Patent Application
20170306515
The present invention generally relates to a composition and method for producing dark coloured chromium coatings by electrodeposition.
Chromium has been used for many years as a decorative coating and has many applications. For decorative purposes, the chromium is generally applied as a thin coating, which is typically less than 1 micrometre in thickness, over a coating of nickel. The chromium provides a hard, wear resistant layer and excellent corrosion performance is also obtained due to the chromium layer being cathodic with respect to the underlying nickel deposit. Thus the underlying nickel becomes the anode in the corrosion cell and corrodes preferentially leaving the chromium layer uncorroded.
Typically, these thin decorative chromium layers have been applied by electrodeposition from electrolytes based on hexavalent chromium, which typically comprise chromic acid. The chromium deposits obtained from these electrolytes are essentially pure chromium and have a uniform and invariant colour. A thin oxide layer forms on the top of the coatings giving a blue/white appearance which is very well known. In addition to the incentive to use alternative electrolytes due to serious health and environmental hazards associated with chromic acid, there is also a market demand for coatings having a darker hue.
An initial solution to providing a darker hued deposit from chromium electrolytes can be obtained by electrodepositing the chromium coatings from electrolytes based on trivalent chromium. Due to the nature of the deposition mechanism from these electrolytes, the chromium coating produced is less pure than that produced from hexavalent electrolytes. This is due to co-deposition of other elements within the coating. Most commonly, these co-deposited elements are iron, sulphur and carbon or combinations thereof. By adjusting the electrolyte formulation of trivalent chromium based processes to maximize the darkness of the deposit produced by the incorporation of these co-deposited elements, coatings of a fairly dark hue can be obtained.
While coatings provided by trivalent chromium electrolytes typically produce darker hued deposits than that of hexavalent chromium electrolytes, the resulting coatings from the prior art are still not dark enough to fulfill the needs of the market and a demand exists to produce darker chromium based coatings. It is the object of this invention to provide a means of producing these coatings.
It is an object of the invention to provide a trivalent chromium plating electrolyte that is capable of providing dark colored chromium deposits on a substrate.
It is an object of the invention to provide amino acids in the trivalent chromium electrolyte that are capable of creating a dark hue in the resulting trivalent chromium deposit.
It is another object of the invention to provide amino acids in the trivalent chromium electrolyte that have nitrogen containing side chains.
It is another object of the invention to provide amino acids that do not contain sulfur in the trivalent chromium electrolyte to provide darker hues of chromium deposits on substrates.
It is yet another object of the invention to provide a darker hued deposit on a substrate using a trivalent chromium electrolyte with select amino acids than the prior art trivalent chromium plating electrolytes can achieve.
It is still another object of the invention to provide a method of plating a trivalent chromium deposit with a dark hue over a nickel deposit.
In one embodiment, a composition is provided for a trivalent chromium electrolyte comprising:
In another embodiment, a method is provided for producing a dark colored chromium deposit on a substrate, comprising the steps of:
The inventors have surprisingly found that the incorporation of select amino acids which comprise nitrogen containing side chains into the trivalent chromium electrolyte results in coatings which are substantially darker than those obtained from the same electrolyte in the absence of these compounds. The amino acids useful in the current invention are additionally at least essentially free of sulfur. By essentially free of sulfur, it is meant that sulfur is not present in any concentration, aside from trace amounts that may occur as contaminants in such compounds. The darkest coatings are obtained when select amino acids are added to electrolytes which have already been optimised to produce dark coatings by the incorporation of other elements such as sulfur, iron, carbon or combinations thereof.
The use of amino acids as complexants in trivalent chromium plating baths has been described in U.S. Pat. No. 4,107,004 to Ward et al. and U.S. Pat. No. 4,157,945 to Barnes et al., which describe the use of glycine as a complexant. U.S. Pat. No. 4,161,432 to Barclay et al. describes the use of glycine, aspartic acid, arginine and histidine as complexants. U.S. Pat. No. 4,448,648 and U.S. Pat. No. 4,448,649 both to Barclay et al., describe the use of aspartic acid as a complexant. The subject matter of each of these patents of which is herein incorporated by reference in its entirety. These patents are all focused on producing coatings which are light in color and do not provide for changing the color of the deposit produced or variation of deposit color associated with different amino acids.
Amino acids fall into several groups as shown in
The inventors of the present invention have surprisingly found that sulfur-free amino acids, including those selected from the group having nitrogen containing cationic side chains, have the desired effect of darkening the deposit in a uniform manner. The side chains are cationic under the normal pH conditions of most commercially available trivalent chromium electroplating baths (pH 2.5-4.0) due to the presence of a nitrogen-containing (amine) functional group in the side chain.
The amino acids that are useful in the current invention include, for example, arginine, histidine, lysine and combinations thereof. It was found that these amino acids will produce significant darkening of the trivalent chromium deposits. Additionally tryptophan, although not listed under the amino acids containing a positively charged group in
The effective concentration range of the preferred amino acids in the trivalent chromium electrolyte is preferably between about 1 g/L and about 50 and more preferably between about 2 g/L and about 20 g/l. The concentration of amino acids in the trivalent chromium electrolyte is most preferably between about 5 g/L and about 10 g/L.
When electroplating from electrolyte solutions described herein, inert anodes, such as carbon anodes, are typically used. Other inert anodes such as platinized titanium, platinum, iridium oxide coated titanium, or tantalum oxide coated titanium may also be used.
The temperature of the trivalent chromium based electrolyte is in the range of 40° C. to 60° C., most preferably around 50° C. The pH of the electrolyte is in the range from about 2 to about 5, most preferably about 3.5. The current used during plating is in the range of about 1 amp to about 10 amps, most preferably about 4 amps. Agitation is not required during the plating of substrates in the trivalent chromium electrolyte.
As used herein, the term “about” refers to a measurable value such as a parameter, or a concentration or the like and is meant to include variations of +/−15% or less, preferably variations of +/−10% or less, more preferably variations of +/−5% or less, and most preferably variations of +/−0.1% or less from the particularly recited value in so far as such variations are appropriate to carry out the invention as described herein. Furthermore, it is also to be understood that the value to which the modifier “about” refers is itself specifically disclosed herein.
In a preferred embodiment, the substrate comprises nickel deposited on the underlying substrate and the chromium is electroplated on the nickel deposit.
In order to demonstrate the scope of the invention, a Konica Minolta CM2600d spectrophotometer was used to determine the “lightness” values of the various deposits by measuring the L* value according to the L*a*b* colorspace system. The colorspace system gives a quantitative value (L*), which can be used to compare the degree of darkening obtained by the various combinations of amino acid additives. The higher the L* value, the lighter the deposit and the lower the L* value the darker the deposit. An L* value of 0 is black and an L* of 100 is white. Lower L* values are the desire of the current invention.
Dark hued coatings produced by the electrodeposition of trivalent chromium using the electrolytes described herein preferably have an L* value, measured according to an L*a*b* colorspace system, of less than that of typical trivalent chromium deposits that produce light colored coatings and those that have already been maximized for darkness in the resulting deposits.
The trivalent chromium based electrolytes may contain thiocyanate ions. The thiocyanate ions may be present in a concentration anywhere from about 0.2 g/L up to about 5.0 g/L.
The trivalent chromium based electrolytes presented herein are at least substantially free of hexavalent chromium salts, wherein no Cr(VI) ions can be detected in the electrolyte by ordinary measurement techniques.
The trivalent chromium electrolyte comprises a source of trivalent chromium ions, one or more complexants (complexing agents) capable of maintaining the trivalent chromium ions in solution, and the select amino acids as described herein. The amino acids have provided darker hues in the plated deposit compared to the use of the same electrolyte without such amino acids.
The trivalent chromium electrolyte used as a standard is an electrolyte designed to produce light colored chromium deposits. The electrolyte and plating process is based on U.S. Pat. No. 4,473,448 to Deeman. This patent is hereby incorporated by reference in its entirety.
Additionally, inventive examples are also provided based on a trivalent chromium electrolyte solution that has already been formulated to produce dark deposits, based on U.S. Pat. No. 4,161,432 to Barclay et al. The amino acid used in the electrolyte composition is aspartic acid. This reference is hereby incorporated by reference in its entirety.
The following non-limiting examples illustrate the effectiveness of the invention. All of the examples were prepared by electroplating a trivalent chromium deposit onto Hull cell panels. The Hull Cell panels had previously been electroplated with 10 microns of a bright nickel deposit and are then placed in a Hull Cell with the trivalent chromium electrolyte being tested. For the trivalent chromium plating step, the conditions are as shown in Table 1. Conditions for the trivalent chromium plating remained consistent regardless of the composition being tested.
The lightness or L* value of the trivalent chromium deposit was measured at a point on the Hull Cell panel corresponding to a current density of 8 amps per square decimeter in all cases, which is representative of a normal working range for chromium plating.
Table 2 provides the L* values measured using the electrolyte based on U.S. Pat. No. 4,473,448 to Deeman as a Standard (1), with various amino acids added that were not effective darkening agents.
As can be seen from the results set forth in Table 2, none of the amino acids tested produced any significant reduction in the L* values when compared to the trivalent chromium electrolyte without amino acids added. The average reduction in L* value was 1.17%.
Table 3 provides the L* values obtained when the amino acids described herein are added to the electrolyte based on U.S. Pat. No. 4,473,448 to Deeman. The same Standard (1) electrolyte is used as in the comparative example above. As previously noted, this electrolyte typically produces light colored trivalent chromium deposits.
In this case, the average reduction in L* value was 12.7% using the examples of the invention of arginine and histidine. This is a significant difference which can easily be seen by eye. The addition of arginine, histidine or lysine gave uniform color across the entire deposit. The addition of tryptophan gave a dramatic effect but produced very uneven and streaky deposits, which although significantly darker, were commercially unacceptable.
Table 4 gives the L* values obtained when amino acids of the invention are added to the electrolyte based on U.S. Pat. No. 4,161,432 to Barclay et al. This electrolyte was used as the Standard (2) to compare the L* values after the amino acids are added. As previously noted, deposits produced using this electrolyte process were previously formulated to provide dark colored trivalent chromium deposits.
In this case, the average reduction in the L* value was 15.02%. This is a very noticeable difference in color. While the electrolyte based on U.S. Pat. No. 4,161,432 to Barclay et al. naturally produces a dark deposit, it was possible to increase the deposit darkness by the addition of the amino acids as described herein.
As it has been shown by way of the examples presented herein, the inventors have surprisingly found that darker hued deposits can be obtained from trivalent chromium electrolytes that contain the amino acids as described within. The result was unexpected as the effective amino acids did not contain sulfur, but the presence of cationic nitrogen containing side chains allowed for darker hues in the deposits obtained using the trivalent chromium electrolyte of the current invention.
It should also be understood that the following claims are intended to cover all of the generic and specific features of the invention as described herein and all statements of the scope of the invention that as a matter of language might fall there between.
