Claims:What is claimed is:

1) A gold electroplating bath, comprising:
indium complex, phosphoric acid, sulphuric acid or hydrochloric acid and trivalent gold cyanide complex [tetracyanoaurate (III)].
2) The gold electroplating bath according to claim 1, wherein the indium is present in the electroplating bath as anamino carboxylic acid or phosphoric acid complex.

3) The gold electroplating bath according to claim 1, further comprising a non-ionic surfactant.

4) The gold electroplating bath according to claim 1, wherein the electroplating bath contains the complexing agents in the range of 0.2-10 g/l.

5) The gold electroplating bath according to claim 1, wherein the electroplating bath contains the phosphoric acid in range of 10-70ml/l.

6) The gold electroplating bath according to claim 1, wherein the electroplating bath contains the sulphuric acid or hydrochloric acid in range of 5-50ml/l.

7) The gold electroplating bath according to claim 1, wherein the electroplating bath contains the trivalent gold cyanide complex [tetracyanoaurate (III)] in range of 0.2-5g/l.

8) The gold electroplating bath according to claim 3, wherein the electroplating bath contains the non-ionic surfactant in range of 0.05-0.5 ml/l.

9) The gold electroplating bath according to claim 1, wherein the temperature of the electroplating bath is maintained between 20-50°C.

10) The gold electroplating bath according to claim 1, wherein the current density of the electroplating bath is maintained between 1-10A/dm2.

11) The gold electroplating bath according to claim 1, wherein the pH of the electroplating bath is maintained between 0.4-0.8.

12) The gold electroplating bath according to claim 1, wherein the rate of deposition of deposits obtained from the electroplating bath 2-5mg/minute.

13) A method for direct gold plating of stainless steel, comprising:
immersing stainless steel in a gold plating solution comprising indium as amino carboxylic complex or phosphoric acid complex, phosphoric acid, sulphuric acid or hydrochloric acid and trivalent gold cyanide complex [tetracyanoaurate (III)]; and
electroplating the stainless steel directly with gold so as to form a coating of gold thereupon.
, Description:FIELD OF INVENTION

[0001] The present invention generally relates to a bath for electroplating of gold, and more particularly to an electroplating bath composition for plating of gold directly onto a stainless steel.

BACKGROUND OF INVENTION

[0002] It has been customary to use gold plated articles in fashion jewelry, watches, cutlery, dental applications and novelty products. Various metal under-coats have been tried to obtain gold plating, excellent in adhesion and appearance. Previously known electroplating solutions used Nickel (Ni) under-coat followed by a coating of gold for electroplating baths. However, the use of Nickel is plagued with problems of biocompatibility and is therefore not considered as an ideal choice in the plating art.
[0003]It is well known that stainless steel containing nickel, chrome or iron as alloy composition, forms a tough oxide layer which prevents corrosion of stainless steel. When adherent gold plating over the tough oxide layer has to be achieved, the regular plating baths are incapable of breaking the oxide layer on the surface of the stainless steel and plate an adherent layer.
[0004] Conventionally known method of Wood’s nickel strike has been historically used for gold plating on the stainless steel. For gold plating of stainless steel with the aforementioned method, a solution is formed by mixing hydrochloric acid solution with nickel chloride at a temperature of 30°C by using a nickel anode and a current density of about 2A/dm2 for 4-5 minutes. The major advantage of the solution is that during the above nickel acid strike, the oxide layers are broken due to cathodic reaction and subsequently, a gold plating excellent in adhesion and uniformity is obtained. Thus, a direct gold plating electrolyte to plate on stainless steel material was developed using the following formulation.
[0005] Accordingly, ideal conditions for direct gold plating of a stainless steel were identified. A solution comprising a mixture of gold (III) potassium cyanide, an acid selected from any of the hydrochloric acid, sulphuric acid and the phosphoric acid, a complexing agent and nickel or cobalt complex at a temperature of 30-40°C, having current density of about 2A/dm2 for about 3-5 minutes was prepared. Amidst existing gold plating practices, a conventional practice was to employ a low pH phosphate bath containing trivalent gold cyanide complex and cobalt or nickel.
[0006] The attempts were also made in recent past to use the gold cyanide complex alone, specifically in absence of cobalt or nickel to minimize the use of alloying metals which are not bio-compatible. However, while this bath appeared ostensibly satisfactory for plating operation, there were some defects which lessened its applicability. The resulting solution deprived of nickel cobalt complex rendered the solution with gradual deposits of brown instead of desired yellow, thereby adversely affecting the appealability of the deposits. Besides, in such a bath the gold gets coated on the anode, in an event the bath is idling. Hence, for the reasons stated, the use of nickel and cobalt in the trivalent gold bath became quiet indispensable.
[0007] It shall, however be understood that although it seems necessary, the use of nickel and cobalt is not desirable for gold plating, particularly for cutlery, fashion jewelry and dental application because of the biocompatibility issues of the two metal alloys. Moreover, the adverse effects are cumulative and characterized by reduced overall brightness range and adherence.
[0008] In view of the challenges described above, it was proposed to replace cobalt and nickel with copper in recent past. The proposition however witnessed difficulties in terms of instability of the bath solution, variability in the color of the deposits from the solution, including changes in deposit composition with aging. While, the bath was not observed to be very stable, the deposits were found to be missing the desired yellow color, which necessitated the coat of gold replenished from a second bath. The process obviously increased the cost of operation and still did not produce the satisfactory formulation.
[0009] In the background of foregoing limitations, there exists a need for a gold electroplating solution that can be directly applied onto stainless steel without requiring any further under-coat and any nickel or cobalt as alloying metals.
OBJECT OF THE INVENTION

[0010] The primary object of the present disclosure is to provide an electroplating bath composition for gold plating.
[0011] Another object of this disclosure is to provide a gold plating bath for plating gold directly onto stainless steel.
[0012] Yet another object of the disclosure is to provide a stable electroplating bath composition for gold plating directly on the stainless steel.
[0013] Yet other object of the present disclosure is to provide an improved electroplating bath composition for gold plating that yields deposits having a bright yellow appearance, ultrafine texture thereby obviating the need for a re-coat from a subsequent bath.
[0014] In yet another embodiment, the disclosure provides an electroplating bath composition for gold plating, which is excellent in its adhesion onto the stainless steel, have a bright appearance, thereby eliminating the foregoing defects of the conventional gold-plating techniques.
[0015] Still another embodiment of present disclosure provides gold plated stainless steel exhibiting aesthetically pleasing yellow color, necessary tarnish and corrosion resistance, and wearability that make it suitable for application in dental restorations, jewelry items and not limiting to cutlery.

[0016] These and other objects will become apparent from the ensuing description of the present invention.

SUMMARY OF THE INVENTION

[0017] The present invention is directed to a gold electroplating bath for plating gold directly onto stainless steel. In one embodiment, the electroplating bath comprises an indium complex, phosphoric acid, sulphuric acid or hydrochloric acid and trivalent gold cyanide complex [tetracyanoaurate (III)], wherein the indium is present in the electroplating bath as an amino carboxylic acid or phosphoric acid complex.

[0018] In another embodiment, an economically viable method for direct gold plating of stainless steel is provided, the method comprising steps of immersing stainless steel in a gold plating solution comprising indium as amino carboxylic complex or phosphoric acid complex, phosphoric acid, sulphuric acid or hydrochloric acid and trivalent gold cyanide complex [tetracyanoaurate (III)], and electroplating the stainless steel directly with gold so as to form a coating of gold thereupon.
[0019] These and other aspects, features and advantages of the present invention will be described or become apparent from the following detailed description of preferred embodiments.
DETAILED DESCRIPTION OF THE EMBODIMENTS

[0020] Before the present gold electroplating bath is described, it is to be understood that this disclosure is not limited to the particular composition and process for preparing the composition, as described, since it may vary within the specification indicated. It is also to be understood that the terminology used in the description is for the purpose of describing the particular versions or embodiments only, and is not intended to limit the scope of the present invention, which will be limited only by the appended claims. The words "comprising," "having," "containing," and "including," and other forms thereof, are intended to be equivalent in meaning and be open ended in that an item or items following any one of these words is not meant to be an exhaustive listing of such item or items, or meant to be limited to only the listed item or items. The disclosed embodiments are merely exemplary methods of the invention, which may be embodied in various forms.
[0021] In accordance with the first embodiment, an electroplating bath for gold plating directly onto a stainless steel is disclosed. The gold electroplating bath comprises use of an indium complex, at least one of sulphuric acid or hydrochloric acid, phosphoric acid and a trivalent gold cyanide complex [tetracyanoaurate (III)].
[0022] Further, in one other embodiment, the electroplating bath may comprise at least one non-ionic surfactant for prevailing acidic conditions in the bath. For example, CRAFOL®or any other non-ionic surfactant can be used for acidic conditions.
[0023] In one preferred embodiment, indium is present in the form of an amino carboxylic acid or phosphoric acid complex.
[0024] The electroplating bath has a composition such that the indium complex existing as an amino carboxylic acid or phosphoric acid complex is contained in an amount of 0.1-2g/l, the phosphoric acid is contained in an amount of 10-70ml/l, at least one of sulphuric acid or hydrochloric acid is contained in amount of 5-50ml/l and the trivalent gold cyanide complex [tetracyanoaurate (III)] is contained in an amount ranging from 0.2-5g/l. In addition the non ionic surfactant is contained in an amount of 0.05-0.5ml/l.
[0025] The disclosure further provides a method for direct gold plating of stainless steel that is employed under working conditions of a temperature ranging between 20-50°C, a pH of 0.4-0.8 and current density maintained between 1-10A/dm2. Under the given working conditions, particularly in “high speed” plating bath, the deposition is observed at a rate of 2-5mg/minute. The “high speed” used herein is intended to mean that a Current Density of almost 10A/dm2 may be employed. Further, the deposits obtained under these conditions have a bright yellow color of approximately 2N on the NIHS scale in accordance with ISO standard 8654. Furthermore, the hardness of deposits can be assessed by Vickers hardness test, which has been measured upto 220 VPN.
[0026] In one preferred embodiment, by using specific concentrations of phosphoric acid, at least one of sulphuric acid or hydrochloric acid, a non ionic or cationic surface active reagent, indium complex, trivalent gold cyanide complex [tetracyanoaurate (III)] under ideal working conditions, as stated above, a synergistic effect of activating the surface of steel and directly plating it with gold without any undercoat can be successfully achieved.
[0027] The aforesaid method for activation of surface of stainless steel can be carried out by immersing the stainless steel in a gold plating solution comprising indium as amino carboxylic complex or phosphoric acid complex, phosphoric acid, sulphuric acid or hydrochloric acid and trivalent gold cyanide complex [tetracyanoaurate (III)] at temperatures maintained between 20-50°C.The step is then followed by electroplating the stainless steel directly with gold so as to form a coating of gold thereupon. Thus, there is no requirement for any further under-coat to be provided onto the surface of stainless steel before applying the gold coat, thereby making the process economically viable and biocompatible.
[0028] It was found that the gold plated stainless steel emanating out of such an electroplating bath composition exhibit excellent adhesion to the underlying surface and uniformity in deposition thereupon. The sample was subjected to bend test to be examined for porosity and cracking, and was found to suitably pass the bend test. The deposits contain approximately 2-3% of indium that provides improved surface lubricity.
[0029] The gold coating formed upon the stainless steel exhibited thickness greater than 1µm, thus making the bath suitable for use as a strike bath as well as a plating bath of thickness upto4µm. The thickness of gold film achieved protects the surface of stainless steel from corrosion of the texture, maintaining the structural integrity of the underlying steel. Thus, the gold plated stainless steel successfully passes the sulphur dioxide corrosion test and results in superior corrosion protection.
[0030] Another significant advantage of the gold electroplating method discussed above is that the bath precludes the use of metallic, semi-metallic or organic compounds as brightener additives to achieve the desired bright and lustrous electrodeposits. In addition, the resistance of electrodeposits to abrasion was measured by Turbola test that yielded controlled results.
[0031] As will be apparent from the foregoing description above, according to the present disclosure, direct gold electroplating of a stainless steel, which has been difficult by the conventional techniques, can advantageously be accomplished by using the above-mentioned specific bath composition, and a gold-plated stainless steel material excellent in various properties such as gloss, adhesion, ductility, strength, hardness, electric conductivity and corrosion & abrasion resistance can be provided.
[0032] The gold-plated stainless steel products prepared by the process of the present disclosure possess an aesthetically pleasing yellow color. Moreover, the alloys according to the present disclosure possess suitable tarnish and corrosion resistance, as well as sufficient strength and wearability for being able to plate onto intricate shapes, enable them to be used in dental restorations and/or jewelry items.
[0033] The foregoing description is a specific embodiment of the present disclosure. It should be appreciated that this embodiment is described for purpose of illustration only, and that numerous alterations and modifications may be practiced by those skilled in the art without departing from the spirit and scope of the invention. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof.