CA1097454A - Coating of metal - Google Patents

Abstract

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. This divisional application is particularly directed to diffusion masking wherein aluminum diffusion coatings are kept from undesired locations by covering those locations with an Ni3Al-type masking layer containing thermoplastic resin over which is applied a powdered nickel masking layer containing thermo-plastic resin. On firing, this masking combination forms a strong shell that effectively masks without contaminating the surrounding coating pack.

Description

~ 7 45 4 ~ACKGROUND OF THE INVENTION

Thls invention relates to the coatin~ of metals to improve their use, particularly in resistin~ corrosion as well as attack by chemicals.

Among the objecis 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.

SUMMARY OF rrHE INVENrrION

The foregoin~ as well as aaditional objects of the present invention will be more full~ understood from the following description of several of its exempIi~
fications.

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~74541 Accordin~ to the pr~sent invention, flake forms of pro-';tective metals form particularly 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.
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' 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 pxotection thus obtainable from layers of less than about 1 or 1.5 milligrams per square centimeter, is greatly ., .~improve* if the aluminum coating is effeotively continuous over '' 'the surace being protected, a result that is obtained ~.~hen leafing-type aluminum particles are applied in amounts that permit the 'individual aluminu~ flakes to partially overlap each other over the entire surace'being protected. It is also helpful, as sug-! gested in Patent 3,787,305, to subject the aluminum-coated ferrous .
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~member to a temperature that causes a.t least a little bit of the aluminum to di~fuse into the ferrous sur~ace.
; I,eafin~ type aluminum particles can be made as described ~in U.S. Patent ~,312,088, and are generally characterized by the presence o~ stearic acid or aluminum stearate or the liXe as a . very thin coating on the surface of each aluminum particle, a con-~dition which maXes it extremely difficult to disperse such aluminum particles in water A substantial amount of wetting agent will effect a s~ita~le dispersion, although it is easier to effect such `;dispersions by also adding diethylene glycol or triethylene glycol ' or more highly polymeric ethyl~ne glycols having a molecular weigh up to about 9000~ as des~ribed in U.SO Patent 3,318,716 grant~d j, i~ `3 ` ' `` , ' ' ~, .... ..... .
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~ay 9, 1967, or by adding glycerine. ~s shown in the last-mcntioned 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 l-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 almos~ any desired aluminum content. Thus a diluted dispersion containing 5%
aluminum, 6% hexa-ethylene glycol and 0.7% para-n-octyl phenyl !l , . ;
,ethex of decaethylene glycol, is readily sprayed onto a stator ~lring 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 I
~air oven until its temperature reaches 800F. The heating first tcauses the glycol and wetting agent to be volatilized off leaving t ia very adherent continuous and shiny coating that resembles polish-.ed aluminum and signiicantly adds to the corrosion resistance of ~the ~tator ring even if the heating temperature goes no higher 'than 600F. The increase in corrosion resistance becomes more ,'significant when the heating carries the coating to temperatures `~f 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 th~ coa~ed stator in an atmosph~re of gaseous aluminuii chloride while it is at temp~ratures above about 700F~ ~
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a7 gL54 The aluminum chloride atmosphere is conveniently provided by a pack treatment as described in E~ample 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 energi~er 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 centimeter coating, 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 energ~ to be driven off. Some fluorides can cause significant attack of the aluminum or of the substrate, and are best avoided bm ~ C

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or used in a high specd operation in whlch the tim~ they contact these m~tals is restricted to minimize th delcterious e~fects of such attack.
The adhesion of 0.1 to 4 milligrams per square c~ntimeter ;layer of a ~laked metal such as aluminum is also improved by incorporating in the layer chromic acid, or a compound 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 2% by weight of the aluminum gives a sharp increase in adhe~
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sion upon heating of a dried 4 milligram per square centimeter layer of such composition on plain carbon steel to 700F for 5 minutes. A 5% addition of the dic~lromate to the dispersion renders such a heat-treated coating complet:ely resistant to wiping off.
Similar results are obtained when t:he heating is at 600 to lG00F
for as little as 20 seconds, and as much as 20 to 30 minutes, although not much is gained by prolonging the heating beyond about 1 mi~ute. The ammonium dichromate content can be as high as 8~o of the weight of the metal, but a~ove this level the ~orrosion ~esistance tends to drop off.
The dichromates are used in amounts corresponding to the fo~egoing amounts of ammonium dichromate, the chromates in amounts a~out 1/5 greater, and chromic acid in amounts about l/S 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 applied. When such coating is covered by any other corrosion-resisting top coatlng, such as thos~
dcscrib~d in th~ working examples, e~ceptionally good corrosion resistance is impartcd, even to plain carbon steels.

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10~7454 ,I The lea~inc3-type of aluminum particles or other protec-¦;tive m~tal used in thc above connection are preferably from about il50 to abou~ 250 microns in maximum size although other sizes can !~ also be used.
~ Non-ionic wetting agents are preferred for dispersing 'Ithe aluminum inasmuch as such wetting agents are more readily I driven off by high temperatures. However other types of wetting agents, including those that are not driven o~f 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 significant to the corrosion l resistance, and as little as 0.1 milligram per square centimeter ' ¦lis helpful although at least about 0.3 milligram per square ¦~centimeter, and better still 1 to 2 milligrams per square centi-¦,meter is preferred~ ¦
j, Inasmuch as ferrous metals, such as plain carbon steel, ~ast iron, low alloy steels, stainless steels and other chromium- ¦
¦'containing steels begin to oxidizè on their surface at the temperatures used for the heating of the flaXe layer, it is help- i ~ful but not essential to conduct that heating in a non-oxidizlng atmosphere. When the energizer used in such heating step is , incorporated in the layer of flaked metal, the volatilization - of the energizer along with the volatilization of any suspending agent present in the dispersion ~rom 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 ~urther oxidation-pr2venting precautions~ However, if !
', desired, the heating can be effected in a closed chamb~r as by batoh heating an opened coil of coated m~tal sheet or wir~ plac~d t , ~

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in the closed chamber which then has its atmosphere flushed out with ar~on, hydrogen or nitrogen, or by continuously passing a continuous strip or wire of the coated metal into and out of th~
chamber through close-fitting slots or holes in the chamber walls, while continuously introducing into the chamber a small stream of protective gas that makes up for losses of gas through the slots or holes.
When the metal flakes to be applied are al~uminum, the ~benefits of a non-oxidizing atmosphere are not significant and an atmosphere of amoient air is ent~rely 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 re~uire frequent agitation and then coatings applied by spraying from such dispersions tend to be of non-uniform thickness The application of a leafing-type aluminum coating particularly improves thè 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 above is accompanied by a little diffusion of the alu~inum into the substrate, and in addition the aluminum coatings thus formed are highly conductive to electricity as well as highly protectiveO

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The leaing aluminum coating also improves the corrosion resistance of a coating obtained from mi~tures of aluminum particles with phosphoric acid, chromic acid and ma~nesium, 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, partLcularly 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 giv~s a produc~
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 volatili7ed away, and no significant reduction of the hexavalent chromium to trivalent condition seems to taXe place~

7~54 . . .

The foregoing improv~ments in corrosion rcsist~nce ~ncl in appc~rance ~re also obtaincd ~hen the las~-mentioned co~tincJ is covered by a similar coating, even one that does not contain ; metallic aluminum. Such top coatings are described in U.S.
~at~nts No;~ 3,948,687 ~nd No. 3,0~18,689~

- . . -~..=. ~~~~-.--: ---. ---- However multiple coating layers each of which contains metallic aluminum are very ef~ec~i~e, particularly when each layer weighs between 0.1 and 0~5 milllgrams per square centimeter.
As shown in the aorementioned applications, the pro~
p~rtions oE the ingr~dients in the chromic acid-phosphoric acid-salt coating mi~ture can range as follows:

Chromat~ ion 0.2 to 1, pr~ferably 0.~ to 0.8 mols per liter - Phosphate ion 0.7 to 4, pre~erably 1.5 to 3.5 . mols per liter Magnesi~m ion 0.4 to 1.7, pre~erably 0.9 to 1.4 mols per liter Polytetrafluoroethylene 2 to 14, preferably 3 to 10 ~; resin grams per liter - The magnesium ion can be replacea by any of the other ions re-ferr~d to above, in the same concentrations.

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~74~;4 Inste~d o directly applying such an ov~rlying coa~ing whether or not it contains Metal]ic aluminum, it can be applied after an intervening coating oE colloidal ~lumina or the like weighing about 0.1 to about 1 milligram per square centimeter, also as described in the aforementioneu ~.S. Patents No. 3,948,687c~d No. 3,048,689 with increases in corrosion resistance as describcd in ~hose patents.~ With or without such an inter-vening coat, the final cured article has a golden sheen, contri-buted by the presence of chromate, that is extremely attractive and quite adherent. The presence of polytetrafluoroethylene ,sparticles in the phosphoric acid-chromic acid-salt mixtures of either or both o~ such laycrs is also helpful ¦ ~` ~-` ¦ ~ = ---------~--~-~~ ¦ and does not detract from the golden appearance. Such presence in a top coating makes that coating very smooth and slippery withou-t detracting significantly from the coating hardness~ These coating combinations with or witho~t the interv~ning coating of colloidal particles are most effecti~e in increasing the corrosion resistance of chromium-free i- and chromium-containing ferrous substrates that have aluminum-diffused surfaces.
Indeed they also have this desirable effect on bulX
- aluminum such as aluminum sheets, foil and bars, as well as on titanium. On aluminum substrates such coatings adhere e~ception-ally well and withstand severe deformation of the surfaces to w~ich they are appli~d. However the gold color contributed by the foregoing top coatings that are fre~ of metallic aluminum, is not provided when mctallic aluminum is included in those top coating _ / ~

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formulations. The intervening coatings of colloidal alumina and the like are not heavy enough to obscure the metallic appearance of the substrate and accordingly do not adversely affect the appearance. On the other hand those intervening layers improve the wettability o~ the aluminum-containing surface by the top coating. Some alumina dispersions are acid and tend to attack a layer o~ 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 acid added to the dis-persion to protect the aluminum.
The following are exam~les of the production of gold-colored highly attractive and very corrosion-resistant steel and aluminum products.
EXAMPLE ~
Ac In~o each of fo~r plain carbon steel retort cups 2 ~eet wide and 14 inches high is poured a powder pacX consisting of 2~fi aluminum by weight and ~0 alumina~ both minus 325 mesh and ~nifor~ly 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 bla~es is thcn covered with more powder till the powder is about 1/2 inch above the vane top~ and another layer o blades i~ then laid down and the layering repeated until the entire packing is 12-1/2 inches deep in cach retort. ~ore p~ck powder is then adcled to eac}l retort to assure there is Abo~t 1 inch of powder ove thc tops of the topmost bl~dos,following which thcre '' : , 1 ~0~745~

is sprinklcd over each ~ very thin stratum of crystalline AlC13 6Ei20 in an amount weighing 0.6' of thc total powder weight.
The retorts are then filled to thcir tops with additional pacX
powder, and they are stacked one above the other on the floor of a gas-fired bell furnace. The stac~ing 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 lO to 15 standard cubic feet per hour. ;
The heating of the furnac:e is started at a rate of about 1~5F per minute, as measured by thermocouples in each retort and connectea 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 lO standard cubic feet per hour.
As the heating-up continues, the temperatures indicated by the thermocoupl~s increase uniformly and gradually and chemical vapors begin to appear in the burni~g outlet gas. 8y 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 heating is set to hold.

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After 16 hours at 875F the furnace heatin~ ~s tcr-minated and the furnace permitted to cool un~il the thermocouplc temperatur~...s reach 300F. The atmosphere in the furnace is then purged by switching the inflow gas eo argon or nitro~en and thc ~urnace 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 particles 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 surace there is sprayed with an air-propelled spray, a uniform ~ery thin layer rom an aqueous dispersion of 3.5% Cro3;
. ` 2.4% ~lgO;
:; - 11% H3PO4;
~ . 5.7% leafing aluminum;
:: 6,8% polyethylene glycol having an average mole-~ . cular weight of 300 and in which the glycols-:~i . range from pentamethylene glycol through ; heptamethylene glycol; and 0,8% para-isonffnyl phenylether o~ dodecaethylene .. . glycol;
: all percentages being by weight ~ The sprayed blades are then air d ff ed and baked at 700~F in an air oven for 30 minutes to give a coating weight from this spray o~ 0,7 milli~ram per square centimeter of ferrous surface.

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1~7~;4 C. The blades coated in steps ~ and B have their coated surfaces given a spray coating of colloidal alumi~a dis-perse~ in a 20 concentration by weight in water to which a little HCl is added to bring the 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 centimeter.
D. The blades with the air-dried coatings are then ',given a top spray coating from an aqueous dispersion of ~'~ 5.8% CrO3;
4% MgO;
18.3% H3PO~; and t ` 0.S% polytetrafluoroethylene particles about 1 , micron in sizé;
this spray being such that upon air drying in an o~en and then baXing at 700F for 30 minutes in an air oven, the final coating weighs 0.5 milligram per square centimeter.
EXAMPLE II
~ he coating steps of Example I are repeated but this time the workpieces are SAE 1010 steel, the diffusion pack peak temperature is 800F, the aluminum picXed up in the diffusion step is 7-1/2 milligrams per square centimeter, the baking in steps 3 and D is at 900F, and the coat weight applied in step B
is 0.9 milligram per square centimeter.

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7~S4 EXAMPL~A III
Discs of low alloy st~el containing 0.5/0 chromium and 0.02% carbon as the only significant alloy ingredients, which discs are used to hold j~t engine compressor blades, are given the coating treatment of Example I, this time the diffusion coating pac~ being held at a peak temperature of 900F, the aluminum picked up in the diffusion being about 8 milliqrams 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 s~gnificant amount 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 Exarnple 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.
EXP*lPLE V
Plates of Type 3S aluminum were coated by the sequence of steps B, C and D o Example I, and the coated plates had a gold sheen of very attractive appearanceO
EXP~IPLE VI
Titanium shcets coatcd by thc stcps 3, C and D of Example I werc also colored with a goldcn shcen.

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~L(;39745 ~xAMPI.r~ VIl Coupons of ~10 s-tainless steel are dipped in the coatin~ mixture of ~xample I B, modified by the addition of , ~ (~) 0.4% fine Teflon particles from an aqueous Teflon ~ispersion.
The coupons were removed from the coating mixture and heated to volatilize off the wetting a~ents, taking care to remove t~e aqueous material running down to the lower edges of the coupons and to thus l;eep the coating more uniform in thickness. After the volatilizatiorl is completed the coupons are quenched in o water and have a very smooth and slippery surface.
Essentially the same results are obtained in Example I
as well as in Examples II and III when anhydrous aluminum ch]oride, bromide or iodide, or hydrated aluminum bromide or iodide is used in place of the hydrated chloride energizer, with the aluminum content of the pack ranging from 100~ down to 2%.
For aluminum diffusion effec-ted below 900 F it is preferre~ that at least ~% aluminum be in -the pack. As disclosed in U.S. Patent No. 3,936,539 issued February 3, 1976 (A.L. Baldi), a conYe~ient amount of hydrated energizer is from 3 to 6 grams for a 6-~ pound pack when the pack is first broken in as well as when the broken-in pack is subsequently used for coatin~.
Instead of using aluminum of relatively pure composition such aluminum can be an alloy containin~ significant quantities of beneficial ingredients such as silicon. A content of 12%
silicon will, by way of example, improve the resistance to hi~h temperature oxidation of ferrous metals sub~ected to diffusion coating by such an alloy.

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The very effective protection imparted to ferrous metals containing less than 1% chromium, such as plain carbon and low alloy steels does not require more than a single layer of the chromic acid-phosphoric acid-salt-aluminum mixture, when preceded by an aluminum diffusion treatment. This is illustrated by the following examples.
EXAMPLE VIII
Panels of SAE lOlO steel are given the diffusion coating treatment of step A in Example I, but using anhydrous AlCl3 energizer. The diffusion coating weighed 7 milligrams per square centimeter, and it was then coated by spraying on an aqueous dispersion containing the chromic-acid-phosphoric acid-salt aluminum mix in the following proportLons:
1.25 moles per liter P04 -0.68 moles per liter Mg~
0.38 moles per liter CrO4-64.5 grams per liter Aluminum 77.0 grams per liter of the polyethylene glycol of Example II r 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 when the dried and oven-heated coating is covered by another layer preferably just like the sprayed-on layer but not containing the metallic aluminum. A porous alumina hm~ -)e -18-. .
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1~7~54 barrler between these two layers gives still further improvement.
Similar results are obtained when aluminu~ ions replace the magnesium ions, as weil as when the baking is at 700F and the baked coating''lightly blasted with very fine glass microspheres about 25 microns in diameter impelled by air blasted at a pressure of 5 pounds per square inch. ~lso the use of hydrated energizer during the diffusion coating produces t~e same Fesults as the use of anhydrous energi'zer.
In the aluminum-containing coating mixtures the concentration of the leafing-type aluminum particles can range from ,about 30 to about 150 grams per liter of mixture, and the remaining ingredients can have the concentation ranges given supra.
The water in these compositions can also be replaced in ~hole or in part by the polyethylene glycols or by any other inert liquid in which the ingredients can be dispersed and sprayed. For combinations 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. ~lowever even such a layer containing the leafing type aluminum of the present inventlon ~' and weigh~ng only 1 milligram per square centimeter or as much' -as t~o milligrams per square centimeter imparts excellent corrosin resistance to plain carbon and low alloy steels as well as other ferrous metals containing less than 1% chromium, when applied over an aluminum ~iffusion coating on the metal.
This corrosion resistance is even further increased when the layer containing the leafing type aluminum has its electrical conductivity increased as by heating to 900F or higher; or by lightly blasting it with fine non-corroding particles such as glass or ground walnut shells or the li~e, or by heating it in the presence of ammonium clloride or other energizes a~
described above~

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Thus panels of steel containing 0.05~ carbon ald 0.3%
titaniurn as th~ only material alloying metal, show unusually , high resistance to salt spray correion when covered by an alum-;inum diffusion coat having an aluminum pick-up of 6.5 milligrams ;per square centimeter, over which is applied the phosphoric acid-chromic acid-salt-aluminum coatin~ of Example IX but baked a~
' 700~F and then given a li~ht blastinq with fine glass micro-spheres in a 5 psi air stream, the blasting removing about 0.1 milligram of the baXed coating pe~ square centimeter.
!Although the thus protected panels show splendid cor-rosion resistance, their eoated surfaces tend ~o turn white or ,~ ' . .
grey af-ter long exposùre to salt spray, indicatinc3 that the aluminum in the top layer is being attacked very 510wly. This wh~tening ox greying can proeeed for a eonsiderable time before the steel is attacked, ev~n w}iere the coatin~ is scratched through to the base metal. However the whitening or greying ean be ~`greatly slowed by covering the phosphoric acid-chromic acid~
salt aluminum layer with a top coating such as the combination o~ an air-dxied colloidal alumina layer weighing 0.1 to 1 milli~
gram per square centimeter and an overlying baXed phosphoric acid-chromic acid-salt-~ellon layer, weighing 0.2 to 1 milligram per square centimeter. It is preferred that the combination of layers on the aluminum diffused surace weigh not more than about

2 milligrams per square centime-terO

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As pointed out a~ove, the coa~ings containing the leaing type of aluminum in accordance with the present inv,ntion are electrically conductive to an appreciable degree when thi_y , have been subjected to baking of at least 900F or when thcy have been burnished as by means of the fin~ 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 , ~atents 3,248,251 and 3,787,305~and ~hus 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 700E` 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 ,ii - ' ' to 90~F or higher, without energizers.
Although Patent 3,248,251 suggests coatings as thin as 0.5 mil, the commercial coating formulation d~rived from it is ;markèted with instructions to apply it in thicXnesses of at.
least 1.5 mil. Those instructions also suggest that those coatings be glass blasted and/or baked to 1000~F or higher for best results. However superior results with the 3,248,251 ~coatings are obtained wh n there iq applied over such coatings a ;very thin layer of flake aluminum~ Thus a 410 stainless steel ` . ,, ~Q~7~LS~ .

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jet engine compr~ssor blade coated with a 2 mil thick layer of the oven-dried (550F) commercial mixture containing granular ;aluminum and corresponding to E~ample 2 oi~ Patent 3,248,251, gave much better protection after there waq sprayed ovex the oven-dried coating a 0.4 milligram per square centimeter layer of flake aluminum from a 2% suspension in an aqueous solution ,of only 2.4% hepta-ethylene glycol and 1/3% p-nonyl-phenoxy ,octadecaethoxy ethanol, and the thus-coated blade again DaXed dry at 550 or 600F. The degree of improvement thus contributed ~by the additional flaXing aluminum layer is approximately the same as that ohtained by baking at 600F without that additional l layer and then glass-blasting the first coating layer.
i 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 e~uipment. In such combinations, the aluminum is con~eniently applied in the form of flakes, as , indica~ed above, and the adhesion of the flakes is improved by '`the dif~sion type heat treatment with or without an energizer, r or by incorporating an ammonium chromate with the aluminum ~laXes ~nd heating to produce the bonding acton, which is `! also describe~ above. ¦
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., Other protcctive metals in flake or leaEing form can be used in place of or in addition to the flake aluminum to also increase the resistance of corrodible metals to o~idation and the like, particularly at high tempera~ures. Examples of such protective leaing metals are nickel, tin, stainless steels of all kinds, and silver. U.S. Patent 3,709,439 describes the making of such ~`lakes; Chromium-bearin~ or austenitic stainless steels are especially e~fective. Indeed flake type 304 or type 316 stainless steel when mixed with from about half to about twice its weight o~ flake aluminum, gives better protection ~., . ~ .
against alkaline attack than flake aluminum alone- Alloys of nickeI 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 1aked metals can be applied as coatin~, 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 thP dispersions containing flaked metal, with or without the chromate, phosphate and salts, can be applied by dippin~, they are preferably applied by spraying or roller coatin~, In general such flake-metal-containing coatings should 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 the 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 coated plain carbon ;

Q'7~54 steel ~s wcll as steel coated with a mi~ure of ~luminum and tin is conveniently made ln this way, as by spraying she~t steel unwinding ~rom a coil, with a dispersion of the flaked metal, then passing the thus coated metal sheet through gas flames to heat it to 700-750F for 30 seconds, after which the sheet is eooled and recoiled.
Dispersions of flake ~etal, such as aluminum, also containing polytetrafluoroethylene, deposit coatings ~hat are not only very decorative by reason of the metallic sheen, but they are also quite hydrophobic and smooth and especially low in frictional resistance to sliding with respect to other objects.
Such coatings 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 su~icient to drive off the dispersing and suspending agents. Top coatings o the oregoing chromate-phosphatle-salt mixtures or other con-version coatings can be applied over such a metal-tetrafluoro- ;
ethylene initial coating, For best resistance to corrosion the flon content should be held down to not over about 1% of the final coating, about 0~5~O being p~rtieularly èffective. Where the :
~eflon~containing coating is heated to about 900F or higher the ~eflo ~ content can before heating be as high as about 2%
without detracting too much from ehe corrosion resistance.

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The foregoing top coatings of chromic acid-phosphoric acid-salt formulations are also helpful when ~pplied over aluminum diffusion coatings that are produced by the inhibited diffusion processes described in U.S, Patents 3~257,230,

3,690,93~ and 3,867,1~4. Those 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 chromium, or of aluminum, chromium 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 mix-ture in finely divided form. To this end the magnesothermic reduction of chromium compounds such as Cr20~ as described in French Patent 1,123,326 and its Addition Patent 70,936, can be . modified by combining an appropriate quantity of alumina with the chromium compound, and such combination mixed and subjected to the magnesothermic reduction as described in those patents. The 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 `~ bm~

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removed by trcatment with an e,~Ce5S o dilute nitric acid having a spccific gravity of about 1.12 to about 1.26. Such acid will not attack chromium-aluminum alloys having as little as 16~,~
chromium by weight, but will readily dissolve metallic magnesium as well as magnesium oxide. Czushing 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 belng coated or to the other ingredients of the coating pack. Where the hot magnesothermic reaction mixture has its `
.
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.
Magnes~hermic reduction can also be used in the same way to directly produce chromium-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 chromium, iron, molyb~enum or tungsten oxides or other compounds ~ ~6-,. , ! 3LC~9745~
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of these metal~ in the presence of aluminum and/or silicon in elcmcntal form. During such reduction the aluminum and/or silicon alloys with the m~tallic chromium, iron, moly~denum and tungsten ' as it is form~d.
-; The following is an example of the dual reduction technique:
! EX~lPLE IX
!
1392 grams of magnesium metal were placed in a plain car~on 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 17~0F where it was held for 25 mïnutes 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 A1203 and S00 grams powdered Cr203 poured over the solidified magnesium. The combination was again heated under ar~on, this time to 1825F for 8 hours, and cooled.
A powdery reaction product remained in the retort. It was re~oved from the retort, treated with excess 2 N HNO3 until there was no further reaction evident, and then washed to neutra-lity with water. The resulting material was a chromium-aluminum in~ermetallic in the form of particles averaging about 1 micron in size. ~t analyzed 81. 2æ chromium and 16.6% aluminum by weight, its yield being 91~. ~hen mixed with alumina and ammonium chloride i~ ga~e very good aluminum diffusion coatings in the process of Canadian Pa~ent 806,618, in place of the mixture of chromium and aluminum ~here suggested.

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7~54 Simil~r r~sults are obt~ined whcn the prellminary melting of the magnesium is not effccted, and where the i.ntcr-metallic is used for diffusion coating steels at lower temper~-tures. Other interm~tallics similarly made and used have the following analyses:

a) 45.5% Al 54.5% Cr b) 44.1% Cr 47.7% Fe 8.5% Al c) 74.5% Cr ; 7.~ A1 , 8~5% Si , All.oy c) contains some unreduced o~i.de, but it still. is ~ery effective for use in the inhibited cli.~usion process~
~ nother type o aluminum clif~usion coatir.g over whi.ch ' the top coa~ings describ~d above can be applied, is a pack dif-' . fusion alumini~ing in which the aluminlzing is inhibited by . .
cobalt. This diffusion coating is illustrat~d by,the following example:
EXAMPLE X
' A pack was made up in parts by weight of cobalt 30 ', ' aluminum 14 alumina(calcined~ 56 NH4Cl , 1/2 Each o~ the foregoing ingredients was a 250 to 360 mesh powder.
The mi~ture was thoroughly blended and then pacXed in a plain carbon steel xetort alo'ng with nickel blow tips used for blowing incandcscent light bulbs.-' .7~54 l'hc rc~tor~ th~ls loadecl was looscly covcred and a lar~cr rctort lowcrcd ovcr it ~s i11-1s~ratc~cl in U,S. Patcn~ 3,7G4,371. Usin~ a hyclrocJen-bathed atmos~herc ~etwccn thc rctorts, as also dcscribcd in 3,7G4~371, th~ pac~cd matcrial was hcatcd to 1975F whcrc it was maintaincd for 20 hours. ~ftcr cooling thc trcated workpicccs and lightly glass-blasting thcm, all showcd an aluminizcd case from about 4 to about 6 mils thick.
. Whcn the same trcatmcnt is applicd to U-700 nickel base blades for the hot scction of a jet engine, somewhat thinn~r dif-~usion 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 show excep-tional resistance to oxidation and sulfidation. In general the resistance to these ef~ects of the U-700 alloy bladès is a littlc better than the resistance of such blades aluminized in a chromium-inhibited dif~usion coating pacX. 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 sup~ralloys such as B-1900,Renè 62, M~R-~l 200 do show this improved resistance when so coated.
Nickel can also ~e used for inhibiting aluminum diffusion as for exampl~ into chromized dispersion-strengthened nickel in the manncr dcscribcd in Patcnt 3,785,854, and will then make a product of outsta~ding oxidation resistancc, particularly with a littlc chromium also prcscnt in thc pack. A vcry suitable alumin-um pac~ in which a largc amount of nicXcl and a small amount of ehromium lowcrs the diffusion ~fcctivcncss i5 discloscd by ~. S.
Scltzer, B. ~. Wilcox an~ J. StrincJcr in Mctallurgical Transactions S~ptcmbc~ 1`972, p~CJCS 23~ 401. Thc Sclt~.cr ct al pack (G00 g.
alumina, ~2 ~. Ni, 17 ~ l, 10.5 ~, Cr, ~ ~.NaCl an~l 6 ~. urca) . . .
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7~s4 .
1 i ¦iat 2210F for 32 hours in a hydrogen or argon-washed atmosphere l deposits on chromized dispersion-strengthened nickel an aluminized --t'layer having an aluminum pick-up of approximately 4 milligrams per' ' square centimeter, a surface aluminum content of only about 4 to , - , a~out 6%, and a case depth of about 4 mils. This product with-.. stands about l90 hours of thermal cycling at 2500F.
¦i Instead of applying the Seltzer et al pack tleatment l directly to a chromized dispersion-strengthened nickel, it can ¦ be more effectively applied to .~ chromized dispersion-strengthened ¦,nickel that is first given an uninhibited aluminizing wi~h a lipick-up of 1.5 to 4 milligrams of aluminum per sauare centi~
j,meters, and the resulting aluminum-containing surface layer i'stripped off in accordance with U. S. Patent 3,622,391. The jiresulting re-aluminiz~ material shows no failure after 190 hours.
.~,of thermal cycling at 2200F, and about one-seventh the weight I! loss of the comparable product produced without the intervening !~ aluminizing and stripping.
I, The ingredients of the Seltzer et al aluminizing pack , !i can be varied plus or minus 20% from the amounts given above ilwithvut significantly detracting from its effectiveness. The jmetallic ingredients of the pack do not have to be pre-alloyed~
but the pack should be given a break-in heat treat~ent prior to .

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In connection with ~xample X, the cobalt and aluminumatorn proportions c~n vary ~rom about 0.4:1 to about 1:0.9 to obtain the adv~ntages of that example; outside these ranges the case thickness~s pxoduced 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 pacX of about 0.1 atom chromium for e~ery atom of aluminum increases the case thicknesses that are produc~d 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 2' ~ .
oxide inclusion otherwise founa 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 pac~, as well as when they are diluted with alumina, kaolin or other inert dilu~nt to the point that the diluent is 90% of the pack. Also the NH4Cl can be replaced with any other energiæer such as ~H4Br, NH4I, NH4HF2, I2 and the like, and the coating temperature varied ~rom 1100F to about 2200F. The atmosphere in 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 retor~ as described in U. S. Patent 3,010,856 granted November 28, 1961.
Moreo~er these Co-Al packs produce very high quality diffusion coatings without requiring a preliminary break-in heat.
They are accordingly simpler to prepare and use.
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~ fe~tur~ o~ the yrcs~nt invention is that ~he fl~ke or leafing aluminum or other metal forms co~tings in which the individual metal flakes overl~p each o~hcr so that the coatings are of continuous character with fewer skips or holes. This improves the protective action of the coatings. Even witho~t s~ch a flake metal overcoat the diffusion coatings from a Co-Al coating pacX either containing or free of chromi.um, are very e~ective to protect iron, stainless steel, nickel and brass as well as bronze members that are used to shape molten glass.
1''-__'''''-''''_`_7_ '``'' ' , . . '~-''`';'~- '' '- -'-'~ DV Mino~ bronze having the ,. . .
f following composition is an example of such a metal that is well protected by such treatments:
Co~per 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 ~his same alloy is also very well protected by pacX alumi.nizing . .
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 th.us obtained and make excellent protective coatings for incan~
d~scent lamp glass-blowing molds made from this alloy.

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~ n~iie so-callccl conve~sion co~tings m~ke very e~ective top coa~ings over the metal-containing coatin~s of the present ; inve~;tion, - a particularly desirablc . . .
conversion coating for use at t~mperatur~s of about 450F or below, is of the chromate~fluoride-iron--cyanide type. ~his is illustrated by ~he following e~amples.
E~PLE XI
Jet el~gine compressor blades of 410 stainless steel were aluminized as described in Example I A, and were then con-version coated by dipping in a bath of 5.6 grams 3 1.3 grams N~I4~2 6~7 grams K3Fe (CN)6 1.2 grams H3B03 diluted to one Liter.
'he bath was Xept at 80F, and after one minute the clipped blades were wi~hdrawn from th~ bath and rinsed in tap water. 'rhe resulting blades have improved lives when Xept from heating up above 450F, as compared to the same blades without the conversion coating.
The ~oric acld doesn't add much to the foregoing bath and can be omitted ~ithout materially reducing its efectiveness.
The ferricyanide and bifluoride can have di~erent alXali metals as cations and the bifluoride can be replaced by fluoride without noticeable c~langes in results. In general the CrO3 content can range from about 0.01 to a~out 0.5 mols per liter, the fluoride ion between about 0.005 and about 0.2 mols per liter and th~

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1~79 f~rricyanide al50 bctween about 0.005 and about 0.2 mols p~r liter. Best results with boric acid has th~ borate ion in a concentration fxom about 0.005 and about 0.1 mols per litcr.
The ~ath tt~mperature can vary from about 40F to its ~oiling 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 ~tut-',- 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 coat.ings generally add about 0.3 '` mîl to the dimensions of the material coated. Suita~.le masking ~ !
technigues are described in Patent 3,801,357, but a preferred technique is ~empllfied as follows:
~' EXAMPLE XII
set of uncoated jet engine blades of IN-100 alloy was c~eaned and their roots immersed in a stirred'slurry of 100 g ~: powdered masking mix in a solution of 3 g. pol~ethylmethacrylate) i resin in 100 g. chloroform. The mas~ing mix wàs ~i3Al containing Z% Cr, this combination then being diluted with equal weight of alumina. The dipped blades were removed from the suspension and air-dried or five minu~esl ~he resulting blades had their roots c~ated with a layer rant-~ing from a~out 300 to 800 milligr~ms Fter g~u~re ce~imet,~r of m~sking powder and resin.

1, !i . , 7~5~L
I, Thc bla~cs with thc dried coating wer~ then dipped for ¦ia few seconds in a 50~O wci~ht dispexsion o powdercd nicX~l in the same xesin solution. After withdrawing the blades were again 1 air-driecl, and ~oth coatings weighed f rom about 500 to about ',~1200 milligrams per square centimcter. They were then packed in I a prefired diffusion aluminizing pacX having the follos~ing ,composition in parts by weight: -I' Al (minus 325 mesh) 10 i Cr (about 10 micxon particle size) 40 l~ 50 Cl 0.3 ,, into which additional N~4Cl was blended to bring its concentration ! to the designated value. The packed assem~ly was heated in a 1 retort as in Example I A to 1900F where it was kept for 5 hours.
l~ 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 car~ied a hard , shell of the originally applied masking layers that appeared ~7 1, sintered in place and did not readily crumble. It was a simple 'Ij matter to remove all the blades from the pack along with all the ¦ masking mixture, leaving the remainder of the pack reusable , without further separation.
The hard shell of masking mix could be broken of with - 1~ an easy hammer blo~ and the cleanly masked blades thus recovered without damaging or even endangering them. Combined masXing coatings which together weighed only about 300 to 2000 milligrams pex sq, ~ timeter were satisfactory for this purpose. Any of th~
,other masking aluminide compositions of Patent 3,801,3S7 can be i used in the first masking layer in place of the Ni3Al to make the I hard shell of the present inv~ntion.
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On t~e other hand whcn using ~um tra~acanth or bentoniteand only one masking la~er, as described in Paten~ 3,801,357, thc high temperatures generally cause such layer to cracX so ~hat good masXing is not obtained unless ehe masking coating weighs about 5 or more ~rams per square centimeter. Moreover such cracXed coatings also disintegrate readily upon removal of the -workpieces from the pacX, and crumbled pieces of the masking layer wind up in the recovered pack. Such pieces must be arduously separated or the entire pack scrapped. -Other acrylic resins such as poly(ethylethacr~late),poly(methylacrylate), poly(butylacrylate), poly(acrylic acid), and other thermoplastic resins s~ch as carbox~ methyl cellulose~
~ellulose nitrate, ethyl cellulose, and even polyethylene, poly-propylene, polystyrene and poly(viny] chloride) can be used in place o the poly(ethylmethacrylate) but with results that are not as good. Readily volatilizable solvents such as those boilîng below 100C are preerred as solvents for the resin.
Obviously many modifications and variations of the pre-sent invention are possible in the light of the above teachinys.
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It is, therefore, to be understood that within the SCQ~e of the appended claims the invention may be practiced otherwise than as specifically described.
This application is a division of copending Canadian application Serial No. 261,461 filed September 17, 1976.

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Claims (15)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A composition for use in the diffusion coating of a metal workpiece to limit the amount of workpiece surface coated, which composition consists essentially of a slurry of powdered diffusion-masking material in a solution in a chloroform solvent of an organic binder that is driven off at diffusion coating temperatures.

2. The composition of claim 1 wherein said diffusion-masking material is nickel.

3. The composition of claim 1 wherein said diffusion-masking material is a nickel aluminide-chromium mixture.

4. The composition according to claim 5, additionally containing an inert refractory diluent.

5. The composition according to claim 1, 2 or 3, characterized in that the binder is an acrylic resin.

6. A method of chemical vapor deposition diffusion coating a metal on a portion of the surface of a metal workpiece by first applying to that surface a localized chemical-vapor-deposition-diffusion-limiting layer and then subjecting the work-piece with that layer to the vapor deposition diffusion coating, characterized in that a slurry essentially of nickel powder and a heat-fugitive binder in a readily evaporated solvent is applied over the limiting layer and the solvent is permitted to evaporate before commencing the chemical vapor deposition diffusion coating and in that the diffusing metal is capable of forming a brittle sheath with the nickel.

7. A method as claimed in claim 6, wherein the organic binder is an acrylic resin.

8. A method as claimed in claim 6, wherein the binder is dissolved in the solvent.

9. A method as claimed in claim 8, wherein the solvent is an organic solvent having a boiling point of less than 100°C.

10. A method as claimed in claim 9, wherein the solvent is chloroform.

11. A method as claimed in claim 6, wherein the localized chemical-vapor-deposition-diffusion-limiting layer comprises an aluminide of nickel or cobalt, together with an organic binder.

12. A method as claimed in claim 11, wherein the organic binder of the chemical-vapor-deposition-diffusion-limiting layer is an acrylic resin.

13. A method as claimed in claim 6, wherein the workpiece is a superalloy and the diffusion limiting layer composition also contains from 0.25 to 3% by weight chromium.

14. A method as claimed in claim 6, 11 or 13, wherein the diffusing metal is aluminum.

15. A method as claimed in claim 6, 11 or 13, wherein the diffusing metal is chromium and aluminum.