US3428472A

US3428472A - Method for forming metal coatings

Info

Publication number: US3428472A
Application number: US403600A
Authority: US
Inventors: Takaaki Shimose; Kenji Mori; Kimio Inoue; Zenichiro Takao
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 1963-10-14
Filing date: 1964-10-13
Publication date: 1969-02-18
Anticipated expiration: 1986-02-18
Also published as: GB1079734A; DE1521346A1; DE1521346B2; FR1602258A; DE1521346C3

Description

Feb. 18, 1969 TAKAAKI SHIMOSE ETAL 3,428,472

METHOD FOR FORMING METAL COATINGS Filed Oct. 13, 1964 Sheet of 2 8. 9 TAKAAKI SHIMOSE ETAL 3,423,472

METHOD FOR FORMING METAL COATINGS Filed 001;. 15, 1964 Sheet 2 of 2' mm, 5;, A J /om [kl/110 fugue BY Z eumH/h FA/ma my w United States Patent 3,428,472 METHOD FOR FORMING METAL COATINGS Takaaki Shimose, Nishinomiya, Kenji Mori, Rokkodaimachi, Kobe, Kimio Inoue, Kobe, and Zenichiro Takao, Nishinomya, Japan, assignors to Kobe Steel Works, Ltd., Kobe, Japan Filed Oct. 13, 1964, Ser. No. 403,600 Claims priority, application Japan, Oct. 14, 1963, 38/55,272; Nov. 7, 1963, 38/60,051 US. Cl. 117-22 9 Claims Int. Cl. C23c 3/00; B44d 1/46 ABSTRACT OF THE DISCLOSURE Clean steel strip is coated with a viscous aqueous solution of potassium polymetaphosphate and sodium polymetaphosphate, and then contacted with powered metal which is adhesively retained by the polymetaphosphate solution. The coated strip is dried, compressed between rolls, and the powder coating is thereafter sintered. It is bright even without a finishing pass between another pair of rollers, and the composite material has high fieXural strength without spalling.

The present invention relates to an improved method for forming a metallic film on the surface of a metallic :body, the film being of a different metal than the metallic body; more specifically this invention relates to a method applicable for coating the surface of iron or steel in the form of plate, wire, bar or tube with aluminum, titanium, zirconium, nickel, chromium or copper, or alternatively with an aluminum base alloy, titanium base alloy, zirconium base alloy or stainless steel.

Various processes for forming metallic coatings on metallic bodies are already known. The most common process is electroplating, but electroplating is limited to only some types of metals and this method requires a relatively long time for forming thick layers. Another known process for forming metallic coatings is the vapor deposition process. This process can be used to form a wide variety of metal coatings, but since this process has to be carried out in a vacuum, the size of the metal bodies to be coated is subject to some limitation which makes the operation impractical for a continuous treatment operation. In addition, the film formed by the vapor deposition process is generally quite thin, so this process can not be used when a thick metallic film or coating is desired. The hot-dip process long employed for forming metallic coatings is, applicable only to low melting metals. The thermal diffusion process is impractical for continuous operation. The metal spraying process can not produce high density coatings having uniform thickness.

Accordingly, one object of the present invention is to provide a method of coating the surface of metallic bodies or substrates with metals different from the metal bodies to be coated while avoiding the disadvantages of the prior art methods.

Another object of the present invention is to provide a novel metal coating method in which powdered materials are employed as coating metals.

A still further object of the present invention is to provide products having any desired thickness of metal coatings formed thereon.

Patented Feb. 18, 1969 According to the present invention, there is provided a metal coating method which comprises the steps of applying a layer of powdered coating material onto the surface of a metallic body or substrate, pressing the powdered metal against the surface of the substrate by mechanical means, and sintering said compressed coating metal so as to bond the particles of the coating layer metal to each other and to the substrate, the bonding of the coating layer to the substrate metal being carried out in the presence of an adhesive medium comprising an aqueous solution which contains sodium or potassium polymetaphosphate.

FIG. 1 is a schematic View of an apparatus suitable for carrying out a preferred method according to the present invention, FIG. 2 is an enlarged cross-sectional view of a steel body carrying a powdered aluminum coating layer formed by the novel method of the present invention. FIG. 3 is a schematic view of an apparatus suitable for carrying out a modified method of the present invention, and FIG. 4 is an enlarged cross-sectional view of a steel body carrying a titanium coating layer formed by the method of the present invention.

The metallic substrates which are to be coated by the method of the present invention include iron, carbon steel or alloy steel in the form of plate, wire, rod or tube, and the coating materials which are to be employed as coatings for these substrate materials include aluminum, titanium, zirconium, nickel, chromium, copper, and alloys containing any one of these materials such as stainless steel. The particle size of the powdered metal used for coating should be less than 200 mesh.

An aqueous solution of sodium or potassium polymetaphosphate has exceedingly high viscosity so that a small amount is sufficient to adhere the powdered coating material firmly to the surface of a metallic substrate, when the coating material is mechanically pressed against the surface of the metallic substrate in a subsequent step. Only a small amount of the dried adhesive material re mains on the coating layer and this has little or no effect on the bond between the metallic substrate and the coating layer. The polymetaphosphate also accelerates the adhesive action between the metallic substrate and coat ing material. The polymetaphosphate may be either in the form of a chain compound or a ring compound, but a chain compound is preferable in the present invention because of its high degree of polymerization. Potassium polymetaphosphate can be easily polymerized to a high degree but is insoluble or sparingly soluble in water, and accordingly, the potassium salt is generally employed in combination with sodium polymetaphosphate or any other suitable sodium salt.

The degree of polymerization of the polymetaphosphate employed is generally within a range from 10 10 However, if the sodium salt is employed with the potassium salt, the polymerization degree of the sodium salt may be subtsantially lower. The preferred concentration of the polyphosphate is 0.01-2.0 g./l. depending upon the polymerization degree of the polyphosphate employed.

In carrying out the method of the present invention, the following conditions may be selectively employed as the situation may require.

The metallic substrate or body to be treated may be in the form of strip, plate, wire or any other suitable form.

The powdered metal must be evenly distributed over the surface of the substrate as by spraying or by passing the substrate through a fluidized bed of the coating material. Finely ground coating material the particle size of which is less than 30 microns may be suspended in the above mentioned adhesive solution prior to its application onto the surface of the substrate metal, and the adhesive solution containing the finely powdered metal coating material may be applied directly to the surface of the substrate by spraying, brushing, roll coating or curtain coating.

The bond obtained between the powdered metal and the substrate largely depends upon the aflinity of the coating material for the adhesive solution. When the powdered metal has a high affinity for the adhesive, the coating layer generally becomes thick and is liable to cover the surface of the substrate metal unevenly.

The affinity of powdered metal to the adhesive can be reduced by adding a hydrophobic material.

Materials which impart hydrophobic properties to the powdered metal include paraflins having more than 4 carbon atoms, olefins having more than 5 carbon atoms, aliphatic monohydric alcohols having more than 8 carbon atoms, monocarboxylic acids having more than 4 carbon atoms or equivalent polar aliphatic compounds such as monohalide compounds, monocarboxyl compounds, monoamines, and mononitriles.

The amount of the powdered metal that adheres to the substrate decreases in proportion to the amount of such organic compounds added to the powder. If desired, the hydrophobic property of the powdered metal coating material may be reduced by adding a suitable amount of surface active agent or low molecular alcohol to the aqueous solution of the adhesive. The surface active agents suitably employed in the method of the present invention include cationic or anionic surface active agents, or nonionic surface active agents, but surface active agents which can be easily pyrolyzed and volatized are preferably employed.

Although a lower alkanol such as ethyl alcohol or propyl alcohol is employed in an amount greater (over 5% by volume) than a surface active agent, the use of alcohol is preferable to the use of the surface active agent because these alcohols volatize at lower temperatures than the surface active agents.

After the powdered metal coating has been applied thereon, the metal substrate is dried at a temperature ranging from 50 to 450 C. whereby the powdered metal is caused to adhere firmly to the substrate metal surface even when the substrate is lightly vibrated. The solvent and adhesive are vaporized during the heat treatment. The heat treatment or drying temperature at which the adhesive is vaporized should be lower than 450 C. in order to prevent oxidation of the powdered metal coating and metal substrate; the critical temperature varies depending upon the type of adhesive employed and the length of the heating time.

The step in which the powdered metal coating is mechanically pressed against the substrate surface constitutes a pretreatment for the sintering step and the pressing operation may be carried out by means of roll pressing, die-drawing or roller-die rolling. In any case, the reduction ratio is suitably selected with regard to the particular type of metal coating powder employed. When a comparatively hard metal powder such as titanium is employed as the coating metal, the reduction ratio should be over 5%, but in case a comparatively soft metal powder such as aluminum is employed as the coating metal, the reduction ratio may be in the order of 1%.

The mechanical pressing causes the particles of the powdery coating metal to adhere to each other and to the metal substrate to form a uniform and bright continuous coating.

A mat coating layer may be formed by a rough roll employed in this mechanical pressing, but such a mat coating does not adhere strongly enough for a final coating layer, so the mat or rough layer is subjected to a sintering treatment. The sintering treatment is carried out in an inert atmosphere or vacuum at a temperature ranging from 4001100 C. for five hours. The sintering temperature and time may vary depending upon the metal coating powder employed. For example, when aluminum is coated on iron or steel, and the temperature is 600 C., an adherent aluminum layer is obtained in five minutes, but if the temperature is 550 C., the same quality of aluminum coating may be obtained in about l030 minutes. This heat treatment serves to sinter the aluminum coating layer and to firmly adhere the layer onto the metal substrate, but this treatment should be carried out in such a manner that no intermediate alloy layer is formed between the aluminum coating layer and metal substrate or, if any, the thickness of such an alloy layer must be held to less than 3 microns. If the heat treatment is carried out at a temperature above 700 C. or the treatment continues for several hours, an intermediate alloy layer comprising iron and aluminum is inevitably formed between the iron substrate and aluminum layer.

If titanium or zirconium is deposited on iron or steel, and if the heat treatment is carried out at 1000 C., a firmly bonded coating is obtained in less than 20 minutes while at 850 C. the same quality of coating may be obtained within 60 minutes.

As is clear from the foregoing, the method of the present invention comprises the steps of applying powdered metal onto the surface of a metal substrate in the presence of an adhesive medium comprising an aqueous solution of polymetaphosphate whereby a coating layer of said powdered material is formed on the substrate, pressing said coating layer against the substrate surface by mechanical means to firmly bond the coating layer thereto, and sintering said coating layer on the substrate surface. The method is applicable to most types of substrate metal and coating metal. The thickness of the coating may be easily controlled over a wide range and the coating layer obtained is of uniform thickness. In addition, this novel coating method can be easily performed without unusual facilities or a vacuum room and is suitable for continuous operation.

For a better understanding of the present invention, several specific examples are given below.

Example 1 The continuous strip coating line schematically illustrated in FIG. 1 was employed with a mild steel strip S having a. thickness of 0.5 mm. and previously cleaned by degreasing and pickling. The pre-treated mild steel strip S was passed through an aqueous adhesive bath 1 of 1.0 g./l. of sodium polymetaphosphate (polymerization degree of 28) and 0.5 g./l. of potassium polymetaphosphate (polymerization degree of 10 -10 The strip S coated with a thin poly-metaphosphate solution layer of uniform thickness was then passed under through a funnel shaped hopper 2 containing aluminum powder (approximately 50 micron particle size) so as to deposit the aluminum powder on the adhesive-coated surface in a layer of uniform thickness. The thus treated strip S was then passed through a drying kiln 3 maintained at 180 C. and the dried strip S was, thereafter passed between pairs of opposing rolls 4 set so as to reduce the thickness of the strip coating to 40-50% and thereafter passed through a heating furnace 5 in which argon gas was flowing at 600 C. for one minute thereby sintering the aluminum powder and bonding the powder more firmly. Strip S was then passed between another pair of opposing rolls 6 to finish the aluminum coating surface and to impart brightness to the surface. The obtained product had an aluminum coating layer of 30 micron thickness. As shown in FIG. 2, the aluminum coating layer has high flexural strength and can withstand bending with a 3.5 cm. radius without damage to the bond between the metal coating and substrate.

Example 2 In this example, the line schematically illustrated in FIG. 3 was employed. Mild steel strip S having a thickness of 0.5 mm. was cleaned by degreasing and pickling before being fed to the line. The pre-treated strip S was passed through a roll coater containing an aqueous solution of 1.0 g./l. of sodium polymetaphosphate (polymerization degree of 28) and 0.5 g./l. of potassium polymetaphosphate. The strip S was coated with an even thickness of polymetaphosphate solution while passing through the roll coater 10 and then passed under a powder applicator -11, containing free-flowing titanium powder (particle size of about 50 microns) so as to distribute the titanium powder continuously and evenly on the adhesive coated surface of the strip which was then passed through a drying kiln 12 maintained at 180 C. for about one minute and the dried strip S was passed through a press 13 set to compress the titanium bearing strip by about 20%, thereby insuring that the titanium powder firmly adhered to the strip. After the pressing operation, strip S was wound on a recoiler 14 and the coiled strip was heated to 850 C. in a bell-type vacuum heating furnace for one hour. The vacuum was 1 10- mm. Hg.

The thickness of the titanium coating layer was about 30 microns as shown in FIG. 4, and the bond between the metal substrate and coating layer was strong enough to withstand bending through 180 at a radius of curvature three times its thickness. The appearance of the coating surface was fine.

Example 3 A specimen of steel strip was cleaned by degreasing and pickling in the conventional way. The pretreated strip was immersed in a solution of 1 g./l. of potassium polymetaphosphate (polymerization degree of 10 -10 and 2 g./l. of sodium polymetaphosphate (polymerization degree of 28) and then dusted with stainless steel powder of 325 mesh thereby producing a steel strip with a stainless powder coating layer of uniform thickness. Thereafter, the coated strip was dried and its stainless coating pressed against the substrate by rollers. The pressed strip was thereafter heated at 950 C. for about one hour to bond the bright stainless steel coating.

Example 4 20 cc. of a benzene solution containing 0.5 g./dl. of solid paraflin was added to 100 g. of atomized aluminum powder of less than 250 mesh and these materials were thoroughly mixed. The mixture was then dried and dusted on the surface of a steel plate covered with a film of an aqueous solution containing 1.5 g./l. of poly-metaphosphate (0.5 g./l. of potassium polymetaphosphate and 1.0 g./l. of sodium polymetaphosphate) thereby distributing the aluminum powder evenly on the surface of the steel plate. The thus coated steel plate was then subjected to the subsequent steps mentioned in Example 1 and a good quality of aluminum coated steel plate was obtained.

Example 5 200 cc. of ethanol solution containing 0.125% of a caproic acid was mixed with 1 kg. of atomized aluminum powder of less than 250 mesh and the mixture was dried and distributed evenly onto the surface of a steel plate previously covered with adhesive solution and further treated in the same manner as in Example 4.

Example 6 A mild steel plate having a thickness of 0.5 mm. was cleaned by degreasing and pickling and covered with a film of aqueous adhesive solution containing a mixture of 1 g./l. of sodium polymetaphosphate and 0.5 g./l. of potassium polymetaphosphate and 0.1% of a surface active agent sold under the trade name Rapizol by Nippon Yushi Co., Ltd. (dialkyl ester of sodium sulfossuccinic acid).

The adhesive-coated steel plate was then evenly dusted with aluminum powder of less than 325 mesh. The plate was then heated at 350 C. for three minutes to de compose the surface active agent and then reduced by rolls to 30% of original thickness and finally heated in an argon atmosphere at 600 C. for three minutes. The thickness of the aluminum coating layer was 50 microns; it made a hood bond with the mild steel and had a bright surface. As an alternative process, an identical steel plate was surface-treated with the same type of adhesive solution without any surface active agent added thereto under the same conditions as mentioned above in this example. The thickness of the aluminum coating layer was 20 microns.

What is claimed is:

1. A method of applying a metal coating to a metallic substrate which comprises:

(a) coating a surface of said substrate with a viscous aqueous solution of a polymetaphosphate of potassium or sodium;

(b) contacting the coated substrate with a powdered metal until a layer of metal powder is adhered to said substrate by said solution;

(c) drying the powder bearing substrate at a temperature lower than the sintering temperature of said powder until the water in said solution is substantialy volatilized;

(d) mechanically pressing said layer against said substrate until the thickness thereof is reduced; and

(e) heating the substrate bearing the pressed layer to said sintering temperature until said powder is sintered.

2. A method as set forth in claim 1, wherein said solution contains 0.01 to 2.0 g./l. potassium polymetaphosphate having a polymerization degree of 10 to 10 3. A method as set forth in claim 2, wherein said powdered metal contains an amount of an organic material sufiicient to impart hydrophobic properties to said powdered metal, and said organic material is a paraffin having more than 4 carbon atoms or a monocarboxylic acid having more than 4 carbon atoms.

4. A method as set forth in claim 2, wherein said viscous solution contains 0.01-0.5% of a surface active agent.

5. A method as set forth in claim 2, wherein said viscous solution contains at least 5% by volume of a lower alkanol.

6. A method as set forth in claim 2, wherein said substrate is ferrous, said sintering temperature is between 400 and 1100 C., and said powder is sintered in an inert atmosphere for less than five hours.

7. A method as set forth in claim 6, wherein said powdered metal is aluminum, titanium, zirconium, nickel, chromium, or copper, and the particle size of said powdered metal is smaller than 200 mesh.

8-. A method of applying a metal coating to a metallic substrate which comprises:

(a) suspending a metal powder having a particle size of less than 30 microns in an aqueuos solution of a polymetaphosphate of potassium or sodium;

'(b) applying the suspension so produced in a uniform layer to a surface of said substrate;

(c) substantially removing the water of said solution from said layer until the latter is substantially dry;

(d) mechanically pressing the metal powder of the dry layer against said surface until the thickness of said layer is reduced; and

(e) heating the substrate bearing the pressed layer to the sintering temperature of said metal powder until the same is sintered.

9. A method as set forth in claim 8, wherein said metal powder is aluminum, titanium, zirconium, nickel, chromium or copper, water is removed from said layer by heating the same to a temperature between 50 and 7 8 450 C., and said metal powder is sintered at 400 to 2,858,600 11/1958 Vigor 117-451 X 1100 C. for less than five hours. 2,895,192v 7/1959 Meissner 11750 X 3,024,128 3/1962 Dawson 117-22 References Cited 3,142,560 7/1964 Storchheirn 117-22 UNITED STATES PATENTS 3,182,395 5/1965 SCOtt 11722 X 1 7 5 11/1929 Dely 117 22 X 5 WILLIAM D. MARTIN, Primary Examiner. 1,922,254 8/ 1933 McCulloch 11722 X PAUL A'ITAGUILE, Assistant Examiner. 2,190,237 2/1940 Koehring 11722 2,261,228 11/1941 Cockrum 117-22 2,289,614 7/1942 Wesley et a1. 117-22 10 117-33,130