US3616279A - Electrolyte method and composition for coloring titanium and its alloys - Google Patents

Abstract

Titanium and titanium alloys are colored by electrolytic action to provide a colored surface of the nature of an anodized film. The colors imparted are controlled by voltage input to the electrolyte so that a wide selection of colors, each corresponding to a specific voltage level, may be obtained and reproduced from one surface to another by use of the same selected voltage levels. The electrolyte is a two-part composition consisting of an organic and an inorganic constituent. The organic constituent is one of a group of amides of which dimethylformamide is preferred. The inorganic constituent is a fluoride-bearing compound of which fluoboric acid is preferred.

Description

United States Patent [72] Inventor Earl W. Kendall Bonita, Calif. [21] Appl. No. 732,032 [22] Filed May 27, 1968 [45] Patented Oct. 26, 1971 [73] Assignee Rohr Corporation [54] IELECTROLYTE METHOD AND COMPOSITION FOR COLORING TITANIUM AND ITS ALLOYS 19 Claims, No Drawings [52] U.S.Cl 204/14 N, 204/15, 204/32 R, 204/56 R, 204/141 [51] Int. Cl C23b 5/48, C23b l/00, C23b 9/00 [50] Field of Search 204/14, 14.1, 56

[56] References Cited UNITED STATES PATENTS 2,909,470 10/1959 Schmidt 204/56 2,934,480 4/1960 Slomin 204/56 3,131,134 4/1964 Micillo 204/14 3,257,295 6/1966 Yoezaki et al. 204/56 3 ,346,469 10/1967 Weigel 204/56 FOREIGN PATENTS 895,695 5/1962 Great Britain Primary ExaminerJohn l-I. Mack Assistant ExaminerT. Tufariello AttorneyGeorge E. Pearson of the same selected voltage levels. The electrolyte is a twopart composition consisting of an organic and an inorganic constituent. The organic constituent is one of a group of amides of which dimethylformamide is preferred. The inorganic constituent is a fluoride-bearing compound of which fluoboric acid is preferred.

ELECTROLYTE METHOD AND COMPOSITION FOR COLORING TITANIUM AND ITS ALLOYS BACKGROUND OF THE INVENTION In the manufacture of aircraft it is customary to provide the same with a corrosion-resistant coating. While various coatings of different types have been found which are generally satisfactory for this purpose they characteristically have added to the weight of the aircraft. With the advent of the higher speed and weight reduction requirements of present-day aircraft organic covering materials as such are becoming inadequate and the need for higher strength metals such as titanium and its alloys required. Accordingly, it is desirable to provide a coloring effect for titanium and its alloys which will not inherently add to the weight of aircraft parts formed in the use of such materials, which will have no adverse effect on the mechanical properties of the substrate metals, and which will provide all of the desired heat and corrosion resistant characteristics of the conventional coatings.

SUMMARY OF THE INVENTION Accordingly to the present invention titanium and its alloys are subjected to electrolytic action to produce a colored surface selectively in accordance with the specific voltages applied to the electrolyte. The colored surfaces are obtained by making the titanium the anode in the electrolytic system, and the maximum intensity of the color is obtained after 10-60 seconds at any particular voltage. The composition of the electrolyte is nonaqueous and consists of a two-part system of which one constituent is one of a group of amides such as dimethylformamide and the other constituent is a fluoridebearing compound such as fluoboric acid.

OBJECTS OF THE INVENTION An object of the invention is to provide a method for imparting coloring effects to titanium and its alloys through electrolytic action.

Another object is to impart any one of a wide variety of colors to titanium and its alloys by electrolytic action through selective control of the applied voltage level.

Another object is to provide a method of coloring titanium and its alloys by electrolytic action in which the time required is of the order of less than a minute.

Still another object is to provide a method of imparting colored surfaces to titanium and its alloys which is of the nature of an anodized film and in which the electrolyte may be used as an immersion bath or applied by an applicator.

Yet another object is to provide a coloring of titanium and its alloys by electrolytic action of an anodizing nature in which the current flow is minimal and the current flow parameter is insignificant in relation to the voltage control parameter in producing the various color effects.

A still further object is to impart a colored surface to titanium and its alloys which is both heat and corrosion resistant.

A further object is to impart a colored surface to titanium and its alloys by an electrolytic action of an anodizing nature which is highly receptive to adhesive bonding resin compositions.

Still another object is to impart a colored surface to titanium and its alloys by electrolytic action of an anodizing nature which is effective to prevent the oxidation thereof under the influence of elevated temperatures upwards of the order of 800 F.

Still another object is to impart electrolytic coloring effects to titanium and its alloys which may readily be removed.

Yet another object resides in the provision of an electrolytic coloring process for titanium and its alloys which lends itself to the production of differently colored areas on the same surface.

A still further object is to provide a coloring process as afore described which lends itself to the coloring of highly polished surfaces of titanium and its alloys without impairing the efficacy of the surfaces to control the reflectance and emissivity therefrom of radiated energy.

Still other objects, advantages and features of the present invention will become more fully apparent as the description proceeds.

MATERIALS TREATED The coloring process and electrolytic composition of the present invention are directed particularly to titanium, and the coloring process is applicable to commercially pure titanium and also applicable, but not limited, to the following titanium alloys upon which the desired surface effects have been produced by application of the process:

Titanium 6A 1 4V Titanium 8AllVlMo Titanium 5Al-2.5Sn

ELECTROLYTE The desired color effects are produced on the surfaces of the titanium and its alloys by an electrolyte action which is of an anodic nature in which the electrolyte is a nonaqueous twopart composition consisting of organic and inorganic materials, these being an amide and a fluoride-bearing compound. For purposes of practicing the electrolytic process, the electrolyte may be efiectively used either upon immersion therein of the articles treated or by brush application of the energized electrolyte to the surface of the articles treated.

The amide employed in the electrolyte may be any one of the group of liquid amides such as dimethylformamide, dimethylacetamide, diethylformamide, formamide, diethylacetamide, t-butylformamide, and ethylformamide, of which dimethylformamide is preferred for most formulations.

The fluoride ion may be supplied from any suitable fluoridebearing compound which is soluble in the amide such as fluoboric acid, hydrofluoric acid, fluosilicic acid, and sodium fluoborate, and other soluble fluoride salts.

The theory of operation of the electrolytic coloring action obtainable from the electrolytic composition and process of the present invention is not known. However, it is thought that the fluoride ion under the influence of applied voltage is responsible for color development and is probably fortified in this effect by such ions as boron and silicon whose presence in the electrolyte tends to make the colors more discretely intense and vivid.

As used in the examples subsequently to be described, the composition of the electrolyte will generally have the following formulation:

ELECTROLYTE COM POSITION Equivalencevolumc percent Amide Fluoride Thus, the concentration of the amide is equivalent to 57-100 volume percent of dimethylformamide as the amide in the composition, and the concentration of the fluoride ion is equivalent to [-43 volume percent of fluobon'c acid as the fluoride-bearing compound in the composition.

As is apparent from the basic formulation, the fluoboric acid content may be approximately I percent of the amide content of the electrolytic composition without impairing the effectiveness of the electrolyte to produce the desired surface coloring effects. 0n the other hand, the fluoboric acid content may be approximately 75 percent of the amide content present without producing an etchant action on the substrate titanium. Etching of a specimen of titanium 6Al-4V occurred in 10 milliliters of fluoboric acid. Dimethylformamide was added until the etching ceased. The ratios of fluoboric acid to dimethylformamide was in the order of 75 percent of the formamide, thus establishing the ratio set forth in the basic formulation. Voltage was applied anodically to a specimen of titanium 6AI-4V immersed in 100 milliliters of dimethylformamide and no current flow was indicated. Fluoboric acid was added to the 100 milliliters of dimethylformamide until a significant coloring effect was apparent on the specimen, this occuring when approximately I milliliter of fluoboric acid had been added to the dimethylformamide thereby establishing the lower limit of the fluoride content as shown in the basic formulation. Thus, the concentration of the fluoride ion in the composition is equivalent to that in which fluoboric acid comprises l-75 percent of the volume of dimethylformamide as the amide in the composition.

It is thus apparent that the amide serves two purposes in the electrolyte, i.e., as an inert carrier for the fluoride ion and as an inhibitor to prevent attack on the substrate metal by the fluoride ion.

APPLIED VOLTAGE When titanium is made the anode in the electrolytic system comprising the aforedescribed electrolyte, colors are imparted to the m'etal which are characteristic of the voltage impressed. When a titanium specimen is to be given a predetermined coior, the specimen is immersed in the electrolyte and the required electrical connections are established making the titanium the anode, the cathode preferably being a carbon electrode which need not be critically spaced relative to the anode. With the electrical connections thus established, the DC voltage is applied and increased from at a uniform rate not to exceed 2 volts per second to the predetermined level of voltage which will produce the color desired. By adhering to this rate, current flow is minimized due to an apparent electri cal resistance inherent in the colored surface imparted to the titanium. If the rate is exceeded, this resistance apparently does not develop as evidenced by an increase in current flow, a lack of coloring of the surface, and in severe cases, a burning through of the surface.

By adding 1 weight percent of picric acid C l-IA NO OH to the electrolyte formulation hereinabove set forth, the rate of voltage may be increased to volts per second without encountering the current increase and possible burn-through conditions. The formulation with the additive thus becomes:

ELECTROLYTE COMPOSITION (with additive) Amide I00 mls. dimethylformamide Fluoride Z rnls. fluoboric acid Additive l gram picric acid The electrolyte may be held within a glass container since the fluoride ion is inhibited by the amide against etchant attack on the glass. In the use of a glass container the electrolytic action in developing the color changes on the surface of a specimen may be observed as the voltage is elevated to the predetermined level required for the specific color to be produced. When the voltage level is reached, continued energy flow should be maintained for a period of from 10-60 seconds.

Different colors may be applied to different surface areas of the same specimen. In one example, a measured volume of electrolyte was placed in the container and a color established at the highest required voltage level. Subsequent to this, additional volumes of electrolyte were successively placed in the container and the voltage decreased by successive decrements to establish a series of different colors on the specimen for the different levels of applied voltage. The same effect can be produced by progressive insertion of the specimen into the electrolyte with corresponding decreases in the level of the voltage applied.

The applied voltage is always increased to the voltage level required to produce a specific color desired since a color once produced cannot be removed by lowering the voltage level. On the other hand, colors produced at selected voltage levels are converted to the next succeeding color in the spectrum when the voltage level is increased.

The surface of the titanium may be colored by the use of an applicator such as a brush or roller which applies the electrolyte to the surface to be colored, the brush serving as the cathode in the electrolytic system and the different voltage levels being applied as before to produce the different colors desired. A unique feature of the brush-on method of producing the colors by electrolytic action is that differently coiored swaths may be laid down adjacent to each other without the one adversely affecting the other. Thus, a colored swath produced at a relatively high voltage level can be laid down adjacent to a colored swath produced at a lower level without converting the first color to the second.

THE COLOR RANGE The colors produced on the surface of titanium and its alloys are those present in the spectrum produced from white light, and the colors produced at successively increased voltage levels appear in the same order of colors as they are found in the spectrum. Thus, when the voltage is increased in increments above a level of about 10 volts, indigo, blue and green are produced in successive order which is the order in which they occur in the spectrum. The following table of colors and corresponding voltage levels indicates further the pattern of colors which are exemplary of the electrolytic process of the present invention.

Voltage Level Color Indigo Dark blue Light blue Green Yellow Salmon The foregoing table is illustrative of the color range which is possible in the use of the electrolytic method and composition of the present invention but is not restrictive since various other colors and shades including irridescent properties are also possible.

CLEANING The specimens preferably are cleaned preparatory to producing the colored surfaces thereon in a nonaqueous bath operated at ambient temperature of the order of 6090 F. for about 1 to 5 minutes and consisting of from lO-75 grams of chromic acid in lOO ml. of sulfuric acid (1.84 sp. gr.). This cleaning composition and method is the subject matter of US. Pat. No. 3,379,645 of Earl W. Kendall for Process and Composition for Removing Protective Paint Films. This cleaning composition is used for removing all forms of surface contamination of titanium and its alloys. Such contaminants include fingerprints, grease, oil, wax, general dirt and paint coatings, In the absence of effective cleaning to remove surface contaminants the aforementioned condition of burnthrough is aggravated by the presence of such contaminants. Once the color has been produced, the titanium articles so colored may also be cleaned using the same composition to remove the contaminants without adversely affecting the colored surface.

COLOR REMOVAL The color imparted to the surface of titanium and its alloys in the manner as aforedescribed may be effectively removed either by immersion in a nonaqueous bath suitable for the purpose or by manual application of the bath to the colored surface to be so reate. A bath which is well suited for this purpose consists of 70 percent acetic acid (glacial), 20 percent sulfuric acid (L84 sp. gr.) and 10 percent hydrofluoric acid (70 percent). This bath whether used for immersion or manual application is effectively operated at ambient temperature of the order of 60-90 F. for about 1 to 2 minutes. This bath forms the subject matter of a copending application of Earl W. Kendall, Ser. No. 600,362, filed Dec. 9, 1966, for Electrolytic Descaling of Titanium and its Alloys now U.S. Pat. No. 3,468,774

SURFACE PREPARATION FOR ADHESIVE BONDING It has been found that the peel and shear tensile strength of the adhesive bond established between articles of titanium is adversely affected by the presence of oxides and other surface conditions which have not been effectively removed by the prior art cleaning process employed. It has been found that a suitable preparation of the titanium parts to be bonded is obtained by first cleaning the parts by immersion for about 1 to 5 minutes in a nonaqueous sulfuric acid-chromic acid composition operated at ambient temperature of the order of 6090 F. and in which the chromic acid is in a range of from l-75 grams per 100 ml. of sulfuric acid (L84 sp. gr.). The parts are next electrolytically cleaned for about I to minutes at a current density of 4.5 to 5 amperes per square foot in a nonaqueous bath consisting of 70 percent acetic acid (glacial), 20 percent sulfuric acid (1.84 sp. gr. and percent hydrofluroic acid (70 percent). This is the same bath composition which is used for color removal as referred to hereinabove. The article treated is the anode in the electrolytic cleaning system which is operated in the manner set forth in the aforementioned patent application disclosing the use of the electrolytic bath. The specific color thereafter produced corresponds to a specific adhesive bonding composition which exhibits maximum adhesion to the surface color.

SURFACE PREPARATION FOR REFLECTANCE AND EMISSIVITY OF RADIANT ENERGY In certain applications involving the reflectance and emis sion of radiant energy from the surfaces of titanium it is desired to impart the surface a maximum capability with respect to reflectance and emissivity of the radiant energy. This is accomplished in accordance with the principles of the present invention wherein the titanium article is first cleaned in the chromic acid-sulfuric acid bath aforementioned. It is then mechanically polished to the highest degreee of reflectance possible as by well-known abrasive methods. The polished surfaces of the article are then imparted with a color by the electrolytic process of the present invention with a particular color corresponding to the wave length of the radiant energy to be reflected SPECIFIC EXAMPLES The utility of the electrolytic process and the composition of the present invention is illustrated in the following specific examples in which various colored surfaces were produced on various titanium specimens utilizing the specific electrolyte formulations and applied voltages. In each of the examples each of the specimens was immersed in the electrolyte composition for a period from 10 -60 seconds at the level of voltage which produced the selected color for the specimen, less time being required to establish the colors at the lower voltages.

EXAMPLE I Several specimens of titanium 6AI4V were connected as the anode in an electrolytic system in which the electrolyte composition consisted of 5 ml. fluoboric acid (HBF and 25 ml. fonnamide (HCONHQ operated at 60-90 F. with the following voltages applied to the several specimens to produce the colors indicated.

Voltage Level Color 20 Indigo 30 Dark blue 40 Light blue 50 Green 60 Yellow Salmon EXAMPLE II Several specimens of titanium 8AllV-|Mo were individually made the anode in an electrolytic system in which the electrolyte composition consisted of 70 ml. dimethylformamide and 1 ml. fluoboric acid operated at ambient temperature (6090 F.) at the following voltage levels to produce the colors indicated.

Voltage Level Color Dark purple Light blue Imitation gold Lavender Green EXAMPLE [I] A first specimen of titanium 8AllVlMo and a second specimen of titanium 6Al-4V were each cleaned in a chromic acid-sulfuric acid bath in accordance with the cleaning method aforedescribed. The titanium 8Al-lV-lMo specimen was then made the anode in an electrolytic system in which the electrolyte composition consisted of I00 ml. dimethylformamide and 2 ml. of fluoboric acid. The cathode consisted of a rectangular section of carbon wrapped with cheesecloth and soaked in the composition as aforementioned. As this electrode was rubbed over one-half of the surface of the titanium specimen, the voltage was increased at the rate of 2 volts per second until a level of 55 volts had been obtained. The color applied at this voltage was yellow, The remaining surface was then swabbed with the voltage raised from 0 to 35 volts to impart a blue color on the surface of the specimen.

The second specimen, titanium 6Al4V, was treated in the same manner as the first except that the initial voltage was established at 70 volts to produce a lavender color on one-half of the specimen and a second voltage was established at 50 volts to produce a green color on the remaining one-half surface of the specimen.

EXAMPLE IV Micrographs were prepared from previously colored specimens of titanium in the manner aforedescribed, and the micrographs were studied to determine the thickness of the applied color. The micrographs disclosed that a relatively uniform penetration of color occurred to a depth of 0.00025 inch. The depth of penetration was independent of the time of immersion of the specimen within the electrolyte. The electrolytic action apparently ceases when the color is established at a selected level of voltage effective to produce that color, this being evidenced by the apparent discontinuance of current flow as indicated by an ammeter which dropped to 0 when the color was established. This uniformity of penetration apparently assures maximum corrosion protection for the substrate metal.

The resultant weight of the color film is calculated to be of the order of 5 oz. per 10,000 square feet of the surface colored. This figure was determined by cleaning the surface of aspecimen of titanium 6Al4 the dimensions of 2X2X0.032

in. and electrolytically coloring the specimen using 100 ml. dimethylformamide and 2 ml. of fluoboric acid at 40 volts for 60 seconds. The difference between the initial weight and the final weight was measured to be 0.0008 gram. This figure then was extrapolated to the more significant figure of 5 oz. per 10,000 square feet.

EXAMPLE V One specimen each of titanium 6Al4V and titanium 8Al-lV-lMo was colored blue at a 40 level in a solution consisting of 5 ml. fluoboric acid and 100 ml. of dimethylformamide. The two specimens were then subjected to elevated temperatures of the order of 700 F. for a period of L hours. No change in color from the original was apparent.

EXAMPLE Vl As in example V, additional specimens prepared in the same manner were continuously immersed in a percent sodium chloride solution at 100 F. for 168 hoursw with no evidence of surface corrosion.

From the foregoing it is now apparent that an electrolytic method and composition has been provided which is well adapted to fulfill the aforestated objects of the invention. The novel principles of this invention transcend the scope of the invention as suggested or implied by the several embodiments hereinbefore described, and the invention may be embodied in other forms or carried out in other ways which have been conceived and reduced to practice during the course of this development, without departing from the spirit or essential characteristics of the invention. The specific examples disclosed herein therefore are to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Having thus described the invention, what is claimed as new and useful and desired to be secured by letters patent is:

I claim:

I claim:

1. A nonaqueous electrolyte composition for anodically producing discrete coloring effects in relation to the level of the applied voltage on the surfaces of titanium and its alloys consisting of a liquid amide and a fluoride-bearing compound soluble therein, the concentration of the amide being equivalent to 57-100 volume percent of dimethylformamide as the amide in the composition, the concentration of the fluroide ion being equivalent to l 43 volume percent of fluoboric acid as the fluoride-bearing compound in the composition.

2. A composition as in claim 1 in which the amide is any one of a group including dimethylformamide, dimethylacetamide, diethylformamide, formamide, diethylacetamide, t-butylformamide, and ethylformamide and in which the fluoride-bearing compound is any one of the group of fluoboric acid, hydrofluoric acid, fluosilicic acid, and sodium fluoborate.

3. The composition as in claim 1 in which the: amide is dimethylformamide and the fluoride-bearing compound is fluoboric acid.

4. A composition as in claim 1 in which the concentration of the fluoride ion is equivalent to that in which fluoboric acid comprises l-75 percent of the volume of dimethylformamide as the amide in the composition.

5. A composition as in claim 1, including an additive, and wherein the amide constitutes 100 ml. dimethylformamide,

the fluoride compound constitutes 2 ml. hydrofluoric acid, and the additive constitutes 1 gram of picric acid.

6. An electrolytic process for coloring the surfaces of articles of titanium and its alloys which comprises the steps of applying voltage of the article to be colored as the anode in a nonaqueous electrolytic system in which the electrolyte consists of a liquid arnide and a fluoride-bearing com ound soluble therein and in which the concentration of e ami e is equivalent to 57-100 volume percent of dimethylformamide as the amide in the composition and the concentration of the fluoride ion is equivalent to 1-43 volume percent of fluoboric acid as the fluoride-bearing compound in the composition, and increasing the voltage from zero to predetermined level corresponding to a specific color to be imparted to the surface of the article.

7. A process as in claim 6 in which the voltage is increased from zero at a rate of the order not exceeding 2 volts per second.

8. A process as in claim 6 in which the predetermined voltage is applied for a duration of the order of l060 seconds.

9. A process as in claim 6 in which the operating temperature of the electrolyte is of the order of 6090 F.

10. A process as in claim 6 in which the voltage is increased from zero and in which discrete color effects are imparted at voltage levels upwards of 10 volts.

11. The process as in claim 10 in which indigo, blue, green and yellow are exemplary of colors evidencing the spectrum and occur progressively in the order named as the voltage is increased in corresponding increments to higher levels.

12. The process as in claim 6 in which the article treated is immersed in the electrolyte.

IS. The process as in claim 12 in which a carbon electrode is used as the cathode in the electrolytic system.

14. The process as in claim 6 in which an electrolyte applicator is made the cathode in the electrolytic system and the electrolyte is applied by the applicator to the article to be treated.

15. The process as in claim 6 in which the article to be treated is cleaned by immersion for about I to 5 minutes in a nonaqueous sulfuric acid-chromic acid composition in which the chromic acid is in the range of from l075 grams per ml. of the sulfuric acid L84 sp. gr.

16. The process as in claim 6 including the further step of removing the color from the treated article by immersion for about 1 to 2 minutes in a nonaqueous bath consisting of 70 percent acetic acid (glacial), 20 percent sulfuric acid l .84 sp. gr.) and 10 percent hydrofluoric acid (70 percent).

17. The process as in claim 16 in which the color is removed by manually applying the removal bath composition to the colored surface with an applicator.

18. The process as in claim 6 and including the additional step prior to coloring of metallurgically polishing the surface to be colored to provide the same with a requisite efficacy with respect to the reflectance and emissivity of radiant ener- E)- 19. The process as in claim 15 including the further step prior to coloring of electrolytically cleaning the article by im mersion for about 1 to 5 minutes and a current density of 455m 5 amperes per square foot in a nonaqueous bath consisting of 70 percent acetic acid (glacial), 20 percent sulfuric acid l .84 sp. gr.) and 10 percent hydrofluoric acid (70 percent) and in which the article treated is the anode in the electrolytic cleaning system, the specific color thereafter produced corresponding to a specific adhesive bonding composition which exhibits maximum adhesion to the surface colored.

Claims (18)

1. 2. A composition as in claim 1 in which the amide is any one of a group including dimethylformamide, dimethylacetamide, diethylformamide, formamide, diethylacetamide, t-butylformamide, and ethylformamide and in which the fluoride-bearing compound is any one of the group of fluoboric acid, hydrofluoric acid, fluosilicic acid, and sodium fluoborate.
2. 3. The composition as in claim 1 in which the amide is dimethylformamide and the fluoride-bearing compound is fluoboric acid.
3. 4. A composition as in claim 1 in which the concentration of the fluoride ion is equivalent to that in which fluoboric acid comprises 1- 75 percent of the volume of dimethylformamide as the amide in the composition.
4. 5. A composition as in claim 1, including an additive, and wherein the amide constitutes 100 ml. dimethylformamide, the fluoride compound constitutes 2 ml. hydrofluoric acid, and the additive constitutes 1 gram of picric acid.
5. 6. An electrolytic process for coloring the surfaces of articles of titanium and its alloys which comprises the steps of applying voltage of the article to be colored as the anode in a nonaqueous electrolytic system in which the electrolyte consists of a liquid amide and a fluoride-bearing compound soluble therein and in which the concentration of the amide is equivalent to 57-100 volume percent of dimethylformamide as the amide in the composition and the concentration of the fluoride ion is equivalent to 1-43 volume percent of fluoboric acid as the fluoride-bearing compound in the composition, and increasing the voltage from zero to predetermined level corresponding to a specific color to be imparted to the surface of the article.
6. 7. A process as in claim 6 in which the voltage is increased from zero at a rate of the order not exceeding 2 volts per second.
7. 8. A process as in claim 6 in which the predeterMined voltage is applied for a duration of the order of 10-60 seconds.
8. 9. A process as in claim 6 in which the operating temperature of the electrolyte is of the order of 60*-90* F.
9. 10. A process as in claim 6 in which the voltage is increased from zero and in which discrete color effects are imparted at voltage levels upwards of 10 volts.
10. 11. The process as in claim 10 in which indigo, blue, green and yellow are exemplary of colors evidencing the spectrum and occur progressively in the order named as the voltage is increased in corresponding increments to higher levels.
11. 12. The process as in claim 6 in which the article treated is immersed in the electrolyte.
12. 13. The process as in claim 12 in which a carbon electrode is used as the cathode in the electrolytic system.
13. 14. The process as in claim 6 in which an electrolyte applicator is made the cathode in the electrolytic system and the electrolyte is applied by the applicator to the article to be treated.
14. 15. The process as in claim 6 in which the article to be treated is cleaned by immersion for about 1 to 5 minutes in a nonaqueous sulfuric acid-chromic acid composition in which the chromic acid is in the range of from 10-75 grams per 100 ml. of the sulfuric acid (1.84 sp. gr.).
15. 16. The process as in claim 6 including the further step of removing the color from the treated article by immersion for about 1 to 2 minutes in a nonaqueous bath consisting of 70 percent acetic acid (glacial), 20 percent sulfuric acid (1.84 sp. gr.) and 10 percent hydrofluoric acid (70 percent).
16. 17. The process as in claim 16 in which the color is removed by manually applying the removal bath composition to the colored surface with an applicator.
17. 18. The process as in claim 6 and including the additional step prior to coloring of metallurgically polishing the surface to be colored to provide the same with a requisite efficacy with respect to the reflectance and emissivity of radiant energy.
18. 19. The process as in claim 15 including the further step prior to coloring of electrolytically cleaning the article by immersion for about 1 to 5 minutes and a current density of 4 1/2 to 5 amperes per square foot in a nonaqueous bath consisting of 70 percent acetic acid (glacial), 20 percent sulfuric acid (1.84 sp. gr.) and 10 percent hydrofluoric acid (70 percent) and in which the article treated is the anode in the electrolytic cleaning system, the specific color thereafter produced corresponding to a specific adhesive bonding composition which exhibits maximum adhesion to the surface colored.