US3446717A - Cathodic treatment of metals in chromate solution to form protective coating thereon - Google Patents

Description

United States Patent US. Cl. 20458 6 Claims ABSTRACT OF THE DISCLOSURE A process for producing a protective coating on meals by cathodic treatment in an acidic chromic acid solution saturated with a metallic chromate. The chromate may be one of the group of calcium, strontium, barium, zinc, cadmium, aluminum, ferric iron or lead chromates.

The invention relates to the treatment of metals for the purpose of inhibiting corrosion. The term metal as used herein includes alloys.

The use of sparingly soluble chromates of calcium, zinc, lead or other metals in priming compositions for the treatment of metal surfaces is widespread. The mechanism by which chromates are capable of protecting labile metal surfaces is complex, but is probably dependent on the oxidizing power of the chromate which maintains a film of oxide on the surface of the metal. It is usually assumed that the chromate employed should have a small degree of water solubility so that in moist conditions traces of chromate can dissolve, and this provides a mechanism by which the chromate can obtain an intimate contact with the metal surface.

The present invention provides a process for the treatment of metal surfaces wherein the metal to be treated is placed in an aqueous acid-reacting electrolyte which is at least saturated with a chromate and a direct current is passedthrough the electrolyte, the metal to be treated constituting the cathode.

This causes a light deposit of a chromate to be laid down on the metal surface under treatment in such a way as to ensure intimate contact between the chromate and the metal. The process also permits the reduction of a portion of the chromate deposited, so that some of the chromium is in the trivalent state and this enhances the bonding of the metal surface.

The nature of the anode is not critical provided that it does not react deleteriously with the electrolyte. The anode may conveniently be lead but other materials such as platinum or carbon may be used.

It is believed that during the process the pH at the surface of the metal under treatment is raised by the normal mechanism of electrolytic conduction and the result is that chromate precipitates on the metal surface. At the same time some degree of reduction of the chromate may take place, but this is advantageous as already stated.

In order to conduct the process tothe greatest advantage it is preferred to carry out the electrolysis in a solution of a chromate in chromic acid. Since we operate under conditions such that the electrolyte is fully saturated with respect to the chromate employed, in theory an infinitely small rise in pH will result in the deposition of chromate and in this way the precipitation takes place right on the metal surface and the chromate thus becomes intimately associated with that surface. We have found that the further from the saturation point of the electro- 3,446,717 Patented May 27, 1969 lyte the poorer are the results obtained. This is no doubt because under unsaturated conditions a time elapses between the liberation of hydroxyl ions at the metal surface and the precipitation of chromate. During this time some difi'usion may occur which will result in the precipitation of chromate at some distance from the metal surface giving rise to poor adhesion.

The process of the invention can be conducted at any convenient temperature up to the boiling point of the electrolyte.

Metal surfaces which have been treated by the process of the invention include those of a variety of grades of iron and steel, zinc and zinc alloys including zinc coated articles such as galvanized steel, cadmium and cadmium plated goods, aluminum both in pure and alloy form and tin plated goods. However, the process of the invention is particularly applicable to ferrous metals, aluminum and magnesium, including alloys containing ose or more of the metals iron, aluminum and magnesium.

One of the advantages of the process is that to obtain the desired effect, treatment can be very rapid. The optimum time of treatment may vary from a few seconds to several hours and no advantage accrues by more prolonged treatment. The time required for treatment is largely dependent upon the current density. Current densities which can be used vary from less than one ampere per sq. ft. in which case treatment would require many hours, to several hundred amperes per sq. ft. allowing treatment in a few seconds.

The chromates which can be used in the process of the invention are those which have a greater solubility in acid than in neutral solution. Preferably sparingly soluble chromates are used and examples of these are the chromates of calcium, strontium, barium, zinc, cadmium, copper, ferric iron and lead.

After treatment by the process of the invention we find that the metals treated resist corrosion to a far greater degree than before treatment. For example mild steel treated according to the invention may be stored under damp conditions for considerable periods before corrosion takes place. In addition we find that metals such as zinc or aluminum or their alloys can be made much more resistant to corrosion in environments in which they would normally corrode. In particular, we find that such materials can be given substantial protection against marine atmospheres by means of the process of the invention.

One of the particular attractions of the process is that metals treated by it may be painted without the necessity of any further pre-treatment such as with a phosphate composition and without the use of any priming paint. We have found that many metals and in particular steel and aluminum may be sprayed with paint after treatment by the process of the invention and the metals are as satisfactory in use as when given conventional and much more expensive treatment prior to painting.

The following examples illustrate the invention.

EXAMPLE 1 A sheet of mild steel 6 in. X 4 in. was suspended in 2 liters of solution containing 250 g. of chromic acid to which 380 g. of calcium chromate had been added. A lead sheet 7 in. x 5 in. was suspended on each side of the steel plate about 1 in. distant from it. A direct current of 60 amperes was passed between the steel and lead sheets for one minute, the steel being used as cathode. The steel sheet on withdrawal from the bath was washed with water and dried in a stream of warm air.

EXAMPLE 2 A sheet of mild steel was treated as described in Example 1 except that the electrolyte consisted of 450 g.

of chromic acid and 150 g. of hydrated lime made up to 2 liters with water and thoroughly stirred.

EXAMPLE 3 A strip of mild steel in. wide was passed continuously through an electrolyte contained in a ceramic trough 6 in. wide, 6 in. deep and 4 ft. long. The strip was kept submerged as it passed through the trough by passing through a system of rollers. Also submerged in the trough were two strips of lead 5 /2 in. wide and 3 /2 ft. long, supported in such a manner that the steel strip passed in a parallel course between, and about half an inch distant from them. The electrolyte consisted of 5 kg. of zinc oxide and 12 kg. of chromic acid thoroughly stirred with water to give approximately liters of solution. A direct current was applied between the steel and lead strips using the former as cathode and a current corresponding to some 275 amperes per sq. ft. of steel surface submerged in the electrolyte was passed. The steel strip was drawn through the bath at approximately twelve feet per minute and as it emerged, it was squeegeed.

EXAMPLE 4 A sheet of mild steel was treated as described in Example 1 except that the electrolyte consisted of 750 g. of chromic acid and 300 g. of copper oxide made up to 2 liters with water and thoroughly stirred.

EXAMPLE 5 A sheet of mild steel was treated as described in Example 1 except that the electrolyte consisted of 350 g. of chromic acid and 100 g. of freshly precipitated aluminum hydroxide made up to 2 liters with water and thoroughly stirred.

EXAMPLE 6 A rod of aluminum metal /8 in. in diameter and 2 ft. long was suspended concentrically inside a lead tube 2 /2 in. in internal diameter and 2 /2 ft. long and closed at its lower end. The lead tube was filled to within 2 inches of the top with a suspension of barium chromate made by mixing g. of barium hydroxide with 400 g. of chromic acid and making up to 4 liters by the addition of water. The aluminum rod was in such a position that it was completely submerged. A direct current of 20 amperes was passed for 5 seconds between the lead tube and the aluminum rod using the latter as cathode. After removing from the electrolyte the aluminum rod was washed, dried and painted directly with a high grade gloss paint. Examination of the painted rod by accelerated corrosion tests and by prolonged exposure to marine atmospheres indicated that the paint layer was not less stable than when applied to a similar aluminum surface previously prepared by conventional means.

4 EXAMPLE 7 A galvanized steel wire (14 gauge) was drawn through a bath consisting of 2 g. of barium hydroxide, 20 g. of chromic acid and 200 mls. of water in a mild steel vessel. The wire entered the foot of the bath through a polytetrafluoroethylene gland and passed vertically upwards over a pulley provided above the liquid level. The length of wire submerged at any one time was about five inches. A current of amperes was passed between the wire and the steel vessel, the wire being the cathode, was drawn through the bath at approximately 30 ft. per minute. On leaving the bath the wrie was washed in water and dried in warm air.

What is claimed is:

1. In the process of cathodically electro coating metal articles having a metal surface selected from the group consisting of ferrous metals, zinc and zinc base alloys, cadmium and cadmium base alloys, aluminum and aluminum base alloys, and tin and tin base alloys, with a non-metallic protective coating, in an aqueous chromic acid electrolyte having an acid pH, the improvement comprising utilizing as the chromic acid electrolyte an acid aqueous electrolyte consisting essentially of chromic acid which is at least saturated with at least one chromate selected from the group consisting of the chromates of calcium, strontium, barium, zinc, cadmium, aluminum, copper, ferric iron, and lead.

2. The process of claim 1 wherein said article is a steel surfaced article.

3. The process of claim 1 wherein said metal article is a zinc surfaced article.

4. The process of claim 1 wherein said metal article is an aluminum surfaced metal article.

5. The process of claim 1 wherein said chromate is selected from the group consisting of calcium, zinc, copper, aluminum, and barium.

6. The process of claim 5 wherein a lead anode is used.

References Cited UNITED STATES PATENTS 2,042,611 1/1936 Lukens 20451 2,445,155 7/1948 Saukaitis 1486.2 2,746,915 5/1956 Giesker et a1. 20456 3,288,691 11/1966 Yonezaki et al. 20456 HOWARD S. WILLIAMS, Primary Exailziner.

G. L. KAPLAN, Assistant Examiner.

US. Cl. X.R. 2045 6