US2283170A - Method of coloring etched stainless steel - Google Patents

Description

Patented ay 19, 1942 2,283,170 METHOD or comma ETCHED STAINLESS STEEL Clements Batcheller, Glens Falls, N. Y.

No Drawing. Application November 30, 1939,

Serial No. 306,974

11 Claims.

My invention relates to a new process-of which the following is a specification whereby stainless steels which are cast, forged or rolled and have a decorative or utilitarian design etched therein may be chemically treated to produce color either in or about the etched zones thereof and thus bring out the design in strong contrast to the surrounding field.

In my copending application Ser. No. 301,805, filed October 28, 1939, I have disclosed a process whereby stainless steel products are first chemically colored throughout, then offset printed in the desired design with a suitable mask in the form of a resist, such as is commonly used in the etching of metals, to protect certain of the colored areas while the color from the exposed areas is removed by an electrolytic stripping process to produce the design. This method works very satisfactorily on stainless steelsurfaces which have not been pre-etched to produce a design thereon. However, I find that where certain portions of a stainless steel surface have been pre-etched, a certain passivity is imparted to the metal in the etched zones which very sub stantially retards the coloration thereof when subjected to my coloration process. I propose to utilize this discovery in an entirely new process for producing a desired design on a stainless steel surface.

The art of etching designs on metals as it has to prepare the specimens by printing either the field or the design thereon with an appropriate resist to protect the areas covered thereby from the action of the etching composition, and then to subject the printed metal to the etching solution. These processes require a subsequent filling in of the etched areas with color in order to accomplish the desired contrast. Such filling in is done either by spraying a colored lacquer onto the etched areas before the surrounding resist is removed from the unetched areas, or is done after the removal of the resist by a troweling operation in which a color paste is packed into the etched areas. In either case a subsequent high temperature baking operation is necessary to harden the color fill. However, even with the best known processes and the most skillful technique it has never been possible to produce permanent color fills in such etched products as are intended to withstand prolonged exposure to the Weather or to the destructive media encountered in many common industrial processes.

The principal object of my invention, therefore, is to provide an improved method of pro- 55 40 ployed. The steel is veying, astronomical and drafting instruments,

clock faces and other dials, name plates, and other plates such as are ordinarily attached to articles of manufacture and machines to indicate the characteristics thereof, wiring diagrams, in-

15 struction charts, etc., are only a few of the many products which may be produced by my process.

My process is only applicable to stainless steels which are iron alloys containing chromium, usually in quantities constituting 6% or more, by

weight, of the composition, and either with or without additions of other elements such, for example, as aluminum, copper, manganese, molybdenum, nickel, silicon, titanium .and the like. By my process either the etched portions may be colored and the field about them left in the contrasting metal color of the steel, or the field about the etched zones may be colored and the etched zones left in the natural color of the metal to produce the necessary contrast.

In accordance with my procedure, the steel surbeen commonly practiced for many years is first face is first prepared for etching by printing the design thereon with any of the resists, such as an asphalt composition commonly used in the etching of metals, which is capable of protecting the metal underneath from attack by the etching composition employed. One of the most common substances used for etching is iron perchloride, although solutionsof sulphuric acid, or mixtures of nitric and hydrofluoric acid are frequently emimmersed in the etching solution until the desired depth of etch has been effected. So far as my process is concerned the depth of the etch is not of major importance and may be as little as 0.0015", because I primarily depend merely on the passivation of the metal in the zones exposed to the etching solution. After the etching, the resist is removed by the application of a suitable solvent such as gasoline, benzol or the like.

Further treatment by my process, I find, is

facilitated if the metal is given a belt sanded finish known in the steel industry as #4 finish. .Such a finish, which is effected mechanically by specimen for rapid and uniform coloration of means of an abrasive belt, initially prepares my those areas surrounding the etch, and since the etched zones are lower than the surrounding zones they are not affected by this treatment. It is to be understood that my coloring process may be applied to the steel without pre-sanding 5 are commonly furnished seem to passivate the surface to some degree and thereby retard the coloration.

The etched zones of the steel, however, are so passivated by the etching solution, and particularly if iron perchloride is employed, that it requires an unusually long time to effect a coloration thereof by my process. In other words, when treated by my coloration process which I shall hereinafter describe, the zones of the steel which are not etched, and particularly if these zones have been pro-treated by giving them a belt-sanded finish, as above described, can be colored a very deep black .while those zones which have been passivated by the etching solution remain substantially in their natural etched color although all of the surfaces are subjected to the same coloring treatment.

In my United States Patent Nos. 2,172,353 and 2,219,554 and in my copendlng applications for Letters Patent Serial No. 206,043, filed May 4,

1938, and Serial No. 241,990, filed November 23, 1938, I have described processes for the coloration of stainless steels by immersion in a solution of concentrated sulphuric acid and water in which one or more of a large number of etching inhibiting oxidizing agents are employed in a quantity sufficient to prevent etching of the steel during treatment in the acid. By these processes a very beautiful deep black color may be 1mparted to the steel.

The steel is colored by treating it, preferably by immersion in a solution of concentrated sulphuric-acid and water in which there is dissolved a sufiicient quantity of an appropriate etchinginhibiting-oxidizing agent to prevent etching of the surface of the steel during the coloring treatment. The acid solutions which I employ are comparatively highly concentrated and of such strength that if used without an etching inhibitor would immediately attack and quickly dissolve 60 the steel. The solutions which I employ may contain from about 23 to about 54 parts, by weight, of sulphuric acid (1.84 sp. g.) and from about 35 parts to about 64 parts, by weight, of

water. In other words, the solution of acid and 55 water may have an acid concentration (by weight) in round numbers from about 25% to about 65%, but for the best results I prefer to use a solution having an acid concentration, by

weight, varying from about 40% to about 55% of the acid-water solution.

In this acid-water solution I. dissolve a sufficient quantity of an appropriate etching-inhibiting-oxidizing agent to prevent etching of the steel surface at the temperature at which the coloring process is to be conducted. Generally speaking, the quantity of etching inhibitor necessary will increase with the acid concentration and also with the temperature, and it is possible to use such a'large proportion thereof as to prevent satisfactory coloration. In the wider ranges of acid concentration given above, from about 4 to about 25 parts by weight of inhibitor, constituting from about 3% to about 30% of the whole solution may be used, but I prefer to use the acid,

water and inhibitor in about the following proportions:

. Parts by weight Etching-inhibiting-oxidizing agent 10-14 Sulphuric acid (1.84 sp. g.) 36-50 Water 40 50.

ily determined for any acid concentration and any temperature by testing specimens of the steel to be colored in the acid-water solution at the temperature to be employed, and adding the inhibitor until attack on the steel ceases.

As an inhibitor I prefer to use sodium or potassium dichromate but I find that any one or more of a large number of substances may be employed. Included among the inhibitors are the chromates and dichromates of aluminum, ammonium, barium, bismuth, cadmium and calcium, cobalt, copper, iron, lead, lithium, magnesium, manganese, mercury, nickel, potassium, sodium, strontium and zinc; the vanadates or metavanadates of ammonium, potassium or sodium, or vanadic acid or metavanadic acid; chromium trioxide; the manganates or permanganates of the alkali metals of group I, the manganates or pennanganates of the alkaline earth metals of group II, and manganese dioxide.

Generally speaking, the coloration treatment may be carried out with the bath at any temperature between normal atmospheric temperature (say 70 F.) and the boiling point of the solution.

hand, with temperatures in excess of about 220 F, the coloration takes place so rapidly that it is difficult to control and, moreover, the color film which is formed is not particularly satisfactory or apparently integral with the surface of the steel in that it may be readily rubbed off. The temperatures which I prefer to employ are between about 180 F. and about 210 F. In this range the best color films are produced, the time required is comparatively short and therefore commercially practicable, and the color changes are not sufiiciently rapid to prevent proper con-' trol. The steel should be removed from the solution when it has attained a black coloration. The time required depends on the-acid concentration, the quantity of inhibitor, the temperature of the solution and the analysis of the steel, and is therefore somewhat indefinite, but with the preferred solutions used at the preferred temperatures the process usually takes from 15 to 20 minutes. If the treatment is appreciably prolonged beyond the time when the steel acquires a black color the steel comes into what I term an overdone'condition. That is to say it loses its intense black color and becomes brown or iridescent. After coloring, the steel may be simply washed and dried.

If it is desired, to produce a black color in the etched zones instead of the zones or field about them the steel is merely subjected to the coloration treatment for a period of time sufficient to overcome the passivity imparted to the steel by the etching solution and thus effect a coloration thereof. Under my preferred procedure this may require as long. as from 30 to 45 minutes. This However, with the lower temperatures, I the time required to produce a satisfactory color 0 may run into hours or even days. On the other prolonged treatment necessary to effect the col-' oration of the etched areas, of course. builds up a color filmin the surface of the unetched portion. Due to the prolonged treatment, however, this oxide film on the unetched zones is not particularly adherent nor is the color a good black color due to the prolonged treatment. Usually the prolonged treatment will produce in the unetched zones either a brown discoloration or a non-uniform iridescent coloration which indicates what I term an overdone condition.

Since the black color in the etched areas is somewhat below the surface of the surrounding zones or areas due to the depth of the etch, the coloration in the zones surrounding the etched areas maybe quickly removed by simply passing the specimen under an abrasive sanding belt thus restoring the natural color of the steel to these zones and leaving the black contrasting coloration only in the etched zones.

The color produced by my treatment is exceptionally fast and is not adversely affected by high temperatures or corrosive media which do not affect the steel itself. It is substantially integral with the surface of the steel, andthe fact that it is in rather than merely upon the surface of the steel makes it far superior to any baked-on lacquers or fills. 7

What I claim is:

1. Those steps in the method of producing a design upon the surface of stainless steel which comprises passivating predetermined zones of said surface by subjecting said zones to the action of an etching medium, and thereafter subjecting said steel to the action of a solution containing sulphuric acid, water and an etching-inhibitingoxidizing agent until the unpassivated portions of said steel have acquired a dark coloration; the sulphuric acid constituting about 25% to about 65%, by weight, of the acid-water solution, and the inhibitor constituting from about 3% to about 30%, by weight, of the whole solution-and being present in a quantity at least suflicient to prevent said solution from etching said steel when immersed therein.

2. The method of producing a design upon the surface of stainless steel which comprises passivating predetermined zones of said surface by subjecting said zones to the action of an etching medium, coloring the surface of said steel including said passivated zones by subjecting said steel to the action ofa solution containing sulphuric acid, water and an etching-inhibitingoxidizing agent, and thereafter removing the color from the unpassivated portions of said surface by abrasion; the sulphuric acid constituting from about 25% to about 65%, by weight, of the acid-water solution, and the inhibitor consti-' tuting from about 3% to about 30%, by weight, of the whole solution and being present in a quantity at least suflicient to prevent said solution from etching said steel therein.

3. The method of producing a design on the surface of stainless steel which comprises coating predetermined zones of said surface with an etching resist, exposing the uncoated zones thereof to an etching medium until said zones are etched to a depth slightly below the level of the coated zones, removing the coating, subjecting those zones formerly covered thereby to an abrasive treatment, and thereafter subjecting when immersed said steel to the action of a solution containing ficient to prevent said solution from etching said steel when immersed therein.

4. The method set forth in claim 1 in which the coloration treatment is conducted with the solution at a temperature between about F. and 220 F.

.5. The method set forth in claim 2 in which the coloration treatment is conducted with the solution at a temperature between about 180 F. and 220 F.

6. The method set forth in claim 3 in which the coloration treatment is conducted with the solution at a temperature between about 180 F. and 220 F.

7. The method of producing a design upon the surface of stainless steel which comprises passivating predetermined zones of said surface by subjecting said zones to the action of an etching medium, and thereafter subjecting said steel to the action of a solution containing sulphuric acid, water and an etching-inhibiting-oxidizing agent until the passivated portions of said steel having acquired a dark coloration; the acid constituting from about 40% to about. 55%, by weight, of the acid-water solution, and the etching-inhibiting-oxidizing agent constituting from about 8% to about 15%, by weight, of the whole solution.

8. The method of producing a design upon the surface of stainless steel which comprises coating predetermined zones of 'said surface with an inhibiting-oxidizing agent; the acid constituting from about 40% to about 55%, by weight, of the acid-water solution, and the etching-inhibitingoxidizing agent constituting from about 8% to about 15%, by weight, of the whole solution.

9. The method of producing a design upon the surface of stainless steel which comprises subjecting predetermined zones of said surface to the action of iron-perchloride to effect a passivation and etching thereof, subjecting the zones of said surface about said first mentioned zones to an abrasive treatment, and thereafter coloring said abraded zones by treating the steel in a solution containing sulphuric acid, water and an etching-inhibiting-oxidizing agent at a temperature between about 180 F. and 220 F.; the

sulphuric acid constituting from about 25% to about 65%, by weight, of the acid-water solution and the inhibitor constituting from about 3% to about 30%, by weight, of the whole solution and being present in a quantity at least suflicient to subjecting said steel to the action of a solution containing sulphuric acid, water and an etchinginhibiting-oxidizing 'agent, and thereafter removing the color iromthe unpassivated portions by abrasion; the sulphuric acid constituting from about 40% to about 55%, by weight, 01 the acidwater solution and the etchlng-inhiblting-oxidizing agent constituting from about 8% to about 15%, by weight, of the whole solution.

11. Those steps in the method of producing a design upon the surface of stainless steel which comprise treating predetermined zones only of said surface with a passivating agent whereby said zones are rendered more resistant to oxidation than the untreated zones of said surface, is