US3869293A - Inorganic coating compositions and a method - Google Patents

Abstract

In accordance with the invention there is provided an improved heat and corrosion resistant protective coating consisting essentially of a dispersion of inorganic solid particulate material in an aqueous solution the solute of which consists essentially of metal, chromate, dichromate or molybdate and phosphate, at least a portion of the particulate material being an alloy containing aluminum and magnesium.

Description

United States Patent Brumbaugh Mar. 4, I975 INORGANIC COATING COMPOSITIONS [56] References Cited AND A METHOD UNITED STATES PATENTS [75] Inventor; Robert J. Brumbaugh, Pottstown, 3,248,251 4/1966 Allen 106/286 FOREIGN PATENTS OR APPLICATIONS [73] Assignee: Teleflex Incorporated, North Wales, 958,787 5/1964 Great Britain 106/14 Pa. Primary Examiner-Allan Lieberman [22] July 1973 Attorney, Agent, or Firm-McGlytnn and Milton [2]] Appl. No.: 381,642

Related US Application Data [57] ABSTRAFJT 63] C an of Ser NO 169 854 Au 6 In accordance with the invention there is provided an g xs f xg improved heat and corrosion resistant protective coating consisting essentially of a dispersion of inorganic solid particulate material in an aqueous solution the [52} Cl 106/14 106/2861 k solute of which consists essentially of metal, chromate, [5]] Int Cl C04b 35/04 cogd 1/00 dichromate or molybdate and phosphate, at least a [58] Field "Mb 11/127 129 portion of the particulate material being an alloy conl17/i69 taining aluminum and magnesium.

15 Claims, N0 Drawings INORGANIC COATING COMPOSITIONS AND A METHOD This application is a continuation-in-part of United States Patent Application Ser. No. 169,854, filed August 6, 1971 and now abandoned.

This invention relates to coating compositions, and methods for coating, which are highly suitable for coating various surfaces, particularly metal, to impart protective characteristics thereto. The most outstanding characteristic attainable by way of the coatings of this invention is that of protection against corrosion.

Reference is here made to US. Pat. No. 3,248,251 issued to Charlotte Allen and assigned to the assignee of the present invention. This patent discloses and covers coating compositions consisting essentially of a slurry of solid inorganic particulate material in an aqueous acidic solution containing substantial amounts of dissolved metal, chromate, dichromate or molybdate, and phosphate. After application of the coating to the substrate it is heated to a temperature upwards of about 500F until the coating is water insoluble. The coating most emphasized in the patent, that which constitutes the most preferred embodiment, incorporates aluminum powder as the particulate material. Coatings made in accordance with this preferred embodiment of the aforesaid patent currently find wide usage and have proven outstandingly successful, particularly for application to aircraft jet engine components to provide increased corrosion protection.

The principal object of the present invention is to provide an improved coating of the type taught in the aforesaid patent.

Briefly, what I have discovered is that if the solid particulate material dispersed in the aqueous solution is or includes an alloy comprising aluminum and magnesium, still further improved corrosion protection can be attained. Whereas I do not purport to have a fully complete and accurate explanation for the improved results attained, l theorize that the improvement derives from two factors. The first factor is that of the relative positions of aluminum and magnesium in the electromotive series of metals. The second factor is that of the relative sintering temperatures of aluminum powder and powdered aluminum-magnesium alloys.

It has been established that the preferred coating of the aforesaid patent that wherein aluminum powder is used as the solid particulate material has the desirable attribute of providing sacrificial corrosion protection. That is, where, for example, the substrate for the coating is ferrous metal, the aluminum powder in the coating, by way of the electrochemical reactions which ensue in a salt spray or the like corrosive atmosphere, gradually sacrifices itself to the end that there is little or no corrosion of the ferrous metal substrate. In terms of beneficial practical results, what this means is that even though there should be a scratch or the like through the coating baring the ferrous substrate metal, nevertheless, there is no substantial corrosion to the barcd substrate metal this because of the sacrificial property of the coating as aforesaid. It has further been established that this property of the sacrificiality of the coating in providing the increased corrosion protection is greatly enhanced by so processing the coating as to render it electrically conductive. The best manner of accomplishing this is to cure the coating at a tempera- V ture of above about 900F.

I have now established that by using a coating wherein the solid particulate material is or includes an alloy of magnesium and aluminum, not only can even greater corrosion protection be attained, but yet at a curing temperature as low as 375F. This, 1 theorize, is because magnesium, by way of its respective position in the electromotive series of metals, is even more sacrificial than is aluminum. Further, the alloys of magnesium and aluminum have a lower sintering temperature than does powdered 100% aluminum and this may be a factor in the full explanation of the improved results attainable by way of the present invention.

It should be pointed out that magnesium, because of its high reactivity and because it lacks the characteristic of aluminum in forming a dense oxide film in the presence of an oxidizing agent, could be expected to become very rapidly dissolved in the aqueous acidic solution, the pH of the solution being from about 1.6 to 2.7. However, what I have further found is that when, in accordance with the present invention, the solid particulate material is an alloy containing both magnesium and aluminum, there is ample time between the mixing of the magnesium-aluminum alloy powder into the solution and the time when the coating, after application thereof to the substrate, is heat cured, that no substantial dissolving of the magnesium into the acidic solution in fact occurs.

The above and other objects, features and advantages of the invention will appear more clearly from the following more detailed description thereof.

The magnesium-aluminum alloy powder can and preferably should consist substantially entirely of aluminum and magnesium in a ratio of anywhere from about 3% to 99.5% by weight aluminum and the remainder magnesium. The most preferred range is from about 30 to by weight aluminum and the remainder magnesium. Examples are: 70% by weight aluminum, 30% magnesium; 50% aluminum, 50% magnesium.

However, alloys consisting in large part of magnesium and aluminum but additionally containing other metal or metals can be used. Typical examples of other metals which can be included in the alloy are zinc, chromium, tin, nickel, silicon, titanium and copper. Indeed, any metal which will' alloy with magnesium and aluminum can be included in the alloy if desired, though there will generally be no advantage to including metals which are not higher than iron in the electromotive force series of the metals. But the key point is that other metals can be present without destroying the advantages attained by the use of the magnesium aluminum combination in the alloy. In general, therefore, preferred alloys for the practice of the invention can contain from about 3% to 99.5% by weight aluminum, from about 0.5 to 97% by weight magnesium, and should preferably consist at least principally of aluminum and magnesium, i.e., more than 50% by weight of the alloy should preferably consist of the aluminum and magnesium. Particularly for the: attainment of a relatively smooth coating, which is desirable and often essential especially for jet engine components which encounter air or other gas flow and hence where roughness could cause turbulence, the particle size of the alloy should be small, preferably 325 mesh (Tyler) or finer and ideally less than 10 microns. It is, of course,

' within the purview of the invention to use the aluminum-magnesium alloy powder in combination with other powder materials as the solid particulate material in the coatingvFor example, for some applications it may be desirable to use a 50-5O mixture of aluminum powder and the aluminum-magnesium alloy powder. The aluminum-magnesium alloy particles, and also the aluminum particles where a mixture is used as aforesaid, should preferably be of generally spherical shape, often referred to as atomized powder this as distinguished from the use of flake material such as flake aluminum commonly used in organic base aluminum pigmented paints. Mixtures of powder containing as little as 1% by weight of the aluminum-magnesium alloy powder and the remainder some other powder find utility depending upon the particular application to be made of the coating and the properties desired, though it is preferred that at least about 5% by weight of the alloy powder be used in the mixture. Of course where a low percentage of magnesium-aluminum alloy powder is used, it is desirable that the alloy contain a relatively high percentage, e.g., 30% or more, magnesium. In general, it is preferred that of the total weight of the total of the powder in the coating, at least about 0.5% thereof be magnesium, i.e., magnesium alloyed with aluminum as described.

It will be understood that where some powder is used in combination with the aluminum-magnesium alloy powder as the solid particulate material in the solution, the other powder used should, in all instances, be such that it does not rapidly dissolve in the acidic solution. In addition to aluminum powder, examples of other powder materials which can be used in combination with the aluminum-magnesium alloy powder are talc, zinc, silica. aluminum oxide, chromium oxide (i.e., Crgoa) and the metal carbides and disilicides. The total solid particulate material included in the coating can be anywhere from to as high as 2000 grams per liter depending upon the application to be made of the coating and also depending upon the atomic or molecular weight of the particulate material. For most applications, however, and particularly where the solid particulate material is totally or at least predominantly the aluminum-magnesium alloy powder or a combination of such alloy powder and aluminum powder, the amount of particulate material included in the coating should preferably be from about 250 to 1000 grams per liter of solution.

As regards the formulation and composition of the aqueous solution, attention is here again directed to U.S. Pat. No. 3,248,251 and all of the teachings of that patent particularly with reference to the solution are incorporated herein by reference. However, it is appropriate that at least sufficient of the teachings be repeated here, sufficient to enable preparation and use of the coatings of this invention without need to refer to the aforesaid prior patent. Before proceeding, however, it is also appropriate to point out that whereas it can be expected that the 6 valence chromium in the solution will be present as dichromate rather than chromate, this because the solution is acidic as indicated above, nevertheless, here, as in the aforesaid patent, the term *chromate" is intended to comprehend dichromate and for purposes of uniformity in defining molar concentration the chromate is assumed to be present as chromate rather than dichromate though in fact the 6 valence chromium be present as dichromate.

In all instances the solution should contain at least about 0.3 mols per liter dissolved chromate or molybdate, at least about 0.5 mols per liter dissolved metal and at least about 1 mole per liter dissolved phosphate. The preferred ranges are from 1 to 4 mols per liter dissolved phosphate, from 0.5 to 3 mols per liter chromate or molybdate and from 1 to 4 mols per liter dissolved metal. In specifying these concentrations reference is made to the specified radical or ion rather than to the salt or acid in the form of which the ingredient might be added to or present in the solution and in these teachings the term ion, for example, phosphate ion is used for convenience to make it clear that reference is being made to the ion or radical even though, depending upon the ionization constant of the particular ingredient, a portion of the dissolved ingredient referred to might not in fact be present in ionized or disassociated form.

The phosphate ion, i.e., anion, can be introduced into the aqueous solution in the form of phosphoric acid or in the form of phosphates of the metal or metals desired to be included as the metal cation, or, it can be added in both forms. It will be understood that the term phosphate" is intended to comprehend not only the P0 ion, but also the HPO and H PO ions. All three, for example, result from the ionization of H PO and the hydrogen phosphate ions generally will, at least to some extent, be present in the compositions of this invention. Where phosphoric acid is used as an additive in making the compositions his much desired to use the ortho acid, H PO though the invention also comprehends the use of the other forms such as the meta or pyro acids, all ionizing in water to provide phosphate anion.

The chromate ion can be added either as chromic acid, or its anhydride CrO or as a chromate or dichromate, ofthe metal or metals desired to be added as the metal cation. Combinations of the acid and the metal salts can of course also be used. If molybdate anion is used, it can be added as molybdic acid, or as the anhydride M00 or as metal molybdate. There is no special advantage to including the molybdate ion for most uses of the compositions of this invention and molybdate has the disadvantage of higher cost. Also, it does not provide composition properties as good as those attainable with chromate.

The metal cation can be added either as a metal phosphate, chromate, dichromate, or, where molybdate ion is desired, as a metal molybdate, all as indicated above. Also, the metal can be added, in whole or in part, in a form such as the oxide, hydroxide or carbonate, which will dissolve in phosphoric acid or chromic acid to produce the metal phosphate or chromate. Of course, if the carbonate is used, carbon dioxide will be evolved. It will be manifest that where the metal is so added, for example, as oxide, at least some of the phosphate, chromate or molybdate should be added as acid, or as acid anhydride, to provide the acidity required for the acid-base reaction. The valence +2 and +3 metal cations are preferred and for most uses of the composition magnesium ion is particularly advantageous.

Numerous examples of preferred formulations in accordance with the above will be set forth hereinafter; however, it is appropriate at this point, for purposes of immediate illustration, to specify one preferred formu lation.

15 Water. to I cc.

The prepared composition, i.e. the mixture of the aluminum-magnesium alloy and the phosphate-chro mate-metaI-ion solution, is applied, as by spraying, dipping, rolling or brushing, to the surface or surfaces to be coated and can subsequently be cured by heating to a temperature of about 600F. The optimum curing temperature will vary depending upon the precise composition. In general, however, the preferred curing temperature is from about 375 to 800F. After complete curing, the coating will be insoluble in water even on prolonged exposure thereto and the resulting coating will be sacrificial to the steel substrate. The time required for curing is dependent upon the temperature used; the higher the temperature the less time needed. For example, for average cure of a given coating at low temperature, about l to 60 minutes at 600F would be desirable. Where a fast cure is required, about 5 to I0 minutes at 800F could be used. For a slow cure, 24 hours at 375F would suffice. For an extremely fast cure radiant heating at even higher temperature can be used to advantage. In general, the compositions cannot be overcured, within relatively wide limits, and hence except for cost and any temperature limitation on the solid particulate material being used or on particular substrate material involved, there is no particular disadvantage in curing for longer periods of time or at higher temperatures than required, say 45 minutes at a temperature on the order of 700F where minutes at 600F would be sufficient.

The thickness of the coating after curing should preferably be from about 0.0005 to 0.02 inch and hence the precise concentration of the liquid composition applied and the amount applied should preferably be adjusted accordingly. In some instances it will be desirable to apply two or more coatings with or without curing between coats. The compositions of this invention exhibit excellent surface tension characteristics and thoroughly wet the substrate. Hence, there is no requirement to use organic wetting agents to obtain a good uniform coating.

As indicated previously, the phosphate, chromate and metal ions may be added to the composition in various forms, i.e., as acids or acidanhydrides, metal salts or. as regards the metal ions, in a form which dissolves in the acids to produce the metal salts in situ. The following additional examples will illustrate.

EXAMPLE 2 MgCrO.;.7H O 266 grams H PQ 98 grams Mg(H PO .3H O 272 grams H.,,() to I000 cc. Alloy 70% (by weight) aluminum and 30% magnesium (spherical 5-l0 micron) 600 grams Coated onto sheet steel and cured at 600"! for 30 minutes.

EXAMPLE 3 CrO 92 grams H -,PO 323 grams MgO 72 grams H O, to I000 cc.

50/50 (by weight) alloy of aluminum and magnesium (spherical, 5-I0 micron) 800 grams Coated onto ferrous base metal compressor blades (for jet engines) and cured at 700F for 30 minutes.

EXAMPLE 4 MgCr O .6H O 348 grams H 98 grams Mg(H PO .3H O 272 grams H O, to I000 cc. Alloy 80% (by weight) aluminum and 20% magnesium (5,I0 micron) 1000 grams Coated onto sheet steel; cured at 800F for 10 minutes.

EXAMPLE 5 CrQ, I00 grams H PO 294 grams LiOH 72 grams H O, to I000 cc.

Aluminum powder (5I0 micron) 400 grams 50/50 (by weight) aluminum-magnesium alloy (5-10 micron) 600 grams Coated onto sheet steel and cured at I000F for I0 minutes.

EXAMPLE 6 MgCr O .6H O I74 grams N112Cf207-2H2O 75 grams MgO 40 grams H PO I96 grams H O, to I000 cc. Alloy (by weight) magnesium and 30% aluminum (5-10 micron) 800 grams Coated onto stainless steel; cured at 700F for 10 minutes.

EXAMPLE 7" AIPO, I22 grams CrO I00 grams H PO 98 grams H O, to 1000 cc.

Aluminum powder (5I0 micron) 300 grams Zinc powder (about I0 micron) 300 grams 50/50 (by weight) aluminum-magnesium alloy (5I0 microns) 700 grams Coated and cured as in Example 6.

EXAMPLE 8 MP0, 122 grams CrO;; 100 grams H;,PO 98 grams H O, to 1000 cc.

Aluminum powder (5-10 micron) 300 grams Zinc powder (about micron) 300 grams Alloy 90% (by weight) magnesium, 7% aluminum, 3% zinc 800 grams Coated onto sheet steel and cured at 600F for minutes.

EXAMPLE 9 MgCr O 6H- O 174 grams Al (Cr O 59 grams MgO 40 grams l-l,-,PO, 196 grams H O to 1000 cc. Alloy 88% Mg (by weight) 8% Al 2% Cr and 2% Zn (about 10 micron) 1000 grams Coated onto sheet steel and cured at 600F for 15 minutes.

EXAMPLE 10 MgCr O;6H- O 174 grams Al (Cr.;O 59 grams MgO 40 grams H;.PO. 196 grams H O to 1000 ccs Alloy 65% Al (by weight) Mg and 10% Zn 800 grams Coat onto ferrous base metal compressor blades (for jet engines) and cure at 700F for minutes.

EXAMPLE 11 The same in all respecte as Example 10 except that the alloy contains 55% Al (by weight) 15% Mg and 30% Zn.

It will be understood that while the invention has been described in its particulars by reference to various preferred embodiments thereof, various changes and modifications may be made all within the scope of the claims, which follow:

The embodiments of the invention in which an exclusive property or privilege is claimed are definedas follows:

l. A composition consisting essentially of a dispersion of inorganic solid particulate material in an aqueous solution the solute of which consists essentially of a combination of inorganic compounds selected from the group consisting of phosphoric acid, chromic acid, molybdic acid and the metal salts of said acids, the combination of compounds in said solution being such as will provide at least 1 mol per liter dissolved phosphate, at least 0.3 mol per liter dissolved material selected from the group consisting of chromate and molybdate, and at least 0.5 mol per liter dissolved metal, said solid particulate material being present in an amount of from about 10 to 2000 grams per liter of said solution, at least about 1% by weight of said solid particulate material being an alloy containing at least about 3% by weight aluminum and at least about 0.5% by weight magnesium, and at least about 0.5% of the total weight of said solid particulate material being magnesium alloyed with aluminum, the aluminum and magnesium constituting more than 50% by weight of said alloy, said composition being heat curable upon drying thereof to a substantially water insoluble material with said solid particulate material being bonded therein.

2. A composition as set forth in claim 1, wherein said alloy consists essentially of from about 30% to by weight aluminum and the remainder magnesium.

3. A composition as set forth in claim 1, wherein said alloy is about 70% by weight aluminum and about 30% by weight magnesium.

4. A composition as set forth in claim 1, wherein a portion of said solid particulate material is metal powder other than said alloy.

5. A composition as set forth in claim 1, wherein a portion of said solid particulate material is aluminum powder.

6. A composition as set forth in claim 1, wherein said solid particulate material is present in an amount of from about 250 to 1000 grams per liter of said solution and at least 50% by weight of said solid particulate material is aluminum.

7. A composition as set forth in claim 1, wherein the combination of inorganic compounds in said solution is such as to provide about from 1 to 4 mols per liter dissolved phosphate, from 0.5 to 3 mols per liter dissolved material from the group consisting of chromate and molybdate and from 1 to 4 mols per liter dissolved metal.

8. A composition as set forth in clalim 1, wherein the metal in said solution is predominantly magnesium.

9. A composition as set forth in claim 1, wherein said solid particulate material has a grain size not exceeding about 325 mesh.

10. A method for coating comprising the steps of applying to a substrate surface a composition consisting essentially of a uniform mixture of powder in an aqueous solution the solute of which consists essentially of a combination of inorganic compounds selected from the group consisting of phosphoric acid, chromic acid, and the metal salts of said acids, the combination of inorganic compounds in said solution being such as will provide at least 1 mol per liter dissolved phosphate, at least 0.3 mole per liter dissolved chromate and at least 0.5 mol per liter dissolved metal, said powder being present in an amount of from about 250 to 1000 grams per liter of said solution and at least about 1% by weight of said powder being an alloy containing from about 3% to 99.5% by weight aluminum and from about 0.5 to 97% by weight magnesium, and at least about 0.5% of the total weight of said powder being magnesium alloyed with aluminum, the aluminum and magnesium constituting more than 50% by weight of said alloy; drying said composition; and then curing the composition by heating to a temperature at which the composition becomes insoluble in water.

11. A method as set forth in claim 10, wherein a portion of the powder in said composition is aluminum powder.

12. An article of manufacture comprising a solid substrate having deposited thereon a layer formed by drying and heat curing to water insolubility a coating consisting essentially of a mixture of powder in an aqueous solution the solute of which consists essentially of a combination of inorganic compounds selected from the group consisting of phosphoric acid, chromic acid, molybdic acid and the metal salts of said acids, the combination of compounds in said solution being such as will provide at least 1 mol per liter dissolved phosphate, at least 0.3 mol per liter dissolved material selected from the group consisting of chromate and molybdate, and at least 0.5 mol per liter dissolved metal, at least about 1% by weight of said powder being an alloy containing at least about 3% by weight aluminum and at least about 0.5% by weight magnesium, and at least about 0.5% of the total weight of said powder being magnesium alloyed with aluminum, the aluminum and magnesium constituting more than 50% by weight of said alloy said powder being present in said solution in an amount of about 250 to 1000 grams per liter of said sothereof is said alloy.

Claims (16)

1. A COMPOSITION CONSISTING ESSENTIALLY OF A DISPERSION OF

1. A PAPER COATING COMPOSITION OF A PIGMENT, STARCH AND A LATENT WATER INSOLUBILIZER FOR SAID STARCH, SAID LATENT WATER INORGANIC SOLID PARTICULATE MATERIAL IN AN AQUEOUS SOLUTION THE SOLUTE OF WHICH CONSISTS ESSENTIALLY OF A COMBINATION OF INORINSOLUBILIZER BEING THE REACTION PRODUCT OF FROM 0.5 TO 0.75 MOLE OF UREA PER MOLE OF GLYOXAL AND HAVING AN AVERAGE MOLECGANIC COMPOUNDS SELECTED FROM THE GROUP CONSISTING OF PHOSPHORETIC ACID, CHROMIC ACID, MOLYBDIC ACID AND THE METAL SALTS ULAR WEIGHT OF FROM 250 TO 325, AND BEING PRESENT IN SAID COMPOSITION AT A CONCENTRATION OF FROM 2 TO 10 PERCENT BY OF SAID ACIDS, THE COMBINATION OF COMPOUNDS IN SAID SOLUTION BEING SUCH AS WILL PROVIDE AT LEAST 1 MOL PER LITER DISSOLVED WEIGHT BASED ON THE WEIGHT OF THE STARCH. PHOSPHATE, AT LEAST 0.3 MOL PER LITER DISSOLVED MATERIAL SELECTED FROM THE GROUP CONSISTING OF CHROMATE AND MOLYBDATE AND AT LEAST 0.5 MOL PER LITER DISSOLVED METAL, SAID SOLID PARTIC ULATE MATERIAL BEING PRESENT IN AN AMOUNT OF FROM ABOUT 10 TO 2000 GRAMS PER LITER OF SAID SOLUTION, AT LEAST ABOUT 1% BY WEIGHT OF SAID SOLID PARTICULATE MATERIAL BEING AN ALLOY CONTAINING AT LEAST ABOUT 3% BY WEIGHT ALUMINUM AND AT LEAST ABOUT 0.5% BY WEIGHT MAGNESIUM, AND AT LEAST ABOUT 0.5% OF THE TOTAL WEIGHT OF SAID SOLID PARTICULATE MATERIAL BEING MAGNESIUM ALLOYED WITH ALUMINUM, THE ALUMINUM AND MAGNESIUM CONSTITUTING MORE THAN 50% BY WEIGHT OF SAID ALLOY, SAID COMPOSITION BEING HEAT CURABLE UPON DRYING THEREOF TO A SUBSTANTIALLY WATER INSOLUBLE MATERIAL WITH SAID SOLID PARTICULATE MATERIAL BEING BONDED THEREIN.

2. A composition as set forth in claim 1, wherein said alloy consists essentially of from about 30% to 70% by weight aluminum and the remainder magnesium.

3. A composition as set forth in claim 1, wherein said alloy is about 70% by weight aluminum and about 30% by weight magnesium.

4. A composition as set forth in claim 1, wherein a portion of said solid particulate material is metal powder other than said alloy.

5. A composition as set forth in claim 1, wherein a portion of said solid particulate material is aluminum powder.

6. A composition as set forth in claim 1, wherein sAid solid particulate material is present in an amount of from about 250 to 1000 grams per liter of said solution and at least 50% by weight of said solid particulate material is aluminum.

7. A composition as set forth in claim 1, wherein the combination of inorganic compounds in said solution is such as to provide about from 1 to 4 mols per liter dissolved phosphate, from 0.5 to 3 mols per liter dissolved material from the group consisting of chromate and molybdate and from 1 to 4 mols per liter dissolved metal.

8. A composition as set forth in clalim 1, wherein the metal in said solution is predominantly magnesium.

9. A composition as set forth in claim 1, wherein said solid particulate material has a grain size not exceeding about 325 mesh.

10. A method for coating comprising the steps of applying to a substrate surface a composition consisting essentially of a uniform mixture of powder in an aqueous solution the solute of which consists essentially of a combination of inorganic compounds selected from the group consisting of phosphoric acid, chromic acid, and the metal salts of said acids, the combination of inorganic compounds in said solution being such as will provide at least 1 mol per liter dissolved phosphate, at least 0.3 mole per liter dissolved chromate and at least 0.5 mol per liter dissolved metal, said powder being present in an amount of from about 250 to 1000 grams per liter of said solution and at least about 1% by weight of said powder being an alloy containing from about 3% to 99.5% by weight aluminum and from about 0.5 to 97% by weight magnesium, and at least about 0.5% of the total weight of said powder being magnesium alloyed with aluminum, the aluminum and magnesium constituting more than 50% by weight of said alloy; drying said composition; and then curing the composition by heating to a temperature at which the composition becomes insoluble in water.

11. A method as set forth in claim 10, wherein a portion of the powder in said composition is aluminum powder.

12. AN ARTICLE OF MANUFACTURE COMPRISING A SOLID SUBSTRATE HAVING DEPOSITED THEREON A LAYER FORMED BY DRYING AND HEAT CURING TO WATER INSOLUBILITY A COATING CONSISTING ESSENTIALLY OF A MIXTURE OF POWDER IN AN AQUEOUS SOLUTION THE SOLUTE OF WHICH CONSISTS ESSENTIALLY OF A COMBINATION OF INORGANIC COMPOUNDS SELECTED FROM THE GROUP CONSISTING OF PHOSPHORIC ACID, CHROMIC ACID, MOLYBDIC ACID AND THE METAL SALTS OF SAID ACIDS, THE COMBINATION OF COMPOUNDS IN SAID SOLUTION BEING SUCH AS WILL PROVIDE AT LEAST 1 MOL PER LITER DISSOLVED PHOSPHATE, AT LEAST 0.3 MOL PER LITER DISSOLVED MATERIAL SELECTED FROM THE GROUP CONSISTING OF CHROMATE AND MOLYBDATE, AND AT LEAST 0.5 MOL PER LITER DISSOLVED METAL, AT LEAST ABOUT 1% BY WEIGHT OF SAID POWDER BEING AN ALLOY CONTAINING AT LEAST ABOUT 3% BY WEIGHT ALUMINUM AND AT LEAST ABOUT 0.5% BY WEIGHT MAGNESIUM, AND AT LEAST ABOUT 0.5% OF THE TOTAL WEIGHT OF SAID POWDER BEING MAGNESIUM ALLOYED WITH ALUMINUM, THE ALUMINUM AND MAGNE SIUM CONSTITUTING MORE THAN 50% BY WEIGHT OF SAID ALLOY SAID POWDER BEING PRESENT IN SAID SOLUTION IN AN AMOUNT OF ABOUT 250 TO 1000 GRAMS PER LITER OF SAID SOLUTION AND BEING BONDED IN SAID LAYER AFTER CURING THEREOF.

13. An article as set forth in claim 12, wherein said alloy contains from about 30% to 70% by weight aluminum and from about 30% to 70% by weight magnesium.

14. An article as set forth in claim 12, wherein at least a portion of the powder in said coating is aluminum powder.

15. An article as set forth in claim 12, wherein of the total powder in said coating at least 5% by weight thereof is said alloy.