US2785091A - Method of protecting metal surfaces, composition therefor, and article resulting therefrom - Google Patents

Description

METHOD OF PROTECTING METAL SURFACES, COMPOSITION THEREFOR, AND ARTICLE RESULTING THEREFROM Carl A. M. Rex, Hoboken, N. 1., assignor of one-half to Jack F. Govan and one-half to Roy H. Govan, Hoboken, N. I.

No Drawing. Application October 23, 1953, V Serial No. 388,069

12 Claims. (Cl. 117-129 The present invention relates to the coating of metals and other materials and the resulting products, and more particularly to coating of metal surfaces of the like to protect such surfaces when exposed to heat, oxidizing conditions, corrosive vapors, mechanical erosion, and the like.

This application is a continuation-in-part of applicants copending application Serial No. 205,035, filed January 8, 1951, entitled Method of Protecting Metal Surfaces, Composition Therefor, and Article Resulting Therefrom, patented November 10, 1953, as Patent Number 2,658,- 834.

Prior to the instant invention, many processes and coating materials have been proposed for use in protecting metallic surfaces. However, it has been found that the articles containing these coatings have been unsatisfactory in one or more of the following respects. At high temperatures, the coatings have been found to decompose and burn and thus lose all of their protective characteristics. The enamel and ceramic glazes, due to the methods of application used, 'have been found incapable of withstanding severe thermal and percussive shock. Such coatings have been found to be incapable of adhering Well to the base metal during usage. None of the coatings nor processes of coating previously proposed has been found capable of being applied to metals and other surfaces of material having widely different expansion coefiicients such as iron, steel, aluminum, copper, stainless steel, tungsten, molybdenum, magnesium, titanium, and the like, and which will adhere to such a variety of surfaces when thermal shock is imparted to the coating, such as by heating followed by sudden cooling. In such cases, the difierences in the expansion of the metals and the coating occurs so suddenly as to set up enormous stresses and strains which dislodge the coatings. Further, the coatings as previously applied will not withstand flexing, bending, forming, swaging, or drawing of the coated surface without dislodging the coating.

An object of the present invention is to provide a coating for metals and other materials capable of protecting such materials during heat treatment. 7

A further object of the present invention is to provide .acoating whichwill protect metals against corrosion and is capable of withstanding thermal and percussive shock regardless of the differences in coefficients of the coating and the metals coated therewith.

A still further object of the present invention is to provide a protective coating for metals and the like which aifords protection to the surface coated therewith fin'either'avitrifiedor non-vitrified form.

still further object of the present invention is to provide .a coating for metals and the like to protect against oxidizing or' corrisive gases by reducing or reacting with'such gases as they diffuse through the coattag so' asito pr'ote'ct thejbase metal or the like.

r 2,785,091 Patented Mar. 12,1957

, ice.

coating for metals and the like which is capable of withstanding flexing of the coated surface without dislodging the coating.

Other objects and the nature and advantages of the invention will be apparent from the following description. r

In applicants copending case referred to above, it was discovered that sodium meta borate in combination with a plastic clay, and an oxidation inhibitor, such as graphite, carbon black, iron or copper, yields a coating material that provided a protection to the surface on-whichit is applied against corrosive vapors, fumes, oxidizing gases and the like. It has now been found that sodium meta borate alone, or in certain combinations other than those specified in the copending case, yields excellent results.

The coating composition of the present invention 7 utilizes sodium meta borate alone or in conjunction with clay, and certain additional ingredients, such as the oxides of titanium, chromium, copper, magnesium, zir conium, aluminum, calcium and the like. When such ingredients are mixed in the proper proportions, it has been found that the resulting material, when applied as a coating by any suitable means, provides a protection against corrosive vapors, fumes, oxidizing gases and the like to a wide variety of materials such as metals, all possessed of widely different characteristics. Further, when the coated metal is subjected to a temperature sufficient for the coating to vitrify and fuse, such heating can be continued considerably beyond such initial fusion point without decomposition, development of gas, formation of bubbles or otheiwise developing a condition that would cause a disruption of the continuity of the coating film. Still further such vitrified coating, when subjected to severe thermal shock, tenaciously adheres to the metal surface.

The sodium'meta borate utilized in the coating can be any of the materials commonly known as sodium meta borate, hydrated or anhydrous. Following are several examples which have been found to work successfully:

NaBO2.2I-I2O NaBOzAHzO Na3B204.4I-IaO Na2B204.8H2O NaHaBOal-IzO NaHzBO3.3H2O

Instead of adding the sodium meta borate, in the above forms, equally good results can be obtained by admixingNazO :and B203 in mol ratios substantially equal to that existing in sodium meta borate. The sodium and boron compounds added can consist of alkalies of sodium together with oxides or acids of boron which, in combination, Will give a sodium meta borate. Examples of the sodium alkali suitable for use are sodium oxide, carbonate, bicarbonate, hydroxide, or any organic sodium salt which upon thermal decomposition or ignition will give an alkaline ash of sodium oxide or carbonate. The boron compound used can be boron oxide, boric'aci'd, borax or the like.

The inclusion of the clay is not essential, since effective coatings can be obtainedfor certain purposes without this ingredient. The inclusion of the clay, however,'resul-ts in a fused vitreous product in which the borate acts as a flux for 'thecl ay. A suitable clay for use in the coating ,ofthe'ceramic industry. Clays which are. not possessed ofthis plasticity arenot suitable for use in the present inventiom The oxide or oxides to be incorporated in the coating must be comrninuted to small particles size so as to distribute itself smoothly throughout the protective layer, to reduce the tendency to separate and stratify while the coating isair-drying, and to present a maximumsurface. The oxides are "those known asbasic oxides and amphoteric oxides, namely, those that will react with acids to form salts. Suitable oxides include titanium dioxide, copper oxide, magnesium oxide, zirconium oxide, aluminum oxide, calcium oxide, chromium oxide and the like or combinations such as in chromite (FeQCrzOs), copper oxide and zirconium oxide, titanium dioxide and calcium oxide, and many others.

In addition to the oxide or oxides, powdered graphite, carbon black, ironor copper can be included in the composition. The resultant films qafter firing at considerably elevated temperatures are significantly free from the bubbles and air inclusions that characterize other compositions for the same purpose.

Additionally, this combination of ingredients appears to result in a vitreous coating possessed of considerable elasticity so that as stresses due to difference in expansions are encountered it successfully withstands any tendency of the surface to crack, check or dislodge itself.

In the application of the material to the surface to be coated, it is preferable that water be used to effect solution of the soluble ingredients and suspension of the insoluble ingredients, using suflicient liquid to form a slurry having the consistency of paint. If, in addition, a proportion of an organic adhesive or binding agent is added thereto either with the original dry materials or with the water used for solution and dispersion, a better adhesion of the wet coating to the work ispsecured, and upon mere drying results in a coating that will withstand more handling and abuse than one prepared without the benefit of such adhesivematerial. Therefore, when applying the coating to the work it is deemed preferable to include an adhesive agent, such asdextride, and the preferred embodiment of the invention makes use of the same. It

is, of course, understood that any similar adhesive such as starch, gum, gum acacia, gum tragacanth, gum arabic, etc., can'be utilized in place of the dextrine quite as effectively. p

The inclusion of such adhesive is for the primary purpose of imparting desirable properties to the coating before heating, for upon heating the adhesive is decomposed at a relatively low temperature. However, by this time the natural adhesiveness of the coating itself begins to come into play and, of course, at high temperature the adhesiveness of the fused mass is brought into full force and effect.

In the application of this invention it has been found that the following range of proportions is satisfactory:

The following specific examples are intended to illustrate the present invention, but are not intended to limit the same:

Example 1 In a mixer, 95 lbs. sodium-meta borate octa hydrate (N212BzO4.8H2O) and 25 gallons Water were mixed until Example 2 26 lbs. powdered titanium dioxide 10 lbs. powdered graphite 20 lbs. dextrine lbs. Kentucky ball clay were mixed in a paddle mixer until all of the ingredients were uniformly distributed. i

In a separate mixer, lbs. sodium meta borate octa hydrate (NaB2O4.8HzO) and 25 gallons water were mixed until the borate dissolved using heat if necessary.

' To this solution, the dry ingredients were added while agitating, a little at a time. When all were added,'the mixture was mixed for 2 /2 hours until all ingredients were thoroughly 'andintimately distributed. In this form, the batch was ready for use.

The surface to be coated was coated with the material in any suitable manner and then dried. The coated material even in this form was afforded protection against corrosion and oxidation, and the coating adhered well to the surface. Upon heating of the coated material, the coating will vitrify and the resulting vitrified coated surface may then be subjected to intense heat for long periods of time with sudden cooling, or alternate and repeated heating and cooling, and there is no indication of fusion, of oxidation or of wasting away' of the material so coated.

It is a common failing of previous coating compositions utilizing sodium oxide-boron oxide mixtures that beyond their fusion points they tend to undergo a decomposition or evolution of gas due to vaporization or some similar phenomenon whereby bubbles are created in the otherwise smooth protective surface and which upon disruption causes a break in the continuity of the coating with exposure .of bare uncoated surface. In accordance with the present invention, the utilization of a ratio of NazO to B203 substantially the same .as that existing in sodium meta borate, in' conjunction with the ingredients specified above, resulted in a coating without this failing, and having a myriad of desirable properties.

The preferred composition results in an increased fusion point as compared to the fusion point of the flnxing agent. Whereas sodium meta borate fuses at 1771* F.,

the coating in accordance with this invention does not fuse until much higher temperatures are reached. Metal coated with sodium meta borate alone and heated to the fusion point of the meta borate results in an immediate evolution of gas and blistering of the. flux along with distinct evidence of the oxidation or burning of the metal. In contrast, when metal is coated withthis coating composition, the coating can be fusedrand heated considerably beyond the fusion point to mu ch higher temperatures without any evolution of gas or other disruption of the film and with full protection of the metal against the corrosive and oxidation irifluences ofair and ga ses.

As illustrative of the wide range of metals having different expansion coetficients towhich this coating material has been applied successfully are the following:

Metal: CoefiicientX 10- Aluminum 18.35 Copper -Q. 14.09 Iron 8-10 Steel 10-13 Stainless steel 9-12 It is to be understood that the inclusion of other ingredients such as various oxides, pigments, etc. to produce a decorative effect, color opacity, or other physical change, such as change in fusion point, will suggest itself to those versed in the art and that any such modification does not detract or alter the basic invention embodied herein.

Further examples of coating compositions that have been used successfully are:

Example 3 Percent Sodium meta borate 37.5

Kentucky ball clay 37.5

Titanium dioxide 12.5

Dextrine 12.5

Example 4 Percent Sodium meta borate 46 Kentucky ball clay 35 Titanium dioxide 12 Dextrine 7 Example 5 Percent Sodium meta bor 33 Kentucky ball clay 39 Titanium oxide 2 Aluminum oxi 15 Dextrine 11 Example 6 In Example 3 above, substitute aluminum oxide, magnesium oxide, zirconium oxide, calcium oxide, or copper oxide for the titanium dioxide.

The field of application of this coating material is extremely varied and diverse and includes such applications as coatings for heat treating metal wire, plates, castings, etc., coating for jet engine exhausts, coatings for metals during welding or brazing, coatings for exhaust manifolds, coatings for casting molds, and many others.

It will be obvious to those skilled in the art that various changes may be made without departing from the spirit of the invention.

What is claimed is:

1. A composition of matter for use as a protective coating for oxidizable metals consisting essentially of 20% to 50% by weight sodium meta borate, 20% to 40% by weight plastic clay and 5% to 40% by weight of at least one metal oxide.

2. A composition of matter for use as a protective coating for oxidizable metals consisting essentially of 20% to 50% by weight sodium meta borate, 20% to 40% by weight plastic clay, 5% to 40% titanium dioxide and up to 20% graphite.

3. A coating composition in accordance with claim 1 wherein sufiicient Water is present in addition to the solid constituents to bring the material to the consistency of paint.

4. A magnesium article having a protective coating on the surface thereof, said protective coating consisting essentially of a solution of sodium meta borate.

5. A magnesium article having a protective coating vitrified on the surface thereof, said protective coating consisting essentially of 20% to by weight sodium meta borate, up to 40% by weight plastic clay, and up to 40% by weight of a metal oxide.

6. A metal article having a protective coating vitrified on the surface thereof, said protective coating consisting essentially of 20% to 50% by weight sodium meta borate, 20% to 40% by weight plastic clay and 5% to 40% by weight of at least one metal oxide.

7. The method of protecting a magnesium surface subjected to elevated temperatures which consists essentially of coating the surface with a composition consisting essentially of a solution of sodium meta borate, and drying the coating thereon.

8. The method of protecting a metal surface subjected to elevated temperatures which comprises coating the surface with a composition consisting essentially of water and as the solid constituents thereof 20% to 50% by weight sodium meta borate, 20% to 40% by weight plastic clay and 5% to 40% by weight of at least one metal oxide, suficient Water being present to render the mixture to the consistency of paint, drying the coating thereon, heating the coated base until the coating melts, and thereafter allowing it to cool until solidified.

9. The method of protecting a metal surface subjected to elevated temperatures which comprises coating the surface with a composition consisting essentially of water and as the solid constituents thereof 20% to 50% by weight sodium meta borate, 20% to 40% plastic clay, 5% to 40% by weight of at least one metal oxide, up to 20% by weight graphite, and up to 20% by weight of an organic viscid gummy adhesive material, sufiicient water being present to render the mixture to the consistency of paint, drying the coating thereon, heating the coated base until the coating melts, and thereafter allowing it to cool until solidified.

10. A molybdenum article having a protective coating on the surface thereof, said protective coating consisting essentially of a solution of sodium meta borate.

11. A molybdenum article having a protective coating vitrified on the surface thereof, said protective coating consisting essentially of 20% to 100% by weight sodium meta borate, up to 40% by weight plastic clay, and up to 40% by weight of a metal oxide.

12. A method of protecting a molybdenum surface subjected to elevated temperatures which consists of coating the surface with a composition consisting essentially of a solution of sodium meta borate, and drying the coating thereon.

References Cited in the file of this patent UNITED STATES PATENTS 2,499,641 Goody Mar. 7, 1950 FOREIGN PATENTS 160,495 Great Britain 1921

Claims (1)

1. A COMPOSITION OF MATTER FOR USE AS A PROTECTIVE COATING FOR OXIDIZABLE METALS CONSISTING ESSENTIALLY OF 20% TO 50% BY WEIGHT SODIUM META BORATE, 20% TO 40% BY WEIGHT PLASTIC CLAY AND 5% TO 40% BY WEIGHT OF AT LEAST ONE METAL OXIDE.