Diffusion coating through narrow passageways

! The present invention relates to the diffusion coating j i of the interior of a hollow workpiece where that interior is !

accessible only through a restricted passageway. ¹

Such coating is highly desirable for example to increase i the resistance of the workpiece to attack. Thus, as described in 1 U. S. Patents 4,132,816 and 4,148,275, jet engine blades and vanes !' that have internal cooling passages frequently require the ן diffusion coating of the surfaces of those passageways to increase !' ־ !

. their resistance to attack by the hot combustion products to i^: : which they are subjected. These patents suggest that such i

׳ coating be effected by forcing a gaseous specially formulated ' diffusion coating composition through the passageways to be j coated while the workpieces are heated to diffusion coating ץ temperature.

.Among the objects of the present invention is the ; provision of a novel technique for diffusion coating through |j restricted passageways.

. Additional objects of the present invention include the • provision of a diffusion coating process which very uniformly 1! coats through restricted passageways without requiring forced | circulation through the passageways.

^! The foregoing as well as additional objects of the present invention will be more fully appreciated from a considerai tion of the following description of several of its exemplifications, reference being made to the accompanying drawings, wherein: I

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I ί:

Sii

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Fig. 1 is a vertical sectional view of a diffusion coating set-up for practicing the present invention; and Fig. 2 is a similar view of a modified set-up pursuant to the present invention.

According to the present invention diffusion coating • of the internal surface of a hollow in a metal workpiece where i

ji that hollow is accessible only through a passageway less than ji about 5 millimeters wide, is readily effected by applying over ! that internal surface an essentially uniform layer of particles : consisting essentially of all the metal to be diffused into that surface, and the workpiece so treated is subjected to diffusion 1:

; coating temperature while the hollow is exposed through the j passageway to a diffusion coating atmosphere.

The layer of particles is conveniently applied as a l.

; layer of a dispersion of the particles in a binder that is ׳ driven off at diffusion coating temperatures. A water dispersion jj of aluminum particles, such as is described in U. S. Patent i 3,318,716, can be used, but it is preferred to use dispersion ;! vehicles in which heavier metals such as chromium can also be I fairly uniformly dispersed. A 1 to 10% by weight solution of an acrylic resin such as ethyl methacrylate in methylchloroform makes a very desirable dispersion medium in which powdered chromium, powdered aluminum, mixtures of these powders, and other metals like powdered cobalt, in particular sizes up to about 150 microns are easily suspended to make a fairly uniform mobile suspension that does not settle out appreciably for the minute or so needed to apply the suspension and then distribute it as a uniform coating.

3.

Settling can be slowed by dissolving in the suspension vehicle a long-chain acid such as ^t0 aliphatic acid, or a copolymer of ethylene and acrylic acid, as described in U. S. patent 4,208,357.

Only about 0.3% to about 0.5% by weight of such additive is very helpful. Low-foaming non-ionic surface active agents such as polyethoxy ethers of linear alcohols like cetyl alcohol or of an alkyl phenol, in amounts as low as 0.1% to 0.3% by weight can also be used to slow the settling of the suspended particles.

With the very narrow passageways involved in the present invention, the mobile dispersion coatings do not spread into uniform layers, but build up in excessive thicknesses by reason of surface effects. Thus a passageway about 1 millimeter in diameter will generally be completely filled with the mobile dispersion. It is accordingly necessary to expel the excess dispersion as by applying suction to the passageway opening to suck out gas as a rapid stream that carries along with it all but a residual thin and quite uniform layer of the dispersion. Suction from a simple water-pump suction generator or from a suction pump that applies a suction of about 1/10 atmospheric pressure or less, as measured on a pressure gauge, is adequate.

Where the passageway whose coating is to be levelled has separate outlets at its opposite ends, the redistribution is easily effected by directing a stream of compressed air into one of t.he outlets. A stream propelled by a 15 pounds per square inch gauge source of air is quite effective.

4.

The excess dispersion can also be expelled by centrir i I fugal force. Spinning the heavily coated workpiece in a centri- I fuge at about 10 to 20 times gravity for a few seconds does a ' I good job of levelling where the centrifugal force is directed ; longitudinally of a filled passageway, for example. For com- j plicated passageways it may be necessary to spin the workpiece , in steps, each step with a different orientation.

The following examples illustrate the present invention more fully.

EXAMPLE 1

Into a short retort box 10 as in Fig. 1, a half-inch layer 12 of a diffusion aluminizing powder mix is poured, following which a perforated flushing tube 14 is placed over the 1m‘ x and then another two־inch־־“layer 16 of the mix covers the perforated tube.

The retort box and the- tube -are made :of -Inconel 600 and the mix has the following formulation by weight:Aluminum powder about 40 micron particles 15 %

Alumina powder about 200 to

300 micron particles 85 7«

NH/C1 powder 3/470 based on the Al plus Α12θ3 total

The tube 14 runs to and fro the length of the box, with each rur) about 1 1/2 inches from the next, and its perforations are 1/16 inch holes. It is connected to an imperforated supply

5.

extension 18 that leads out of the retort box to a source of argon. The retort wall opening through which extension 18 passes,¹can be sealed as by welding or filled with tamped powder or fiber to permit a pressure build-up within the retort. Very fine I 1 alumina or ceramic fibers is suitable. i

Suspended by sturdy nickel wires 22 hooked over the tops of the side walls of the box, are a series of blocks 20 of nickel-base alloy having 77« aluminum, 14.57« molybdenum and 77« j tungsten, the balance being essentially nickel. Each block is I about an inch high and has a central cylindrical bore 24 about 23

I mils in diameter penetrating its entire height. !

Also placed in the retort is a thermocouple 30 inserted in a thimble 32 welded to an inside wall and opening to the exterior through a perforation in the wall.

Before the blocks 20 are placed in the retort, they ^;first have their passageways 24 filled with a dispersion of 30 i grams 325 mesh aluminum powder in 40 cc. of a 57« by weight solu- ¹tion of poly(ethyl acrylate) resin in methyl chloroform. A suction hose is then promptly applied to one end of aperture 24 to suck out excess dispersion. The blocks so treated are permitted to stand a few minutes to set the residual coating.

The exteriors of the blocks are then painted with a 10 ^;milligram per square centimeter layer of the masking slurry of : N13AI powder as described in U. S, Patent 3,801,357, and the slurry coating permitted to dry. ;

After being loaded, the retort box 10 is covered with a lid 36 which can also be made seni-ti^it by asbestos fibers tamped around its periphery. The covered box is placed inside an outer retort ,

1

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which is then covered by a furnace as shown in U. S, Patent

3,801,357, and heated to 1900°F where it is held for nine hours while argon is fed into the perforated tube at a rate that takes about one hour to supply an argon volume equal to the box volume.

The heat is then turned off, the furnace lifted off the outer retort, and the retorts permitted to cool. The retort box 10 is opened when sufficiently cool, and the blocks 20 removed and cleaned of the masking layer. They then show a very uniform aluminized case about 2 mils thick over the entire internal surface of passageway 24. No cleaning is needed in that passageway, other than blowing air through it to clear out any ash, and the rinsing off of residual halide with water.

The same results are obtained when the blocks are held 1/16 inch or 2 inches from the top of layer 16, and when the blocks are positioned in the retort box 10 with their passageways horizontally oriented. With such orientation the blocks can simply be laid on top of layer 16 so that no special work-supporting equipment is needed.

It is not necessary to force the energizer-containing atmosphere through the narrow passage 24 as described in U.S. Patent 4,148,275, nor is it necessary to use complex energizers with special throwing power, as described in U.S. Patent 4,132,816. However those complex energizers as well as fluoride energizers in general do a very effective job in the present invention.

EXAMPLE 2

In this example a group of jet engine blades with internal cooling passages have the walls of the passages heavily chromaluminized while the airfoil surfaces are lightly chromaluminized and the roots are given little or no external coating.

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Such a blade is schematically illustrated at 120 in Fig. 2 and has a number of passages 124 extending the entire length of its airfoil section 120 from the airfoil tip 123 to the opposite face of the mounting flange 125. At their extreme ends the passages are about 1 mil by 2 mils in cross section, and in their intermediate portions their cross section is a little larger.

The blades, which are made of B-1900 alloy, are cleaned by mild blasting with fine aluminum grit, followed by degreasing. There is then introduced into the cooling passages, with the help of a medicine dropper, a suspension of 40 grams 325 mesh aluminum powder and 5 grams 325 mesh chromium powder in 50 cc. of a 77־ by weight solution of poly(methyl ethacrylate) resin and a 0.5 by weight solution of stearic acid in methyl chloroform. Suction from a water-pump is then promptly applied to each end-of each passage in the airfoil for a few seconds, and the blade permitted to stand to cause the suspension remaining.in_.the passages to .dry Excess suspension on the-outside surface of the blade i-s removed with the help of a cloth wet with a little methyl chloroform, and a group of blades so prepared is loaded into previously prepared retort box 110. This box is similar to box 10, but made of type 304 stainless steel and it has shelves 135 welded onto its endwalls and carrying spaced rods 137 that span the box length. The blades are fitted between the bars with their airfoils 121 extending downwardly and their flanges 125 supported by the bars. The bars can be plain carbon steel heavily aluminized beforehand, and for example have a diffusion-aluminized case at least about one mil thick with a maximum aluminum content of at least about

357» in the case.

In addition to the fitting of the aluminized rods, box 110 is prepared with layers 112, 116 having the same composition as layers 12 and 16. After loading the blades, the box is I inserted in an outer retort and heated to 1950°F, while a slow stream of hydrogen is fed through perforated tube 114 at a rate | that requires about 1/2 hour to supply an amount of hydrogen equal to the volume of the box. Before the heating is started the hydrogen stream is temporarily speeded up to more effectively, replace the previous atmosphere in the box by hydrogen.

The 1950^eF temperature is maintained for 8 1/2 hours, : I and the box then cooled. After sufficient cooling the hydrogen atmosphere is replaced by argon, and the box opened. The internal surfaces of the passages in the blade show an extremely uniform i aluminized case about 2 to about 2.3 mils thick. The airfoil surfaces have an aluminized case about half as thick, and the blade root 126 has a less than 0.4 mil thick case.

The flange 125 has its lower face aluminized to about the same extent as the airfoil surface, and has its upper face aluminized to about the same extent as the root. The aluminized lower face does not show a drop in aluminum content where that face rested on the bars 137. Quite the contrary it appears that the heavily aluminized bar surfaces help to aluminize the upper j portions of the airfoil as well as the flange, and thus compen- I sate for the greater distance of these surfaces from the powder 116.

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The varying distance of the internal passageway portions from the powder 116 seems to have no significant effect ׳ inasmuch as the metal being diffused into the passageway surfaces is located at those surfaces. Such diffusion takes place rela- I tively rapidly when a diffusion atmosphere reaches those surfaces after travelling 4 to 6 inches or more. Such an atmosphere need ¹only be a vaporized diffusion energizer, such as a halogen or halogen compound, but the action of such an atmosphere is im- j proved if it also contains a halide of the metal being diffused. ; Such an improved atmosphere is the usual atmosphere produced : during diffusion coating, and powders 12, 16, 112, 116 are usual prior art diffusion coating powders. 1

The chromium present with the aluminum in the disper- j sion applied to the internal passages, diffuses into the passage-, way surfaces along with the aluminum and further improves the ן resistance of those surfaces to attack. .The proportion of ! chromium can be increased and the; aluminum completely eliminated to provide a chromized surface rather than an aluminized or chromaluroinized surface. The chromium and aluminum particles can be pre-alloyed together if desired, or they can be mixtures of. the separate metals.

For diffusion coating nickel-base superalloys with aluminum !preferred that the aluminum content of aluminum-chromium ן

I dispersions be greater than twice the weight of the chromium. ' ’ The metal particles in the metal dispersions should be not over about 3 mils in size, preferably not over 2 mils, where the passageway walls they are diffused into are to remain very . I smooth. 1

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The diffusion coating heat should be maintained at least as long as needed to cause all of the dispersion metal particles to diffuse into the passageway surfaces. This leaves those sur- j faces clean and ready for service without further treatment. When the workpiece being coated is a nickel-based superalloy and the ׳ metal being diffused in is aluminum or chromium or mixtures of the two, at least about two hours is needed for every 0.1 mil of dis- I persed metal when the diffusion is effected at 1800°F, although [ i somewhat shorter times can be used when the diffusing metal is I aluminum alone. Silicon, cobalt, iron and other metals used to I make diffusion coatings can be used in addition to or in place of j ' the aluminum and/or chromium. Some combinations of metals are known not to coat very well if at all. :

• Cobalt-based superalloy workpieces require about twice ,

1 ז ׳ i the diffusion time that nickel-based superalloys take, but iron- I base alloys such as RA 330 and Incoloy 800 take less time than the ¹ nickel-based superalloys. A cobalt-base superalloy MAR M 509 vane also with cooling passages, when subjected to treatment as in

I Example 2 but at 2000°F for 20 hours provides excellent results.

; EXAMPLE 3 ' First stage hot section jet engine vanes made of the

I' ין nickel -based IN 100 alloy and with cooling passageways about 30 mils in diameter, are treated in the manner described in Example : 2, but with the following specific differences:׳ (a) The powder on the floor of the retort is;

a chromizing powder mixture of 20/0 ultrafine chromium!

powder (particles less than 20 microns in size), 807» 325 mesh alumina, and 17» NH^Br based on the total weight of the chromium and alumina.

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(b) The coating slurry is a dispersion of 15 grams of the ultrafine chromium in 20 cc. of the binder solution of Example 1.

(c) The vanes are held in their horizontal position about 1 inch above the powder on the floor.

(d) The inner retort box has its cover loosely ! applied without any attempt to seal its edges.

(e) The rods 137 are chromized Inconel 600.

i (f) The diffusion coating was maintained at

I 2000°F for 15 hours, the flow of hydrogen into the : inner retort box is stopped when that temperature was reached and a flow of argon is started when the temperature reaches 300°F in the cool-down.

(g) A slow flow of hydrogen is maintained ’ through the outer retort throughout the heat, but is stopped when the argon flow starts in the inner i retort.

i Both the external surfaces of the vanes and the sur׳ faces of its cooling passageways are very effectively and uniI

I formly chromized.

The powder on the retort floor need not have the same metal components as the powder in the passageways. Thus by having chromium as the only metal in the passageway powder and aluminum as the only metal in the powder on the retort floor, the passageways can be chromized while the exterior of the

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I workpiece treated are aluminized. A little aluminum may appear in the chromized case on the passageway surfaces, particularly if the diffusion treatment is prolonged. The Al and Cr can be reversed in position.

Omitting all metal particles from the powder on the retort floor seems to slow down the diffusion case formation on the passageway surfaces, but good aluminizing, chromizing, and chromaluminizing is still obtained.

It is not essential that the process of the present invention be carried out with a flushing gas fed through the perforated tube 14 or 114. A little flushing does help flush away any vapors formed by the decomposition of whatever binder is j fi used to hold the dispersed metal layer in place, but by the time j i ן |i the retort box reaches about 600°F during heat-up,—the flushing | |i I j can be stppped. Where the retort box is surrounded by־another . I ' retort having a closely controlled atmosphere,, such as ־is— i maintained when a stream of hydrogen־, argon or other inert gas ן is flushed through the outer retort chamber only, flushing of the i inner retort box need not be resumed except when the outer retort atmosphere is hydrogen or other combustible gas. In that event it is helpful to flush an inert gas through the inner and outer ' retorts to sweep away combustible gas before the retorts are opened.

During the dwell at diffusion-coating temperature, any flushing of the inner retort should not be so rapid as to sweep ן out too much activator from its atmosphere. The activator present; in the powder on the retort floor is all converted to vapor by i i i

13.

the time the heat-up brings the powder to about 700°F, and after . such vaporization the flushing gas should not be supplied any I i faster than required to equal the volume in the inner retort ' space when flowing for a time corresponding to about one-twentieth the diffusion-coating time. The flushing action is not complete,ן particularly with a light gas such as hydrogen, so that with such i maximum flow there is still some activator present at the end of the diffusion-coating heat.

Any halogen or halogen compound vaporized at diffusion-! coating temperature can be used as an energizer. Where the I diffusion is effected at relatively low temperatures, such as 1600|^oF 0 below aluminum chloride is a very desirable energizer, parti- I cularly when aluminum is being diffused into a workpiece. Other i energizers (sometimes called activators) are listed in U. S. Patent 3,764,371. !

The methyl chloroform solvent of Examples 1 and 2 can 1 be replaced by other solvents such as methyl ethyl ketone,ן chloroform, toluene, isopropyl alcohol and the like. However,| methyl chloroform is a particularly safe material to work withj because it does not burn and its hazard to health is extremely low. Water can also be used as a solvent with water-soluble ! 1 binders, but it is generally not desirable to keep finely divided metal particles in contact with water for a long period of time. !

Other acrylic resins that make effective binders ! include poly(methyl methacrylate) and the various polymeric acrylic and methacrylic esters of to Cg alcohols, as well as

14.

polyacrylic acid and mixtures or copolymers of the monomers from;

which these are made. Other binders that can be used include rosin, polyethylene, polystyrene, methyl cellulose and even | i dimethyl silicone oils. The acrylic resins are driven off quite ; cleanly during the diffusion heat, but some binders might leave ;

a little carbon behind and this would also diffuse into the :

I workpiece surface.!

The rods 137 of Fig. 2 preferably have their surfaces !:

j heavily chromized beforehand when chromizing the .workpiece;

jj interiors. Similarly when diffusion coating workpieces withj zinc, cobalt, or other metal, these rods or other work-engaging surfaces are preferably correspondingly precoated.

^! The powders 12 and 16 are not required to be located on the retort floor, but can be held in baskets below or evenί • !

I .j j above the workpieces. The activating vapors generated by these j i־ I ; powders have a throwing power of as much as six inches, and when

I;

j the inner retort is not flushed during the diffusion temperature I ! dwell good diffusion coatings form even further away from the I ί nearest powder. An easily vaporized metal halide such as aluminum chloride can be introduced into the inner retort as a vapor carried by the flushing gas, and no powder is needed other ' than in the fine passageways. ί

i

15. i

Some substrates, such as age-hardenable stainless steels do not take uniform diffusion coatings, particularly when the diffusion is conducted at low temperatures. At temperatures of 1200°F or below, such coatings tend to form a fairly rough surface. Coating uniformity is improved by pre-plating a nickel or cobalt flash not over about 0.1 mil thick on the walls to be coated.

Such improvement in uniformity and smoothness is obtained with coatings whether or not in narrow passageways. This is shown in the following examples.

EXAMPLE 4

A group of AM 355 last stage compressor blades about 9/16 inch wide, 2 inches long, and about 30 mils in thickness, for a J-85 jet engine, were cleaned by anodic treatment at 50 amperes per suqare foot in a 160°-180°F water solution of sodium carbonate (1 oz./gal.) and sodium hydroxide (1 oz./gal.) for one minute, followed by water rinse and then a dip in 18% HCI.

After cleaning these blades showed a. surface roughness of 17 to 20 micro-inches. They were given a four minute electroplating treatment by applying a long magnet to the roots of a row of individual blades, immersing the airfoils of the blades so held in. a solution of 426 g. of NiC126־H2O and 70 cc. concentrated HCI in enough water to make one liter, and connecting the magnet as a cathode with respect to a nickel anode also immersed in the same solution. The cathode current density was 50 amperes per square foot, and the bath temperature about 27°C.

The electrolysis was then terminated, the plated blades were rinsed with water, dried and inspected. A bright coating was observed over the entire airfoil surfaces of the blades, and one of them on sectioning showed a nickel plate thickness of about 0.04 to about 0.09 mil. The remaining dried blades were then packed in a plain carbon steel diffusion-coating retort previously used for aluminizing. The packing was with a powder pack having the following composition by weight:

Powdered aluminum - about 10 micron particle size 20 parts Powdered alumina ־ minus 325 mesh 79.7 parts

Aluminum chloride, anhydrous •3 parts

The aluminum, and., alumina were ־in־־the form of a׳ mixture that had.been previously used as an aluminizing pack.

The packed retort was then placed in an outer retort as described in U. S. Patent 3-,801,357 and under the bathing action of hydrogen was heated to bring the pack to a temperature of 850° to 870°F as measured by a thermocouple also inserted in the pack. The temperaturewas then maintained for 25 hours, after which the retorts were permitted to cool and the blades unpacked. As removed from the pack they showed a surface roughness from about 24 to about 30 micro-inches and presented a very good appearance.

One of the thus-treated blades was sectioned and examined microscopically. It showed an average aluminide case about 0.4 mil thick, the outer layer of the case having a high

17.

nickel structure that extended into the case about one-fifth the case depth. A salt-spray test showed a little better corrosion resistance for these treated vanes as compared with corresponding blades aluminized without the nickel plate. The ductility of the aluminized cases was about the same with the nickel plate as without it, as indicated by deforming such blades.

Additional AM 355 blades of the same type were subjected to the same sequence of treatment steps except that the electrolytic plating time was extended to 12 minutes. These showed that before aluminizing a nickel plate thickness of about 0.2 mil was deposited, and after aluminizing the case was much more brittle than the cases applied over the thinner nickel plating. This 0.2 mil nickel plate thickness is the minimum such thickness suggested in U. S. Patent 3,859,061.

The nickel plating can be applied by vapor deposition, or by ion deposition as described in U. S. Patent 4,039,416 or in the Society of Automotive Engineers, Paper No. 730546, by Gerald W. White, entitled Applications of Ion Plating or by sputtering as described in the paper RF Sputtering by the same author and presented at the 8th Annual FAA International Aviation Maintenance Symposium, Oklahoma City, Oklahoma, November 28, 1972. Electroless plating can also be used with somewhat poorer results, inasmuch as the electroless platings contain phosphorus or boron or the like. The minimum suitable nickel plating thickness is about 0.01 mil. Electroplating in narrow passageways is readily accomplished with the help of an anode in wire form penetrating through the center of the passageways.

18.

The aluminizing can be effected with the workpieces embedded in a diffusion-coating pack as in Example 4, or with the workpieces kept out of contact with, but adjacent to the pack as in Examples 1, 2 and 3. The lowest practical aluminizing temperature is about 700°F, and other activators can be used in place of the aluminum chloride.

EXAMPLE 5

The processing of Example 4 is repeated with the following changes :

The activator is anhydrous aluminum bromide instead of the aluminum chloride.

The diffusion-bathing atmosphere is argon rather than hydrogen.

The initial cleaning of the blades was by solvent degreasing in place of the anodic electrolytic cleaning.

The aluminizing is conducted at 880°-900°F to yield a case about 0.7 mil thick.

The surface roughness after aluminizing is about 28 to 35 micro-inches. Other cleaning steps such as simple glass blasting can also be used with similar results.

EXAMPLE 6

The processing of Example 4 is repeated but CoC126־H20 was substituted for the ΝΐΟ^-δΙ^Ο of Example 1, the quantity

19.

being unchanged. The resulting aluminized vanes have a surface roughness about the same as the Example 4 products, and showed even greater resistance to corrosion.

EXAMPLE 7

The processing of Example 4 is repeated but AM 350 airfoils are used, the nickel chloride is replaced by a mixture of 107 g. NiC12-6H₂O and 107 g. CoC126־H20, the HC1 content of the electroplating solution is increased 50%, the cathodic electroplating current density is 100 amperes per square foot, the electroplating temperature is 35°C, and the electroplating time 2 minutes. The roughness of the final product is only about 5 to 10 micro-inches more than the untreated airfoils.

The aluminized blades can be used with or without the top coatings described in U. S. Patents 3,859,061, 3,958,046, 3,948,6873,764,371 ,־ and 4,141,760. These top coatings after drying and firing generally provide a surface somewhat smoother than that of the surface on which they are applied. Thus a top coating containing leafing aluminum as described in column 6 of U. S. Patent 3,958,046, applied as a 0.3 milligram per square centimeter layer over the aluminized product of Example 4 in the present specification and fired at 700°F, improves the smoothness by about 2 to 5 micro-inches. Such a top coating over a rougher similarly aluminized workpiece which did not have the thin nickel electroplate, brought the top smoothness down to close to 30 micro-inches.

20.

Increasing the number of top coating layers on the workpiece further improves the smoothness, but will generally not get the smoothness much below about 24 micro-inches. A series of three layers of the above-noted flake aluminum coating on the product of Example 4 builds up the total top coating weight to 0.8 to 0.9 milligrams per square centimeter and shows a surface roughness as low as about 20 micro-inches.

Some top coating formulations when cured form hydrophobic surfaces over which it is difficult or impossible to apply a uniform overlying layer. The teflon-containing formulations of U. S. Patent 3,948,687 are examples of such difficult materials. However top coatings that contain-at least about 5% leafing aluminum by weight, or contain at least about 0.17־ by weight wetting agent not destroyed or driven off by a curing operation, will accept overlying coatings fairly well.

One type of coating seems unique in that when applied over a top coating containing flake aluminum, has an exceptional smoothing effect. Thus an aqueous dispersion of colloidal silica containing 147־ of the silica, and also containing 157־ of a bonding agent such as magnesium chromate or mixtures of magnesium phosphate and magnesium chromate or such mixtures that also contain a little free phosphoric or chromic acid, when applied over other top coatings or other layers of the same top coating, will get'the smoothness down to 10 to 15 micro-inches. Such a smoothness does not appear obtainable from other top coating layers regardless of how many are applied.

21.

Thus an improvement of 14 micro-inches is obtained יי when coating an unaluninized Type 304 stainless steel compressor 1׳ j! blade having an original roughness of 42 micro-inches after glass bead blasting to clean it, using the following coating treatment:

EXAMPLE 8 (a) Spray on the blade surface a suspension of the aluminum paste of Example I in U. S. Patent 3,318,716 dispersed in 30 times its weight of a 4% water solution of MgCrO^, the coating residue after drying weighing about 0.25 milligrams per square centimeter.

(b) Dry and then bake the coated blade at 700800°־F for 10 minutes.

(c) Repeat steps .(a) and (b) on the baked blade.

(d) Repeat steps (a) and ־(b)-again...

(e) Spray on the resulting coated blade a 57־ suspension of colloidal alumina in the teflon-free magnesium phosphate-chromite acid solution of Example II- in U. S. Patent 3,948,687, the alumina particles having a particle size below 10 millimicrons, to leave a stratum that after drying weighs about 0.6 milligram . per square centimeter.

(f) Repeat the drying and baking step (b).

(g) Repeat step (e) on the thus baked blade.

(h) Repeat the drying and baking.

22.

(i) Repeat step (e) again.

(j) Repeat the drying and baking.

The final coated blade shows a roughness of about 28 micro-inches and makes a very effective compressor blade for jet engines.

This exceptional top smoothness is provided by dispersions containing about 1 to 207־ of silica or alumina particles no larger than about 25 millimicrons in size and a water-soluble bonding agent in an amount at least equal to that of the dispersed particles. However magnesium chromate is a particularly desirable bonding agent inasmuch as it has strong corrosioninhibiting effects on a metal workpiece it covers. . As much as . . . half the magnesium chromate can be replaced by magnesium phosphate and/or chromic acid and/or phosphoric acid. The hardness and mar-resistance of-aluminum flake coatings is also markedly increased by such colloidal over-coatings.

The foregoing smoothing effect of top coatings is provided on other־substrates such as on type 410 stainless steel airfoils that have been aluminized without the help of the thin nickel or cobalt flash electroplate, but such electroplates at least 0.01 mil thick make for a much smoother product on agehardenable stainless steels.

• The compositions of AM 355 as well as of other typical age-hardenable steels suitable for the present invention is given below, taken from ASTM Data Series Publication No. DC 9d,

October 1967.

23.

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I7״PH .

״.)PH .

Fl HUM phmum־

Pyioiritl X״li

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H)ium P.. Xliiutn PD

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<td> ״Μί‘</td><td> LOO</td><td> ‘ β.’ώ ‘'</td><td></td><td> 4.25 '</td><td> ....</td><td> 1:n .</td><td></td><td></td><td></td><td> ...</td><td></td><td></td><td></td><td> S.l.</td><td> ».ION</td>

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<td> ow D.M</td><td> 0.50 0 jn</td><td> 0 40 0.6C</td><td> it 00 H OC</td><td> 4 40 4 23</td><td> ..</td><td></td><td> ...</td><td> 0.23 0.25</td><td></td><td> ...</td><td> *P</td><td></td><td> ...</td><td rowspan="2"> Bal. fid 3»1.</td><td> F.JoCu l.WC־</td>

<td> 0.07</td><td> 0.5¢</td><td> 044</td><td> 1L00</td><td> 740</td><td> ....</td><td></td><td></td><td> ....</td><td></td><td></td><td></td><td></td><td> ______</td><td></td>

<td> 0.04’</td><td> 0.0J</td><td> 0.05</td><td> UJJ </td><td> 0 40</td><td></td><td> Li</td><td> ...</td><td></td><td></td><td></td><td> 140</td><td></td><td></td><td> Li.</td><td></td>

<td> 0 04</td><td> 0.3¢</td><td> 040</td><td> I4.3S</td><td> US</td><td> ....</td><td> 24</td><td></td><td> ....</td><td></td><td></td><td> 140 1 &gt;0</td><td></td><td></td><td> Rak</td><td></td>

<td> 0.07</td><td> 0.SO</td><td> 0.30</td><td> H 10</td><td> 7.10</td><td></td><td> 24</td><td></td><td> • ...*</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td> 0 03</td><td> c. 10</td><td> 040</td><td> H.OO</td><td></td><td> 2¢</td><td> 5.C</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td> 0 40V</td>

<td> OH</td><td></td><td></td><td> 1440</td><td rowspan="2"> 2.®</td><td> 12</td><td> 5 0</td><td></td><td></td><td></td><td rowspan="2"> Tr־</td><td> ”lf</td><td></td><td></td><td rowspan="2"></td><td rowspan="2"> 0.2N</td>

<td> 042</td><td></td><td></td><td> 17.00</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

<td> 0.»</td><td> OJS</td><td> 0.73</td><td> ax</td><td> • ®</td><td></td><td> 2.23</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td rowspan="2"> 44 _______________i</td><td> LUC.</td>

<td> fi.i«</td><td> 043</td><td> ¢.75</td><td> 26.®</td><td> 5 ®</td><td> A.J</td><td> 2.25</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td>

* For roe״״. io 100 .Ml 1000 hr. Sol lor dn.p porpr»«.

* C».! .W- * Matimum *r.iper.meMil .Hoy. ., , t- I- ___1 rirurt.'iMTr

When a stainless steel workpiece is to be aluminized, a very effective pre-cleaning is accomplished by the following sequence , or by grit blasting with 220 mesh alumina grit.

EXAMPLE 6

First subject the workpiece to 1/2 minute cathodic treatment at about 50 amperes per square foot in a 107־ solution of sodium carbonate in water, then anodically treat it in the same solution at about the same current density for about the same time, after which the workpiece is rinsed with water, dipped in 10% NaOH solution in water to remove any residual smut,

24.

1!

1' _; then in cold 1:1 concentrated HCI diluted with water, followed ' by another water rinse. I ! The resulting cleaned workpiece with a surface rough’ ness of about 18 micro-inches is ready for plating in an acid nickel salt bath to a pick-up of about 1/2 milligram per square 1- centimeter producing a nickel flash about 0.07 mil thick. After rinsing and drying it can then be aluminized in the powder pack of Example 4 for 30 hours at 870 to 890°F to yield an aluminized case about 0.7 mil thick and having a surface roughness of about 22 to 23 micro-inches .

The aluminizing step in the above examples can be effected in very short times by heating a workpiece embedded in an activated powder pack, with a thermal input that brings it to diffusion coating temperature and completes the diffusion coating all in about 50 minutes or less. During this short interval the activator present in pack begins to be volatilized at a relatively rapid rate that persists about 45 minutes, even if only present in the pack at a concentration of 0.57־ by weight, and the formation of the diffusion coating case is extremely rapid. Thus a 2 mil aluminized case is produced only about 30 minutes after starting to heat a workpiece to 1800°F in a pack .of weight percent Aluminum powder about 100 microns in size weight percent Chromium powder about 10 microns in size weight percent AI2O3 about 100 microns in size

25.

p with 0.57־ NH^Cl mixed in based on the weight of the pack, if I: the workpiece reaches 1800°F in 15 minutes.

It is preferred to have the workpiece covered by no more than about 1/2 inch of activated pack when it is heated, inasmuch as the pack acts as thermal insulation and slows down the penetration of the heat to the workpiece from the walls of the retort in which it is held during the heating. With the workpiece embedded in a pack held in a cylindrical retort having a 7 inch length and a 2 inch diameter, so that about 1/2 inch pack thickness envelopes the workpiece, heat supplied at the rate of at least about 200,000 BTU per hour per pound of workpiece will effect the.desired heat-up to temperatures as high as 1800°F. During such heat-up the retort can have one or both its ends loosely covered to permit escape־ of gases ,־ and can be held in larger retort through which hydrogen or argon'is flowed at a slow rate to flush out the escaping gases.

It is not necessary to arrange the workpiece so that it comes to within 1/4 inch of the retort as described in U. S. Patent 3,824,122. Indeed the presence of a 1/2 inch thick pack covering is preferred when practicing the rapid diffusion coating of the present invention inasmuch as it assures the presence of . sufficient energizer even when the energizer content of the pack is only 0.57־ or less by weight. The energizer content can be increased, for example to 17־, or 27־, and energizer can be additionally or alternatively added to the metal powder deposited on the wall of a narrow passageway to be diffusion coated.

26.

A retort packed in accordance with the rapid diffusion coating technique of examples 4, 5 and 6 can contain a number of workpieces and there is no need to position each workpiece into its own carefully dimensioned closely fitting retort as in U. S. Patent 3,824,122.

Low temperature diffusion coating, as in Example 4 is even more readily accomplished in short periods of time -not over 45 minutes of heating is generally needed to bring the workpieces to temperature and obtain an aluminized case at least 1 mil thick. Thinner cases require only about 30 minutes or even less.

To further save time the retort cooling is best effected by withdrawing it from the furnace in which the heating is carried out. Exposed to the ambient air and with the help of the flushing gas stream between the retorts, the cylindrical retort assembly described above cools in about 15 minutes to the point that the outer retort can be opened and the inner retort withdrawn,exposed to the atmosphere and emptied. In this way the entire diffusion coating sequence including the completion of the cool-down takes only about an hour or 65 minutes. This compares with the 1 1/2 hours disclosed in U. S. Patent 3,824,122 for just the heating time. The cool-down can also be accelerated by blowing air over the cooling retort assembly or by lowering it into a quenching liquid such as water.

27.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.