US 3506499 A
Description (OCR text may contain errors)
United States Patent US. Cl. 148-62 14 Claims ABSTRACT OF THE DISCLOSURE A method is provided for treating the surfaces of zinc, aluminum, and their alloys to obtain a product having excellent corrosion resistance, paint adhesion properties and appearance. In accordance with the process an article having a surface of zinc, aluminum or an alloy thereof is treated with an aqueous solution of chromic acid and colloidal silica.
This invention relates to methods of treating the surfaces of zinc, aluminum, and their alloys and more particularly to surface-treating methods for preventing the formation so-called white rust on the zinc surfaced articles such as galvanized iron or steel sheets during storage in highly humid areas and shipment.
An object of the present invention is to provide a novel and improved method for the treatment of an article having a surface of a metal selected from the group consisting of zinc, aluminum and their alloys to protect the same from white rust or other corrosion by a simple treatment without baking at a high temperature after applying a treating solution of the present invention to the said articles or without rinsing after drying them.
Another object of the present invention is to provide an improved method for the utilization of chromic acid or related chrominum compounds to protect zinc-surfaced and aluminum-surfaced articles against white rust" or other corrosion.
A further object of the present invention is to provide a zinc-surfaced article which has no such surface coloring as is seen in any conventional treating method, is very excellent in the adherence of paints and has a high commodity value.
Of the metallic coating available for the protection of iron and steel, zinc is the most desirable, because of (a) the excellent resistance of zinc to atmospheric corrosion, (b) the anodic relationship of zinc to iron which provides sacrificial protection to exposed base metal and (c) the availability and low cost of zinc metal.
Specifically an iron or steel sheet plated with zinc is well known as a galvanized iron or steel sheet. The hot dipped galvanized process is one of the cheapest methods of anticorroding iron or steel.
A zinc product has excellent resistance to atmospheric corrosion, but in a humid place, white rust will be produced on the surface of such zinc product and not only the metallic luster on the surface will be lost but also the corrosion resistance for the metal of the base will be reduced. Such white rust will impair the appearance and will reduce the commodity value. Specifically in such highly warm and humid land as Japan, white rust or other corrosion will occur on the surface of such product during the transportation and will be also a cause of the return of the product.
On the other hand, aluminum coating or aluminized ferrous metal has been also recently developed and is applied as a corrosion resistant coating for iron and steel. But the same corrosion problem as in the case of zinc has occurred in aluminum coating or aluminized ferrous metal.
3,506,499 Patented Apr. 14, 1970 For the prevention of formation of white rust on zinc, there has been carried out a chromate treating method wherein chromic anhydride, or various soluble chromate is mostly used. For another method of the prevention of white rust, it is carried out to absorb an oil layer .on the surface of zinc. But a zinc product subjected to such treatment is so slippery as to be difficult and dangerous to handle.
The present invention relates to the above described chromate treatment. Such chromate treatment has been widely used for a method of antirusting nonferrous metals. The reaction mechanism in such treating method shall be considered. In an anode part, zinc will be dissolved or oxidized. In a cathode part, hexavalent chromium ions will be reduced and a chromate film mainly consisting of trivalent chromium will be formed on the surface. In such case, if such anion accelerating the dissolution of zinc as a sulfate ion is added, the reaction of both cathode and anode will be accelerated and a relatively thick, corrosion resistant and colored chromate film consisting of trivalent chromium will be formed on the said surface.
In the surface-treatment of ordinary zinc-electroplating, the above mentioned method forming a relatively thick and colored chromate film from a chromic acid bath containing sulfate ion is adopted and a favorable, corrosion resistant film having a good appearance is obtained by rinsing after the chromate treatment and, as required, by neutralizing the treated surface with sodium carbonate or the like. On the other hand, in the case of a galvanized sheet, since the producing process is different, that is to say, a hot dipped galvanized process and a chromic acid treatment are carried out continuously, it is necessary to carry out the surface-treatment as simply as possible. Therefore, a treating bath in which an excellent anticorrosive film is obtained only steps of chromic acid treatment, roll squeezing and drying without baking at high temperature and water rinsing is most desired.
Typical of the surface treating methods for galvanized sheets wherein said steps are well utilized is a method wherein there is used a dilute aqueous solution of pure chromic acid. When this method is used, the surface will not be altered from the original appearance and a corrosion resistant film to some extent will be obtained. On the other hand, as the reaction between the zinc surface and a chromic acid will not proceed Well, though the appearance may be satisfactory, the corrosion resistance will not be sufiicient.
In view of the application to the simple steps of only the above mentioned dipping, roll-squeezing and drying, it is desirable to contain no detrimental anion in the chromic acid bath but it is also necessary to contain as few detrimental cations as possible. If such cations as alkali or alkaline earth metal ions or such anions as sulfate and chloride ions are present in the treating bath, they will have a bad influence on the time variation of the surface of zinc, will impair the appearance and others with the lapse of time, will produce a white rust or other corrosion.
A chromic acid-water glass series has been recently proposed. However, Na ions are contained in water glass and will have a bad influence on the time variation of the treated metal surface in the simple steps of roll squeezing and drying.
Accordingly, the water rinsing after drying the film is an indispensable operation. If the rinsing step is omitted, the corrosion resistance and paint adhesion of the film will deteriorate. Such deterioration of the corrosion resistance and paint adhesion of the film is considered to be due to the Na ions in the water glass.
If a desired corrosion resistance is to be given to the treated film by omitting the rinsing after the drying by using the chromic acid-water glass series treating solution, there may be considered the application of a soealled two solution type operation wherein the surface is dipped first in water glass and then in a chromic acid solution. However, in such case, the process will be complicated and the loss of the treating solution will be apt to increase.
Here, if electro chemical consideration is applied to the reaction in a minute part of the zinc surface it will be possible to read oil? a suggestion whereby the difficulties in the case of applying the above described chromic acid-water glass series treating solution can be solved.
That is to say, in the reaction in the minute part of the zinc surface, in the cathode part, Cr will be reduced and, in the anode part, zinc will be dissolved. Therefore, if a substance which will be adsorbed in the anode part and will give an anticorrosive film is added instead of ions to a pure chromic acid solution, it should be possible to improve the corrosion resistance of the film and at the same time to reduce the treating time.
As conforming to such condition is selected in the present invention a chromic acid-colloidal silica series treating bath. Such uniformly dispersed colloidal silica will be negatively charged in the solution, will effectively compensate the defects of the chromate film produced in the cathode part and will improve not only the corrosion resistance but also the paint adhesion of the treated film and will reduce the dipping treating time.
The treating solution according to the present invention is a solution prepared by adding a colloidal silica substance to an aqueous solution of chromic anhydride, or various soluble chromate. For the colloidal silica substance is used a solution prepared by removing sodium in a s lution of NaSiO (water glass) with an ion exchange resin or such commercial colloidal silica as, for example, Ludox produced by E. I. du Pont de Nemours & Co., Inc., Wilmington, Del. Further, there can be used a silica acid colloid obtained by dialysis, electro dialysis, dissolution of elemental silicon, etc.
Anyhow, SiO must be uniformly dispersed in the form of a colloid in the solution. Further, in the method wherein a part of hexavalent chromium is reduced so that no detrimental oxidation product may be produced in the treating solution, in case trivalent chromium is contained, it will be more effective. For such method, there is used a method wherein such reducing agent as saccharose is used or it is electrolytically reduced with a carbon electrode.
The proper concentration of hexavalent chromium ions in the treating solution is 0.5 to 10 g./l. as metallic chromium or preferably 1 to 8 g./l. Specifically, with the concentration of 1.2 to g./l., the appearance will be favorable and at the same time the corrosion resistance will be sufiicient. In the above, if the concentration is so high as to be, for example, more than g./l., the surface will be colored and will be stained with the nonuniform deposition of chromic acid or various soluble chromate and further the amount of chromic acid or various soluble chromate taken out and deposited on the treated material will be so large that such high concentration will be eco nomically disadvantageous. Further, with a concentration of less than 0.5 g./l., no favorable corrosion resistance will be shown.
The prior concentration of the colloidal silica is 1 to 70 g./l. as SiO Thereby, not only the corrosion resistance will be improved but also the treating solution will be uniformly deposited and a favorable film will be obtained. In the above, with the addition of more than 70 g./l., there will be a tendency of coloring and the corrosion resistance will reduce. With less than 1 g./l., the above mentioned effect will not come out favorably. The range of addition most effective to give corrosion resistance is about 5 to 50 g./l. Specifically, in the case of addition of 20 g./l., the paint adhesion will be remarkable.
According to present treating method, a zinc or aluminum product is degreased, is rinsed so that the surface may be clean and is then dipped in the treating bath at the room temperature for several seconds to about 30 seconds, the excess of the treating solution deposited on the product being treated is removed with rubber rolls or any other wiping mechanism and the product is dried.
If the present treating solution is used, after the product is dried, there will be no need of rinsing it to improve the paint adhesion.
If the method of the present invention is adopted, even though no rinsing is carried out after coating and drying, the anticorrosiveness and paint adhesion of the film will be very excellent.
Further, even if a two-solution method is applied to the present treating method and the material is treated first with only a colloidal silica sol and then with chromic acid, there will be shown an effect not so different from the result of a one-solution treatment. As a result, in the treating method of the present invention, there is no need of specifically carrying out the two-step drying process in the case of applying the two-solution method, the treating process can be simplified and the drying time can be reduced very much.
The treating solution may be deposited on the surface of the material to be treated not only by dipping but also by spraying or brush-coating the excess treating solution may be removed and the material may be dried.
Examples of the present invention are given in the following.
EXAMPLE 1 Chromic anhydride5 g./l. Colloidal silica-20 g./l. Water--Rest.
Solution temperature25 C. Dipping time5 seconds.
EXAMPLE 2 Chromic anhydride-5 g./l. Saccharose1 g./l.
Colloidal silica (as SiO )20 g./l. Water'Rest.
Solution temperature-25 C. Dipping time5 seconds.
This treating solution was prepared by making chromic anhydride and saccharose react well with each other by warming and then cooling them.
When a galvanized iron sheet treated with the above mentioned treating solution was subjected to a salt water spray test, no white rust was seen to be produced at all in hours of salt water spraying.
For comparison, when the sheet treated with pure chromic acid to which no colloidal silica had been added was subjected to the salt water spray test, white rust was seen to be produced already in 10 to 12 hours and 25 to 30% white rust was produced in 24 hours.
When the paint adhesion of the above mentioned materials was tested with a melamine alkyd series by a Frichsen cup test, the material treated with the chromic acid bath containing the colloidal silica sol was very excellent in the paint adhesion and showed satisfactory results.
Further, the amount of deposition of chromium was 0.2 mg./dm. in each of Examples 1 and 2 and was also about 0.2 mg./dm. in case no colloidal silica was added.
When the above mentioned treating bath was applied to aluminum, the same excellent corrosion resistance and paint adhesion as in zinc were shown.
What is claimed is:
1. A method of surface-treating metals consisting essentially of the steps of coating the surface of a metal selected from the group consisting of zinc, aluminum, and their alloys with a mixed solution of chromic acid containing 0.5 to 10 g./l. as metallic chromium and a colloidal silica containing 1 to 70 g./l. of SiO and drying the applied solution to form thin film on the said surfaces.
2. A method of surface-treating metals consisting essentially of the steps of coating the surface of a metal selected from the group consisting of zinc, aluminum, and their alloys with a mixed solution of chromic acid containing 1 to 8 g./l. as metallic chromium and a colloidal silica containing 5 to 50 g./l. of SiO and drying the applied solution to form a thin film on the said surfaces.
3. A method of surface-treating metals consisting essentially of the steps of coating the surface of a metal selected from the group consisting of zinc, aluminum, and their alloys with a mixed solution of chromic acid containing 1.2 to 5 g./l. as metallic chromium and a colloidal silica containing 5 to 50 g./l. of SiO and drying the applied solution to form a thin film on the said surfaces.
4. A method of surface-treating metals consisting essentially of the steps of coating the surface of a metal selected from a group consisting of zinc, aluminum and their alloys first with a colloidal silica solution containing 1 to 70 g./l. of SiO and then with chromic acid solution containing 0.5 to g./1. as metallic chromium and drying the applied solution to form a thin film on the said surfaces.
5. The method according to claim 1 wherein the coating is by dipping, spraying or brush-painting.
6. The method according to claim 1 wherein the coating comprises dipping said metal in the treating solution at the room temperature for substantially 1 to 30 seconds.
7. The method according to claim 1 wherein the surface of the metal is a surface of zinc, aluminum or its alloy with which is coated or plated the surface of iron or steel.
8. A method of surface-treating metals consisting essentially of the steps of coating the surface of a metal selected from the group consisting of zinc, aluminum and their alloys with a mixed solution of chromic acid containing 0.5 to 10 g./l. as metallic chromium and a colloidal silica, and drying the applied solution to form a thin film on the said surfaces.
9. A metal having a dry coating in intimate contact with a surface thereof, the metal being a member selected from the group consisting of zinc, aluminum and their alloys. the dry coating being in the form of a thin film from an aqueous solution consisting essentially of water, chromic acid and from 1 to grams per liter of colloidal silica, the concentration of hexavalent chromium ions in the aqueous solution being from 0.5 to 10 grams per liter.
10. A coated metal according to claim 9 wherein the metal is zinc.
11. A coated metal according to claim 9 wherein the metal is aluminum.
12. A method comprising (a) coating a surface of a metal selected from the group consisting of zinc, aluminum and their alloys with a solution consisting essentially of water, chromic acid and from 1 to 70 grams per liter of colloidal silica, the concentration of hexavalent chromium ions in the solution being from 0.5 to 10 grams per liter, and (b) drying the coated surface to form a thin film thereon.
' 13. A method according to claim 12 wherein the metal is zinc.
14. A method according to claim 12 wherein the metal is aluminum.
References Cited UNITED STATES PATENTS 2,030,601 2/1936 McDonald 148-6.16 2,680,081 6/1954 Probert et al 117135.1 2,989,418 6/1961 Harbaugh 117135.1 X 3,013,897 12/1961 Cupery et al. 117135.1 X 3,133,829 5/1964 Cupery et al. 117-1351 3,150,015 9/1964 Boyer et al. 148-6.2 2,850,415 9/1958 Harrison 148-6.2 X
RALPH S. KENDALL, Primary Examiner US. Cl. X.R.