PROCESS AND COMPOSITION FOR TREATING METAL SURFACES USING TRIVALENT CHROMIUM COMPOUNDS

Abstract

Aqueous compositions useful as pretreatments prior to painting to reduce the formation of rust in the uncoated condition. The compositions consist essentially of water, a trivalent chromium compound with the formula Cr(H.sub.xPO.sub.4).sub.3, where x can be 1.5 or 2, a silica compound, and optionally hydrogen peroxide. The composition may have a pH between about 1 and about 4. A process for treating a metal surface includes contacting the surface with such aqueous compositions. The compositions and the processes provide benefits in comparison to the zinc phosphate metal pretreatment thought to be the standard in the industry.

Claims

1. An aqueous pretreatment composition for treating a metal surface, the composition comprising: water; silica; a trivalent chromium compound with the formula Cr(H.sub.xPO.sub.4).sub.3, where x can be 1.5 or 2.

2. The aqueous pretreatment composition of claim 1, wherein the pretreatment composition is acidic.

3. The aqueous pretreatment composition of claim 1, wherein the pretreatment composition has a pH of between about 1 and about 4.

4. The aqueous pretreatment composition of claim 1, further comprising hydrogen peroxide.

5. The aqueous pretreatment composition of claim 1, wherein the trivalent chromium compound is present in an amount of between about 10% wt. % to about 0.5% wt. %.

6. The aqueous pretreatment composition of claim 1, wherein x is 1.5.

7. The aqueous pretreatment composition of claim 1, wherein x is 2.

8. An aqueous pretreatment composition for treating a metal surface, the composition consisting of: water; silica; a trivalent chromium compound with the formula Cr(H.sub.xPO.sub.4).sub.3, where x can be 1.5 or 2.

9. The aqueous pretreatment composition of claim 8, wherein the pretreatment composition is acidic.

10. The aqueous pretreatment composition of claim 8, wherein the pretreatment composition has a pH of between about 1 and about 4.

11. The aqueous pretreatment composition of claim 8, wherein the pretreatment composition is acidic.

12. The aqueous pretreatment composition of claim 8, wherein the trivalent chromium compound is present in an amount of between about 10% wt. % to about 5% wt. %.

13. The aqueous pretreatment composition of claim 8, wherein x is 1.5.

14. The aqueous pretreatment composition of claim 8, wherein x is 2.

15. A method for preparing a metal pretreatment composition consisting of: reducing a hexavalent chromium compound, with hydrogen peroxide, in the presence of phosphoric acid to create a trivalent chromium compound with the formula Cr(H.sub.xPO.sub.4).sub.3, where x can be 1.5 or 2, at a pH of between about 1 and about 4; and adding silica after the hexavalent chromium compound has been fully reduced.

16. The method of claim 15, wherein the silica is Aerodisp W7512S.

17. A method for coating a metal surface comprising: roll coating the metal surface with the composition of claim 1; spraying the metal surface for a period of up to about 300 seconds in the composition of claim 1; or dipping the metal surface for a period of up to about 300 seconds with the composition of claim 1.

18. The method of claim 17 wherein the metal surface is selected from the group consisting of zinc, zinc alloys, aluminum, aluminum alloys, steel, zinc coated steel, and zinc with aluminum alloy.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0018] The invention is best understood from the following detailed description when read in conjunction with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures:

[0019] FIG. 1 is a side-by-side photograhic comparison of the results from Composition 1 as compared to the Control composition for panels scribed and exposed to neutral salt spray for a period of 250 hours per ASTM B117;

[0020] FIG. 2 is a side-by-side photographic comparison of the results from Composition 1 as compared to the Control composition for panels scribed and exposed to neutral salt spray for a period of 250 hours per ASTM B117;

[0021] FIG. 3 is a side-by-side photographic comparison of the results from Composition 1 and Composition 2 as compared to the Control composition for panels scribed and exposed to neutral salt spray for a period of 500 hours per ASTM B117;

[0022] FIG. 4 is a side-by-side photograph of the results from. Composition 2 for panels scribed and exposed to neutral salt spray for a period of 250 hours per ASTM B117;

[0023] FIG. 5 is a side-by-side photographic comparison of the results from Composition 1 as compared to the Example 1 composition disclosed in the Rausch patent for panels scribed and exposed to neutral salt spray for a period of 250 hours per ASTM B117; and

[0024] FIG. 6 is a side-by-side photographic comparison of the results from Composition 1 as compared to the Example 1 composition disclosed in the Rausch patent for panels scribed and exposed to neutral salt spray for a period of 250 hours per ASTM B117.

DESCRIPTION OF THE INVENTION

[0025] The present invention is directed to compositions and processes for treating a metal surface. Compositions according to the present invention comprise, preferably consist essentially of or most preferably consist of water, a trivalent chromium compound with the formula Cr(H.sub.xPO.sub.4).sub.3, where x can be 1.5 or 2, a silica compound, and optionally hydrogen peroxide. Processes according to the present invention include contacting a metal surface with the above composition, including: water, a trivalent chromium compound with the formula Cr(H.sub.xPO.sub.4).sub.3, where x can be 1.5 or 2, a silica compound, and optionally hydrogen peroxide.

[0026] Aqueous compositions of the present invention are applied to a metal surface after cleaning but before some final coat is applied to the metal surface, contributing to at least one of the following: (1) improving the paint adhesion of the metal surface; (2) improving the corrosion resistance of the metal surface; and (3) maintaining or reducing the resistivity of the metal surface. Compositions of the present invention include compositions which significantly improve one or two of these characteristics, even though at least one of the others is worsened to a lesser extent. The improvement could be due to the compositions alone or the compositions in combination with other process steps. Such compositions are referred to herein or in the metal treatment industry as pretreatment compositions, conversion coatings, or working compositions. Resistivity is defined as resistance per unit surface area; typical units of resistivity are microhms per square inch.

[0027] As used herein, the term hexavalent chromium compound means compounds, namely salts, of chromium in which the chromium has a valence of plus 6. A wide range of anions could be used, and more than one hexavalent chromium compound could be used. Preferably, the hexavalent chromium compound is anhydrous chromic acid (CrO.sub.3), chromic acid (H.sub.2CrO.sub.4), or chromium chromate (Cr.sub.5O.sub.12).

[0028] As used herein, the term metal, used for example in the phrase metal surface, includes copper, brass, magnesium, aluminum, iron, zinc, cadmium, or alloys thereof. Each metal listed includes both the elemental metal and alloys thereof; for example, the term aluminum means aluminum and aluminum alloys. The term alloy is a metal in which the primary element has a higher content than every other element or a content equal to the highest content of every other element (e.g., an aluminum alloy being a metal in which aluminum is present in an amount at least equal to that of any other element). Iron alloys include cold rolled steel, electro-galvanized steel, and hot-dipped galvanized steel. Preferably, compositions of the present invention are used to treat a range of metals including alloys of copper, brass, magnesium, aluminum, and iron.

[0029] As used herein, the term pretreatment composition means any composition which improves the paint adhesion, lowers electrical contact resistance, and improves corrosion resistance of a metal surface. Aqueous pretreatment compositions are used as a pretreatment before painting and may be used as a passivation treatment to reduce the formation of corrosion in the uncoated (unpainted) condition. Thus, although the composition may be called a pretreatment composition for convenience, it is a composition used to improve the adhesion of subsequently applied paint and resist corrosion of the unpainted surface.

[0030] As used herein, the term treating means applying a treatment or cleaning, rinsing, and applying a pretreatment. The pretreatment also functions as a sealant to seal the metal surface, so the term treating shall optionally include the step of sealing the metal surface. Further, treating optionally can include process steps up through and including painting. For example, treatment steps may also include a step of applying a decorative coating, such as painting. After applying the pretreatment, the pretreatment may be rinsed first or dried-in-place before application of the paint. Each of these steps play a role in a final product's ability to resist corrosion and minimize paint loss. As mentioned above, the treatment composition can be used as a pre-paint treatment.

[0031] As used herein, the term trivalent chromium compound means compounds, namely salts, of chromium in which the chromium has a valence of plus 3. No hexavalent chromium (or at worst a de minimus, inconsequential amount of it) is present in such compounds. A wide range of anions could be used, and more than one trivalent chromium compound could be used. Preferably, the trivalent chromium compound is a compound with the formula Cr(H.sub.xPO.sub.4).sub.3, where x can be 1.5 or 2.

[0032] In an alternative embodiment, the composition of the present invention consists essentially of water, a compound with the formula Cr(H.sub.xPO.sub.4).sub.3, where x can be 1.5 or 2, a silica compound, and optionally hydrogen peroxide. While such a composition could include additives, it excludes halide ions which promote undercutting. If undercutting occurs, reduced performance could follow.

[0033] Appropriate concentration ranges of the various components are dependent upon their solubilities. At concentrations too low, there are insufficient amounts of the constituents to cover the metal surface and perform their functions. Above the solubility limits, the solute may begin to come out of the solution. In an embodiment of the invention, the trivalent chromium compound is present in an amount of between: about 10% wt. % to about 5% wt. %, about 10% wt. % to about 2.5% wt. %, about 10% wt. % to about 1% wt. %, or about 10% wt. % to about 0.5% wt. %. In an embodiment of the invention, the silica is present in an amount of less than 2% wt. %; less than 1.75% wt. %; or less than 1.5% wt. %.

[0034] In an embodiment of the invention in which the precursor hexavalent chromium compound is anhydrous chromic acid (CrO.sub.3), the chromic acid is first reduced in the presence of phosphoric acid to create a trivalent chromium compound: chromium dihydrogen phosphate (Cr(H.sub.2PO.sub.4).sub.3). Hydrogen peroxide is the reducing agent. After the chromium is reduced, silica is added. The following pH range has been found to be preferred given certain other conditions: 1 to 4.

[0035] The compositions given above are of the working bath. It is desirable, of course, to transport the product in the form of a concentrate, namely up to a 10 to 100 fold increase in concentration of the above working bath concentrations. The upper limit of such concentrates is the solubility limit of the first constituent to meet or exceed its solubility limit.

[0036] The pH of the compositions is preferably, when the composition is used to treat cold rolled steel, between 1 to 4. More preferably, the pH is about 1.75.

[0037] Compositions according to the invention may be made by mixing the ingredients in any of a number of sequences. In a non-limiting embodiment where a hexavalent chromium compound is a precursor material, the hexavalent chromium compound is first reduced in the presence of phosphoric acid to create a trivalent chromium compound: chromium dihydrogen phosphate (Cr(H.sub.2PO.sub.4).sub.3). Hydrogen peroxide is the reducing agent. After the chromium is reduced, silica is added. In another non-limiting embodiment where a hexavalent chromium compound is a precursor material, the hexavalent chromium compound is metered into a solution containing phosphoric acid and hydrogen peroxide until the hexavalent chromium compound is fully reduced. After the chromium is reduced, silica is added.

[0038] In one non-limiting embodiment, a concentrate (i.e., master batch) is created by combining the trivalent chromium compound with the formula Cr(H.sub.xPO.sub.4).sub.3, where x is between 1.5 and 2, with water. A silica compound can then be added to the solution. The concentrate can then be diluted, preferably with deionized water, to create the desired concentration at the metal treatment site prior to use.

[0039] In a non-limiting embodiment, upon completion of the reaction some residual hydrogen peroxide may remain. Conversely, in another non-limiting embodiment, the hydrogen peroxide is fully utilized. Preferably, upon completion of the reaction the hydrogen peroxide concentration is within the following range: from 0 to about 0.85 grams H.sub.2O.sub.2 (35%) per gram of Cr(H.sub.2PO.sub.4).sub.3.

[0040] Additional components that are well-known in the art could be included in compositions of the present invention. For example, wetting agents, such as fluorosurfactants, may be included to improve wetting. In some cases, thickeners might also be included if an application requires a higher viscosity.

[0041] In a process of the present invention, a metal surface is coated with a pretreatment composition of the present invention. In this coating step, the composition may contact the metal surface by any number of techniques known in the art. One such method is immersion coating in which the metal is immersed in the bath of pretreatment. Other techniques known in the art including spraying, roll coating, or reverse roll coating, as well as manual application (e.g., brushing). The coating step is done for a time sufficient to achieve the desired coating weight on the metal surface, which can be determined empirically. This desired coating weight will depend on a number of factors well-known in the art. In one embodiment, the amount of coating is sufficient to leave from about 0.1 to about 30 milligrams of chromium and phosphate per each square foot of dried metal surface as determined by the weight-strip-weight method. In another embodiment, the amount of coating is sufficient to leave from about 1 to about 10 milligrams of chromium per each square foot of dried metal surface as determined by x-ray fluorescence, and most preferably about 2.5 to about 3.5 milligrams of chromium per each square foot of dried metal surface as determined by x-ray fluorescence. By using a solution with a higher concentration of the included elements, it is possible to leave the desired amount of the dried coating with less residence time.

[0042] A process for treating a metal surface to improve corrosion resistance, improve paint adhesion, and/or maintain low electrical contact resistance comprises: (1) cleaning the metal surface to form a cleaned metal surface; (2) rinsing the cleaned metal surface with water to form a rinsed metal surface; and (3) contacting the rinsed metal surface with a composition comprising water, a trivalent chromium compound with the formula Cr(H.sub.xPO.sub.4).sub.3, where x can be 1.5 or 2, a silica compound, and hydrogen peroxide.

[0043] The cleaning step may be carried out in any manner known in the art. The types of cleaners suitable for use in the present invention will vary with a number of factors, including the metal being treated, the desired application, and the amount and type of impurities and soils on the metal surface. As such, the preferred cleaners can be determined empirically based on these factors. In one non-limiting embodiment, an alkaline cleaner is used. An exemplary alkaline cleaning agent which can be used in connection with the present invention is Bulk Kleen 841MC cleaner, an alkaline liquid cleaner sold by Bulk Chemicals, Incorporated of Reading, Pa. In another non-limiting embodiment, a phosphoric acid cleaner is used. In general, the cleaning step may be accomplished by contacting the metal surface with a bath of an alkaline cleaning solution to form a cleaned metal surface. The alkaline cleaning solution may be an aqueous solution of an alkaline cleaning agent. The cleaning bath cleans the metal surface by removing oil and other contaminants from the metal surface. The cleaning bath is effective to remove the loose impurities and surface soils. Thus, the cleaning bath removes soils and certain impurities from the metal surface. If the metal surface is heavily soiled, a detergent cleaner additive may be included in the cleaning step.

[0044] A metal surface which has been contacted by an alkaline cleaning solution is referred to herein as a cleaned metal surface. It is cleaned in the sense that it has been exposed to a cleaning bath. It may not be completely cleaned, however, in the sense that substantially all of the impurities have been removed such that it is ready to be exposed to a pretreatment composition. In some cases, it may be adequately cleaned, but in other cases, it should first be rinsed with water before being contacted with a pretreatment composition (i.e., substantially all of the impurities are removed by that point).

[0045] The rinsing step is well-known in the art, and deionized water is preferably used. The use of deionized water avoids the introduction of any deleterious ions, such as chloride ions, into the system. The rinsing step can be two-fold, with a first rinsing step done using tap water and then rinsing with deionized water.

[0046] After step (3) above, contacting the metal surface with the composition, the metal surface may be rinsed with water once again, as is well-known in the art. The rinsed metal surface can then be sealed. Any chemical sealing composition well-known in the industry can be used. In a preferred embodiment, the pretreatment composition includes just a composition comprising water, a trivalent chromium compound with the formula Cr(H.sub.xPO.sub.4).sub.3, where x can be 1.5 or 2, a silica compound, and hydrogen peroxide. When a sealing composition is used, an intervening rinsing step is preferably applied.

[0047] After step (3) or any subsequent sealing step, the metal surface may be dried, or rinsed and dried, and then a decorative coating may be applied to it. For example, the metal surface may be painted or lacquered, or first primed and then painted. Such steps, priming and painting, are known in the art as finishing steps, and any known and suitable finishing steps may be used. Suitable paints include acrylic paints and fluorocarbon paints, among others.

[0048] As can be inferred, after step (3) above or any subsequent sealing step, the metal surface can be dried and then a decorative coating (a paint layer) is applied, without an intervening rinsing step between these steps. This alternative process is known as a dried-in-place pretreatment. Regardless of whether the pretreatment is dried-in-place or there is an intermediate rinsing step, any known method of drying may be employed. The coating may be dried by, for example, using an oven, forced air, etc.

[0049] Determining the times of treatments of the metal surfaces with the baths of the various steps is well-known in the art. They need only be long enough to permit a sufficient time for cleaning (in the case of the cleaning step) or reaction (in the case of the pretreatment or sealing steps). They can be very short or as long as thirty minutes and depend on the stage of treatment, the type of application (e.g., immersion, spray), the type of metal surface, and the desired coating weight, among other factors. The immersion time of a substrate into the composition solution will vary with the stage, and generally varies between approximately 1 minute up to about 10 minutes. The times for immersion are typically longer than when spray is used as the method of contact. Rinse times in general can be fairly short, e.g., 30 seconds to one minute. The specific times of treatment may vary over wide ranges and can be readily determined by one of ordinary skill in the art.

[0050] The desirable performance characteristics of the present invention can be achieved by the processing steps described above to produce a pretreated metal surface with good paint adhesion and corrosion resistance. These characteristics are obtained on the metal surface without a decorative coating. Accordingly, the treated metal surface can be used as unpainted products and will exhibit corrosion resistance even if there is a delay between the treatment steps and any subsequent painting.

[0051] The compositions and processes of the present invention provide the stated benefits without the use of additional components which affect the basic and novel characteristics of the invention. When added to the composition in sufficient amounts, other components may affect the novel characteristics. For example, certain components may make the compositions unstable. Such components may affect the shelf-life of the treatment. Other components may degrade the performance of the compositions and processes of the present invention.

[0052] In summary, the present invention provides environmentally friendly compositions and processes for treating metal, while still maintaining excellent paint adhesion and corrosion resistance. More particularly, the present invention avoids the use of hexavalent chromium, and its associated health hazards and disposal problems.

EXAMPLES

[0053] The following examples are included to more clearly demonstrate the overall nature of the invention. FIGS. 1-6 illustrate the improved results obtained by employing aqueous compositions of this invention. These examples are exemplary, not restrictive, of the invention.

[0054] In the below experiments, a batch of Cr(H.sub.2PO.sub.4).sub.3 was first synthesized via the combination of the raw materials identified in Table 1 below.

TABLE-US-00001 TABLE 1 Raw Material Mass (g) Wt. % CrO.sub.3 (40%) 63.5 29.1 H.sub.3PO.sub.4 (75%) 99.5 45.6 H.sub.2O.sub.2 (35%) 55.1 25.3

[0055] A total of 87.1 grams of chromium dihydrogen phosphate (Cr(H.sub.2PO.sub.4).sub.3) was synthesized. Additional water was added to the solution and the final concentration of the master batch was determined to be 39.2% wt. %. This master batch was further diluted with water to create two separate compositions identified in. Table 2 below.

TABLE-US-00002 TABLE 2 Composition 1 Composition 2 (Identified as #4 (Identified as #6 in the Figures) in the Figures) Raw Material Wt. % Active % Wt. % Active % Cr(H.sub.2PO.sub.4).sub.3 (39.2%) 22.96 9.00 15.31 6.00 Water 63.84 89.42 75.89 92.94 Aerodisp W7512S (12%) 13.20 1.58 8.80 1.06

[0056] Preparation of Test PanelsComposition 1

[0057] Cold rolled steel panels were treated via the following spray process. First, the panels were cleaned with a 3% v/v solution of Bulk Kleen841MC for 5 seconds at 165 F. Bulk Kleen841MC is an alkaline cleaner designed to clean steel, zinc, and aluminum parts. Second, the panels were rinsed at ambient temperature for 30 seconds. Third, the panels were again cleaned with Bulk Kleen841MC at 3% v/v for 5 seconds at 165 F. Fourth, the panels were rinsed with deionized water at ambient temperature for 30 seconds. Fifth, Composition 1 was applied to the panels using a roll coater. Sixth, the panels were dried using hot air. Seventh, all panels were painted using a high reflectance white, single coat polyester at a dry film thickness of 0.7-0.8 mils.

[0058] Preparation of Test PanelsComposition 2

[0059] Cold rolled steel panels were treated via the following spray process. First, the panels were cleaned with a 3% v/v solution of Bulk Kleen841MC for 5 seconds at 165 F. Second, the panels were rinsed at ambient temperature for 30 seconds. Third, the panels were again cleaned with Bulk Kleen841MC at 3% v/v for 5 seconds at 165 F. Fourth, the panels were rinsed with deionized water at ambient temperature for 30 seconds. Fifth, Composition 2 was applied to the panels using a roll coater. Sixth, the panels were dried using hot air. Seventh, all panels were painted using a high reflectance white, single coat polyester at a dry film thickness of 0.7-0.8 mils.

[0060] Preparation of Test PanelsControl Formula

[0061] Cold rolled steel panels were treated via the following spray process. First, the panels were cleaned with a 3% v/v solution of Bulk Kleen841MC for 5 seconds at 165 F. Second, the panels were rinsed at ambient temperature for 30 seconds. Third, the panels were again cleaned with Bulk Kleen841MC at 3% v/v for 5 seconds at 165 F. Fourth, the panels were rinsed with deionized water at ambient temperature for 30 seconds. Fifth, Bulk Bond780 12% v/v was applied at 165 F. in a spray washer for 8 seconds. Bulk Bond780 is a liquid chemical used in coil coating processes to produce a corrosion-resisting, paint-bonding, iron phosphate coating on steel. Afterwards the panels were rinsed with deionized water. Sixth, the panels were dried using hot air. Seventh, the control panels were then sealed with Bulk BondNP250 35% v/v using a roll coater. Bulk BondNP250 is a liquid chrome/silica product designed to be mill applied as a passivation/pretreatment for multi-metal surfaces. Eighth, all panels were dried and painted using a high reflectance white, single coat polyester at a dry film thickness of 0.7-0.8 mils.

[0062] Preparation of Test PanelsRausch Formula

[0063] Cold rolled steel panels were treated via the following spray process. First, the panels were cleaned with a 3% v/v solution of Bulk Kleen841MC for 5 seconds at 165 F. Second, the panels were rinsed at ambient temperature for 30 seconds. Third, the panels were again cleaned with Bulk Kleen841MC at 3% v/v for 5 seconds at 165 F. Fourth, the panels were rinsed with deionized water at ambient temperature for 30 seconds. Fifth, the composition outlined in Example 1 of the Rausch et al. patent was applied to the panels using a roll coater. Sixth, the panels were dried using hot air. Seventh, all panels were painted using a high reflectance white, single coat polyester at a dry film thickness of 0.7-0.8 mils.

[0064] All test panels were then exposed to neutral salt spray for a period of 250 or 500 hours per ASTM B117. Photographic comparisons of the results obtained are displayed in FIGS. 1-6. As can be seen, Composition 1. and Composition 2 provided comparable protection to the control formula (i.e., Bulk Bond780) and outperformed the Rausch et al. formula. Specifically, Composition 1 and Composition 2 exhibited similar corrosion and pitting as the control formula, whereas the Rausch et al. formula exhibited increased corrosion and pitting.

[0065] Without being bound by any specific theory, it is believed that Composition 1 and Composition 2 outperformed the Rausch et al. formula because of the additional ions other than chromium, phosphate, and silica present in the Rausch et al. formula.

[0066] Although illustrated and described above with reference to certain specific embodiments and examples, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention. It is expressly intended, for example, that all ranges broadly recited in this document include within their scope all narrower ranges which fall within the broader ranges. It is also expressly intended that the steps of the methods of using the various compositions disclosed above are not restricted to any particular order.