US2898253A

US2898253A - High temperature protective coating for metals

Info

Publication number: US2898253A
Application number: US723634A
Authority: US
Inventors: Edward B Schneider; Everett G Stevens
Original assignee: North American Aviation Corp
Current assignee: North American Aviation Corp
Priority date: 1958-03-25
Filing date: 1958-03-25
Publication date: 1959-08-04
Anticipated expiration: 1976-08-04

Description

United States Patent HIGH TEMPERATURE PROTECTIVE COATING FOR METALS Edward B. Schneider, Canoga Park, and Everett G. Stevens, El Segundo, Califl, assignors to North American Aviation, Inc.

No Drawing. Application March 25, 1958 Serial No. 723,634

16 Claims. (Cl. 148 13.1)

This invention relates to a high temperature protective coating composition for metals. More particularly this invention relates to a coating composition which protects metal surfaces from oxidative and corrosive deterioration at elevated temperatures.

At temperatures required to heat treat metals such as titanium and its alloys, the metal is subject to gaseous contamination. This contamination results in a brittle layer on the surface and renders the material unsuitable for forming processes and structural use. This brittle layer must be removed before the material can be further fabricated or used as a structural member. Sandblasting, grinding and/or acid etching are among the methods which have been employed to remove this contaminated layer. Sandblasting can result in uneven metal loss; grinding produces sharp notches (stress risers); and acid etching often raises the hydrogen content above a tolerable amount. These procedures in addition to destroying expensive metal are time consumingand give rise to many production problems. Controlled atmospherefurnaces or encapsulating the titanium sheet in a steel bag are among the methods employed to prevent this contamination. These methods are inconvenient, complex, and in some cases costly. Therefore a more simplified and economical method for the high temperature processing of titanium is needed.

It is therefore an object of this invention to provide a coating which will protect metals at high temperatures. It is also an object of this invention to provide a coating which will protect metals during heat treatment at elevated temperatures. Another object is to provide a coating which will protect titanium during heat treatment. It is likewise an object ofthis invention to provide a coating composition which will prevent gaseous;

contamination of titanium and its alloys during treatment at temperatures in excess of 1100 F. Still another object of this invention is to provide a coating composition which will substantially reduce warpage of titanium and titanium alloy sheet stock when water quenched from temperatures in excess of 1100 F. It is also an object to provide a process for heat treating of metals so as to prevent contamination of the surface by the gases of the surrounding atmosphere. It is like- Wise an object of this invention to provide coated metal articles which are not subject to corrosive attack by atmospheric elements. Still other objects will be apparent from the discussion which follows.

The above and other objects of this invention are ac complished by providing a high temperature protective coatingcomposition for metal comprising 100 parts by weight of a ceramic material, from about 10 to about 200' parts by weight aluminum flakes, from about 1 to about 100 parts by weightof a bentonite clay, and an amount of diluent sufficient to give the composition a spreadable consistency. When a coating composition of this nature is applied to the surface of a metal article, the article may then be subjected to heat treatment at elevated temperatures without contamination of gaseous 2,898,253 Patented Aug. 4, 19 59 P CC . 2" elements of the surrounding atmosphere. An example is the heat treatment of titanium covered with a coat-j ing of this composition at temperatures in excess of 1100" F. There is no embrittlement of the titanium a r-I ticle. When, however, titanium is heat treated without a protective coating the surface oxidizes and becomes brittle which renders the material unsuitable for forming processes and structural use. H v p The ceramic material that is employed in the composition of this invention is a ceramic vitreous frit, sometimes also referred to as a porcelain enamel frit or a ceramically maturable vitreous enamel frit composition. An example of a frit of this type is Percent by wt.

Quartz 34.1 Borax 21.8 Feldspar 19.9 Soda ash 11.2 Fluorspar Sodium nitrate 4.9 Lithium manganite 2.1

hereinafter referred to as frit A, which is found to be satisfactory. An NBS ceramic coating frit designated as No. 332, hereinafter referred to as frit B is also found to give satisfactory results. It has the following 00111 Another vitreous ceramic composition which is found to give satisfactory results; is a frit, hereinafter referred to as frit C, whose composition is V p Percent by wt.

SiO 37.50 Alumina 1.00 "Boric acid 6.50 Calcium oxide 3.50 f Barium oxide 44.00 Zinc oxide V 5.00 Zirconium oxide 2.50

While the above are examples of various frits, composition of this invention is not limitedito the'use of these specific frits. Any ceramic frit which is niaturable into a vitreous compositionmay be used.

Best results are obtained if the frit, prior to use, put through a ball mill and the particle sizereduced so that it will pass through about a 70-mesh screem However, frit of particle, size which pass through a screen of about 20 to about mesh can also beused in the composition of this invention. 'One method accomplishing this is; to subject the an to. the action of a dry stage pebble mill for a. period of. about 2.4.- hours. x p

The aluminum that,v is used in the'composition: of this. invention is in the form of aluminu'rii flakes; Aluminum is reduced to alumiuum fiakes. by subjecting, alumi num powder to the action of a ball mill which, reduces the powder to fine flake-like particles. One form in which the aluminum may be employed is as an al'umi num paste which is prepared by mixing aluminum pigment with a liquid thinner such as toluene or with petroleum fraction hydrocarbons of the naphtha group, together with a leafing agent such as stearicor palmitie acids and subjected to the action of a ball mill. When used in this form the aluminum solids content of the paste is usually from about 50 to about 75 weight percent. The composition of the liquid thinner and leafing agent is immaterial since these hydrocarbon compounds are burned off before the fusion temperature to which the coating is subjected prior to and during the heat treatment of the coated metal. An example of a suitable aluminum paste is one containing 65 weight percent aluminum flakes in a naphtha solvent. The amount of coarse particles is such that not more than 0.5 weight percent are retained on a 100 mesh screen. Aluminum pastes in which not more than from about 0.1 to about 1.0 weight percent are retained on a 325 mesh screen are also satisfactory.

' 'I'he bentonite clay composition which is used in the coating of this invention is a well-known clay composition which swells upon the absorption of water and organic diluents and has strong absorbing properties. Bentonite is sodium montmorillonite or sometimes referred to as sodium bentonite. The bentonite may be used either in its original clay form or in the form of an organic substituted ammonium bentonite composition which is the product of the interaction between sodium bentonite and an organic ammonium compound. Thus, the term bentonite clay composition embraces boththe bentonite in its original clay form and also the organic ammonium bentonite. The organic ammonium bentonite compounds may be represented by the general formula:

wherein the N is nitrogen; Y is a clay substituent such as bentonite which includes montmorillonite, a component of bentonite; the R groups may be the same or different and represent hydrocarbon groups which have replaced hydrogen atoms attached to the nitrogen atom and are selected from the class consisting of alkyl, cycloalkyl, alkenyl, cyclo-alkenyl, aryl, arylkyl and alkaryl groups having from 1 to about 20 carbon atoms.

Non-limited examples of mono-hydrocarbon substituted ammonium bentonite compounds are 2-ethylhexylammonium bentonite, hexadecylammonium montmorillonite, dodecylpyridinium bentonite, octadecylammonium bentonite, etc. Examples of dihydrocarbon substituted ammonium clay compounds are di-Z-ethylhexylammonium bentonite, methylnaphthylammonium bentonite, didodecylammonium bentonite, etc. Examples of tri-hydrocarbon substituted ammonium bentonite compounds are trioctadecylammonium bentonite, methylcyclohexyldodecylammonium bentonite, dimethyloctadecyiammonium montmorillonite, etc. Non-limiting examples of tetra-hydrocarbon-substituted ammonium bentonite compounds are tetramethylammonium bentonite, dimethyldiethylammonium montmorillonite, dimethyldioctadecylammonium bentonite, methyltributylammonium montmorillonite, dimethyldodecylbenzylammonium bentonite, tetrabutylammonium bentonite, methyltrieicosylammonium bentonite, diethyldiphenylammonium bentonite, trimethylnaphthylammonium montmorillonite, dibutyldicyclohexylammonium bentonite, diethyldiisohexenylammonium bentonite, dipropenyldioctadecylammonium bentonite, dimethyldipropenylammonium bentonite, dimethyldi(2,4-di-heptylphenyl)ammonium bentonite.

Other organic ammonium clay compounds which are employed in the process of this invention are compounds in which the organic substituent on the nitrogen atom is an ether substituent-containing hydrocarbon, i.e., the substituent contains one or more -C-OC groups therein, as for example, ethylhexoxypropylammonium bentonite, octylphenoxyethoxyethyldimethylammonium bentonite, etc. Other ammonium clay compounds in- .clude rosinammonium bentonite, pyridinium bentonite,

etc.

The amine:clay ratio in the organic ammonium clay compound can have a value of 50 to 200 milliequiv. amine per 100 grams of clay. When an ammonium bentonite compound is employed, it is preferred to use a clay composition in which the amine:clay ratio is 75-ll5 milliequiv. amine per 100 grams of clay in order to obtain better protection upon heat treating of metals coated with a composition of the clay compound. Especially preferred is a clay compound in which the amine:clay ratio is substantially 80 milliequiv. amine per 100 grams of clay, the theoretical base exchange value for the combination.

The above hydrocarbon substituted ammonium bentonite compounds are made by the process substantially as described in Kolloid-Zeitschrift, 137. Band, Heft 1, Seite 40 (1954), and in US. 2,531,427. The hydrocarbon substituted ammonium bentonite is in a powdered form of a particle size substantially such that it will pass through a 200 mesh screen. However, particle sizes from about 0.002 to about 100 microns in diameter are suitable.

A coating composition containing a ceramic material, aluminum, and a bentonite clay composition affords pro tection to metal surfaces from deterioration due to attack by gaseous components during heat treatment at elevated temperatures. However, it has been found that occasionally the surface of a heat treated coated article has become slightly contaminated due to the action of oxygen and other components in the air, indicating that flaws or weak points may develop in the coating. It has also been found that the addition of nickel powder to the coating composition eliminates any failure in the protection rendered by the coating at all temperatures ranging from i about 1100" F. to about 1850 F. This is especially true in the case of heat treatment of titanium. Therefore, a coating composition as described above which contains in addition from about 1 to about parts by weight of nickel powder constitutes a preferred embodiment of this invention. The particle size of the nickel powder that is used is such that the powder will pass through a screen having a mesh of from about 200 to about 600 mesh. For example, good results are obtained when the particle size of the nickel powder is such that it will pass through a 325 mesh screen.

While the coating composition may be used in the form described above, it is advantageous to add a binding composition which will impart a better spreadable consistency for application purposes. The presence of a binder is also found to give added protection to the coated surface while it is being heated up to the fusion temperature of the coating. An embodiment of this invention is thereforea casting composition-as described abovewhi'ch conrains in' addition from about 1' to about 200 parts; by weight of a compatible organic binding composition based on 100 parts by weight of ceramic material. The type of composition employed is not critical so that any compatible binding composition may be used. It may be any polymeric, resinous, or plastic material which will serve to bind the solid particles in the coating together so as to form a continuous film on the surface of the metal to" which it is applied. Hence, the binding composition can be composed of acrylate resins, polymeric epoxy resins, polyurethans, alkyd resins, copolymers of various resins, and the like. The preparation of these resins is well known to those skilled in the art and can be found in such textbooks as Organic Chemistry, by Fieser and Fieser, published by D. C. Heath & 60., Boston. Nonlimiting examples of acrylate resins which can be used in the coating are methylmethacrylate, ethylrnethacrylate, n' propylmethyacryl'ate, diethylen'eglycolmethacrylate, methylethacrylate, etc. An example of an alkyd resin is the condensation product of glycerol and phthalic anhydride. Other alkyd resins are well known to those skilled in the art. An example of an epoxy resin is a glycidyl polyether resin obtained by the reaction upon heating of epichlorohydrin with 2,2-bis( 4-hydroxyphenyl) propane in the presence of sodium hydroxide asa catalyst. This produces a polymer in which the molecular units having terminal epoxy groups. Other epoxy resins prepared by the reaction of epichlorohydrin with a polybyd' c c ol such as. l., .3- hv roxvp op ne re. well known in the art. An example of a polyurethan is a diisocyanate of propylene glycol of the general formula wherein n is a number taken from the series 0, 1, 2, 3, and having an average molecular weight of substantially 2500.

The diluent used in the preparation of the coating of this invention is any compatible diluent including Water. This includes any of the well known diluents employed with resins and polymers" in the paint industry. Nonlimitin'g examples of diluents include lower aliphatic alcohols, lower aliphatic ketones', lower alkyl esters of lower aliphatic acids, and lower hydrocarbons such as benzene and lower alkyl substituted benzenes, all containing up to about 14-. carbon atoms. Non-limiting examples of these diluents are acetone, methylethyl ketone, diethyl ketone, diisopropyl ketone, octyl hexyl ketone, methylacetate, butylacetate, octylacetate, methyl propionate, octylhexanoate, benzene, toluene, xylene, ethyl benzene, tern-butyl-benzene, e tc. p v V I The resin may contain various customary compatible plasticizers, the nature and amounts of which are Well known to those skilled in the art and will not be discussed in this writing.

The amount or diluent employed with the coating can vary froml to times the. amount of combined ceramic frit, aluminum bentonite and resin compounds in parts by: weight. The amount of diluent, can be adjusted to suit the particular application, namely, brush application, spraying, dipping or other appropriate means for spreading the coating on the surface. A ratio of combined solid components-to-diluent of 1:1 is found to be satisfactory when' the coating is spread with a spatula on the surface to be protected. For spray application, it is found that the composition is of the proper consistency when the ratio of solids to diluent is about 1:10. Still greater amounts of diluent maybe employed if desired, however, ordinarily amounts in excess of that which would give a ratio of solidsto-diluent of 1:10 give. no additional advantage and only increase the amount of diluent that must be evaporated from the coating.

Small amounts of low boiling organic compounds having from about 2 to about. 8 carbon atoms. such as from about 0.1 to about 5. weight percent ethyl alcohol based on the weight ofth dilu nt-may be added to aid in the subsequent evaporation of the diluent fromthe coating. The lower alkyl alcohols can be employed as diluent either alone or in combination with: other diluents. These alcohols can have from about lto abouts carbon atoms and includes such compounds as methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, butyl alcohol, amyl alcohol, octyl alcohol, phenylethyl alcohol, etc.

In' addition to a; process for heat treating metals; an embodiment of this invention provides coated metal articles in which the coating protects the surface from oxidation at ambient atmospheric conditions as well as at cle v'ated' temperatures. In this manner, metal articles which are to be heat treated may be protected from oxidation for long periods of time prior to the actual treating of the articles. Therefore, an embodiment ofthis invert tion includes a metal article having a coating consisting of a composition which" is more particularly described elsewhere in this writing. For example, an embodiment of this invention comprises a metallic article having a surface coating consisting essentially of a mixture of a ceramic material, aluminum and a bentonite clay composition. A preferred embodiment of this invention is a. metal article having a surface coating as described above containing nickel powder in addition thereto. Another embodiment of this invention is a metallic article havinga surfacecoating consisting essentially of a ceramic material, aluminum, a bentonite clay composition, a cornpatible organic binding composition, with or without nickel powder. 1 V

When the coated metallic article has been initially heated to a temperature sufficient to decompose the orgame material in the composition to'form a film of noncombustible material on the surface of the article, it is then heated to a temperature suflicient' tof fuse the Thereafter the coated article is heat tr'eated at a prodeter'rnin'ed temperature. I

The surface coating upon fusion consists essentially of a fused mixture of ceramic material, aluminum, bentonitei clay composition, with or without the presence of nickel. The proportions of these various components of the fused coating depends on the amount that was originally added in the preparation of the coating composition as described elsewhere in this Writing. A metallic article containingsuch a fused surface, coating constitutes an embodiment of this invention since. it provides a metal; li'c article having a protected surface. The article can be put into service in certain applications where high temperature surface protection is required without removal of the coating. For example, such a coated tita nium article will be protected from'oxygen contamination after the heat treatmentv cycle attemperatures upv to the heat treating temperature, and higher. p I o 'While the amount of aluminum that is used per 10.0 parts by weight of ceramic materialcan vary from about 1 t about 1mm by we t... it. s fe n h t po sibility of failure of the protection afiorded by the coat ing can be greatly reduced when the amount of alumi: num is from about 50 to about 150 parts by weight per parts of ceramic material. Therefore, a coating composition containing an amount of aluminum Within the latter designated range constitutes a preferred embodi; ment of this invention. In like manner, it is found that a preferred amount of bentonite clay composition varies from about 2 to about 50 parts by weight per 100 parts of ceramic material. The preferred amountof nickel powder for better protection is from about 2 to about 50. parts by weight per 100 partsof ceramic material.

It is also found that in order to insure cohesiveness'ofthe coating material upon application to the. surface of the metallic, article, it is advisable to employ from about 50 to about parts by weight of; an organic binding composition per 100 parts of ceramic. material. All the above ranges of components constitute a preferred em-: bodiment of invention.

An especially preferred composition which gives optimum protection to metallic articles both before and after fusion is one comprising substantially 100 parts by weight of a ceramic material, substantially 70 parts by weight aluminum flakes, substantially 10 parts by weight dimethyldidodecyl ammonium bentonite, substantially 10 parts by weight nickel powder, substantially 100 parts by weight methylmethacrylate resin and sufficient volatile diluent to give the composition a spreadable consistency. An example of diluent is about 220 parts by weight of methyl isobutyl ketone.

The general process for the preparation of the coating composition of this invention consists of mixing together the solid components, namely the ceramic material, the aluminum flakes, the bentonite clay composition, and the nickel powder if it is used, together with the binding composition if employed and whatever diluent is used to give the composition a spreadable consistency. The mixing can be accomplished by stirring, tumbling, milling or any other appropriate means well known to the art. One method of thoroughly mixing the components is to subject them to the action of a ball mill for a period of from about 4 to about 16 hours. Alternatively, each component may be reduced to the proper particle size by grinding, comminuting, or other appropriate means such as by the action of a ball mill. One method is to charge the ceramic material, the bentonite clay composition, together with the nickel powder and acrylic resin if the latter two are employed, and a small amount of diluent to a ball mill and subject the components to the grinding action of the mill for a period of from about 4 to about 24 hours. The components are then removed and aluminum flakes added either in the dry or paste form and blended in by mixing, stirring, or other appropriate means until each component is evenly dispersed throughout the entire composition. The following examples will more clearly illustrate the composition. and process of this invention.

- EXAMPLE I EXAMPLE II The process of Example I is repeated with the modification that about 1 part of nickel is added to the ball mill.

EXAMPLE In The procedure of Example I is repeated with the modification that about 1 part of nickel powder together with about 1 part of methylmethacrylate resin is added.

EXAMPLE IV The procedure of Example III is repeated with the modification that a 50/50 mixture of ethyl alcohol and acetone is substituted in place in the water as a diluent.

EXAMPLE V To a ball mill are added substantially 100 parts by weight of the ceramic frit C, substantially 70 parts by weight of aluminum flakes, substantially 10 parts by weight of dimethyl didodecyl ammonium bentonite, substantially 10 parts by weight of nickel powder, substantially 30 parts by weight of methylmethacrylate resin, and about 220 parts by weight of methyl isobutyl ketone. The contents are subjected to the grinding action of a ball mill for a period of about 16 hours. The contents are then removed and the ball mill washed with an additional'amount of about 70 parts of methyl isobutyl ke tone and the composition with added washing is stirred until a uniform consistency is obtained.

EXAMPLE VI To a ball mill were added 50 parts by weight of ceramic frit C, 5 parts by weight of nickel powder, 5 parts by weight of dimethyldidodecyl ammonium bentonite, about 60 parts by weight of an aluminum pigment paste consisting of aluminum flakes in an n-heptane vehicle which contained about 5 weight percent stearic acid as a leafing agent with the aluminum flake content of the paste being about 65 weight percent, about 15 parts by weight of methylmethacrylate resin, dissolved in about 35 parts of methyl ethyl ketone and about 45 parts of methyl isobutyl ketone. The components were subjected to the grinding action of the ball mill for a period of about 16 hours after which time the components were removed and the mill washed out with an additional 15 parts of methyl isobutyl ketone and the washing added to the removed contents. The ground composition and added washings were stirred until a composition of uniform consistency was obtained.

EXAMPLE VII The procedure of Example VI was repeated with the modification that the aluminum pigment paste was not added to the ball mill, but was added to the milled components after removal from the ball mill. The milled components together with the aluminum paste and added washing fluid were stirred until a composition of uniform consistency was obtained.

The following table contains examples of still other coating compositions of this invention in which the components are present in various proportions.

Table Parts by weight Composition No.

Frit A 100 100 1 Frit B 00 100 Frit O 100 Aluminum flakes 150 100 75 200 Bentonite 50 25 Dimethyldidodecl ammonium bentonite 50 15 50 2 100 20 75 100 200 50 Methyl isobutyl ketone. 100 2, 000 Toluene 200 50 5, 000 20 Ethylacetate 100 n-Heptane 100 50 100 l Resin D is an alkyd resin obtained by the condensation of ethylene glycol with phthallic anhydride.

2 Resin E is an Epoxy resin obtained by heating together equimolar quantities of epichlorohydrin and 2,?Fbis(4-hydroxyphenyl) propane in the presence of a sodium hydroxide catalyst, and having an epoxide equivalent; of about -210 grams of resin per gram equivalent of epoxide The metal article which is to be heat treated is coated with the composition of this invention by spreading, brushing, or spraying so as to provide a coat having a thickness of from about 0.25 to about 20 mils (thousandths of an inch), preferred 0.5 to 6 mils. The coating is then allowed to air dry for a short period of time of from about 1 to about 20 minutes in order to allow the excess diluent to evaporate. The coating is sufficiently dry when it is firm to the touch and exhibits no tackiness. The coated metal article is then ready for heat treatment. In one embodiment of this invention the coated metal article is placed in a furnace originally at ambient temperature and then heatis applied to bring the temperature of the coated article up to a point at which the organic material in the coating is decomposed and burned off, leaving an anhydrous film having no carbon, hydrogen or ammonium therein. This usually occurs at a temperature of substantially from about 500 F.'to about 700 F. Following this more heat is applied sutfia cient tofuse the coating and then additional heat is applied to bring the article to the predetermined heattreating temperature of the metal in question. The metal is heat treated at a temperature of substantially from about 1100 F. to about 1850 F. for periods of time of from about 0.5 to about 4.0 hours. In the case of alloys of titanium such as alloys containing 6 weight percent aluminum and 4 weight percent vanadium, the article is subjected to heat treatment at a temperature of substantially 1700 F. for a period of about 30 minutes to'eifect what is known as solution treatment which consists of maintaining the alloy at this high temperature so as to form a solid solution of aluminum and vanadium in the titanium. After this heat treatment the titanium article is immediately water quenched. Articles of other metals may be brought to ambient temperatures by slow cooling in an airstream.

The film left on the metal article after heat treatment is readily removed by subjecting it to the action of aqueous mineral acid solutions such as aqueous solutions of hydrofluoric and nitric acids for a period of from about 10 to about 30 minutes. When, however, the metal is heat treated without the coating composition of this invention on its surface, the outside scale which is formed is much greater than the coating composition thickness and cannot be removed by acid solutions. Instead, it must be sandblasted, vapor blasted, or machined off.

The following examples will more clearly illustrate the protection afforded metallic articles during heat treatment. I

EXAMPLE VIII The coating composition of Example V was reduced with an additional 150 parts by weight of methyl isobutyl ketone in order to give a consistency applicable for spray application. The composition was then sprayed onto a titanium article consisting of 90v Weight percent titanium, 6 weight percent aluminum and 4 weight percent vanadium, after the surface had first been cleaned with methyl ethyl ketone solvent to remove any grease or foreign matter. applied to the surface with a subsequent air drying period of about 15 minutes. This provided a coating substantially 6 mils (0.006 inch) in thickness. The coated titanium article was then placed in an oven maintained at 600 F. for 30 minutes during which time the organic material volatilized or oxidized and decomposed leaving a film of combined ceramic frit aluminum bentlonite and nickel. The titanium specimen with the coating was then placed in a furnace maintained at a temperature of substantially 1700 F. for a period of about 30 minutes. The article was then removed from the furnace and immediately quenched in water.

The surface film or scale on the coated titanium article' which had been heat treated was found to be substantially 1 mil thick. Analysis or the surface scale shows there is no hydrocarbon, nitrogen or water present and that only aluminum oxide, aluminum silicate, silica, nickel oxide and the components of frit C make up the film indicating that the film consists mainly of the fused components of nickel, aluminum bentonite clay and ceramic frit. The article was then immersed in an aqueous solution containing 50 weight percent sodium hydroxide and 1 weight percent sodium chromate maintained at 275 F. followed by an aqueous acid solution containing 3 weight percent hydrofluoric acid and 35 weight percent nitric acid for a period of substantially 10 minutes. This removed the surface film leaving a shiny smooth surface.

EXAMPLE IX A commercially pure titanium metal article 1.0 inch x 10.0 inches x 0.032 inch was covered with the composition of Example I by meansof a brush to provide a surface film of approximately 20 mils in thickness. The

A total of two spray coats were I0 film was allowed to dry in the air for about 45 minutes. The film-coated article was then placed in the furnace maintained at 1400 F. and kept there for a period of substantially 4 hours. The article was then removed from the furnace and immediately quenched in water. The titanium article did not warp upon quenching.

A similar specimen of titanium treated as described in Example IX without the protection of a coating of this composition, warped upon quenching to a degree of 6 percent to 10 percent variation from flat.

EXAMPLE X A specimen of 6Al4V titanium containing 6 percent aluminum, 4 percent vanadium, and measuring 36 inches by inches by .040 inch was coated with the composition of Example V according to the procedure described in Example VIII to form a film 4.0 mils thick. After air drying and baking at 500 F. for a period of substantially 30 minutes at which temperature the organic material in the coating decomposed and the coating formed a film on the surface of the article, the specimen was heat treated in an oven at 1700 F. for 30 minutes and then water quenched immediately. The coating which fused at the heat treating temperature was removed by vapor blasting. The sheet was then heat aged at 1000 F. for 4 hours. Warpage was from 2.5 to 3.5 percent variation from flat.

Several specimens were prepared according to the procedure described in Example IX and subjected to various tests. A specimen treated according to Example IX with the protection of the coating of this composition was subjected to a bend test. It was found that a radius of bend of 3.8T was obtained with no failure in the specimen material. T is the thickness of the material and in this case was 0.040 inch. However, a specimen, which has been treated in like manner but without the coating of this composition on its surface, cracked upon being bent to a bend radius greater than 10T.

A 6A1-4V titanium specimen which had been coated and treated as described in Example IX was subjected to tensile tests. It was found that it had a transverse yield strength at 157,500 p.s.i., an ultimate tensile strength at 171,100 p.s.i. and an elongation of 7.8 percent, whereas a similar article treated in like manner, but without a coating of this composition had a yield strength of 166,000 p.s.i., an ultimate strength of 179,000. p.s.i., and an elongation of 2.7 percent. It is seen that there is an increase of 35 percent in elongation on the coated specimen.

EXAMPLE XI a An AllOAT titanium disk 24 inches in diameter and 0.187 inch in thickness composed of titanium containing 5 weight percent aluminum and 2.5 weight percent of tin, was coated with a composition of Example V to a thickness of 5 mils. The coating was dried in air for a period of 30 minutes and then subjected to a temperature of 700 F. for 30 minutes. The coated titanium specimen was next annealed at l600 F. for a period of 30 minutes and then air cooled. The coating was then removed from the titanium specimen by subjecting it to an aqueous caustic solution containing 50 weight percent of NaOH and 1 percent Na CrO maintained at 2751:5" F., followed by an aqueous acid solution containing 3 weight percent hydrofluoric acid and 35 weight percent nitric acid, for a period of substantially 10 minutes. This removed the film leaving a shiny smooth surface. The specimen was then formed into a 16 inch diameter hemisphere on a 7000 ton hydroform press. No flaws or cracks appeared in the titanium during this forming process.

A disk of titanium similar to that employed in Example XI which had been treated without a coating composition, cracked severely during the pressing in the hydroform press and could not, be drawn out into a hemispherical shape. This indicates that the surface of the uncoated specimen was contaminated by oxygen and other gaseous impurities during the annealing cycle and this caused the specimen to fail upon being subjected to the forming process in the hydroform press.

A titanium specimen measuring 0.032 x 1.0 x 10.0 coated with the composition of Example VI to form a film 0.25 mil thick and subjected to heat treating temperature of 1850 F. for a period of 30 minutes showed acceptable bend and tensile strength characteristics. In like manner the specimens heat treated while covered with coatings of Examples I-VII and with the coatings shown in the table, exhibited satisfactory tensile and bend characteristics.

Specimens of steel and Inconel X when heat treated with a coating composition as described in Examples I-VII or with one of the compositions shown in the table, are found to be adequately protected from oxidization and gaseous surface contamination during heat treatment.

Titanium articles are important in the manufacture of structural parts for high-speed aircraft. It is vital that such titanium possess maximum tensile and yield strength. Therefore, contamination by oxygen or other gases in the atmosphere during annealing cannot be tolerated. For example, two of the 16 inch diameter spherical or dome-shaped articles described in Example XI are welded together to form a tank construction used to house fuel on high-speed aircraft. It is seen that without the use of applicants coating composition such a dome-shaped titanium member cannot be manufactured unless other inconvenient, complex, and costly methods are employed to remove or prevent the contamination of the titanium alloy as discussed hereinabove.

EXAMPLE XII A titanium billet measuring 6.0 x 6.0" x 18.0" is covered with the composition marked No. in the table, by means of brush application to a thickness of about 20 mils. The coating is air dried for a period of 1 hour and then baked at 600 F. for a period of 15 minutes. The billet is then heated to 1850 F. and squeezed down in a forging press to a 20 percent reduction in crosssection. The surface film is then removed from the reduced billet by sandblasting to a smooth clear surface with a surface metal loss of approximately 0.002 inch.

When the experiment of Example XII is repeated Without a coating, an oxide scale 5 mils thick develops which on removal results in a surface metal loss of 0.005 to 0.010 inch.

In like manner a 4130 carbon steel specimen coated with composition No. 4 of the table to a depth of 0.5 mil prevented scale formation on the steel specimen while being normalized at 1800 F. for 30 minutes. The specimen with no composition to protect it developed a scale 0.006 inch thick which could not be removed by pickling and had to be sandblasted ofi leaving an uneven or rough surface.

EXAMPLE XIII A specimen of 6A1-4V titanium, coated to a depth of 5 mils with composition No. 2 of the table by means of dipping, when subjected to an aging treatment at 1100 F. for 4 hours develops a negligible amount of scale on the surface which is readily removed upon immersion in an aqueous sodium hydroxide bath containing sodium chromate, followed by an acid bath.

When the heat aging process of Example XIII is repeated without the use of a coating of this invention, the scale formed is thicker than in Example XIII and cannot be removed by a sodium hydroxide-sodium chromate bath, but requires a more drastic technique such as sandblasting which not only is more costly and time consuming, but results in the loss of an appreciable amount of metal from the surface.

Although the invention has been described and illustrated in detail, it is to be understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

We claim:

1. A high temperature protective coating composition for metal consisting essentially of parts by weight of a ceramic material, from about 10 to about 200 parts by weight aluminum, from about 1 to about 100 parts by weight of a bentonite clay composition and an amount of diluent sufiicient to give the composition a spreadable consistency.

2. The composition of claim 1 containing in addition from about 1 to about 100 parts by weight nickel powder.

3. The composition of claim 1 containing in addition from about 1 to about 100 parts by weight nickel powder and from about 1 to about 200-parts by weight of an organic binding composition.

4. The composition of claim 1 containing in addition from about 1 to about 100 parts by weight of nickel powder and from about 1 to about 200 parts by weight of an organic binding composition and wherein the diluent is selected from the class consisting of water and volatile organic compounds in an amount of from about 1 to about 10 times the total solid content.

5. A high temperature protective coating composition for metal consisting essentially of 100 parts by weight of a ceramic material, from about 50 to about parts by weight of aluminum, from about 2 to about 50 parts by weight of a bentonite clay composition, and an amount of diluent sufficient to give the composition a spreadable consistency.

6. The composition of claim 5 containing in addition from about 2 to about 50 parts by weight of nickel powder.

7. The composition of claim 5 containing in addition from about 2 to about 50 parts by weight nickel powder and from about 50 to about 150 parts by weight of a compatible organic binding composition.

8. A high temperature protective coating composition for metals consisting essentially of substantially 100 parts by weight of a ceramic material, substantially 70 parts by weight aluminum flakes, substantially 10 parts by weight dimethyl didodecyl ammonium bentonite, sub stantially 10 parts by weight nickel powder, substantially 30 parts by weight of methyl methacrylate resin and substantially 290 parts by weight of methyl isobutyl ketone.

9. A process of heat-treating metallic articles comprising applying to the surface of said article the high temperature coating composition of claim 1 to form a coated article, initially heating said coated article to ture suflicient to fuse said coating, and thereafter heattreating said metallic article at a predetermined temperature.

10. A process of heat-treating metallic articles comprising applying to the surface of said articles the high temperature coating composition of claim 8, to form a coated article, initially heating said coated article to a temperature sufficient to decompose the organic material of said composition to form a film of non-combustible material on the surface of said article, heating said film to a temperature sufiicient to fuse said film and thereafter heat-treating said metallic article at a predetermined temperature.

11. The process of heat treating a metal article comprising applying to the surface a high temperature coating composition of claim 3, initially heating said coated article to a temperature sufficient to decompose the organic matter in said coating to form a film on the surface of said article, and thereafter heat treating said metal article at a predetermined temperature.

12. The process of heat treating a metal article comprising applying to the surface a high temperature coating composition of claim 8, initially heating said coated 13 article to a temperature suf'ncient to decompose the organic material in said composition to form a film on the surface of said article and thereafter heat treating said metal article at a predetermined temperature.

13. The process of heat treating a metal article comprising applying to the surface of said article the composition of claim I initially heating said article to a temperature suificient to fuse said composition on said surface, and subsequently heat treating said article at a temperature of from about 1100 to about 1850 F. for a period of from about 15 to about 60 minutes, cooling said article and removing said coating from surface.

14. A process of heat treating titanium metals and a1- loys thereof comprising coating said titanium with the composition of claim 8, heating said coated titanium articles to a temperature of substantially 575 F. for a 14 period of substantially minutes and then heat treating said article at a temperature of from about 1400 to about 1850 F. for a period of substantially 30 minutes, and quenching said heat treated article.

15. A metal article having a coating consisting of essentially parts by weight of a ceramic material, from about 10 to about 200 parts aluminum, and from about 1 to about 100 parts of a bentonite clay composition.

16. A metallic article having a coating consisting essentially of 100 parts by weight of a ceramically maturable vitreous enamel composition, from about 50 to about parts by weight of aluminum, from about 2 to about 50 parts by weight of a bentonite clay composition, from about 2 to about 50 parts by weight of nickel.

No references cited.

Claims

1. A HIGH TEMPERATURE PROTECTIVE COATING COMPOSITION FOR METAL CONSISTING ESSENTIALLY OF 100 PARTS BY WEIGHT OF A CERAMIC MATERIAL, FROM ABOUT 10 TO ABOUT 200 PARTS BY WEIGHT ALUMINUM, FROM ABOUT 1 TO ABOUT 100 PARTS BY WEIGHT OF A BENTONITE CLAY COMPOSITION AND AN AMOUNT OF DILUENT SUFFICIENT TO GIVE THE COMPOSITION A SPREADABLE CONSISTENCY.