US3404045A - Conversion coating containing partially reduced chromic acid, a metal dichromate and phosphoric acid - Google Patents

Description

United States Patent CONVERSION COATING CONTAINING PARTIAL- LY REDUCED CHROMIC ACID, A METAL DI- CHROMATE AND PHOSPHORIC ACID William J. T. Coulman, Jr., Novelty, Ohio, assignor to The Lubrizol Corporation, Wickliife, Ohio, a corporation of Ohio No Drawing. Filed Oct. 17, 1963, Ser. No. 317,045

13 Claims. (Cl. 148-616) ABSTRACT OF THE DISCLOSURE A composition consisting essentially of a dilute aqueous solution of partially reduced chromic acid, a calcium, zinc or nickel dichromate, and a phosphoric acid is useful for corrosion-proofing metal articles, especially ferrous metal articles. Surfaces which are thus treated and then coated with a siccative top-coat are less susceptible to undercuttingthat is the loosening of the top-coat in areas adjacent to a scratch or score. Ordinarily, the metal article is first phosphated and then treated with the chromium-containing composition.

The present invention relates to a composition adapted for the corrosion-proofing of metal articles. In a more particular sense, it relates to a convenient and economical method for inhibiting the corrosion of metal articles, especially ferrous metal articles and galvanized and/or phosphated ferrous metal articles.

It is known to treat metal articles, especially ferrous metal and galvanized ferrous metal articles, with aqueous phosphating solutions, aqueous chromic acid solutions, aqueous metal nitrite solutions, and the like to improve the corrosion resistance of the metal article and extend the useful life of a subsequently applied siccative topcoat such as a topcoat of paint, lacquer, primer, enamel, varnish, synthetic resin, etc. One major shortcoming of known methods of corrosion-proofing has been noted, for example, in the automotive and household appliance industries, where ferrous metal, galvanized ferrous metal, or phosphated ferrous metal parts .are invariably provided with a siccative top-coat of lacquer or enamel. It has been observed that when such a top-coat is scratched or scored during, for example, handling, forming, or assembling operations, the metal substrate becomes a focal point for corrosion and for a phenomenon known as undercutting. Undercutting, or the loosening of the top-coat in areas adjacent to a scratch or score, causes a progressive flaking of the top-coat from the affected area. In severe cases, the undercutting may extend an inch or more from each side of the scratch or score, causing aloosening and subsequent flaking of the top-coat from a substantial portion, if not all, of the metal article.

Another shortcoming, particularly troublesome to the household washing machine industry, is the tendency of top-coated 'metal parts which come in contact with aqueous cleaning solutions such as aqueous solutions of common soap or synthetic detergent to develop blisters in the top-coat due to the corrosion of the metal substrate. Such blisters grow in size as the corrosion progresses and, in some instances, flake from the rest of the top'coat and thereby expose portions of the bare metal substrate.

It is an object, therefore, of the present invention to provide an improved corrosion-proofing composition for metal articles.

Another object is to provide a method for reducing the incidence of undercutting and blistering in top-coated metal articles.

Still another object is to provide metal articles which exhibit improved resistance to the ravages of corrosion.

These and other objects of the invention are realized 3,404,045 Patented Oct. 1, 1968 by the provision of a composition adapted for the corrosion-proofing of metal articles which consists essentially of a dilute aqueous solution of (A) partially reduced chromic acid in which the ratio of hexavalent chromium to trivalent chromium is within the range from about 0.5 to about 5, preferably from about 0.8 to about 3,

(B) a polyvalent metal dichromate selected from the group consisting of calcium dichromate, zinc dichromate and nickel dichromate; and

(C) a phosphoric acid in an amount to provide from about 0.01 to about 0.1 part of phosphorus per part of chromium in said solution;

in which solution from about 20 to about percent and preferably from about 30 to about 70 percent of the chromium content thereof is contributed by component A, the remainder of the chromium content being contributed by component B.

Such dilute aqueous solution will generally contain from about 0.01 to about 0.3 percent and more often from about 0.03 to about 0.15 percent of chromium present as components A and B.

In a preferred embodiment, the invention is directed to a composition adapted for the corrosion-proofing of metal articles which consists essentially of a dilute aqueous solution of (A) partially reduced chromic acid prepared by reducing chromic acid with methanol, in which partially reduced chromic acid the ratio of hexavalent chromium to trivalent chromium is about 1,

(B) calcium dichromate or zinc dichromate; and

(C) orthophosphoric acid in an amount to provide about 0.03 part of phosphorus per part of chromium in said solution;

which solution contains from about 0.03 to about 0.15 percent of chromium and in which solution about twothirds of said chromium content is contributed by component A.

It has been found that the combination of components A, B, C, which combination characterizes all of the compositions of this invention, is more effective in reducing corrosion than any of such components used alone or in pairs.

COMPONENT A As indicated previously, this component is partially reduced chromic acid in which the ratio of hexavalent chromium to trivalent chromium is within the range from about 0.5 to about 5, preferably from about 0.8 to about 3. It is known, as exemplified by US. Patents Nos. 2,768,104 and 3,063,877, to reduce chromic acid partially by means of oxidizable compounds such as formaldehyde, ethanol, glycol and other polyalcohols, phenol, hydroquinone, potassium iodide, etc., and then to use a dilute solution of such partially reduced chromic acid as a corrosion-proofing composition. The preparation of component A does not constitute the invention claimed herein and such preparation may be effected by known methods set forth in the cited patents or elsewhere in the literature. Example I of US. Patent 3,063,877, for example, describes the preparation of a partially reduced chromic acid (ratio of hexavalent chromium to trivalent chromium: 1.27) by the reaction below 180 F. of a solution of chromic acid (prepared by dissolving pounds of CrO in 40 gallons of water) with a solution of 48 pounds of 36.6 percent aqueous formaldehyde in 21 gallons of water. From the standpoint of convenience and economy, however, it is generally preferred to use methanol as the reducing agent for chromic acidin preparing component A. Since component C (a phosphoric acid; to be described in detail hereinafter) is an essential 3 component of the compositions of this invention, it is ordinarily most convenient to partially reduce the chromic acid in the presence of a phosphoric acid. The latter acid does not participate in or interfere with the reduction reaction. Its presence in the chromic acid solution .is de sired solely for the purpose of minimizing the number of acidic solutions which must be prepared and handled during the compounding of the compositions of the present invention.

The following examples are presented to illustrate additional specific modes of preparing partially reduced chromic acid (with or without added component C) useful for the purposes of the present invention. These examples are presented for purposes of illustration only and are not to be construed as limiting the scope of the invention, except as the appended claims may require. Unless otherwise indicated, all parts and percentages are by weight.

Example l.-432 parts of water is introduced into a reaction vessel and 550 parts of CrO is added thereto over a period of 20 minutes at 2732 C. The whole is heated to 93 C. over a period of 1.5 hours and a mixture of 29 parts of methanol with 29 parts of water is added thereto beneath the surface of the chromic acid solution over a period of 4 hours at 90-99 C. A very exothermic reaction occurs and the addition must be made carefully to avoid a too vigorous reaction. Approximately 40 parts of carbon dioxide is formed as a consequence of the complete oxidation of the methanol. After all of the methanol has been added, the whole is stirred for 1 hour at 96 C. The product, a concentrated aqueous solution of partially reduced chromic acid, shows the following analyses:

Chromium percent 28.45 Hexavalent chromium do 19.55 Trivalent chromuim do 8.90

Ratio of hexavalent chromium to trivalent chromium 2.2

Example 2.This example illustrates the preparation of component A by the reduction of chromic acid in the presence of component C. 350 parts of CrO is dissolved in 212 parts of water at 2732 C. and then 35 parts of technical 75% orthophosphoric acid is added. The whole is heated to 87 C. and a mixture of 25.5 parts of methanol and 76.5 parts of water is added over a period of hours at 8796 C., cooling water being applied to the reaction vessel to control the highly exothermic reaction. After all of the aqueous methanol has been added, the whole is stirred for one hour at 93-96 C. The resulting product, an aqueous solution of orthophosphoric acid and partially reduced chromic acid, shows the following analyses:

Chromium percent 27.4

Ratio of hexavalent chromium to trivalent chromium 1.19

Phosphorus percent 1.2-

Example 3.To 1100 grams of CrO dissolved in 2500 grams of water and heated to 90l00 C., there is added dropwise a mixture of 128 grams of methanol and 128 grams of water. The highly exothermic reaction maintains the temperature at 90-100 C. After all of the aqueous methanol has been added (approximately '8 to 10 hours required), the whole is stirred for an additional hour to insure complete reaction. The material in the reaction vessel is then diluted with suflicient water to yield 3 liters of solution. The resulting product, a concentrated aqueous solution of partially reduced chromic acid, shows the following analyses:

Chromium percent 14.75 Ratio of hexavalent chromium to trivalent chromium "1" 1.15

Example 4.An experiment is carried out in the same manner set forth in Example 3, except that a mixture of 89.6 grams of methanol and 89.6 grams of water is employed. The resulting product, a concentrated aqueous solution of partially reduced chromic acid, shows the following analyses:

Chromium percent 14.9 Ratio of hexavalent chromium to trivalent chromium 1.68

Example 5.'An experiment is carried out in the same manner set forth in Example 3, except that a mixture of 48 grams of methanol and 48 grams of water is employed. The resulting product, a concentrated aqueous solution of partially reduced chromic acid, shows the following analyses:

Chromium percent 15.45 Ratio of hexavalent chromium to trivalent chromium COMPONENT B As indicated earlier, this component is a polyvalent metal dichromate selected from the group consisting of calcium dichromate, zinc dichromate, and nickel dichromate. By reason of their excellence, availability, and low cost, calcium dichromate and zinc dichromate are particularly preferred. It is not to be assumed that the several polyvalent metal dichromates are equivalent for the purposes of the invention. As shown by the test data hereinafter, the choice of the particular metal dichromate employed will depend in large measure on the kind of metal article to be corrosion-proofed.

Although the anhydrous polyvalent metal dichromates CaCr O ZnCr O and NiCr O may be employed as component B, it is generally more convenient to use a concentrated aqueous solution thereof, because such solution dissolves immediately in water upon mixing. For example, the reaction of a slurry of 1.72 parts of calcium hydroxide in 6.5 parts of water with 5.1 parts of CrO for about 8 hours at 110120 F. yields an aqueous concentrate containing 45 percent of calcium dichromate. An aqueous concentrate containing 49 percent of zinc dichromate can be prepared from 65 parts of water, 18.9 parts of zinc oxide, and 40 parts of CrO The zinc oxide and chromic acid are added portionwise to the water (so as to avoid formation of a gel) over an 8-hour period at 140 F. Nickel dichromate can be made in a similar manner by preparing a slurry of 500 parts of water and 300 parts of basic nickel carbonate and adding slowly thereto a solution of 1048 parts of CrO in 650 parts of water. After all of the aqueous chromic acid has been added, the whole is heated at the reflux temperature for one hour. The material in the reaction vessel is an aqueous concentrate containing 29 percent of nickel dichromate.

COMPONENT C This component, as indicated earlier, is a phosphoric acid. Any of the several available phosphoric acids such as polyphosphoric, orthophosphoric, metaphosphoric, hypophosphoric, or pyrophosphoric acid may be used either alone or in admixture as this component. Because of its availability and relatively low cost, orthophosphoric acid (H PO is preferred. As noted above in the section dealing with Component A it is generally most convenient to have Component C present in the partially reduced chromic acid (Component A) so as to minimize the number of acidic solutions which must be prepared and handled.

As noted hereinabove, the corrosion-proofing composition of this invention consists essentially of a dilute aqueous solution of Components A, B, and C. Such dilute solutions will ordinarily contain from about 0.01 to about 0.3 percent of chromium and from about 0.01 to about 0.1 part of phosphorus (as a phosphoric acid) per part of chromium. When less than about 0.01 percent of chromium is present, the corrosion-proofing characteristics of the solution fall oli quite rapidly. Somewhat more than 0.3 percent of chromium can be present; however, the use of such larger content of chromium is uneconomical and, in most instances, is not attended by any substantial increase in the corrosion-proofing characteristics of the solution. Preferably, the chromium content of the solution will be within the range from about 0.03 to about 0.1 percent. Each of Components A and B contributes to the chromium content of the solution. In order for the objects of the invention to be achieved, it is important that from about 20 to about 80 percent and preferably from about 30 to about 70 percent of the chromium content of the solution be contributed by Component A, with the remainder being contributed by Component B. When Components A and B are present in such amounts, they provide optimum corrosion-proofing characteristics. It is likewise important that the ratio of hexavalent chromium to trivalent chromium in Component A be within the range from about 0.5 to about 5, preferably from about 0.8 to about 3, and most desirably about 1.

A number of specific examples of aqueous corrosionprooiing compositions of this invention are given in Table I. They are prepared by simply dissolving the indicated components in water.

ventional means such as spraying, brushing, dipping, rollercoating, etc., and then dried either by exposure to air or by means of a baking technique, depending on the nature of the siccative top-coat material.

The following experiments were carried out to determine the corrosion-proofing characteristics of compositions of this invention. They are presented for purposes of illustration only and are not intended to limit the scope of the invention, except as the appended claims may require. Unless otherwise specified, all parts and percentages are by weight.

Example 21.Three clean, 4 inch by 12-inch panels of satin finish, galvanized 20-gauge SAE 1020 steel were spray-phosphated for one minute at 150 F. with an aqueous solution containing 0.054% of calcium ion, 0.381% of phosphate ion, 0.375% of nitrate ion, and 0.04% of nickel ion, and having a total acidity of 9.8 points (points total acidity represents the number of milliliters of 0.1 normal sodium hydroxide solution required to neutralize a 10 milliliter sample of the phosphating solution in the presence of phenolphthalein indicator). The phosphated panels were spray-rinsed with Water for 15 seconds at room temperature and then sprayed for 15 seconds at room temperature, respectively, with a corrosionproofing composition of this invention (Example 6; contains 0.05% of chromium) and two different control solu- TABLE I.-AQUEOUS OORROSION-PROOFING COMPOSITIONS OF THE INVENTION Component A Component B Example No.

Identity Percent 1 Identity Percent 1 Id ntity Percent 14 6-. Product of Example 2 0.033 Calcium dichromate .017 orthophosphonc acid (present 0. 029

in product of Example 2). 7 do d 0.029 8 ..do.. 0.029 9.". Zinc dichromat 0.029 l0 0. 029 ll 0.051 0.029 l2 0. 017 0. 029 13 0. 034 0. 029 14. 0. 051 0. 029 15- 0.025 Hypophosphoric a d 0. 016 16 0.025 Metaphosphoric acid 0. 05 17 0.09 Pyrophosphoric acid 0. 09 l8- 0. 18 Orthophosphoric acid. 0. 04 19 0.01 ....do 0.01 20 Product of Example 4 Zinc dichromatem- 0.03 do 0. 02

1 Component present in an amount to supply the indicated percent of chromium. 2 Component present in an amount to supply the indicated part of phosphorus per part of chromium in the composition.

The treatment of the metal article with the corrosionproofing composition of this invention may be accomplished in any one of several ways. The composition may be contacted with or applied to the article by spraying, brushing, dipping, flow-coating, roller-coating, etc., techniques, and the temperature of the composition may vary within Wide limits, e.g., from room temperature or below to about 212 F. Higher temperatures such as 250 F., 300 F., or more may also be employed by the use of superatmospheric pressures. In general, best results are obtained when the temperature of the composition is within the range from about 60 F. to about 150 F. The time of contact between the metal article and the com-- position of this invention is not critical and may vary from as little as 5 or 10 seconds to as much as 10 minutes or more. In most instances a contact time between about 15 seconds to about 1 or 2 minutes suffices. For best results, the metal article should be thoroughly clean before it is contacted with the corrosion-proofing composition. Cleaning, if required, may be accomplished by known physical and/or chemical means 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. This can be done 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, etc. The treated and dried metal article is then generally provided with a siccative top-coat such as a top-coat of paint, enamel, varnish, lacquer, synthetic resin, etc., to provide further protection and/or decorative effects. Such top-coat may be applied by contions outside of the scope of the invention. For convenient identification the panels were labeled, respectively, as panels A, B, and C. The treated panels were dried in a current of hot (300 F.) air for 10 seconds and then sprayed with a white, alkyd resin-based bake enamel. The top-coated panels were baked in an oven for 20 minutes at 320 F., scribed with a sharp steel instrument to yield a vertical score extending one inch from the top of the panel to one inch from the bottom thereof, and subjected to the Salt Fog Corrosion test. The apparatus used for this test is described in ASTM Procedure Bl17-57T. It consists of a chamber in which a mist or fog of 5 percent aqueous sodium chloride is maintained in contact with the test panels for a predetermined time (in the present case, 120 hours) at i2 F. Upon removal from the test chamber, the top-coated panels are inspected and rated for undercutting. The undercut rating is the average loss of enamel from each side of the score expressed as a value which represents the number of thirty-seconds of an inch of such loss.

The results obtained in this test are shown in Table II.

isol-ution similar to that used in treating panel A, but omitting the orthophosph oric acid. Solution contains 0.05% of chromium.

"Solution similar to that used in treating panel A, but omitting the calcium dichromate and increasing the amount of partially reduced chromic acid employed so that the solution contains 0.05% of chromium.

It will be noted that the panel which had been treated with a corrosion-proofing composition of this invention showed no undercutting, whereas panels which had been treated with solutions outside of the scope of the invention suffered loss of enamel near the score.

Example 22.An experiment like that described in Example 21 was carried out, with the exception that spangle finish galvanized steel panels (labeled A, B, and C, respectively) were used in lieu of the corresponding satin finish galvanized steel panels. The test results are shown in Table III.

TABLE III. SALT FOG CORROSION TEST, 120 HOURS Top-coated panel: Undercut rating A (of the invention) 1.0 B (control 3.0 C (control 3.5

1 See footnote to Table II.

'- See footnote to Table II.

Again it will be noted that the corrosion-proofing composition of the invention was superior to solutions outside of the scope of the invention in reducing loss of enamel from the panel.

Example 23.-Three clean, 4-inch by 12-inch panels of spangle finish, galvanized ZO-gauge SAE 1020 steel were spray-phosphated for one minute at 150 F. With the aqueous phosphating solution described in Example 21. The phosphated panels were spray-rinsed with water for 15 seconds at room temperature and then sprayed for 15 seconds at room temperature, respectively, with a corrosion-proofing composition of the invention (Example 6; contains 0.05% of chromium) and two dififerent control solutions outside of the scope of the invention. For convenient identification, the panels were labeled, respectively, as panels D, E, and F. After the panels had been treated, they were dried in a current of hot (300 F.) air for 10 seconds and dipped in a gray, epoxy resin-based bake enamel. The top-coated panels were then baked in an oven for 10 minutes at 400 F. and subjected to the Tide Corrosion test. In this test, panels to be evaluated are immersed for 48 or 96 hours at 160 F, in an aqueous detergent solution consisting of water plus 1 percent of a synthetic detergent composition consisting of 53 parts of sodium pyrophosphate decahydrate, 19 parts of sodium sulfate, 20 parts of sodium alkylbenzene sulfonate, 7 parts of sodium metasilicate, and 1 part of sodium carbonate. Upon removal from the test solution, the panels are rated for blistering of the top-coat by means of ASTM Procedure D7l4-56. The rating provides for the designation of the size of any blisters present on a scale of to 10, wherein 0 represents large blisters and represents no blisters. It also provides for the frequency of any blisters present as follows: dense=D, medium dense=MD, medium=M, and few=F.

The test results obtained on the several panels of this example are set forth in Table IV.

TABLE IV.TIDE CORROSION TEST, 48 HOURS 1 Solution similar to that used in treating panel A, but. omitting the orthophosphoric acid. Solution contains 0.05% of chromium.

2 Solution similar to that used in treating panel A, but omitting the calcium dichromate and increasing the amount of partially reduced chromic acid employed so that the solution contains 0.05% of chromium.

Example 24.-Three clean, 4-inch by l2-inch panels of ZO-gauge SAE 1020 plain steel were phosphated in the manner set forth in Example 21. Thereafter, the panels were spray-rinsed with water for seconds at room temperature and then sprayed for 15 seconds at room temperature, respectively, with a corrosion-proofing composition of the invention (Example 7; contains 0.1% of chromium) and two different control solutions outside of the scope of the invention. For convenient identification, the panels were labeled, respectively, as panels G,

H, and I. The treated panels were then top-coated with enamel and subjected to the Salt Fog Corrosion test in the same manner set forth in Example 21.

The results obtained in such test are shown in Table V.

TABLE V.-SALT FOG CORROSION TEST, HOURS Top-coated panel: Undercut rating G (of the invention) 0.0 H (control 1.0 I (control 0.5

Solution similar to that used in treating panel G, but omitting the orthophosphoric acid. Solution contains 0.1% of chromium.

Solution similar to that used in treating panel G, but omitting the calcium dichromate and increasing the amount of partially reduced chromic acid employed so that the solution contains 0.1% of chromium.

It will be noted that the corrosion-proofing composition of this invention is also effective in preventing loss of enamel from phosphated and enameled plain steel panels.

Example 25.-Three clean, 4-inch by 12-inch panels of spangle finish, galvanized ZO-gauge SAE 1020 steel were phosphated in the manner set forth in Example 21. The phosphated panels were spray-rinsed with water for 15 seconds at room temperature and then sprayed for 15 seconds at room temperature, respectively, with a corrosion-proofing composition of the invention (Example 8; contains 0.15% of chromium) and two different control solutions outside of the scope of the invention. For convenient identification, the panels were labeled, respectively, as panels I, K, and L. The treated panels were then top-coated with enamel and subjected to the Tide Corrosion test in the same manner set forth in Example 23. The test results are presented in Table VI below.

TABLE VL-TIDE CORROSION TEST, 48 HOURS 1 Solution similar to that used in treating panel J, but omitting the orthophosphoric acid. Solution contains 0.15% of chromium.

2 Solution Similar to that used in treating panel J, but omitting the calcium dichromate and increasing the amount of partially reduced chromic acid employed so that the solution contains 0.15% of chromium.

It will be noted that once again a corrosion-proofing composition of the invention was superior to solutions of like chromium concentration but outside of the scope of the invention.

Example 26.-Two clean, 4-inch by l2-inch panels of spangle finish, galvanized 20-gauge SAE 1020 steel were phosphated in the manner set forth in Example 21. The phosphated panels were spray-rinsed with water for 15 seconds at room temperature and then sprayed for 15 seconds at room temperature, respectively, with a corrosion proofing composition of the invention (Example 9; contains 0.05% of chromium) and a control solution like that of Example 9, except that the zinc dichromate was omitted and the amount of partially reduced chromic acid employed was increased so that the solution contained 0.05% of chromium. For convenient identification, the panels were labeled, respectively, as panels M and N. The treated panels were then enameled and subjected to the Tide Corrosion Test in the same manner set forth in Example 23.

The results obtained in this test are given in Table VII below.

TABLE VII.TIDE CORROSION TEST, 48 HOURS Top-Coated Panel Blister Size Blister Frequency M (of the invention) 10 None N (control) 6 M were phosphated in the manner set forth in Example 21. The phosphated panels were spray-rinsed with water for 15 seconds at room temperature and then sprayed for 15 seconds at room temperature, respectively, with a corrosion-proofing composition of the invention (Example 12; contains 0.05% of chromium) and a control solution similar to that of Example 12, except that the nickel dichromate was omitted and the amount of partially reduced chromic acid employed was increased so that the solution contained 0.05% of chromium. For convenient identification, the panels were labeled, respectively, as panels and P. The treated panels were enameled and subjected to the Tide Corrosion test in the same manner set forth in Example 23.

The results obtained in this test are given in Table VIII.

TABLE VIIL-TIDE CORROSION TEST, 48 HOURS The data obtained in this test are shown in Table X.

TABLE X.-TIDE CORROSION TEST, 96 HOURS It will be noted that for the particular application de scribed in this example, the several polyvalent metal dichromates (calcium, zinc, and nickel) present, respectively, in the solutions applied to panels S, T, U, are of equal effectiveness.

Example 30.Four clean, 4-inch by 12-inch panels of plain (i.e., not phosphated or galvanized) ZO-gauge SAE 1020 steel were spray-rinsed with water for 15 seconds at room temperature and then sprayed for 15 seconds at room temperature, respectively, with the solutions set Top-Coated Panel Blister Size Blister Frequency f h h f 11 held 1 ort 1 too OWlI1SC ue:

0 (of the invention) 10 None n a P (control) 6 M SOLUTION Panel Identity Percent Chromium It will be noted that nickel dichromate 15 effective as PmductofExamp1e6 0'05 component B m a corrosion-proofing composition of this Product of Example 9. 0. 05 invention Product of Example 12 0. 05

' Control solution 0.05

Example 28.-Two clean, 4-inch by 12-inch panels of spangle finish, galvanized ZO-gauge SAE 1020 steel were phosphated in the manner set forth in Example 21. The phosphated panels were spray-rinsed with water for 15 seconds at room temperature and then sprayed for 15 seconds at room temperature with a corrosion-proofing composition of the invention (Example 14; contains 0.15% of chromium) and a control solution similar to that of Example 14, except that the nickel dichromate was omitted and the amount of partially reduced chromic acid employed was increased so that the solution contained 0.15% of chromium. For convenient identification, the panels were labeled, respectively, as panels Q and R. The treated panels were enameled and subjected to the Salt Fog Corrosion Test in the same manner set forth in Example 21.

The results obtained in this test are shown in Table IX below.

TABLE IX. -4SALT FOG CORROSION TEST, 1 20 HOURS Top-coated panel: Undercut rating Q (of the invention) 0.0 R (control) 1.5

Again, it will be noted that nickel dichromate is effective as component B in a corrosion-proofing composition of this invention.

Example 29.--Four clean, 4-inch by 12-inch panels of satin finish, galvanized 20-gauge SAE 1020 steel were spray-phosphated for one minute at 110 F. with an aqueous phosphating solution containing 0.13% of calcium ion, 0.055% of zinc ion, 0.957% of phosphate ion, 0.175% nitrate ion, and 0.078% of nickel ion, and having a total acidity of 16.5 points. Thereafter the panels were spray-rinsed with water for 15 seconds at room temperature and then sprayed for 15 seconds at room temperature, respectively, with the solutions shown in the following schedule:

1 Similar to solutions used in treating panels S, T, and U except for the omission of the polyvalent metal dichromate and the use of an increased amount of partially reduced chromic acid.

The treated panels were enameled and subjected to the 96-hour Tide Corrosion test in the manner set forth in Example 23.

1 Similar to solutions used in treating panels W, X, and Y, except for the omission of the polyvalent metal dichromate and the use of an increased amount of partially reduced chromic acid.

T op-coated panel: Undercut rating W (of the invention) 0.5 X (of the invention) 3.5 Y (of the invention) 4.0 Z (control) 8.0

It will be noted that for the particular application described in this example, the several polyvalent metal dichromates (calcium, zinc, and nickel) present, respectively, in the solutions applied to panels W, X, and Y, are not of equal effectiveness; the solution of the invention containing calcium dichromate (applied to panel W) being the most effective. All of the solutions of the invention are superior, however, to a solution outside of the scope of the invention.

Example 31.Three clean, 4-inch by l2-inch panels of aluminum were phosphated in the same manner set forth in Example 21. The phosphated panels were spray-rinsed with water for 15 seconds at room temperature and then sprayed for 15 seconds at room temperature, respectively, with a composition of the invention and two different control solutions outside of the scope of the invention, as set forth in the following schedule:

2 Solution similar to that of Example 7, but omitting the calcium dichromate and increasing the amount of partially reduced chromic acid employed.

The treated and dried panels were then sprayed with a red, modified alkyd resin-based bake enamel and baked in an oven for 30 minutes at 250 F. The coated panels were then subjected to the Butler Abrasion Test. In this test, coupons measuring 1.5 by 3 inches are cut from the coated panels and taped to the interior of a porcelain ball mill with the long axis of the coupons parallel to the cylindrical axis of the ball mill. The ball mill is charged with 25 cylinders, each measuring 0.81-inch diameter by 0.81-inch length, of hard ceramic (89-91 Rockwell A Hardness), 180 grams of white sand, and 250 grams of water. The ball mill is then sealed and rotated for 45 minutes at 36 rpm. Thereafter, the mill is opened and the coupons are removed, washed with water, and dried. Adhesive cellophane tape is applied to the exposed surface of the coupon and then removed, thereby stripping any non-adherent coating from the coupon. The surface of the coupon is then inspected to determine the percent of the coating which still adheres to the metal substrate, and such value is reported as percent adhesion.

The results obtained in this test are given in Table XII.

TABLE XII. BUTLER ABRASION TEST Top-coated coupon: Percent adhesion A2 (of the invention) 85 B2 (Control) 63 C2 (control) 70 It will be noted from these test data that-a composition of this invention may be used to advantage to treat aluminum aricles as well as ferrous metal articles.

In addition to their utility as corrosion-proofing compositions for application to the surfaces of ferrous metal articles, galvanized ferrous metal articles, phosphated metal articles, and aluminum articles, the corrosionproofing compositions of this invention are also useful in I protecting other metals and alloys such as beryllium, magnesium, cadmium, copper, brass, bronze, white metal, etc., against corrosion. They are also useful in protecting plated metal surfaces such as copper-plated, nickel-plated,

and cadmium-plated ferrous metal articles against corro- I sion.

What is claimed is:

1. A composition adapted for the corrosion-proofing of metal articles which consists essentially of a dilute aqueous solution of (A) partially reduced chromic acid in which the ratio of hexavalent chromium to trivalent chromium is within the range from about 0.5 to about 5;

(B) a polyvalent metal dichromate selected from the group consisting of calcium dichromate, zinc dichromate, and nickel dichromate; and

(C) a phosphoric acid in an amount to provide from about 0.01 to about 0.1 part of phosphorus per part of chromium in said solution;

in which solution from about 20 to about 80 percent of the chromium content thereof is contributed by component A.

2. A composition in accordance with claim 1 which contains from about 0.01 to about 0.3 percent of chromium.

3. A composition in accordance with claim 1 wherein the ratio of hexavalent chromium to trivalent chromium in component A is within the range from about 0.8 to about 3.

4. A composition in accordance with claim 1 wherein from about 30 to about 70 percent of the chromium content thereof is contributed by component A.

5. A composition in accordance with claim 1 wherein component A is prepared by partially reducing chromic acid with methanol.

6. A composition adapted for the corrosion-proofing of metal articles which consists essentially of a dilute aqueous solution of (A) partially reduced chromic acid prepared by reducing chromic acid with methanol, in which partially reduced chromic acid the ratio of hexavalent chromium to trivalent chromium is about 1, (B) calcium dichromate; and (C) orthophosphoric acid in an amount to provide about 0.03 part of phosphorus per part of chromium in said solution; which solution contains from about 0.03 to about 0.15 percent of chromium and in which solution about twothirds of said chromium content is contributed by component A.

7. A composition adapted for the corrosion-proofing of metal articles which consists essentially of a dilute aqueous solution of (A) partially reduced chromic acid prepared by reducing chromic acid with methanol, in which partially reduced chromic acid the ratio of hexavalent chromium to trivalent chromium is about 1, (B) zinc dichromate; and (C) orthophosphoric acid in an amount to provide about 0.03 part of phosphorus per part of chromium in said solution; which solution contains from about 0.03 to about 0.15 percent of chromium and in which solution about twothirds of said chromium content is contributed by component A.

8. A method of corrosion-proofing a metal article which comprises contacting said metal article with the composition of claim 1.

9. A method in accordance with claim 8 wherein the metal article is a ferrous metal article.

.10. A method in accordance with claim 8 wherein the metal article is a galvanized ferrous metal article.

11. A method in accordance with claim 8 wherein the metal article is a phosphated metal article.

12. A metal article which has been corrosion-proofed in accordance with the method of claim 8.

13. A metal article in accordance with claim 12 which has subsequently been provided with a siccative top-coat.

References Cited UNITED STATES PATENTS 2,314,565 3/1943 Thompson 148--6.16 2,412,543 12/1946 Tanner 1486.16 X 2,418,608 4/1947 Thompson et al. 1486.16 2,477,310 7/1949 McLean et al. 148-6.16 X 2,901,385 8/1959 Curtain 1486.16 2,901,385 8/1959 Curtin 1486.16 2,793,932 5/1957 Kahler et al. 3

3,063,877 11/ 1962 Schiffman 1486. 16 3,282,744 11/ 1966 Goldsmith 1486.2

ALFRED L. LEAVITT, Primary Examiner. J. R. BATTEN, Assistant Examiner.