US2917818A

US2917818A - Aluminum coated steel having chromium in diffusion layer

Info

Publication number: US2917818A
Application number: US478481A
Authority: US
Inventors: Robert F Thomson
Original assignee: Motors Liquidation Co
Current assignee: Motors Liquidation Co
Priority date: 1954-12-29
Filing date: 1954-12-29
Publication date: 1959-12-22
Anticipated expiration: 1976-12-22

Description

United States Patent ALUMINUM COATED STEEL HAVING CHRO- MIUM IN DIFFUSION LAYER Robert F. Thomson, Grosse Pointe Woods, Mich., as-

signor to General Motors Corporation, Detroit, Mich, a corporation of Delaware No Drawing. Application December 29, 1954 Serial No. 478,481

4 Claims. (Cl. 29-1962) This invention relates to aluminum coated ferrous metal parts and particularly to such parts having high ductility due to the presence of chromium in the diffusion layer between the aluminum and ferrous metal.

During the past few years, the oxidation resistance of ferrous base metal articles has been increased appreciably by coating these articles with a thin layer of aluminum or aluminum alloy. A thin layer of an alloy of aluminum and the ferrous base metal is preferably provided at or adjacent the surface of the coated article to provide optimum oxidation resistance, particularly at elevated temperatures. United States Patent No. 2,569,097, Grange et al., discloses and claims a process for providing such a coating on ferrous metal articles.

Although the use of an aluminum coating on ferrous metal has proved to be highly satisfactory for increasing oxidation resistance, the interfacial layer of complex ironaluminum which is formed next to the steel or other ferrous base metal is relatively brittle. Since this layer may have a tendency to spall or flake off the base metal in some applications, it is desirable to increase the ductility of aluminum coated articles by reducing the brittleness of the interfacial layer.

Accordingly, a principal object of the present invention is to provide an aluminum coated ferrous metal article having improved ductility at both elevated temperatures and at room temperature. A further object of this invention is to provide an inexpensive process for forming an article which is both oxidation resistant and sufficiently ductile to permit its use in applications for which aluminum coated ferrous metals were inappropriate heretofore.

These and other objects are attained in accordance with the present invention by introducing chromium into the diffusion layer of an aluminum coated ferrous metal article, steel being preferably meployed as the base metal. Superior results are obtained if the aluminum coating is applied by a hot dipping operation. The subsequent diffusion of the aluminum coating results in a thin, ductile aluminum-iron-chromium layer on the surface of the steel or other ferrous metal article.

Other objects and advantages of this invention will more fully appear from the following detailed description of a preferred embodiment of the invention.

The surfaces of the articles to be coated are preferably cleaned prior to the coating and alloying operations. One satisfactory method is to clean the articles in a molten electrolytic caustic salt (such as the commercially available product known as Kolene) at a temperature of about 900 F. The articles may then be washed in Water and thereafter preferably further cleaned by acid pickling. A suitable acid pickling bath is an aqueous solution containing about 2% hydrofluoric acid, 7% sulfuric acid and 10% nitric acid. In some instances the articles may require only a simple degreasing treatment in a chlorinated solvent before being coated. Mechanical cleaning methods, such as grit blasting, sand blasting, hydroblasting,

2,917,819 Patented Dec. 22, 1959 2 etc., may be employed in some cases to supplement the chemical treatment. V A x The steps of degreasing and pickling are not essential to the process, however, and frequently the articles to be coated are not contaminated to a sufficient extent to warrant this treatment.

After the articles have been cleaned, the portions there-' of which are not to be coated may be treated with a suitable stop-off coating to prevent the coating metals from bonding to or alloying with the base metal at such sur= faces. A suitable stop-off material for this purpose is a sodium silicate soluitomsuch as an aqueous solution con= taining 20% to 50% sodium silicate. v

The chromium coating may be applied to the ferrous metal article by any apropriate method. Electrodeposition is normally most convenient and least expensive. Moreover, inasmuch as it is desirable to apply a very thin layer of chromium to the base metal, electrodeposition is most satisfactory because it permits precise control of the coating thickness. A chromium coating between approximately 0.00025 inch and 0.001 inch has proved to be satisfactory.

After the chromium coating has been formed on the ferrous metal, the outer layer of aluminum or aluminum base alloy is provided on the chromium coating. When an alloy of aluminum is used, it should contain or more of aluminum. Hence, the word aluminum, when hereinafter used in the specification and claims to refer to the outer coating material, is intended to include not only pure aluminum or commercially pure aluminum, but also aluminum base alloys containing at least approximately 80% aluminum.

Alloying of the surface portion of the ferrous metal with the chromium layer may be carried out in any desired manner. The preferred method is to apply molten aluminum to the chrominum plated ferrous metal article under conditions such that the chromium will partially diffuse into the iron or steel and the aluminum will partially diffuse into the chromium. As hereinafter explained, the resultant intermeditae alloy layer should be relatively thin.

Best results are obtained when the aluminum is applied by any of the procedures described in the aforementioned Patent No. 2,569,097, Grange et al. An especially advantageous method comprises preheating the chromium plated ferrous metal article to a temperature between approximately 1280 F. and 1400 F. in a fused salt bath consisting essentially of 37% to 57% KCl, 25% to 45% NaCl, 8% to 20% Na AlF and 0.5% to 12% AIF The preheated article is thereafter immersed for a short time in a molten bath of aluminum or aluminum base alloy at a temperature of about 1250 F. to 1325 F. Subsequently the aluminum coated article is removed from the aluminum bath and rinsed for a short period of time not in excess of approximately 15 seconds in the fluxing salt. The excess coating material is then removed by means of an air blast or by rapidly vibrating or rotating the coated article during or immediately after removal from the aluminum bath. If vibration or rotation of the coated article is resorted to, it is preferable to perform this step while the article is immersed in the molten salt bath. 7

After the excess aluminum coating metal has been removed, the coated ferrous base article is cooled or permitted to cool. The excess flux may be removed by washing, for example, of the coated article. Water or other quenching media may be employed to cool the coated part. Generally it is preferable to air cool the part below 800 F. followed by a water quench.

A relatively short period of immersion in the aluminum bath is all that is necessary or desirable. Ordinarily, the article being coated need be retained in the molten, alu- 'minum or aluminum base alloy not more than approximately 10 seconds, an immersion period between 5 and seconds being preferred at present.

insomc instances, particularly where small-ferrous base parts are to be coated in accordance with this invention, the aluminum may be applied by the procedure described in ctr-pending patent application SN. 459,093, tiled September 29, 1954, in the names of Robert P. Thomson, Albert A. Shoudy, Jr.,-and Dean K. Hanink. This procedure involves applying the aluminum in the form of a paint or paste containing aluminum powder or applying aluminum by a hot spraying operation. The latter practice is commonly referred to as metallizing. ln'eitl or event the aluminum coating is thereafter melted and diffused into the'chromium by the subsequent plication of heat, preferably by'immersion in the molten salt flux hereinbefore' described.

As thus treated, the ferrous metal article is provided with an extremely thin and uniform layer of aluminum bonded to the ferrous base metal by an intermediate thin and uniform layer of chromium which has partially alloyed with both the aluminum and ferrous base metal.

The aluminum and alloy layers should be relatively thin in all cases. In general, the aluminum-iron-chromium layer should have a thickness from approximately 0.0005 inch to 0.002 inch, the outer aluminum layer having a thickness not in excess of about 0.004 inch. It is preferred to have the outer aluminum layer between 0.0005 inch and 0.0015 inch thick, however, resulting in a' combined thickness of the aluminum-iron-chromium layer and the'aluminum overlay of approximately 0.001 and 0.0035 inch. A coating layer of this thickness possesses superior yielding characteristics when subjected to repeated contact with other materials and also has little tendency to crack, check or spall.

in some instances it may be desirable to electrodeposit a thin layer of nickel on the surface of the ferrous article before the chromium plating operation in order to improve oxidation resistance at elevated temperatures. For most applications, however, plating with nickel is unnecessary. if a nickel layer is employed between the chromium and the ferrous metal, its thickness should not exceed a few thousandths of an inch.

Bend tests were conducted on panels treated in the above-described manner, and the results indicated that these panels possess considerable ductility.

While this invention has been described by means of certain specific examples, it will be understood that the scope of the invention is not to be limited thereby except as defined in the following claims.

I claim:

1. A method of providing a ductile, oxidation-resistant coating on a ferrous metal article, said method comprising electroplating a layer of chromium having a thicknessof approximately 0.00025 inch to 0.001 inch on a ferrous metal article, thereafter immersing said chromium plated ferrous metal article in a fused salt bath heated to a temperature of about 1280 F. to1400 F.

and activatedby aluminum in contact therewith to-flux saidaa'rtiole, retaining said plated article in said salt bath until the surface temperature of said article substantially reaches the temperature of said bath to partially diffuse the chromium into-the ferrous metal and to dissolve oxides of chromium in the salt, subsequently immersing said plated article in a molten coating metal selected fromthe class consisting of aluminum and aluminum base alloys-heated to a temperature of about 1250 F.

'to'l1325 vF., retaining said article in said molten coating :metal for a period of time not exceeding ten seconds to thereby form on said article a ductile aluminum-ironchromium interlayer having a thickness of approximately 0.0005 inch to 0.002 inch and an aluminum overlay having a thickness between about 0.0005 inch and 0.004 inch, and removing the coated article from said molten coating metal.

2. A method of providing an oxidation-resistant coating on a ferrous base metal article while retaining the ductility thereof, said method comprising electroplating a layer of chromium having a thickness of approximately 0.0005 inch to 0.0001 inch on a ferrous base metal article, thereafter immersing said chromium plated article in a fused salt bath comprising, by Weight, approximately 37% to 57% KCl, 25% to 45% NaCl, 8% to 20% NagAlFs and 0.5% to 12% A11 said fused salt bath being heated to a temperature of about 1280 F. to 1400 F.-and activated by aluminum in contact therewith, retaining said plated article in said salt bath until the surface temperature of said article substantially reaches the temperature of said bath to partially diffuse the chromium into the ferrous metal and to dissolve Oxides of chromium in the salt, subsequently immersing said plated article in molten aluminum heated to a temperature of about 1250 F. to 1325 F., retaining said article in said molten-aluminum for five to ten seconds to thereby form on said article a continuous intermediate layer of ductile aluminum-iron-chromium having a thickness of approximately 0.0005 inch to 0.002 inch and an outer aluminum layer having a thickness of about 0.0005 inch to 0.0015 inch, and thereafter removing the resultant aluminum coated article from said molten aluminum.

3. A method of providing an oxidation-resistant coating on a ferrous base metal article while retaining the ductility thereof, said method comprising electrodepositing a thin layer of nickel on the surface of a ferrous base metal article, electroplating a layer of chromium having a thickness'of approximately 0.00025 inch to 0.0001 inch over said nickel layer, thereafter immersing said plated article in a fused salt bath comprising, by weight, approximately 37% to 57% KCl, 25%- to 45% NaCl, 8% to 20% Na AlF and 0.5% to 12% A11 said fused salt bath being heated to a temperature'of' about 1280 F. to 1400 F. and activated by aluminum-in contact therewith, retaining said plated article in said salt bath until the surface temperature of said article substantially reaches the temperature of said bath to partially diffuse the chromium and nickel into the ferrous metal and to dissolve oxides of chromium in the salt, subsequently immersing said plated article in molten coating metal containing at least aluminum heated to a temperature of about 1250 F. to 1325 F., retaining said article in said molten coating metal for a period of time not exceeding ten seconds to thereby form on said article an intermediate layer of ductile aluminumiron-chromium-nickel having a thickness of approximately 0.0005 inch to 0.002 inch and an outer aluminum layer having a thickness of about 0.005 inch to 0.004-inch, and then removing the coated article from said'molten coating metal.

4. A ferrous metal article having a protective coating characterized by high ductility and oxidation resistance, said protective coating consisting of an intermediate diffusion layer of an aluminum-iron-chromium alloy having a thickness of about 0.0005 inch to 0.002 inch and i an aluminum overlay having a thickness of approximately 0.0005 inch to 0.0015 inch, said article having a continuous chemical bond at the interface of the ferrous metal and the aluminum-iron-chromium alloy layer.

References Cited in the file of this patent UNITED STATES PATENTS 2,569,097 Grange et a1 Sept. 25, 1951 2,752,268 Whitfield June 26,1956

FOREIGN PATENTS 698,897 Germany Nov. 19, 1940 OTHER REFERENCES I Burns -and Schuh: Protective Coatings for Metals, Reinhold Publishing Corp, 1939, pp. 212.

Claims

1. A METHOD OF PROVIDING A DUCTILE, OXIDATION-RESISTANT COATING ON A FERROUS METAL ARTICLE, SAID METHOD COMPRISING ELECTROPLATING A LAYER OF CHROMIUM HAVING A THICKNESS OF APPROXIMATELY 0.00025 INCH TO 0.001 INCH ON A FERROUS METAL ARTICLE, THEREAFTER IMMERSING SAID CHROMIUM PLATED FERROUS METAL ARTICLE IN A FUSED SALT BATH HEATED TO A TEMPERATURE OF ABOUT 1280* F. TO 1400* F. AND ACTIVATED BY ALUMINUM IN CONTACT THEREWITH TO FLUX SAID ARTICLE, RETAINING SAID PLATED ARTICLE IN SALT BATH UNTIL THE SURFACE TEMPERATURE OF SAID ARTICLE SUBSTANTIALLY REACHES THE TEMPERATURE OF SAID BATH TO PARTIALLY DIFFUSE THE CHROMIUM INTO THE FERROUS METAL AND TO DISSOLVE OXIDES OF CHROMIUM IN THE SALT, SUBSEQUENTLY IMMERSING SAID PLATED ARTICLE IN A MOLTEN COATING METAL SELECTED FROM THE CLASS CONSISTING OF ALUMINUM AND ALUMINUM BASES ALLOYS HEATED TO A TEMPERATURRE OF ABOUT 1250* F. TO 1325* F., RETAINING SAID ARTICLE IN SAID MOLTEN COATING METAL FOR A PERIOD OF TIME NOT EXCEEDING TEN SECONDS TO THEREBY FORM ON SAID ARTICLE A DUCTILE ALUMINUM-IRONCHROMIUM INTERLAYER HAVING A THICKNESS OF APPROXIMATELY 0.0005 INCH TO 0.002 INCH AND AN ALUMINUM OVERLAY HAVING A THICKNESS BETWEEN ABOUT 0.0005 INCH AND 0.004 INCH, AND REMOVING THE COATED ARTICLE FROM SAID MOLTEN COATING METAL.