US2467114A - Vitreous coatings for light metals - Google Patents

Description

Patented Apr. 12, 1949 UNITED STATES PATENT OFFICE vrrnaous coa'rmcs ron LIGHT METALS Alden J. Deyrup, Westiield, N. J., assignor toll. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application December 18, 1945, Se-

rial No. 635,765. In Canada January 8, 1943 and 478,648, filed January 28, 1942 and March 10, 1943, respectively, both now abandoned.-

Aluminum and its alloys consisting principally of aluminum are uniquely suitable for many purposes because of characteristic strength and relative lightness. For many purposes, however, the use of aluminum is restricted by chemical reactivity, characteristic of aluminum and even more so of. its alloys, in general. Exposed surfaces of aluminumv will react with numerous chemical agents, including even such relatively mild substances as are found in water and in the air. Aluminum and its alloys corrode rapidly in contactwith sea water. Even slight corrosion has tended to restrict in some cases the application of these metals. For example, centrifugal washing machine drums can advantageously be made of the strong alloys of aluminum, where strength and lightness of such alloys is desirable, but corrosion by mild alkaline detergents results in black or dark colored corrosion products which are undesirable in the washing of clothes.

From the standpoint of appearance, many pleasing buifed, burnished, etched or other surface treatments of the metal can be'applied to these metals. Many of the surfaces so prepared are, however, readily scratched and abraded and are poor in resistance to weathering, as the metal tarnishes and corrodes. There are anodic treatments for protecting and coloring 'aluminum, and paints and lacquers may be used for decoration and protection against weathering. All

of these are quite readily scratched or abraded and generally are susceptible to fading or weathering.

Various attempts have been made in the past to protect and decorate aluminum and its alloys by application of vitreous protecting layers or enamels. The Warga Patents Nos. 1,230,958 and 1 346,475 are illustrative of such previous attempts. However, these attempts have not yielded a suitable vitreous enamel which would mature to a glossy surface at temperatures low enough to avoid deterioration of the metal article yet possessing substantial resistance to 16 Claims. (Cl. 106-49) weathering and other chemical action, as well as adherence to' the metal.

Although pure aluminum does not melt below about 1200 F., it is well known that aluminum articles may deform and lose tensile strength well below this temperature. Many of the high strength aluminum-rich alloys, 1. e., alloys containing at least 80% of aluminum exhibit changes due to eutectic melting at the grain boundaries attemperatures above 920 to 980 F., depending on the particular alloy, resulting in loss of strength and promotion of intergranular corrosion. The temperatures at which enameling would be permissible, 920 to 980 F., are much lower than those whose may be used in enameling cast iron and steel, viz., 1300 to 1500- F. For this reason it has hitherto been regarded as impossible to develop enamels for aluminum and its alloys with any substantial degree of chemical resistance, since it is well known that chemical resistance must usually be sacrificed to secure even a small lowering of the maturing temperature of an enamel. For example, those enamel compositions available to the art which could be matured at a temperature sufllciently low to avoid damaging the aluminum article were always found to be severely lacking in chemical resistance, to the action of dilute acids and alkalies, and even to water.

Surprisingly, I have now discovered certain new and improved enamel compositions which may be satisfactorily used to cover and protect surfaces of aluminum and aluminum-rich alloys, whether the metal is cast, rolled, or drawn, these vitreous compositions supplying hard, smooth, glossy, durable coatings, which provide excellent protection against attack by the usually encountered chemical agents. Moreover, these enamels may be applied to the metal at temperatures which are not only below the melting point of the aluminum or alloy of aluminum, but in fact are so low that the application of the enamel to the metal need not damage or destroy any special metallurgical properties which may have been developed therein by previously applied heat treatments.

It is an object of this invention to provide new and improved vitreous enamel compositions which will furnish hard, smooth, glossy, durable coatings on the surfaces of aluminum and aluminum-rich alloys.

It is another object of this invention to provide new and improved vitreous enamel compositions which can be fused and matured at a temperature between 920 and 980 F. and which will furnish a hard, smooth, glossy, durable coatin! on the surface of aluminum and aluminum-rich alloys.

It is a further object of this invention to provide new and improved enamel compositions which can be fused and matured at a temperature between 920 and 980 F. and which will furnish a hard, smooth, glossy, durable coating of any desired color, including white and clear coatings, on the surface of aluminum and alu minum-rich alloys.

It is still another object of this invention to provide a vitreous enamel of the above-described properties which can be applied to aluminum and aluminum-rich alloys from an aqueous suspension thereof.

It is yet another object of this invention to provide an aluminum article having a coating of vitreous enamel which is chemically resistant to water, dilute alkalies and acids, has a high degree of adhesiveness to the aluminum, and has a high resistance to buckling, bending and impact.

Other objects of the invention will appear hereinafter. V

The new and improved vitreous enamel of the present invention has a firing and maturing temperature of between 920 F. and 980 F. and contains, as essential ingredients, lead oxide, silica, lithium oxide, and at least one other alkali metal oxide taken from the group consisting of sodium oxide and potassium oxide, a portion of the silica being replaceable with titanium oxide. If the vitreous enamel of this invention is to be used in light colors or white for their decorative effect, it is preferred that it also contain a small amount of antimony oxide.

The above-mentioned constituents of the enamel must be present in accordance with the following ranges and relative proportions:

Mole per cent Lead oxide (PbO) to 18 Silica (S102) 38 to 65 Lithium oxide (Li2O) 5 to 12 Sodium oxide (NaaO) 0 to 22 Potassium oxide (K) 0 to 20 Titanium oxide (T102) 0 to 11 The total alkali metal oxide content (LizO+Na20+K2O) of the enamel composition must be between 25 and 36 mole per cent, and the ratio of the silica content, or the silica content plus twice the titanium oxide content, of the composition, in mole per cent, to the total alkali metal oxide content, in mole per cent, must be between 1.8 and 3.0, i. e.,

All quantities of constituents are expressed in mole per cent, i. e., the number of gram molecular weights of a constituent in relation, percentagewise, to the total number of gram molecular weights of all the constituents in the composition.

The enamel constituents are set forth in ranges expressed in mole per cent since this defines the invention more accurately and narrowly than could be done by expression as weight per cent.

Although the enamels characterized above are entirely suitable for forming vitreous corrosion protective coatings on aluminum, they are not altogether suitable for use where decoration is desired in light colors or white because they are susceptible to darkening during application firing by traces of organic matter. Such darkening or, staining can be obviated by eliminating all traces of organic matter, but this is considered impracticable in most cases except where black or dark, hues are desired. Where light hues or white or clear enamel coatings are desired, I have dis covered that an addition of 0.3 to 1.0 mole per cent of antimony oxide (SbzOa) to the above compositions before melting the batch obviates the undesirable staining by traces of organic matter and permits readily secured vitreous coatings in any desired hue without impairing any of the properties secured in the absence of antimony oxide from the batch.

It is generally suitable and economical to apply these enamels in the form of a suspension in water. For this purpose, it is preferred to maintain the sodium oxide content at 14 to 22 mole per cent and the potassium oxide content at not more than 12 mole per cent (0 to 12 mole per cent).

Preferred typical exemplary enamelfritcompositions are given in Table I. The correspond-' ing weight percentages are embodied in Table II, and suitable batches for making these frits in Table III, in weight per cents These enamel com- I is not so fluid in the requisite firing range, but

possesses excellent acid resistance. Enamel H is particularly suited for preparation of red and SiO +2TiO 8 to 3 0 orange enamels with cadmium sulphoselenide Li,0+Na O+K,O pigments.

Table! v [Mole per cent composition of enamel irits] ABCDEFGHIIKLMN 5.9 NazsiFc 1.4 3.2 Li=O+Na O+KiO 29.0 26.2 27.2 23.9 28.7 29.6 30.0 27.9 26.5 30.6 29.8 29.5 2&6 26.9

' Antimony oxide ($1,103)

TM 1! [Weight per cent composition of enamel kits] A B C D E F G H I J K L M N 42.0 41.0 38.1 34.7 35.4 30.4 401 39.3 40.9 30.5 41.4 27.8 27.3 25.3 31.45 an 34.9- 2&3 32.3 39.3 30.4 34.1 2.3 2.3 2.2 2.0 2.0 22 2.4 2.4 3.5 1.8 2.3 9.7 0.0 10.2 11.1 0.5 13.4 14.8 14.0 14.6 120 11.3 7.4 7.2 8.3 1 7.7 2.2 24 2.4 7.4 1.0 9.7 9.5 3.7 7 a1 9.3 10.3 as 1.1 3.1 2. 1.8 1.6 1.7 .1.7 1.7 20 1.7 4. 4.0 o o I NmSlFn .7

Table III [Weight per cent composition of batches ior enamel irits] Red lend (Pb;0 37.0 40.0

Flint z 24 Lithium carbonate (L140 0;).

Soda Ash (NagCOal. l4 4 Potassium carbonate (ZKaCOa-liHzO) 11.1

Titanium oxide (Ti0).. 8 3

Boric acid (H 801) Sodium zirconium silicate (14% NuzO), (29% $10,),

210:) Cadmium oxide (CdO) Sodium fluosilicate (NflzSlFo) It will readily be understood that the composition ranges cannot sharply diiferentiate all practical compositions from less useful compositions, even with the additional limitations stated. Accordingly, there are given below some general rules to facilitate securement of optimal within the above-mentioned range of 1.8 to 3.0, is generally somewhat detrimental to securement of maximum chemical resistance combined with good gloss and leveling of the enamel. Decrease of this ratio within said range is beneficial in the latter respect, but in the immediate vicinity of the lower limitcited for this ratio, 1.3, compositions are secured which are less stable to aging as a water suspension. Thus, this ratio should be kept low for optimum results, unlessit is intended that enamel slips should remain stable for long periods after preparation. Increased? L120 content at the expense of NazO and K20 is favorable to stability of resultant enamel suspension on aging, whereas increase. 0f-K2O content at expense of LizO and NazO content is unfavorable in this respect. Although SiOz can be replaced by T102, as indicated, little gain in resistance to acids is secured, but rather a gain in stability on aging water suspensions. In fact, the best acid resistance, at equal gloss, has been secured with little or no titanium content in the enamel, which is surprising in view of the fact that titanium oxide is commonly regarded in enamel formulation as enhancing acid resistance. The antimony oxide content, used batches where enamels are desired that are little subject to darkening by traces of organic matter during firing on aluminum, may be used as stated in amounts from 0.3 to 1.0 mole per cent. Smaller amounts than 0.3 mole per cent are. ineifective.

Throughout the range 0.3 to 1.0 mole per cent batch to provide a transparent blue enamel.

Surprisingly, this has no efiect on the truly remarkable adherence of these'enamels to aluminum and its alloys. Up to 8 mole per cent of cadmium oxide (CdO) may be added to the batch for the purpose of stabilizing cadmium sulphoselenide pigments, in accordance with the process disclosed in Huber and Felton U. S. P. 1,673,679. Up to 3 mole per cent of sodium fluosilicate (NazsiFe) may be added to the batch to produce opacity when white enamels are de- .raw materials used. precautions should be taken sired, although it is generally preferable to secure such white opacity by titanium oxide added as to limit the content thereof in the enamel composition to such amount, generally less than 5 mole per cent, as willmaintain the maturing temperature of the enamel at less than 980 F.

Fluorides such as those of sodium and calcium. or fluorides of other alkali metals and alkaline earth metals, have also been found detrimental when present in substantial. quantity, and are atomic preferably omitted and, in any event, restricted to less than 6 mole per cent.

It will be understood that other suitable batch compositions than those shown in Table 111 can be used in making the frits of Tables I and II. Thus, red lead may be substituted by litharge or lead nitrate or other compounds commonly known to yield PbO in enamel frits, it being understood that the amounts of such altered raw materials are readily calculated to yield the desired frit composition.

In preparing these frits, the batch is weighed,

mixed, heated in a crucible until the mix is completely melted to a homogeneous glass. Crucible temperatures may vary within wide limits, but 900 C. to 1200 C. is a particularly suitable range. The molten glass is fritted by running into water which shatters it into small pieces forming the frit. The frit is advantageously freed of water by low-temperature drying, for example, at a temperature less than 50 C.

-The frit is then placed, for grinding, in a ball mill or other mill, with water or other suitable liquid, or in some cases it may be dry-ground. Dry grinding may be preferred if the frit is to be stored for long periods because it obviates any atl tack of water on the comminuted frit and eliminates the operations of filtering and drying. If

the frit is to be used directly inenamel, it isadvantageously ground in water which serves as suspending medium for application to the metal articles. During the grinding, it is convenient to include with the frit'white pigments (opacifying agents) such as titanium dioxide, or colorin pigments such as cobalt aluminuate blue, in amounts up to 10% by weight of the total enamel compositions. These are, of' course, omitted if a clear, colorless, vitreous coating is desired. It is advantageous also to include suitable dispersing and bonding agents as additions to the frit in the grinding mill.

The dispersion or suspension of the powdered enamel in a suitable vehicle may be applied to the aluminum by the well-known procedures of spraying, dipping, stencilling, brushing, etc. As an example of a convenient vehicle for use in the spray application, one composed of methyl cellulose 0.75%, sodium chloride 1%, octyl alcohol 0.05%, and water 98.2% (all parts by weight) may be used; or commercial water glass (sodium silicate solution) 0.4%, sodium fluosilicate 0.4%, and water 99.2%. A suitable vehicle for applying the vitreous enamel by the squeegee-stencil application procedure is one composed of ethyl cellulose (25 C. P. S. viscosity grade) 4%, soya lecithin 0.25%, and tei'pineol 95.75%. When using aqueous vehicles for application, itis frequently desirable to include in the vehicle corrosion inhibitors to prevent the possible corrosion of the metal article by the enamel suspension, since such corrosion evolves hydrogen which may mechanically break up the enamel film before firing. Sodium silicate and fluosilicate may be used for this purpose, as in the vehicle cited above; or a small amount of soluble chromate such as 0.1% by weight of potassium chromate may be incorporated in the aqueous vehicle.

It is desirable to apply the enamel to the surface of the aluminum in any manner which will obtain a uniform and smooth coatin before firing. The thickness of the coat may be varied within wide limits according to the type of effect desired, although 15 to grams (based on dry weight of enamel) per square foot per coat is sary to apply any preliminary treatment to the surface of the aluminum or alloy other than removal of grease or excessive amounts of corrosion products. In general, however, these enamels may be applied to any aluminum or aluminum alloy surfaces without preliminary clean-. ing, provided the surface to be enameled is not contaminated. Cast metal articles to be enameled should be made with proper care to ensure dense castings free of undue porosity. In general, articles for enameling should be designed in accordance with the well-known principles obtaining in other metal-enameling arts, particularly to avoid excessively sharp edges and corners in design. If such sharp edges, corners or burrs are present, theyshould be lightly rounded off by filing, grinding or other suitable means.

After application of the enamel to the metal, the composition is fired in a suitable furnace in order to fuse the powdered enamel to a continuous glassy coating. The firing conditions are not critical, except that the article must be brought to a temperature high enough to melt the enamel but not high enough to injure the metal article. The temperature necessary de-' pends to some extent on the particular aluminum alloy being coated and on the particular enamel composition. However, it is not necessary to em-- ploy a temperature above 980 F. and in some cases the enamel may be satisfactorily melted as low as 920 F. It is merely necessary to keep the article at the selected temperature for a few minutes to permit the enamel to fuse completely to a uniform, adherent, vitreous layer. Several coats may be applied, if desired. The firing, of course, should be effected in a furnace free of sulphate or chloride, or other objectionable fumes which might impair the desired gloss. In some cases, the enameling may be advantageously combined with the heat treating of heat-treatable alloys. Lon heat-treating periods, even of several hours duration, generally do not adversely affect these enamels. When thin sheet stock is enameled, particularly of soft metals in this group such as pure aluminum sheet, it may be found to warp on cooling because of the differential contraction of metal and enamel during cooling. This may be obviated by enameling both sides of the sheet. Or, if desired, I find that, surprisingly, such warped sheet may be mechanically bent back to its original shape without any adverse effect on the enamel whatever. Or, if desired, warping diffiiculties may be minimized by use of the stronger alloys in fabricating articles for enameling.

The coating properties obtained by the use of my new and improved vitreous enamels are most surprising in that the combination of desirable properties in a single enamel is most unexpected and unusual. The enamels have extremely low fusion points, and yet maintain a degree of resistance tochemical agents which hitherto have been believed possible only with enamel compositions having fusion points of at least 0. higher. In addition to low fusing point and high resistivity to chemical agents, the vitreous enamels will adhere to the aluminum or the aluminum alloy surface even when the member is subsequently exposed to thermal shock or to mechanical flexing. While it is generally known that very thin enamel layers will ordinarily with-' stand flexing and thermal shock, it is indeed surprising to obtain satisfactory resistance to both flexing and thermal shock with relatively thick enamel layers, coatings thick enough .to provide enamel aluminum and its alloys, enamel layers having thicknesses sufiicient to avoid surface imperfections have invariably chipped off, either spontaneously, or as the result of relatively slight mechanical or thermal shock. My new and improved enamels, however, may be subjected to considerable mechanical abuse without any danger of spalling or cracking off of the vitreous protective coating. For the first time there can be secured protective coatings onaluminum and its alloys providing satisfactory protection to the surface of the metallic structure on which they are placed. The adherence of these enamels to aluminum is considered even more surprisingin view of the fact that their linear expansion coefilcients are generally only approximately twothirds of that of aluminum.

As examples of the resistance properties, remarkable in enamels of so low maturing temperature, the following examples may be cited.

Example I An aluminum test piece, enameled with an enamel comprising frit G, was exposed out in the weather for twenty months. At the end of this time, inspection showed it retained substantially its originally pleasing appearance. A specimen of commercially pure aluminum similarly exposed lost its metallic appearance and showed a corroded, gray appearance.

Example II An aluminum test piece enameled with an enamel comprising 93 parts, by weight, of frit I and 7 parts, by weight, of titanium dioxide, fired at 950 F., exhibited a pleasing glossy white appearance. Immersion of this test piece in 10% citric acid solution for 48 hours did not affect its appearance. A similar test piece, coated on one side only, was immersed for two hours in 10% sodium hydroxide solution at 85 0. The metal dissolved away, leaving the enamel behind. The enamel itself was substantially unafiected.

Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to said details except as set-forth in the appended claims.

I claim:

1. A vitreous enamel frit, suitable for use as a protective coating on the surface of aluminum and aluminum-rich alloys, having a firing and maturing temperature of between 920 F. and

980 F., and which comprises I Mole per cent Pbo 10 to 18 S102 38 to'65 L120 to 12 NazO 0 to 22 K20 0 to 20 TiOz 0 b0 11 2. A vitreous enamel frit, suitable'for use as a protective coating on the surface of aluminum and aluminum-rich alloys, having a firing and "maturing temperature of between 920"F. "and 980 F., and which comprises i Mole per cent Pbo Q 10 to 18 S102 3a to 65 LizO 5 to,12 NazO 0 to 2 K20 0 to 20 TiOz 0 to 11 SbzOs 0.3 to 1.0

the total alkali metal oxide content (LizO+NazO+KzO) I Mole per cent PbO 10 to 18 S102 38 to 65 L 5 to 12 NazO 0 to 22 K20 0 110.20 T102 0 to 11 the total alkali metal oxide content of the enamel composition being between 25 and 36 mole per cent, and the ratio of the sum of said silica content and twice the said titanium oxide content of the composition to the said total alkali metal oxide content of the composition being between 1.8 and 3.0, all of said oxides being melted into said enamel composition, and in admixture tlserewith a ceramic pigment in an amount up to l 4. A pigmented ,enamel composition, suitable for use as a protective coating on the surface of aluminum and aluminum-rich alloys, having a firing and maturing temperature of between 920 F. and 980 F., and which comprises Mole per cent PbO 10 to 18 S102 38 to 65 L120 5 to 12 NazO 0 to 22 K20 0 to 20 T102 0 I50 11 SbzOa 0.3 to 1.0

the total alkali metal oxide content (Li2O+Na2O+K2O) of the enamel composition being between 25 and 36 mole per cent, and the ratio of the sum of said silica content and twice the said titanium oxide content of the composition to the said total alkali metal oxide content of the composition being between 1.8 and 3.0, all of said oxides being melted into said enamel composition, and in admixture tlerewith a ceramic pigment in an amount up to 1 5. A vitreous enamel frit, suitable for use as a protective coating on the surface of aluminum 11 and aluminum-rich alloys, having a iiring and maturing temperature of between.920 F. and 980 R, and which comprises Mole per cent 10 to 18 PhD 8101 38 to 65 L120 to 12 NazO 14 to 22 K20 0 to 12 T: 0 to 11 the total alkali metal oxide content of the enamel composition being between 25 and 36 mole per cent, and the ratio of the sum of said silica content and twice the said titanium oxide content of the composition to the said total alkali metal oxide content of the composition being between 1.8 and 3.0, all of said oxides being melted into said enamel composition.

6. A vitreous enamel frit, suitable for use as a protective coating on the surface of aluminum and aluminum-rich alloys, having a firing and maturing temperature of between 920 F. and 980 F., and which comprises Mole per cent P50 J I to 18 S10: 38 to 65 L120 5 to 12 No.20 14 to 22 K 0 to 12 T102 0 to 11 313203 0.3 to 1.0

the total alkali metal oxide content of the enamel composition being between 25 and 36 mole per cent, and the ratio of the sum of said silica content and twice the said titanium oxide content of the composition to the said total alkali metal oxide content of the composition being between 1.8 and 3.0, all of said oxides being melted into said enamel composition.

7. A pigmented enamel composition, suitable for use as a protective coating on the surface firing and maturing temperature of between 020 of aluminum and aluminum-rich alloys, having a firing and maturing temperature of between 920 F. and 980 F., and which comprises Mole per cent PbO 10 to 18" S102 38 to 65 L120 5 to 12 NazO 14 to 22 &0 0 to 12 T102 the total alkali metal oxide content F. and 980 1",, and which comprises Molepercent PbO 10 to 18 S10: 38 to 85 L120 5 to 12 NazO 14 to 22 K20 Onto 12 T102 0 to 11 SbzO: 0.3 to 1.0

the total alkali metal oxide content (L1z0+Na0+K=0 of the enamel composition being between 25 and 36 mole per cent, and the ratio 01' the sum of said silica content and twice the said titanium oxide content of the composition to the said total alkali metal oxide content of the composition being between 1.8 and 3.0, all of said oxides being melted into said enamel composition, and in admixture therewith a ceramic pigment in an amount up to 10%.

9. A process for applying a protective coating on the surface of aluminum and aluminum-rich alloys which comprises fusing thereon a vitreous enamel, having a firing and maturing temperature of between 920 F. and 980 F., and comprising Mole per cent Pbo 10 to 18 S102 38 to 65 L120 5 to 12 NazO 0 to 22 K20 0 to 20 T10: 0 to 11 the total alkali metal oxide content (Li2O+Na:O+KaO) of the enamel composition being between 25 and 36 mole per cent, and the ratio of the sum of said silica content and twice the said titanium oxide content of the composition to the said total alkali metal oxide content of the composition being between 1.8 and 3.0, all of said oxides being melted into said enamel composition.

10. A process for applying a protective coating on the surface of aluminum and aluminum-rich alloys which comprises fusing thereon a vitreous enamel, having a firing and maturing temperature of between 920 F. and 980 F., and comprising Mole per cent PbO 10 to 1 S102 38 to 6 L 5 to 12 NazO 0 to 22 K20 0 to 20 T10: 0 to 11 SbzOs 0.3 to 1.0

the total alkali metal oxide content (LiaO-I-NazO-I-KzO) of the enamel composition being between 25 and 36 mole per cent, and the ratio of the sum of said silica content and twice the said titanium oxide content of the composition to the said total alkali metal oxide content of the composition being between 1.8 and 3.0, all 01 said oxides being melted into said enamel composition.

' 13 11. A process for applying a. protective coating on the surface ofaluminum and aluminum-rich alloys which comprises fusing thereon a vitreous enamel, having a firing and maturing temperature of between 920 F. and 980 F., and comprising Mole per cent PhO 10 to 18 S102 38 to 65 L120 5 to 12 NazO 14 to 22 K20 to 12 TiOz -e 0 to 11 the total alkali metal oxide content (LizO-l-NazO-i-KzO) of the enamel composition being between 25 and 36 mole per cent, and the ratio of the sum of said silica content and twice the said titanium oxide content of the composition to the said total alkali metal oxide content of the composition being between 1.8 and 3.0, all of said oxides being melted into said enamel composition. V

12. A process for applying a protective coating on the surface of aluminum and aluminum-rich alloys which comprises fusing thereon a vitreous enamel, having a firing and maturing temperature between920" F. and 980 F., and comprising Mole per cent PbO 10 to 18 S102 s.. 38 to 65 L120 to 12 NazO 14 to 22 K20 o to 12 TiOz 0 to 11 SbzOs 0.3 to 1 0 th total alkali metal oxide content (LizO-I-NazO-I-KzO) of the enamel composition being between 25 and 36 mole per cent, and the ratio of the sum of said silica content and twice the said titanium oxide content of the composition to the said total alkali metal oxide content of the composition being between 1.8 and 3.0, all of said oxides being melted into said enamel composition.

13. Aluminum and aluminum-rich articles containing a protective coating comprising a vitreous enamel having a firing and maturing temperature of between 920 F. and 980 F. and having the composition Mole per cent PbO to 18 I S102 38 to 65 LizO 5 to 12 NazO 0 to 22 m0 0 to T10: 0 to 11 the total alkali metal oxide content (LizOQ-NazO-i-KzO) of the enamel composition being between and 36 mole per cent, and the ratio of the sum of said silica content and twice the said titanium oxide content of the composition to the said total alkali metal oxide content of the composition being between 1.8 and 3.0, all of said oxides being melted into said enamel composition.

14. Aluminum and aluminum-rich I articles containing a protective coating comprising a vitreous enamel having a firing and maturing temcontent of the composition to the said total alkali metal oxide content of the composition being between 1.8 and 3.0, all of said oxides being melted into said enamel composition.

15. Aluminum and aluminum-rich articles containing a protective coating comprising a vitreous enamel having a firing and maturing temperature of between 920 F. and 980 F. and having the composition Mole per cent PbO 10 to 18 SiOz 38 to L 5 to 12 N320 14 to 22 K20 0 to 12 T102 0 to 11 the total alkali metal oxide content of the enamel composition being between 25 and 36 mole per cent, and the ratio of the sum of said silica content and twice the said titanium oxide content of the composition to the said total alkali metal oxide content of the composition being between 1.8 and 3.0, all of said oxides being melted into said enamel composition.

16. Aluminum and aluminum-rich articles containing a protective coating comprising a vitreous enamel having a firing and maturing temperature of between 920 F. and 980 F. and having the composition Mole per cent PbO 10 to 18 S102 38 to 65 LizO 5 to 12 NazO 14 to 22 K20 0 to 12 T102 0 to 1 1 SbzOa 0.3 to 1.0

the total alkali metal oxide content (LizO-i-NazO +K20) of the enamel composition being between 25 and 36 mole per cent, and the ratio of the sum of the sum of said silica content and twice the said titanium oxide content of the composition to the said total alkali metal oxide content of the composition being between 18 and 3.0, all of said oxides being melted into said enamel composition.

ALDEN J. DEYRUP.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Deyrup Jan. 4, 1944 Number Certificate of Correction Patent No. 2,467,114. April 12, 1949.

ALDEN J. DEYRUP It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 5, lines 40 and 41, for resistances read resistance; column 8, line 9, after the word not insert too; column 14, line 60, claim 16, after ratio strike out 'of the sum;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 13th day of September, A. D. 1949.

JOE E. DANIELS,

Assistant Uommissioner of Patents.