US3279958A - Method of rinsing conversion coatings with chromium complex solutions from chromic acid - Google Patents

Description

. spraying or dipping.

United States Patent "ice METHOD OF RINSING CONVERSION COATINGS WITH CHROMIUM COMPLEX SOLUTIONS FROM CHROMIC ACID James I. Maurer, St. Clair Shores, Richard E. Palmer,

Farmington, and Vinod D. Shah, Detroit, Mich., assignors to Hooker Chemical Corporation, New York, N.Y., a corporation of New York No Drawing. Filed Oct. 15, 1962, Ser. No. 230,729

The portion of the term of the patent subsequent to Dec. 7, 1982, has been disclaimed 2 Claims. (Cl. 1486.16)

This invention relates to improvements in metal coating and more particularly relates to solutions for and a method of improving chemically formed coatings on metal surfaces as a base for paint, laquer, varnish or other organic finishes.

In the art of preparing metal surfaces to receive paint, it is now widely understood that the application of a dilute chromic acid rinse to phosphate coatings and oxide coatings on the metal surface improves that coating as a base for paint, usually both with respect to corrosion resistance and humidity resistance of the painted surface. The use of such rinses is now quite conventional and widely used. Such use is not without difficulty, however, and one of the long standing problems has been the avoidance of uneven concentrations of the chromic acid on different areas of a rinsed metal surface. Such uneven concentrations produce blistering, peeling or other premature failure of the paint applied thereover. A number of solutions have been proposed to overcome this problem including the use of specially controlled and formulated rinses subsequent to the conventional chromic acid rinse, the application of the dilute chromic acid rinse by atomization on a hot bare or chemically coated surface, and the use of dried on coatings containing hexavalent chromium compounds, reduced hexavalent chromium compounds or mixtures thereof. While these proposals represent advances in this art and have utility in specific applications, they each have substantial drawbacks from the commercial standpoint. There is still an important need for an inexpensive, simple process which is adaptable for use in conventional apparatus in high speed production line operation to improve the corrosion and humidity resistance of conversion coatings on metal surfaces.

In commercial high speed production line processing of metal parts for painting, the predominant processes apply the phosphate or other chemical coating from an aqueous bath and after water rinsing the formed chemical coating, it is subjected to an aqueous chromic acid rinse, either by In many of these processes, the chromic acid rinsed coating is thereafter water-rinsed to remove uneven concentrations of chromic acid on certain areas of the surface. It is Well known that the final water rinse dissolves a substantial portion of the applied chromic acid rinse and yet the final water rinse continues to be employed because non-uniform distribution of chromic acid on the surface causes more serious paint failure than ,results from the reduction in the chromic acid content in the final rinsing step. Moreover, the final water rinse has the advantage that it prepares the surface for use with all types of paints and painting systems, even including those which are susceptible to contamination by excess chromic acid in the coating, since the rinse eliminates any water soluble chromic acid.

This invention provides a process which includes the conventional steps which are predominantly employed in high speed production line operation, namely, conventional cleaning, phosphate or other conversion coating forming steps and thereafter rinses the phosphate or conversion coating with the modified aqueous rinse solution of this 3,279,958 Patented Oct. 18, 1966 invention, and finally employs the step of water rinsing the rinsed conversion coating to insure the absence of uneven concentrations of rinse material on the surface.

This invention is based on the discovery that certain chromium-containing solutions are capable of forming water-insoluble chromium-containing complexes with certain integral chemical coatings on a metal surface by a simple rinsing step in a short time, which complexes remain on and in the coating during a subsequent water rinsing step. It was also found that the application of such rinse solutions, under the herein below specified conditions, forms such water-insoluble chromium-containing complexes in unexpectedly large quantities as an integral part of the conversion coating. The evidence at hand clearly demonstrates that coatings such as phosphates, mixed phosphate-oxides, chromates, oxalates and arsenates on the surfaces of metals which are susceptible to corrosion such as iron, steel, zinc, aluminum and the like, after painting, show substantially better corrosion resistance, humidity resistance or paint adhesion, when they are rinsed in the solutions of this invention than when they are rinsed in the heretofore used dilute chromic acid solutions.

In accordance with this invention, it has been found that in order to control the reaction or interaction between the chemical conversion coating and the applied rinse solution which produces the water insoluble chromium complex in that coating the rinse solution should be a dilute aqueous solution containing chromium complexes which include an anion portion that is displaceable upon contact by an anion in the chemical coating. When such a chromium complex contacts the chemical coating, the complex is modified to contain the anion of the surface coating, and the chromium complex thus becomes attached to and an integral part of the coating. It has been found that in order to form dilute aqueous solutions which contain such a chromium complex, it is necessary to provide a solution containing at least about 0.001% w./v. of the trivalent chromium ion and having a pH within the range of about 3.8 to about 6.0. As used herein and in the appended claims, concentrations are expressed as percent w./v. or percent weight per unit volume of the solution, and pH refers tot he numerical values obtained from solutions prepared with deionized water. The pH should be adjusted in the rinse solution prior to the application of the solution to the surface since the necessary chromium complex must be present in the rinse solution at the moment of contact of the chemical coating.

The chromium complex which becomes attached to the chemical coating is water insoluble and the entire complex remains in place through the subsequent water rinse step. When using the rinse solution of this invention it has been found that there is a substantial increase in the chromium concentration in the coating after water rinsing relative to that which is present in the coating after the use of the heretofore known dilute chromic acid solutions. It has also been found that the improvement in corrosion and humidity resistance after painting which is obtained correlates with the increase in the integral water insoluble chromium complex which is found in that coating. However, in order to form the needed chromium complexes in the solution to enable this interaction with the coating on the surface it has been found that the pH of the rinse solution containing at least about 0.001% tri-valent chromium ion must be adjusted to within the range of about 3.8 to 6.0. When the pH of the rinse solution is below about 3.8, the tendency for the formation of the necessary complexes in the rinse solution is reduced and the 'use of such solutions produces no commercially signifimodified dilute chromic acid rinse solutions. When the pH of the rinse solution of this invention exceeds about 3.8, however, it is possible to detect a significant increase in the concentration of chromium which remains on the surface in a water insoluble form relative to that which remains on a similar surf-ace treated with a conventional dilute chromic acid rinse solution containing an identical concentration of chromium, and the corrosion resistance to salt spray is measurably improved relative to the conventionally rinsed coatings. When the pH of the rinse solution of this invention exceeds about 6, there is a tendency for the complex to become insoluble. It has been observed, however, that the solubility of the complex varies not only with the numerical pH value but is also a function of the additional anions which are present in the solution, for example, oxalate or the like, and gelation or precipitation does not always occur precisely at a pH of 6. The upper limit of pH referred to in the claims as about 6 means that value at which the chromium complex remains stable in the dilute aqueous solution.

As above indicated, the dilute aqueous rinse solutions of this invention should contain chromium complexes which include an anion that is'displaceable by the anion portion of the coating to be rinsed. While the most commercially important coating of this type is a phosphate coating, it is to be understood that the coating may be of the phosphate-oxide type such as is obtained from the use of alkali metal dihydrogen phosphate solutions, e.g., sodiu-m or potassium or ammonium dihydrogen phosphate solutions, or may be a chromate coating, an oxalate coating, or an arsenate coating. The method is adaptable for use in conjunction with such chemical coatings on the surfaces of any metal which is susceptible to corrosion in the atmosphere or under other corrosive conditions and particularly including iron, steel, zinc, aluminum, copper, brass, bronze, magnesium, titanium and the like. It has been found that the anion of phosphate coatings most effectively displaces from a chromium complex one or more of the anions nitrate, chloride or chromate. The P0, anion will displace other anions to a somewhat lesser degree, such as sulfate ion, the formate ion and the acetate ion. When the coating is other than phosphate, for example, one which contains the oxalate anion, the dilute rinse solution containing the chromium complex may satisfactorily be one which contains any one of the anions nitrate, chloride, sulfate, phosphate, formate, acetate or sulfite, and when the coating is chromate or arsenate, the preferred anions are nitrate and chloride or mixtures thereof. When rinsing any of the above specified surface coatings, the dilute chromium complex rinse solution may contain certain other anions as well as those above mentioned, such as the nitrite, phosphate, chromate, oxalate or chlorate anions, and it is to be understood that in any case the rinse solution may contain chromium complexes which are mixtures of complexes containing one or more of the above named anions. The presence of such other anions does not prevent the interaction between the chromium complex and the coating and in the case of the chromate anion the concentration may even exceed the chromium complex concentration and yet the benefits of the invention are still obtained, although the degree of insoluble complex formation in the coating does vary as the concentration of the other anion is increased, or decreased, and ordinarily only relatively small amounts of such other anions should be present.

Therinse solutions of this invention may contain trivalent chromium in concentrations within the range of about 0.001% w./v., up to the limit of solubility of the selected chromic salt. A preferred operating concentration is within the range of 0.01% to 0.25% and for typical commercial phosphate coating rinse applications no advantage has been found from the use of trivalent chromium concentrations exceedingabout 0.1% w./v.

The rinse solutions of this invention are those which are prepared from aqueous chromic acid solutions by reducing the chromic acid with any of a large number of known materials which are capable of reducing the hexavalent chromium ion to the trivalent state. Materials which contain an active hydroxyl, aldehyde or carboxyl group are suitable for this purpose and the materials of this type which are capable of relatively fast reduction of the hexavalent chromium ion are preferred. Suitable rinse solutions are formed so long as the quantity of reduction is insufficient to form a gel.

Examples of such materials include the mono hydroxy alcohols including methyl, ethyl, propyl, isopropyl and butyl alcohols, etc., the dihydroxy alcohols, such as glycol, polyethylene glycols, and the polyhydroxy alcohols such as glycerine, mannitol, sorbitol; the aldehydes including aliphatic and aromatic aldehydessuch as formaldehyde, acetaldehyde, be'nzaldehyde; phenol and carboxylic acids including citric acid, tartaric acid, etc. The reducing agent is preferably one which is capable of causing reduction of the hexavalent chromium ion to the trivalent chromium ion in a relatively short time.

Excellent results have been obtained from the use of the product obtained by reducing chromic acid with methyl alcohol in an amount in the range of about 1% to about 60% reduction. The procedure employed for producing the desirable reduction product includes the steps of slowly adding an aqueous methyl alcohol solution to an aqueous chromic acid solution, and after preliminarily mixing, the mixture is further reacted, with agitation, preferably at a temperature in the range of about F. to about F. for about 5-36 hours, depending upon the time required to complete the reduction for the quantity of methyl alcohol present. As the temperature is increased, the speed of reduction or rate of the reaction similarly increases. The specific degree of reduction has not been found to be critical and good results have been obtained in rinsing zinc phosphate coatings or phosphate-oxide coatings prior to painting with rinse solutions having between about 1% and about 55% reduction of the hexavalent ion to the trivalent ion.

In solutions within \a range of pH of about 3. 8 to about 6.0, the formation of the needed chromium complexes occurs fairly rapidly so that it is unnecessary to age the rinse solution for an extended period of time prior to use. For example, a solution prepared at normal room temperature and adjusted to within the above stated pH range will have formed sufficient r rorni-um complexes to be effective as a rinse in the interval of time which is required to confirm that the pH is within the desired range.

The method of this invention simply comprises the application of the dilute rinse solution to the preliminarily formed chemical coating on the metal surface to be prepared to receive paint or other organic finish in a conventional manner such as by spraying, dipping, brushing or the like. After the rinse solution is permitted to drain from the treated chemically coated metal surface, the surface is subjected to a water rinse to insure the absence from uneven concentrations of primarily water-soluble chromium in land on the coating and after drying, the surface is ready to receive an organic finish coating. The after rinse solution is preferably deionized water and excellent results are obtained when it is applied in accordance with the method disclosed in Richards United States Patent No. 3,304,933. The benefits of the invention are obtained even though the final rinse is tap water or other water which is not contaminated with unusually high concentrations of undesirable anions such as chlorides, sulfates, etc. After the final water rinse the coating may be permitted to dry in air or, if desired, may be dried in an oven or with forced Where greater speed is necessary.

It has been found that when using the rinse solutions of this invention on a continuous basis that the rinse solutions tend to become 1 ore acid, and if the solutions are not adjusted periodical-1y by the addition of alkaline material, the pH of the rinse solution will migrate below 3.8. It is necessary to check the pH of the solution during use and in order to obtain the improvements which characterize this invention to adjust the pH so that it is Within the lherein claimed range. It has also been observed that the chromium complex rinse solutions of this invention have the same tendency upon mere standing and if substantial periods of time occur subsequent to the preparation of a rinse solution it is important to check the pH of that rinse solution before it is used, and if necessary, to adjust it within the claimed nange.

The fiollowing examples illustrate the invention in somewhat greater detail but it is to be understood that the specific compositions, conditions of treatment and products produced are given for purposes of i-llustnation only and are not intended to set forth the definitive limits of the invention which have been given hereinabove.

Example I An aqueous acidic zinc phosphate coating solution, modified for coating zinc, was prepared in a conventional manner and upon analysis found to contain 0.27% zinc, 0.23% nickel, 0.98% P 0.21% fluoride, added as silicofluoride, 0.2% N0 and having a total acid of 26 points.

A number of 4" x 12" continuous hot dipped galvanized panels were cleaned in a conventional titanated cleaner, and phosphate coated by spraying the above solutions, at 160 F on the surfaces for 1 minute and therafter cold water rinsing for 30 seconds. A number of these phosphate coated panels were then rinsed in a dilute aqueous chromic acid solution containing 0.05% CrO having a pH of approximately 3.5 fior 30 seconds at 125 F., then rinsed in deionized water by spraying for 5 seconds at room temperature and dried.

A dilute trivalent chromium containing solution was prepared by slowly adding 2,010 ml. of 20%, by volume, aqueous methyl alcohol solution to 51 lbs. of 25.5% by weight CrO aqueous solution. The rate of alcohol addition was controlled to insure that the temperature in the reaction solution did not exceed 175 F. and thereafter the total volume was increased to gallons by adding additional water. The dilute material was then heated for 6 hours, with stirring, at 170 F.l75 F. and at the end of this reaction period water was added to form a total of 93 lbs. reduced chromic acid concentrate. A portion of this concentrate was tested and found to have a CrO content of 9.6% by weight. When deionized water was added to a portion of the concentrate to produce a concentration of 0.016% trivalent chromium, the pH was found to be 3.3. A port-ion of the 3.3 pH solution was adjusted by adding sodium hydroxide solution thereto to form a solution having a pH of 4.6.

A number of the panels coated with zinc phosphate, as above described, were rinsed in the 4.6 pH rinse solution by spraying for seconds at 125 F., and then rinse-d in deionized water by spraying for 5 seconds at room temperature and dried.

All of the panels were then coated with paint using the commercial two-coat enamel, Dulux 7076741, and subjected to the standard 5% sodium chloride salt spray test.

After 672 hours, an inspection of the panels rinsed in the chromic acid solution having a pH of 3.5 showed that the panels were corroded an average of A" to /2 from the diagonal scratch marks and contained localized spots along the scratch ma-rlcs having diameters of about After 672 hours, the panels which were rinsed in the methyl alcohol reduced chromic acid solution having a pH of 4.6, upon inspection, were found to have no general cree-page from the scratch marks and to have only a few spots along the scratch marlcs having diameters of about Example 11 An aqueous acidic chroma-ting solution for coating aluminum was prepared and upon analysis found to contain 0.25% CrO 0.35% fluoride, added as HP, 0.1% aluminum, 0.08% K Fe (CN) and having a pH of 1.6. A number of 4" x 12" 3003 type aluminum alloy panels were conventionally cleaned in a non-etching cleaner and ehromatecoated with the above described solution by spraying the solution on the panels for 15-20 seconds at F. The coating produced was a mixed oxidechromate coating having an average weight of about 3040 rnillignams per square ft. A portion of the coated panels were then rinsed in a dilute chromic acid rinse solution containing 0.09% CrO and aving a pH of approximately 3.5 by immersing the panels in the solution for 30 seconds at F., and thereafter the panels were rinsed in deionized water by flushing the surface and the panels were then dried in an oven at 200 F. for 2 minutes.

Another series of the chromate-coated panels were rinsed in the reduced chromic acid rinse solution described in detail above in Example I, by immersion for 30 seconds at 125 F., and thereafter rinsed in deionized Water by flushing and thereafter dried in an oven at 220 F. for 2 minutes.

All of the panels were then painted with the two-coat enamel, Dulux 707-6741, and subjected to the scratch adhesion test. The panels which were rinsed in the chromic acid rinse solution having a pH of 3.5 had an adhesion rating of 7-8, whereas the panels rinsed in the reduced chromic acid rinse solution having a pH of 4.6 had an adhesion rating of 8-9. The adhesion test rates the adhesion of the paint to the surface, the test comprising an attempt to scrape the paint from the surface by a knife-blade applied thereto at constant angle and pressure. The numeral 10 represents excellent adhesion, 8, good adhesion, 6, fair adhesion, etc.

Example III A dilute trivalent chromium containing solution was prepared by slowly adding 28 grams of a 25% aqueous solution of formaldehyde to 364.4 grams of a 25% CrO by weight aqueous deionized water solution. The highest temperature reached during mixing was 174 F. and after partial cooling the reaction was continued at F. for 2 /2 hours and a determination showed that the reaction had elfected a 27% reduction of the CrO to trivalent chromium. A portion of the reduced concentrate was diluted with ordinary tap water to produce a solution containing 0.0138% trivalent chromium and having a pH of 3.9.

A number of 4" x 12" cold rolled steel panels were coated with a zinc phosphate coating by using a solution containing 0.23% zinc, 0.45% P0 0.56% calcium, 2.48% nitrate, 0.17% ferrous iron, having a total acid of 15.1 and a free acid number of 1.0, by spraying for 60 seconds at 177 F. followed by a 30-second cold water rinse. A series of such coated panels were then rinsed in a dilute aqueous chromic acid rinse solution containing 0.05% CrO and having a pH of approximately 3.5 by immersion at room temperature for 30 seconds. After withdrawal from the chromic acid rinse solution the panels were sprayed for 10 seconds with deionized water and dried for 3 minutes at 375 F.

Another series of similarly phosphate coated panels were rinsed in the trivalent chromium containing solution having a pH of 3.9 by immersion at room temperature for 30 seconds, followed by a 10-second spray with deisonized water and drying for 3 minutes in an oven at 37 P.

All of the panels were then coated with paint using the commercial two-coat enamel system, Dulux 7076741, and subjected to the standard sodium chloride salt corrosion test.

After 792 hours an inspection of the panels rinsed in the chromic acid solution having a pH of 3.5 showed that the migration of the corrosion from the diagonal scratch marks was between and /2 and that the 7 panels had from 10-12 spots of corrosion on their surfaces. After 792 hours the panels which were rinsed in the formaldehyde reduced chromic acid solution having a pH of 3.9, upon inspection, were found to have migration of the corrosion from the diagonal scratch marks of zero and Me" and to have 3 spots of corrosion on their surfaces.

Example IV A dilute trivalent chromium containing solution was prepared by slowly adding grams of benzaldehyde to 404.3 grams of a 25% by weight, CrO -deionized water solution. There was no discernible rise in temperature which resulted from the mixing. The admixture was then heated to 160 F.170 F. and maintained at that temperature for 18-20 hours. At the end of this time, analysis of the product revealed that approximately 3.5% of the CrO had been reduced to trivalent chromium. A portion of this reduced concentrate was diluted with ordinary tap water to form a solution containing 0.001236% trivalent chromium.

A number of 4" x 12" cold rolled steel panels were coated with a zinc phosphate coating by using the solution and procedures described above in detail in Example III. A series of these coated panels were rinsed in a dilute chromium aqueous rinse solution containing 0.05% CrO and having a pH of approximately 3.5 by immersion at room temperature for 30 seconds, and thereafter deionized-water rinsing for seconds by spraying at room temperature and then drying the panels for 3 minutes at 375 F.

Another series of the same phosphate coated panels were rinsed in the trivalent chromium containing solution, at a pH of 5.0, by immersion at room temperature for 30 seconds, spray rinsed in a deionized water spray for 10 seconds at room temperature, and thereafter dried for 3 minutes in an open at 375 P.

All of the panels were then coated with paint, using the commercial two-coat enamel system, Dulux 707-6741, and subjected to the standard sodium chloride salt corrosion test.

After 792 hours, an inspection of the panels rinsed in the chromic acid rinse solution having a pH of 3.5 showed that the migration of the corrosion from the diagonal scratch marks was between and /2" and that the panels had from 10-12 spots of corrosion on their Surfaces. After 792 hours the panels which were rinsed inthe benzaldehyde chromic acid solution having a pH of 5.0, upon inspection, were found to have a migration of the corrosion from the diagonal scratch marks of between and and to have 2-3 spots of corrosion on their surfaces.

What is claimed is:

1. A method of preparing integral chemical coatings, selected from the group consisting of phosphate coatings, phosphate-oxide coatings, chromate coatings, oxalate coatings and arsenate coatings, on a metal surface to receive an organic finish which comprises the steps of applying to said chemical coating a dilute aqueous solution containing at least about 0.001% trivalent chromium in the form of a chromium chromate complex, said solution having a pH in the range of about 3.8 to about 6, and thereafter water rinsing said surface.

2. A method of preparing integral chemical coatings, selected from the group consisting of phosphate coatings, phosphate-oxide coatings, chromate coatings, oxalate coatings and arsenate coatings, on a metal surface to receive an organic finish which comprises the steps of applying to said chemical coating a dilute aqueous solution containing at least about 0.01% to about 0.25% trivalent chromium in the form of a chromium chromate complex, said solution having a pH in the range of about 3.8 to about 6, and thereafter water rinsing said surface.

References Cited by the Examiner UNITED STATES PATENTS 900,597 10/1908 Salzer 20451 1,922,853 8/ 1933 Kissel. 2,768,104 10/1956 Schuster et al. 1486.16 2,846,342 8/1958 Curtin 148-616 2,882,189 4/1959 Russell et a1 1486.16 2,911,332 11/1959 Schuster et a1. 1486.2 3,063,877 11/ 1962 Schilfman 1486.2 X 3,094,441 6/ 1963 Curtin 148-616 OTHER REFERENCES Mellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry, Longmans Green Co., 1931, vol. 11, p. 235.

ALFRED L. LEAVITT, Primary Examiner.

WILLIAM D. MARTIN, RICHARD D. NEVIUS,

R. S. KENDALL, Assistant Examiners.

Claims (1)

1. A METHOD OF PREPARING INTEGRAL CHEMICAL COATINGS, SELECTED FROM THE GROUP CONSISTING OF PHOSPHATE COATINGS, PHOSPHATE-OXIDE COATINGS, CHROMATE COATINGS, OXALATE COATINGS AND ARSENATE COATINGS, ON A METAL SURFACE TO RECEIVE AN ORGANIC FINISH WHICH COMPRISES THE STEPS OF APPLYING TO SAID CHEMICAL COATING A DILUTE AQUEOUS SOLUTION CONTAINING AT LEAST ABOUT 0.001% TRIVALENT CHROMIUM IN THE FORM OF A CHROMIUM CHROMATE COMPLEX, SAID SOLUTION HAVING A PH IN THE RAGE OF AOUT 3.8 TO ABOUT 6, AND THEREAFTER WATER RINSING SAID SURFACE.