US 3785854 A
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United States Patent 1191 Baldi 1*Jan. 15, 1974 DIFFUSION COATING  Inventor: Alfonso L. Baldi, Drexel Hill, Pa.
[731' Assignee: Alloy Surfaces Co., Inc., Wilmington, Del.
[ Notice: The portion of the term of this patent subsequent to Oct. 9, 1990,
has been disclaimed.
 Filed: May 18, 1972 21 Appl. No.: 254,403
Related US. Application Data  Continuation-in-part of Ser. Nos. 219,514, Jan. 20, I972, and Ser. No. 90,682, Nov. 18, I970, each is a continuation-in-part of Ser. No. 837,811, June 30, 1969, abandoned.
 US. Cl. 117/1072 P, l48/6.l6, 29/194,
Berkley l l7/l07.2 P X Primary Examiner-Ralph S. Kendall AttorneyConnolly and Hutz  ABSTRACT Rich chromium diffusion coating on dispersionstrengthened nickel or nichrome is obtained in one step by embedding work in chromium diffusion coating pack containing nickel with or without some cobalt, and held in unsealed retort cup at least 15 inches high. Pack can also contain metallic iron to reduce coating temperature. Masking is arranged by covering the sites to be masked with a layer of a mixture of nickel powder and inert filler. For better high temperature oxidation resistance the chromium-rich coating is covered by an aluminum diffusion coating from a simple aluminum diffusion pack or one that has the aluminum mixed with chromium. Low temperature aluminum diffusion is more uniform when pack energizer is aluminum chloride or other material that does not generate nitrogen, and gives good protection against marine corrosion of steels, and particularly when there is a chromate-type coating applied over the aluminizing. Highly effective chromate-type coating mixture consists essentially of aqueous solution of chromic and phosphoric acid also containing magnesium salts of said acids and dispersed polytetrafluoroethylene particles. Aluminized superalloy can be heated in air to whitenit, then cleaned to give product having more ductile case. Such coated superalloy can also be stripped of coating by aqueous l-INO -HF-CrO bath.
13 Claims, 2 Drawing Figures PAIENTEUJAN 1 51914 DIFFUSION COATING The present application is in part a continuation of applications Ser. No. 219,514, filed Jan. 20, 1972, and Ser. No. 90,682, filed Nov. 18, 1970, each of which is in part a continuation of application Ser. No. 837,81 1, filed June 30, 1969 and subsequently abandoned.
The present invention relates to the coating of TD nickel and other high temperature materials to make them more resistant to exposure.
Thoria-dispersed nickel, more commonly called TD nickel, and thoria-dispersed nichrome, similarly called TD nichrome, are unusual materials, particularly at high termperatures. They have greater structural strength at temperatures of about 2,300F than nickeland cobalt-based superalloys. However it has been difficult to adequatelyprotect the TD nickel and TD nichrome against excessive oxidation at such high temperatures.
It has been known that good oxidation protection is provided by first diffusion coating the TD nickel with 40 to 60 milligrams chromium per square centimeter of surface, followed by a diffusion coating of 1.5 to 4 milligrams aluminum per square centimeter. Unfortunately uniform heavy smooth chromium coatings of desirable structure have been difficult to obtain .in a readily reproducible satisfactory form without using two separate chromium diffusion operations.
Among the objects of the present invention is the provision of improved coating techniques for TD nickel and related materials.
The foregoing as well as other objects of the present invention will be more fully understood from the following description of several of its exemplifications, reference being made to the accompanying drawings in which:
FIG. 1 is a vertical sectional view of a diffusion coating set-up pursuant to the present invention; and
FIG. 2 is a similar view of a related masked diffusion coating operation.
According to the present invention a rich chromium layer of high quality is diffused into TD nickel or TD nichrome articles in one step from a pack held in an unsealed retort cup at lcast l5 inches high, and the pack contains 5 to 40 percent chromium by weight as well as nickel in a proportion from about one-third toabout one-twentieth of the chromium by weight. The pack can also contain cobalt in an amount from about onetwentieth to about one-fifth the chromium by weight. Any places where the articles are not to be coated can be masked as by an intervening layer of nickel powder diluted with alumina.
A layer of aluminum can then be diffused in over the chromium-diffused case. This aluminum diffusion can be a simple pack aluminizing or can be the more complex aluminizing from packs that contain chromium as well as aluminum, such as shown in Canadian Pat. No. 806,618 or U. S. Pat. No. 3,257,230. Some of the chromium from such complex packs diffuses along with the aluminum into the surface being coated, but the extra chromium is not significant where that surface already contains a large content of chromium. For low temperature aluminizing, that is where aluminum diffusion is carried out at 1,300F or below, more uniform coatings are obtained when nitrogen is excluded, as by using an activator such as aluminum chloride that does not liberate nitrogen. Manganese can also be added to analuminum diffusion coating pack for coating ferrous metals such as stainless steels, and it helps provide a coating that is protective against marine forms of corrosion, expecially when such coating is covered by a chromatetype of coating. Even without the manganese addition very good protection is obtained.
- Turning now to the drawings, the apparatus of P16. 1 has been found very effective for diffusion coating in one step more than 40 milligrams chromium in each square centimeter of the surface of TD nickel jet engine burner rings, and to produce a high quality dense coated surface with very few or no inclusions. This apparatus has a bell-type furnace shell 10 placed around an outer retort 12 also of bell type and with its open bottom set on a sealing strip 14 over a retort base 16 embedded in a concrete floor 18. The outer rim 20 of the base is a water-cooling jacket to protect the sealing strip 14, and the center 22 of the base provides a bottom closure for the outer retort 12. Base center 22 has conduits 31, 32, 33 sealed through it for passing gas flushing lines 41, 42 as well as thermocouple leads 43, and can be made hollow so that it can be pressurized with gas and connected to a pressure gauge that indicates when any leak develops.
Above the retort base 16 there is placed a spacer 50 which can merely be in the form of a spider-like set of welded-together metal walls radiating from a center and spaced from each other to permit passage of the gas flushing and thermocouple lines. Above the spacer is mounted a plurality of inner retorts 51, 52 and 53 each in the form of an annulus with an open top 59. These inner retorts are shown as having cylindrical outer and inner walls 61 62 and an annular bottom sheet 63, the parts being dimensioned so that the jet engine deflector rings will be received in them. Such rings can have a wall thickness of about 40 mils with diameters of from about 10 to about 30 inches, and for such sizes the diameter of inner cylindrical wall 62 should be significantly less than 10 inches while the diameter of the outer cylindrical wall 61 should be significantly greater than 30 inches. The upper edges of these inner retorts have grooves or notches 67 that can act as vents, and the inside height of an inner retort between its floor 63 and the bottom of notches 67 is at least 15 inches, preferably at least 18 inches. lt is also helpful to have a loosely fitted cover 71 over each inner retort.
A chromium diffusion operation is carried out by loading the inner retorts with the burner rings packed in a chromium diffusion pack such as one containing by weight 20 percent chromium powder, 3 percent nickel powder, 3 percent cobalt powder, 0.5 percent ammonium chloride powder, the balance being alumina powder. The packed retorts are then stacked on the spacer 50, the outer retort l2 lowered over them, a stream of argon started through the flushing line 41, and the furnace shell 10 then placed around the assembly.
When the air in the retorts has been displaced by the argon the furnace is started and the flushing line 41 switched from an argon supply to a supply of dry hydrogen. The flushed gases exiting through line 42 can then be lit where they discharge from the exit conduit. The
-rsts ls a brsss t "P. r ysth tvtnsss t N to 2,200F and kept there for 20 to 30 hours under the control of one or more thermocouples. The heat is then shut off and the assembly permitted to cool. When the temperature in the inner retorts drops below 300F the hydrogen flush is replaced by an argon flush. After the hydrogen is flushed out the retorts can be opened and unloaded. The coated burner rings can then be given a light dry blast with 100 grit aluminum oxide at 20 to 30 pounds per square inch, to clean up their surfaces, leaving them very smooth with a surface chromium content about 35 to 40 percent by weight and a chromium pick-up of about 50 milligrams per square centimeter of surface. Little or no alpha chromium phase and inclusions are present in the coating, and it is highly suited for receiving an aluminum diffusion coatmg.
The foregoing results are obtained with inner retorts 15 inches high. However when the heights of these retorts are inches and the process otherwise identical, the pick-up of chromium is only about half the foregoing amount. With retort cups deeper than inches the chromium pick-up is slightly higher than with the 15 inch depths. The coating temperature should be at least 2,l50F to keep the coating time from being too long. After about 22 hours at 2,l50 to 2,200F the coating rate diminishes sharply and further treatment tends to post-diffuse the deposited chromium rather than deposit significantly more chromium.
With the cobalt-nickel-chromium pack mixture the coating temperature need not be any higher than 2,150F to obtain best results. When nickel is used in the pack without cobalt, it is preferred to have the coating carried out at 2,200F. Unless nickelis present in the pack in an amount at least about one-twentieth the weight of the chromium, the diffusion-coated case is apt to contain undesirable inclusions.
The presence in the coating pack of about I to 10 percent metallic iron by weight speeds up the chromium deposition, and permits the use of coating temperatures 25 to 100F lower than indicated in the above example. However the coating then tends to be somewhat rougher and also contains some inclusions. The iron can be added in the form of pure metal such as carbonyl deposited iron powder, as ferrochrome or Ni-Fe or Cr-Ni-Fe alloy.
Varying the chromium content of the pack from about 5 to about 40 percent changes the surface chromium content of the coatings but does not detract appreciably from the quality of the coatings. Varying the nickel content between about one-twentieth to about one-third of the chromium by weight generally shows that the coating has fewest inclusions when the nickel content is between about one-tenth to about one-fourth of the chromium by weight. However better oxidation resistance and a faster coating rate is provided by the added presence of cobalt in an amount from about onetwentieth to about one-fifth the weight of the chromium. Because of such faster coating the coating temperature can be 50F lower with the cobalt-containing packs than with corresponding cobalt-free packs. The cobalt diffuses into the coating with the chromium, when both are present in the pack, and its presence in the coating is evidenced by the typical beige color of cobalt aluminide which appears after the subsequent aluminizing. The alumina in the pack can be replaced by any other inert filler such as kaolin, magnesium oxide or C 0 and such fillers can be used singly or in any combination. The coating step can also be performed in other types of equipment, so long as the minimum retort height is observed.
TD nichrome containing about 20 percent chromium, 2% Th0 the balance nickel, can be similarly diffusion coated with chromium. Good results are obtained with the TD nichrome when the chromium pickup in the coating is only about 30 to about 40 milligrams per square centimeter. However, the diffusion of chromium into the TD nichrome is slower than in TD nickel so that about the same treating temperatures and times are used. Other forms of dispersion-strengthened nickel and nichromes such as zirconia-strengthened and hafnia-strengthened nickel show the same type of improvement when coated in the above manners.
activator that greatly speeds up the coating operation.
Any other activator such as ammonium iodide, ammonium bromide, ammonium fluoride, ammonium bifluoride, elemental iodine, elemental bromine, hydrogen bromide, or the higher aluminum halides (chloride, bromide and iodide) can be used individually or in any combination, and in concentrations from about 0.05 percent to about 1 percent by weight. With a pack that is not perfectly anhydrous, the minimum activator content should be 0.1 percent.
The pack, or at least those pack ingredients that remain solid during the coating, is of relatively fine particle size. The maximum particle size is desirably microns and preferably less than 40 microns, although the activator and scavenger particles can be up to about 1 millimeter in size without detracting from the quality of the coating. Best results have been obtained with metal particles less than 10 microns in size and with filler particles up to about 40 microns in size.
The chromium diffuion packs also give better results for the second and subsequent coating treatments after they are freshly mixed. lf desired the freshly mixed packs can be subjected to a blank run without work pieces before they are placed in service. Used packs are simply reused with the addition of another charge of activator so long as the metal content is adequate. About 1% to 2 percent chromium can also be added to used packs to keep the metal content substantially unchanged through successive coating runs. Where cobalt is a pack ingredient substantial amounts are consumed by the coating and should be replaced as by adding one-eighth to one-half percent after each coating run. Any nickel additions should be kept very low inasmuch as there is very little nickel consumed during the coati W M. W "a, a
It is not necessary to use a hydrogen flush or hydrogen atmosphere during the diffusion. An argon flush maintained throughout the coating treatment also gives good results although it is more awkward to monitor the argon flow because it does not burn. Indeed no flush whatever is needed in which case the outer retort can be sealed against its cover as by a molten glass seal.
The YSFeEbihEEhmmium coating operation adds enough chromium to the work piece surface to significantly increase its height. At locations such as attach- .ment sites and the like where the greatest dimensional accuracy is needed, the work pieces can be masked as by a layer or pocket of a mixture of equal parts by weight inert filler and nickel powder.
FIG. 2 illustrates one masking arrangement in the coating of jet burner nozzles 100 having threaded shanks 102 which are to remain uncoated. In an inner retort cup 104 there is placed a layer 106 of masking compound deep enough to receive the entire shank 102 of a nozzle. This layer can be a mixture of to 60 percent nickel powder and 40 to 90 percent alumina or other inert filler, by weight. Over layer 106 another layer 108 of metal-free inert filler is placed, this being a thin layer to keep particles from sintering in a groove 103 in the nozzle sidewall. Another thin layer 110 having a nickel-containing composition like that of layer 106 tops off the masking combination.
A group of nozzles 100 is first pressed with th eir threaded shanks down into the layer 106, layer 108 is sprinkled and rolled over the surface of layer 106 and then covered with layer 110 until the shanks are completely submerged, leaving the upper portions of the nozzles protruding up from layer 110. There is then poured-in a layer 112 of diffusion coating material that completely covers the nozzle tops.
Where the nozzles are TD nickel or TD nichrome and a heavy diffusion coating of chromium is desired, the retort cup 104 should be at least l5inches high or should be set into retort cups at least inches high as explained above. With other materials or for lighter coatings the retort cup need only be shallow enough to hold a single layer of nozzles. Layer 108 of FIG. 2 is free of metal and used where the work pieces have recesses that might be difficult to clean up after the coating.
Where the nozzles are not subjected to an extremely Even where the work pieces to be masked are nickelbase or cobalt-base superalloys, it is sometimes desirable to use a thin layer such as layer 110 of masking mixture containing only nickel and filler at the edge of the masked area adjacent the area to be coated. The diffusion coatings ordinarily applied have a substantial ability to throw a short distance and it only takes about a H152 to about a 1/16 inch layer of such nickelfiller mixture to provide a sharp coating boundary and keep the diffusion coating from throwing where it is not wanted. The nickel-tiller layer might have a tendency to weaken the superalloy but the extreme thinness of such a barrier layer minimizes the weakening effect.
After the heavy chromium coating above has been completed and the work piece surface thus coated cleaned up, the material is ready for the diffusion coating of aluminum. This is readily accomplished with an aluminum-containing pack such as a mixture of 50 to 98 percent alumina and 2 to 50 percent aluminum by weight, using a coating temperature of 800 to l,l00F and a coating time of about 4 to about 24 hours. Such aluminizing is an alternative to the more complex aluminizing referred to above as described in Canadian Pat. No. 806,618 and U. S. Pat. No.
3,257,230. The more complex type of treatment is preferably conducted at l,450 to 1,550F for about 6 hours when it follows the above chromizing.
A good form of simple aluminizing is accomplished with a pack consisting of percent alumina and 30 percent aluminum, both 325 mesh, activated with A percent aluminum chloride, using a coating temperature of 850 F for 20 hours. Another good example of a pack contains percent of.the alumina and 20 percent of the aluminum powder, with'the same activator in the same concentration, used at 800 F. It is particularly desirable to keep the temperature below 900 F during this coating treatment. There is no minimum retort height preference for aluminizing, but the aluminizing can be carried out in the apparatus of FIG. 1, or in any other form of diffusion-coating apparatus. The coating produced by a simple aluminizing pack gives better results when the pack has been previously used in a coating run. It is accordingly helpful when starting with a fresh pack to give it a break-in treatment with a dummy work piece, or even with no work piece at all.
The simple aluminizing described above does not produce a consistently uniform coating when an ammonium halide is used as the energizer and the material being coated is an age-hardenable or a martensitic stainless steel. The lack of uniformity appears to be due to the presence of nitrogen in the retort atmosphere during the coating, and the resultant erratic formation of nitrides. The aluminum chloride energizer does a good job of flushing out residual air without introducing nitrogen, but other energizers such as elemental iodine and bromine, iodine trichloride or similar nitrogen-free halogen compounds including other higher halides of aluminum (chloride, bromide or iodide), halides of silicon, colombium, titanium, boron, zirconium, hafnium, tantalum, chromium, molybdenum, tungsten, iridium, osmium, platinum, gallium, germanium, tin and phosphorus will do the same although they are not preferred. Whichever energizer is used is preferably in an amount from about 0.1 to about 1 percent of the pack weight. Also better results are obtained if the unvaporized energizer is isolated from the work pieces as by enclosing all the energizer in a container that permits the escape of vapor. A container for this purpose can be made of fine screening or with an open top or with a loosely fitted top and several of such containers can be distributed throughout the mix. Such a container or containers can be embedded in the coating pack and will release vapors of energizer as the pack is heated up to coating temperature, such vapors accomplishing the same flushing and depositaccelerating results expected of an energizer, but without the coating flaws experienced when solid aluminum chloride is mixed into the entire pack. The container holding the energizer can be made of plain carbon steel or other suitable metal such as aluminized steel or low alloy chromium steel, or even martensitic stainless steel. The retort itself can also be made of any of the foregoing materials.
As an alternative packing technique all the energizer can be confined to a stratum of the pack below the work pieces, with the remainder of the pack being a uniform mixture of filler and diffusing material. Thus good results are obtained when the diffusion retort is first packed with about a k to 1 inch deep layer of the pack material, all the energizer is then sprinkled over that layer, another 1 inch deep layer of energizer-free pack placed over the foregoing, and the retort then filled with work pieces and additional pack. However it is simpler to pack the retort with the separately contained energizer, and work pieces cannot be inadvertently inserted in such a separately contained energizer. Should a work piece be accidentally pushed into the separately stratified energizer of the alternative packing technique, a good coating will not form on the portion of the work piece that has penetrated into that stratum.
In general the simple aluminizing as well as the more complex aluminizing are effectively used to cause an aluminum pick-up of about 0.5 to 7.5 milligrams per square centimeter of surface coated, giving a coating case about 0.1 to about 1.5 mils thick. A preferred pick-up range is from about 1 to about 5 milligrams per square centimeter. The coating packs used can be replenished as by adding 1 percent aluminum after every use, even after a break-in use.
A useful diffusion aluminizing of stainless steel and chromium steels is also effected by incorporating with the aluminum about one-fourth to three-fourths metallic manganese calculated on the weight of the aluminum. Thus a diffusion coating pack of 60 percent alumina, 30 percent aluminum and 10 percent manganese will give at 875 F over a period of 10 hours an alumi' nized coating on age-hardenable or martensitic stainless steels that provide good protection, particularly against marine-type corrosion. Type 410 stainless steel jet engine compressor blades or gas generator housings given a 1 mil thick coating case from a manganese-free aluminum pack at temperatures from 800 to 1,l F will however withstand corrosion in salt air for a particularly long period of time.
A chromate-type coating applied over the manganese-containing aluminum diffusion coating or the manganese-free aluminum coating, further increases corrosion resistance. A particularly effective chromatetype coating for this purpose is one that is made by dipping the aluminized compressor blade after vapor honing to clean the surface, into an aqueous solution of phosphoric acid and chromic acid containing per liter about to 100 grams phosphoric acid and about 1 to 25 grams chromic acid, removing the dipped blade and permitting the solution to drain, followed by calcining the blade with the residual coating solution thereon at 800 F for minutes. So-called conversion coatings such as described in U. S. Pat. No. 3,385,738 are not sufficiently protective at elevated temperatures, that is at about 800 F or higher. The chromic acidphosphoric acid coatings of U. S. Pat. application Ser. No. 90,682 filed Nov. 18, 1970, with or without the related treatments there disclosed are much better in this respect and provide protection at temperatures that reach as high as 1,200F.
Very effective results on aluminized greek ascoloy are obtained with l0 to grams CrO and 57 grams orthophosphoric acid per liter, the calcining being at 600 F for 40 minutes. In general calcining temperatures can vary from about 450 to about 900 F, and should be long enough to cause the chromate-type coating to become almost completely (at least about 90 percent insoluble in water.
Even better results are obtained with age-hardenable and martensitic stainless steels when the chromate-type coating also contains magnesium as well as particles of polytetrafluoroethylene, as in the following example:
EXAMPLE 180 grams CrO 130 grams MgO 410 cc 85% l-l PO (by weight in water) 15 cc aqueous dispersion of polytetrafluoroethylene particles less than 1 micron in size, containing 13.5 grams of the resin, and
Water to make up 3 liters of coating bath.
The MgO dissolves in the acid and remains dissolved upon dilution, while the resin particles remain undissolved but dispersed. If the stock resin dispersion is sensitive to acid, it is added after the MgO is dissolved inasmuch as this sharply lowers the acidity. The resin particles need not be very stably dispersed, although such stability is improved through the use of a small amount of a dispersing agent that is not sensitive to acid or oxidizers. Non-ionic surface-active agents or quaternized imidazoline surface-active agents such as N CH; R-PJ Ik-CIhCOOH are suitable for this purpose. In any event the bath can be agitated to assure uniformity of dispersion.
Dipping an aluminized or uncoated greek ascoloy compressor blade in the bath of this example at room temperature, followed by oven heating at 700 F for minutes provides a cured coating weighing about 0.27 milligrams per square centimeter that gives excellent protection in marine environments.
The ingredients of the bath of the foregoing example can vary as follows:
Magnesium 0.4 to 1.7, preferably 0.9 to 1.4 mols per liter Chromate ion 0.2 to l, preferably 0.4 to 0.8 mols per liter Phosphate ion 0.7 to 4, preferably 1.5 to 3.5 mols per liter Resin 2 to 14, preferably 4 to 10 g per liter Coating weights above about 0.5 milligram per square centimeter tend to craze, and below about 0.2 milligram per square centimeter are not as effective although as little as 0.05 milligram of coating per square centimeter givesnoticeably improved corrosion resistance. This improvement increases with increased coating weight, and two coats can be used if desired, as by going through a second such coating treatment after a first coating is applied and cured, to make a total chromate-type coating weight of about 1 milligram or more per square centimeter.
Phosphorous acid and other phosphorus acids like pyrophosphoric acid can be substituted for part or all of the orthophosphoric acid without significantly lowering the effectiveness of the chromate-type coating.
The foregoing chromate-type coatings greatly prolong the useful lives ofjet engine compressor blades of martensitic stainless steels that have a simple aluminized case, particularly in the salty air of marine use where higher temperatures are encountered. Other ferrous metals such as austenitic stainless steels and even low alloy and plain carbon steels that have aluminum diffusion cases are also made much more resistant to marine corrosion by the foregoing chromate-type coating. However with low alloy and plain carbon steels the resistance to marine corrosion is further increased if before the aluminizing the metal is given a chromium or mixed chromium-nickel diffusion case or even a chromium plating. ln general it is preferred for the ferrous-surface to contain at least percent chromium before aluminizing, but sharply improved results are obtained with AlSl 4140 steel or when as little as l percent chromium is present in the surface of a steel.
Applyingdiffusion coatings on some high strength metals like type 410 stainless steel can cause loss of strength, particularly if the coating is applied at l,000
. F or higher, and the coated material is slowly cooled fected by the process disclosed in U. S. Pat. application Ser. No. 159,175 filed July 2, 1971.
First stage turbine vanes of cobalt-base or nickelbase superalloys that have been aluminized with either the complex or simple aluminizing, are also improved by heating in air at 2,050" to'2,l00 F until their surface is uniformly whitened, generally about 10 to 20 hours, then glass blasting after cooling to remove the white skin (which appears to be aluminum oxide). The resulting vane looks very much like the untreated aluminized vane',.except for a loss of some color'where the vane is a cobalt-base superalloy like WI 52, but the aluminized case is more adherent and less subject to spalling and the like on handling. Where the case is damaged it can be stripped off by the processes described in U. S. Pat. Nos. 3,622,391 granted Nov. 23, 1971 and 3,458,353 granted July 29, 1969. Those processes are further improved by modifying the aqueous HF-HNO; stripping baths thus used so that CrO is also present in those baths. A solution of g HF, 80 g HNO and 5 g CrO in 920 g water makes a very effective stripping bath for this purpose when used at 85 F. The aluminized case dissolves in the bath and the base metal is not significantly attacked. Polished surfaces of the substrate survive the stripping bath treatment without much loss of polish.
The Cro -containing stripping baths can also contain other ingredients that do not detract from its effectiveness. Ammonium, alkali metal and alkaline earth metal cations as well as acetate and phosphate anions are examples of such allowable addition to the baths. In general the HF content can vary from about 0.1 to 5 percent by weight, the HNO content 3 to percent by weight, and the ratio of HF to CrO from 15:] to 1:5 by weight. Preferred ranges are HF 0.5 to 3% HNO 5 to 15% HFzCrO 7:2 to 1:2 all calculated by weight. All of the stripping baths work well at from 50 to 140 F.
Any attack on the superalloy base caused by the CrO -containing or CrO -free baths is further minimized by keeping the work piece being stripped in contact with metallic nickel or cobalt. Thus the stripping can be carried out by placing the work pieces to be stripped and the stripping bath in a nickel-surfaced container, or holding the work pieces in a nickel wire mesh basket while they are dipped in the stripping bath.
Pickling inhibitors can also be added to the stripping bath.
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.
What is claimed:
1. In the process of diffusion coating articles of dispersion-strengthened nickel or dispersion-strengthened nickel alloys containing about 20% chromium, with a deep layer rich in chromium from a diffusion coating powder pack in which the coating is effected with the pack held in an unsealed retort cup at least l5 inches high, and the pack contains about 5 to about 50% chromium as well as nickel in a proportion from about onethird to about one-twentieth of the chromium.
2. The combination of claim 1 in which all the pack ingredients that remain solid during the coating have particle sizes smaller than microns.
3. The combination of claim 1 in which the metalingredients of the pack have particle sizes smaller than 40 microns.
' 4C Th e contain-anon of clainii'inwhi h the backin contains cobalt in an amount from about one-twentieth to abOutpne fifththe weight of the chromium.
5. In the process of protecting articles of dispersionstrengthened nickel by diffusion coating them with chromium and then with aluminum, the improvement according to which the chromium coating is applied as a single coating step inaccordance with claim 1.
The combination of claim 1 in which the pack also contains cobalt in an amount from about one-twentieth to about one-f fth the weight of the chromium.
9. Dispersion-strengthened nickel which has been coated bythe process of claim f8,
l0. ln the process of protecting articles of dispersionstrengthened nickel or dispersion-strengthened nickel alloys containing about 20 percent chromium by diffusion coating them with chromium and then with aluminum, the improvement according to which the diffusion coating with chromium is effected with a pack containing about 5 to about 50 percent chromium, nickel in a proportion about one-third to about onetwentieth of the chromium, and cobalt in a proportion about one-fifth to about one-twentieth of the chromium.
11. The product produced by the process of claim l0.
12. In the process of protecting dispersionstrengthened nickel articles by diffusion coating them with chromium, the improvement according to which the diffusion coating with chromium is effected with a pack containing about -5 to about 50 percent chromium, nickel in a proportion about one-third to about one-twentieth of the chromium, and cobalt in a proportion about one-fifth to about one-twentieth of the chromium.
/ l3. The product produced by the process of claim 12. |K 1