US20080169199A1

US20080169199A1 - Trivalent chromium electroplating solution and an electroplating process with the solution

Info

Publication number: US20080169199A1
Application number: US11/654,265
Authority: US
Inventors: Ching An Huang; Chun Ching Hsu; Ui Wei Leu
Original assignee: Chang Gung University CGU
Current assignee: Chang Gung University CGU
Priority date: 2007-01-17
Filing date: 2007-01-17
Publication date: 2008-07-17
Also published as: US20100155256A1

Abstract

A trivalent chromium electroplating solution in accordance with the present invention contains at least one trivalent chromium salt for electroplating a chromium coating layer on a workpiece. By using the low toxic trivalent chromium to substitute highly toxic hexavalent chromium, an electroplating process of the present trivalent chromium electroplating solution has less pollution.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to an electroplating bath contained with low toxic trivalent chromium ions to obtain chromium deposit through electroplating.
2. Description of the Prior Art
Because of high hardness, excellent wear and oxidation resistance, the chromium deposit is commonly used in industrial applications. Nowadays, the chromium deposit is mainly obtained from the plating bath containing hexavalent chromium ion which is highly toxic in nature. Thus, to develop alternative surface coatings for replacement of hard and decorative chromium deposits has been much attracted in recent years. The plating bath with low toxic trivalent chromium ions could be a suitable alternative electroplating method for obtaining chromium deposit. However, lack of stability and difficult to obtain thick chromium deposit are presently the main drawbacks for trivalent chromium electroplating. In this invention, a stable trivalent chromium electroplating bath and its electroplating process will be stressed.

SUMMARY OF THE INVENTION

The main objective of this invention is to provide a low toxic trivalent chromium electroplating bath, which can be used to achieve chromium deposit by electroplating with high current efficiency and high electroplating current density; meanwhile, an electroplating process of using the trivalent chromium electroplating solution is provided.
To achieve foregoing main objective, the trivalent chromium electroplating bath comprises an aqueous solution added with trivalent chromium salt, a complex agent, conductive salt, a buffering agent, and an additive, wherein the additive is a mixture of ammonium bromide, sodium bromide, and potassium bromide.
The electroplating process comprises the steps of: (a) degreasing the workpiece; (b) washing the workpiece; (c) surface activating to enhance the binding ability between the surface and workpiece; (d) trivalent chromium electroplating on the workpiece; and (e) post drying.
Benefits and advantages of the present invention will become apparent after a careful reading of the detailed description with appropriate reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 demonstrates the flow chart of an electroplating process using the trivalent chromium electroplating solution in accordance with the present invention;

FIG. 2 shows a SEM (Scanning Electron Microscopy) micrograph of a chromium layer on steel surface by using above-mentioned electroplating process with the trivalent chromium electroplating solution;

FIG. 3 is an EDS (Energy Dispersive X-ray Spectrometer) spectrum of the chromium layer obtained from the electroplating process with the trivalent chromium electroplating bath;

FIG. 4 is a SEM micrograph of cross-sectional chromium layer obtained from the electroplating process with the trivalent chromium electroplating solution; and

FIG. 5 presents the figure showing the current efficiency of the trivalent chromium electroplating at different temperatures and electroplating current densities.

DETAILED DESCRIPTION OF THE INVENTION

A trivalent chromium electroplating solution in accordance with the present invention contains at least one trivalent chromium salt for electroplating a chromium deposit on a workpiece. To obtain the chromium deposit on a workpiece, electroplating is conducted with the bath containing low toxic trivalent chromium ions to substitute the bath with highly toxic hexavalent chromium ions. Therefore, the electroplating process of the present trivalent chromium electroplating bath is an environmental friendly method.
Owing to its high hardness, superior oxidation and wear resistance, chromium coating is widely used for surface finish of precision mechanical parts, mould, and surgical tool. According to the thickness, two types of chromium deposits xould be classified. The first type is decorative chromium deposit and the other type is hard chromium deposit. The decorative chromium deposit usually has a thickness of about 0.1-0.5 μm. The decorative chromium is mainly used for decoration and protection because its varnish appearance. The hard chromium deposit has a thickness higher than 1 μm and is used for wear and corrosion resistance. The trivalent chromium electroplating bath and its electroplating process in the present invention can be applied to obtain both the decorative and hard chromium deposits through electroplating.
The trivalent chromium electroplating solution substantially comprises:
(a) trivalent chromium salt: 0.2-1.4 mole/L of trivalent chromium salt that is selected from the group consisting of chromium chloride, chromium sulfate and hydrates of the foregoing components;
(b) complex agent: 0.2-4.2 mole/L of the complex agent that is selected from the group consisting of urea (carbamide), glycine (aminoacetic acid), hydroxyacetic acid, formic acid, dissoluble salts of acids, and dissoluble salts of urea;
(c) conductive salt: 1.0-3.0 mole/L of the conductive salt component that is a mixture having at least two components selected from the group consisting of ammonium chloride, sodium chloride, potassium chloride, magnesium chloride, ammonium sulfate, sodium sulfate, potassium sulfate, and magnesium sulfate;
(d) buffering agent: 0.1-0.8 mole/L of the buffering agent that is selected from the group consisting of boric acid, aluminum chloride, aluminum sulfate, and hydrates of foregoing components; and
(e) additive: 0.01-0.25 mole/L of the additive that is selected from the group consisting of ammonium bromide, sodium bromide, potassium bromide, and a mixture of the foregoing components.
According to above description, some preferred compositions of the trivalent chromium electroplating solution are shown in the following table:


	component

		Complex	Conductive
Num.	Cr⁺³	agent	salt composition	Buffering agent	Addative

1	1.2 mole/L	4.8 mole/L	1.5 mole/L	0.8 mole/L	0.3 mole/L
2	0.1 mole/L	0.4 mole/L	1.0 mole/L	0.4 mole/L	0.05 mole/L
3	0.2 mole/L	2.4 mole/L	2.0 mole/L	0.6 mole/L	0.25 mole/L
4	0.8 mole/L	0.4 mole/L	1.0 mole/L	0.5 mole/L	0.05 mole/L
5	0.6 mole/L	4.0 mole/L	1.2 mole/L	0.8 mole/L	0.2 mole/L
6	0.8 mole/L	2.4 mole/L	1.5 mole/L	0.8 mole/L	0.1 mole/L
7	1.1 mole/L	3.8 mole/L	2.4 mole/L	0.8 mole/L	0.3 mole/L
8	0.6 mole/L	1.2 mole/L	1.2 mole/L	0.6 mole/L	0.05 mole/L
9	0.5 mole/L	2.4 mole/L	1.8 mole/L	0.8 mole/L	0.1 mole/L
10	1.0 mole/L	1.8 mole/L	1.5 mole/L	0.6 mole/L	0.2 mole/L
11	0.9 mole/L	0.5 mole/L	0.8 mole/L	0.4 mole/L	0.15 mole/L
12	0.4 mole/L	0.8 mole/L	1.2 mole/L	0.7 mole/L	0.05 mole/L
13	1.6 mole/L	5.4 mole/L	1.2 mole/L	0.8 mole/L	0.3 mole/L
14	0.7 mole/L	2.4 mole/L	0.8 mole/L	0.3 mole/L	0.15 mole/L
15	0.5 mole/L	1.05 mole/L	1.0 mole/L	0.8 mole/L	0.1 mole/L
16	1.4 mole/L	4.2 mole/L	3.0 mole/L	0.8 mole/L	0.2 mole/L
17	0.6 mole/L	3.6 mole/L	2.1 mole/L	0.8 mole/L	0.1 mole/L
18	0.3 mole/L	0.6 mole/L	1.5 mole/L	0.7 mole/L	0.05 mole/L
19	1.5 mole/L	1.5 mole/L	1.5 mole/L	0.8 mole/L	0.1 mole/L
20	1.1 mole/L	3.0 mole/L	2.4 mole/L	0.7 mole/L	0.25 mole/L
21	0.7 mole/L	1.8 mole/L	1.8 mole/L	0.8 mole/L	0.15 mole/L
22	1.3 mole/L	0.8 mole/L	1.0 mole/L	0.5 mole/L	0.25 mole/L
23	0.4 mole/L	2.4 mole/L	1.8 mole/L	0.6 mole/L	0.1 mole/L
24	0.9 mole/L	1.35 mole/L	1.2 mole/L	0.8 mole/L	0.15 mole/L
25	1.0 mole/L	4.0 mole/L	2.4 mole/L	0.6 mole/L	0.1 mole/L

Compositions of the trivalent chromium electroplating solution in number of 6, 8, 10, 12, 15, 18, 21 and 24 in the table are mostly preferred.
With reference to FIG. 1, an electroplating process is demonstrated and adopted to electroplate a chromium deposit on a workpiece, for example harden high-carbon tool steel, by using the trivalent chromium electroplating solution in the present invention. The electroplating process comprises steps of:
(a). Degreasing:
The prepared workpiece is degreased with a degreasing agent to remove oil and dirt from its surface.
(b). Washing:
The prepared workpiece is washed to remove the degreasing agent and to keep the surface clean.
(c) Surface Activating:
The workpiece is dipped into acid or alkaline solution to activate the surface by adding oxidant or providing electricity so as to enhance the binding efficiency between the surface of the workpiece and the chromium deposit.
(d) Trivalent Chromium Electroplating:
The workpiece, auxiliary electrodes, and the trivalent chromium electroplating solution are set in a cell tank. A fixed current is applied across the workpiece and the auxiliary electrodes by an additional power supply to start electroplating operation. No extra membrane, for example Nafion, is necessary to separate electroplating solutions between cathode and anode.
(e) Drying:
After electroplating, residual electroplating solution is removed from the surface of the workpiece by clean agent dipping and hot-air blaster drying.
Wherein, the auxiliary electrodes are made of material selected from the group comprising with the platinized Ti mesh or plate, platinum, graphite and stainless steel.
Wherein, the fixed current provided by the additional power supply has a range from 5 to 95 ampere per square decimeter.
Wherein, the temperature for the trivalent chromium electroplating on the workpiece is in the range of 1 to 60° C.
As presented in FIG. 1, a preferred flowchart of an electroplating process to achieve the trivalent chromium deposit is shown. Before electroplating of trivalent chromium, the workpiece is prepared (10), degreased (11) and washed (12) to make its surface clean. Then, the surface of the workpiece is activated (13) and washed (14) again. After inputting the workpiece, auxiliary electrodes, and the trivalent chromium electroplating solution into a tank, electroplating is mechanically stirred (15). By applying a fixed current from an additional power supply, the ions of chromium complex obtain electrons to reduce and deposit on the surface of the workpiece so that the workpiece obtains a uniform chromium deposit. After electroplating, the workpiece is washed (16) again and dried (17) to obtain the final product.
With reference to FIGS. 2, 3 and 4, the final product is observed by Scanning Electronic Microscopy (SEM) and Energy Dispersive X-ray Spectrometer (EDS) to exam the surface and a cross-section of the deposited workpiece. According to those figures, the workpiece indeed obtains a uniform chromium deposit on the surface thereof.
With reference to FIG. 5, the graph shows that deposition rate of the chromium coating layer is influenced by the temperature and the plating current density.
In summary, the trivalent chromium electroplating bath is an aqueous solution added with a trivalent chromium salt, a complex agent, a conductive salt, a buffering agent, and an additive, wherein the additive is a mixture of ammonium bromide, sodium bromide or potassium bromide.
Wherein, the additive is of 0.01 to 0.25 mole/L in the trivalent chromium electroplating solution.
Wherein, the buffering agent is selected from the group consisting of boric acid, aluminum salt, dissolvable salt of foregoing components, a mixture of at least two of the foregoing components, and the dissolvable salt.
Wherein, boric acid and/or the dissolvable salt are of 0.1-0.8 mole/L in the trivalent chromium electroplating solution.
Wherein, the aluminum salt is selected from the group consisting of aluminum chloride, aluminum sulfate, and hydrates of the foregoing components.
Wherein, the aluminum salt and/or the hydrates of the aluminum salt are of 0.1-0.6 mole/L in the trivalent chromium electroplating solution.
Wherein, the complex agent is selected from the group consisting of urea (carbamide), glycine (aminoacetic acid), formic acid, dissoluble salts of urea, and dissoluble salts of acids.
Wherein, urea and/or its dissoluble salt are of 0.2-4.2 mole/L in the trivalent chromium electroplating solution.
Wherein, glycine and/or its dissoluble salt are of 0.1-2.8 mole/L in the trivalent chromium electroplating solution.
Wherein, hydroxyacetic acid and/or its dissoluble salt are of 0.1-2.8 mole/L in the trivalent chromium electroplating solution.
Wherein, formic acid and/or its dissoluble salt are of 0.1-1.4 mole/L in the trivalent chromium electroplating solution.
Wherein, the conductive salt composition is a mixture having at least two components selected from the group consisting of ammonium chloride, sodium chloride, potassium chloride, magnesium chloride, ammonium sulfate, sodium sulfate, potassium sulfate, and magnesium sulfate.
Wherein, the conductive salt composition is of 1.0-3.0 mole/L in the trivalent chromium electroplating solution.
Wherein, the trivalent chromium salt is selected from the group consisting of chromium chloride, chromium sulfate, and hydrates of the foregoing components.
Wherein, the trivalent chromium salt and/or its hydrate are of 0.2-1.4 mole/L in the trivalent chromium electroplating solution.
Although the invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed.

Claims

1. A trivalent chromium electroplating solution comprising an aqueous solution added with trivalent chromium salt, a complex agent, conductive salt, a buffering agent, and an additive,

wherein the additive is a mixture of ammonium bromide, sodium bromide, and potassium bromide.

2. The trivalent chromium electroplating solution as claimed in claim 1, wherein the additive is of 0.01 to 0.25 mole/L in the trivalent chromium electroplating solution.

3. The trivalent chromium electroplating solution as claimed in claim 1, wherein the buffering agent is a mixture comprising boric acid, aluminum salt, dissolvable salts of boric acid, and aluminum salt; and

the boric acid and the dissolvable salt are of 0.1-0.8 mole/L in the trivalent chromium electroplating solution.

4. The trivalent chromium electroplating solution as claimed in claim 3, wherein the aluminum salt is selected from the group consisting of aluminum chloride, aluminum sulfate, and hydrates of aluminum chloride and aluminum sulfate; and

the aluminum salt and the hydrates of the aluminum chloride and aluminum sulfate are of 0.1-0.6 mole/L in the trivalent chromium electroplating solution.

5. The trivalent chromium electroplating solution as claimed in claim 1, wherein the complex agent is selected from the group consisting of urea (carbamide), glycine (aminoacetic acid), formic acid, dissoluble salts of acids, and dissoluble salts of urea.

6. The trivalent chromium electroplating solution as claimed in claim 5, wherein the urea and a dissoluble salt of the urea are of 0.2-4.2 mole/L in the trivalent chromium electroplating solution.

7. The trivalent chromium electroplating solution as claimed in claim 5, wherein the glycine and a dissoluble salt of glycine are of 0.1-2.8 mole/L in the trivalent chromium electroplating solution.

8. The trivalent chromium electroplating solution as claimed in claim 5, wherein hydroxyacetic acid and/or a dissoluble salt of hydroxyacetic acid are of 0.1-2.8 mole/L in the trivalent chromium electroplating solution.

9. The trivalent chromium electroplating solution as claimed in claim 5, wherein the formic acid and a dissoluble salt of formic acid are of 0.1-1.4 mole/L in the trivalent chromium electroplating solution.

10. The trivalent chromium electroplating solution as claimed in claim 1, wherein the conductive salt is a mixture having at least two components selected from the group consisting of ammonium chloride, sodium chloride, potassium chloride, magnesium chloride, ammonium sulfate, sodium sulfate, potassium sulfate, and magnesium sulfate.

11. The trivalent chromium electroplating solution as claimed in claim 10, wherein the conductive salt is of 1.0-3.0 mole/L in the trivalent chromium electroplating solution.

12. The trivalent chromium electroplating solution as claimed in claim 1, wherein the trivalent chromium salt is selected from the group consisting of chromium chloride, chromium sulfate, and hydrates of the foregoing components; and

the trivalent chromium salt and a hydrate of the trivalent chromium salt are of 0.2-1.4 mole/L in the trivalent chromium electroplating solution.

13. An electroplating process for using a trivalent chromium electroplating solution comprising:

(a) degreasing, wherein a prepared workpiece is degreased with a degreasing agent to remove oil and dirt from the surface of the workpiece;

(b) washing, wherein the prepared workpiece is washed to remove the degreasing agent and to keep the cleaned surface of the workpiece;

(c) surface activating, wherein the workpiece is dipped into one of acid and alkaline solution to activate the surface by adding oxidant or providing electricity so as to enhance the binding efficiency of the surface of the workpiece;

(d) trivalent chromium electroplating, wherein the workpiece, auxiliary electrodes, and the trivalent chromium electroplating solution are set in a tank and a fixed current is applied across between the workpiece and auxiliary electrodes by an additional power supply to start electroplating operation; and

(e) drying.

14. The electroplating process as claimed in claim 13, wherein the auxiliary electrodes are made of material selected from the group comprising plantized titanium mesh, titanium plate, platinum, graphite, and stainless steel.

15. The electroplating process as claimed in claim 13, wherein the fixed current provided by the additional power supply has a range from 5 to 95 ampere per square decimeter.

16. The operational method as claimed in claim 13, wherein the temperature for a surface treatment of the workpiece is in a range of 1 to 60° C.

17. The electroplating process as claimed in claim 13, wherein the trivalent chromium electroplating solution comprises an aqua solution added with trivalent chromium salt, a complex agent, conductive salt, a buffering agent, and an additive,

wherein the additive is a mixture of ammonium bromide, sodium bromide and potassium bromide.

18. The electroplating process as claimed in claim 17, wherein the trivalent chromium salt is selected from the group consisting of chromium chloride, chromium sulfate, and hydrates of the foregoing components; and

19. The trivalent chromium electroplating solution as claimed in claim 1, wherein the buffering agent is a mixture comprising boric acid, aluminum salt, dissolvable salts of boric acid and aluminum salt; and

one of the boric acid and the dissolvable salt is of 0.1-0.8 mole/L in the trivalent chromium electroplating solution.

20. The trivalent chromium electroplating solution as claimed in claim 3, wherein the aluminum salt is selected from the group consisting of aluminum chloride, aluminum sulfate, and hydrates of aluminum chloride and aluminum sulfate; and

one of the aluminum salt and the hydrates of the aluminum chloride and aluminum sulfate is of 0.1-0.6 mole/L in the trivalent chromium electroplating solution.

21. The trivalent chromium electroplating solution as claimed in claim 5, wherein one of the urea and a dissoluble salt of the urea is of 0.2-4.2 mole/L in the trivalent chromium electroplating solution.

22. The trivalent chromium electroplating solution as claimed in claim 5, wherein one of the glycine and a dissoluble salt of glycine is of 0.1-2.8 mole/L in the trivalent chromium electroplating solution.

23. The trivalent chromium electroplating solution as claimed in claim 5, wherein one of the formic acid and a dissoluble salt of formic acid is of 0.1-1.4 mole/L in the trivalent chromium electroplating solution.

24. The trivalent chromium electroplating solution as claimed in claim 1, wherein the trivalent chromium salt is selected from the group consisting of chromium chloride, chromium sulfate, and hydrates of the foregoing components;

one of the trivalent chromium salt and a hydrate of the trivalent chromium salt is of 0.2-1.4 mole/L in the trivalent chromium electroplating solution.

25. The electroplating process as claimed in claim 17, wherein the trivalent chromium salt is selected from the group consisting of chromium chloride, chromium sulfate, and hydrates of the foregoing components;