P207, 3.0 g/L; K413207.3H20, 450 g/L; K2HPO4-3H20, 60 g/L, with a pH value of 8.5, a temperature of 25 C; and as the auxiliary complexing agent B is 0.3 g/L, the plating components under different current densities are as shown in FIG. 6, which is also shown to the influence of the auxiliary complexing agent B on the plating components.
In FIG. 6, the horizontal axis represents the current density, and the vertical axis represents the mass percentage of copper in the plating. The curve B shows the plating components of plating liquid under different current densities without the addition of the auxiliary complexing agent B, and the curve C shows the plating components of plating liquid under different current densities with the addition of the auxiliary complexing agent B. It can be known from FIG. 6 that, with the increase of the current density, the copper content in the plating is gradually increased; as the current density is 0.1 A/dm2, the copper content in the plating is 87.62%; and as the current density is 2 A/dm2, the copper content in the plating is decreased to 84.33%. With the addition of the auxiliary complexing agent B, it has a small influence on the copper content in the plating in the high-current density zone; while in the low-current density zone, the precipitation of copper is effectively inhibited, and as the current density is 0.1 A/dm2, the copper content in the plating is 86.21%, which indicates a drop of 1.41%. It can be seen that, the addition of the auxiliary complexing agent B
can effectively inhibit the difference in copper content in the plating under different current densities, thereby increasing the uniformity of the plating.
3. Synergistic effect of the auxiliary complexing agent A and the auxiliary complexing agent B:
As the plating liquid has a composition of Cu2P207-3H20, 25 g/L; Sn2P207, 3.0 g/L;
K4P207=3H20, 450 g/L; K2HPO4.3H20, 60 g/L, with a pH value of 8.5 and a temperature of 25 C; and as the auxiliary complexing agent A is 0.3 g/L and the auxiliary complexing agent B
is 0.3 g/L, the plating components under different current densities are as shown in FIG 7, which is also shown to the influence of addition of auxiliary complexing agents on the plating components.
In FIG 7, the horizontal axis represents the current density, and the vertical axis represents the mass percentage of copper in the plating. The curve B shows the plating components of plating liquid under different current densities without the addition of the auxiliary complexing agents, and the curve C shows the plating components of plating liquid under different current densities with the addition of the auxiliary complexing agent of 0.3 g/L
and the auxiliary complexing agent B of 0.3 g/L. It can be known from FIG 7 that, in the case that the auxiliary complexing agent A and the auxiliary complexing agent B
exist simultaneously, and in the low-current density zone, the auxiliary complexing agent A
effectively inhibits the precipitation of copper, thereby decreasing the mass percentage of copper in the plating; while in the high-current density zone, the auxiliary complexing agent B
effectively inhibits the precipitation of tin, thereby decreasing the mass percentage of tin in the plating, and increasing the mass percentage of copper. The auxiliary complexing agent A and the auxiliary complexing agent B in the plating liquid inhibit the electrodeposition of copper and tin in zones of different current densities and generate the synergistic effect, so that the copper-tin alloy plating can maintain relatively stable plating components under different current densities.
The above descriptions are merely preferred embodiments of the present invention, but not any limitations in the form and substance on the present invention. It should be noted that, those of ordinary skill in the art can further make a number of improvements and supplements without departing from the method of the present invention, and these improvements and supplements should also be considered as falling within the protection scope of the present invention. Various alternations, modifications, evolutions and equivalent changes made by those skilled in the art based on the technical contents disclosed above without departing from the spirit and scope of the present invention are equivalent embodiments of the present invention;
simultaneously, any equivalent alternations, modifications and evolutions made on the embodiments according to the technical essence of the present invention fall within the scope of the technical solutions of the present invention.

Claims

What is claimed is:

1. A pyrophosphate electroplating solution of multi-layer cyanide-free electroplated copper-tin alloy plating, comprising:
a cyanide-free brass-tin brightening agent consisting of (1) 1 to 10 g/L of Mirapol.TM. WT and (2) 0.05 to 0.5 g/L of 2-mercapto benzimidazole, wherein the concentration of the cyanide-free brass-tin brightening agent in the pyrophosphate electroplating solution is 3 to 20 ml/L.

2. The pyrophosphate electroplating solution according to claim 1, wherein the pyrophosphate electroplating solution has a pII of 8.0 to 10.0, and a density of 1.25 to 1.45 g/cm3.

3. The pyrophosphate electroplating solution according to any one of claims 1 to 2, further comprising:
350 to 450 g/L of pyrophosphate;
20 to 35 g/L of soluble copper salt;
1.8 to 3.0 g/L of soluble tin salt;
0 to 80 g/L of conductive salt ; and to 50 ml/L of cyanide-free brass-tin adjuvant .

4. The pyrophosphate electroplating solution according to claim 3, wherein the cyanide-free brass-tin adjuvant consists of (1) 5 to 10 g/L of an auxiliary complexing agent A and (2) 5 to 10 g/L of an auxiliary complexing agent B.

5. The pyrophosphate electroplating solution according to claim 4, wherein the pyrophosphate is selected from potassium pyrophosphate and sodium pyrophosphate; the soluble copper salt is selected from copper pyrophosphate, copper sulfate, copper chloride, basic copper carbonate, copper methane sulfonate and copper sulfamate; the soluble tin salt is selected from stannous pyrophosphate, stannous sulfate, stannous chloride, tin fluoborate and tin alkylsulfonate; the conductive salt is selected from potassium chloride, sodium chloride, dipotassium hydrogen phosphate, ammonium chloride, potassium sulphate, sodium sulphate, potassium carbonate and sodium carbonate; and each of the auxiliary complexing agent A and the auxiliary complexing agent B is selected from glycolic acid, sodium gluconate, HEDP (hydroxy ethylidene diphosphonic acid), citric acid, sodium citrate, ammonium citrate, potassium sodium tartrate, methanesulfonic acid, triethanolamine, oxalic acid and glycine, the auxiliary complexing agent A
and the auxiliary complexing agent B being different.

6. The pyrophosphate electroplating solution according to claim 3, further comprising 0.01 to 0.05 g/L of a stabilizer.

7. An electroplating method for multi-layer cyanide-free electroplated copper-tin alloy plating, comprising:
sequentially electroplating 2 to 4 plating layers of copper-tin alloy on a coin substrate; and subjecting the plating layers on the coin substrate to a high-temperature treatment, to form a coin with multi-layer cyanide-free electroplated copper-tin alloy platting;
wherein each even layer of the plating layers and the surface layer are electroplated by adopting the pyrophosphate electroplating solution of any one of claims 1 to 6.

8. The electroplating method according to claim 7, wherein the high-temperature treatment comprises treatment at a temperature of 600°C to 800°C.

9. The electroplating method according to claim 8, comprising:
1) electroplating a first layer, comprising:
providing a coinage blank of low-carbon steel as the coin substrate, removing oil from, pickling and activating, the coinage blank, placing the coinage blank in a first electroplating liquid, to electroplate a layer with a thickness of about 1 to about 5 micrometers at a temperature of 20°C
to 30°C, so as to obtain the first layer of copper-tin alloy with a tin content of less than 2%, and washing the first layer on the coinage blank with water;
2) electroplating a second layer on the first layer, comprising:
placing the coinage blank with the water-washed first layer in the pyrophosphate electroplating solution, to electroplate a layer with a thickness of about 10 to about 20 micrometers at a temperature of 25°C to 35°C, so as to obtain the second layer of copper-tin alloy with a tin content of 14% to 18%, and washing the second layer on the coinage blank with water; and 3) drying the water washed coinage blank with two layers of plating and subjecting the dried coinage blank to a high-temperature heat treatment, to obtain a coin of multi-layer cyanide-free electroplated copper-tin alloy plating.

10. The electroplating method of claim 8, comprising:
1) electroplating a first layer, comprising:
providing a coinage blank of low-carbon steel as the coin substrate, removing oil from, pickling and activating, the coinage blank, placing the coinage blank in a first electroplating liquid, to electroplate a layer with a thickness of about 1 to about 5 micrometers at a temperature of 20°C
to 30°C, so as to obtain the first layer of copper-tin alloy with a tin content of less than 2%, and washing the first layer on the coinage blank with water;
2) electroplating a second layer on the first layer, comprising:
placing the coinage blank with the water-washed first layer in the pyrophosphate electroplating solution, to electroplate a layer with a thickness of about 10 to about 20 micrometers at a temperature of 25°C to 35°C, so as to obtain the second layer of copper-tin alloy with a tin content of 14% to 18%, and washing the second layer on the coinage blank with water; and 3) electroplating a third layer on the second layer, comprising:
placing the plated and water-washed coinage blank of 2) in the first electroplating liquid, to electroplate a layer with a thickness of about 3 to about 5 micrometers at a temperature of 20°C to 30°C, so as to obtain the third layer of copper-tin alloy with a tin content of less than 2%, and washing the third layer on the coinage blank with water;
4) electroplating a fourth layer on the third layer, comprising:
placing the plated and water-washed coinage blank of 3) in the pyrophosphate electroplating solution, to electroplate a layer with a thickness of about 10 to about 12 micrometers at a temperature of 20°C to 30°C, so as to obtain the fourth layer of copper-tin alloy with a tin content of 14% to 18%, and washing the fourth layer on the coinage blank with water; and 5) drying the coinage blank with four layers of plating and subjecting the dried coinage blank to a high-temperature heat treatment, to obtain a coin of multi-layer cyanide-free electroplated copper-tin alloy plating.

11. The electroplating method according to claim 9 or claim 10, wherein, in Step 1), a current density for electroplating the first layer is 0.5 to 1.5 A/dm2; and an electroplating time is 30 to 60 min;
in Step 2), a current density for electroplating the second layer is 0.5 to 1.5 A/dm2; and an electroplating time is 200 to 550 min; and the washing comprises rinsing in deionized water at room temperature.

12. The electroplating method of claim 10, wherein in Step 3), a current density for electroplating the third layer is 0.5 to 1.5 A/dm2; and an electroplating time is 60 to 90 min;
in Step 4), a current density for electroplating the fourth layer is 0.5 to 1.5 A/dm2; and an electroplating time is 200 to 270 min; and in Step 1) to Step 4), the washing comprises rinsing in deionized water at room temperature.

13. The electroplating method according to any one of claims 9 to 12, wherein the removing oil step in Step 1) includes sequentially removing an alkaline oil and an electrolytic oil; the pickling and activating step in Step 1) comprises pickling and activating on the coinage blank with a hydrochloric acid solution.

14. The electroplating method according to claim 9, wherein the plating has a thickness of 20 to 24 micrometers.

15. The electroplating method according to claim 10, wherein the plating has a thickness of 25 to 31 micrometers.

16. A coin obtained by using the electroplating method for multi-layer cyanide-free electroplated copper-tin alloy plating according to any one of claims 7 to 13, wherein the content of tin in the plating of the coin is 11% to 14% by weight.

17. The coin according to claim 16, wherein the coin plating has a thickness of 20-24 micrometers or 25-31 micrometers.