US3794572A - Process for coating steel - Google Patents

Abstract

PROCESS FOR DEPOSITING CHROMIUM HYDROXIDE ON STEEL IS PROVIDED. A SMALL AMOUNT OF ONE OR MORE OF A THIOSULFATE, DITHIONITE, BISULFITE, SULFITE, THIOSULFITE, PYROSULFITE, SULFIDE AS AN AUXILIARY AGENT IS ADDED TO A MAIN ELECTROLYTE SOLUTION OF CHROMIC ACID AND A CATHODE ELECTROLYTIC REDUCTION THEREOF IS CARRIED OUT IN A CONVENTIONAL CELL.

Description

United States Patent 3,794,572 PROCESS FOR COATING STEEL Hidehisa Yamagishi and Hiroshi Takano, Kawasaki,

Japan (both Nippon Kokan Kabushiki Kaisha Technical Institute, 2730 Miuamiwataridachio, Kawasaki-sh], Kanagawa-ken, Japan) No Drawing. Filed June 1, 1970, Ser. No. 42,505 Claims priority, application Japan, Apr. 28, 1967, 42/27,117 Int. Cl. C23h 11/02 US. Cl. 204-56 R 10 Claims ABSTRACT OF THE DISCLOSURE Process for depositing chromium hydroxide on steel is provided. A small amount of one or more of a thiosulfate, dithionite, bisulfite, sulfite, thiosulfite, pyrosulfite, sulfide, as an auxiliary agent is added to a main electrolyte solution of chromic acid and a cathode electrolytic reduction thereof is carried out in a conventional cell.

FIELD OF INVENTION This invention relates to a process for producing a coating having excellent adhesion properties and/or corrosion resistance using an ordinary stationary cell and electrolytic reduction with an aqueous treating solution comprising chromic acid as the primary agent and an auxiliary agent selected from thiosulfate, sulfite, dithionite, bisulfite, thiosulfite and sulfide.

BACKGROUND OF INVENTION 'Heretofore, in producing a corrosion resisting -film on the surface of steel with a hydrated chromium oxide as the essential film component, a baking treatment or electrolytic reduction has been employed. Chromic acid or chromate in relatively low concentration in solution has been used as the primary agent, with various additives as the auxiliary agents. In electrolytic reduction, typical auxiliary agents which have been used include: sulfuric acid, tartaric acid, maleic acid, oxalic acid, aromatic sul- :fonic acid having a hydroxyl group, sucrose, and glycol. Such electrolytic plating solutions have not been completely satisfactory in that chromium metal which is produced in the film is only 70-80% of the total quantity of chromium deposited, and the remaining chromium of 20- 30% exists as hydrated chromium oxide. Therefore, the coating film produced by conventional electrolytic treatment generally comprises a double layer consisting of metallic chromium and hydrated chromium oxide layers deposited on the surface of steel, notwithstanding the difference in some of the electrolytic conditions or the difference in auxiliary agents. It is true that such a double layer contributes to the corrosion resistance, but problems remain with regard to adhesion or color tone of the surface. The object of the present invention is to provide a process for forming easily a coating which is primarily chromium oxide.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a process for depositing chromium hydroxide on a steel surface which comprises contacting a steel surface with an aqueous chromium plating solution for from 1 to 5 seconds and a current density of to 50 A/dm. the solution containing from about 50 to about 100 grams per liter of chromic acid and from about 0.5 to about 1 gram per liter of an auxiliary agent selected from the group consisting of an alkali metal thiosulfate, sulfite, dithionite, bisulfite, thiosulfite, pyrosulfite, and sulfide.

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SPECIFIC EMBODIMENTS OF THE INVENTION It is a feature of this invention that the aqueous chromium solutions employed have an usually low concentration of chromic acid. The concentration ranges from about 50 to about 100 grams per liter (g./l.). Concentrations below about 50 g./l. are insuflicient to provide a suitable film on the steel surfaces which are treated. More than about 100 g./l. results in considerable deposit of metallic chromium, rather than the desired chromium hydroxide. With excessive deposits of metallic chromium, the resulting films suffer in adhesion properties.

The aqueous chromium solutions also contain one or more auxiliary agents. These agents include water-soluble thiosulfates, sulfites, dithionites, bisulfites, thiosulfites, pyrosulfites and sulfides, particularly the alkali metal compounds of these groups. The concentration of such an agent or agents in the solutions is also controlled. Such concentrations range from about 0.5 to about 1 g./l. of solution. Concentrations of less than about 0.5 g./l. are insufficient to provide the desired characteristics to the steel surfaces which serve as the cathode and which are treated. And concentrations in excess of about 1 g./l. cause unsatisfactory surface efiects.

Current density employed ranges from about 10 to about 50 A/dmfi, with about 30 preferred. Plating or treating time with such a current density is extremely short, from about 1 to about 5 seconds, with about 3 preferred.

In treating a steel surface, it is subjected to conventional cleaning treatments to remove any fats or oils. It can be contacted with a conventional pickling solution to remove any rust which may be on the surface, and then washed with water. It is then contacted in a conventional stationary chromium plating cell, under the conditions of this invention.

Steel such as strip steel, ordinary carbon steel sheet and the like are suitable for use in the process of this invention.

The invention is illustrated by the following examples.

EXAMPLE 1 Chromic acids having dilferent concentrations, were electrolyzed in a conventional stationary plating cell using specified auxiliary agents under the conditions shown in Table 1 below and using a workpiece as the cathode.

TABLE 1 Concentration of chromic acid, g./l.

Current density,

Auxiliary agent aJdm.

EXAMPLE 2 Samples of Experiment Nos. 1 and 3 in Table 1 and a sample obtained by Experiment No. 1, using 0.5 g./l. of H instead of Na S O for comparison, were tested for their physical and chemical properties. The results of the tests are shown in Table 2.

TABLE 2 Adhesio o 1: Antiw bending property Antiimpaet Erichsen D D W test Synthetic mark Experiment N 0.

Square test N orE.The numerals denote marks of evaluation.

Bending test was carried out in accordance with test method 115 Z 2203 and impact test in accordance with test method 115 Z 2242.

In the Erichsen test, the samples, the coated surfaces of which are on the outside, are pushed out of the inside of an Erichsen tester by a circular rod having a certain diameter at a definite speed, and the distance by which the samples are pushed is measured when cracks or exfoliations appear.

In the square test, the coated surface of the samples is cut with a knife in such a way as to make 100 squares, each of which has an area of 1 mm. so deeply as to reach the matrix, and the state of the exfoliation of the film is observed.

In any of the tests shown in Table 2, values obtained as a result of the tests are converted into evaluated marks for convenience, and the synthetic marks are obtained by adding all the marks in each experiment.

Table 3 shows the result of chemical property tests.

NOTE-Numerals denote marks of evaluation.

Salt spray test is carried out in accordance with test method JIS Z 2371. The result of the test is evaluated as 1 when rust appears in more than 51% of the area of the surface of the sample, and as 10 when rust does not appear (i.e. of the area).

Shade test is carried out with regard to the colors of rusts. A sample on which the color of the rust does not change at all is evaluated as 10, and one whereon the characteristic reddish brown rust appears is evaluated as 1.

As demonstrated with the above-described examples, involving the specified auxiliary agents of this invention, the anti-corrosive property of a film on the surface of the samples is substantially superior to that of a film wherein H was used in the treating solution.

What is claimed is:

1. A process for depositing chromium hydroxide on a steel surface which comprises contacting a steel surface as a cathode with an aqueous chromium plating solution for from about 1 to about 5 seconds and a current density of from about 10 to about 50 A/dmfi, the solution containing from about 50 to about grams per liter of chromic acid and from about 0.5 to about 1 gram per liter of an auxiliary agent selected from the group consisting of an alkali metal thiosulfate, sulfite, dithionite, bisulfite, thiosulfite, pyrosulfite and sulfide.

2. A process of claim 1, wherein the chromic acid concentration is about 50 grams per liter.

3. A process of claim 1, wherein the chromic acid concentration is about 100 grams per liter.

4. A process of claim 1, wherein the agent is Na S O 5. A process of claim 1, wherein the agent is Na S O 6. A process of claim 1, wherein the agent is Na SO 7. A process of claim 1, wherein the agent is NaHS0' 8. A process of claim 1, wherein the concentration of said agent is from about 0.5 to about 0.65 gram per liter.

9. A process of claim 1, wherein the current density is about 30 A/dmfi.

10. A process of claim 1., wherein the time is about 3 seconds.

References Cited UNITED STATES PATENTS 2,976,193 3/1961 Pimbley 148-621 2,746,915 5/ 1956 Giesker et a1. 204-56 R 3,486,990 12/1969 Fukuzuka et al. 204-56 R 2,898,250 8/1959 Pimbley 148-621 3,679,554 7/1972 Yamagishi et a1. 204-56 R 3,479,260 11/ 1969 Rauch et a1 204-56 R 2,998,361 8/1961 Kitamura 204-56 R 2,812,296 11/ 1957 Neish 204-56 R 1,233,633 7/1917 Allen 204-56 R JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner 9 I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 72 Dated February 26, 1974 'lnventor(s) HIDEHISA YAMAGISHI et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 7 Replace "Filed. .42, 505" by the following:

--Continuationin-part of abandoned application Ser. No. 724,991, April 29, 1968. This application Jnne'l", 1970, Ser. No. 42,505--.

Signed and sealed this 15th day of August 197A.

(SEAL) Attest:

McCOY M; GIBSON, JR. (2. MARSHALL DANN Attesting Office-r Commissioner of Patents USCOMM-DC 603764 69 w u.s. covznuugwr nmmu; omc: 2 an oan-s:u.

F ORM PO-I 050 (10-69)