US3418220A

US3418220A - Electrodeposition of chromium and duplex micro-crack chromium coatings

Info

Publication number: US3418220A
Application number: US390463A
Authority: US
Inventors: Roggendorf Wilhelm
Original assignee: Individual
Current assignee: Individual
Priority date: 1963-08-26
Filing date: 1964-08-18
Publication date: 1968-12-24
Anticipated expiration: 1985-12-24
Also published as: DE1496967A1; CH441919A; NL6409261A; GB1081932A; FR1400157A

Description

United States Patent Otlfice 3,418,220 Patented Dec. 24, 1968 3,418,220 ELECTRODEPOSITION F CHROMIUM AND DU- PLEX MICRO-CRACK CHROMIUM COATINGS Wilhelm Roggendorf, 3 Alte Zollstrasse,

7460 Kehl am Rhine, Germany No Drawing. Filed Aug. 18, 1964, Ser. No. 390,463 Claims priority, application Germany, Aug. 26, 1963, R 35,983 17 Claims. (Cl. 204-41) ABSTRACT OF THE DISCLOSURE A chromium pl-ating aqueous solution is used in which tetrachromate of the type of sodium tetrachrornate (Na Cr O or which may also be expressed as Na O-4CrO is the principal, predominant, basis or major constituent and in which a lesser but appreciable amount of free chromic acid is also a constituent, This primary part of the plating solution may be obtained by adding chromic acid to water and then adding sodium hydroxide or carbonate to convert a greater part of the chromic acid to sodium tetrachromate. The solution also employs a chemical complex of dissolved materials in relatively minor amounts. Such materials include a sulfate which may be provided by adding sulfuric acid; cations of metals of the class of titanium and zirconium (essential) and fluoride which may be provided by the addition of a sodium fluoride of one or both of the metals; soluble silicon; and selenium. The silicon may be provided by adding silicon fluoride and selenium by adding selenium dioxide. Where silicon is to be used, it may be added as silicon fluoride and the titanium and zirconium may be added in the form of potassium titanate and zirconate. The solution is maintained within a, relatively low operating temperature range of about 18 to 39 C., preferably about 18 to 32 C. when the solution is used without selenium and 30 to 32 C. when it is used with selenium. The selenium is particularly useful when a dual coating is to be provided on a metal workpiece. The solution has a ratio of total chromic acid to free chromic acid content of about 1 to 3 up to about 1 to 5.

This invention relates to the provision of an improved chromium plated surface on metal articles or objects and particularly, to. a process for providing a corrosion-resistant, crack and pore-free chromium plating on a metal surface. A phase of the invention relates to providing a micro-cracked, chromium plated surface of improved characteristics and particularly, to a double chromium layer type of metal surface coating or deposit that has a crackfree first or initial layer or deposit and a second microcracked layer or deposit.

An object of my invention has been to provide an improved bright chromium plated metal surface or coating which will essentially have an improved resistance to corrosion.

Another object of the invention has been to provide a stress-free chromium coating on a metal surface that is porefree and bright.

A further object of the invention has been to provide an improved chromium plating process and one which is more efficient and effective from the standpoint of the action of its electrolytic solution or bath in depositing chromium on a metal surface.

A still further object has been to provide a dual coating or deposit of chromium whose first coating will be crackfree and whose second or final coating will have a microcracked surface pattern, such that the overall coating will have an improved resistance to corrosion.

These and other objects of my invention will appear to those skilled in the art from the following description and the appended claims.

Heretofore, chromic acid basis electrolytes with catalysts have been used for electroplating corrosion-resistant chromium coatings or layers, but such solutions for effective results have been used at temperatures above 40 C.

It has also been known that for the deposition of crackfree chromium, using a sulphate catalyzed acid-chromium plating solution, that the number of cracks per square centimeter of surface area will depend upon the ratios of the amount of chromic acid to the catalyst, and on the temperature of the electrolyte used. As an example, the number of cracks per square centimeter of surface area will be reduced in an electrolyte operating at a ratio of chromic acid to sulfuric acid of 100 to 1, if the operating temperature is raised from the norm-a1 range of to higher temperatures of or C. Also, a reduction of cracks per square centimeter of surface area will occur if the temperature is held at the normal range of 45 C., if the ratio of chromic acid to sulfuric acid catalyst is raised from to 1 to -150 to 1.

It is also known that more or less crack-free chromium layers may be deposited from a chromic acid solution containing as the catalyst, sulfuric acid or mixed catalysts of sulfuric acid and silicofluoric acid to which electrolyte magnesium oxide is added and temperature of the solution is raised above about 40 C.

The detrimental or disadvantageous results of the abovementioned so-called crack-free solutions is that, in reducing the number of cracks per square centimeter of surface area, the porosity will be increased. Thus, the advantage of a reduced number of cracks per unit of surface area is offset by the increased porosity and the resultant loss of corrosion resistance incident thereto.

It is also known that when the above-mentioned socalled crack-free chromic acid plating solutions are used, the deposit is plated under high internal stress. Thus, when the chromium plated objects or articles are mechanically deformed, as for example, during assembly with pressure or pull, when subjected to large variations in temperature, or when subjected to deformation by bending in use or by abrasive action of stones hitting the plated objects, the original so-called crack-free surface is destroyed. In addition, in view of the inherent internal stress of the chromium coating or plating, it will show a coarsely distributed crack pattern per unit of surface area.

Another factor is that chromic acid plating solutions containing silicofluoride catalysts or mixed catalysts, such as sulphate and silicofluoride catalysts, will tend to age. The use of an aged electrolyte produces a bright chromium plating with a coarsely cracked pattern.

Chromic acid plating solutions heretofore used for micro-porous chromium depositing have only a narrow plating range insofar as cathode current densities are concerned in which the micro-porous deposit will appear, and under and over which range micro-porosity that is desired is not accomplished. In view of the well-known bad throwing power of chromic acid plating solutions, the ability to maintain micro-porous deposits on prominent and, at the same time, on deeply recessed areas on the same article or object becomes very difficult if not impossible to achieve.

In accordance with my invention, I have discovered that it is possible to achieve an electro-deposit of chromium that is crack and pore-free as well as bright or that is additionally micro-cracked and is exceptionally useful for double chromium plating. This is accomplished by using a sodium tetrachromate basis electrolyte that contains brighteners or complexes of soluble titanium and Zirconium compounds, with the preferred or optimum complex being a fluoride of the named metals and an additional metal selenium.

I have determined that the addition of 1 /2 grams of titanium fluoride (TiF or 4 grams of zirconium fluoride (ZrF per liter of electrolyte will provide a tetrachromate electrolyte with the same effects of reduction in cracking tendencies as occurs in a sulphate-catalyzed chromium plating solution having a chromic acid to sulphate ratio of 100 to 1 when the temperature of the solution is raised from 40 to 60 C. However, the bright chromium deposit obtained from the use of my tetrachromate electrolyte will provide a very low value of internal stress or, in other words, a substantially stress and strain-free structure, as compared to a deposit gained from the abovementioned known plating solutions. Further, additions in the magnitude of 3 grams of titanium fluoride or 8 grams of zirconium fluoride in the electrolyte will lead to an even greater reduction in tendency for crack formation or, in other words, to an optimum stress-free structure.

A bright chromium plating solution of a tetrachromate basis containing complexes of titanium and zirconium, with and without selenium, operates at an electrolyte temperature below 32 C. and, as an optimum, in the range of about 29 to 31 C. It will provide bright electro deposits in a wide current density range of about 10 to 20 amperes per square deeimeter, and will produce a brillant and micro-cracked appearance and a coating having an extremely good corrosion resistance. 1 have discovered, in view of the unsurpassed high throwing power of an electrolyte of a tetrachromate type, that the chromium deposit Will be accomplished on objects having high prominence and deep recesses without the necessity of resorting to special techniques, such as high current strikes or internal anodes, etc.

In accordance with my invention, a bright plating solution of a tetrachromate type is most suitable for double chromium plating of bright nickel plated objects or of objects that were earlier double nickel plated. Such objects may be chromium plated by using the solution of Example I, in which they may be chromium plated to a thickness of about 0.25 micron to produce a crack-free deposit or coating. The work pieces or objects may then be re moved in a manner avoiding anodic polarity by cathodic charging during the transfer, at a potential below the deposition potential of chromium, for example at about 1 to 2 volts. That is, the objects to be provided with a second coating are removed from the solution of Example I and may then immediately, without rinsing, be immersed in the solution of Example II.

In the second solution, the Work pieces, objects or articles may be subjected to a step-wise increased current density and plated at a final current density within a range of about 10 to 20 amperes per square deeimeter at a solution temperature of about 29 to 31 C. to an electroplated thickness of, for example, about 0.5 micron. This produces a final, micro-cracked, bright chromium coating or deposit. The drag-in of electrolyte from solution I into the electrolyte of solution II is not harmful, but has been found to be helpful, since both electrolyte compositions or solutions are compatible.

German patent DRP 608,757 to Bornhauser indicates that his tetrachromate basis or type of solution will yield a chromium deposit which is matte and never bright, requiring subsequent finishing; it is not crack and pore free or micro cracked. Such solutions are characterized as operating at higher than usual current densities for chromium plating (200 to 900 amp/sq. ft.), and at low temperatures Within a range of about 16 to 22 C. and that should not exceed 24 C. According to Bornhauser, sodium tetrachromate will decompose spontaneously if heated above 40 C. into complexesof lower molecular sizes. It is indicated that at higher temperatures such a solution loses its characteristic advantages.

As contrasted to the above, I have found that electroplating solutions of my formulations of sodium tetrachromate type or basis are extremely stable under high temperatures and will not decompose when the temperature is raised to 30 C. or even higher, about up to 60 C. In fact, I have determined that a good upper limit of their working range is about 32 C. Further, my electrolytes do not lead to a sudden drop of cathode current efliciency. I have found that plating at 2 amperes per square centimeter with an electrolyte temperature of 30, the cathode current efliciency was 24% of the theoretical. This efliciency was reduced at a current density of 20 amperes per square deeimeter and an operating temperature of 35 C. by only 2%, with an average current efliciency of 22% of the theoretical that I have found when operating in this range. This is considerably less reduction in current efficiency than previously reported as characteristic of a tetrachromate basis chromium plating solution and was not and could not have been predicted.

In all the examples and the discussion of my invention, I have reference to grams per liter of solution when grams are mentioned, to a solution of a sodium tetrachromate type, and to an aqueous solution that is made up from water to the required chromic acid concentration.

Example I A compatible metal work piece or object, such as a bright nickel plated object, was chromium plated in a solution of my tetrachromate basis of the following composition per liter of electrolyte:

In this example, a solution temperature of 25 C. with a cathodic current density of 12 amperes per square decimeter was employed for 14 minutes. It provided a chromium deposit free of cracks and pores, with an asdeposited, commercially acceptable brightness.

Example II A workpiece having a compatible surface, such as one nickel plated with a double nickel layer, was chromium plated, using an aqueous electrolyte solution of a tetrachromate type containing the following composition per liter of solution:

Grams Chromic acid anhydride 280-320 Free chromic acid -120 Sulfuric acid 1.7-1.9 Titanium fluoride 0.1-0.5 Zirconium fluoride 0.3-1.2

Selenium dioxide 0.01-0.02

The above solution thus contains 1.66 to 1.86 grams of sulfate ions, .03 to .15 gram of titanium ions or .13 to .53 gram of zirconium ions and .07 to .67 gram of fluoride ions, as calculated from the amounts of the sulfuric acid and titanium or zirconium fluoride present.

The solution was maintained at a temperature of about 30 C. and the plating was effected at a cathode current density of 16 amperes per square deeimeter for ten minutes. The resultant chromium coating or deposit showed a micro-cracked appearance with more than 1,000 cracks per square centimeter of surface areas. A double chromium plating accomplished by using the solutions of Examples I and II in succession has the additional advantage that the throwing power is maximal in obtaining the first crack-free chromium deposit at :a solution temperature of about 25 C. (using solution I). Thus, the covering ability is greatly enhanced for a subsequently produced microcracked chromium deposit (using solution 11).

In carrying out my invention, I make use of a tetrachromate electroplating solution containing a major amount or quantity of chromic acid converted to sodium tetrachromate (Na Cr O as the predominant or basic chromium containing constituent, and a lesser but appreciable intermediate amount of free chromic acid I have been able to provide a greatly improved chromium coating or deposit on a metal surface of a workpiece and to increase the effective throwing power of the solution by employing a chemical complex represented by a catalyst containing a minor amount of a sulphate and a fluoride, and brightness represented by a minor but effective amount or quantity of a soluble metal compound of the class consisting of titanium and zirconium, with and Without silicon or selenium, also used in the form of a soluble compound. That is, I use a minor amount of a chemical catalyst complex, as, for example, provided by sulfuric acid or sodium sulphate for the sulphate, and as provided by alkali metal fluorides of the class of metals consisting of titanium and zirconium or by fluorides of the class of elements consisting of sodium and silicon for the fluoride, to not only provide a more effective throwing power and a lesser current density, but also to employ a wider range of temperature and particularly, to effectively employ a lower range of temperature of the plating solution. Further, the chromium coating as applied is pore-free, highly resistant to corrosion and to strain and stress produced cracking, is crack-free for any desired initial deposited coating thickness, and is bright as directly deposited on the metal surface of the work article or piece.

In such a solution, it appears that the fluoride and the sulphate work as catalysts, that the metals of the class of zirconium and titanium, with or without soluble silicon or selenium additions, work in providing a bright surface and in producing a pore-free, stress-free, crack and corrosion resistant coating. The increased throwing power appears to result from the complex of chemicals and particularly, from the complex of fluoride and sulphate catalysts with the metal cations. It will be noted that the titanium and zirconium cations may be provided separately or as a mixture in the solution, but should not exceed specified quantities for each, whether present individually or as a mixture.

1 have further discovered that selenium may be used in a relatively minute quantity in such a plating solution to augment the action of the metals of the class of zirconium and titanium. Further, I have discovered that selenium may be used in a slightly larger amount in a solution used for applying a coating or coatings to produce a single micro cracked deposit or dual coated chromium surface whose outer or last coating is desirably provided with highly minute or micro cracks.

By using a combination or complex of fluoride and sulphates with the brightening metals in soluble form, I have been able to eliminate the disadvantageous effects of these two compounds when used individually in a plating solution and, surprisingly, without any disadvantageous offsetting eifects as used with the brightening metals and with a tetrachromate solution, and to operate effectively at relatively cold or low solution temperatures (below about 32 C.). I enable operation at higher temperatures without spoiling the effectiveness of the solution or its chemical complex. In this connection, the tetrachromate solution is made up by adding sodium hydroxide or carbonate to a chromic acid aqueous solution, to thereby convert a greater part of the chromic acid to sodium tetrachromate, leaving free chromic acid in the range of about to below 50%.

As shown by examples, metals of the class of titanium and zirconium may be used, as an optimum, in proportionate amounts together, and may also be used individually. Such metals may be introduced in the form of fluorides, potassium titanate (K TiO potassium zirconate (K ZrO etc. The fluoride portion of the complex may, as indicated, be provided by sodium or silicon fluorides (to also provide silicon cation), or by fluorides of the metals of the class of titanium and zirconium. The

selenium, if used, may be introduced as selenium dioxide. The following are further examples of elfective plating solutions employing the principles of my invention:

Example 111 Total of chromic acid (CrO grams 150-600 Free chromic acid (H CrO with reference to the total chromic acid percent 20-50 Sulphate with reference to the total chromic acid percent .07-.4 Fluoride grams .1-13 Titanium do .03-3 Zirconium do .04-6

Solution or bath operating temperature of about 18 to 39 C.

EXampleIV Same as Example HI, but also containing:

Silicon per liter of the electrolyte solution grams .02-2

Solution operating temperatures of about 30 to 39 C.

Example VI Same as Example V, but additionally containing:

Silicon per liter of electrolyte solution grams .02-2

The following are examples of chemical compounds that may be used in providing a solution in accordanm with my invention:

Example .A

Grams Total chromic acid (CrO 200-270 Free chromic acid (H CrO 40-135 Sulfuric acid (H .2-.8 Titanium fluoride (TiF .1-3 Zirconium fluoride (ZrF .1-8 Sodium fluoride (NaF) .1-8

Example B Grams Total chromic acid 420-600 Free chromic acid -210 Sulfuric acid I .4-l.8 Potassium titanate (K TiO .1-10 Potassium zirconate (K Zr-O .1-13 Silicon fluoride (SiF .l-10

Example C Grams Total chromic acid 280-300 Free chromic acid 60-140 Sulfuric acid 1.5-2 Titanium fluoride .1-.5 Zirconium fluoride .l-1.2 Selenium dioxide (SeO .007-.02

Example D Grams Total chromic acid 400-600 Free chromic acid 120-220 Sulfuric acid 1.8-2.8 Potassium titanate .1-1 Potassium zirconate .1-2

Selenium dioxide .007-.02 Silicon fluoride .1-10

It will be noted that quantities or amounts of compounds containing metals of the class of titanium and zirconium are reduced when a selenium compound is used in the electrolyte solution or bath. Examples I to VI and A to D all relate to tetrachromate solutions in which the proportioned or indicated ranges of total chromic acid (anhydride) and free chromic acid (H CrO are obtained by the employment of a sodium compound of a type such as previously mentioned. All, in specifying grams, have reference to grams per liter of the aqueous solution or electrolyte.

Since my procedure provides a first or initial chronium deposit on a metal surface that is crack-free in view of the low residue stress in the deposit, it will retain this freedom from cracking and thus, provide greater corrosion protection. This is also true when the coated work piece or article is subsequently subjected to a second chromium deposit in further accordance with my invention. Such a second deposit is also bright but has a micro-cracked surface pattern that will predictably have at least 1,000 micro-cracks per linear centimeter of surface, as viewed withabout a 50 power lens. In this manner, increased corrosion resistance is assured in view of the multiplicity of pore sites where corrosion may occur, thereby limiting corrosion current at individual corrosion sites and thus minimizing the effectiveness of such action. If desired, a solution such as represented by Example Il may be used to apply a first coating that has a minimum of 1,000 micro-cracks per linear centimeter of surface.

When employing selenium in the solution, I have found that a slightly higher operating range is desirable, of about 30 to 32 C., as compared to about 18 to 32 C. where it is not used. As indicated by the examples, smaller amounts of metals of the class of titanium and zirconium, at least as to their upper limits, are used when selenium is employed. Although the titanium and zirconium have been designated as brighteners, they appear to have an influence on throwing power, control of stress, etc., as used with catalysts. In fact, the upper limits of these metals are governed by their effectiveness in this connection and particularly, with reference to throwing power. A good, inexpensive, and workable solution in accordance with my invention employs soluble titanium and silicon compounds, with or without selenium. Selenium, when used, has the ability to partially displace metals of the class of titanium and zirconium; in a smaller quantity it serves as a brightener and in a larger quantity it serves as a brightener and a producer of micro-cracks. Silicon also has a brightening effect as used I in the solution. My solution operates well at a much lower current density than an ordinary tetrachromate basis solution. In this connection, it may be operated from about 5 to about 50 to 60 amperes per square decimeter,

with about to 20 amperes as a good normal operating density. An optimum solution temperature is below about 32 C. Also the good qualities of my deposit including its high adherency are not limited to a maximum depth, and it may be applied to any suitable clean work piece surface, such as nickel, copper, brass, steel, cobalt, aluminum, etc. The work piece will be the cathode, the anode may be of tin-lead or antimonial lead, and any suitable voltage of, for example, 3 to 6 volts may be used in the plating operation.

Ordinarily the solution of my invention may be prepared by first adding chromic acid (H CrO to water, then adding the sodium compound to partially neutralize the chromic acid and form the tetrachromate which is in the form of sodium tetrachromate but which is measured by testing for the free chromic acid (H CrO content and calculated as anhydride acid constant (CrO The brightening metals, such as titanium and zirconium, as well as the silicon and selenium additions may be made by dissolving soluble comp unds in water and then adding to the solution, or by dissolving them directly in the solution. The sulphate content may be provided by adding sulfuric acid or sodium sulphate to the partially neutralized tetrachromate solution before the addition of the complexes. The fluoride may be added with the complexes in the form of soluble compounds. The selenium can be either directly dissolved in the chromic acid solution or separately dissolved in hot water and then added to the solution.

Essentially, the solution of my invention enables the electro-chemical deposition of chromium metal on the surface that is always bright as deposited, is corrosion and strain resistant, is highly adherent, is effected with increased throwing power, and is crack and pore-free, but that may be provided with micro-cracks, if desired. The coating is stress-free as applied. My solution enables avoidance of heretofore undesirable limitations as to temperature of the solution, depth of application, current density, etc., in that the chemical complexes, as used with the catalysts, in a tetrachromate solution in combination, produce entirely new and improved results including an asdeposited brightness.

In carrying out my invention, I employ a complex of soluble compounds in a proportionated basis in a tetrachromate solution and that may have about 0.16 to 6 grams per liter of sodium fluoride, such as to provide a content of free chromic acid to total chromic acid of a range of about 1 to 3 up to about 1 to 5. My proportioned solution has an unusually good covering and bright throwing power at a high current efficiency of a range of about 18 to 22% of the theoretical, and deposits an adherent pore-free, single or double deposit (crack-free or microcracked), and does so effectively at lower temperatures. It is completely stable up to a maximum higher temperature of about 39 C. and has good stability up to a temperature of about C.

What I claim is:

1. A process for electrolytically depositing a chromium coating on a metal surface of a work piece which comprises, preparing an aqueous sodium tetrachromate electrolytic plating solution having a free chromic acid to total chromic acid content of about 1 to 3 up to about 1 to 5 and which also contains: cations selected from the group consisting of titanium and zirconium in sufiicient amount to produce a bright chrominum plate and sulfate and fluoride ions in .an amount sufiicient to increase the throwing power of the electrolyte, dissolving silicon in the plating solution, introducing the work piece into the plating solution as a cathode, applying electroplating current thereto and directly depositing a chromium coating on the surface of the work piece which is bright, resistant to corrosion, resistant to cracking, pore and stress free.

2. A process as defined in claim 1 wherein silicon is introduced in soluble form into the plating solution in a minor .amount of about .02 to 2 grams/ liter.

3. A process as claimed in claim 1 wherein the workpiece is thereafter treated as cathode in a second plating solution, said plating solution having a sufiicient but small amount of a soluble selenium compound to form microcracks in .a second deposited coating, and applying electroplating current thereto until a micro-cracked chromium coating is deposited.

4. A process as defined in claim 1 wherein the tetrachromate of the solution is provided by reacting chromic acid with a compound of the class consisting of sodium hydroxide and sodium carbonates.

5. A process as defined in claim 1 wherein selenium is dissolved in the solution.

6. A process as defined in claim 1 wherein the plating solution is maintained below a temperature of about 32 C.

7. A process for electrolytically depositing a chromium. coating on a metal surface of a work piece which comprises, preparing an aqueous sodium tetrachromate electrolytic plating solution of increased throwing power hav ing a free chromic acid to total chromic acid content of about 1 to 3 up to about 1 to 5 Which also contains:

cations selected from the group consisting of titanium and zirconium in an amount of about .03 up to about 3 grams/ liter when the cation is titanium and in an amount of about .04 up to about 6 grams/liter when the cation is zirconium, about .07 up to about .7% sulfate ions with respect to the total chromic acid anhydride, and about .07 up to about 13 grams/liter of fluoride ions; introducing the work piece into the plating solution as a cathode, applying electroplating current thereto. and directly depositing a chromium coating on the surface of the work piece which is characterized by its brightness, improved resistance to corrosion, freedom from pores, resistance to cracking and freedom from internal stress and strain.

8. A process as defined in claim 7 wherein a small minor amount of selenium is introduced in soluble form into the plating solution to further improve the deposited coating.

9. A process as defined in claim 7 wherein about .02 to 2 grams of silicon per liter of electrolyte are provided in the plating solution.

10. A process as defined in claim 7 wherein, the plating solution is provided with a content of metals of the class consisting of titanium and zirconium in an amount of about .03 to .3 gram per liter for the titanium and about .04 to .9 gram per liter for the zirconium, about .005 to .015 gram per liter of selenium per liter of the plating solution, and the plating solution is maintained at a temperature of about 30 to 39 C. during the deposition of the chromium coating.

11. A process as defined in claim 10 wherein the plating solution is additionally provided with about .02 to 2 grams of silicon in soluble form per liter of plating solution.

12. A process as claimed in claim 7 wherein the workpiece is thereafter treated as cathode in a second plating solution, said plating solution having .a sufiicient but small amount of a soluble selenium compound to form microcracks in a second deposited coating, and applying electroplating current thereto until a micro-cracked chromium coating is deposited.

13. A process as claimed in claim 7 wherein the workpiece is thereafter treated in a second plating solution, said second plating solution being an aqueous tetrachromate electrolytic plating solution having a free chromic acid to total chromic acid content of about 1 to 3 up to about 1 to which also contains: cations selected from the group consisting of titanium and zirconium in an amount of about .03 up to about .15 gram/liter when the cation is titanium and in an amount of about .13 up to about .53 gram/liter when the cation is zirconium, about 1.66 up to about 1.86 grams/liter of sulfate ions, about .07 up to about .67 gram/liter of fluoride ions, and about .01 up to about .02 gram/liter of a selenium compound; and introducing the work piece into the second plating solution as a cathode, applying electroplating current thereto, and directly depositing a micro-cracked chromium coating.

14. In a process as defined in claim 7, providing the plating solution with a content of about 150 to 600 grams/ liter total amount of chromic acid anhydride and about 20 to 50% of free chromic acid with respect to the total of chromic acid anhydride, and maintaining the temperature of the plating solution at about 18 to 32 C. during the depositing of the chromium coating.

15. A process for electrolytically depositing a chromium coating on a metal surface of a work piece wherein the coating is characterized by its as-plated brightness, its high adherency, by its resistance to cracking, and by its freedom from pores which comprises, preparing .an aqueous tetrachromate plating solution containing about 200 to 270 grams per liter of total chromic acid anhydride, about 70 to 90 grams per liter of free chromic acid, about .6 to .8 gram per liter of sulfuric acid, a metal compound of the class consisting of titanium and zirconium fluorides in an amount of about 1.5 to 3 grams per liter of titanium fluoride and about 4 to 8 grams per liter of zirconium fluoride, and selenium in soluble form in an amount of about .003 to .005 gram per liter; maintaining the plating solution at a temperature below about 32 C., introducing the work piece into the plating solution and employing it as a cathode while applying electric current thereto, directly depositing a bright chromium coating on the work piece in the solution, and removing the work piece from the solution.

16. A process for electrolytically depositing a microcracked coating on .a metal surface of a work piece which comprises, preparing an aqueous tetrachrom ate plating solution containing about 280 to 320 grams per liter of chromic acid anhydride, about to grams per liter of free chromic acid, about 1.7 to 1.9 grams per liter of sulfuric acid, a metal compound of the class consisting of titanium and zirconium fluorides in an amount of about .1 to .5 gram per liter of titanium fluoride and about .3 to 1.2 grams per liter of zirconium fluoride, and about .01 to .02 gram per liter of selenium in a soluble form; maintaining the plating solution at a temperature below about 39 C., introducing the work piece into the plating solution and employing it as a cathode while applying electric current thereto, directly depositing a bright chromium coating on the work piece in the solution, and removing the Work piece from the solution.

17. A process for electrolytically depositing a microcracked chromium coating on a compatible metal surface of a work piece which comprises, preparing an aqueous sodium tetraohromate electrolytic plating solution having a free chromic acid to total chromic acid content of about 1 to 3 up to about 1 to 5 which also contains: cations selected from the group consisting of titanium and zirconium in an amount of about .03 up to about .15 gram/ liter when the cation is titanium and in an amount of about .13 up to about .53 gram/liter when the cation is zirconium, about 1.66 up to about 1.86 grams/liter of sulfate ions, about .07 up to about .67 gram/ liter of fluoride ions, and about .01 up to about .02 gram/liter of a selenium compound; and employing the work piece as a cathode within the plating solution while applying electric current thereto and directly depositing a micro-cracked chromium coating on the compatible metal surface of the work piece.

References Cited UNITED STATES PATENTS 1,985,308 12/1934 Bornhauser 204-51 2,686,756 8/1954 Stareck et al 20451 2,800,443 7/1957 Stareck et a1. 204-51 2,9,52,590 9/1960 Stareck et a1 204-51 2,962,428 11/ 1960 Passal 2045l 3,157,585 11/1964 Durham 204-41 FOREIGN PATENTS 798,590 7/ 1958 Great Britain. 1,106,575 5/1961 Germany.

654,782 12/1962 Canada.

OTHER REFERENCES Safranek, W. H., et al.: Use of Selenic Acid for Plating Microcracked, Protective and Decorative Chromium Plate, Plating, pp. 1027-1031, September 1960.

HOWARD S. WILLIAMS, Primary Examiner. G. KAPLAN, Assistant Examiner.

US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,418,220 December 24, 1968 Wilhelm Roggendorf It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 31, after 1.5-3" insert or line 51, after "0.1-0.5" insert or Column 6, line 11, after ".03-3" insert or line 28 after 03-. 3" insert or line 46, after ".l-3" insert or line 55, after ".1- l0" insert or line 64, after ".l-.5" insert or line 72, after ".l-l" insert or Column 7, line 12, "chronium" should read chromium line 71, "constant" should read content Signed and sealed this 10th day of March 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr. E.

Attesting Officer Commissioner of Patents