US3900601A - Treatment of thin metallic films for increased durability - Google Patents

Abstract

Thin metallic films, particularly transparent films, on substrates such as glass are heat treated and then are contacted with alkali metal dichromate solutions or the like to superficially oxidize the films making them more resistant to corrosion in a moist environment.

Description

United States Patent 11 1 Franz et a1.

1 1 TREATMENT OF TI-IIN METALLIC FILMS FOR INCREASED DURABILITY [75] Inventors: I-Ielmut Franz, Pittsburgh; David E.

Lecocq, New Kensington, both of Pa.

[73] Assignee: PPG Industries, Inc., Pittsburgh, Pa.

[22] Filed: Sept. 28, 1973 [21] Appl. No.: 401,756

[52] US. Cl. 427/108; 427/123; 427/125; 427/380; 148/62; 427/169 [51] Int. Cl. C03C 17/10 [58] Field of Search 117/211, 47 A, 71 R, 227, 117/217, 118, 124 A, 124 C, 33.3; l48/6.2

[56] References Cited UNITED STATES PATENTS 2,690,402 9/1954 Crehan 117/71 1 Aug. 19, 1975 Primary Examiner-Cameron K. Weiffenbach Assistant ExaminerRalph E. Varndell Attorney, Agent, or FirmE. Kears Pollock 57 ABSTRACT Thin metallic films, particularly transparent films, on substrates such as glass are heat treated and then are contacted with alkali metal dichromate solutions or the like to superficially oxidize the films making them more resistant to corrosion in a moist environment.

11 Claims, No Drawings TREATMENT OF THIN METALLIC FILMS FOR INCREASED DURABILITY BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the. coating ofsubstrates with metallic films. More particularly, this invention relates to the treatment of chemically or electrochemically deposited films on non-conductive substrates, such as glass, to improve the durability of such films.

2. Description of the Prior Art Metal-coated articles have long been produced by both electroplating and electroless or wet chemical coating or plating techniques.

Electroplating techniques generally involve the immersion of a substrate to be coated into a bath of electrolyte containing ions of the metal to be deposited on the substrate. The substrate acts as an electrode and has applied to it an electric potential relative to another electrode so that the metal ions in the bath are transported to the substrate. Upon contact with the substrate, the metal ions gain electrons and form a metallic coating on the substrate.

Electroless or wet chemical plating or coating techniques are also well known. These techniques generally involve the contacting of an article to be coated with suitable electroless coating solutions to deposit a metal film thereon by reducing the metal from a metal saltin the coating solution. Both autocatalytic coating techniques and exhaustive coating techniques are known. Principal among the autocatalytic techniques are those wherein an alkali metal hypophosphite, formaldehyde or like reducing agents are employed. In such techniques the reducing agent and the metal salt to be reduced are typically present in a single solution. This solution is generally not subject to rapid reaction until placed into contact with a catalytic or sensitive surface. Exemplary autocatalytic techniques are those described in U.S. Pat. No. 2,532,283 and U.S. Pat. No. 2,532,284, both to Brenner and Riddell. Other techniques of this type are described in patents such as U.S. Pat. No. 2,956,900 to Carlson et al.

Exhaustive electroless coating techniques are also known. Principal among these techniques are those wherein a boron-containing reducing agent is used to reduce a metal from a metal salt-containing solution. In these techniques the reduction reaction will proceed rapidly, once the reducing agent is present together with the metal salt-containing solution. It is therefore usually necessary to apply separate solutions of these reactants substantially simultaneously to a substrate to be coated.

The preparation of thin, transparent films on transparent substrates, particularly on large sheets of glass for architectural use, presents particular problems which are not present in the coating of opaque articles, particularly opaque metallic articles. In the making of large, transparent, coated articles for architectural use, it is of extreme importance that films of uniform thickness, transmittance and reflectivity be produced in order for the articles to have a uniform esthetic appearance. Various metals have been successfully deposited on glass and other transparent substrates by electroless coating techniques. For example, films comprising one or more of the following metals have been successfully deposited on large substrates suitable for architectural use: iron, cobalt, nickel, copper, silver and the like.

A few of the known patented electroless coating techniques have been successfully employed to produce high-quality films, each having an attractive esthetic appearance and a high reflectivity for energy in the infrared range. Coated glass produced according to these known techniques has been effectively used in buildings where it isdesired to provide attractive windows or curtain walls which will effectively reject solar energy and minimize the load imposed on environmental conditioning systems servicing such structures. Commercially significant architectural products having such characteristics have been produced according to the teachings of U.S. Pat. No. 3,457,138 to R. G. Miller.

This patent to R. G. Miller describes. methods for making copper-silver films that are highly-efficient refiectors for infrared radiation. Following the teachings of this patent, films that are rich in copper have been produced, and these films have a pink appearance. Such films have both a pleasing and a high environmental efficiency. As described in U.S. Pat. No. 3,457,138, the copper films are generally protected from the environment in order to insure a useful life for them. A glass sheet coated with copper is glazed with a protective enclosure facing the film or laminated with a protective film or the like over the copper film. Even so, due to pinholes in a protective film or breathers as in some double glazed windows, the film is exposed to the outside environment. Corrosion of the film can occur in such instances, and the present invention is intended to improve the corrosion resistance of the metal film itself.

Iron, cobalt and nickel may be deposited as thin, transparent films on glass or other non-conductive substrates according to the teachings of U.S. Pat. Nos. 3,674,517 and 3,723,158, also to R. G. Miller.

Solutions containing chromate ions have been used to treat metal plated metal workpieces as taught by U.S. Pat. No. 3,088,846. Nickelphosphorus coatings have been treated by soaking in solutions of alkali metal chromates, dichromates and the like as disclosed in this patent. The treatment of films on nonconductive substrates is more difficult than the treatment of plated metal workpieces or substrates for such films are easily stripped from their substrates.

The present invention provides methods for improving the corrosion resistance of thin metallic films on non-conductive substrates.

SUMMARY OF THE INVENTION A non-conductive substrate, such as an organic polymer, a ceramic or a glass substrate, is prepared to receive a metallic film. This is accomplished in a conventional manner. For example, a substrate such as a sheet of glass, is thoroughly cleaned and is then subjected to a sensitizing treatment to make a surface of the glass receptive to a film. If the coating step to follow is an electroless coating step, the glass will usually be contacted with a tin salt solution and a noble metal salt solution in the manner of Bergstrom, U.S. Pat. No. 2,702,253.

Once a substrate has been prepared for coating, a metallic film is deposited on the prepared surface of the substrate. This may be by an electroplating technique carried out in a conventional manner, but preferably isby an electroless plating or coating technique. The preferred electroless coating technique will depend upon the particular metal or mixture of metals to be deposited.

For the deposition of copper films, silver films or copper-silver films, the methods described in US. Pat. No. 3,457,138 are preferred. Silver films are preferably deposited from aqueous, ammoniacal silver solutions. Copper films arepreferably deposited from solutions, each containing a copper salt, a reducing agent and a tartrate salt along with a small amount of a nickel or cobalt salt. US. Pat. No. 3,457,138 is specifically incorporated by reference herein.

For the deposition of iron films, nickel films, cobalt films or films of mixtures of such metals, the methods described in US. Pat. Nos. 3,723,158 and 3,674,517 are preferred. Such metallic films are deposited by simultaneously contacting a substrate with a metal salt solution and a boron-containing reducing agent solution in the presence of a nitrogen-hydrogen compound that controls the metal deposition rate permitting the deposition of uniform films. US. Pat. Nos. 3,723,158 and 3,674,517 are specifically incorporated by reference herein.

After depositing a metallic film on a substrate, it is further treated to increase its durability and improve its resistance to corrosion when exposed to a moist environment.

The metallic film is contacted with a composition that includes a weak oxidizing agent, such as, for example, potassium dichromate or the like. A sufficient amount of the weak oxidizing agent is used for a'sufficient time to superficially oxidize the surface of the metallic film. Care is exercised to avoid so completely oxidizing the film that its optical properties are substantially changed from the optical properties existing for the film prior to treatment.

The contacting step is of short duration of the order of 30 seconds to minutes, preferably of the order of 2 minutes. It is accomplished by spraying or dripping the weak oxidizing agent composition onto the film to be treated. Thereafter the film is usually rinsed to stop the action of the treatment composition.

The composition comprising the weak oxidizing agent is preferably an aqueous solution of the oxidizing agent, although organic solvent solutions or dispersions may be used. The pH value of such a solution may be adjusted to obtain optimum oxidation results during the contacting step. Such parameters as choice of oxidizing agent, concentration of oxidizing agent, duration of contacting and temperature of contacting may be adjusted in coordination, one with another, to insure an appropriate extent of superficial oxidation for a particular metallic film.

The step of contacting a metallic film with a weak oxidizing agent composition is preferably carried out within the temperature range of from about C. to about 100C. and more preferably within the range from about C. to about 70C. for copper and from about 80C. to about 100C. for iron, cobalt or nickel. The contacting composition may be heated and applied hot to a film of lower temperature, but, preferably, both the film and its substrate, as well as the contacting composition, are at about the same temperature immediately prior to contacting the film with the weak oxidizing agent composition.

The preferred weak oxidizing agent is potassium dichromate, followed by the other alkali metal dichromates or ammonium dichromates. Other weak oxidizing agents having closely equivalent disassociation constants and coupled oxidation potentials may also be employed. The weak oxidizing agent may be an ammonium or alkali metal permanganate or perborate in sufficiently dilute solutions. As will be understood by any skilled chemist, an equilibrium exists in solution among chromate, dichromate and other chromic oxide species so that the present characterization of the weak oxidizing agent as a dichromate is not intended to so limit this invention as to exclude these related species. Rather, the characterization as dichromate merely provides a convenient way of expressing quantitatively effective treatment concentrations.

As already stated, the contacting composition is preferably an aqueous solution of potassium dichromate. The concentration of the weak oxidizing agent in solution is preferably from about 0.1 to about 10 percent by weight and more preferably from about 1.0 to about 5.0 percent by weight. Particularly preferred solutions have been prepared containing 1 to 50 grams of potassium dichromate per liter of water. The pH of potassium dichromate contacting solutions may range widely while each solution is effective for treating metallic films. Nevertheless, solutions having pH values within the range of from about 4 to about 7 are preferred, and those having pH values from about 4 to about 5 are particularly preferred. The durability of films contacted with such acidic solutions is noticeably greater than that of films contacted with alkaline solutions. The films contacted with alkaline solutions are substantially more durable than untreated films.

The durability of metallic films, particularly iron, cobalt or nickel films, may be further increased by heat treating such films prior to contacting them with a weak oxidizing agent composition. For example, a nickel film may be heated to a temperature within the range of from about 150C. to about 500C. for from about 5 minutes to about 20 minutes prior to contacting it with a weak oxidizing agent composition. The resulting film is even more durable than films receiving no heat treatment. lron, cobalt or nickel films may be heat treated in air or preferably in an inert or even a re ducing atmosphere. Copper films are preferably treated in a reducing atmosphere or environment since they tend to oxidize most rapidly in air with a consequent change in optical properties. Preferred heat treatment conditions are temperatures of about 400C. and treatment times on the order of about 10 minutes. The upper temperature limit is defined by the temperature at which the substrate will distort due to softening.

The heat-treatment step apparently causes a consolidation and ordering of the film structure as evidenced by a decrease in electrical resistance of a film following heat treatment. Increases in crystalinity and in abrasion resistance are also noted following heat treatment. The duration of heat treatment is preferably that sufficient to cause the electrical resistance of a film to decrease by at least about one-half of its initial value prior to heat treatment and preferably to decrease as much as to percent of its initial value prior to heat treatment. The electrical resistance of a film may be determined using aconventional ohmmeter and a pair of point probes or a conventional spring loaded, circumferential, multiple point probe.

The optical properties of a film include, first, its appearance to the eye and, second, its quantitative properties of transmittance and reflectance as detected using conventional spectrophotometric devices in accordance with the conventions .of the International Council on Illumination. The lack of optical property change during treatment is that tolerable change that is undetected by the normal human eye. In terms of quantitative properties, changes of less than i 5 nanometers dominant wavelength, 5 percent excitation purity and i 2 percent visible light or luminous transmittance and reflectance are considered tolerable or undetectable changes and, thus, represent substantially no change in optical properties.

This invention may be further understood from the specific illustrative examples which follow. In the following examples the substrates are clear, soda-limesilica, flat glass sheets such as produced by the float process. Other substrates such as organic polymers, ceramics or siliceous or calcareous base compositions may be employed. For example, a substrate for coating and treatment according to this invention may constitute one of the following types of glasses: soda-limesilica glasses; alkali-alumina-silica glasses, such as those containing lithia as a component alkali; alkali zirconiasilica glasses, alkali-alumina--zirconia-silica glasses; borosilicate glasses; etc. Bearing this in mind, the present invention is described hereinbelow with specific reference to soda-lime-silica glass.

The soda-lime-silica glass to be treated can be a clear glass or it can be a colored glass tinted by the introduction of various conventional glass colorants such as metal oxides or metal oxide mixtures into the glass forming batch. These latter glasses are often referred to as heat absorbing glasses especially when they contain iron oxide. Representative soda-lime-silica glass bases which can be treated in accordance with this invention usually contain 65 to 75 percent by weight SiO to 18 percent by weight Na O, 5 to percent by weight CaO, l to 5 percent by weight MgO, 0 to 1.0 percent by weight Na SO 0 to 5 percent by weight aluminum oxide (A1 0 0 to 8 percent by weight K 0, 0 to 8 percent by weight B O 0 to 1 percent by weight iron oxide (Fe O and 0 to 0.7 percent by weight of NaCl, S0 AS205, BaO, NiO, C00 and Se and combinations thereof.

A representative range of composition for soda-limesilica glasses is listed as follows (wherein the given amounts of metals listed are determined as their oxides, except as otherwise noted):

glass as is conventional of glass produced by this method. The approximate composition of the glass follows:

Percent by weight Component:

SiO, Na o CaO 8 MgO 3 2 3 Na,SO 0. NaCl 0 FQQ, 0

EXAMPLE I Glass sheets of the composition listed above are cleaned using the conventional cleaning procedures used to prepare glass surfaces for mirroring, and are then rinsed with demineralized water.

Then the glass surface to be filmed with copper is coated with a stannous chloride aqueous solution containing 0.1 percent by weight stannous chloride salt by pouring it onto the glass surface. The stannous chloride salt is prepared by dissolving 1 weight part of stannous chloride in 1,000 weight parts of demineralized water at room temperature to form the stannous salt solution. The stannous salt solution is allowed to contact the glass surface to be filmed for a period of l to 3 minutes, followed by rinsing the stannous salt treated glass surface with demineralized water for 0.5 to 1 minute rinse period.

Then the thus stannous salt treated glass surface is immediately contacted while still wet from the demineralized water rinse with an aqueous solution of ammoniacal silver nitrate containing 0.25 percent by weight silver nitrate. This ammoniacal silver nitrate aqueous solution is prepared prior to use by dissolving 5 weight parts silver nitrate in 100 weight parts of demineralized water followed by the addition of 15 volume parts of aqueous ammonium hydroxide. Then sufficient water is added to bring to a volume of 1,000 volume parts.

Then 10 weight parts of dextrose sugar is dissolved in 1,000 weight parts of demineralized water to form a reducing solution. The silver film is applied by intermingling the two solutions in equal parts and spraying the intermingled volume solutions onto the previously prepared glass surfaces. The previously rinsed stannous salt treated glass surface is contacted at 80F. (25C) with the above aqueous silvering solution for a period of 0.5 to 5 minutes of spraying the silvering solution onto the stannous salt treated glass surface, to deposit 1 6 In these examples the glass sheets that are coated and treated are cut from glass produced by the float method and having tin in the extreme surface regions of the transparent metallic silver films having thicknesses ranging from 4 to 20 X 10 inch (10 50 X 10 cm). Then the excess silver solution is rinsed with demineralized water.

Next while the glass surfaces are still wet with the silvering solution, the thus silvered glass surfaces are immediately subjected to contact with a copper salt film forming solution previously prepared by mixing (A) a copper solution containing 34.6 weight parts cupric sulfate, 8.6 weight parts nickel sulfate, 275 volume parts of a 37 percent by weight aqueous formaldehyde solution (previously prepared by addition of the formaldehyde to demineralized water) and 1,000 volume parts of de-ionized water with (B) an aqueous alkaline tartrate salt solution containing 175 weight parts of sodium-potassium tartrate, 50 parts by weight of sodium hydroxide, and sufficient demineralized water to arrive at a total volume of 1,000 volume parts. Upon mixing of the copper solution A and the alkaline tartrate solution B, the resulting solution is then diluted by mixing with 4 volume parts of de-ionized water. This solution is then flowed onto the previously silvered glass surfaces by pouring repeatedly and allowed to contact it for periods ranging from 2 to 3 minutes to form thin, transparent metallic copper film having an average thickness of 0.75 X 10- inches (30 to 35 milligrams of copper per square foot of glass surface) uniform in texture and homogeneous in color indicating good uniformity in the deposition of the copper films. Thereafter, the thus formed copper filmed surfaces are rinsed with demineralized water to remove excess copper film forming solution. The copper film surface is then dried in the air.

The thus formed transparent copper filmed glass articles possess a luminous transmittance rangingfrom 25 to 40 percent, and show a solar energy reflection of 50 to 60 percent (coated side) as compared to a solar energy reflection of 7 to 8 percent for unfilmed glass sheets having the same composition and thickness. These copper films are tenaciously adherent to the glass base when dry, immediately after being formed.

A portion of the coated glass sheets are set aside and a portion are further treated. .Of the sheets further treated, the sheets of a. first portion are immediately treated with a solution of triaminotriazole as is known in the art (see US. Pat. No. 3,382,087 to Ostrowski); the sheets of a second portion are heat treated'in an inert atmosphere for 10 minutes at 300C. and then contacted with a 2 percent aqueous solution of potassium dichromate at 60C. for 3 minutes; the sheets of a third portion are treated immediately with an identical potassium dichromate solution under identical conditions of time and temperature. All resulting films have substantially the same optical properties to the eye of an observer.

All films are then subjected to accelerated corrosion testing. Some coated sheets are placed in a Cleveland Condensation Chamber-OCT Environment Tester manufactured by the Q-Panel Company, 15610 Industrial Parkway, Cleveland, Ohio 44135 and subjected to cyclic humidity conditions with the temperature of the chamber being cycled between 140F. (60C.) and 160F. (71C.) over a period of 3 hours per cycle. Other coated sheets are placed in the same type of chamber for condensation corrosion testing. They are subjected to continuous condensation at 160 170F. (71 77C). These tests are those commonly known from the Official Digest of the Cleveland Society for Paint Technology, December 1963, June 1965 and November 1965. Still other coated plates are immersed in demineralized water held at room temperature.

After 18 hours of condensation exposure, the untreated and triaminotriazole treated films are almost completely removed from the glass while the treated films remain, although affected by about percent removal of film in scattered areas throughout the film. Similar results are experienced by the films subjected to the other accelerated corrosion tests.

EXAMPLE II The silver-coating step of Example I is practiced to prepare silver films on glass. Individually coated sheets are treated and tested as in Example I with similar results.

EXAMPLE III Sheets of glass are coated with nickel-boron films. The sheets of glass are first cleaned and rinsed with water. A tin solution of the following composition is then sprayed on each glass surface to be coated.

TIN SOLUTION grams milliliters Stannous chloride Hydrochloride acid (12 N) Water, added to 1 liter.

Following this, each sheet of glass is rinsed with water, preferably water containing an alkaline buffer, and then each sheet of glass is sprayed with a palladium solution.

PALLADIUM SOLUTION Palladious chloride grams 0.02 Hydrochloric acid (12 N) milliliters 0.04

Water, added to 1 liter.

Each sheet of glass is again rinsed with water and then simultaneously sprayed with a nickel solution and a reducing solution.

NICKEL SOLUTION V Grams Nickelous acetate 5 Boric acid 2.5 Sodium gluconate 9.0 Hydrazine sulfate 0.5 Water, added to 1 liter. Ammonium hydroxide, added to pH 7.4. Ethomeen C-20 0.06

Ethomeen C-20 (trademark of Annour and Company) is a cocoamine having an average molecular weight of 645 and the following generalized formula:

(C1-1 CH 0),,H wherein R is derived from a cocoamine and X y equals 10.

REDUCING SOLUTION Grams Sodium borohydride 0.5 Water, added to 1 liter. Sodium hydroxide, added to pH 1 1.6. Ethomeen C-20 0.03

--the advancing plate and has its centermost portion spaced about 48 inches (122 cm) from the final rinse spray. The air knife is operated at about 5 p.s.i.g. 1.34 atm absolute) and a flow rate of about 350 c.f.m. (9.88

m /min). The ambient air temperature is about 82F. (25C.), while the temperature of the demineralized and tap water used throughout is about 63F. (18C.). The glass plate is advanced at a rate of 3% feet per minute (1.17 m/min).

The temperature of each of the solutions is about 70F. (20C.). The pH of the intermixed nickel and borohydride solutions is about 7.7. A nickel film is formed which contains about 5 percent by weight boron and the resulting coated plate has a luminous transmission of about 23 percent. The film is very adherent to the glass plate and is very uniform in appearance. The film has an initial resistivity of 300 ohms per square.

The procedure is utilized to prepare several samples. The results are reproducible.

A portion of the coated glass sheets are set aside for testing; sheets of a second portion are treated with triaminotriazole solution; sheets of a third portion are heated treated at about 400C. for about 15 minutes in air and then treated with 2 percent aqueous potassium dichromate solution at about 90C. for 10 minutes; sheets of a fourth portion are heat treated as described but not treated with potassium dichromate and sheets of a fifth portion are not heat treated but are treated with potassium dichromate solution.

The films are subjected to accelerated corrosion testing as described in Example I with the following results:

CONDENSATION TEST 16 Hours 24 Hours A. No treatment or triazole treatment Complete Not left in film retest moval B. Heat treatment alone 40% film 75% film removal removal C. Heat treatment and potassium Negligible Slight attack dichromate treatment effect on film WATER IMMERSION TEST 17 Hours 41 Hours 120 Hours A. No treatment or tria- Complete Not left in Not left in zole treatment film retest test moval 8. Heat treatment alone 50% film 80% film Complete removal removal film removal C. Heat treatment and No effect Slight sur- Slight film potassium dichromate face attack removal treatment CYCLlC HUMIDITY TEST A. No treatment or triazole treatment B. Heat treatment alone C. Heat treatment and potassium dichromate treatment 85% film removal 75% film removal 5% film removal HOURS OUTDOOR EXPOSURE (Vicinity of Pittsburgh, Pa.)

Complete film removal 85% film removal 25% film removal The films that were treated with potassium dichromate and not heat treated perform much better than those heat treated alone but not so well as those having both treatments.

EXAMPLE IV A cobalt film is produced in a manner like that described in Example Ill except using the following coating solutions.

The resulting cobalt film comprises about 96 percent by weight cobalt and about 4 percent by weight boron, and the resulting coated plate has a luminous transmission of about 21 percent.

Individual coated sheets are treated and tested as in Example III with similar results.

EXAMPLE V A nickel cobalt film is produced as in Example III using the following metal solution.

METAL SOLUTION Grams Cobalt acetate 4 Nickel propionate l0 Boric acid 2.5 Sodium gluconate 7 Hydrazine sulfate 0.7 Water, added to 1 liter. Ammonium hydroxide, added to pH 7.2. Ethomeen C-15 0.06

Ethomeen C-lS (trademark of Armour and Company) is a cocoamine having an average molecular weight of 422 and the following generalized formula:

c .cH. wherein R is derived from a cocoamine and x y equals 5. A transparent film was obtained that was very uniform and adherent. The light transmission of the coated plate was about 23 percent.

Individual coated sheets are treated and tested as in Example III with similar results.

EXAMPLE VI An iron film is produced as in Example III using the following coating solutions:

IRON SOLUTION Grams Ferrous sulfate 10 Boric acid 3 Sodium gluconate 7 Hydrazine sulfate 0.6 Water, added to 1 liter. Ammonium hydroxide, added to pH 7.5. Ethomeen C-20 0.06

REDUCING SOLUTION Grams Potassium borohydride 0.75 Water, added to 1 liter. Sodium hydroxide, added to pH 1 1.3. Ethomeen C-20 0.03

sium dichromate has a pronounced effect upon the durability of treated films with the already indicated range of treatment temperatures being preferred.

The resulting iron film comprises about 94 percent by weight iron and about 6 percent by weight boron. The coated plate has a luminous transmission of about 26 percent.

Individual coated sheets are treated and tested as in Example 111 with similar results.

Further experiments are conducted to determine the effects of individual process parameters.

Nickel coatings are treated with potassium dichro- Use of potassium dichromate solutions having pH values of 4 to 5 yields films that are noticeably more durable than those treated with solutions having pH values of 9 to 10.

The temperature of a contacting solution of potas- Copper films, in general, are to be treated under milder conditions than nickel films. Ideal temperatures for treating copper films are from 10C. to 60C. lower than those for treating nickel films.

Even extremely low concentrations of an appropriate weak oxidizing agent, such as potassium dichromate, are useful to provide some improved film durability. For example, even solutions containing less than 0.1 percent potassium dichromate cause a nickel film to have increased durability.

Although the present invention has been described by specific illustrative examples and by specific detailed descriptions of preferred embodiments, those skilled in the art will recognize other embodiments of this invention upon consideration of this disclosure. Accordingly, the applicants do not intend to unduly limit the scope of the accompanying claims by the specific disclosure provided.

We claim: 1. A method of making a transparent metal coated glass article comprising the steps of contacting the metallic film with an aqueous solution comprising water and a weak oxidizing agent selected from the group consisting of alkali metal dichromates, ammonium dichromate, alkali metal perborates, ammonium perborate, alkali metal permanganates and ammonium permanganate present as 0.1 to 10 percent by weight of the solution, the solution having a pH of at least about 4.

2.-The method of claim 1 wherein the weak oxidizing 10 agent is selected from the group consisting of alkali metal dichromates and ammonium dichromate.

3. The method of claim 1 wherein the weak oxidizing agent is potassium dichromate.

4. The method of claim 3 wherein the aqueous solution of potassium dichromate comprises on the basis of 1 liter of water: n

1 liter 1 to 50 grams.

water potassium dichromate 5. The method of claim 3 wherein the step of contacting the metallic film with an aqueous solution containing potassium dichromate is carried out at a temperature between about 20C and about 100C.

6. The method of claim 5 wherein the metal of the film is copper and wherein the contacting step is carried out at a temperature between about 25C. and about C. v

7. The method of claim 5 wherein the metal film is iron, cobalt, nickel or mixtures thereof and wherein the contacting step is carried out at a temperature between about C. and C. r

8. The method of claim 1 wherein the combination is maintained within the temperature range from at least about C. to below a temperature at which the glass is distorted for a time sufficient to decrease the electrical resistance of the metallic film to below about 50 percent of its initial electrical resistance prior to heating.

9. The method of claim 8 wherein the temperature of the combination is maintained until the electrical resistance of the metallic film has decreased at least about 90 percent of its initial electrical resistance prior to heating.

10. The method of claim 8 wherein the step of heating is carried out in a reducing atmosphere.

11. The method of claim 1 wherein the aqueous solu tion comprising a weak oxidizing agent has a pH of from about 4 to about 5.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. I 3 900 601 DATED August 19, 1975 |NVENTOR(S) Helmut Franz et a1.

it is certified that error appears in the ab0ve-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 1, line 4, after "copper" insert si lver.

Claim 1, line 4, "mixture" should be --mixtures-- Claim 4, line 3, delete "n".

Signed and Scaled this Attest:

RUTH 'C. MASON C. MARSHALL DANN Arresting Officer Commissioner oflarents and Trademarks

Claims (11)

1. A METHOD OF MAKING A TRANSPARENT METAL COATED GLASS ARTICLE COMPRISING THE STEPS OF DEPOSITING A TRANSPARENT METALLIC FILM OF IRON, COBALT, NICKEL, COPPER, OR MIXTURE THEREOF ON A SURFACE OF A GLASS SUBSTRATE, HEATING THE METALLIC FILM AND GLASS SUBSTRATE COMBINATION TO A TEMPERATURE WHICH IS AT LEAST 150*C. BUT BELOW A TEMPERATURE AT WHICH THE GLASS IS DISTORTED AND MAINTAINING THE COMBINATION WITHIN THAT TEMPERATURE RANGE FOR A TIME SUFFICIENT TO SUBSTANTIALLY REDUCE THE ELECTRICAL RESISTANCE OF THE METALLIC FILM, AND THEREAFTER CONTACTING THE METALLIC FILM WITH AN AQUEOUS SOLUTION COMPRISING WATER AND A WEAK OXIDIZING AGENT SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL DICHROMATES, AMMONIUM DICHROMATE, ALKALI METAL PERBORATES, AMMONIUM PERBORATE, ALKALI METAL PERMANGANATES AND AMMONIUM PERMANGANATE PRESENT AS 0.1 TO 10 PERCENT BY WEIGHT OF THE SOLUTION, THE SOLUTION HAVING A PH OF AT LEAST ABOUT 4.

2. The method of claim 1 wherein the weak oxidizing agent is selected from the group consisting of alkali metal dichromates and ammonium dichromate.

3. The method of claim 1 wherein the weak oxidizing agent is potassium dichromate.

4. The method of claim 3 wherein the aqueous solution of potassium dichromate comprises on the basis of 1 liter of water: n

5. The method of claim 3 wherein the step of contacting the metallic film with an aqueous solution containing potassium dichromate is carried out at a temperature between about 20*C and about 100*C.

6. The method of claim 5 wherein the metal of the film is copper and wherein the contacting step is carried out at a temperature between about 25*C. and about 70*C.

7. The method of claim 5 wherein the metal film is iron, cobalt, nickel or mixtures thereof and wherein the contacting step is carried out at a temperature between about 80*C. and 100*C.

8. The method of claim 1 wherein the combination is maintained within the temperature range from at least about 150*C. to below a temperature at which the glass is distorted for a time sufficient to decrease the electrical resistance of the metallic film to below about 50 percent of its initial electrical resistance prior to heating.

9. The method of claim 8 wherein the temperature of the combination is maintained until the electrical resistance of the metallic film has decreased at least about 90 percent of its initial electrical resistance prior to heating.

10. The method of claim 8 wherein the step of heating is carried out in a reducing atmosphere.

11. The method of claim 1 wherein the aqueous solution comprising a weak oxidizing agent has a pH of from about 4 to about 5.