US3532609A - Process for the preliminary treatment adapted for the electrolytic formation of aluminum coatings on metallic surfaces in molten salt bath - Google Patents

Description

United States Patent O PROCESS FOR THE PRELIMINARY TREATMENT ADAPTED FOR THE ELECTROLYTIC FORMA- TION OF ALUMINUM COATINGS ON METALLIC SURFACES IN MOLTEN SALT BATH Akira Miyata, Chikayoshi Tomita, and Akio Suzuki, Tokyo, and Hideyo Okubo and Masahiko Nagakuni, Kanagawa-ken, Japan, assignors to Nippon Kokan Kabushiki Kaisha, Tokyo, Japan N Drawing. Filed Nov. 9, 1965, Ser. No. 507,033

Int. Cl. C23f 17/00; C23b /50, 5/22 US. Cl. 204-38 7 Claims ABSTRACT OF THE DISCLOSURE SUMMARY OF THE INVENTION This invention relates generally to improvements in or relating to steel, copper or other metal sheets or surfaces with aluminum by the electrolytic process utilizing molten salts containing aluminum halide and the like. More specifically, it relates to a novel treating process to be carried out in advance of the electrolysis, and capable of providing a compact and well-bonded aluminum coating on the final products which is free of pinholes.

Although the application of an aluminum coating has been conventionally and broadly employed in various industries in the past, none of the metal stock used has been ideally suited for coating, since it always contained nonmetallic inclusions or other defects on the surface. When this kind of stock is subjected to the electrolytic coating process, the n0nconducting particles on the surface will not be covered by the metallic deposit and, as a result, pin holes will be formed in the coating. Further, it should be noted that to date there has been no practical process for the removal of these inclusions.

Therefore, it is the main object of this invention to provide a preparatory treating process of the kind above referred to, for providing effective means for overcoming these disadvantageous surface defects with which all of the stock is supplied.

A further object of the invention is to provide a preparatory treating process of the above mentioned nature, providing the possibility of the subsequent electrolytical deposition of a nonporous and compact aluminum coating on the stock.

A still further object of the invention is to provide a process of the above kind, for obtaining an electrolytic coating with much higher current densities without the adverse effects of the conventional process.

Further objects, features and advantages of the present invention will be more clearly understood as the description proceeds.

According to the novel technique proposed by the invention, an intermediate and compact metallic layer without pin-holes can be formed for eliminating the detrimental surface defects in the stock. The metallic layer utilized for this purpose is exclusively of zinc, the hydrogen overvoltage of which, as commonly known, is higher than that of iron or steel and will afford a subsequent adhesive coating.

3,532,609 Patented Oct. 6, 1970 When the stock thus treated is subsequently subjected to an electrolytic coating process of the above kind, a faster bonding and more compact aluminum coating is formed on the stock than is obtainable by the conventional direct-coating technique without the use of the intermediate zinc layer. In addition, higher current densities permit much higher production output per unit time.

It is highly recommended that absorbed gases be driven off from the surface of the stock in preparation for the intermediate layer. For this purpose, electron bombardment upon the surface of the stock under vacuo, say at 10 mm. Hg, may be most advantageously employed after the degreasing treatment. This preparatory bombardment is, according to our experiments, highly effective to strengthen the bond between the intermediate zinc layer and the stock material.

The intermediate zinc layer may be formed by vacuum evaporation. Alternatively, a zinc foil may be pressed on at an elevated temperature and thus made integral with the surface of the stock. Further, when necessary, a zincgalvanizing process can be employed, provided that the size of the deposited particles is intentionally controlled.

Further details of the invention can be well understood from the following examples thereof, however, the details are disclosed in a non-limiting sense of the invention.

EXAMPLE 1 A steel sheet, 50 mm. x 150 mm. x 0.5 mm., was degreased by dipping in an aqueous caustic soda solution at 70 C. for about 3 hours.

The sheet was then taken out from the bath, and dipped in another bath comprising a 2%-sulfuric acid aqueous solution for several seconds, cleansed by water, and dried in a hot air bath.

The stock, which was cleaned in this way, was placed and retained in a vacuum vessel for about 20 minutes at 10 mm. Hg. Electron bombarding treatment in which a high DC. voltage such as 5 kilovolts was impressed between the stock and an electrode made of tungsten, positioned at a distance of 2030 cm. below the stock, for directing an intense electron beam upon its surface for driving the absorbed gases out of the stock material.

The stock was kept in the vessel until it cooled down to a temperature lower than C., and then granules of zinc, positioned below the stock, were heated electrically to about 400 C. and kept at that temperature for about 2 minutes, to effect their sublimation or evaporation. In this way, a thin zinc film, 0.3-0.5 micron thick, was deposited on the surface of the steel sheet in vacuum.

The zinc-coated stock was taken out of the vessel and dipped in a fused bath comprising aluminum chloride 60 mole percent, sodium chloride 25 mole percent and potassium chloride 15 mole percent, at l60-180 C., and subjected to electrolysis, using an aluminum plate as the anode with a cathodic current density of 2 amperes per square decimeter at about 0.6 volt. Upon being treated in this Way for about 10 minutes, a film of aluminum, 1.9 microns thick in the mean, was formed on the both sides of the steel sheet. This coated film had a beautiful silverlike appearance, and was smooth and compact, and of a very good quality. The corrosion resistance of the product was found excellent as shown in line II of Table 1 set forth hereinafter.

EXAMPLE 2 In the similar way as set forth in Example 1, a steel sheet having the same dimensions as before was degreased, pickled and washed with water and dried. This stock was then electroplated with a zinc layer, in an alkaline solution containing a small amount of mercury for preventing the possible generation of hydrogen bubbles, with care being taken to minimize the size of the deposited zinc particles. The final aluminum coating, 1.7 microns in thick- 3 ness, was formed on the stock in the manner as set forth in Example 1. Instead of an alkaline electrolyte, an acid one may be employed. In this case, however, caution must be exercised; before to avoid the possible generation of hydrogen bubbles at points on the steel surface where in What We claim is:

1. In a process for coating aluminum on an iron or steel stock comprising the steps of applying a non-porous coating of zinc on the stock, and forming a coating of aluminum n the stock by placing the Zinc coated stock clusions exist, such as carbon particles, because at such 0 in a molten salt bath consisting essentially of aluminum points hydrogen bubbles tend to generate owing to their chloride, sodium chloride and potassium chloride at a lower hydrogen overvoltage and then break up causing the temperature range of about 160180 C. and electrolyticaldeposit to become porous. ly depositing aluminum on the stock using an aluminum The aluminum-coated steel sheet according to the presplate as the anode and the stock as the cathode. ent Example 2 has superior corrosion-resistance as indi- 2. In a process, as set forth in claim 1, wherem prior cated in the third line III of Table l. to applying the zinc coating on the stock, the steps of dereasin th s ck and rivin absorbed ases rom EXAMPLE 3 b g a to d g g f the stock.

Anon-porous zinc fOll, 100 micronsthick, was prepared 3, I a ro a et fo th in claim 2, the step of and cladded on a steel sheet having dimensions as set driving off the absorbed gases being accomplished by forth in Example 1, by pressing at an elevated temperaplacing the stock in a vacuum vessel under vacuum conture, for instance 400 C. Further treatment Was Carried ditions and effecting electron bombardment of the metal out as disclosed in Example 1. In this way, an aluminum t k, coated steel sheet was obtained having similar properties 4. In a process, as set forth in claim 2, wherein the to those of the product in the foregoing example. step of applying a coating of zinc on the stock compris- TABLE 1 Thiek 11055 of Al- Ilumidity test Exposure to in- Icst layer, Water uumerslon (RH. 08%, industrial Heating test Water innncrsion No. Q, Surface treatment microns test 0.) atmosphere at 650 C. test after bending 1 Alplateddireetlyonstcel 2.0 Redrustappearcd Redrusta-ppearcd Redrustappeared after 7 days. after 3 days. after 7 days. II Al plated on Vacuum de- 1. 9 No changes for No changes for No changes for Durable for Durable for posited Zn-film 6 months. 30 days. 3 months. longer than longer than (Example 1). hours. 240 hours. III Al plated on eleetro- 1. 7 1o N0 considerable No considerable ..do Do.

deposited Zn film changes for 30 changes for 3 (Example 2). days; slight months; slight change of color. color change to gray. IV IIot dip Al-eoating 25 Redrust appeared Red rust appeared Discolored after after one day. after one day. days.

It will be recognized from the foregoing that the products, processed according to our teaching of the present invention, have, despite their thin coating, a remarkable corrosion resistance. Even after severe mechanical deformation, our products preserve their corrosion resistance as can be understood from the results of the water immersion tests made after bending, as disclosed in Table 1.

Even though the foregoing examples were directed exclusively to steel sheets as the stock, this invention is also applicable to other various kinds of steel stock, as well as metals other than steel.

From the foregoing, it will be noted that with the r zinc-coated stock as proposed by this invention, the electrolysis for depositing the desired aluminum coating may be carried out under high current densities for mass production. To show this more clearly, results of a comparative test wherein the cathodic current density was continuously increased, are shown hereinbelow in Table 2.

TABLE 2 ing effecting the deposition of the zinc coating layer by vacuum evaporation.

5. In a process, as set forth in claim 4, wherein cooling the electron bombarded stock within the vacuum vessel to a temperature lower than 100 C., and effecting the vacuum evaporation of the Zinc coating on the stock by electrically heating granules of zinc within the vacuum vessel to about 400 C. for about two minutes for forming a zinc vapor for deposition on the surface of the stock.

6. In a process, as set forth in claim 2, wherein the step of applying the zinc coating layer on the stock is effected by pressing a non-porous zinc foil on the surface of the stock at an elevated temperature in the range of about 400 C.

7. In a process, for coating aluminum on an iron or steel stock, comprising the steps of degreasing the stock, driving off absorbed gases from the stock by placing the stock in a vacuum vessel under vacuum conditions and Cathodic current density, amp./d.m'-

Stock, Coated steel sheet 2 5 10 as per the invention Bare steel sheet Good in appearance... Gray dendrite appeared; Black dendrite appeared; inferior adherence.

interior adherence. D0 .do Good in appearanee White silver tone; smooth and beautiful.

It should be understood that the foregoing specific examples have been disclosed only by way of example. Various changes and modifications may occur to those skilled in the art upon reading through the foregoing detailed description. These changes and modifications should be included in the scope of the invention, provided that they do not lie outside of the proper interpretation of the appended claims.

effecting electron bombardment of the stock, cooling the electron bombarded stock within the vacuum vessel to a temperature lower than C., electrically heating granules of zinc within the vacuum vessel to about 400 C. for about two minutes for forming a zinc vapor for deposition on the surface of the stock and forming a zinc coating layer thereon, and dipping the zinc coated stock in a molten salt bath consisting essentially of aluminum 5 6 chloride 60 mole percent, sodium chloride 25 mole per- 3,247,086 4/1966 Goldstein et al. 204--34 XR cent, and potassium chloride 15 mole percent, at a tem- 3,268,422 8/1966 Smith et al. 204-34 XR perature range of about 160 l80 C. and electrolytically 3,307,936 3/ 1967 Smith 75---65 XR depositing aluminum on the stock using an aluminum plate as the anode and the stock as the cathode with a cathodic r FOREIGN PATENTS current density of about 2 amperes per square decimeter 15,831 1886 Great Britain at about 0.6 volt.

References Cited O H R REFERENCES UNITED STATES PATENTS Journal of Electrochemical Society, Preprint 78-17, The 2 44 349 -3 1943 i et 1 204 14 10 Deposition Potentials of Metals From Fused Alkali Chlo- 2 544 70 3 1951 Grange et 1 29 19 5 XR ride-Aluminum Chloride Baths, Wade et al. 2,763,058 9/1956 McCullou h et al. 29196.5 XR 2,807,575 9/1957 Charlton e t al. 2o4 39 HOWARD WILLIAMS, Prlmary Exammfir 3,103,472 9/ 1963 sla'lin 204-67 15 W. VAN SISE, Assistant Examiner 3,438,754 4/ 1969 Shepard et al. 2043-8 XR 2,968,723 1/1961 Steigerwald 14813 XR U.S. C1.X.R.

3,205,086 9/1965 Brick et al. 204-34 XR 204-29, 39, 67

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 532 ,609 October 6 1970 Akira Miyata et a1 It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

insert coating intermediate,

--. Column 3, line 4,

- easily Column 1, line 30, before "steel" line 66, intermediate and" should read Column 2, line 7, before "much" insert a cancel 3 before"; line 70 before "occur" insert Signed and sealed this 30th day of March 1971 (SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

EDWARD M.PLETCHER,JR. Attestaing Officer Commissioner of Patents