GB2058844A - Diffusion coating of metal workpiece - Google Patents

Abstract

Diffusion coating of an internal surface of a workpiece accessible only a through very narrow passageway, is simply effected by coating that surface with a uniform layer of particles of the metal to be diffused into it, and then holding the thus- prepared workpiece at diffusion- coating temperature in a diffusion- coating atmosphere.

Description

SPECIFICATION Diffusion coating of metal workpieces The present invention relates to the diffusion coating of metal workpieces.

Such coating is highly desirable for example to increase the resistance of the workpiece to attack.

Thus, as described in U.S. Patents 4,132,816 and 4,148,275, jet engine blades and vanes that have internal cooling passages frequently require the diffusion coating of the surfaces of those passageways to increase their resistance to attack by the hot combustion products to which they are subjected. These patents suggest that such coating be effected by forcing a gaseous specially formulated diffusion coating composition through the passageway to be coated while the workpieces are heated to diffusion coating temperature.

The invention provides a method for the diffusion coating of the internal surface of a hollow in a metal workpiece where that hollow is accessible only though a passageway less than about 5 millimeters wide, which method comprises applying over that internal surface an essentially uniform layer of particles of the metal to be diffused into that surface, and heating the workpiece so treated to diffusion coating temperature under conditions suitable for causing the said internal surface to become diffusion coated with the said metal.

According to one embodiment, diffusion coating of the internal surface of a hollow in a metal workpiece where that hollow is accessible only through a passageway less than 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 coating temperature while the hollow is exposed through the passageway coating atmosphere.

The layer of particles is conventiently applied as a layer of a dispersion of the particles in a binder that is driven off at diffusion coating temperatures. A water dispersion of aluminum particles, such as is described in U.S. Patent 3,31 8,71 6, can be used, but it is preferred to use dispersion vehicles in which heavier metals such as chromium can also be fairly uniformly dispersed.A 1 to 10% by weight solution of an acrylic resin such as ethyl methacrylate in methylchlorofrom 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 1 50 microns are easily suspended to make a fairly uniform mobile suspension that does not settle our appreciably for the minute or so needed to apply the suspension and then distribute it as a uniform coating.

Settling can be slowed by dissolving in the suspension vehicle a long-chain acid such as C12 to C50 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 narrown 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 residuel 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 the outlets. A stream propelled by a 1 5 pounds per square inch gauge source of air is quite effective.

The excess dispersion can also be expelled by centrifugal force. Spinning the heavily coated workpiece in a centrifuge at about 10 to 20times gravity for a few seconds does a good job of levelling where the centrifugal force is directed longitudinally of a filled passageway, for example. For complicated passageways it may be necessary to spin the workpiece in steps, each step with a different orientation.

A number of embodiments of invention are described in the following Examples which refer to the accompanying drawings, in which Fig. 1 is a vertical sectional view of an apparatus suitable for practicing the present invention; and Fig. 2 is a similar view of alternative apparatus.

The invention includes within its scope a method for the rapid high temperature diffusion coating in a retort of a workpiece embedded in diffusion coating powder containing energizer that is vaporized off at diffusion coating temperature which method comprises supplying heat to the retort at a rate that brings the workpieces to diffusion-coating temperature and then completes the diffusion coating all in less than 50 minutes, and then cooling the retort, an excess of energizer being present in the powder and this is also illustrated by the Examples.

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 mix 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% NH4Cl powder 3/4% based on the Al plus Awl203 total The tube 14 runs to and fro the length of the box, with each run about 1 1/2 inches from the next, and its perforations are 1/1 6 inch holes. It is connected to an unperforated supply extension 1 8 that leads out of the retort box to a source of argon. The retort wall opening through which extension 1 8 passes, can be sealed as by welding or filled with tamped powder or fiber to permit a pressure build-up within the retort. Very fine alumina or ceramic fibers is suitable.

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 7% aluminum, 14.5% molybdenum and 7% tungsten, the balance being essentially nickel. Each block is about an inch high and has a central cylindrical bore 24 about 23 mils in diameter penetrating its entire height.

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

Before the blocks 20 are placed in the retort, they first have their passageways 24 filled with a dispersion of 30 grams 325 mesh aluminum powder in 40 cc. of a 5% by weight solution 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 ot the masking slurry of Ni3AI 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 semi-tight by asbestos fibers tamped around its periphery. The covered box is placed inside an outer retort which is then covered by a furnace as shown in U.S. Patent 3,801,357, and heated to 1 9000F 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 cleaing 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 the layer 1 6 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. 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-1 900 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 7% 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 is 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 end-walls"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 1 mil thick with a maximum aluminum content of at least about 35% in the case.

In addition to the fitting of the aluminized rods, box 110 is prepared with layers 112, 11 6 having the same composition as layers 12 and 16. After loading the blades, the box is inserted in an outer retort and heated to 19500 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 1 9500F temperature is maintained for 8 1/2 hours, 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 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 portions of the airfoil as well as the flange, and thus compensate for the greater distance of these surfaces from the powder 11 6.

The varying distance of the internal passageway portions from the powder 11 6 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 relatively rapidly when a diffusion atmosphere reaches those surfaces after travelling 4 to 6 inches or more. Such an atmosphere need only be a vaporized diffusion energizer, such as a halogen or halogen compound, but the action of such an atmosphere is improved if it also contains a halide of the metal being diffused. Such an improved atmosphere is the usualatmosphere produced during diffusion coating, and powders 12, 1 6, 112, 11 are usual prior art diffusion coating powders.

The chromium present with the aluminum in the dispersion applied to the internal passages, diffuses into the passageway surfaces along with the aluminum and further improves the 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 chromaluminized 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 it is preferred that the aluminum content of aluminum-chromium dispersions be greater than twice the weight ofthe 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 smooth.

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 surfaces 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 dispersed metal when the diffusion is effected at 1 8000F, although somewhat shorter times can be used when the diffusing metal is aluminum alone.

Silicon, cobalt, iron and other metals used to make diffusion coatings can be used in addition to or in place of the aluminum and/or chromium. Some combinations of metals are known not to coat very well if at all.

Cobalt-based superalloy workpieces required about twice the diffusion time that nickel-based superalloys take, but iron-base alloys such as RA 330 and Incoloy 800 take less time than the nickelbased superalloys. A cobalt-based superalloy MAR M 509 vane also with cooling passages, when subjected to treatment as in Example 2 but at 20000F for 20 hours provides excellent results.

EXAMPLE 3 First stage hot section jet engine vanes made of the 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% ultrafine chromium powder (particles less than 20 microns in size), 80% 325 mesh alumina, and 1% NH4Br based on the total weight of the chromium and alumina.

(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.

(f) The diffusion coating was maintained at 20000F for 1 5 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 3000F 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 retort.

Both the external surfaces of the vanes and the surfaces of its cooling passageways are very effectively and uniformly 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 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 used to hold the dispersed metal layer in place, but by the time the retort box reaches about 6000F during heat-up, the flushing can be stopped. Where the retort box is surrounded by another retort having a closely controlled atmosphere, such as is maintained when a stream of hydrogen, argon or other inert gas is flushed through the outer retort chamber only, flushing of the 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 out too much activator from its atmosphere. The activator present in the powder on the retort floor is all converted to vapor by the time the heat-up brings the powder to about 7000F, and after such vaporization the flushing gas should not be supplied any faster than required to equal the volume in the inner retort space when flowing for a time corresponding to about one-twenthieth the diffusion-coating time. The flushing action is not complete, particularly with a light gas such as hydrogen, so that with such 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 diffusion is effected at relatively low temperatures, such as 1 6000F or below aluminum chloride is a very desirable energizer, particularly when aluminum is being diffused into a workpiece. Other energizers (sometimes called activators) are listed in U.S. Patent 3,764,371.

The methyl chloroform solvent of Examples 1 and 2 can be replaced by other solvents such as methyl ethyl ketone, chloroform, toluene, isopropyl alcohol and the like. However, methyl chloroform is a particularly safe material to work with because it does not burn and its hazard to health is extremely low. Water can also be used as a solvent with water-soluble 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 C1 to C8 alcohols, as well as 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 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 diffilSA intn the workpiece surface.

The rods 137 of Fig. 2 preferably have their surfaces heavily chromized beforehand when chromizing the workpiece interiors. Similarly when diffusion coating workpieces with 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 even above the workpieces. The activating vapors generated by these powders have a throwing power of as much as six inches, and when the inner retort is not flushed during the diffusion temperature dwell good diffusion coatings form even further away from the 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.

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 1 2000F or below, such coatings tend to form a fairly rough surface. Coating uniformity is improved by pre-plating a nickel or cabalt flash 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 9/1 6 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 square foot in a 1 60-1 800F 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 electro-plating 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 NICK, 6H20 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 270C.

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 mii. 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 particles 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 8500 to 8700F as measured by a thermocouple also inserted in the pack. The temperature was 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 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 4,039,41 6 or in the Society of Automotive Engineers, Paper No. 730546, by Gerald W. White, entitled "Applications of lon 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, Oklahomaa, November 1 972. Electroless plating can also be used with somewhat poorer results, inasmuch as the electroiess 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.

The aluminizinga 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 7000F, 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 cleanings of the blades was by solvent degreasing in place of the anodic electrolytic cleaning.

The aluminizing is conducted at 8800--9000F 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 CoCI2. 6H20 was substituted for the NiCI2 6H2O of Example 1 the quantity 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 process of Example 4 is repeated but AM 350 airfoils are used, the nickel chloride is replaced by a mixture of 107 g. NiCI2. 6H20 and 107 g. CoCl2 - 6H2O, the HCI content of the electroplating solution is increased 50%, the cathodic electroplating current density is 100 amperes per square foot, the electroplating temperature is 350C, 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, 3,948,687, 3,764,371 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 7000F, improves the smoothness by about 2 to 5 micro-inches. Such a top coating over a rougher aluminized workpiece which did not have the thin nickel electroplate, brought the top smoothness down to close to 30 micro-inches.

Increasing the number of top coating layers on the workpiece further improves the smoothness, but will generally not get thesmoothness much below about 24 micro-inches. A series of three layers ol 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 aluminium by weight, or contain at least about 0.1% 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 aluminium, has an exceptional smoothing effect. Thus an aqueous dispersion of colloidal silica containing 14% of the silica, and also containing 1 5% 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 1 5 micro-inches. Such a smoothness does not appear obtainable from other other top coating layers regardless of how many are applied.

Thus an improvement of 14 micro-inches is obtained when coating an unaluminized Type 304 stainless steel compressor 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 MgCrO4, the coating residue after drying weighing about 0.25 milligrams per square centimeter.

(b) Dry and then bake the coated blade at 700-8000F 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 5% 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 particles 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.

(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 20% 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 corrosion-inhibiting 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 phosporic 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 1 967. GROUP 1 FERRITIC (MARTENSITIC) STEELS)

Nominal Chemical Composition, per cent Alloy C Mn Si Cr Ni Co Mo W Cb Ti Al B Zr Fe Other Age-Hardening Stainless Steels AM-350 0.10 1.00 0.40 16.50 4.25 2.75 Bal.

AM-250 AM-355 0.15 1.00 0.40 15.50 4.25 2.75 Bal. 0.10N AM-355 AM-363 0.04 0.15 0.05 11.00 4.00 0.25 Bal.

15.5PH 0.04 0.30 0.40 15.00 4.60 0.25 Bal. 3.30Cu 17.4PH 0.04 0.30 0.60 16.00 4.25 0.25 Bal. 3.30Cu 17.7PH 0.07 0.30 0.30 17.00 7.10 1.10 Bal.

17.7PH PH13-8 Mo 0.04 0.05 0.05 12.75 8.10 2.2 1.10 Bal.

PH14-8 Mo 0.04 0.30 0.40 14.35 8.15 2.2 1.10 Bal.

PH15-7 Mo 0.03 0.10 0.10 15.00 20 3.0 Bal.

Pyromet X-15 0.03 0.10 0.10 15.00 20 3.0 Bal.

AFC-77 0.15 14.50 13 5.0 Bal. 0.40V Stainless W 0.12 17.00 7.00 1.0c 1.0c Bal. 0.2N Illium P 0.20 0.75 0.75 28.00 8.00 2.25 56.8 3.25Cu Illium PD 0.10 0.75 0.75 26.00 5.00 6.5 2.25 58.0 a For rupture in 100 and 1000 br. Not for design purposes.

b Cast alloy.

c Maximum.

d Experimental alloy.

e Alloy known not to be in commercial production.

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 10% 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, then in cold 1:1 concentrated HCI diluted with water, followed by another water rinse.

The resulting cleaned workpiece with a surface roughness of about 1 8 micro-inches is ready for plating in an acid nickel salt bath to a pick-up of about 1/2 milligram per square 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 8900F 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 0.5% 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 1 8000F in a pack of 10 weight percent Aluminum powder about 100 microns in size 45 weight percent Chromium powder about 10 microns in size 50 weight percent Awl203 about 100 microns in size with 0.5% NH4CI mixed in based on the weight of the pack, if the workpiece reaches 1 8000F 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 cyclindrical 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 1800if. 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 a 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 0.5% or less by weight. The energizer content can be increased, for example to 1%, or 2%, and energizer can be additionally or alternatively added to the metal powder deposited on the wall of a narrow passageway to be diffusion coated.

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 aboput 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 1 5 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 cooldown 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.

Obviously many mofifications 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.

Claims (14)

1. A method for the diffusion coating of the internal surface of a hollow in a metal workpiece where that hollow is accessible only through a passageway less than about 5 millimeters wide, which method comprise applying over that internal surface an essentially uniform layer of particles of the metal to be diffused into that surface, and heating the workpiece so treated to diffusion coating temperature under conditions suitable for causing the said internal surface to become diffusion coated with the said metal.

2. A method as claimed in Claim 1 in which the passageway is less than 2 millimeters wide.

3. A method as claimed in Claim 1 or Claim 2 in which the layer of particles is applied as a layer of a dispersion of the particles in a binder that is driven off at diffusion coating temperature.

4. A method as claimed in any one of the preceding claims in which the essentially uniform layer is applied by first applying a coating of a mobile dispersion of the metal particles in a solution of an organic binder in a liquid solvent and then expelling coating to leave the desired essentially uniform layer.

5. A method as claimed in Claim 4 in which the binder is an acrylic resin.

6. A method as claimed in Claim 4 in which the expulsion is accomplished by propelling a stream of gas against the mobile coating so that thickened portions of the coating are expelled out of the passageway.

7. A method as claimed in Claim 4 in which the expulsion is accomplished by applying suction to the passageway to suck out thickened portions of the coating.

8. A method as claimed in any one of the preceding claims in which the workpiece is a superalloy.

9. A method as claimed in Claim 8 in which the diffusing metal is aluminuma or chromium or a mixture of the two containing more than twice as much aluminum as chromium by weight.

10. A method as claimed in any one of the preceding claims in which the workpiece is a jet engine blade and the hollow is a cooling passageway in the interior of the blade.

11. A method as claimed in any one of the preceding claims, wherein the said conditions include exposing the hollow through the passageway to a diffusion coating atmosphere.

12. A method as claimed in Claim 11 wherein the diffusion coating atmosphere includes a diffusion energizer.

1 3. The combination of Claim 1 in which the diffusion coating atmosphere is that produced by an energizer in contact with the diffusing metal at diffusion coating temperature.

14. A method as claimed in any one of the preceding claims wherein essentially all of ther metal to be diffused into the said surface is present in the said layer.

1 5. In the diffusion coaying of the internal surface of a hollow in a metal workpiece where that hollow is accessible only through a passageway less than about 5 millimeters wide, the improvement according to which there is applied 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 coating temperature while the hollow is exposed through the passageway to a diffusion coating atmosphere.

1 6. A method for the rapid high temperature diffusion coating in a retort of a workpiece embedded in diffusion coating powder containing energizer that is vaporized off at diffusion coating temperature which method comprises supplying heat to the retort at a rate that brings the workpieces to diffusioncoating temperature and then completes the duffision coating all in less than 50 minutes, and then cooling the retort, an excess of energizer being present in the powder.

1 7. A method of diffusion coating a workpiece, substantially as hereinbefore described in any one of the specific examples.