|Publication number||US3210220 A|
|Publication date||Oct 5, 1965|
|Filing date||Jul 30, 1962|
|Priority date||Jul 30, 1962|
|Publication number||US 3210220 A, US 3210220A, US-A-3210220, US3210220 A, US3210220A|
|Inventors||Norman E Clegg, William J Greening|
|Original Assignee||Norman E Clegg, William J Greening|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (8), Referenced by (11), Classifications (14)|
|External Links: USPTO, USPTO Assignment, Espacenet|
United States Patent 3,210,220 PROCESS FOR COATING STAINLESS STEEL Norman E. Clegg, 12522 Foster Road, Los Alamitos, Calif., and William J. Greening, 6815 Monlaco Road, Long Beach, Calif. No Drawing. Filed July 30, 1962, Ser. No. 214,450 5 Claims. (Cl. 148-621) I This invention relates to metal coating and, more particularly, to a method adapted for the coating of stainless steel.
The process of the invention is particularly adapted to the coating of stainless steel to provide a tightly adhering oxide film. The process is limited in its application to stainless steel alloys of iron and chromium. The process is not adaptable to the coating of iron or iron alloys containing no significant amount of chromium. Iron alloys known as stainless steel normally contain from to 30% chromium and may contain from 1 to 30% nickel. The process of the invention may be used to coat chromium steel alloys having less than 10% chromium, for example, chromium steel alloys containing 6% chromium; but, for the most part, the process will be found to have its greatest advantages in the coating of alloys of higher chromium content. The presence of smaller amounts of other alloy materials, such as molybdenum, copper, aluminum, cobalt, titanium, manganese and the like, is permissible, provided a significant amount of chromium is contained in the steel alloy composition.
The process of the invention comprises subjecting a steel-chromium alloy surface of an object to the action of a concentrated sulfuric or nitric acid water solution of chromium trioxide, CrO for a length of time necessary to form thereon the desired thin oxide coating. The solution is held at an elevated temperature and must be highly acid with the sulfuric or nitric acid making up at least of the aqueous acid solution on a weight basis. It has been found that if less than 20% acid solutions are employed, the metal surface being treated will be corroded by the acid bath. One of the advantages of the process is that a color coating can be produced while maintaining a mirror-like finish on the surface of the stainless steel base metal. Also, solutions containing less than about 20% acid fail to provide the required concentration of tetrachromic acid which latter acid is thought to be responsible for the film formation. Preferably, the mineral acid is used in the process bath in an amount of to 70% on a Weight basis. Mineral acid solutions in excess of 70% are to be avoided, though operative, as such solutions will cause the rejection of some of the chromium trioxide from the solution. Sulfuric acid is preferred to nitric acid and is generally used. Hydrochloric acid is not suitable for use in the process because the chloride ion is very detrimental to stainless steel.
The temperature of the acid bath for the formation of the film should be in excess of 150 F. and is usually within the range of 175 to 210 F. A temperature within the more limited range of 190 to 200 F. is generally optimum, but this will vary with the composition of the bath to some extent. The relative low temperature of the acid bath protects the surface finish of the stainless steel and avoids alteration of the physical characteristics of the base metal.
The immersed alloy surface is left in the sulfuric acid solution to form the oxide coating, being removed when the coating has acquired the desired color. It has been found that almost an unlimited number of shades of blacks, blues, golds, violets, greens and yellows (in that order) can be procured by varying the time of immersion and the temperature of the bath. The color change is fairly rapid and normally the alloy surface need be imice mersed only a few minutes, for example, two to six minutes. The particular length of time required to obtain a coating of a desired color is dependent upon bath composition and temperature, but seldom needs to exceed ten or fifteen minutes.
It has been found that a small amount of ammonium molybdate (NH MoO is desirably included in the acid bath to improve the luster of the finished product. Preferably, the ammonium molybdate is used in an amount from a trace to an optimum of 6.5 to 8.5 grams per liter of the sulfuric acid bath.
The effectiveness of the processing bath requires that the bath in its preparation be held at an elevated temperature for a fairly long period of time before use. The purpose of the prolonged heating is to permit what is believed to be the formation of tetrachromic acid. The processing bath after a period of time at an elevated temperature takes on a dark reddish-black color which is though to be an indication of the presence of the tetrachromic ion Cr O= As an example, in the preparation of a typical bath, 500 ml. of 9 normal H 50 is added slowly to 500 ml. of water. 7.5 grams of ammonium molybdate is added.
The solution is heated to 190F. and 850 grams of chromium trioxide (CrO added, this being in excess of the amount that will dissolve. Amounts of chromium trioxide less than that which will dissolve may be used but with the result that proportionally less of the tetrachromic acid will be produced. Desirably, at least 775 grams of chromium oxide is added to a liter of the acid solution. 775 to 850 grams of chromium trioxide is a desired range. An amount of about 275 grams is at the lower limit and will require an exorbitant length of time to obtain results. The bath is then held at the elevated temperature for at least six hours, and generally for eight to twenty-four hours with increased advantages being obtained up to twenty-four hours. The bath is then decanted to separate the chromium trioxide crystals that did not go into the solution. In the processing bath preparation, the prolonged heating is carried on at a temperature of at least F., and preferably at a temperature within the range of 200 F. Temperature in excess of 200 F. may be used, for example, as high as 250 F., but generally to little advantage. The prolonged heating favors the formation of the tetrachromic acid.
The presence of the tetrachromic acid in the process bath is believed to be responsible for the production of the thin oxide coating, the thickness of which is dependent upon time of immersion, composition and temperature of the bath. The characteristic colors of the varying film thicknesses are believed to be produced by an interference effect between two trains of light waves reflected by opposite surfaces of the thin oxide film.
The process of the invention provides a highly desirable passivation of :the stainless steel surface in the forming of a continuous, tightly adhering, oxide film. The oxide film resulting from the process of the invention affords a greater corrosion protection to the basic metal than is possible with, for example nitric acid passivation. Because of the color development of the oxide film, which color is indicative of film thickness, passivation is obvious to visual inspection. In nitric acid treatment, there being no visual indication of the degree of passivation, passivation is checked by a separate test. 65
The oxide film possesses desirable antigalling prop erties. There is a decided tendency for contacting stainless steel parts to gall or sieve. The oxide coating being nonmetallic, actually serves as a lubricant to prevent selfwelding from occurring during times of high bearing loading, either in static or dynamic situations.
In addition to its own inherent lubricating properties,
the oxide coating will also serve as a lake or holding area for oils or dry lubricants such as graphite. Where used as a holding area for lubricants, the oxide coating desirably has a thickness greater than ZA/n, where A is the wavelength of the light produced by a sodium flame and n is the index of refraction of the oxide formed. The alloy content of the stainless steel has some effect on the reffaction index.
The oxide film formed by the process is essentially a nonconductor of electricity. This property of the oxide film permits its use in various controlled thicknesses as an insulator, semiconductors, capacitors, diodes, resistors and other electronic uses.
As pointed out above, it is possible with the process of the invention to obtain various colors in the coating by controlling the coating thickness. This-may be done visually, removing the object from the processing bath when the desired color is reached. Colors of varying degrees of black, blue, violet, gold, green and yellow are obtained in accordance with the formula At'y/n with the light normal to the surface of the coating. Here '7 is the wavelength of the color desired and n is .the index of refraction.
It has been found that oxide coating of the invention enhances the wettability of glass to stainless steel, resulting in a superior bonding of the glass to the metal. It is believed that the oxide coating of the invention provides a well or lake for the glass to bond to the stainless steel. The coating may also be applied to stainless steel as a pretreatment to improve the adherence of paints and plastics such as Teflon.
The u/e ratio (where or is the solar 'absorptivity and e is the emittance) may be selected to give the desired reflectivity or absorptivity characteristics to the coated stainless steel. -In one such application, a Venetian blind type louver is made of the coated stainless steel with the thickness of the oxide coating being selected to accept or reject heat as desired. A very fiat reflectance curve can be obtained through the ultraviolet, visible and infrared light ranges through use of the thin film properties of the oxide coating.
The coating may be applied .to stainless steel in selective areas or applied to entire surface and selectively removed. The surface of a sample on which it is desired to place a design is coated with an acid resistant lacquer or photographic resist after the part has been coated to obtain the desired color. The piece with the pattern thus established is immersed in an electrolyte such as a 5% solution of sulfuric vacid and an electric alternating or direct current is applied to a lead electrode and the piece. This procedure removes the coating in all areas that are not masked with the resist and thus produces the design.
It is possible with the process of the invention to produce multicolored samples. The following is a descrip tion of the technique used. The surface is, for example, first exposed to a processing bath for approximately two minutes to impart to the surface a black coating. The optimum immersion time to obtain a black coating will depend upon the composition of the bath and its temperature. Following the application of the black coating, the piece is rinsed in hot water and dried with hot air. The piece is then coated with a photographic resist and exposed to a negative of the desired pattern. The image of the pattern is then developed in accordance with conventional techniques. The piece is then immersed in the bath for an additional two minutes or so and the exposed area which was formerly black now changes to blue. The photographic resist is removed by an appropriate solvent such as xylene and the piece once again rinsed in hot water and dried with hot air. Another pattern is applied to the work piece as before by first coating the work piece with photographic resist. The resist is then exposed to the negative of a desired pattern and thereafter the image developed. The piece with the image thereon is then immersed for an additional two minutes (making a six minute total) and the area of the new pattern takes on a gold color. The piece Will now have patterns aligned in black, blue and gold. To produce additional colors, this technique is repeated over and over. The next color to be produced will normally be violet followed by green.
Although exemplary embodiments of the invention have been disclosed herein for purposes of illustration, it will be understood that various changes, modifications, and substitutions may be incorporated in such embodiments without departing from the spirit of the invention as defined by the claims which follow.
1. A method of forming a coating on a steel-chromium allow surface of an object, said method comprising: subjecting the alloy surface of the object to the action of a highly acid solution containing tetrachromic ion and the anion of sulfuric acid, said solution being at a temperature within the range of to 210 F.; and leaving the alloy surface in the sulfuric acid solution until a desired color is produced at which time the object is removed from the sulfuric acid solution.
2. A method in accordance with claim 1 wherein the acid solution is prepared by providing chromium trioxide in the amount of at least 275 grams per liter of the sulfuric acid solution and heating the solution for at least eight hours at a temperature in excess of 180 F.
3. A method of forming a coating on a steel-chromium alloy surface of an object, said method comprising: sub jecting the alloy surface to the action of a treating solution and forming thereon a desired thin coating, said solu tion having been prepared by heating at least for six hours a concentrated mineral acid aqueous solution of chromium trioxide at a temperature in excess of 180 F., said mineral acid being selected from the group consisting of sulfuric acid and nitric acid with the mineral acid making up at least 20 percent of the aqueous solution on a weight basis.
4. A method in accordance with claim 3 wherein the heating of the solution used for the treatment of the chromium alloy surface is carried on at a temperature within the range of -200 F. for a period of eight to twenty-four hours.
5. A method in accordance with claim 3 wherein the mineral acid is present in the aqueous solution in the amount of 25-70 percent on a weight basis.
References Cited by the Examiner UNITED STATES PATENTS 2,283,170 5/42 Batcheller 148-621 2,312,066 2/43 Batcheller 148-62 2,773,623 12/56 Schuster et al 148-62 2,858,244 10/58 Long 148-62 2,902,394 9/59 Jeremias 148-621 2,991,205 7/61 Lincoln 148-62 FOREIGN PATENTS 275,781 8/27 Great Britain. 483,551 4/38 Great Britain.
RICHARD D. NEVIUS, Primary Examiner.
WILLIAM D. MARTIN, Examiner.
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|U.S. Classification||148/264, 148/266|
|International Classification||C23C22/05, F24J2/00, F24J2/48, C23C22/24, C23C22/43|
|Cooperative Classification||F24J2/487, C23C22/24, Y02E10/40, C23C22/43|
|European Classification||C23C22/24, C23C22/43, F24J2/48F2|