US3257295A

US3257295A - Method of chemically treating metals

Info

Publication number: US3257295A
Application number: US251002A
Authority: US
Inventors: Yonezaki Shigeru; Kamata Minoru; Watanabe Tsuneyasu; Sakai Kango; Ikegami Kazuo
Original assignee: Yawata Iron and Steel Co Ltd
Current assignee: Yawata Iron and Steel Co Ltd
Priority date: 1962-01-20
Filing date: 1963-01-14
Publication date: 1966-06-21
Anticipated expiration: 1983-06-21
Also published as: DE1521096A1; DE1521096B2; GB1031792A

Description

United States Patent 3,257,295 METHOD OF CHEMICALLY TREATING METALS Shigeru Yonezaki, Yawata, Minoru Kamata and Tsuneyasu Watanabe, Tobata, and Kango Sakai and Kazuo Ikegami, Yawata, Japan, assignors to Yawata Iron & Steel Co., Ltd, Tokyo, Japan, a corporation of Japan No Drawing. Filed Jan. 14, 1963, Ser. No. 251,002 7 Claims priority, application Japan, Jan. 20, 1962,

' 37/ 1,867 6 Claims. (Cl. 204-56) This invention relates to methods of chemically treating metal surfaces.

. Generally, when a cathode electrolysis is carried out in an aqueous solution containing hexavalent chromium ion, a treating time of more than 20 seconds Will be required in order to obtain an expected electrodeposited film;

The present invention is a method of chemically treating metals in which the above mentioned electrolytic treating time can be remarkably reduced and which is characterized by carrying out a cathode electrolytic treatment in a solution'containing hexavalent chromium ion and a slight amount of halogen ion. In spite of the very short treating time, the metal surface treated according to the present invention has a high anticorrosiveness and paint adhesiveness and shows a beautiful interference color. There are three elements which constitute the important part of the present invention; the hexavalent chromium ion, halogen ion and cathode electrolysis. Therefore, if any one of them is lacking, no satisfactory product will be obtainable. The presence of a slight amount of halogen ion suchas chlorine ion or fluorine ion is of especially important significance.

When a cathode electrolysis was carried out in a chromic acid electrolyte containing no halogen ion at all, as described above, an electrolyzing time of more than 20 seconds was required. It is also known to add halogen ion in a chromic acid treatment, but not in an electrolytic A ode electrolysis is carried out by using an aqueous solutionprepared by adding halogen ion'to a solution of a hexavalent chromium compound. When 25, 50 and 100 p.p.m. (parts per million) of a halogen ion are added, the same amount of deposition will be obtainable by elec- With the increase in the amount of addition of a halogen ion which is to be an adjuvant the amount of production of an electrodeposited film will increase. But, if the amount of addition is too large, the film will be likely to peel off. Therefore, the range of addition of 10 to 200 p.p.m. is preferable' As a means of supplying halogen ion, there is added such acid, salt or soluble complex salt as dissociates to yield halogen ion as, for example, hydrochloric acid, bydrofiuoric acid chloride or fluoride, fluotitanate or boron fluoride.

When the cathode electrolysis is continued for a long time in the treating solution of the present invention, a trivalent chromiumion will come to be produced in the solution. However, a trivalent chromium ion may be contained in the treating solution in advance for improving the dryability of the solution.

Phosphoric acid will give a property of improving the paint adhesiveness and the uniform electrodepositability of the electrodeposited film and is therefore a favorable additive. It is necessary to keep the amount of addition of phosphoric acid to below 10 g./l. If this amount is exceeded, no adhesive film will be produced.

When the pH of the solution is above- 1.3, the electrodeposited film will become very hard to produce. Therefore, the pH should be kept below 1.3.

The lower the liquid temperature, the larger the amount of the film produced by the electrolysis. With the rise of the liquid, temperature the amount of production of the film will gradually decrease. However, the treatment in a range of 10 to 80 C. is feasible.

The anticorrosiveness of the product depends most upon the amount of deposition of the film, and the latter is again mostly determined by the amount of addition of the halogen ion, the current density and the electrolyzing time. The amount of film produced will increase almost trolysis for 4, 2 and 1 second, respectively. Without the halogen ion, no chemical treatment can be achieved in such a short time. Such high effect of a halogen of accelerating the formation of a film in a cathode electrolytic treatment has been heretofore quite unknown.

The treating method accordingto the present invention shall now be explained in detail. The present invention is a chemical treating method wherein a cathode elec trolysis is carried out in a solution in which 5 to 50 g./l. of hexavalent chromium ions and 10 to 200 p.p.m. of chlorine or fluorine ions are added, the pH is adjusted to less than 1.3 and the liquid temperature is kept at 10 to 80 C. I

First of all, the composition of the present treating solution shall be described. As supply sources of the hexavalent chromium ion which is to be a main ingredient of the present treating solution can be considered a chromic anhydride, dic-hromate and chromate. But usually a chromic anhydride is used. In case a film is to be formed by using a dichromate or chromate, it will be necessary to adjust the pH to below 1.3 by adding nitric acid or phosphoric acid.

in proportion to increase in current density and to increase in electrolyzing time.

Any condition may be selected for the current density and electrolyzing time but they are in a relation of an inverse proportion. That is to say, in order to obtain the same amount of deposition, the electrolyzing time may be shorter, if the current density is higher and vice versa. Thus, by adjusting the amount of addition of the halogen ion, the current density and the electrolyzing time, a film having any amount of deposition can be obtained.

The color of the treated product will vary depending on the amount of deposition of the electrodeposited film. With the increase in the amount of deposition of the electrodeposited film, the color will change to light brown (below 10 mg./ft. bluish brown (at 10 to 20 mg./ft. blue (at 20 to 30 mg./ft. yellowish blue to yellow (at 20 to 40 mg./ft. red (at 40 to 50 mg./ft.) and green (at 50 to mg./ft. in turn. Therefore, the present invention can be utilized also as a method'of coloringmetals.

However, practically by carrying out the treatment to such a degree as will produce blue or yellowish blue (at 10 to 30 mg./ft. suflicient anticorrosiveness and paint adhesiveness may be obtained.

As a drying method after the electrolytic treatment, the

product may be squeezed with rolls and then dried to 1 dry the residual solution or may be washed with water after the electrolysis to wash off the solution.

When drying the solution by squeezing, the solution is easier to dry, if a trivalent chromium ion is present in the solution. Defatting must be carried out as a pretreatment. But, in case the material is not rusted, acidwashing is not necessary. And the anticorrosiveness Will be improved, if the steel sheet is painted With a hexavalent chromium ion before the electrolytic treatment is carried out. As such hexavalent chromium ion solution may be used either the same solution as the treating solution or a separate chormic anhydride solution.

The nature of the film obtained by the method of the present invention is not yet clarified enough. However, in the observation by using an electron microscope and an electron diffraction, apparatus neither crystalline film nor metallic chromium has been recognized. Thus the film is considered to be an a morphous gel. Judging from this result, the mechanism of producing the film by the method of the present invention is considered to be that, when a metal is cathode-electrolyzed in a solution containing 21 hexavalent chromium ion, a trivalent chromium ion will be produced on the metal surface as a result of the reduction of hexavalent chromium ion, at the same time the pH will rise on the surface and the trivalent chromium ion will be deposited on the surface in the form of a gel as chromium chromate.

In what part of the film production the slight amount of the halogen ion has a catalytic effect is not yet known. However, it is clear that, in case no halogen ion is contained, chromium chromate will be very hard to deposit and that, with the addition of the slight amount of the halogen ion, the production of the film will be accelerated very much.

Even when the thus produced film is washed with water, it will not peel ofi. When it is dried, it will further stick to the steel sheet and will become high in hardless. Further, with the lapse of time, the film will gradually show water repellency and its anticorrosiveness will come to improve.

Example 1 A steel sheet was electrolyzed and defatted with an aqueous solution of an alkali, was then washed with water, was dipped at the normal temperature in a 1%- chromic acid solution and was cathode-electrolytically treated under the following conditions by using pure wa- .ter:

Chromic anhydride g./l 35 Phosphoric acid g./l 7 Sodium chloride g./l 0.05 Temperature of the treating solution C 60 Current density of the cathode a./ft. 200 Electrolyzing time second-s 3 Example 2 Chromic anhydride g./l 5O Hydrochloric acid g./l 0.05 Temperature of the treating solution C 30 Current density of the cathode a./ft. 150 Electrolyzing time seconds 2 Example 3 Chromic anhydride g./l 50 Sodium fluoride g./l 0.1 Temperature of the treating solution C 50 Current density of the cathode a./ft. 200 Electrolyzing time seconds 3 Example 4 Chromic anhydride g./ 1 40 Sodium chloride g./l 0.12 Temperature of the treating solution C 50 Current density a./ft. 250 Electrolyzing time -second 1 In case the steel sheets treated as mentioned in the above Examples 1 to 4 were left to stand in an ordinary room, they were not damaged by rusting even after the lapse of 6 months. Even when they were tested in a moisture tank at a humidity of 100% and a liquid temperatu're of 49 C. for one week, they were not seen to rust. And even in the dew point corrosion test for one week in an outer tank at 49 C. and an inner tank at 39 C., they did not rust. Further, when they Were subjected to a salt water spray test according to J ISZ 2371 (wherein salt water was sprayed against a wall through a nozzle at a salt water concentration of 5%, a tank temperature of 35 C. and a test piece angle of 15 degrees so that the rising salt water mist might be sprinkled on the surface of the test piece), even after the lapse of 6 hours, they were not seen to rust.

In the same salt water spray test, a steel sheet treated with a phosphate and a steel sheet not treated at all usually produced red rust on the whole surfaces in about 30 and 5 minutes, respectively. On the other hand, the' Example 5 A steel sheet was cathode-electrolyzed under the following conditions, the product then squeezed with rolls and the residual solution dried:

Chromic anhydride g./l 30 Chromium acetate (as trivalent chromium ion) g./l 2.5 Sodium chloride (as Cl) p.p.m 50 Temperature of the treating solution C 50 Current density of the cathode a./ft. Electrolyzing time seconds" 4 As a drying condition in this case, the steel sheet could be dried in 4 seconds in a drying furnace at 500 C. But, when no trivalent chromium ion was present, even when the steel sheet was dried for 4 seconds in a drying furnace at 800 C., it could not be Well dried and showed a hy-grosco-pi-city.

Example 6 An electrogalvanized steel sheet was cathode-electrolyzed under the following conditions:

Chromic anhydride g./l 35 Sodium chloride (as Cl) p.p.rn 70 Temperature of the treating solution C 60 Current density of the cathode a./ft. 100

Electrolyzing time "seconds" 4 Example 7 An electr-ogalvanized steel sheet was cathodeelectrolyzed under the following conditions:

Chromic anhydride g./l 30 Sodium chloride (ascl' p.p.rn 40 Temperature of the treating solution C 40 Current density of the cathode 'a./ft. 600 Electrolyzin g time second 0.5

When the thus treated galvanized steel sheet was subjected to a salt water spray test for hours, it was not seen to produce any red rust. But an untreated galvanized steel sheet produced red rust in 2 hours.

Example 8 A molten-aluminum plated steel sheet was cathodeelectrolyzed under the following conditions:

Chromic anhydride g./l 35 Sodium chloride (as Cl-) p.p.m 50 Temperature of the treating solution C 40 Current density of the cathode a./ft. 600 Electrolyzing time seconds 2 The film produced by this treatment showed no interference color and was colorless.

When an aluminum plated steel sheet not treated in the above mentioned manner was subjected to a salt water spray test for 120 hours, it produced white rust on the whole surface. But the sheet treated in the above mentioned manner produced no white rust at all.

What we claim is:

1. A method of chemically treating a metal article which comprises electrolytically treating said metal article in an aqueous solution, making said metal article a cathode, said aqueous solution consisting of a hexavalent chromium compound yielding hexavalent chromium ions in an amount of 5-50 :g./l. as its main ingredient and a substance producing halogen ions in an amount of 10 to 200 ppm. as adjuvant, the pH of said solution being below 1.3 and the temperature thereof being 10 to 80 C.

2. A method according to claim 1 wherein said aqueous solution also has present phosphoric acid in an amount of less than 10 g./ 1.

3. A method according to claim 1 wherein said aqueous solution also contains trivalent chromium ions.

References Cited by the Examiner UNITED STATES PATENTS 2,733,199 1/1956 Wick 20456 2,812,296 11/1957 Neish 204--56 X 2,998,361 8/1961 Kitamura 20456 3,032,487 5/1962 Yo-nezaki et a1. 20456 3,118,824 1/ 1964 Yonezaki et al. 20456 3,175,964 3/1965 Watanabe et a1. 20456 X JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner.

Claims

1. A METHOD OF CHEMICALLY TREATING A METAL ARTICLE WHICH COMPRISES ELECTROLYTICALLY TREATING SAID METAL ARTICLE IN AN AQUEOUS SOLUTION, MAKING SAID METAL ARTICLE A CATHODE, SAID AQUEOUS SOLUTION CONSISTING OF A HEXAVALENT CHROMIUM COMPOUND YIELDING HEXAVALENT CHROMIUM IONS IN AN AMOUNT OF 5-50 G./L. AS ITS MAIN INGREDIENT AND A SUBSTANCE PRODUCING HALOGEN IONS IN AN AMOUNT OF 10 TO 200 P.P.M. AS ADJUVANT, THE PH OF SAID SOLUTION BEING BELOW 1.3 AND THE TEMPERATURE THEREOF BEING 10 TO 80*C.