IL27753A - Electrodeposition of chromium-containing coatings on a conductive metal - Google Patents

Description

27753/2 as*Via nsna *?p ona Electrodeposition of chromium-containing coatinge on a conductive metal DIAMOHD SHAMROCK CORPORATION This Invention relates to a composition and method for the electrodeposltion of chromlum-eontaining coatings on a conductive metal substrate as the eathode and more particularly relates to the electrodeposltion of chromium coatings on such substrates in such physical and chemica form as to produce a black surface having a high degree of absorptivity for both heat and visible light.

Black chromium deposits find use In areas where their heat and ligh absorbing properties are important, one example of which is the military field where such items as firearms, communications equipment, personnel ornament, etc. are so coated* Such deposits are also valuable for decorative purposes such as on metal furniture, automobile parts, plumbing fixtures, etc*, where their corrosion resistance coupled with their appearance make them superior to other black finishes such as paint* While several processes and plating bath compositions have heretofore been disclosed in the art for producing black chromium electrodeposits, they have not received wide commercial acceptance for a variety of reasons. For example in one process, a grey-black deposit is obtained from a chromic acid bath having a low (less than 0.07$) sulfate content to which has been added a small amount of a carboxylie acid, preferably acetic acid* The chief disadvantages of this bath arise from the conditions for electrodeposlting the black chromium whereby very high current densities, i.e., 10,000 to 20,000 amperes per square meter (935 to 1370 amperes per square foot), must be used at relatively low temperatures, i.e., about 20* C. Therefore, in It has also been disclosed that dark grey to black aleetrodeposits may be obtained from a chromic acid bath from which the sulfate has been removed and which contains large amounts of acetic acid. While this bath is said to operate at low current densities, its effective current density range is quite limited, i.e., 40 to 90 amperes per square foot, and this bath also has the further disadvantages that steam coils must be provided to maintain the desired temperatures for plating 90* to 115* P., and that an exhaust fan is required to remove the noxious acetic acid vapors.

In a more recent process* black chromium deposit is obtained from an aqueous bath consisting of chromic acid and a fluoride catalyst. While the current density and temperature ranges recommended are satisfactory, reproduction of good results is still quite difficult to attain because of the eritlcallty of the fluoride catalyst concentration and the necessity accordin to this teaching of eliminating all other catalytic ions from the bath, ¾hus, in addition to precluding the presence of sulfate in the bath, it is also necessary to use delonized or distilled water for bath make-up and pre«bath rinsing procedures in order that n foreign ions be introduced. , Consequently this type of bath is difficult to control and not suitable for large scale commercial applications* It is an object of the present inventio to provide an improved bath composition and easily operable process whereby uniform, black, chromium-containing ©lectrodeposita may be obtained.

It is another object of the present invention to provide an improved bath composition which is highly effective over a wide range of operating conditions for elee rodepositing These and other objects of this invention will become apparent from the specification and claims that follow.

It has now been found that uniform, corrosion resistant, black, chromium-containing coatings may be electrodeposited on an electrically conductive member by making said member the cathode in an aqueous solution consisting essentially of from at least 60 grams per liter up to saturation of chromic acid, a fluoride-containing catalyst in an amount sufficient to supply from about ¾ 0.03 to about 1 gram fluoride in solution per liter and an additive compound selected from the group consisting of an inorganic nitrogen-containing compound in an amount sufficient to supply the equivalent of from about 0.35 to 3.5 grams of (NOg) radical per liter, about 1 to 25 grams of trivalent chromium per liter and mixtures thereof, said aqueous solution being free of sulfate ions, and passing a direct current between said cathode and an anode immersed in said solution at a current density of from about 30 to 1500 amperes per square foot while maintaining the solution at a temperature of from about 60° to l 0°F.

While solutions containing chromic acid and a fluoride-containing catalyst have previously been used to obtain black, chromium-containing electrodeposits, it has now been found that the inclusion of relatively small amounts of an additive compound selected from the group consisting of an inorganic nitrogen-containing compound, trivalent chromium and mixtures thereof affords substantial advantages. With the use of such additive compounds a uniform, black, chromium-containing electrodeposit is readily obtained over a wide range of current densities. The deposit itself is also improved in appearance in that it is darker in color and has a glossier finish than that obtained Any commercially Available grade of chromic anhydride (CrOg) may be used in the practice of this invention, but since commercial chromic anhydride generally contains significant quantities of sulfates which interfere with the formation of the desired black, chromium-containing deposits* it is necessary that the chromic acid solutions described herein be treated before use to remove these sulfates. This treatment may be easily effected by the addition to the solution of a source of barium ion such as barium carbonate or barium oxide* The addition of 5 to 20 grama per liter of barium carbonate will generally provide a sufficient excess to insure a sulfate-free plating solution* The amount of chromic anhydride used may be within the range of from at least 60 grans per liter up to saturation* preferably about 300 to 500 grams per liter* Particularly preferred at this time is a concentration of 50 grams per liter* Any of the fluoride-containing chromium plating catalysts known to the art which supply fluoride in solutions of chromic acid may be used in the bath compositions of this invention. Examples of these catalysts are hydrofluoric acid* fluoborlc acid* fluosilicic acid and water soluble alkali metal, alkaline earth metal, heavy metal and ammonium salts thereof* Preferred fluoride-containing catalysts are those obtained by reacting a hexavalent chromium compound such as chromic acid, an organic reducing agent such as sucrose and a fluoride-silicon compound such as fluosilicic acid as described In If. S. Patent Ho. 2,84l,50. The amounts of fluoride-containing catalyst useful in this invention will var according to the amount of fluoride which the catalyst can supply to the chromic acid solution, and the amount of fluoride in solution may be varied from about 0.03 to about 1 gram per liter, preferably from The inorganic nitrogen-containing compounds effective in the solutions of the present invention are nitric acid and nitrous acid and the alkali metal, alkaline earth metal and ammonium salts thereof, nitric acid and sodium nitrate being preferred at present by reason of their ready availability and comparatively low price. These inorganic nitrogen-containing compounds are effective for the purposes of the present invention in amounts sufficient to supply the -equivalent of about 0.35 to about 3-5 grams of (N02) radical per liter, preferably about 0.7 to 2 grams of (N02) radical per liter.

While amounts in excess of 3.5 grams per liter (NOg) may be used, they are without apparent additional beneficial effect.

Thus, by way of example, if sodium nitrite is used in the practice of this invention amounts within the range of from about 0.52 to about 5.2 grams per liter, preferably 1 to 3 grams per liter will be effective; if sodium nitrate is used amounts within the range of from about 0.65 to about 6.5 grams per liter, preferably 1.3 to 3.7 grams per liter will be effective.

The manner of providing the trivalent chromium in the solutions of this invention is not critical. Conveniently, this may be accomplished by the addition of a reducing agent, such as sucrose or hydrogen peroxide, to the solution prior to its use. If sucrose is to be used as the reducing agent, about 1 gram of sucrose is required for each 2.2 grams of trivalent chromium to be formed at a chromic acid concentration of,fifcout 50 grams per liter. As stated hereinabove, 0amounts in the range of from about 1 to about 25 grams of trivalent chromium per liter of solution are effective in the practice of this invention. Especially preferred at this time are amounts within the range of about 4 to about 10 grams per liter. In addition to the above described use of a reducing agent to obtain the desired trlvalent chromium concentration it is also possible to obtain same by electrolysis of the solution in a special manner. This "pre-electrolysis" is carried out under the normal operating conditions of the bath but using a cathode to anode surface area ratio of greater than 1:1, e.g. 20:1. The amount of trlvalent chromium present in the bath may be readily determined for purposes of addition and control by any of the known analytical techniques.

The plating solutions disclosed herein for electro-depositing black, chromium-containing coatings are easier to control than prior art solutions in that it is not necessary to use deionized or distilled water in either the make-up of the solutions or in the rinsing steps prior immersion of the article to be coated in the solution. The solutions may be used effectively in producing electrodeposits for long periods of time without detrimental effects due to decomposition products and employing only the precautions used in the operation of conventional :chromium plating solutions to prevent the introduction of foreign ions.

Uniform, black, chromium-containing deposits; are readily obtained over a current density range of about 35 to 1500 amperes per square foot. The temperature of the bath may be maintained within the range from about 60° to l 0° P., preferably from about 70° to 95° thereby eliminating the need for either heating or cooling means in most cases.

While the time required to electrodeposit the black, chromium-containing coatings will vary somewhat depending on the solution composition, temperature, and the thickness of coating desired, 1 to 5 minutes at a current density within the above mentioned range will generally give a uniform, black, Any of the insoluble anodes used with conventional chromium plating baths may be employed. Especially to be preferred are lead or lead alloy anodes. Corrosion and erosion of these lead or lead alloy anodes when used in the practice of this invention is on a level equivalent with that experienced in conventional chromium plating solutions . Tanks employed for containing the solutions of the present invention may be lined with any suitable corrosion resistant material such as glass, ceramiccma:t.<-trlal; ,ρρ1;γ*η3ϋ- chJbdri-de.,-: , le.addandctbfeolIR&c .

While the best results are obtained by subjecting the:r electrically conductive member to.be plated by the process of this invention to a nickel strike treatment, i.e., cathodically plating nickel on said member for a short period at any conventional current density, quite satisfactory coatings may be obtained by plating directly onto copper, zinc, iron, steel, stainless steel, conventional chromium deposits and other conductive bases.

As "is the practice in conventional chromium plating, mist suppressants may be added to the solution before plating. Many of the... commercially available products for this purpose (which are generally proprietary surfactant-containing compositions) are acceptable.

In order that the compositions and process of this invention may be more readily understood by those skilled in the art, the following specific examples are provided. Unless otherwise, noted, the examples that follow are the result of evaluation of the compositions and process of this invention in a modified 267 milliliter Hull cell using as the cathode brass panels, 2.5 by 4 inches, which have been given a thin, uniform nickel coating by electrodeposition from a commercial cell is a trapezoidal box of non-conductive material in the opposite ends of which are positioned anode and cathode plates as is more particularly described in U, S„ Patent No. 2, 149, 3 . By the use of this device it is possible to easily determine the effective plating range of a plating composition under varying conditions. The current density at any point on the cathode is determined according to the formula A = 0 (27 - 7-48.7 log L) wherein A is the current density at the selected point, C is the total current applied to the cell and L is the distance of the selected point from the high current density end of the plate. In the modified version used herein, holes are provided in the sides of the Hull cell adjacent the anode and cathode whereby, upon immersion of the cell in a container of plating solution, improved electrolyte circulation and consequently improved temperature control is afforded as is more particularly described in an article by J. Branciaroli appearing on page 257., March, 1959 issue of Plating, Vol. 46, No. 3, (a publication of the American Electroplater !s Society, Inc.).

EXAMPLE 1 An aqueous plating solution is made containing 4 0 grams per liter of CrQ^, 1 gram per liter of fluoride-contaih$ng catalyst (the reaction product of a chromium compound, an organic reducing agent and fluosilicic acid as more particularly described in ϋ. S„ Patent No. 2, 84l,540 and containing 25$ by weight chromium, 2 by weight fluoride and 16$ by weight silicon) and 7.5 grams of BaCO^ to precipitate the sulfate. A panel is electroplated for 3 minutes at 10 amperes and a bath temperature of 96° P o A dull black electrodeposit is obtained from the high current density end of the panel to a current density of about To the above bath is added 1.5 ml. of concentrated nitric acid (specific gravity 1.42 gm/ml.). A second panel is now electroplated for 3 minutes at 10 amperes and a temperature of 96° F. The black deposit range is now found to extend to about 40 amperes per square foot or 78 coverage. In addition to the substantially improved plating range the deposit itself is more uniform, is darker and has a glossier appearance than the deposit obtained without the addition of the nitric acid.

EXAMPLES 2-9 A series of Hull cell panels is run as indicated in Table 1. In each instance the solution is treated with an excess of BaCO^ to precipitate the sulfate and the fluoride-containing catalyst is the same as described in Example 1.

The column titled "Range"' indicates the effective plating range obtained, i.e., it indicates that a uniform black deposit is obtained from the extreme high current density. end of the panel to the area on the panel corresponding to the stated value.

TABLE 1 EXCrOc. F-Catalyst HNO ** Temp, Range AMPLE (gram/liter) (gram/liter) (ml/liter) (eF.) (a.s.f 2 4 0 1 1 "90 30 3 450 1 1 95 30 4 4 0 1 1 105 60 450 1 1.5 112 40 6 450 1 1.5 117 40 7 450 1 1.5 120 50 8 450 1 1.5 125 50 9 450 1 3.0 123 60 * amperes per square foot ** specific gravity 1.42 gm/ml.

These examples show the effect of increasing temperature on the effective plating range of the compositions of this invention and indicate that while a loss of coverage in the low current density areas occurs with increasing temperatures, this may be corrected by increasing the amount of nitrogen-containing compound. At the high temperature of Example 8, a grey band appeared in the deposit at the low current density area. This band disappeared in Example 9 wherein more HNO3 was added, with only a slight loss in coverage. Thus, the present invention is shown to yield a uniform black electrodeposit over a broad current density range even with widely varying plating solution temperatures.

EXAMPLE 10 A panel is electroplated in a plating solution containing 450 grams per liter CrO^, 1 gram per liter of the fluoride-containing catalyst of Example 1, 1.5 ml. of concentrated HNO^ (specific gravity 1.42 gm./ml.) and BaCO^ in excess of that required to precipitate the sulfate present. The bath is maintained at 112° P. for 3 minutes while applying 30 amperes direct current. A uniform, black, chromium-containing deposit is obtained over an effective plating range of from greater than 1500 amperes per square foot to about 30 amperes per square foot.

EXAMPLE 11 To illustrate the use in this invention of inorganic nitrogen-containing compounds other than HNO^ a plating solution is prepared as follows. 450 grams per liter CrO^j 1 gram per liter fluoride catalyst (as in Example l); 2.5 grams per liter over that required to precipitate the sulfate. A panel is plated from this bath in a Hull cell at Although there is some evidence of burning at the extreme high current density end of the panel, a uniform black deposit is obtained to a current density of about 70 amperes per square foot.

EXAMPLE 12 To show the effectiveness of the compositions of this invention over a wide range of C O^ concentrations the following solution is used: 300 grams per liter CrO^, 1 gram per liter fluoride catalyst (as in Example l), 2.9 grams per liter of excess BaCOg over that required to precipitate the sulfate present. The panel is plated for 3 minutes at 72°F. and 10 amperes applied current. A black deposit is obtained to the area on the panel corresponding to about 120 amperes per square foot with only a slight burn at the extreme high current density edge .

Now 300 grams per liter of CrO^ is added (total 600 grams per liter) and another panel is plated at 8l° P. for 3 minutes. An excellent black deposit is obtained to βθ amperes per square foot and the burned area is greatly decreased.

EXAMPLE 13 To *show the effectiveness of inorganic nitrogen-containing compounds other than nitric acid and salts thereof the following plating solution is used: 50 grams per liter CrO^, 1 gram per liter fluoride catalyst (as in Example l) and excess BaCOg to ensure complete sulfate precipitation. A panel is plated for 3 minutes at 78° P. and 10 amperes applied current. A black deposit is obtained only to about 200 amperes per square foot with considerable burning on the high current density end of the panel. This corresponds to a coverage of about 0$.

Now 1 gram per liter of NaNOg is added and a panel is deposit is obtained to a current density of about 60 amperes per square foot with no burn on the high current density end of the panel. The total area covered is now about 70$ of the panel.

EXAMPLE 14 To show the use of another fluoride-containing catalyst in the practice of this invention an aqueous solution is used as follows: 50 grams per liter CrO^, 2 ml. of concentrated HN0g (specific gravity 1 .42 grams per milliliter), 0.5 ml. of HF (49$ aqueous solution) and an excess of BaCO^ over that required to precipitate the sulfate present. A Hull cell panel is plated for 3 minutes at 73° F. and an applied current of 10 amperes. A black deposit is obtained from the high current density end of the panel to about 70 amperes per square foot.

EXAMPLE 15 An aqueous plating solution is made containing 450 grams per liter of CrO^, 1 gram per liter of fluoride-containing catalyst (the reaction product of a chromium compound, an organic reducing agent and fluosilicic acid as is more particularly described in U. S. Pat. No. 2, 841, 540 and containing 25$ by weight chromium,; 2 $ by weight fluoride..ahd .16$ 'by;,.weight' silicon):.. and about 8 grams of barium..carbonate to precipitatenthe sulfate. A panel is electroplated for 3 minutes at 10 amperes applied current and a bath temperature of 89°F„ A black electrodeposit is obtained from the high current density end of the panel to a current density ο about 200 amperes per square foot, a value corresponding to 37$ coverage.

To the above bath is added, with agitation for dissolui tion, reaction and dispersion, 4 grams per liter of granular sucrose, an amount sufficient to give about 10. grams per liter- and a temperature of 94°F0 The black deposit range is now found to extend to about 70 amperes per square foot or 62$ coverage. Thus the addition of trivalent chromium is shown to greatly extend the effective plating range of the solution.

EXAMPLE 16 An aqueous plating solution is made containing 50 grams of. CrO^ per lite?, 0.125 grams of the fluoride catalyst described in Example 1 per liter, 4 grams of sucrose per liter and an excess of BaCO^ over that required to precipitate the sulfate present. A panel is plated in the solution for 3 minutes at an applied current of 10 amperes and a temperature of 1100F o A black deposit is obtained from the high current density edge of the panel to an area corresponding to a current density of about 90 ampere per square foot. This example shows that even at relatively high temperatures and low fluoride catalyst concentrations the plating solutions of this invention are effective for obtaining the desired black deposit.

EXAMPLE 17 To show the use of the compositions, of the present invention containing the additive compounds in varying proportions) Hull Cell panels as defined hereinabove are electroplated at an applied current of 10 amperes. The actual compositions of the baths, the conditions used and the results obtained are shown in Each of the solutions indicated is treated with an amount of BaCO^ In excess of that required to precipitate the sulfate present. 50 I 1 : 95 s 350 : , 20 45Ο : 1 : 2.5 97 Ϊ 125 ί 53 450 : 1 1 : 2.5 : 106 : 30 ί 82 * 45Ο : 1 i 1 s 4. 7 : 104 t 40 : 78 450 : 1 : 1 : " IO.5 95 i 35 : 80 * 450 : 1 : 1 ! 22 ! 97 s 60 Ί 71 450 ; 1 s 2 1 10.5 i 84 s 40 % 78 450 1 · 3 Ϊ 10.5 I 100 1 0 ; 75 450 i 1 : 5 s 10.5 ! 110 s 100 % 59 (1) Catalyst as in Example 1 (2) Specific gravity 1.42 (3) Formed by adding sucrose to the bath (4) Figures in this column indicate that an acceptable black deposit is obtained from the high current density edge of the panel to an area corresponding to the stated current density. (5) This column reports the percent of the total surface area of the panel that is covered by the black deposit.

A distinct improvement is shown using the combination of additives of this invention as opposed to a solution containing only chromic acid and fluoride. Additionally it is shown that the compositions are effective over a wide range of additive concentrations .

EXAMPLE 1§ A -solution isΊmade' i¾0nsisting Of ;4$O"gramsS e'r^lite of BaCC>2 in excess of that required to remove the sulfate by precipitation. A panel is made the cathode therein for 3 minutes at an applied current of 10 amperes and a temperature 72°F. A black, chromium-containing electrodeposit is obtained from the high current density edge of the panel to an area corresponding to a current density of about 60 amperes per square foot. This shows that with a different fluoride catalyst and a lower temperature than in previous examples, black deposits are still obtained over a wide current density range .

EXAMPLE 19 An aqueous solution is made consisting of 300 grams per liter CrO^, 1 gram per liter of the fluoride catalyst as described in Example 1, 2 grams per liter sucrose (2.7 grams per liter Cr+3 ) and about 7 grams per liter BaCO^. A panel is plated for 3 minutes at 10 amperes and a temperature of 75°P.

Although there is evidence of some burning at the extreme high current density edge of the panel, even at this low Cr03 concentration a black deposit is obtained to an area corresponding to about 275 amperes per square foot.

EXAMPLE 20 To the solution of Example 5 are added 300 grams per liter of CrO^ and 4 grams of sucrose to give a total of 600 grams per liter CrO^ and 14, 5 grams per liter trivalent chromium. A panel is now electroplated for 3 minutes at 10 amperes applied current and a temperature of 110°P. Upon inspection of the plated panel it is found that there is no evidence of burning in the high current density: area and that the plating range is extended to about 150 amperes per square foot. This shows the EXAMPLE 21 To an aqueous solution containing 450 grams per liter CrO^ and 1 gram per liter of the luoride catalyst of Example 1 and which has been treated with an excess of BaCO^ to remove the sulfate is added 8 milliliters per liter of a 30$ aqueous solution of 2Q2' Tn*s is an amount sufficient to form about 1.2 grams per liter of trivalent chromium. A panel, plated for 3 minutes at 10 amperes and 80°F in this solution, shows a black, chromium-containing deposit from the high current density edge of the panel to about 150 amperes per square foot with evidence of only a slight burning at the extremeuhigh current density edge of said panel. This shows the use of agents other than' sucrose to obtain the desired trivalent chromium concentration.

EXAMPLE 22 A series of experiments is conducted to show" the effect of temperature on the compositions of the present invention. In all instances the panels are electroplated at an applied current of 10 amperes and the solutions have been treated with an excess of BaCO^ to precipitate the sulfate.

The source of the fluoride, nitrate and trivalent chromium are as in Example 1.

Current (1) (1) CrOo P HNO Cr+3 Temp. Density . Coverage .

G/L3 G/L l./L G/L °F. A.SoPc % 5Ο 1 1 10.5 81 30 82 50 1 3 10, 5 125 50 75 450 1 3 10, 5 140 80 65 (l) As in Example 1 .

It can be readily seen that while a slight loss in coverage occurs with an increase in temperature, the results EXAMPLE 23 A plating solution is made up containing 600 grams of CrO^ per liter, 1 gram of the fluoride catalyst of Example 1 per liter, 5 grams of Cr+3 per liter and 2 „ 9 grams of being added to precipitate the sulfate. A panel is plated for 3 minutes at an applied current of~ 10 amperes and a temperature of 90°F. A uniform, glossy black deposit is obtained from the high current density edge of the panel to an area on the panel corresponding to 50 amperes per square foot, i.e., 75$ coverage of the panel.

This shows the operability of the compositions of the instant invention at higher C O^ concentration and using an inorganic nitrogen-containing compound different from the previous examples .

EXAMPLE 24 To illustrate the use of another inorganic nitrogen-containing compound, 1 .5 grams of NaNO^ per liter is added to a solution containing 4 0 grams of CrOg per liter, 1 gram of the fluoride-containing compound of Example 1 per liter and 4.7 grams of C + per liter which has been treated with 10 grams of BaCO^ per liter. A panel is plated therein for 3 minutes at 10 amperes and a temperature of 98°F. The resultant black deposit extends from the high current density edge of the panel to an area corresponding to 60 amperes per square foot.

EXAMPLE 25 A further example of the use of various inorganic nitrogen-containing compounds in the compositions of this invention is afforded by the substitution of 1 gram of HaN02 per liter for the NaNOg of the previous example. A panel electroplated in EXAMPLE 26 A panel is electroplated for 3 minutes at 25 amperes applied current and 100°F„ in a solution containing 50 grams of CrO^ per liter, 1 gram of fluoride catalyst (as in Example l) per liter, 3 ml. of HNO^ (specific gravity 1.42) per liter, Cr +3 .5 grams of per liter and a sufficient excess of BaCO^ to insure removal of the sulfate .

Owing to the high applied current it is possible to estimate that the effective plating range of the solution, as evidenced by the extent of the deposit, extends from 50 to a value in excess of 1300 amperes per square foot.

EXAMPLE 27 To illustrate the use of a fluoride-containing compound other than that of Example 1, 0.5 milliliters per liter of HP ( 49$) is added to a solution containing 50 grams of Cr03 per liter, 2 ml. of HNO^ (specific gravity 1.42) per liter, 4.7 grams of Cr+3 per liter and an excess of BaCO^ to precipitate the sulfate. A panel is electrolyzed therein for 3 minutes at 80°F„ and at 10 amperes applied current. A black deposit is obtained from the high current density edge of the panel to an area corresponding to 90 amperes per square foot.

EXAMPLE 28 As a further illustration of a useful fluoride compound, 1 ml. of HBF^ (48^) is substituted for the HP of Example 13. A panel plated in this solution under the same conditions but at 72°P. has a uniform black deposit extending to an area corresponding to 120 amperes per square foot.

EXAMPLE 29 previous examples, a solution containing 50 grams of GrO^ per liter, 1 gram of the fluoride catalyst of Exampls 1 per liter, 1 ml. of HNOg (specific gravity 1.42) per liter, 10.5 grams of Cr per liter and an excess of BaCO^ is prepared. A number of panels 4 by 2.5 inches in size and composed of various metals are prepared for electroplating. Panel A is a brass panel having a conventional chromium-plated surface. Panel B is made of brass and Panel G is stainless steel. Each of the panels is made the cathode in the above solution and electroplated at a current density of 1 ampere per square inch and a temperature of 85°F. for 3 minutes. The resultant deposit on each of the panels, while not as uniform as that obtainable on a nickel substrate, is quite acceptable for most purposes.

It is to be understood that although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended c claims . - 21 - 27753/3

Claims (15)

1. : A composition for use in the electrode^jposition of a black chromium-containing deposit, being an aqueous solution substantially free from sulphate ions and containing from about 60 grams per liter u to saturation of chromic anhydride, a fluoride-containing catalyst in an amount sufficient to supply from about 0.03 to about 1 g of fluoride per liter of solution and an additive compound comprising (l) an inorganic nitrogen-containing compound in an amount sufficient, to supply the equivalent of about 0.35 to 3.5 g of H02 radical per liter or (2) about 1 to 25 g of. a trivalent chromium compound per liter or (3) a mixture of (1) and (2).

2. A composition according to Claim 1, containing the chromic anhydride in an amount in the range from 300 to 500 grams per liter.

3. A composition according to Claim 2, containing 450 grams per liter of chromic anhydride*

4. A composition according to any of Claims 1 to 3, containing an amount of fluoride in solution of 0.1 to 0.25 grams per liter.

5. - A composition according to any preceding Claim, wherein the fluoride-containing catalyst is hydrofluoric acid, fluoboric acid, fluosilicicacid, a water-soluble alkali metal, alkaline earth metal or ammonium salt of hydrofluoric, fluoboric or fluosilicic acid or a reaction produc.t of a hexavalent chromium compound, an organic - - 22 - 27753/3 any of

6. . A composition according to/Claims 1 to 5» containing the equivalent of 0.7 to.2 gram3 per liter of . (NOijj) radical.

7. - A composition according to Claim 6, wherein the nitrogen-containing compound is nitric acid, nitrous acid or- a water-soluble alkali metal, alkaline earth metal oi* ammonium salt thereof.

8. .8. . A composition according to any of Claims 1 to 7» containing 4 to 10 grams of trivalent chromium per liter.

9. " A composition according to any preceding Claim, wherein the trivalen chromium is formed in the solution from hexavalent chromium by the addition of sucrose as a reducing agent.

10. Compositions accordin to Claim 1, substantially as hereinbefore described.

11. A process for the electrodeposltion of a black chromium-containing deposit, whieh comprises disposing an electrically-conductive member to be plated as a cathode in ah aqueous solution consisting of a composition as defined in any preceding Claim and passing direct curren between the cathode and an anode at a current density of from 30 to 1500 amperes per square foot.

12. process according to Claim 11, wherein electro-deposition is effected at a temperature of 60° to 14G°F.

13. A process according to Claim 12, wherein the temperature is 70° to 95°Ρ. - 23 - 27753/3

14. A process according to Claim 11, substantially as described with reference to the foregoing Examples.

15. An article, when electroplated by a process according to any of Claims 11 to 14. For the Applicants NERS PC/rb