US3535213A

US3535213A - Method of surface-treating metals

Info

Publication number: US3535213A
Application number: US691925A
Authority: US
Inventors: Hideya Okada
Original assignee: Yawata Seitetsu KK
Current assignee: Yawata Seitetsu KK
Priority date: 1967-12-20
Filing date: 1967-12-20
Publication date: 1970-10-20
Anticipated expiration: 1987-10-20

Description

METHOD OF SURFACE-TREATING METALS Hideya Okada, Tokyo, Japan, assignor to Yawata Iron & Steel Co., Ltd., Tokyo, Japan N Drawing. Continuation-impart of application Ser. No.

357,726, Apr. 6, 1964. This application Dec. 20, 1967,

Ser. No. 691,925

Int. Cl. C23b 5/50, 5/56 U.S. Cl. 204-41 3 Claims ABSTRACT OF THE DISCLOSURE This application is a continuation-in-part of copending application Ser. No. 357,726, now abandoned.

This invention relates to methods of surface-treating metals and more particularly to methods of surface-treating iron and steel.

Generally chromium plating has various advantages, e.g., it is pleasing in appearance, does not discolor in the atmosphere and is high in heatproofness, hardness and wear-resistance. Therefore, the field of its application is expanding more and more. But it has a defect in that the chromium film is so likely to produce pores or cracks that the ground metal can not be perfectly coated. It is therefore usual to apply copper or nickel plating as an intermediate layer onto the ground metal to be chromium plated and then to apply a very thin chromium plating thereto. However, even with such a chromium plating method, the anticorrosiveness is far from being perfect.

According to the present invention, by using a chromate film as a ground without applying such copper or nickel plating as is used as a ground for conventional chromium plating, there can be obtained a protective film which shows excellent anticorrosiveness even under very severe conditions and is high in workability. That is to say, the present invention is directed to a method of treating metal surfaces characterized by applying a chromate film onto the metal surface to be treated and then chromium-plating said chromate film.

Chromate treatment is used to prevent rust and has a wide range of applicability. Such film is composed of an oxide of chromium, has a substantially amorphous structure and is therefore very high in anticorrosiveness and workability. By using it as a ground for chromium plating, the anticorrosiveness and workability of the chromuim-plated film can be remarkably improved.

In the conventional methods that, for example, use nickel plating as a ground for chromium plating, generally the nickel layer used as the ground will be exposed through cracks in the chromium plating. Therefore, when the chromium plating is left in a corroding environment, a potential difference will be produced between chromium plating and nickel plating and corrosion will be accelerated. On the other hand, according to the method of the present invention, a chromate film exists as a ground for chromium plating and therefore, even though a chromate film is exposed through cracks in the chromium plating, no potential difference will be produced between the chromate film and chromium plating and therefore corrosion will be remarkably prevented.

United States Patent 0 Such adhesion between the chromate film and chromium plated layer as is obtained by the method of the present invention is so high as to endure severe working. In addition, the plating of the present invention is so high in paint adhesiveness as to be able to be advantangeously used as a ground for painting.

In the method of the present invention, the surface of the metal to be treated is first degreased by any normal process, is pickled and is then coated with a chromate film thereon. The chromate treatment itself can be done with any known process. The chromate film can be applied onto the surface of the metal to be treated, for example, by carrying out an electrolysis with the metal as the cathode in an aqueous solution containing chromic acid or by dipping the metal in an aqueous solution of chromic acid containing a reducing agent, squeezing it with rolls and then heating and baking it. For example, in the case of chromate-treating a steel sheet by the former process, the electrolysis may be carried out with the steel sheet as a cathode in an aqueous solution containing about 0.1 to 3 moles of pure chromic acid. It is preferable that the current density is about 15 to 100 a./dm. the electrolyzing time is about 1 to 60 seconds, and preferably 1 to 5 seconds, the bath temperature is about 20 to 50 C. and the thickness of the chromate film is about 100 to 5000 A. However, depending on the circumstances, other conditions then are mentioned above can be used. In some cases, the chromate-treating bath to be used may be prepared by adding such chromate film production accelerators as, for example, selenic acid, potassium zirconium fluoride, potassium titanium fluoride or perchloric acid to an aqueous solution of chromic acid.

The typical bath composition in the case of treating, for example, a steel sheet with a bath prepared by adding a reducing agent to an aqueous solution of chromic acid in the latter process is about 15% chromic acid, about 6% zinc oxide, about 15% ammonium molybdate and about 5% cane sugar, the rest being water. When the steel sheet is dipped in this bath, is squeezed with rolls and is then heated at a temperature of about 550 C. for about 1 minute, a chromate film Will be produced on the surface of the steel sheet.

Needless to say, the process for applying a chromate film onto the surface of a metal according to the method of the present invention is not limited to the above mentioned process but may be any other chromate-treating process.

The metal onto which a chromate film has been applied as in the above is then chromium plated in the second step of the present invention. This chromium plating step itself can be applied by any known process. Specifically, it is preferred to carry out the chromium plating by using a current density of about 20 to 60 a./dm. at a bath temperature of about 45 C. for about 1 to 60 seconds in an ordinary Sargent bath (of l to 3 mols of CrO and 0.01 to 0.03 mole of H The specifically preferable range is a current density of 25 to 35 a./dm. and a treating time of l to 5 seconds. The thickness of the chromium-plated layer may preferably be about to 5000 A. However, when applying the present method on tin-free steel, a total thicknes of between 100 and 500 A for both the chromate film on the chrome plating layer is preferred.

Needless to say, the method of the present invention can be applied to such final metal product. Further, the chromium-plated layer obtained by the method of the present invention is so high in anticorrosiveness, workability and paint adhesiveness as shown by the data below that it can be advantageously applied to such material as a metal sheet.

The method of the present invention shall be explained with reference to examples in which it is applied to nickel-plated surface of steel sheets. However, the subject matter of the present invention is not limited to such application but can be applied to such various metal-plated surfaces as zinc, iron and copper.

EXAMPLE 1 A mild steel sheet was degreased and pickled and was then first treated under the below mentioned conditions in a chromate bath to apply a chromate film onto it.

Solution0.5 mol of CrO Electrolyzing conditions:

Current density30 at/din. Temperature30 C. Treating time seconds The steel sheet having the thus obtained chromate film was chromium plated under the below mentioned electrolyzing conditions in the below mentioned plating bath. The chromium-plated surface on the steel sheet obtained after water-rinsing and drying showed a high anticorrosiveness, workability and paint adhesiveness.

Chromium plating bath 3 mols of CrO +0.03 mol of H 80 Electrolyzing conditions:

Current densitya./dm. Temperature- C. Treating time30 seconds Example 2 A mild steel sheet was degreased and pickled and then a chromate film was applied onto said steel sheet under the below mentioned electrolyzing conditions in the below mentioned chromate bath.

Chromate bath-0.5 mol of CrO +0.5 g./l. of H SeO Temperature-45 C. Treating time-30 seconds When the steel sheet having the thus obtained chromate film was chromium-plated under the chromium-plating conditions shown in Example 1, the chromium-plated surface showed properties high in anticorrosiveness, workability and paint adhesiveness.

Example 3 The plated surface of a steel sheet nickel-plated by a known treating process by using a Watts bath was cleaned, was then chromium-treated as shown in Example 1 and was chromium-plated on the chromate film under the same conditions as in Example 1 except that the treating time was 1 minute. The thus obtained chromium-plated surface showed the same very high anticorrosiveness, workability and paint adhesiveness as in Examples 1 and 2.

The results of the anticorrosiveness tests of the chromium-plated layers on the mild steel sheets as obtained by the method of the present invention and conventional methods are shown in Table 1.

TABLE l.ItESULTS OF ANTICORROSIVENESS TESTS Sample No. N-30: The treating conditions were as mentioned in Example 2.

Sample No. N-31: The plating was made for 1 minute under the conditions in Example 1.

Sample No. N-33: The nickel plating was made in a Watts bath containing a brightener for 30 seconds at a current density of 10 a./dm. The chromium plating was made under the same conditions as in Example 2.

The results obtained with the mentioned respective samples after salt spray tests for 24 hours are shown in Table I. It will be seen that the anticorrosiveness of sample No. N-30 as obtained by the method of the present invention is much higher than of the Samples Nos. N-31 and N-33 by conventional methods.

Further, even when the sample by the method of the present invention was subjected to an outdoor exposing test, no rust was produced for several months.

The results of the workability test and paint adhesiveness test of the sample obtained by the method of the present invention are shown in Table 2.

TABLE 2RESULTS OF WORKABILITY AND PAINT ADHESIVENESS TESTS Paint adhesiveness test (melamine alkyd paint) workability test (Ericsen Picture drawing Sample N 0. test) Du Pont test test Sample obtained N o peeling was No painted N o painted by Example 2. caused at all film peeled film peeled by extrusion oil. oil". of 7 mm.

EXAMPLE 4 A mild steel sheet was degreased and pickled and was then first treated under the below mentioned conditions in a chromate bath to apply a chromate film onto the surface thereof.

Bath composition-C10 0.5 mol Bath temperature-45 C. Current density50 a./dm. Time--1 second The steel sheet having the thus obtained chromate film was chromium plated under the below mentioned electrolyzing conditions in the below mentioned plating bath.

Bath composition-CrO 2.5 mol; H 2.5 g./l. Bath temperature45 C.

Current density50 a./dm.

Time-4 seconds The chromium plated steel sheet obtained by the above method showed a beautiful metallic luster in appearance. Its anticorrosive properties in a salt water spray test for 6 hours was only 10%. Further, even when the chromium plated steel sheet was dipped in 1% sulfuric acid for 24 hours, only 40,41. g./ml. of iron was dissolved.

EXAMPLE 5 A mild steel sheet was degreased and pickled and was then first treated under the below mentioned conditions in a chromate bath to apply a chromate film onto it.

Bath compositionCrO 0.5 mol; H 80 1.2 g./l.

Bath temperature-45 C.

Current density50 a./dm.

Time3 seconds m l The steel sheet having the thus obtained chromate film was chromium plated under the below mentioned electrolyzing conditions in the below mentioned plating bath.

Bath compositionCrO 2.5 mol; H 50 2.5 g./l. Bath temperature-45 C.

Current density-50 a./dm.

Time2 seconds 6 The film obtained by the above method showed a beaube obtained very quickly by the method of the present tiful metallic luster in the appearance and, had excellent invention. anticorrosive properties. For example, when the steel sheet The example below illustrates that the appearance or was subjected to a salt water spray test for 6 hours, no luster of the chromium plated metal surface depends on rust was produced and, when the chromium plated steel the conditions utilized in producing the chromium film sheet was dipped in 1% sulfuric acid for 24 hours, only 80 g./ml. of iron was dissolved.

prior to the plating operation.

EXAMPLE 8 The following examples lllustrate the fact that when a ground metal is first coated with a chromium film and A m1ld Steel Sheet was degreased and Plckled w subsequently plated with chromium, a chromium plated then first treated under the below mentioned conditions ground metal having an excellent metallic luster can be in a chromate bath to apply a chromate film Onto obtained in a much shorter time than if no chromium B th c0rnp0siti0n-CrO 0.5 mol; H 80 0.5 to 1.2 g./l. film is formed on the surface of the metal prior to the B h tem erature 4j C,

chromium Plating 0f Said ground metal; Current density50 a./dm. with Pb anode Electrolyzing time2 to 3 seconds EXAMPLE 6 The steel sheet was then chromium plated according to the procedure and conditions as shown in Example 6 A m1ld steel sheet was degreased and pickled and was above.

then first treated under the below mentioned conditions h electrolyzing i f h h i 1 i in in a chromate bath to pp a chromate Plating On the onds and the variation of the appearance may be seen in surface thereof. the table below.

Electrolyzing time of chromium-plating in seconds 1 2 3 4 5 10.

Appearance No metallic No metallic There was There was There was There was luster. luster. a metallic a metallic a metallic a metallic luster. luster. luster. luster. Reflection rate 98 98 85 80.

Bath composition-CrO 2.5 mol; H 80 2.5 g./l. Bath temperature45 C. Current density-50 a./dm.? with Pb anode It should be noted that when selenic acid is added to the bath composition for making the chromate film, a metallic luster will be shown more quickly during the The appearance of the metal thus treated may be seen electroplating operation. in the table below: The fact that a chromate layer is thus electrodeposited Electrolyzing time in seconds 1 3 .2 5 7 10.

Appearance N0 metallic No metallic No metallic No metallic There was luster. luster. luster. luster. a metalic luster. Reflection rate 85.

As can be seen by the above showings, it requires at below and then chromium-plating is made is epochal as least 10 seconds for a chromate film to be plated on the a method of not only increasing the anticorrosiveness but surface of the stainless steel sheet when no chromium film also obtaining an appearance having a metallic luster is first filmed on the surface of the steel sheet. within a very short time. In case a chromium-plating bath of 250 to 300 g./l. of CrO +2.5 to 3.0 g./l. of H EXAMPLE 7 which is most generally used is used, unless a method of This example demonstrates the variance of appearance making a chromate layer below is used, it will be nearly of a metal sheet, when a metal sheet is treated according 5O impossible to obtain on iron or steel an appearance having to the method of the present invention. a metallic luster within a few seconds.

A mild steel sheet was degreased and pickled and was However, once a chromium-plating bath of a low conthen first treated under the below mentioned conditions centration is used, that is, a bath composition containing in a chromate bath to apply a chromate film onto it. 100 to 150 g./l. of CrO is used, an appearance having a Bath composition cros, 0.5 mol metallic luster will be able to be obtained within a com- Bath temperature paratrvely short t1me. However, m such case, as 1s well Current ajdmz known, the anticorrosiveness Wlll be reduced to as much Electrolyzing to 2 seconds lower value than in the case of using a bath composltion having 250 to 300 g./l. of CrO as a main ingredient.

The steel sheet was then chromium plated according to Therefore, even if a chromate layer is made below, in

the procedure and conditions shown in Example 6 abovecase the bath composition of the chromium plating bath The appearance of the metal thus obtained may be to be electrodeposited thereon has to g./l. of

seen in the table below: CrO as a main ingredient, the anticorrosiveness will be Thus, as can be seen above, in the case wherein a 79 low. Even in the case there is a chromate layer below, it

chromate layer is produced below, a plated surface having will be preferable to use a bath composition having 250 to a metallic luster can be obtained very quickly. Even if 300 g./l. of CrO as a main ingredient for the chromiumthe time required to make the chromate layer and the plating bath to be electrodeposited thereon.

time for electrodepositing the chromium-plated layer are From the above, it may be seen that in making a added together, it can be seen that a metallic luster may 75 product high in anticorrosiveness and metallic luster on an iron or steel plate, it is preferable to first make a chromate layer on iron or steel and then electrodeposit chromium from a bath composition having 250 to 300 g./l. of CrO as a main ingredient. In such case, it is considered best to electrodeposit a chromate layer under the conditions of a bath temperature of 30 to 50 0, current density of 20 to 70 a./dm. and electrolyzing time of 1 to 3 seconds from a bath composition having 25 to 70 g./l. of chromic acid as a main ingredient (to which 0.1 to 5 g./l. of selenic acid may be added) and then electrodeposit chromium under the conditions of a bath temperature of 30 to 45 C., current density of 20 to 70 a./dm. and electrolyzing time of 1 to 4 seconds from a bath containing a bath composition having 250 to 300 g./l. of chromic acid as a main ingredient and a small amount of an additive (such as a sulfuric acid radical).

As pointed out above, the concentration of the chrmium plating bath is preferably 250 g./l. of CrO For example, when a steel sheet is electroplated for 15 seconds with 250 g./l. of CrO and another is plated with 100 g./l. of CrO for 15 seconds and both are subjected to a salt water spraying test, the following diiferences in rusting may be shown below:

Rusting in salt water spray test for 24 hours: Percent 100 g./l. of CrO 1 g./l. of H 50 70 250 g./l. Of CrO 2.5 g./l. Of H SO 10 What is claimed is:

1. A method of surface-treating metals comprising electrolytically treating an iron or steel product as a cathode in a chromate-treating bath having chromic acid as a main ingredient so that a film consisting of chromium oxide may be deposited on the surface of said iron or steel product and then cathode-electrolytically treating it in a chromium-plating bath containing sulfuric acid for 5 seconds at most at a current density of about 2060 a./dm. so that a chromium-plated coating having a metallic luster may be formed on the surface of the iron or steel product through the above mentioned chromate film within a very short time.

2. A method of surface-treating metals comprising electrolytically treating an iron or steel product as a cathode in a chromate-treating bath containing chromic acid as a main ingredient therein, at a current density of about 15 to 100 a./dm. so that a film consisting of chromium oxide may be deposited on the surface of said iron or steel product, and then cathode-electrolytically treating it in a chromium-plating bath consisting of an aqueous solution of 2.5 to 3.0 g./l. of sulfuric acid and 250 to 300 g./1. of chromic acid for 1 to 5 seconds at a current density of about 20-60 a./dm. so that a chromiumplated coating having a metallic luster may be formed on the surface of the iron or steel product through the above mentioned chromate film within a very short time.

3. A method of surface-treating metals comprising electrolytically treating an iron or steel product as a cathode at a current density of 20 to 70 a./dm. for 3 seconds at most in a chromate-treating bath at a bath temperature of 30 to C. having 25 to 70 g./l. of chromic acid as a main ingredient and containing 0.1 to 5 g. /l. of selenic acid so that a very thin chromate film may be deposited on the surface of said iron or steel product and then cathode-electrolytically treating it in a chromium-plating bath consisting of an aqueous solution of 2.5 to 3.0 g./l. of sulfuric acid and 250 to 300 g./l. of chromic acid for 1 to 5 seconds at a current density of 20 to a./dm. so that a chromium-plating coating having a metallic luster may be formed on the surface of the iron or steel product through the above mentioned chromate film within a very short time.

References Cited UNITED STATES PATENTS 2,603,593 7/ 1952 Blickensderfer 20416 2,812,297 11/1957 Stareck et a1. 20434 3,081,238 3/1963 Gurry 20434 3,288,691 11/1966 Yonezaki et a1 20456 3,316,160 4/1967 Uchida et a1. 20441 ROBERT K. MIHALEK, Primary Examiner W. B. VAN SISE, Assistant Examiner US. Cl. X.R. 20429, 36