Title:
Corrosion resistant coating system for ferrous metal articles having brazed joints
United States Patent 3922396


Abstract:
Ferrous metal articles, such as stainless steel aircraft components, e.g. turbine vanes, vane/shrouds and the like, characterized by at least one brazed joint, in which the braze is a non-ferrous alloy are aluminized by first uniformly plating onto the entire surface of said articles including the braze, a coating of an aluminide-forming metal selected from the group consisting of iron, nickel, cobalt and chromium and then thermally diffusing aluminum over the entire surface of the article, including the braze, such that a sacrificial corrosion resistant coating is produced characterized by the presence of an aluminide compound selected from the group consisting of iron aluminide, nickel aluminide, cobalt aluminide and chromium aluminide. The aluminide coatings are further enhanced by the application of a non-metallic coating, e.g a conversion coating.



Inventors:
Speirs, Kenneth K. (San Antonio, TX)
Weinstein, Martin (San Antonio, TX)
Dean, Michael F. (San Antonio, TX)
Application Number:
05/509049
Publication Date:
11/25/1975
Filing Date:
09/25/1974
Assignee:
CHROMALLOY AMERICAN CORPORATION
Primary Class:
Other Classes:
148/273, 148/528, 205/194, 205/197, 205/228, 427/250, 427/295, 427/304, 427/328, 427/405
International Classes:
C23C10/02; (IPC1-7): B44D1/14; C03C7/02; C23C9/02; C23D5/04; C23F17/00
Field of Search:
117/71M,13E,17.2P,7C,7S,7A 148
View Patent Images:
US Patent References:
3837894PROCESS FOR PRODUCING A CORROSION RESISTANT DUPLEX COATING1974-09-24Tucker, Jr.
3811959N/A1974-05-21Weinstein et al.
3640815METHOD FOR SURFACE TREATMENT OF NICKEL AND COBALT BASE ALLOYS1972-02-08Schwartz et al.



Primary Examiner:
Rosdol, Leon D.
Assistant Examiner:
Varndell, Ralph E.
Attorney, Agent or Firm:
Hopgood, Calimafde Kalil
Parent Case Data:


This is a division of copending application Ser. No. 353,800, filed Apr. 23, 1974, now U.S. Pat. No. 3,759,061.
Claims:
What is claimed is

1. A method of aluminizing a brazed ferrous metal article characterized by at least one brazed joint, the braze of said joint being formed of a non-ferrous brazing alloy of melting point ranging from about 1125°F to 1925°F which comprises:

2. The method of claim 1, wherein said non-ferrous brazing alloy is either a copper-base, silver-base or gold-base alloy, and wherein said brazing alloy has a melting point ranging from about 1175°F to 1850°F.

3. The method of claim 1, wherein said coating of aluminide-forming metal is obtained by plating from an aqueous bath.

4. The method of claim 1, wherein following the production of the thermally diffused aluminum surface, a non-metallic coating is applied to said surface comprising the steps of applying a coating of a solution of a soluble silicate salt selected from the group consisting of sodium silicate, potassium silicate, lithium silicate and ethyl silicate at a temperature ranging up to 100°C, removing excess liquid from said surface so as to form a uniform layer of said silicate salt covering said surface and then drying and curing said non-metallic layer, said curing being carried out at a temperature of about 150°C to 430°C.

5. The method of claim 4, wherein the step of applying said silicate layer is repeated a plurality of times and dried and wherein the layers are thereafter cured by heating at a temperature of about 150°C to 430°C.

6. The method of claim 4, wherein following the formation of silicate coating on the aluminum surface, a conversion coating is applied comprising the steps of applying a solution containing by weight about 5% to 30% of phosphoric acid, about 3% to 8% chromic acid (CrO3), and at least about 0.5% of at least one chromate and phosphate forming metal and the balance essentially water, drying said conversion coating and then thermally curing said conversion coating.

7. The method of claim 6, wherein said chromate and phosphate-forming metals are aluminum in the amount of about 0.02% to 3% and magnesium in the amount of about 0.75% to 6%.

Description:
This invention relates to the protection of ferrous metal articles against corrosion in highly saline and/or marine or other corrosive environments by employing thermally diffused aluminum as the main sacrificial coating, the invention being particularly applicable to ferrous metal articles, such as stainless steel articles, characterized by at least one brazed joint, wherein the braze is a non-ferrous alloy. The invention also relates to the production of aluminized coatings on ferrous metal articles comprising brazed joints, in which the aluminized layer also has a barrier-type non-metallic overcoat.

FIELD OF THE INVENTION

Jet and gas turbine engine compressor components are subject to corrosion in highly saline environments at the air intake end of the engine and also to the direct impact of abrasive particulate matter, such as coral dust. Additionally, the compressor blades are subjected to tremendous mechanical stresses from centrifugal forces, thermal shock, vibration and other sources of stresses. Thus, corrosion can accelerate catastrophic failure, since pits and other corrosion defects can act as stress raisers.

High strength ferrous alloys are employed in the construction of compressor blades and other aircraft engine components such as vane/shrouds (e.g. Society of Automotive Engineers alloy designation AMS 5508, AMS 5616, AMS 6304 and others), but, because of their low resistance to saline corrosion, they are generally provided with a protective surface treatment. One in particular is the provision of an aluminum-base diffusion coating on the ferrous substrate by pack-aluminizing at coating temperatures ranging up to 1000°F and preferably not higher so as to avoid undesired crystallographic or metallurgical changes in the substrate during coating which might have an adverse or undesired effect on the mechanical property of the parts. Such coatings have provided advantageous oxidation and erosion resistance and have minimized the production of pulverous corrosion by-products and have been very useful in extending the operating life of jet engine components.

However, where the jet and gas turbine engine components are comprised of welds and/or at least one brazed joint, such as vane/shroud assemblies in which the braze is a non-ferrous alloy based on a metal selected from the group consisting of copper, silver and gold, e.g. copper-, silver-, and/or gold-base brazing alloys, certain problems arise in the production of a uniform aluminized coating having requisite physical characteristics. The braze, being markedly different than the ferrous metal surface, reacts differently during the diffusion of aluminum therein. The extent of diffusion of aluminum in different materials is governed mainly by the aluminide phases that are formed which act as diffusion barriers. Generally speaking, the lower the melting point of a particular braze alloy, the deeper is the diffusion of aluminum into it. Moreover, some metal substrates do not provide good adherent aluminide coatings.

Thus, in the case of aluminum diffusion in a brazed component in which stainless steel elements are brazed together to form the joints, the aluminum case depth on the stainless steel element might be 0.0005 inch, whereas the aluminum may penetrate as much as 0.007 inch deep in the brazed portions of the joints for a braze alloy comprising by weight 50% Ag, 18% Cd, 16% Zn and 16% Cu; or as much as 0.002 inch in a braze composition comprising by weight 54% Ag, 40% Cu and 6% Zn. The ratio of penetration between the braze metal and stainless steel for example, may vary between 3:1 to 15:1, depending upon the braze composition.

Mechanical tests have indicated that the uncontrolled diffusion of aluminum into the braze alloy tends to degrade the brazed joint as evidenced by fatigue test data.

Thus, it would be desirable to provide a method of controlling the diffusion at the brazed joint commensurate with the diffusion at the unbrazed portion of the component such as to insure uniform properties over the whole surface of the component.

OBJECTS OF THE INVENTION

It is thus the object of the invention to provide a sacrificial coating of aluminum on ferrous metal parts characterized by the presence of at least one brazed joint comprising a non-ferrous brazing alloy and wherein the properties of the coating are substantially uniform at both the unbrazed areas and the brazed joint.

Another object of the invention is to provide a method of aluminizing a ferrous metal part having at least one brazed joint, wherein the entire metal part including the braze is coated with an aluminide forming metal selected from the group consisting of iron, nickel, cobalt and chromium prior to aluminizing of the part such that the fatigue properties at the joint are maintained at acceptable levels or are enhanced.

A further object is to provide, as an article of manufacture, an aluminized ferrous metal part, characterized by at least one brazed joint of non-ferrous metal or alloy, the entire metal part being first provided with a layer of aluminide-forming metal, such that the aluminum coating on the article including the braze is characterized by the presence of a substantially uniform layer of an aluminide compound selected from the group consisting of iron aluminide, nickel aluminide, cobalt aluminide and chromium aluminide.

These and other objects will more clearly appear from the following disclosure and the accompanying drawing.

DRAWINGS

FIG. 1 represents a vane/shroud assembly of stainless steel in which the vanes are brazed across the annular space between inner and outer rings making up the shroud;

FIG. 2 is a close-up of a fragment of the shroud showing the plurality of brazed joints making up the vane/shroud assembly;

FIG. 3 is a representation of a photomicrograph taken at 100 times magnification showing the contour of an aluminide coating based on an applied nickel plate followed by thermal diffusion therein of aluminum, the coating being disposed uniformly over the braze and along the steel surface;

FIG. 4 is an electron microprobe trace showing the distribution of nickel and aluminum in the coating with respect to the braze metal joint to which the coating adheres;

FIG. 5 is a representation of a photomicrograph at 500 times magnification showing a nickel-aluminum coating adjacent the steel substrate;

FIG. 6 is an electron microphone trace showing the distribution of the nickel and aluminum in the coating adhering to the steel substrate,

FIG. 7 is a representation of a photomicrograph at 500 times magnification showing a chromium-aluminum coating adjacent the steel substrate;

FIG. 8 is an electron microprobe trace showing the distribution of chromium and aluminum in the coating adhering to the braze;

FIG. 9 is a representation of a photomicrograph at 500 times magnification showing a chromium-aluminum coating adjacent to the steel substrate; and

FIG. 10 is an electron microprobe trace showing the distribution of chromium and aluminum in the coating adhering to the steel substrate.

STATEMENT OF THE INVENTION

One aspect of the invention resides in a method of aluminizing a brazed ferrous metal article, such as an aircraft engine component comprised of at least one brazed joint. The brazing material in the joint is made of a non-ferrous brazing alloy having a melting point ranging from about 1125°F (607°C) to 1925°F (1052°C), such as an alloy based on at least one metal selected from the group consisting of copper, silver and gold. Because the aluminum tends to diffuse more deeply into the non-ferrous brazing alloy than in the steel substrate, the method resides in coating the entire article including the braze with an aluminide-forming metal and then thermally diffusing the aluminum, preferably by pack cementation, into the entire surface of the ferrous article thereby producing a substantially uniform coating of aluminum over substantially the entire surface, wherein the thermally diffused aluminum coating is characterized by the presence of a substantially uniform layer of an aluminide compound selected from the group consisting of iron aluminide, nickel aluminide, cobalt aluminide and chromium aluminide. Fatigue tests on cantilevered specimens have indicated that the fatigue properties of the aluminized surface at particularly the braze are comparable to uncoated specimens and superior to aluminized brazed specimens.

The foregoing method is also applicable to the coating of ferrous metal articles which uses a protective duplex coating system, that is, a system in which following the production of a thermally aluminized coating on the ferrous metal substrate, a non-metallic silicate overcoat is applied to the aluminum coating which is highly adherent. With respect to the non-metallic overcoat, reference is made to co-pending application Ser. No. 143,842, filed May 17, 1971. The disclosure of said application relating to the production of such non-metallic overcoats on aluminized ferrous metal surfaces is incorporated herein by reference.

As stated hereinbefore, the non-ferrous brazing alloys which present the problem of uncontrolled diffusion of aluminum therein include those which are based on one or more metals selected from the group consisting of copper, silver and gold. When the expression "based on one or more metals of the group copper, silver and gold" is employed, what is meant is that the nonferrous brazing alloy contains at least one of the foregoing copper group metals as a main ingredient, with one or more of other non-ferrous metals making up substantially the balance, such as zinc, nickel, palladium, cadmium, tin, manganese and the like.

Apparently, the lower the melting point of the braze alloy, the deeper is the aluminum diffusion into it. Thus, it may be stated generally that the non-ferrous brazing alloys include those having melting points ranging from about 1125°F (607°C) to 1925°F (1052°C) and preferably 1175°F (635°C) to 1850°F (1010°C). As will be understood by those skilled in the art, the non-ferrous brazing alloy must have a lower melting point than the metal substrates being joined and yet must be high enough to resist softening at elevated temperatures to which the aircraft part (e.g. vane/shroud assemblies) is subjected during use.

Examples of non-ferrous brazing alloy compositions commonly employed for producing brazed joints in aircraft components made of ferrous metals, e.g. stainless steel are by weight as follows: (1) 50% Ag, 18% Cd, 16% Zn and 16% Cn [designated as AMS4470 C]; (1)54% Ag, 40% Cu, 6% Zn and up to 1% Ni [designated as AMS 4722B]; (3) 82% Au, 18% Ni [designated as PWA 698], (4) 54% Ag, 25% Pd and 21% Cu [designated as PWA 706], and (5) 55% Cu, 35% Mn and 10% Ni, among many others. The diffusion of aluminum in alloy (1) may proceed up to a depth of 0.007 inch and in the others up to 0.002 inch. Such diffusion by aluminum into the braze has a weakening effect on the joint and adversely affects its resistance to fatigue.

Illustrative brazing alloy compositions range in weight percent (for brazing stainless steel) including their solidus, liquids and brazing temperatures are as follows:

Example Temperature °F Nos. % Ag % Cu % Zn % Cd % Ni Solidus Liquidus Brazing __________________________________________________________________________ 1145- 1A 44 - 46 14 -16 14 - 18 23 - 25 -- 1125 1145 1400 1175- 2A 49 - 51 14.5-16.5 14.5 - 18.5 17 - 19 -- 1160 1175 1400 1270- 3A 49 - 51 14.5-16.5 13.5 - 17.5 15 - 17 2.5-3.5 1170 1270 1500 1435- 4A 39 - 41 29-31 26 - 30 -- 1.5-2.5 1240 1435 1650 1370- 5A 44 - 46 29-31 23 - 27 -- -- 1250 1370 1550 1575- 6A 54 bal. 5 -- 1 1325 1575 1785 1325- 7A 60 bal. -- -- 10% Sn 1115 1325 1550 1610- 8A 92.5 bal. -- -- 0.2 Li 1435 1635 1800 __________________________________________________________________________

Another brazing alloy based on gold comprises 81.5% Au and the balance nickel. This alloy has a solidusliquidus temperature of 1740°F and is employed over a brazing temperature of about 1740°F to 1840°F.

Most stainless steels can be brazed by any one of several different filler metals, including silver-base alloys, gold-base alloys, copper-base alloys and the like. Stating it another way, the non-ferrous brazing alloy may contain at least about 40% by weight of the copper group metals and preferably at least about 50% and the balance non-ferrous alloying ingredients so long as the melting point of the alloy ranges from about 1125°F to 1925°F and preferably from about 1175°F to 1850°F. The melting point is generally taken as the liquidus temperature of the alloy. The minimum of at least 40% of the copper group metal is met where the alloy contains at least 40% Ag, or 40% Cu or 40% Au, or at least 40% of a combination of two or more of the foregoing copper group metals. As stated hereinbefore, the other non-ferrous alloying elements may comprise one or more of Zn, Cd, Ni, Sn, Mn, Pd and the like. Thus, the brazing alloy may contain 40 to 95% of at least one of the copper group metals and the balance at least one other non-ferrous metal.

When the foregoing brazing alloys are employed in the production of certain aircraft components, it has been found essential, in order to assure substantially uniform corrosion and oxidation resistant properties over the entire surface thereof, to coat said surface including the braze with an aluminide-forming metal prior to thermally aluminizing the whole component. Various methods may be employed in coating the component. A preferred method is electroplating. Another method is to deposit the metal from a chemical plating bath, such as a nickel hypophosphite bath, where nickel is the aluminide-forming metal.

In plating complex shapes like a vane/shroud component, an insoluble anode is employed which has a configuration corresponding to the overall shape of the shroud so as to assure substantially uniform throwing power of the bath to the complex surface being plated.

The plating metals selected are those which combine with aluminum to form thermally stable aluminides. Metals which are particularly preferred are iron, nickel, cobalt and chromium. The most useful systems are nickel or chromium plate followed by aluminum diffusion therein. The sacrificial behavior of nickel aluminide and chromium aluminide can be evaluated by salt spray testing and also indicated by EMF measurements.

For example, coated coupons of stainless steel with 1/4 inch strips of the coating removed have been placed in a salt spray cabinet operated in accordance with ASTM B117. Examination of the specimens have shown production of corrosion products on the coating with the base stainless steel protected against corrosion. As illustrative of the EMF readings obtained for various substrats using a calomel electrode in a 3% sodium chloride solution, the following data are given: Substrate EMF Reading (Volts) ______________________________________ 410 Stainless Steel -0.30 Chromium Aluminide -0.58 Nickel Aluminide -0.43 AMS 4770 Braze -0.14 AMS 4772 Braze -0.23 Aluminum Diffusion In AMS 4770 -0.68 Aluminum Diffusion In AMS 4772 -0.91 ______________________________________

The foregoing values indicated that chromium aluminide and nickel aluminide are both sacrificial (that is, anodic) relative to the stainless steel substrate. While the diffusion of aluminum into the braze indicates sacrificial properties; nevertheless, such diffusion adversely affects the fatigue properties at the brazed joint. However, by plating the whole component including the braze with an aluminide-forming metal, substantially uniform chemical and physical properties are assured for the treated component. The thickness of the applied aluminide-forming metal is at least about 0.0002 inch.

The plating of the entire surface including the braze is advantageous in that the results obtained can now be independent of the prevailing ferrous substrate being coated. For example, a ferrous substrate of one analysis might react differently with diffused aluminum as compared to a ferrous substrate of another analysis, such that the aluminized layer might have different characteristics. By coating the surface substantially uniformly with an aluminide-forming metal, such as nickel or chromium, the process is rendered independent of the metal substrate being protected.

As stated hereinbefore, the aluminide-forming metal may be applied by various methods, such as by electroplating, electroless plating from a chemical plating bath, vacuum plating from a vapor, etc. For example, nickel may be applied electrolytically from a sulfamate bath or from an electroless nickel bath. Following the plating or coating of the part with the aluminide-forming metal, the part, e.g. the vane/shroud component, is embedded in an aluminum-containing cementation pack comprising by weight, for example, 80% aluminum powder and 20% aluminum oxide having mixed therein about 2% of dry aluminum chloride.

One method comprises preparing an aluminizing pack comprised, for example, of 800 lbs. of -60+140 mesh aluminum powder blended with 200 lbs. of Al2 O3, also -60+140 mesh size. To the 1000 lb. mixture is added 20 lbs. of dry AlCl3 under a humidity preferably not exceeding 45%.

The pack is mixed in a vibrating blender under dry conditions for about 5 to 10 minutes. If the charge is a fresh charge, it is subjected to burn-out at 795°-825°F (425°C to 440°C) for 36 hours. However, where a charge has already been used and is recycled for another pack, burn-out is not required. The pack is placed in a dry condition in a retort with the vane/shroud component of AMS 5616 steel to be treated, the vane/shroud being completely embedded in the pack using vibration to fill in the spaces between the vanes. The cover is sealed to the retort body with multiple layers of aluminum foil in the form of a gasket sufficient to prevent air from getting in but to allow out-gassing of gaseous by-products.

The retort is placed in an oven at ambient temperature and the temperature allowed to rise to the desired coating temperature by the application of heat. As the temperature rises, it is preferred that it go through an endothermic arrest at about 350°F (176°C) due to vaporization of AlCl3 to effect further cleansing of the surface of the component of any oxide film thereon and then allowed to reach a temperature not exceeding about 1000°F (538°C), for example, a range of about 795°F to 825°F (425°C to 440°C) and the retort maintained at substantially that temperature range for about 36 hours. Upon completion of the heating cycle, the retort is removed from the oven and allowed to cool approximately to 400°F (205°C), after which it is placed in a dry environment for cooling to ambient temperature.

The cooled retort is then placed in a humidity control cabinet, the cover removed and the aluminum-coated vane/shroud taken out of the cementation pack. The part is cleaned of adhering coating compound by blowing with dry air and immersed in water to remove fine dust and other residues to provide a very clean aluminum deposit containing an iron aluminide intermetallic compound, such as FeAl3, on the braze-free portion of the component and a nickel aluminide intermetallic compound on the nickel-plated brazed portion of the component. The aluminized surface, like other thermally diffused aluminum coatings, is characterized by sacrificial properties in that it will corrode in preference to the ferrous substrate in saline environments and, therefore, substantially protect the ferrous substrate against corrosion.

Generally speaking, the pack composition may comprise by weight about 60% to 100% aluminum, about 1% to 5% dry AlCl3 and the balance essentially an inert refractory metal oxide, such as oxides of aluminum, magnesium, titanium, zirconium, etc.

An electroplating bath that may be employed in providing a ferrous metal component with a substantially uniform plate of nickel is as follows:

Nickel Sulfamate [Ni(NH2 SO3)2 ] -- 450 gpl* Boric Acid -- 30 gpl pH -- 3.0-5.0 Temp. -- 40°-60°C Current Density -- 2-30 Amps/dm2 *gpl is grams per liter?

Another bath comprises:

Nickel Sulfamate [Ni(NH2 SO3)2 ] -- 300 gpl Nickel Chloride (NiCl2.6H2 O) -- 6 gpl Boric Acid -- 30 gpl pH -- 3.5-4.2 Temp. -- 28°-60°C Current Density -- 2-25

With regard to plating whith nickel and other aluminide-forming metals, reference is made to the book "Modern Electroplating" edited by Frederick A. Lowenheim (published 1942, 1953, 1963 by John Wiley & Sons, Inc.).

A typical chromium plating bath is as follows:

(1) Chromic Acid (CrO3) -- 400 gpl Sulfate (SO4=) -- 4 gpl

The sulfate ion is employed as an acid radical catalyst. Another composition is:

(2) Chromic Acid (CrO3) -- 250 gpl Sulfate (SO4 =) -- 2.5 gpl

Most commercial baths contain 150 to 400 gpl of chromic acid. Thus, bright deposits are produced from bath (2) above at 40°C and cathode current densities of 3.1 amps/dm2 to 15.5 amps/dm2. A plating speed of about 1 mil (0.001 inch) per hour can be achieved.

One electroless plating bath for producing a substantially uniform coating of nickel over the whole surface of a ferrous metal part comprises the following:

Nickel Chloride (NiCl2.6H2 O) -- 25 gpl Sodium Hypophosphite (NaH2 PO2.H2 O) -- 25 gpl Sodium Pyrophosphate (Na4 P2 O7) -- 50 gpl pH -- 10-11 NH4 OH -- for neutralizing

A cobalt electroless plating bath is as follows:

Cobalt Chloride (CoCl2.6H2 O) -- 30 gpl Sodium Hypophosphite (NaH2 PO2.H2 O) -- 20 gpl Sodium Citrate (Na3 C6 H5 O2.5-1/2H2 O) -- 35 gpl Ammonium Chloride (NH4 Cl) -- 50 gpl pH -- 9-10 NH4 OH -- for neutralizing

A stainless steel part that has been successfully treated in the foregoing manner is the vane/shroud component shown in FIGS. 1 to 3. FIG. 1 shows a vane/shroud 10 of AMS 5616 steel comprising inner and outer rings 11 and 12, respectively, with vanes 13 brazed therebetween.

The brazed joints 14 are clearly shown in FIG. 2. FIG. 3 shows the steel substrate 15 as well as the fillet 16 of the braze at the joint coated uniformly with nickel by electroplating into which nickel coating 17 has been diffused with aluminum to form a protective layer of substantially nickel aluminide in coating 17. In FIG. 3, an overplate 18 of nickel and copper is shown which is added to the specimen to preserve the coating during mounting and polishing of the specimen.

FIG. 4 is a microprobe trace of the coating of FIG. 3 showing a coating up to approximately the braze interface of thickness of about 0.0008 inch. As will be noted, the outer 0.0005 inch thick coating contains about 36.7% Al and about 50 to 57.5% nickel, the nickel aluminide indicated being Ni2 Al3. The small amount of copper in the coating is the result of outward diffusion from the braze. The aluminum did not penetrate into the braze.

The micrograph of FIG. 5 at 500 times magnification adjacent the steel substrate 15 shows more clearly the nickel aluminide layer 17A adjacent unreacted nickel layer 17 next to the steel substrate.

The microprobe trace of FIG. 6 is taken across the coating to the steel substrate, certain portions of the trace being indicated with the approximate composition of the diffused coating. It will be noted that the whole coating thickness, including the unreacted nickel layer, is approximately 0.0008 inch.

FIGS. 7 to 10 are similar figures illustrating the use of chromium as an aluminide-forming metal; FIGS. 7 and 8 showing the effect of chromium at the braze and FIGS. 9 and 10 as to the steel substrate.

In FIG. 7 (500 times magnification) braze 18 is depicted showing a portion of unreacted chromium plate 19 thereon and a layer of chromium aluminide 20 (Cr4 Al9) adjacent the unreacted chromium. Referring to the microprobe trace of FIG. 8, the combination of the chromium aluminide layer and the unreacted chromium layer adjacent the brace provides a thickness of about 0.0007 inch. the composition of the aluminum and chromium at indicated portions of the coating is indicated, the aluminide layer containing approximately 53.3% aluminum, 42.7% chromium, 3% copper and the balance residuals.

The coating on the stainless steel portion of the article is shown in FIG. 9 (500 times magnification) as comprising the chromium aluminide layer 20A, unreacted chromium layer 19A on steel substrate. The various layers are indicated graphically in the microprobe trace of FIG. 10 together with the approximate analysis of each portion.

As will be noted from the foregoing, a substantially uniform sacrificial coating is provided over substantially the whole surface and particularly at the braze where the aluminum is inhibited from diffusing into the braze and thus adversely affect the fatigue properties of the joint as aluminide precipitates in the braze tends to weaken the braze structure.

As stated hereinbefore, unless the whole surface of the ferrous component, including the braze, is plated with an aluminide-forming metal prior to aluminizing said component, the fatigue properties at the joint are generally degraded. This has been confirmed by tests in which specimens of the joint have been subjected to cantilever loading to provide maximum bending stress at the joint during fatigue testing. In a test series conducted, the fatigue samples were shaped into the form of a "tee" from type 410 stainless steel and a joint produced with braze AMS 4772. This was done by brazing a machined cantilever arm to the sample. Prior to brazing, the specimens were solution treated, quenched and tempered.

One sample was aluminized without first applying a layer of aluminide-forming metal. Another was substantially covered with nickel and another with chromium and subsequently aluminized. The specimens were then subjected to cantilever fatigue testing at a stress of 50,000 psi. The following results were obtained:

Fatigue Cycles Item Specimens to Failure Remarks ______________________________________ (1) As brazed 300,000 Average of 7 samples (2) Aluminum-coated 180,000 Average of 3 samples (3) Nickel-Plated + 350,000 Average of Aluminum 3 samples (4) Chromium Plated + 240,000 1 sample Aluminum ______________________________________

As will be noted from Item (1), the as brazed specimen exhibits good resistance to fatigue until it is thermally coated with aluminum as shown in Item (2) wherein the specimen failed at 180,000 cycles, a drop of about 40% from the higher value.

However, when the specimen is first nickel plated and then aluminized as shown by Item (3), the number of cycles at failure is greatly enhanced to 350,000. In Item (4), chromium plate protects the brazed joint from aluminum to the extent that the fatigue life is substantially less degraded and, in fact, is much better than when the part is only aluminized.

A more telling test is the oxidation/corrosion fatigue test. This test involves the following cycle:

1. 10,000 fatigue cycles at 50,000 psi root load

2. 5 hours oxidation at 700°F

3. 16 hours salt spray

The salt spray test used is ATSM B 117.

The foregoing cycle is repeated until the sample fails. The results obtained on the bare brazed joint, the aluminized joint and the nickel-plated aluminized joint are as follows:

Fatigue Cycle Salt Oxida- Total No. to Failure at Spray tion Specimen of Cycles 50,000 psi Hours Hours ______________________________________ Braze Joint 5 58,000 80 25 Aluminized 10 106,000 160 50 Joint Nickel-Plated Aluminized 20 210,000* 320 100 Joint ______________________________________ *No Failure? Improvements were also indicated with chromium as the aluminide-forming metal. Thus, aluminide-forming metals may be the iron-group metals iron, nickel and cobalt and also chromium.

One of the attributes of an aluminide coating on a metal substrate is its ability to absorb readily a silicate liquid in the production of a protective non-metallic overcoat.

As pointed out in the aforementioned copending application Ser. No. 143,842, it is believed that the high affinity of the thermally aluminized coating or surface for the silicate is associated with the physical-chemical character of the aluminized surface arising out of the method of growth of the aluminide. The expression "thermally aluminized coating or surface" is meant to cover the thermal diffusion of aluminum in a metal surface in which an aluminide compound is formed at the surface.

In one embodiment, the non-metallic overcoat or barrier layer is formed by applying to the thermally aluminized surface of the article a solution of a soluble silicate salt at a temperature ranging up to about 100°C, for example, about 70°C to 95°C (about 160°F to 200°F), removing excess liquid from the surface, such as by blowing it off with air, to form a uniform layer of said silicate salt, and then drying the layer on said surface.

While the aluminized surface in and of itself exhibits resistance to saline corrosion, a typical salt spray test shows that sacrificial products form on the aluminized surface after approximately 15 hours of testing, whereas times in excess of 200 hours have been obtained when the aluminized surface is coated with a uniform silicate layer.

A wide range of sodium silicate solutions can be employed in producing the non-metallic overcoat. For example, the solutions can be prepared from solutions of 50 to 100% concentrations of Na2 O.3.22 SiO2. Various other sodium silicates can be employed to prepare solutions such as Baume 40, 45, 47 and 50. Potassium silicate may be similarly employed. Lithium silicate and also organic silicates can be used, such as ethyl silicate.

A preferred solution for producing a uniform pre-coat or barrier layer on the intermetallic iron aluminide substrate is one containing by weight about 0.05 to 2% SiO2 equivalent, for example, a soluble silicate in the form of Na2 O . 3.22 SiO2. The temperature of the substrate during application should preferably range from about 70°C to 95°C.

A preferred method for applying the silicate solution pre-coat at the foregoing concentration comprises immersing the thermally aluminized ferrous component in a tank maintained at a temperature of about 70°C to 95°C with sufficient time in the bath to bring the component to the temperature and assure absorption of the solution into the aluminized surface. The excess liquid is then blown off with air and the part allowed to dry. It is immersed again for a brief period for a time sufficient to allow the article to be covered with liquid, after which it is removed, blown off with air and air dried. The steps may be repeated until the desired thickness is obtained. It has been found that when using the air-drying technique, only short dips in the tank need be employed to ensure a continuous build-up of the silicate layer. Leaving the part in the bath too long can result in the layer being redissolved in the solution. To assure wettability of the coating on the intermetallic substrate, an ionic surfactant or wetting agent may be employed, for example, an anionic phosphate surfactant such as Triton QS-30 (manufactured by Rohm & Haas).

An alternate method which yields a more stable silicate coating resides in applying a succession of layers as described hereinabove followed by curing in an oven. Infra-red or forced air heated ovens may be employed in the temperature range of about 150°C to 430°C (about 300°F to 805°F) with enhanced corrosion protection. The silicate coating applied by any of the methods described herein will produce a uniform layer with a thickness of approximately 0.0001 inch (0.1 mil) while avoiding as far as is possible areas of excess silicate on the surface. A preferred method is to apply at least one pre-coat from a dilute silicate bath containing 0.05% to 2% by weight of SiO2 equivalent by a series of dipping, drying and curing steps followed by at least one spray coating of silicate from a more concentrated solution containing about 2.5% to 17.5% (e.g. 6.8%) by weight of SiO2 equivalent.

The advantage of curing the silicate coating which allows multiple layers to be formed is that the cured coating can withstand ten oxidation-corrosion cycles comprising heating the coated substrate to 1000°F (about 538°C) for 1 hour followed by 5 hours of salt spray testing, the foregoing test being repeated for 10 cycles. Applications of the silicate coating followed by curing at about 400°F (205°C) have yielded high degrees of protection and, in many cases, very little sacrificial products have been observed after 10 cycles of heating to 1000°F (538°C) followed by the salt spray test. The foregoing tests are helpful as controls in assuring the quality of the silicate coating before the next coating treatment is applied.

The corrosion resistance of the intermetallic layer is further enhanced by applying a conversion coating to the cured silicate layer. The conversion coating in turn may be covered by a silicate layer. The conversion coating may be applied by spraying, using commercially available reciprocating guns. Following the application of the conversion coating, the silicate solution containing 6.8% by weight equivalent of SiO2 and containing about 0.002% by weight of an anionic phosphate surfactant may optionally be sprayed over the conversion coating having a surface temperature not exceeding about 150°F (65°C) followed by curing at temperatures from about 300°F (150°C) to about 805°F (430°C) for 10 minutes in an infra-red furnace. Another method of covering the conversion coating is to dip the article in a hot solution of about 180°F to 200°F (82°C to 93°C), using a sodium silicate concentration of about 0.9% to 2.4% by weight of SiO2 equivalent with a 0.002% addition of an anionic phosphate surfactant.

The article or component is immersed in the bath and allowed to come to temperature and excess liquid removed rapidly by means of an air gun. It is then immersed again and immediately pulled out of the solution and air dried. A third application is made in the same manner. It is important that excess liquid be removed from the part to avoid foaming during curing. The purpose of repeated immersion and drying is to assure uniform coating of the surface. As stated above, the curing is preferably carried out at about 800°F in an infra-red furnace.

A simple production procedure which has been found successful for applying uniform layers of silicate is as follows:

A component of AMS 5616 steel is subjected to four cycles of treatment in the solution by supporting the component, e.g. a vane/shroud component, on a rack which is immersed in the solution and immediately withdrawn. The liquid is allowed to drain for approximately 15 seconds, after which it is immersed again and withdrawn. Following the second dip, air pressure means is disposed about the rack to blow off the excess liquid. This group of steps constitutes one cycle. Four cycles are employed to produce the desired silicate layer. If necessary, an air gun can be used to remove excess liquid from the root of the blades. After the fourth cycle, the blades are dried free of moisture by, for example, blowing with air. The temperature at which the silicate layers are applied may range from about 160°F (70°C) to 200°F (90°C). Following completion of the four cycles, the coating on the dried blade is then cured at about 800°F (425°C) in an infra-red oven.

Apparently the combination of the silicate coating and the aluminide compound in the aluminized surface markedly improves the resistance of the aluminized surface to corrode sacrificially, wherein the life of the sacrificial coating is unexpectedly extended for longer periods of time in saline environments than obtained with the aluminized surface alone.

However, as stated in co-pending application Ser. No. 143,842, the life of the silicated sacrificial aluminized coating is further enhanced by the application of a conversion coating from a solution in substantially the manner in which the silicate coating is applied. An aqueous conversion coating solution which is preferred may range by weight from about 5% to 30% phosphoric acid (preferably 10 to 30%), about 0.0235% to 3% aluminium, about 3% to 8% chromic acid (CrO3), about 0.75% to 6% magnesium, and the balance essentially water. A formulation found particularly preferred in producing the solution is as follows:

Wt. % Phosphoric acid 15.0 Aluminum powder 0.225 Chromic acid (CrO3) 5.0 Magnesium turnings 1.5 A non-anionic surfactant compris- ing a condensation product of ethylene oxide with an alkyl- phenol (Triton X-100 by Rohm & Hass 0.1 Water 78.175 100.00

The aluminum and magnesium are dissolved in the solution by virtue of the free acid present.

In conversion coating a silicated steel substrate, the substrate is sprayed, then dried and cured in the oven which heats the substrate to a temperature of about 800°F (427°C). The substrate is then cooled prior to the next application of the coating. The steps of spraying, baking and cooling constitute one spray cycle. Three spray cycles are normally used in applying the conversion coating.

The application of the conversion coating as described above results in a smooth uniform surface layer which provides oxidation-corrosion protection without the need for supplementary surface finishing. A build-up of approximately 0.1 mil can be obtained by employing a plurality of silicate and conversion coating applications.

It is believed that the baking of the duplex silicate-conversion coating results in a reaction product which provides new and improved resistance to corrosion in saline environments. While the silicate is preferably first applied to the thermally aluminized surface, it is appreciated that it can be applied as a solution together with the conversion coating materials. Thus, the conversion coating solution prior to spraying may contain about 0.05 to 2% by weight SiO2 equivalent as sodium silicate, potassium silicate, ethyl silicate, and the like.

It will be noted from co-pending application Ser. No. 143,842 that in addition to the conversion coating formulation described herein, various conversion coatings of the phosphate-chromate types may be employed in conjunction with the soluble silicate salt. Stating it broadly, the conversion coating comprises phosphates and chromates of at least one metal, for example, Al, Mg, Zn, Be, Ba, Sr, Ce, group metals and other metals. As a preferred embodiment, a conversion coat containing phosphates and chromates of aluminium and magnesium is particularly desirable.

Broadly speaking, the conversion solutions may range in composition by weight of at least about 0.5% of at least one phosphate and chromate-forming metal, e.g. about 0.5 to 10%, about 5% to 30% phosphoric acid, about 3% to 8% chromic acid (CrO3) and the balance essentially water. A preferred conversion solution is one containing by weight about 0.02% to 3% dissolved aluminum, about 0.75% to 6% dissolved magnesium, about 5% to 30% phosphoric acid (preferably 15% to 30%), about 3% to 8% chromic acid and the balance essentially water. A more specific composition is one containing by weight about 1/4% aluminum, about 1.5% magnesium, about 15% phosphoric acid, about 5% chromic acid and the balance essentially water.

Examples of steels to which the invention is particularly applicable includes AMS 5616. This steel comprises 13% Cr, 2% Ni, 3% W and the balance essentially iron; type 410 comprises about 11.5 to 13.5% Cr, 1% Si max., 1% Mn max., 0.15% C max., and the balance essentially iron; and 17-4 PH comprises about 17% Cr, 4% Ni, 3% Cu, small amounts of Co, Mn, Si, etc., and the balance essentially iron. Broadly speaking, the steels may comprise about 5% to 25% Cr, up to 5% W, up to 25% Ni, up to 4% Cu, up to 3% Al, up to 2% Ti and the balance essentially iron.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.