US3477882A - Method of and composition for preventing "white rust" formation - Google Patents

Description

United States Patent 3,477,882 METHOD OF AND COMPOSITION FOR PREVENT- ING WHITE RUST FORMATION William J. T. Coulman, Novelty, Ohio, assignor to The Lubrizol Corporation, Wickliffe, Ohio, a corporation of Ohio No Drawing. Filed Dec. 13, 1965, Ser. No. 513,587

Int. Cl. C23f 9/ 02, 7/26 US. Cl. 148-6.16 6 Claims ABSTRACT OF THE DISCLOSURE Aqueous solutions containing partially reduced chromic acid, a fluoride or fluorine-containing complex and phosphoric acid are effective for forming conversion coatings on metal surfaces, especially hot dip galvanized steel surfaces. The ingredients in these solutions are effective in very low concentrations.

This invention relates to improved conversion coatings for metal, and more particularly to an improved composition and method for passivation of galvanized steel before applying siccative coatings thereto.

The manufacture of hot dip galvanized steel sheeting has mushroomed in the past several years. Because of its chemical resistance, galvanized steel is extremely useful for a number of uses, and it furnishes an ever-increasing percentage of the output of most steel mills. The methods used for forming hot dip galvanized steel usually are confidential and are jealously guarded by the steel companies which have developed them. In general, however, they involve passing steel sheeting through a bath of molten zinc to form a zinc coating thereon, and subsequently annealing the coated sheet to form chemical or alloy bond between the steel and the zinc. The zinc surface reacts rapidly with oxygen to form a passive coating which protects the sheeting from corrosion and chemical attack.

In the presence of atmospheric moisture, the oxidation of galvanized steel sometimes results in the formation of a powdery surface film of zinc oxide. This film, commonly known as White rust, imparts an unattractive streaked appearance to the surface and decreases the marketability of the sheeting. Moreover, the presence of white rust impairs the adhesion of paints and other siccative coatings to the metal surface. Often, white rust appears on galvanized steel surfaces during the short time required for shipment of the steel from the mill to the purchaser, and since it is the manufacturers responsibility to furnish high-quality material, the purchaser may reject the shipment and subject-the manufacturer to considerable financial loss. For this reason, it is of interest to develop methods for the formation of passive coatings on hot dip galvanized steel, which coatings adhere tenaciously to the surface and promote the adhesion of siccative coatings thereto, while at the same time preventing the formation of white rust.

It has long been known that the application of chromate conversion coatings to galvanized and other metal surfaces promotes adhesion of siccative coatings thereto. While the effect of these chromate coatings is not fully understood, it is believed that the chromate is at least partially reduced by other constituents of the coating bath to form a gel-like film of trivalent chromium which is impervious to oxygen and chemical attack, thereby passivating the metal. It is also known that the addition of fluoride-containing substances to the chromate bath promotes the formation of effective conversion coatings, probably by etching the metal to form a rough surface to which the chromate coating adheres tenaciously. De-

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spite their many advantages, however, fluoride-containing chromate coating compositions have not been found entirely satisfactory for treating galvanized steel. One reason for this is that the concentration of these compositions must be so high, in order to form an effective coating, that the metal surface is discolored by the coating solution. Since galvanized surfaces are expected to be colorless, this discoloration is a commercial disadvantage.

A principal object of the present invention, therefore, is to provide new and improved chromate conversion coatings for ferrous and non-ferrous metal surfaces.

A further object is to provide conversion coatings which effectively protect galvanized steel surfaces from white rust formation.

A further object is to provide conversion coatings which promote the adhesion of siccative organic coatings to galvanized metal surfaces and inhibit formation of adhesion-suppressing oxide coatings on said metal surfaces.

Another object is to provide coating compositions which are effective at low concentrations and do not discolor the metal surface.

Another object is to provide an improved method for passivation of metal surfaces, especially galvanized metal surfaces.

Still another object is to provide a new and improved method for protecting the surfaces of galvanized metal objects during the period between the fabrication of such objects and the time they are coated with paint or a similar siccative coating by the purchaser thereof.

Other objects will in part be obvious and will in part appear hereinafter.

It has been discovered that the above objects can be attained by providing an aqueous coating composition containing chromium, fluoride and phosphoric acid, the chromium being partially in the hexavalent and partially in the trivalent state. An aqueous composition thus constituted can be applied to galvanized steel in extremely dilute solution and forms a conversion coating thereon which is impervious to atmospheric moisture and to abrasion, and which therefore protects the metal surface against formation of white rust and also improves paint adhesion.

Accordingly, the present invention is directed to a composition for the formation of conversion coatings on metal surfaces which comprises (A) partially reduced chromic acid in which the ratio of hexavalent to trivalent chromium is between about 0.5 and 8.0, (B) a fluoride or fluorine-containing complex in an amount to provide about 0.1 to 1.0 part by weight of fluorine per part of chromium, (C) phosphoric acid in an amount to provide about 0.01 to 0.10 part by weight of phosphorus per part of chromium, and (D) water. More particularl the present invention is directed to an aqueous coating composition as described above which consists essentially of the aboveidentified ingredients and which contains about 0.02-0.20 percent by weight, preferably 0.10-0.18 percent, of chromium; about 0.01-0.09 percent, preferably 0.03-0.09 percent, of fluorine; and about 0001-0010 percent, preferably 0.004-0.010 percent, of phosphorus. Also present may be other metal ions such as zinc, calcium, and the like.

While it is not necessary, and in many cases not possible, that all ingredients of the coating composition be in solution, it is naturally preferred that the composition be homogeneous for convenience in handling as Well as for other reasons. Accordingly, the preferred ingredients (especially the fluoride components) are generally those with the highest water solubility.

The use of component (A), partially reduced chromic acid, in conversion coating compositions is described in the prior art; for example, in U.S. Patents 2,768,104 and 3,063,877. These partially reduced chromic acid compositions are made by reducing an aqueous solution of chromic acid with an oxidizable compound such as formaldehyde, ethanol, ethylene glycol or other polyalcohols, phenol, hydroquinone, potassium iodide or the like. For the purpose of the present invention, methanol has been found to be a particularly useful reducing agent. The reduction may be carried out in the presence of phosphoric acid which is also an essential ingredient of the compositions of this invention; the presence of the latter acid facilitates reduction of the hexavalent chromium and its addition at this time minimizes the number of acidic solutions which must be prepared and handled during the compounding of the compositions. The partially reduced chromic acid solution, as finally used in the compositions of this invention, has a ratio of hexavalent to trivalent chromium of about 0.5-8.0, preferably about 0.8-3.0.

The following examples illustrate typical methods for the preparation of solutions useful as component (A) in this invention.

EXAMPLE 1 A solution of 550 parts of chromic oxide in 432 parts of water is heated to 90-99" C., and a mixture of 29 parts of methanol and 29 parts of water is added slowly beneath the surface of the chromic acid solution, over 4 hours. Approximately 40 parts of carbon dioxide is evolved as the methanol is substantially completely oxidized. After all of the methanol has been added, the mixture is stirred for one hour at 96 C. The product contains 28.45% chromium, and the ratio of hexavalent to trivalent chromium chromium is 2.2.

EXAMPLE 2 A solution of 1100 grams of chromic acid in 2500 grams of water is heated to 90-100 C., and a mixture of 128 grams of methanol and 128 grams of water is added dropwise. About 8-10 hours is required for the addition of the aqueous methanol. The mixture is then stirred for an additional hour and diluted with water to yield three liters of solution. The product thus obtained contains 14.75% chromium, and the ratio of hexavalent to trivalent chromium is 1.15.

EXAMPLE 4 The procedure of Example 3 is followed except that a mixture of 89.6 grams of methanol and 89.6 grams of water is employed. The product contains 14.9% chromium, and the ratio of hexavalent to trivalent chromium is. 1.68.

EXAMPLE 5 The procedure of Example 3 is followed except that 48 grams of methanol and 48 grams of water are added. The product contains 15.45% chromium, and the ratio of hexavalent to trivalent chromium is 3.0.

Fluorine-containing substances suitable for use according to this invention include salts and acid salts of hydrogen fluoride and complex fluorides. The latter, while they do not provide fluoride ion as such in an aqueous medium,

do not provide fluorine in a form suitable for use in the compositions of the invention. Examples of suitable substances are sodium fluoride, sodium hydrogen fluoride, ammonium hydrogen fluoride, sodium fluoborate, sodium fluosilicate, zinc fluosilicate, sodium fluocobaltate and the like.Since high Water-solubility is desired whenever possible, highly soluble fluorides such as the sodium or ammonium hydrogen fluorides are preferred.

The phosphoric acid content of the compositions of this invention may be provided by any of the several available phosphoric acids such as polyphosphoric, orthophosphoric, metaphosphoric, or pyrophosphoric acid, either individually or in admixture. Because of its avail ability and relatively low cost, orthophosphoric acid (H PO is preferred. As noted above, it is generally convenient to have component (C) present in the partially reduced chromic acid solution.

It has been found that the active ingredients of the compositions of this invention may be employed in extremely low concentration for the formation of conversion coatings on metal surfaces. As pointed out hereinabove, the chromium concentration of the coating medium may be as low as about 0.02-0.20 percent and preferably 0.10- 0.18 percent. correspondingly low fluorine and phosphorus concentrations are also suitable, but the preferred concentrations are 0.03-0.09 percent fluorine and 0.004- 0.010 percent phosphorus.

In preparing the coating compositions of this invention, it is possible merely to combine the active ingredients with the necessary amount of water to form the aqueous coating bath. Preferably, however, a concentrate is formed and this concentrate is diluted with water to the desired ingredient level. Typically, a concentrate is prepared containing about 15-25 percent chromium and other ingredients in proportion, and this concentrate is diluted with water to the extent of about 3-4 pints of concentrate per gallons of solution, or about 5.5-8.0 grams of concentrate per liter of solution. The preparation of a concentrate has the advantage that separate batches of ingredients need not be handled; all of the various ingredients may be combined in a single composition which is then diluted to form the coating bath.

The metals suitable for treatment with the compositions of this invention include both ferrous and nonferrous metals. Typical examples are aluminum and steel. The method is particularly applicable to galvanized steel, and especially hot dip galvanized steel; the coating is conveniently applied to such steel immediately following its emergence from the galvanization operation to protect it from chemical and atmospheric attack prior to its shipment to and fabrication by the purchaser.

The coating composition may be applied to the metal article in any one of several ways such as spraying, brushing, dipping, flow-coating, roller-coating and the like, and the temperature of application may vary within wide limits, e.g., from about room temperature to about 212 F. (100 C.). Higher temperatures may also be used if superatmospheric pressures are employed, but such higher temperatures are rarely necessary or desirable. In general, best results are obtained when the temperature of the composition is about 60-150" F. The time of contact between the metal article and the coating composition is not critical and may vary from as little as 5 or 10 seconds to 10 minutes or more. In most instances, a contact time between about 15 seconds and l or 2 minutes is sufficient.

For best results, the metal article should be thoroughly clean before it is contacted with the coating composition. Cleaning, if required, may be accomplished by known physical or chemical methods which remove grease, dirt, oxides and other surface contaminants.

After the metal article has been treated with the composition of this invention, it is dried by allowing it to drain and dry at ambient temperature, by subjecting it to a current of hot air, by passing it through a heated zone or by a similar method. The treated and dried article may then be provided with a siccative top coat such as paint, enamel, varnish, lacquer, synthetic resin or the like to provide further protection and/or decorative effects. This top coat may be applied by conventional means such as spraying, brushing, dipping, roller-coating, etc.. and

Example Product of Example 1, percent 75. 20 56. 30 Product of Example 2, percent; 78.16 72. 48 Ammonium hydrogen fluoride, percent"... 8. 40 7. 28 Sodium fluoborate, percent 06 Sodium hydrogen fluoride, percent Phosphoric acid (75%), pcrcentn. Water, percent The percentages and weight ratios of the various elements in these compositions are given in the following table.

Percent Weight Ratio Cr F P F:C1 P Cr Example:

The superiority of the coating compositions of this invention over known conversion coating compositions containing hexavalent chromium and fluoride is shown by the following experiments. A coil of freshly prepared full-spangle galvanized steel is the substrate to be coated; coating takes place immediately after the hot dip galvanizing operation. The compositions of Examples 6 and 7 are compared with a commercial aqueou bonding coat concentrate sold under the trademark Iridite which contains 17% hexavalent chromium, 7% fluoride and 1.73% ammonia, together with silicic acid and a wetting agent. The Iridite composition is combined with water in a concentration of about 66 lbs. per 100 gallons, or about 79 grams per liter. The compositions of Examples 6 and 7 are diluted to about 3 or 4 pints, respectively, per 100 gallons, or about 5.81 and 7.75 grams per liter, respectively.

The following table gives the approximate percentages of chromium, fluoride and phosphorus in the final compositions.

Upon emergence from the galvanizing unit, the steel sheet is passed through the coating bath at 140 F. at a rate of 50 feet per minute. The treated strip is dried by blowing with warm air, after which 20-foot panels are severed and subjected to the tests described below.

(1) Bare Salt Fog Corrosion Test-The panels are treated according to ASTM Method B117-57T; this procedure, described briefly, involves exposing the panels to a mist or fog of 5% aqueous sodium chloride at 95:t2" F. for a predetermined time. Substantial corrosion of the Iridite-treated panel occurs after 48 hours exposure, while the panels treated with the compositions of this invention are still substantially corrosion-free after 144 hours.

(2) Panels are painted with a white baking alkyd paint, and the painted panels are soaked in distilled water for sixteen hours at 160 F. The soaked panels are scribed with a set of eleven blades to form a cross-hatched pattern of one hundred -inch squares, and adhesive tape is applied to the scribed area and removed until no more paint is removed. The surface is then rated between 0 and 10, with 10 representing paint adhesion. The control panel gives a rating of 0 (complete removal) while the panels treated with the composition of Examples 6 and 7 give ratings of 8.5.

(3) Panels are painted as described above and scribed with a single line by means of an instrument which penetrates to the bare metal. They are then subjected to the Salt Fog Test for hours, and the amount of paint removed is measured in 32nds of an inch from the scribed line. The control panel gives a reading of 2.5, while the panels treated according to this invention both give zero readings. All panels are blistered, however.

The results of the above-described experiments show that the compositions of this invention are substantially more efiective in protecting metal surfaces than comparable compositions containing only hexavalent chromium, even when the former contain much lower concentrations of chromium and other active ingredients.

What is claimed is:

1. A composition suitable for preventing white rust on a zinc or galvanized metal surface which comprises (A) partially reduced chromic acid in which the ratio of hexavalent to trivalent chromium is between about 0.5 and 8.0, in an amount to provide about 0.02-0.20% by weight of chromium, (B) a fluoride or fluorine-containing complex in an amount to provide about 0.01- 0.09% of fluorine and about 0.1 to 1.0 part by weight of flourine per part of chromium, (C) phosphoric acid in an amount to provide about 0.001-0.0l0% of phosphorus and about 0.01 to 0.10 part by weight of phosphorus per part of chromium, and (D) the balance water.

2. A composition according to claim 1 which consists essentially of 0.10-0.18% of chromium, 0.03-0.0 9% of fluorine, 0.004-0.010% of phosphorus, and the balance water, and in which the ratio of hexavalent to trivalent chromium is between about 0.8 and 3.0.

3. The composition of claim 2 wherein the fluoride is selected from the group consisting of ammonium hydrogen fluoride and sodium fluoborate.

4. The composition of claim 2 wherein the fluoride is ammonium hydrogen fluoride.

5. A method of preventing the formation of white rust on a zinc or galvanized metal article which comprises applying to the surface of said article the composition of claim 1 and subsequently drying said article.

6. A method of preventing white rust formation on a galvanized steel article which comprises applying to the surface of said article the composition of claim 4 and subsequently drying said article.

References Cited UNITED STATES PATENTS 2,438,877 3/1948 Spruance 148-616 2,494,908 1/1950 Spruance 148-616 2,494,910 l/1950 Spruance 148-616 2,507,956 5/1950 Bruno et a1 148-627 X 2,894,865 7/1959 Pryor 148-616 2,927,874 3/1960 Pimbley 148-627 2,928,763 3/1960 Russell 148-616 2,967,791 1/1961 Halversen 148-616 3,278,343 10-/1966 Leuzinger 148-616 3,382,111 5/1968 Tongyai et al. 148-616 3,391,032 7/1968 Hansen et al. 148-616 3,410,707 11/1968 Pocock et a1. 148-616 X RALPH S. KENDALL, Primary Examiner I. R. BATTEN, JR., Assistant Examiner U.S. Cl. X.R. 148-627