Corrosion resistant coating system for ferrous metal articles having brazed joints

Abstract

Ferrous metal articles, such as stainless steel articles, characterized by at least one brazed joint, in which the braze is a non-ferrous alloy are aluminized by first selectively coating the braze alloy with an aluminide-forming metal, such as nickel, and then thermally diffusing aluminum over the entire surface of the article including the selectively coated braze, such that a sacrificial corrosion resistant coating is produced wherein the aluminum coating on the ferrous metal surface is characterized by the presence of iron aluminide, and wherein the aluminum coating on the metal-coated braze is characterized by the presence of metal aluminide, such as nickel aluminide. The aluminide coatings are further enhanced by the application of a non-metallic coating, e.g., a conversion coating.

Parent Case Text

This application is a continuation-in-part of application Ser. No. 353,677, filed Apr. 23, 1973.

Description

This invention relates to the protection of ferrous metal articles 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 and also having 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 1,000.degree.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 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.

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 into 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 for a braze composition comprising by weight 54% Ag, 40% Cu and 6% Zn.

Mechanical tests have indicated that the uncontrolled diffusion of aluminum into the braze alloy tends to degrade the brazed joint as evidence 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 braze at the joint is selectively coated with a layer of aluminide-forming metal, e.g., nickel, prior to aluminzing of the whole 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, such as a stainless steel aircraft part, characterized by at least one brazed joint of non-ferrous metal or alloy, the braze having been first provided with a layer of an aluminide-forming metal, such as nickel, such that the aluminum coating in the unbrazed area at the steel substrate is characterized by the presence of iron aluminide and in the brazed areas by the presence of a metal aluminide.

A still further object is to provide a method of preparing a ferrous metal part having at least one brazed joint for aluminizing wherein the brazed joint is selectively plated with a coating of nickel while inhibiting nickel from plating out in the unbrazed areas.

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

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; and

FIG. 3 is a representation of a micrograph of a brazed joint at 100 times magnification.

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 1,125.degree.F (607.degree.C) to 1,925.degree.F (1,052.degree.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 selectively coating the brazed area with a barrier metal selected from the group consisting of nickel, cobalt, iron, titanium, chromium, manganese, molybdenum and vanadium and then thermally diffusing the aluminum, preferably by pack cementation, into the entire surface of the ferrous component, including the selectively coated brazed area and thereby produce a substantially uniform coating of aluminum, wherein the thermally diffused aluminum coating on the ferrous metal surface outside the brazed area is characterized by the presence of iron aluminide and wherein the thermally diffused aluminum coating on the barrier metal coated braze is characterized by the presence of metal aluminide, e.g., nickel aluminide. Fatigue tests on cantilevered specimens have indicated that the fatigue properties of aluminized nickel-coated brazed specimens are comparable to uncoated specimens and superior to aluminized brazed specimens without the nickel protective layer.

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 copending application Ser. No. 143,842, filed May 17, 1971, now U.S. Pat. No. 3,729,295. The disclosure of said patent 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 preferably 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 non-ferrous 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 1,125.degree.F (607.degree.C) to 1,925.degree.F (1,052.degree.C) and preferably 1,175.degree.F (635.degree.C) to 1,850.degree.F (1,010.degree.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% Cu [designated as AMS 4770 C] ; (2) 54% Ag, 40% Cu, 6% Zn and up to 1% Ni [designated as AMS 4772 B] ; (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, liquidus and brazing temperatures are as follows: Exam- ple Temperature .degree.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 solidus-liquidus temperature of 1,740.degree.F and is employed over a brazing temperature of about 1,740.degree. to 1,840.degree.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 1,125.degree. to 1,925.degree.F and preferably from about 1,175.degree. to 1,850.degree.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 to selectively coat the brazed areas with said aluminide-forming barrier metal before thermally aluminizing the whole component. In the case of nickel, various methods may be employed in selectively coating the brazed joint therewith. One method is to apply a resist to the unbrazed areas (e.g., a thin wax coating) while leaving the brazed portion of the joint exposed for nickel plating. Following nickel plating of the braze, the resist is removed from the component and the component then embedded in an aluminizing pack containing alumina, some aluminum powder and a small but effective amount of a halide, e.g., AlCl.sub.3. The use of the foregoing halide is advantageous in that very good aluminum coating can be produced at temperatures ranging up to about 1,000.degree.F (538.degree.C) while avoiding the production of undesired crystallographic or metallurgical changes in the substrate. The aluminizing temperature should not exceed the melting point of the braze, otherwise, the brazed joint will be deleteriously affected during aluminizing.

The same method may be applied to such aluminide-forming metals as cobalt, iron, palladium, chromium, manganese and the like. In the case of titanium, molybdenium and vanadium, these metals can be selectively applied as powder slurries dispersed in a fugitive organic binder, the coating dried and sintered in place at a temperature below the melting point of the braze alloy in the joint.

The aluminized surface produced in the foregoing manner is characterized metallographically by the presence of iron aluminide in the unbrazed areas adjacent the ferrous substrate outside of the brazed portions and by the presence of a metal aluminide at the metal-plated braze portions, e.g., nickel aluminide.

Where the aircraft component is made of stainless steel, such as those bearing the designations AMS 5508, AMS 5616, AMS 6304, 17-4PH, type 410 and others, we prefer to use nickel as a barrier metal which is applied as a nickel mask on the brazed areas by chemical plating. We find this method to be very advantageous in that we can passivate the exposed stainless steel substrate while activating the brazed areas, such that during the subsequent step of chemical plating, the nickel selectively coats only the brazed areas and not the stainless steel substrate. The method is also applicable to cobalt and iron.

Of the foregoing steels, the composition of AMS 5616 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.

The passivation of the steel substrate is achieved by employing a chemical anodizing solution comprised of cupric chloride, mercuric chloride, bismuth chloride, hydrochloric acid, alcohol and water. Broadly, the aqueous solution may have the following composition calculated in grams per liter (gpl) and milliliters per liter (ml/l):

10 to 70 gpl cupric chloride

20 to 140 gpl mercuric bichloride

10 to 70 gpl bismuth chloride

100 to 400 ml/l of ethyl alcohol and

100 to 400 ml/l concentrated hydrochloric acid balance essentially water to make 1 liter equivalent.

Examples of actual bath compositions are as follows:

I II III IV ______________________________________ Cupric chloride (gpl) -- 60 30 20 12 Mercuric Bichloride (gpl) -- 120 60 40 28 Bismuth Chloride (gpl) -- 60 30 20 12 Ethyl Alcohol (ml/l) -- 300 150 100 60 Hydrochloric Acid (ml/l) -- 375 188 125 75 Water (ml/l) -- bal. bal. bal. bal. ______________________________________

The foregoing solutions are used at room temperature, the steel parts being immersed therein for generally about 1/4 to 10 minutes.

Following preparation of the solution, the cleaned stainless steel component (e.g., in the grit-blasted honed condition) is immersed in it. The steel surface tends to darken while mecury reduction occurs cathodically (due to galvanic action) on the braze at immersion times of 1/4 to 10 minutes. However, chemical or electroless plating of nickel will not occur on the surfaces of the component unless the steel substrate is further treated. This is done by subsequent nitric or chromic acid dipping at room temperature to remove any observable mercury, thereby activating the braze surface.

Typical passivating solutions for this treatment are: (A) 50% nitric acid solution, (B) saturated chromic acid solution, and (C) or varying combinations of both. The chromic acid solution may range in concentration from 100 grams/liter to saturation, the steel substrate being immersed for 10 to 30 minutes followed by immersion in 10 - 30 volume % nitric acid for 30 to 60 seconds.

Following nitric acid dipping, the stainless steel component is then preferably nickel plated in a chemical or electroless plating bath of nickel, such as nickel baths based on dimethylamine borane or sodium hypophosphite. The aqueous sodium hypophosphite may comprise the following:

Nickel sulfate 15 to 30 gpl Sodium Hypophosphite 15 to 30 gpl Sodium Glycolate 20 to 40 gpl Sodium Succinate 10 to 20 gpl The pH is adjusted to 4.5 to 6.

The temperature is preferably 180.degree.-195.degree.F.

A typical aqueous solution is one containing 25 gpl nickle sulfate, 25 gpl sodium hypophosphite, 30 gpl sodium glycolate and 17 gpl sodium succinate.

Chromium-containing steel components have been immersed for one hour to deposit approximately 0.0005 inch of nickel and for as long as 2 hours to produce a nickel plate approaching 0.001 inch thick on the braze, while inhibiting nickel plating out in the stainless steel substrate. Nickel plates found adequate for the purpose generally exceed about 0.0002 inch.

A stainless 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. Referring to FIG. 3, vane 13 is shown attached by braze 14 to inner ring 11. The fillet of the AMS 4772B brazed joint has been selectively coated with nickel such that following aluminizing of the part by pack cementation, a layer 15 containing nickel aluminide is provided following the contour of the fillet. As will be noted, an aluminide layer 16 is shown adjacent the surface of the inner ring comprising iron aluminide by reaction with the steel substrate during pack cementation. As will be noted in FIG. 3, the specimen has an overplate of copper and nickel 17 to provide the necessary support for mounting and polishing said specimen without adversely affecting the coating.

In thermally aluminizing the selectively nickel-plated vane/shroud component shown in FIG. 1, a preferred pack cementation method is employed as follows:

The method comprises preparing an aluminizing pack comprised for example of 800 lbs. of -60+140 mesh aluminum powder blended with 200 lbs. of Al.sub.2 O.sub.3, also -6+140 mesh size. To the 1,000 lb. mixture is added 30 lbs. of dry AlCl.sub.3 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.degree.825.degree.F (425.degree. to 440.degree.C) for 35 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.degree.F (176.degree.C) due to vaporization of AlCl.sub.3 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 1,000.degree.F (538.degree.C), for example, a range of about 795.degree. to 825.degree.F (425.degree. to 440.degree.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.degree.F (205.degree.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 FeAl.sub.3, 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 AlCl.sub.3 and the balance essentially a stable inert refractory oxide.

As stated hereinbefore, unless the braze of the joint is plated with an aluminide-forming metal, such as nickel, prior to aluminizing the stainless steel 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 shpaed in the form of a "tee" from type 410 stainless steel by brazing a machined cantilevered arm to the surface of the sample with alloy AMS 4772B. Prior to brazing, the specimens were solution treated, quenched and tempered.

In one specimen, the brazed joint was aluminized in the manner described hereinbefore without applying a nickel plate. That is to say, the braze was bare prior to aluminizing. In other specimen, the braze was first selectively nickel plated and subsequently aluminized. The aluminized specimens are then subjected to an oxodation/corrosion/fatigue screening test using the following cycle:

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

2. 5 hours oxidation at 700.degree.F

3. 16 hours salt spray

The salt spray test used is ASTM B117.

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

Total Fatigue Cycles Salt Oxida- Number to Failure at Spray tion Specimen of Cycles 50,000 psi Hours Hours ______________________________________ Bare Joint 5 58,000 80 25 Nickel- Plated Joint 19 200,000* 304 95 ______________________________________ *This specimen did not fail.

The failed joint showed general oxidation and corrosion attack to a depth of 0.0035 inch. The nickel-plated joint showed no evidence of braze corrosion after 19 test cycles. A bare joint without corrosion ran 300,000 cycles to failure at a load of 50,000 psi.

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 U.S. Pat. No. 3,729,295, 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 iron and/or nickel aluminide 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.degree.C, for example, about 70.degree. to 95.degree.C (about 160.degree. to 200.degree.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 Na.sub.2 O.sup.. 3.22 SiO.sub.2. 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% SiO.sub.2 equivalent, for example, a soluble silicate in the form of Na.sub.2 O.sup.. 3.22 SiO.sub.2. The temperature of the substrate during application should preferably range from about 70.degree. to 95.degree.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 abouot 70.degree. to 95.degree.C with sufficient time in the bath to bring the component to 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 anionic 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 ranges of about 150.degree. to 430.degree.C (about 300.degree. to 805.degree.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 SiO.sub.2 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 SiO.sub.2 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 1,000.degree.F (about 538.degree.C) for 1 hour followed by 5 hours of salt spray testing, the foregoing test being repeated for ten cycles. Applications of the silicate coating followed by curing at about 400.degree.F (205.degree.C) have yielded high degrees of protection and, in many cases, very little sacrificial products have been observed after 10 cycles of heating to 1,000.degree.F (538.degree.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 SiO.sub.2 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.degree.F (65.degree.C) followed by curing at temperatures from about 300.degree.F (150.degree.C) to about 805.degree.F (430.degree.C) for 10 minutes in an infra-red furance. Another method of covering the conversion coating is to dip the article in a hot solution of about 180.degree. to 200.degree.F (82.degree. to 93.degree.C), using a sodium silicate concentration of about 0.9 to 2.4% by weight of SiO.sub.2 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.degree.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 4 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 mositure by, for example, blowing with air. The temperature at which the silicate layers are applied may range from about 160.degree.F (70.degree.C) to 200.degree.F (90.degree.C). Following completion of the four cycles, the coating on the dried blade is then cured at about 800.degree.F (425.degree.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 U.S. Pat. No. 3,729,295, the life of the silicated sacrificial aluminized coating is further enhanced by the application of a conversion coating from a solution in substantially the same 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% aluminum, about 3 to 8% chromic acid (CrO.sub.3), about 0.75 to 6% magnesium and the blance 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 (CrO.sub.3) 5.0 Magnesium turnings 1.5 A non-anionic surfactant comprising a condensation product of ethylene oxide with an alkylphenol (Triton X-100 by Rohm & Haas) 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 and then dried and cured in the oven which heats the substrate to a temperature of about 800.degree.F (427.degree.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 SiO.sub.2 equivalent as sodium silicate, potassium silicate, ethyl silicate, and the like.

it will be noted from copending U.S. Pat. No. 3,729,295 that, in addition to the conversion coating formulation described herein, various conversion coatings of the phosphatechromate 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 aluminum 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 (CrO.sub.3) 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.

While specific examples are directed to nickel as the aluminide-forming metal, the invention is applicable to the other stated aluminide-forming metals. Thus, in applying chromium to the braze area, the steel substrate is selectively coated with a resist, e.g., wax, while leaving the braze exposed and chromium then plated on the brazed areas making the steel substrate the cathode in an electrolyte containing 250 to 400 grams/liter chromic acid (CrO.sub.3) and 2.5 to 4 grams/liter of sulfate ions. The current density may range from 10 to 45 amps/dm.sup.2. Following formation of the chromium layer on the braze, the steel substrate is de-waxed, cleaned and then aluminized as described hereinbefore, the aluminide at the braze being chromium aluminide. The same method may be employed for cobalt and iron. As for molybdenum, titanium and vanadium, these may be applied to the braze as a powder slurry and dried prior to aluminizing.

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.

Claims

What is claimed is:

1. A stainless steel article having at least one brazed joint,

the braze of said joint being formed of a non-ferrous brazing alloy of melting point ranging from about 1,125.degree. to 1,925.degree.F,

said braze being characterized by the presence of an aluminide-forming metal selected from the group consisting of nickel, cobalt, iron, titanium, chromium, manganese, molybdenum and vanadium at the surface thereof,

the stainless steel article having a thermally diffused aluminum coating on substantially the entire surface of said article,

the coating on the stainless steel surface outside the braze being characterized by the presence of iron aluminide,

the aluminum coating on the braze being characterized by the presence of an aluminide of said metal group,

the thermally diffused aluminum coating also having bonded thereto a cured non-metallic barrier layer formed from a silicate selected from the group consisting of sodium silicate, potassium silicate, lithium silicate and ethyl silicate.

2. A stainless steel article having at least one brazed joint,

the braze of said joint being formed of a non-ferrous brazing alloy of melting point ranging from about 1,125.degree. to 1,925.degree.F,

said braze being characterized by the presence of nickel at the surface thereof,

the stainless steel article having a thermally diffused aluminum coating on substantially the entire surface of said article.

the coating on the stainless steel surface outside the braze being characterized by the presence of iron aluminide,

the aluminum coating on the braze being characterized by the presence of nickel aluminide,

the thermally diffused aluminum coating also having bonded thereto a cured non-metallic barrier layer formed from a silicate selected from the group consisting of sodium silicate, potassium silicate, lithium silicate and ethyl silicate.

3. The stainless steel article of claim 2, wherein the non-ferrous brazing alloy has a melting point of about 1,175.degree. to 1,850.degree.F and wherein said brazing alloy is either a copper-base, silver-base or gold-base alloy.

4. The staniless steel article of claim 2, wherein said non-metallic barrier layer also includes a chromate and a phosphate of at least one metal.

5. The stainless steel article of claim 4, wherein said chromates and phosphates of said at least one metal are chromates and phosphates of aluminum and magnesium.

6. The article of manufacture of claim 5, wherein the silicate of the non-metallic barrier layer is derived from sodium silicate.

7. The article of manufacture of claim 5, wherein the silicate of the non-metallic barrier layer is derived from potassium silicate.

8. A stainless steel article having at least one brazed joint,

the braze of said joint being formed of a non-ferrous brazing alloy of melting point ranging from about 1,125.degree. to 1,925.degree.F,

said braze being characterized by the presence of an aluminide-forming metal selected from the group consisting of nickel, cobalt, iron, titanium, chromium, manganese, molybdenum and vanadium at the surface thereof,

the stainless steel article having a thermally diffused aluminum coating on the whole surface of said article,

the coating on the stainless steel surface outside the braze area being characterized by the presence of iron aluminide,

the aluminum coating on the braze being characterized by the presence of an aluminide of said metal group.

9. A stainless steel article having at least one brazed joint,

the braze of said joint being formed of a non-ferrous brazing alloy of melting point ranging from about 1,125.degree. to 1,925.degree. F,

said braze being characterized by the presence of nickel at the surface thereof,

the stainless steel article having a thermally diffused aluminum coating on the whole surface of said article,

the coating on the stainless steel surface outside the braze area being characterized by the presence of iron aluminide,

the aluminum coating on the braze being characterized by the presence of nickel aluminide.

10. The stainless steel article of claim 7, wherein the non-ferrous brazing alloy has a melting point of about 1,175.degree. to 1,850.degree.F and wherein said brazing alloy is either a copper-base, silver-base or gold-base alloy.