CA1075980A - Coating of metal - Google Patents

Abstract

COATING OF METAL
Abstract of the Disclosure Aluminum diffusion can be effected from continuous coatings of leafing-type aluminum particles and such leafing coatings in very thin layers are more effective than coatings of non-leafing aluminum, with or without diffusion. Other protective metals in flake or leaf form can be substituted for or added to the leafing aluminum. Adhesion of the flakes to the substrate is greatly improved and can be effected at lower temperatures if the flakes are applied from a dispersion containing a vola-tilizable halogen-type carrier or an ammonium chromate. The leafing coatings can be sprayed on from aqueous dispersion containing wetting agents and if desired a polyethylenetrtra-fluoroethylene and/or mixtures of phosphoric acid, chromoic acid and magnesium, aluminum, calcium or zinc salts of these acids.
A protective second coating of such mixtures can be applied as a cover layer over the layer containing the leafing aluminum, and this combination works best on a ferrous metal that has an aluminum diffusion coating, particularly a ferrous metal that contains less than 1% chromium and has such an aluminum diffu-sion coating. It also works very well on aluminum diffusion coatings from packs containing chromium, or chromium and sili-con, in addition to the aluminum.

Description

1075s80 :
BACKGROUND OF THE INVENTION

This invention relates to the coating of metals to improve their use, particularly in resisting corrosion as well as attack by chemicals.

Among the objects of the present invention is the provision of novel coating methods and compositions, as well as novel coated metals, that are simple to manufacture and use and are highly effective.
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. SUMMARY OF THE INVENTION
,: i :`'., . 10The foregoing as well as additional objects of the present invention will be more fully understood .from the following description of several of its exempli-fications.

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'' ' ' ' , , 1~75980 ' ', According to the present invention, ~lake forms of pro-tective metals form particular}y effective protective coatings for corrodible metals when partially diffused into the surface to be protected or when combined with special binders or added over other coatings.
As shown in U.S. Patents 3,248,251 granted April 25, 1966 and 3,787,305 granted January 22, 1974, powdered aluminum has been suggested for use in applying protective layers over corro-dible metals. The protection thus obtainable from layers of less than about 1 or 1.5 milligrams per square centimeter, is greatly improved if the aluminum coating is effectively continuous over the surface being protected, a result that is obtained when leafing-type aluminum particles are applied in amounts that permit the individual aluminum flaXes to partially overlap each other over the entire surface being protected. It is also helpful, as sug-gested in Patent 3,787,305, to sub~ect the aluminum-coated ferrous member to a temperature that causes at least a little bit of the aluminum to diffuse into the ferrous surface.
~ eafing type aluminum particles can be made as described 'in U.S. Patent 2,312,098, and are generally characterized by the presence of stearic acid or aluminum stearate or the like as a very thin coating on the surface of each aluminum particle, a con- j dition which makes it extremely difficult to disperse such aluminum particles in water. A substantial amount of wetting agent will effect a suitable dispersion, although it is easier to effect such dispersions by also adding diethylene glycol or triethylene glycol ;
or ~ore ~ighly polymeric ethylene glycols having a molecular weight up to about 9000, as described in U.S. Patent 3,318,716 granted ' i . . t 3- i :: ~ . . . -,: . . : .

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May 9, 1967, or by adding glycerine. As shown in the last-mentioncd patent, very effective dispersions of leafing-type aluminum can be made from a concentrate that consists essentially of the leafing aluminum, the polymeric ethylene glycol and a wetting agent, the aluminum being present in an amount about 1/4to about 1-1/2 parts by weight for every part of the polymeric ethylene glycol by weight, and the wetting agent concentration from about 5% to about 25%
by weight of the concentrate. ', The foregoing concentrate readily mixes with water in all proportions to provide an aqueous dispersion of almost any desi~ed aluminum content. Thus a diluted dispersion containing 5%
aluminum, 6% hexa-ethylene glycol and O.r/O para-n-octyl phenyl ether of decaethylene glycol, is readily sprayed onto a stator ring of a jet engine compressor to leave a coating weighing 0.5 milligram per square centimeter after drying in air to evaporate most of the water. The stator thus coated is then heated in an air oven until its temperature reaches 800F. The heating first causes the glycol and wetting agent to be volatilized off leaving a very adherent continuous and shiny coating that resembles polish-ed aluminum and significantly adds to the corrosion resistance of ~the stator ring even if the heating temperature soeq no higher :................... ::
~,~ ' than 600F. The increase in corrosion resistance becomes more ,'significant when the heating carries the coating to temperatures of about 900F, where some diffusion of the aluminum into the : - ;
;ferrous surface of the stator begins. The rate of diffusion and the degree of resulting corrosion resistance is further increased by confining the coated stator in an atmosphere of gaseous aluminum chloride while it iq at temperatures a~ove about 700F.

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-`-` 1075980 me aluminum chloride atmosphere is conveniently provided by a pack treatment as described in Example I of U.S. Patent # 3,958,046 (May 18, 1976) or Example I A (infra) of the present application, but with no aluminum in the pack, However the stator ring containing the leafing aluminum coating can merely be hung on a wire in a retort containing a little energizer and no pack, and fired in this way in an otherwise inert atmosphere.
Other aluminum halides such as aluminum bromide and aluminum iodide also behave like aluminum chloride and indeed other well known energizers for low temperature aluminum diffusion coatings can be used instead of the aluminum halides with corresponding results, A list of such energizers is given in U,S~ Patent No, 3,948,687, Instead of adding the energizer to the atmosphere in which the heating is effected, they can be added to the dispersion from which ~, the metal flake is applied, Thus the above-mentioned diluted dispersion of aluminum deposits on 410 stainless steel or on plain carbon steel an adherent 0,1 milligram per square oentimeter coatingr after heating in air to about 600 or 700F for as little as 20 seconds, Other ammonium halides and similar high temperature or low temperature -aluminizing energizers that are driven off at 600 to 1000F can be used to give adherent protective films weighing as much as 4 milligrams per - square centimeter when so heated, For this purpose the energizer content should be no greater than about 80~ and at least 1%! of the weight of , the flaked metal. Larger quantities can be used but merely require more time and heat energy to be driven off. Some fluorides can cause significant attack of the aluminum or of the substrate, and are best avoided `:

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107S91~0 or used in a high spc~d opcration in which the time they contact these metals is restrictcd to minimize thc dcletcrious e~fccts of such attac~.
The adhesion of 0.1 to 4 milligrams per square ccntimcter layer of a flaXed metal such as aluminum is also improved by incorporating in the layer chromic acid, or a compoùnd such as ammonium chromate or dichromate which is a salt of chromic acid with a volatile base, or magnesium chromate or dichromate, or water-soluble chromates or dichromates of other divalent metals.
Dissolving in the aluminum dispersion an amount of ammonium dichro-mate 2Yo by weight of the aluminum gives a sharp increase-in adhe-sion upon heating of a dried 4 milligram per square centimeter layer of such composition on pl~in carbon steel to 700F for 5 minutes. A 5% addition of the dichromate to the dispersion renders such a heat-treated coating completely resistant to wiping off.
Similar results are obtained when the heating is at 600 to 1000F
for as little as 20 seconds, and as much as 20 to 30 minutes, although not m~ch i~ gained by prolonging the heating beyond about 1 minute. The ammonium dichromate content can be as high as 8~/o of the weight of the metal, but above this level the corrosion resistance tends to drop off.
The dichromates are used in amounts corresponding to the foregoing amounts of ammonium dichromate, the chromates in amounts .,~ . . .
about 1/5 greater, and chromic acid in amounts about 1/5 less.
Each of these chromium compounds improves the aluminum coating so that it provides good salt spray resistance to plain carbon steel on which such a coating is applicd. When such coat'ng is covered by any other corrosion-resisting top coating, such as thosc described in the working examples, e~ceptionally good corrosion resistance is impartcd, ~vcn to plain carbon steels.

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! ~he lcafing-type of aluminum particlcs or other protec-tive metal used in thc above connection are preferably from about i~50 to about 250 microns in maximum size although other sizes can also be used.
Ij Non-ionic wetting agents are preferred for dispersing "the aluminum inasmuch as such wetting agents are more readily driven off by high temperatures. However other types of wetting i agents, including those that are not driven off or not completely driven off at 600 to 900F or 1000F, can be used. Making the aluminum coatings heavier than about 4 milligrams per square centimeter does not add anything slgnificant to the corrosion resistance, and as little as 0.1 milligram per square centimeter j is helpul although at least about 0.3 milligram per square centimeter, and better still 1 to 2 milligrams per square centi-meter is preferred.
. 1, . i Inasmuch as ferrous metals, such as plain carbon steel, - l~cast iron, low alloy steels, stainless steels and-other chromium-.::. j, - i ¦ containing steels begin to oxidize on their surface at the ~ temperatures used for the heating of the flake layer, it is help- i - j ful but not essential to conduct that heating in a non-oxidizing atmpsphere. When the energizer used in such heating step is incorporated in the layer of flaked metal, the volatilization of the energizer along with the volatiliz~tion of any suspending ~;agent present in the dispersion from which the flaked metal deposits, provides an atmosphere of reduced oxidation potential, , and heat treatments that only extend for half a minute or less ,~need no further oxidation-preventing precautions. 8Owever, if desired, the hoating can be effected in a closed chamber as by batch heating an opened coil of coated metai sheet or wire placed ' ~' .i ` ~
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-~ ` 075980 in the closed ch~mber which then has its atmosphere flushcd out with argon, hydrogen or nitrogen, or by continuously passing a continuous strip or wire of the coated metal into and out of the chamber through close-fitting slots or holes in the chamber walls, while continuously introducing into the chamber a small stream of protective gas that maXes up for losses of gas through the slots or holes.
When the metal flakes to be applied are aluminum, the benefits of a non-oxidizing atmosphere are not significant and an atmosphere of ambient air is entirely adequate.
It is not essential to have the polyglycol present in the flake dispersions in the foregoing proportions, or at all, although such presence is helpful. Reducing its concentration leaves less of it in the layer of metal flakes, so that less has to be driven off or converted to innocuous residue by the heating operation. Without the polyglycol, the dispersions require frequent agitation and then coatings applied by spraying from such dispersions tend to be of non-uniform thic~ness.
The application of a leafing-type aluminum coating particularly improves the corrosion resistance of plain carbon steel or other ferrous surfaces that contain less than 1%
chromium, when such surfaces have a diffusion coating of aluminum.
The adhesion promotion obtained with the energizer treatments described ab~ve is accompanied by a little diffusion of the aluminum into the substrate, and in addition the aluminum coatings thus formed are highly conductive to electricity as well as highly p~otective.
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' The leafing aluminum coating also improves the corrosion resistance of a coating obtained from mixturcs of aluminum particles with phosphoric acid, chromic acid and maqnesium, aluminum, calcium or zinc salts of these, as described in Patent 3,248,251. Thus substituting the leafing aluminum, along with sufficient wetting agent and with or without the polymeric ethylene glycol, for the spherical aluminum in the formulations described in that patent contributes a significant increase in corrosion resistance, particularly in cured layers weighing not more than about 1 milligram per square centimeter. In such mixtures firing of an aluminum-containing coating does not effect significant diffusion of aluminum into a ferrous substrate so long as the firing temperature is not over 1000F. Above that temperature the . .
firing tends to adversely affect ferrous metals, particularly those used in jet engine compressor sections.
Another feature of the use of leafing-type aluminum .. :
is the improved appearance that the workpieces are given. Sub-stituting this type of aluminum for that shown in the composition of Example I in Patent 3,248,251 with the help of the foregoing .. ,, ~ :
polyglycol-wetting agent formulation, not only gives a product having somewhat better corrosion resistance, but with a bright aluminum sheen. During the heating of the new compositions to cure them, fumes are given off indicating that the polyglycol and the wetting agent are being volatilized away, and no significant reduction of the hexavalent chromium to trivalent condition seems to taXe place.
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, The foregoing improvements in corrosion resistance and in . appearance are also obtained when the last-mentioned coating is covered by a similar coating, even one that does not contain metallic aluminum.
Such top coatings are described in U S, Patents No 3,948~687 and No.
3,048,689. However multiple coating layers each of which contains metallic aluminum are very effective, particularly when each layer weighs between 0.1 and 0.5 milligrams per square centimeter.
As shown in the aforementioned applications, the proportions of the ingredients in the ehromic acid-phosphorie acid-salt coating mixture ean range as follows:

Chro~ate ion 0 2 to 1, preferably 0.4 to 0,8 mols per liter Phosphate ion 0 7 to 4, preferably 1.5 to 3.5 mols per liter Magnesium ion 0.4 to 1,7~ preferably 0,9 to 1.4 mols per liter ! Polytetrafluoroethylene 2 to 14, preferably 3 to 10 . resin grams per liter The magnesium ion ean be replaced by any of the other ions referred to above, in the same eoneentrations, bm: .
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; Instead of directly applying such an overlying coating whether or not it contains metallic aluminum, it can be applied afte~ an intervening coating of colloidal alumina or the like weighing about 0.1 to about 1 milligram per ~quare centimeter, also as described in the aforementioned U.S. Patents No, 3,948,687 and No. 3,048,689 . with increases in corrosion resistance as described in those patents,With or without such an intervening coat, the final cured article has a golden sheen, contributed by the presence of chro.mate, that is extremely attractive and quite a&erent, The presence of poly-: 10 tetrafluoroethylene particles in the phosphoric acid-chrGmic acid-salt mixtures of either or both of such layers is also helpful and doe s not detract from the golden appearance, Such presence in a top coating m~kes that coating very smooth and slippery without detracting . significantly from the coating hardness, These coating co.mbinations with or without the intervening coating of colloidal particles are . most effective in increasing the corrosion resistance of chromaum-freeand chromium-containing ferrous substrates that have aluminum-diffused surfaces, Indeed they also have this desirable effect on buIk aluminum such as aluminum sheets, foil and bars, as well as on titanium, On aluminum substrates such coatings adhere exceptionally ~ well and withstand severe deformation of the surfaces to which they : are applied. However the gold color contributed by the foregoing top coatings that are free of metallic aluminum, is not provided when metallic aluminum is included in those top coating ~11-bm:

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10759~0 formulations. The intervening coatings of colloidal alumina andthe like are not heavy enough to obscure the metallic appearance of the substrate and accordin~ly do not adversely affect the appearance. On the oth~r hand those intervening layers improve the wettability of the aluminum-containing surface by the top coating. Some alumina dispersions are acid and tend to attack a layer of aluminum on which they are applied. To alleviate this situation the alumina dispersion used can be neutral or even some-what alkaline, or the acidity of such dispersion can be so low in strength and the dispersion so rapidly applied and dried that any attack is immaterial, or a little chromic ac~d added to the dis-persion to protect the aluminum.
The following are examples of the production of gold-colored hi~hly attractive and very corrosion-resistant steel and aluminum products.
EXAMPLE I
A. Into each of four plain carbon steel retort cups 2 feet wide and 14 inches high is poured a powder pack consisting of 2~ aluminum by weight and 8~o alumina, both minus 325 mesh and uniformly mixed together. After the retort bottoms are covered with about 1/2 inch of powder, jet engine compressor blades made of AISI 410 stainless steel are laid over the powder layer, the vanes being spaced about 1/8 inch apart. This layer of blades is thcn covered with more pow'er till the powdcr is about 1/2 inch above the vane tops, and another layer of blades is then laid down and the layering repeated until the entire packing is 12-1/2 inches deep in cach retort. More pack powder is then added to each retort to assure there i9 about 1 inch of powder above thc tops of the topmost blades,follow~ng which thcre .' . . ' ~.' 12.
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-- : : , is sprin~led over each a very thin stratum of crystalline AlC13 61;2O in an amount weighing 0.6" of thc total powder weight.
The retorts are then filled to their tops with additional pack powder, and they are stacked one above the other on the floor of a gas-fired bell furnace. The stacking does not seal any of the retorts shut. The top of the furnace equipped with gas inlet and outlet flush lines is lowered over the stack and sealed against the furnace floor, and a slow flow of argon gas is passed through the furnace interior to start flushing out the air within it.
After the argon purge hydrogen is substituted for the argon, and is introduced at a rate that permits it to be burned with a small flame as it emerges from the end of the outlet tube. Only a very low flow rate is necessary, about 10 to 15 standard cubic feet per hour.
The heating of the furnace is started at a rate of about 1.5F per minute, as measured by thermocouples in each retort and connected to external meters, and when the thermocouples reach 300F the flow of hydrogen can be reduced so that the outlet flame is very tiny. At this point the hydrogen inflow can be less than 10 standard cubic feet per hour.
As the heating-up continues, the temperatures indicated by the thermocouples increase uniformly and gradually and chemical vapors begin to appear in the burning outlet gas. By the time the thermocouple temperatures reach about 450F the discharge of chemical vapors has subsided, the gas flow continuing till the temperatures reach 875F where the furnace hea~ing is set to hold.

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After 16 hours a~ 87SF che furnace heating is tcr-minated and the furnace ~ermitted to cool until the thermocouplc temperaeures reach 300F. The atmosphere in the furnace is then purged by switching the inflow gas to argon or nitrogen and thc furnace shell then removed from the retorts, permitting the retorts to cool further in air. The contents of the retorts are then poured out, washed, dried and finally lightly blasted with fine glass ~articles propelled by an air stream supplied at 5 to 10 pounds per square inch, giving an aluminum pick-up of 4.0 milligrams per square centimeter of ferrous surface.
~_.' B, On the lightly blasted aluminized surface there is sprayed with an air-propelled spray, a uniform very thin layer from an aqueous dispersion of 3.5% CrO3;

2.4% MgO;
11% H3P04i 5.7% leafing aluminum;
6`8% polyethylene glycol having an average mole--~ cular weigkt of 300 and in which the glycols-range from pentamethylene glycol through , hep;amethylene glycol; and :; 0,8% para-isononyl phenylether of dodecaethylene . glycol;
: ~ ~ all percentages bein~ ~yjweight.
.~ . The sprayed blades are then air d~sr~and baked at 700F in an: air oven for 30 minutes to give a coating weight from this spray of 0.7 milligram per square centimeter of ferrous surface.

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C. The blades coated in steps A and B have their ;coated surfaces given a spray coating of colloidal alumina dis-persed in a 20 concentration by weight in water to which a little HCl is added to bring thc pH down to about 4. A very fine spray is used to leave a light coating which after drying in air weighs 0.5 milligram per square cen~imeter.
D. The blades with the air-dried coatings are then j given a top spray coating from an aqueous dispersion of , .
5.8% CrO3:
4% MgO:
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~- ~3~ ; 18-3% H3Po4 an ~ 0.5% polytetrafluoroethylene particles about 1 ; - micron in size this spray being such that upon air drying in an oven and then baking at 700F for 30 minutes in an air oven, the final coating weighs 0.5 milligram per square centimeter.
, EXAMPLE II
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` The coating steps of Example I are repeated but this time the worXpieces are SAE 1010 steel, the diffusion pack peak . temperature is 800F, the aluminum picked up in the diffusion step is 7-1/2 milligrams per square centimeter, the baking in ',- steps B and D is at 900F, and the coat weight applied in step B
is 0.9 milligram per square centimeter.
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Discs of low alloy steel containing 0.5% chromium and 0.0~/~ carbon as the only significant alloy ingredients, which discs are used to hold jet engine compressor blades, are given the coating treatment of Example I, this time the diffusion coating pack being held at a peaX temperature of 900F, the aluminum picked up in the diffusion being about 8 milligrams per square centimeter; the coating step B is followed by a light blasting with very fine glass microspheres about 5 microns in diameter propelled by an air stream from a blast supplied at 5 pounds per square inch gauge, and care being taken to make sure that no significant a unt of the leafing aluminum in this coating is removed during such blasting.
EXAMPLE IV
Sheets of 18-8 stainless steel are given the coating sequence of steps B, C and D of Example I except that the sheets with coating B are baked at 800F for 30 minutes and after such baking that coating weighs 1 milligram per square centimeter.
''Coating D is also baked at 800F for 30 minutes with its weight being 0.7 milligram per square centimeter.
EXAMPLE V
Plates of Type 3S aluminum were coated by the sequence of steps B, C and D of Example I, and the coated plates had a gold sheen of very,attractive appearance.
EXAMPLE VI
Titanium sheets coated by the steps B, C and D of Example I were also colored with a golden shecn.

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E~YP~E VII
Coupons of 410 stainless stecl are dipped in thc coatin~
mixture of Example I B, modified by the addition of 0.4~,' fine teflon particles from an aqueous teflon dispersion. The coupons were removed from the coating mixture and heated to volatilize off the wetting agents, taking care to remove the aqueous material running down to the lower edges of the coupons and to thus keep the coating more uniform in thickness. After the volatilization is completed the coupons are quenched in ~ater and have a very 'smooth .and slippery surface. -. .: .
Essentially the same results are obtained in Example Ias well as in Examples II and III when anhydrous aluminum chloride, bromide or iodide, or hydrated aluminum bromide or iodide is used in place of the hydrated chloride energizer, with the aluminum content of the pacX ranging from 10~/o down to 2%.
For aluminum diffusion effected below 900F it is preferred that at least 4% aluminum be in the pack. As disclosed in Serial No.
304,220, a convenient amount of hydrated energizer is rom 3 to 6 grams for a 6-1/2 pound pack when the pack is first broken in as well as when the broken-in pack is subsequently used for coating.
Instead of using aluminum of relatively pure composi-tion such aluminum can be an alloy containing significant quanti-ties of beneficial ingredients such as silicon. A content of 12%
silicon will, by way of example, improve the resistance to high temperature oxidation of ferrous metals subjected to diffusion coating by such an alloy.

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~ he very effcctive proeection imparted to ferrous mctals containing less than 1~ chromium, such as plain carbon and low alloy steels does not require more than a single layer of thc chromic acid-phosphoric acid-salt-aluminum mixture, whcn prcccdcd by an aluminum diffusion treatment. This is illustrated by thc ~ollowing examples.
EX~tPLE VIII
Panels of SAE 1010 steel are given the diffusion coating treatment of step A in Example I, but using anhydrous AlC13 ener-gizer. ~he diffusion coating weighed 7 milligrams per square centi-meter, and it was then coated by spraying-on an aqueous dispersion containing the chromic-acid-phosphoric acid-salt aluminum mix in ;th~ following proportions:
1.25 moles per liter P04 0.68 moles per liter M~+~
0.38 moles per liter CrO4 64.5 grams per liter Aluminum 77.0 grams per liter of the polyethylene glycol of Example II, and 10.0 grams per liter of para-isooctyl phenyl ether of tetradecaethylene glycol The sprayed-on layer was dried and heated in an air oven to 900F
for 25 minutes to give a 1 milligram per square centimeter coating weight.
The thus-coated panel withstood 10 cycles of alternately heating to 900F for 6 hours in air followed by 16 hours exposure to a 5% salt-spray at 95F without showing attack of base metal, and substantially no attack nor spalling of the coating. Even better results are produced whcn the dried and ov~n-heated coating ls covered by another layer preferably just liXe the spraycd-on layer but not containing the metallic aluminum. A porous alumina barrier betwcen thcse two layers givcs still further improvcmcnt.

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SLmilar results are obtained when aluminum ions replacc ehe magnesium ions, as well as when the baking is ac 700F and the bakcd coating lightly blasted with very fine glass micro-spheres aboue 25 microns in diameter impelled by air blasted at a pressure of 5 pounds per square inch. Also the use of hydra~ed energizer durin~ the diffusion coating ?.roduces the same results as the use of anhydrous energizer.
In the aluminum-containing coating mixtures the con-centration of the leafing-type aluminum particles can range from about 30 to about 150 grams per liter of mixture, and the re-maining ingredients can have the concentration ranges given supra.
The water in these compositions can also be replaced in whole or in part by the polyethylene glycols or by any other inert liquid in which the ingredients can be dispersed and sprayed. For com-binations in which only a single chromic acid-phosphoric acid-salt layer is used such layers can advantageously weigh as much as 1.5 milligrams per square centimeter. However even such.a layer containing the leafin~ type aluminum of the present inven-tion and weighing only 1 m'lligram per square centimeter or as much as two milligrams per square centimeter imparts excellent corrosion resistance to plain carbon ant low alloy steels as well as other ferrous metals containing less than 1% chromium, when applied over an aluminum diffusion coating on the metal.
This corrosion resistance is even fureher increased when the layer containing the leafing type aluminum has its electrical conductivity increàsed as by heating to 900F or higher; or by li~htly blasting it with finc non-corroding particlcs such as glass or ground walnue shells or the like; or by heating it in ehe presence of ammonium chloride or ochcr energizers as described abovc.

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:. . , m us panels of steel containing 0.05% carbon and 0,3% titanium as the only material alloying metal, show unusually high resistance to salt spray corrosion when covered by an aluminum diffusion coat having an aluminum pick-up of 6.5 milligrams per square centimeter, over which is applied the phosphoric acid-chromic acid-salt-alumin,um co,ating of Example IX but baked at 70QF and then given a light blasting wi.th fine -glass micro-spheres in a 5 psi air streamt the blasting removing about 0.1 milligram of the kaked coating per square centimeter, Although the thus protected panels show splendid corrosion resistance, their coated surfaces tend to tUXn whi,te or greX after long exposure to salt spray~ indicating that the aluminum in the top layer is being attacked very slowly, This whitening or greying can proceed for a considerable time before the steel is attackedl even where the coating is scratched through to the base ~etal~ ~owever the whitening or greying can be greatly slo~ed by covering the phosphoric acid-chromic acid-salt-aluminum layer with a top c,oating such as the combination of an air-dried colloidal alumina layer weighing 0~1 to 1 milligram per square centimeter and an overlying baked phosphoric acid-chromic acid-salt-Teflon~layer~ weighing 0.2 to 1 milligram per square centimeter. It is Preferred that the combination of layers on the aluminum diffused surface weigh not more than about 2 milligrams per square centimeter.

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A, pointed out above, the coatings containing the leafing type of aluminum in accordance with the present invention arc electrically conductive to an appreciable degree when they have been subjected to baking of at least 900F or when they ; have been burnished as by means of the fine glass blasting, or when they are heated to at least about 600F in the presence of an energizer or an ammonium chromate or chromic acid. The greater their electrical conductivity, the greater the corrosion resistance they impart, particularly to ferrous substrates. These coatings ' are also smoother and more effective in thinner layers than com-parable coatings containing granular aluminum as described in Patents 3,248,251 and 3,787,305)and thus much more suitable for use ` an air foils, particularly of turbines.
! A silver flake coating adheres well to ferrous metal - when heated to 600 to 700F in the absence of energizers. Indeed - , the application of energizers to silver flake coatings detracts ... ~.
from their effectiveness. On the other hand tin flake does not disperse very well and tin flake coatings are best heated ' to ~00F or higher, without energizers.
Although Patent 3,248,251 suggests coatings as thin as 0.5 mil, the commercial coating formulation derived from it is ., , ;marXeted with instructions to apply it in thicknesses of at least 1.5 mil. Those instructions also suggest that those 1:
coatings be glass blasted and/or baked to 1000F or higher for ,'best results. However superior results with the 3,248,251 .
coatings are obtained when there is applied over such coatings a ` very thin layer of flake aluminum. Thus a 410 stainless steel '' ` ~,'' ` , . '~

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10759~0 jet engine compressor blade coated with a 2 mil thick layer of the oven-dried ~550F) commercial mixture containing granular aluminum and corresponding to ~xample 2 of Patent 3,248,251, gave much better protection after there was sprayed over the , oven-dried coating a 0.4 milligram per square centimeter layer of flake aluminum from a ~/O suspension in an aqueous solution of only 2.4% hepta-ethylene glycol and l/3% p-nonyl-phenoxy octadecaethoxy ethanol, and the thus-coated blade again baked dry at 550 or 600F. The degree of improvement thus contributed iby the additional flaking aluminum layer is approximately the ., ,~ :.
same as that obtained by baking at 600F without that additional layer and then glass-blasting the first coating layer.
Continuous aluminum coatings as thin as 0.1 milligrams per square centimeter, and even thinner, are particularly , ~ :
desirable for coating titanium riv~ts used in aircrafts or ~spacecraft or similar eguipment. In such combinations, the ~aluminum is conveniently applied in the form of flakes, as indicated above, and the adhesion of the flakes is improved by the dif,fusion type heat treatment with or without an energizer, 'or by incorporating an ammonium chromate with the aluminum flakes and heating to produce the bonding acton, which is '~
!also described above.
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Other protective metals in flake or leafing form can be used in place of or in addi~ion to the flake aluminum to also increase the resistance of corrodible metals to o~idaeion and the like, particularly at high temperatures. E~amples of such pro~ective leafing metals are nickel, tin, stainless steels of all kinds, and silver. U.S. Patent 3,709,439 describes the making of such flakes. Ohromium-bearing or austenitic stainless steels are especially effective. Indeed flake type 304 or type 316 stainless stèel when mixed with from about half to about twice its weight of flake aluminum, gives better protection against alkaline attack than flake aluminum alone- Alloys of nickel and aluminum such as NiAl, and alloys of iron or silicon with aluminum are also suitable for use in flake form in place of the aluminum flake to provide improved protection in accordance with the present invention. Mixtures of flaked metals can be applied as coating, and heated to cause the mixed metals to alloy with each other as well as with the substrate, and NiAl is readily formed in this way to make a very effective high-temperature-resistant coating.
While the dispersions containing flaked metal, with or without the chromate, phosphate and salts, can be applied by dipping, they are preferably applied by spraying or roller coating. In general such flake-metal-~ontaining coatings shou~d be baked at a temperature of at least 550F for a few minutes, and preferably for at least 30 minutes at as high a temperature as the coated combination will withstand, to provide best results.
Alloying of che metals in the coating with each other and with the metal substrate is also desirably effected in the manner described in U~S. Patent 3,720,537. Aluminized or tin coatet piain carbon .
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qteel ~s well as seeel coated wi~h a mix~ure of nluminum and ein is convenien~ly made in this way, as by spraying sheec s~cl unwinding from a coil, with a dispersion of ~he flaked me~al, then passing the thus coated metal sheet through gas flames to heae it to 700-750F for 30 seconds, after which ehe sheet is cooled and recoiled.
Dispersions of flake metal, such as aluminum, also containing polytetrafluoroethylene, deposit coatings ehat are not only very decorative by reason of the metallic sheen, but they are also quite hydrophobic and smooth and especially low in . frictiona~ resistance to sliding with respect to other objects.
Such coatin~s are also quite adherent to plain carbon steel or stainless steel or aluminum or aluminized substrates, even when cured at temperatures as low as 500F. or barely sufficient to drive off the dispersing and suspending agents. Top coatings - of the foregoing chromate-phosphate-salt mixtures or oth~r con-version coatings can be applied over such a metal-tetrafluoro-ethylene initial coaeing. For best resistance to corrosion the teflon content should be held down to not over about 1% of the ~ ~ .
final coating, about 0.5% being particularly effective. Where the teflon-containing coating is heated to about 900F or higher the teflon coneent can before heating be as high as about 2%
' w~thout detracting too much from the corrosion resistance.

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)75980 e foregoing top coatings of chromic acid-phosphoric acid-salt formulations are also helpful when applied over aluminum diffusion coatings that are produced by the inhibited diffusion processes described in U.S. Patents 3,257,230, 3,690,934, and

3,867,184. m ose processes are generally conducted at temperatures well above 1100F with cobalt- and nickel-based superalloys, but can also be conducted with ferrous substrates at lower temperatures, particularly to diffuse less aluminum.
In such inhibited diffusion it is desirable to use extremely fine particles of pre-fired alloys such as alloys of aluminum and chrominum, or of aluminum, chrcmium and silicon.
Particle sizes of from about 1 to about 10 microns are particularly suitable.
The separate step of pre-firing the chromium and aluminum mixture can be avoided by directly preparing such a mixture in finely divided form. To this end the magnesothermic reduction of chrcmium compoNnds such as Cr203 as described in French Patent 1,123,326 and its Addition Patent 70,936, can be modified by oombining an appropriate quantity of alumina with the chromium oompound, and such combination mixed and subjected to the magnesothermic reduction as described in those patents. This simultaneous reduction takes place at about the same temperatures and times as is shown for the reduction of the chromium compound alone and with the same equipment, producing a chromium-aluminum alloy having a particle size of about 1 micron. Residual magnesium as well as magnesium oxides present in the reduced material is ''' .

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removed by treatment with an excess of dilute nitric acid having a spccific gravity of about 1.12 to about 1.26. Such acid will not attacX chromium-aluminum alloys having as little as 16%
chromium by weight, but will readily dissolve metallic magnesium as well as magnesium oxide. Crushing the alloy to a fine powder helps the acid dissolve all the magnesium rapidly. It is not essential to remove any magnesium oxide present in the reduced mixture inasmuch as this compound is essentially inert during a coating operation and does not tend to sinter or adhere to the workpieces being coated or to the other ingredients of the coating pack. Where the hot magnesothermic reaction mixture has its ~; c `
vapor flushed out at high temperatures to flush out the relative-ly volatile magnesium metal remaining after the reduction is completed, t,he crude reaction product can after crushing be direct-ly used for diffusion coating. Where nitric acid washing is carried out, the washed material is rinsed with water, preferably to neutrality, filtered and dried before use.
Magnesothermic reduction can also be used in the same J ~ way to directly produce chr~mium-silicon, chromium-aluminum-silicon, chromium-aluminum-iron, molybdenum-silicon and tungsten-silicon alloys in the extremely finely divided form so highly desirable for diffusion coating workpieces. Silica makes a con-venient source of silicon for such purposes and can be directly substituted for or added to the mi~ture being reduced without materially changing the reduction rate or temperature. The finely divided alloys can also be produced by magnesothermically reducing chrom1um, iron, molybdenum or tungsten oxid~s or oth~r compounds .' ,".' ' ' ' ' :
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; i 1075~80 of these metals in the presence of aluminum and/or silicon in elemcntal form. During sueh reduction the aluminum and/or silicon alloys with the metallic chromium, iron, molybdenum and tungsten as it is formed.
The following is an example of the dual reduction techni~ue:
EX~MPLE IX
1392 grams of magnesium metal were placed in a plain carbon steel retort cup 8 inches in diameter and 7 inches deep, the retort uncovered with an inverted outer inconel retort and the combination heated in a furnace under an argon atmosphere to 1700F where it was held for 25 minutes to melt the magnesium.
The molten metal was then permitted to cool, still under argon, to room temperature, when the covering retort was removed, and replaced after 104 grams powdered A12O3 and 500 grams powdered Cr2O3 poured over the solidified magnesium. The combination was again heated under argon, this time to 1825F for 8 hours, and cooled.
A powdery reaction product remained in the retort. It was removed from the retort, treated with excess 2 ~ HNO3 until there was no further reaction evident, and then washed to neutra-lity with water. The resulting material was a chromium-aluminum intermetallic in the form of particles averaging about 1 micron in size. ~t analyzed 81~2-~o chromium ana 16.6% aluminym by weight, its yield being 91%. When mixed with alumina and ammonium chloride it gave very good aluminum diffusion coatings in the process of Canadian Patent 806~618~ in place of the mixture of chromium and aluminum there suggested.

Similar results are obtained when the preliminary melting of the magnesium is not effected, and where the inter-metallic is used for diffusion coating steels at lower temperatures, Cther intermetallics similarly made and used have the follcwing analyses:

a) 45,5% Al 54,5% Cr b) 44,1% Cr 47,7% Fe 8,5% Al c) 74,5% Cr 7,0% Al 8,5% Si Alloy c) contains some unreduced oxide~ but it still is very effective for use in the inhibited diffusion process.
Another type of aluminum diffusion coating over which the ~ top coatings described a~ove can be appliedr is a pack diffusion ';r aluminizing in which the aluminizing is inhibited by cobalt, m is . diffusion coating is illustrated by the following example: -':
' EX~LE: X
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A pack was made up in parts by weight of cobalt 30 ;` aluminum 14 '~ alumina (calcined~ 56 :
: . NH4Cl 1/2 ::
.. Each of the foregoing ingredients was a 250 to 360 mesh pcwder. m e ` mixture was thoroughly blended and then packed in a plain carbon steel retort along with nickel blow tips used for blowqng incandescent light bulbs~

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-; , , ~(~75980 m e re-tort thus loaded was loosely covered and a larger retort lowered over it as illustrated in U.S, Patent 3,764,371, Using a hydrogen-bathed atmosphere between the retorts, as also described in 3,764,371, the packed material was heated to 1975 where it was maintained for 20 hours. After cooling the treated workpieces and lightly glass-blasting them, all showed an aluminized case frcm about 4 to about 6 mils thick.
~ hen the same treatment is applied to U-700 nickel base blades for the hot section of a jet engine, somewhat thinner diffusion coating cases are produced, While leafing aluminum coatings can be applied over the foregoing diffusion aluminized blades, even without such top coating the aluminized blades shcw exoeptional resistance to oxidation and sulfidation. In general the resistance to these . .: .
effects of the U-700 alloy blades is a little better than the resistance of such blades aluminized in a chrominum-inhibited diffusion coating pack. This improvement seems to be due to the ` introduction of some cobalt into the case from the pack, and is not shown by the cobalt-base substrates. Other nickel-based - superalloys such as B-1900, Rene 62, MAR-M 200 do show this improved 20 resistance when so coated.
Nickel can also be used for inhibiting aluminum diffusion as for example into chromized dispersion-strengthened nickel in the manner described in Patent 3,785,854, and will then make a product of outstanding oxidation resistance, particulæly with a little chromium also present in the pack A very suitable aluminum pack in which a large amount of nickel and a small amount of chromium lowers the diffusion effectiveness is disclosed by M S, Seltzer, B. A, Wilcox and J. Stringer in Metallurgical Transactions, September 1972, pages 2391-2401 The Seltzer et al pack (600 g. alumina, 82 g. Ni, 17 g.
Al, 10,5 g. Cr, 6 g NaCl and 6 g, urea) bm:

ii -^ 1075980 I,at 2210F for 32 hours in a hydrogen or argon-washed atmosphere i deposits on chromized dispersion-strengthened nickel an aluminized~
¦'layer having an aluminum pick-up of approximately 4 milligrams per¦
square centimeter, a surface aluminum content of only about 4 to about 6%, and a case depth of about 4 mils. This product with-stands about l90 hours of thermal cycling at 2500F, ¦; Instead of applying the Seltzer et al pack treatment J
i directly to a chromized dispersion-strengthened nickel, it can be more effectively applied to a chromized dispersion-strengthened !
ilnickel that is first given an uninhibited aluminizing with a ¦ipick-up of 1.5 to 4 milligrams of aluminum per sauare centi-,meters, and the resulting aluminum-containing surface layer stripped off in accordance with U. S. Patent 3,622,391. The iresulting re-a~uminiz~ material shows no failure after 190 hours , of thermal cycling at 2200~F, and about one-seventh the weight ?
loss of the comparable product produced without the intervening ¦,aluminizing and stripping.
¦ The ingredients of the Seltzer et al aluminizing pack ! can be varied plus or minus 20/~ from the amounts given above without significantly detracting from its effectiveness. The ¦metallic ingredients of the pack do not have to be pre-alloyed, ¦but the pack should be given a break-in heat treatment prior to use.
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In connection with Example X, the cobalt and alum~num atom proportions can vary from about 0.4:1 to about 1:0.9 to obtain the advantages of th~t example: outside these ranges the case thicXnesscs produced are substantially smaller and not as desirable. Best results seem to be provided in the Co:Al range 0.8:1 to 1:0.9. The addition to the pack of about 0.1 atom chromium for every atom of aluminum increases the case thicknesses that are produced and increasing the chromium content to about 0.5 atom for every atom of aluminum further increases the thick-nesses. The addition of the chromium also reduces the amount of oxide inclusion otherwise found in the aluminized cases, but those inclusions are not particularly harmful even when in the amounts formed in the absence of chromium.
The foregoing results with the Co-Al pack compositions are obtained when these two metals are the only ingredients of the pack, as well as when they are diluted with alumina, kaolin or other inert diluent to the point that the diluent is 90% of the pack. Also the NH4Cl can be replaced with any other energizer such as ~H4Br, NH4I, NH4HF2, I2 and the like, and the coating temperature varied from 1100F to about 2200F. The atmosphere ~n the pack can be bathed with argon or other inert gas instead of nitrogen, or can be unbathed as by using a so-called glass sealed retort as described in U. 5. Patent 3,010,856 granted November 28, 1961.
Moreover these Co-Al packs produce very high quality diffusion coatings without requiring a preliminary break-in heat.
ey are accordlngly slmp r to prepare and use.

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1C~75980 A feature of the present invention is that the flake or leafing aluminum or other metal forms coatings in which the individual metal flakes overlap each other so that the coatings are of continuous character with fewer skips or holes. This improves the protective action of the coatings, Fven without such a flake metal overcoat the diffusion coatings from a Cb-Al coating pack either containing or free of chramium, are very effective to protect iron, stainless steel, nickel and brass as well as bronze members that are used to shape molten glass, DV
Minox bronze having the following composition is an example of such a metal that is well protected by such treatments:
. Copper 63.5 - 68,5%
Nickel 15.5 - 17,5%
Zinc 8 - 10 %
. Aluminum 6,5 - 8.5%
Iron 1.0% maximum Lead plus Tin 0,1% maximum This same alloy is also very well protected by pack aluminizing at 800 - 900F for 20 hours in a simple diffusion coating pack such as described in the above-mentioned U,S, Patents 3,764,373, or 3,785,854. Diffusion coating cases about 4 mils thick are thus obtained and make excellent protective coatings for incan-descent lamp glass-blowing molds rade from this alloy.

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While so-called conversion coatings make very effective tcp coatings over the metal-containing coatings of the present invention, a particularly desirable conversion coating for use at temperatures of about 450F or belcw, is of the chrcmate-fluoride-iron-cyanide type.
mis is illustrated by the following examples.
EXAMPLE XI

. _ _ Jet engine compressor blades of 410 stainless steel were .
- aluminized as described in Example I A, and were then conversion coated by dipping in a bath of 5.6 grams CrO3 1.3 grams NH4HF2 6 7 grams K3Fe (CN)6 1.2 grams H3EO3 diluted to one liter.
me bath was kept at 80F, and after one minute the dipped blades were withdrawn from the bath and rinsed in tap water. m e - resulting blades have improved lives when kept from heating up above 450F, as compared to the same blades without the conversion coating.
m e boric acid doe sn't add much to the foregoing bath and can be omitted without materially reducing its effectiveness. m e - ferricyanide and bifluoride can have different alkali metals as cations and the bifluoride can be replaced by fluoride wit~out - noticeable changes in results. Ih general the CrO3 content can range from about 0.01 to about 0 5 mols per liter, the fluoA de ion between about 0.005 and about 0.2 ls per liter and the .~

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- ferricyanid~ also betwcen about 0.005 and about 0.2 mols per liter. Best results with boric acid has the borate ion in a concentration from about 0.005 and about 0.1 mols per litcr.
The bath temperature can vary from about 40F to its boiling point at atmospheric pressure.
,The leafing aluminum or other leafing metal of the coatings of the present invention are generally applied in such small thicknesses that they do not significantly change the di-.: .. ..
'' , mensions of the substrate being coated. Even where dimensional accuracy is of very close tolerance, as in the roots and but-tresses of jet engine blades and vanes, such coatings can be applied over the entire substrate in the form of coatings 0.1 mil - thick or thinner. Close tolerances will generally first require masking of the substrate to keep the diffusion coating from de-, position inasmuch as diffusion coatings generally add about 0.3 ' mil to the dimensions of the material coated. Suitable maskingtechniques are described in Patent 3,801,357, but a preferred ' technique is exemplified as follows:
'' EXAMPLE XII
- ' A set of uncoated jet engine blades of IN-100 alloy .. I . . I
was cleaned and their roots immersed in a stirred'slurry of 100 g ' I . !
;~ ~ powdered masking mix in a solution of 3 g. pol~ethylmethacrylate) .; I , .
i resin in 100 g. chloroform. The masking mix was Ni3Al containing ' 2% Cr, this combination then being diluted with equal weight of alumina. The dipped blades were removed from the suspension and air-dried for five minutes. The resulting blades had their roots !
coated with a layer ranging from about 300 to 800 milligrams per ;. :
- square centimeter of masking powder and resin.
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The blades with thc dried coating were thcn dippcd for iia few seconds in a 5~O weight dispersion of powdercd nicXel in , ~ i j the same resin solution. After withdrawing the blades werc again air-dried, and both coatings weighed from aoout S00 to about ' 1200 milligrams per square centimeter. They were then packed in ! a prefired diffusion aluminizing pack having the following ,composition in parts by weight: ;
Al (minus 325 mesh) 10 i Cr (about 10 micron particle size) 40 Al 23 S
~H4C1 0.3 ,into which additional NH4Cl was blended to bring its concentration to the designated value. The pacXed assembly was heated in a retort as in Example I A to 1900F where it was kept for 5 hours.
Upon cooling down and opening the retort, the aluminizing pack could be sucked out to the point that the blades could then be individually pulled out from the pack. The blades carried a hard , i shell of the originally applied masking layers that appeared sintered in place and did not readily crumble. ~t was a simple matter to remove all the blades from the pack along with all the ~asking mixture, leaving the remainder of the pack reusable without further separation.
he hard shell of masking mix could be broken off with ¦, an easy hammer blow and the cleanly masked blades thus recovered t without damaging or even endangering them. Combined masking !. coatings which together weighed only about 300 to 2000 milligram~ , per sq,centimeter were satisfactory for this purpose. Any of the ,-other masking aluminide compositions of Patent 3,801,357 can be I uscd in the first masking layer in place of the Ni3Al to makc the - ! hard shcll of the prescnt invention.
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On the othcr hand when using gum tragacanth or bcntonite and only one masking layer, as described in Patent 3,801,357, the high temperatures generally cause such layer to crack so that good masking is not obtained unless the masking coating weighs about 5 or more grams per square centimeter. Moreover such cracked coatings also disintegrate readily upon removal of the workpieces from the pack, and crumbled pieces of the masking laycr wind up in the recovered pack. Such pieces must be arduously separated or the entire pack scrapped.
Other acrylic resins such as poly(ethylethacrylate), poly(methylacrylate), poly(butylacrylate), poly(acrylic acid), aad other thermoplastic resins such as carboxy methyl cellulose, cellulose nitrate, ethyl cellulose, and even polyethylene, poly-propylene, polystyrene and poly(vinyl chloride) can be used in place of the poly(ethylmethacrylate) but with results that are not as good. Readily volatilizable solvents such as those Doiling below 100C are preferred as solvents for the resin.
Obviously many modifications and variations of the pre-sent invention are possible in the light of the above teachings.
It i9, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as speclfically described.
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Claims (5)

1. THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
   PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
   l. In the combination of a corrodible stainless steel member having a diffusion-aluminized surface, the improvement according to which the protective coating is covered with a separate continuous adherent layer of aluminum in flake form.
2. 2. The combination of claim l in which the corrodible member is a jet engine intake component.
3. 3. The combination of claim 1 in which the separate layer is adhered with magnesium chromate and weighs not more than about 1 milligram per square centimeter.
4. 4. The combination of claim 3 in which the separate layer is adhered with a mixture of chromic acid, phosphoric acid and their magnesium salts.
5. 5. In a metal member having a surface coated with a cured mixture of finely-divided aluminum, phosphoric acid, chromic acid, and the magnesium salts of these acids, which mixture protects the surface against corrosion, the improvement according to which the metal surface is a diffusion-aluminized surface of AISI 410 stainless steel jet engine intake member, the finely-divided aluminum is aluminum leaf that forms a substantially continuous layer over said surface, and the cured coating weighs not more than about 1.5 milligrams per square centimeter.