US2927046A - Coated metals and solutions and process for making the same - Google Patents

Description

United States COATED METALS AND SGLUTIONS AND PROCESS FOR MAKING THE SAME Donald S. Andrade, Parma' Heights, Ohio, assignor to Parker Rust Proof Company, Detroit, Mich, a corporation of Michigan No Drawing. Application November 20, 1958 Serial No. 775,115

8 Claims. Cl. 117-427 The present invention relates generally to metals, metallic articles and surfaces having an adherent, protective coating thereon. More particularly, the invention relates to metallic articles, the surfaces of which are provided with an integral. coating comprising the heat reaction product of tertiary butyl alcohol and chromic acid, which coating is tightly adherent, protects the metal against corrosion and improves the adherence of paints, enamels, lacquers, varnishes and other protective finishes thereto, and to the method of formingsuch coatings.

Heretofore, metal articles and metal surfaces have been coated with insoluble reacted chemical coatings such as phosphates, oxalates, sulfides, oxides, etc, to protect the surface against corrosion, as drawing aids and to improve the adherence of paints and other finishing materials. For best results, phosphate coatings have been rinsed in dilute aqueous chromic acid solutions and dried prior to painting.

An object of the present invention is to provide a bare metal surface with a coating which makes the surface resistant to corrosion and to which paint and other finishing materials will tenaciously adhere, all without provision of further coatings or treatments.

Still another object is to provide a process of only two steps which provides a corrosion-resistant coating on bare metal to which paint and other materials will tenaciously adhere.

Yet another object is to. provide a solution for accomplishing the above and other objects and which is less expensive than known coating materials.

The above and other objects and advantages, according to this invention, are obtained by applying to a bare metallic surface a solution containing as the essential coating-forming ingredients tertiary butyl alcohol and chromic acid, and forming a coating on that surface which comprises the heat reaction product of the tertiary butyl alcohol and the chromic acid. A metal article, the surface of which is coated with an integrally bound coating comprising the heat reaction product of tertiary butyl alcohol and chromic acid, constitutes the improved article of this invention.

The metal surfaces which can be protected by the coatings of this invention include the surfaces of all of the industrial metals including iron, steel, stainless steel, aluminum, zinc, zinc coated iron and steel, copper and its alloys including brass and bronze, magnesium, cadium, "taniurn, zirconium and surfaces in which one or more of these metals constitutes the predominant ingredient. The solutions of and the method of this in vention are particularly satisfactory and useful when applied to the surface of metals in which the predominant metallic constituent is iron, zinc or aluminum.

The solutions of this invention contain as the essential coating-forming ingredients a hexavalent chromium compound, preferably chromic acid, and tertiary butyl alcohol. It has been found that the solutions should not contain water. When water, or other high boiling organic solvent is present the application of heat to the cn V g 2,927,046 Patented Mar. l, 1968 'filrn of the solution on the surface evaporates the tertiary butyl alcohol leaving the hexavalent chromium dissolved in water or the high boiling solvent. It is necessary that the tertiary butyl alcohol be present in conjunction with the hexavalent chromium ion on the metal surface during the latter stages of the heating cycle employed to dry and cause the tertiary butyl alcohol and hexavalent chromium to inter-react and become integrally hound-to the metal surface. The exact nature of the insoluble coatings of this invention is not fully understood. It is known, however, that the hexavalent chromium is present in the coating in an insoluble or protected form in which it is either reacted with, dissolved in or dispersed in the tertiary butyl alcohol. Tests have also revealed the presence of trivalent chromium in the coating indicating that a portion at least of the hexavalent chromium has been reduced in some manner. In addition, neither the tertiary butyl alcohol nor the, hexavalent chromium can be easily washed off or extracted from the metal surface by aqueous or organic solvents, the metal is rendered corrosion resistant to a remarkable degree, and the adhesion of paint, plastic coatings and other finishing materials to the metal is markedly increased. The metal is improved in corrosion resistance to such a degree, for example, on iron, steel, zinc, copper, brass, aluminum and other metals and especially on zinc, copper and aluminum, that the unpainted coated metal can be stored in a humid atmosphere without further protection such as oiling. The resistance of the bare, coated metal, as shown in salt spray and humidity tests, is frequently con.- siderably better. than comparable surfaces coated with phosphate coatings. When painted, the coated metal usually exhibits better salt spray and humidity resistance and adhesion of paints than similar painted surfaces which have been coated first with an oxalate or aqueous chromic acid rinsed-phosphate coating. These remarkable results are obtained by a two-step process comprising (1) wetting the surface with the solution, and (2) heating to fix or insolubilize the coating, whereas phosphate and oxalate coating processes include 3 or more steps, such as (l) prolonged standing in contact with the phosphate or oxalate solution, (2) rinsing in water, (3) dilute.

aqueous chromic acid rinse, and (4) drying.

The hexavalent chromium compound in thecoatings and in the solutions of this invention may be derived from chromium trioxide or chromic acid anhydride (Cl g), 0r chromic acid itself. Chromic trioxide is most preferred since it is appreciably soluble in tertiary butyl alcohol, is conveniently handled, and does not contain water of hydrationto be driven off during drying and insolubilization.

The solutions of this invention should contain hexavalent chromium equivalent to that derived from at least about 1% of chromic anhydride, CrO in the solution, wcight/ volume, that is at least about 1 gram of CrO per ml. of the solution, the specific gravity of the solution being assumed to be unity, for convenience. As used hereinafter'and in the appended claims proportions indicated. in percent are intended to refer to percent weight/volume. In some cases slightly smaller amounts of CrO can be utilized to form an insoluble coating. In other cases slightlylarge amounts of CrO will be required to form such a coating and both. good corrosion resistance and good adherence for finishing materials. In most cases the equivalent of at least 1% of CrO in the solution will be'suflicient to form good coatings. Still greater proportions of hexavalent chromium of, for example, up to 10% CrO may beemployed and proportions as high as the saturation value of the particular hexavalent chromium compound in the tertiary butyl alcohol can be used if desired. Usually, however, from 1% to about 4% of CrO or its equivalent, weight/ adherent thereto.

volume, will be satisfactory under commercial operating conditions.

The method of this invention is effected by the steps of applying to a clean metallic surface a solution containing as the essential coatingproducing ingredients tertiary butyl alcohol and hexavalent chromium, in the above indicated proportions, by immersion of the metal in the solution, by spraying the solution on the metal surface, by roller coating the surface or by any other conventionally employed method which is adapted to form a 1111!- form film of the solution on the surface, and thereafter heating the coating to drive off a part of the tertiary butyl alcohol and cause the above described inter-reaction and bonding of the heat reaction product to the metal surface. It is unnecessary to employ any unique cleaning procedure for preparing the metal surface to be coated to receive the solutions of this invention. In general, the condition of the metallic surface does not appear to be as critical as it is in the more conventional phosphate and oxalate coating processes. If the surface of the metal is dirty or oily, or covered with scale or corrosion prodnote, it is desirable to remove the oil by spraying, immersion, wiping or exposing it to the vapors of a solvent, and to loosen and dissolve away other types of dirt and corrosion with conventional acid and/ or alkaline cleaners. Insoluibilization of the coatings, or the formation of the heat reaction product between the tentiary butyl alcohol and hexavalent chromium, occurs at temperatures above about 200 F. and in the range of about 200 F. to about 500 F. in as little as about /2 minute to as much as 3 or 4 minutes, or slightly more. In no case should the coatings be so strongly heated, or heated for such an extended period as to result in appreciable chairing of the coating.

For use as a paint base the reacted coating should be uniformly distributed and have a weight of about to about 50 mg./sq. ft., preferably about 10 to about 40 mg./sq. ft. For bare corrosion protection the coating weight can satisfactorily extend as high as about 200 rug/sq. it, but coatings having a weight in the range of 50-400 mg./sq. ft. are usually satisfactory for this purpose. The insoluble coating thus produced imparts extremely good bare, unpainted, corrosion resistance and painted corrosion resistance to most metals and makes most paints, plastics and finishing materials tenaciously The coatings are effective for these purposes in their as-obtained condition and do not require further treatment.

The following examples more fully describe the solutions, method and articles of this invention, but it is to be understood that the particular solutions disclosed therein and the conditions of treatment employed are illustrative only and that the above given ranges of composi; tion and conditions represent the controlling conditions of this invention.

Example 1 Solutions were made up by adding 1, 2 and 4% (wt/vol.) of chromic trioxide (CrO as a stock solution in tertiary butyl alcohol to undiluted tertiary butyl alcohol. By analysis, the CrO content of the resulting solutions was found to be, respectively, 0.98%, 1.96% and 3.56%, weight/volume. Panels of two difierent types of hot-dipped zinc coated steel sheet, one known as Armco Stabilized Zincgn'p and the other as Armco Zincgri panels of ordinary cold rolled steel; and panels of an aluminum alloy known as 243 were dipped in the solutions and dried in an air oven at 250 F. In every case a uniform, attractive and insoluble coating was obtained. Prior to immerson, the panels were cleaned in either of cleaning solutions A or B. Solution A consisted of an aqueous solution of sodium orthosilicate made up to a concentration of 48 grams per liter. Solution B had a pH of 5.15 and consisted of 12.5 grams per liter of NaH PO and 0.5% C10 The panels were immersed in the cleaning solutions for about 5 minutes at about F. and then rinsed in warm water and dried. The chromic acid tertiary butyl alcohol coatings on zinc-coated steel and cold rolled steel were compared to controls prepared by phosphate coating panels of the some stock which had been cleaned in the same fashion and, after phosphate coating, conventionally rinsed with a dilute aqueous solution of chromic acid having a pH of about 2.5 to 3 and containing about 0.095 to 0.1% CrO then dried and finally painted in the same fashion. The phosphate solution employed in the preparation of the control panels contained zinc phosphate and minor amounts of copper, nickel, nitrite, and fluoborate ions as accelerators and analyzed as having a pH of 2.6 and a total acid of 19.8 to 20.1 points. The aluminum panels coated with the insoluble chromate-alcohol coatings were 7 not painted but were compared for bare corrosion resistance against bare phosphate-coated controls prepared as described. The insoluble chromate tertiary butyl alcohol coatings on steel and zinc-coated steel obtained from the solutions containing 1, 2 and 4% chromic trioxide were better than the phosphate coated, chromic acid rinsed control panels in painted salt spray resistance, painted humidity resistance, and physical adhesion of the paint. On aluminum, the coatings of this invention were especially resistant in the bare corrosion test, the metal in some cases showing less corrosion after 1176 hours exposure than the phosphate coated controls showed after only 504 hours. 4

Example 2 A first solution was prepared to contain 4% CrO in tertiary butyl alcohol, weight/volume.

A series of solutions were prepared containing 4% CrO and mixtures of tertiary butyl alcohol and a high boiling hydrocarbon solvent in the proportions of 75 tertiary butyl alcohol 25 hydrocarbon solvent, 60 tertiary butyl alcohol 40 hydrocarbon solvent, 50 tertiary butyl alcohol 50 hydrocarbon solvent.

The high boiling solvent employed is commercially known as Amsco solvent No. 140, which has a boiling range of C.-20'7 C., a flash point of 143 F. an evaporation speed of 58 minutes and a specific gravity of .803. A plurality of 4" x 6" cold rolled steel panels which were preliminarily mineral spirits and dry rag wiped were dipped in each of the 4 baths, removed and heated for 2 minutes at 250 F. Some of the panels were painted with a commercial paint system designated Nubelite 3820 and others were left unpainted. Separate painted panels, as treated in each bath were then scratched diagonally and subjected to 20% salt spray for 504 hours. The panels processed in the solutions containing the hydrocarbon solvent were in all cases complete failures after 504 hours whereas no corrosion was detected on the panels from the bath containing only tertiary butyl alcohol and chromic acid. Moreover the panels from the tertiary butyl alcohol-chromic acid bath were more adherent as determined both by bending and scratching tests which are conventional in the art.

A plurality of 4" x 6" aluminum panels, aluminum 3S, were mineral spirits and dry rag wiped and dipped in the same solutions used above with the steel panels, at room temperature, removed and heated at 250 for two minutes. These panels were then subjected to salt spray corrosion testing for 504 hours in 20% salt fog to 552 hours in a humidity cabinet and were also evalu ated for adhesion. The panels from the solution con taining 75% tertiary butyl alcohol and 25% hydrocarbon solvent were inferior in salt spray resistance to the panels from the solution containing only tertiary butyl alcohol and chromic acid. The panels from the bath containing 60 tertiary butyl alcohol and 40 hydrocarbon solvent were much worse in salt-spray resistance, and the panels from the solution containing 50 tertiary butyl alcohol and 50 hydrocarbon solvent were worse than the panels from the baths containing 25 and 40% hydro carbon solvent and were much poorer than the panels obtained from the solution containing only tertiary butyl alcohol and hexavalent chromium. In humidity resistance the panels from the solutions containing 75 tertiary butyl alcohol and 25 hydrocarbon solvent were approximately the same as that of the panels obtained from the solution containing only tertiary butyl alcohol and chromic acid, but the panels containing 40% and 50% of hydrocarbon solvent were inferior in humidity resistance to those obtained from the solution containing only tertiary butyl alcohol and chromic acid. The panels from the solutions containing only tertiary butyl alcohol and chromic acid were superior in adhesion to the panels obtained from each of the other three baths as measured both by bending and scratch tests of conventional type.

Example 3 In this example aqueous solutions of chromic acid and tertiary butyl alcohol were prepared containing about 0.1% (wt/vol.) of a polyoxethylene type of non-ionic dispersant known as Igepal (Igepal CA-630, an isooctylphenolpolyoxyethylene with approximately 8 to 9 oxyethylene groups) and 0.01% of triethanol amine, both based on the total volume of solution. The dilution of the alcohol with water ranged from as much as 95% (i.e. 5% alcohol) to as little as 60% (i.e. 40% alcohol). In these solutions, the CRO content was about 1%, 2%, 4% and 6% by weight based on solution volume. Cleaned panels of cold rolled steel, zinc-coated steel (Armco Zincgrip), and aluminum alloys 24S and 38 were dipped in the bath at room temperature and dried in an oven at 250 F. At this temperature all solutions gave insoluble coatings except those containing only 5% tertiary butyl alcohol and 4% to 6% chromic acid. When the dry off temperature of the latter was raised to about 350 F., insoluble coatings were obtained. The properties of the coatings prepared from aqueous solutions on steel were not as good as those prepared from undiluted tertiary butyl alcohol chromic trioxide solutions in salt spray resistance, and the coatings on aluminum were inferior in bare corrosion resistance to those obtained from the solutions containing only tertiary butyl alcohol and chromic acid. While both the bare and painted salt spray resistance, the painted humidity resistance and the physical adhesion of paint were much better than that of the bare metal in each case, the properties were inferior to those of the phosphate-coated controls.

In the absence of the Igepal wetting agent the coatings were not uniformly distributed and occurred as beads scattered over the surface.

This application is a continuation-in-part of my copending application, Serial No. 519,619, filed July 1, 1955, now abandoned.

What is claimed is:

1. A composition for coating metallic surfaces which consists essentially of tertiary butyl alcohol and chromic acid, said chromic acid being dissolved in said tertiary butyl alcohol in an amount between about 1% and saturation.

2. A composition for coating metallic surfaces which consists essentially of tertiaiy butyl alcohol and chromic acid, said chromic acid being dissolved in said tertiary butyl alcohol in an amount between about 1% and about 4%.

3. A composition for coating metallic surfaces consisting of tertiary butyl alcohol and chromic acid, said chromic acid being dissolved in said tertiary butyl alcohol in an amount between about 1% and saturation.

4. A composition for coating metallic surfaces consisting of tertiary butyl alcohol and chromic acid, said chromic acid being dissolved in said tertiary butyl alcohol in an amount between about 1% and about 4%.

5. A method of forming a coating on a metallic surface which comprises the steps of (l) contacting the said metallic surface with a solution consisting essentially of tertiary butyl alcohol and chromic acid, said chromic acid being dissolved in said tertiary butyl alcohol in an amount between about 1% and saturation to form a uniform film of the said solution on the surface and (2) heating the said film to a temperature in the range of about 200 F. to 500 F. for a time sufiicient to form a uniformly distributed adherent coating on said surface, said coating having a weight in the range of about 10 ing/sq. ft. and about 200 rug/sq. ft.

6. A method of forming a coating on a metallic surface which comprises the steps of (l) contacting the said metallic surface with a solution consisting essentially of tertiary butyl alcohol and chromic acid, said chromic acid being dissolved in said tertiary butyl alcohol in an amount between about 1% and 4% to form a uniform iilm of the said solution on the surface, and (2) heating the said film to a temperature in the range of about 200 F. to 500 F. for a tirne sufiicient to form a uniformly distributed adherent coating on said surface, said coating having a weight in the range of about 10 mg./sq. ft. and about 200 mg./sq. ft.

7. A metallic article having on its surface an integrally bonded coating comprising the reaction product of heating on said surface a uniformly distributed film of a solution consisting essentially of tertiary butyl alcohol and chromic acid, said chromic acid being present in said solution in an amount between about 1% and saturation, said coating having a weight in the range of about 10 rug/sq. ft. to about 200 mg./sq. ft.

8. A metallic article having on its surface an integrally bonded coating comprising the reaction product of heating on said surface a uniformly distributed film of a solution consisting essentially of tertiary butyl alcohol and chromic acid, said chromic acid being present in said solution in an amount between about 1% and 4%, said coating having a weight in the range of about 10 rug/sq. ft. to about 200 mg./sq. ft.

References (Iited in the file of this patent UNITED STATES PATENTS 630,867 Dickey Aug. 15, 1899 1,926,766 Englund Sept. 12, 1933 2,206,064 Thompson July 2, 1940 2,762,732 Somers Sept. 11, 1956

Claims (1)

1. A COMPOSITION FOR COATING METALLIC SURFACES WHICH CONSISTS ESSENTIALLY OF TERTIARY BUTYL ALCOHOL AND CHROMIC ACID, SAID CHROMIC ACID BEING DISSOLVED IN SAID TERTIARY BUTYL ALCOHOL IN AN AMOUNT BETWEEN ABOUT 1% AND SATURATION.