Composition and method for treatment of phosphated metal surfaces

Abstract

A rinse solution for the treatment of conversion-coated metal substrates for improving the adhesion and corrosion resistance of siccative coatings, comprising an aqueous solution of zirconium ion and an organosilane selected from the group consisting of 3-glycidoxypropyltrimethoxysilane, methyltrimethoxysilane, γ-methacryloxytrimethoxysilane, phenyltrimethoxysilane, and mixtures thereof, with the zirconium ion concentration selected to provide a pH about 2.0 to 9∅ A method for treating such materials by applying the rinse solution to the substrate.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending application Ser. No. 08/106,070, filed Aug. 13, 1993.

BACKGROUND OF THE INVENTION

This invention relates to the treatment of metal surfaces prior to a finishing operation, such as the application of a siccative organic coating (also known as an "organic coating", "organic finish", or simply, "paint"). Specifically, this invention relates to the treatment of conversion-coated metal with an aqueous solution comprised of a selected organosilane and a zirconium ion. Treatment of conversion-coated metal with such a solution improves paint adhesion and corrosion resistance.

The primary purposes of applying siccative coatings to metal substrates (e.g., steel, aluminum, zinc and their alloys) are protection of the metal surface from corrosion and for aesthetic reasons. It is well-known, however, that many organic coatings adhere poorly to metals in their normal state. As a result, corrosion-resistance characteristics of the siccative coating are substantially diminished. It is therefore a typical procedure in the metal finishing industry to subject metals to a pre-treatment process whereby a conversion coating is formed on the metal surface. This conversion coating acts as a protective layer, slowing the onset of the degradation of the base metal, owing to the conversion coating being less soluble in a corrosive environment than is the base metal. The conversion coating is also effective by serving as a recipient for a subsequent siccative coating. The conversion coating has a greater surface area than does the base metal and thus provides for a greater number of adhesion sites for the interaction between the conversion coating and the organic finish. Typical examples of such conversion coatings include, but are not limited to, iron phosphate coatings, zinc phosphate coatings, and chromate conversion coatings. These conversion coatings and others are well-known in the art and will not be descried in any further detail.

Normally, the application of an organic finish to a conversion-coated metal surface is not sufficient to provide the highest levels of paint adhesion and corrosion resistance. Painted metal surfaces are able to reach maximum performance levels when the conversion-coated metal surface is treated with a "final rinse", also referred to in the art as a "post-rinse" or a "seal rinse", prior to the painting operation. Final rinses are typically aqueous solutions containing organic or inorganic entities designed to improve paint adhesion and corrosion resistance. The purpose of any final rinse, regardless of its composition, is to form a system with the conversion coating in order to maximize paint adhesion and corrosion resistance. This may be accomplished by altering the electrochemical state of the conversion-coated substrate by rendering it more passive or it may be accomplished by forming a barrier film which prevents a corrosive medium from reaching the metal surface. The most effective final rinses in general use today are aqueous solutions containing chromic acid, partially reduced to render a solution comprised of a combination of hexavalent and trivalent chromium. Final rinses of this type have long been known to provide the highest levels of paint adhesion and corrosion resistance. Chromium-containing final rinses, however, have a serious drawback due to their inherent toxicity and hazardous nature. These concerns make chromium-containing final rinses less desirable from a practical standpoint, when one considers such issues as safe handling of chemicals and the environmental problems associated with the discharge of such solutions into municipal water streams. Thus, it has been a goal of the industry to find chromium-free alternatives which are less toxic and more environmentally benign than chromium-containing final rinses. It has also been desirous to develop chromium-free final rinses which are as effective as chromium-containing final rinses in terms of paint adhesion and corrosion resistance properties.

Much work has already been done in the area of chromium-free final rinses. Some of these have utilized either zirconium chemistry or organosilanes. U.S. Pat. No. 3,695,942 describes a method of treating conversion-coated metal with an aqueous solution containing soluble zirconium compounds. U.S. Pat. No. 4,650,526 describes a method of treating phosphated metal surfaces with an aqueous mixture of an aluminum zirconium complex, an organofunctional ligand and a zirconium oxyhalide. The treated metal could be optionally rinsed with deionized water prior to painting. U.S. Pat. No. 5,053,081 describes a final rinse composition comprising an aqueous solution containing 3-aminopropyl triethoxysilane and a titanium chelate. In all of the above examples, the treatment method described claimed to improve paint adhesion and corrosion resistance.

The levels of paint adhesion and corrosion resistance afforded by the treatment solutions in the above examples do not reach the levels desired by the metal finishing industry, namely the performance characteristics of chromium-containing final rinses. I have found that aqueous solutions containing selected organosilane compounds and zirconium ion provide paint adhesion and corrosion resistance characteristics comparable to those attained with chromium-containing final rinses. In many cases, the performance of conversion-coated metal surfaces treated with organosilane-zirconium solutions in accelerated corrosion tests exceeds that of conversion-coated metal treated with chromium-containing solutions.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a method and composition of an aqueous rinse which will impart an improved level of paint adhesion and corrosion resistance on painted conversion-coated metal. The composition is comprised of an aqueous solution containing a selected organosilane and zirconium ion and provides levels of paint adhesion and corrosion resistance comparable to or exceeding those provided by chromium-containing final rinses.

It is a further object of the invention to provide a method and rinse composition which contains no chromium.

The presently preferred embodiment of the invention includes a rinse solution for the treatment of conversion-coated metal substrates for improving the adhesion and corrosion resistance of siccative coatings, comprising an aqueous solution of zirconium ion and an organosilane selected from the group consisting of 3-glycidoxypropyltrimethoxysilane, methyltrimethoxysilane, 3 2.5 2.0 4.6 4 2.2 8.9 3.2 5 2.3 10.8 3.2 6 1.7 7.4 3.0 7 2.6 5.4 3.4 ______________________________________

TABLE II
______________________________________
Final      Alkyd       Urethane   Polyester
Rinse No.  (168 hr)    (216 hr)   (240 hr)
______________________________________
1          2.3 mm      1.8 mm     2.1 mm
2         36.3        23.2       40.0
8          0.9         1.9        2.2
9          1.2         1.1        1.1
10          1.2         1.8        1.2
11          1.8         2.3        2.3
12          1.3         2.6        1.6
13          1.6         2.4        2.3
______________________________________

TABLE III
______________________________________
Final      Alkyd       Urethane   Polyester
Rinse No.  (168 hr)    (216 hr)   (240 hr)
______________________________________
1          2.3 mm      1.8 mm     2.1 mm
2         36.3        23.2       40.0
14          1.5         2.0        1.1
15          0.9         1.8        1.2
16          1.5         3.8        1.6
17          0.8         2.0        0.9
18          1.1         5.5        1.3
19          1.0         3.9        1.2
20          0.5        10.9        0.8
21          0.3        11.6        1.0
22          2.6         2.6        1.7
______________________________________

TABLE IV
______________________________________
Final                   High-Solid
Melamine-
Primer-
Rinse Epoxy    Enamel   Polyester
Polyester
Topcoat
No.   (504 hr) (168 hr) (243 hr) (216 hr)
(262 hr)
______________________________________
1    1.3 mm   3.8 mm   1.5 mm   2.2 mm  2.6 mm
23 1.1      0.9      1.9      1.7     1.9
24    1.4      0.5      1.1      0.7     5.8
25    1.4      0.3      0.6      0.4     1.6
______________________________________

TABLE V
______________________________________
Final          Acrylic  High-Solid
Melamine-
Baking
Rinse Epoxy    Urethane Polyester
Polyester
Enamel
No.   (502 hr) (191 hr) (169 hr) (262 hr)
(214 hr)
______________________________________
1    2.2 mm   2.8 mm   5.4 mm   3.1 mm 3.1 mm
24    2.0      1.8      0.5      0.8    1.3
25    1.6      1.6      1.1      1.1    1.1
______________________________________

TABLE VI
______________________________________
Alkyd
Final Epoxy              High-Solid     Primer-
Rinse Melamine  Enamel   Polyester
Acrylic
Topcoat
No.   (607 hr)  (266 hr) (170 hr) (216 hr)
(266 hr)
______________________________________
1    2.0 mm    13.4 mm  4.7 mm   3.4 mm
4.6 mm
25    1.2        0.8     0.6      1.9   1.5
26    1.4        0.7     1.0      3.8   2.9
______________________________________

TABLE VII
______________________________________
Final  Urethane  Epoxy    Alkyd Polyester
Melamine
Rinse  Powder    Powder   Urethane   Polyester
No.    (502 hr)  (672 hr) (168 hr)   (264 hr)
______________________________________
1     0.9 mm    1.7 mm   5.6 mm      5.0 mm
27     4.1       N/A*     N/A        24.1
28     0.9       N/A      N/A        N/A
29     0.9       1.6      4.4         4.2
______________________________________
*Data not available.

TABLE VIII
______________________________________
Final      Alkyd       Urethane   Polyester
Rinse No.  (168 hr)    (240 hr)   (240 hr)
______________________________________
1         2.8 mm      1.6 mm     2.4 mm
30         2.7         1.1        1.9
31         2.3         1.0        1.3
32         2.5         2.0        2.6
33         2.3         1.5        1.9
34         2.7         1.0        1.5
35         3.5         0.9        1.5
36         3.2         0.6        2.3
______________________________________

Claims

1. A rinse solution for the treatment of conversion-coated metal substrates for improving the adhesion and corrosion resistance of siccative coatings, comprising an aqueous solution of zirconium ion and an organosilane in a concentration of about 0.1 to 6.0% w/w and selected from the group consisting of 3-glycidoxypropyltrimethoxysilane, methyltrimethoxysilane, γ-methacryloxytrimethoxysilane, phenyltrimethoxysilane, and mixtures thereof, with the zirconium ion concentration selected to provide a pH for the entire solution about 2.0 to 9∅ 2. A rinse solution as defined in claim 1 wherein the zirconium ion concentration in the rinse solution is at least about 0.005% w/w and the organosilane is about 0.1 to 4.0% w/w γ-methacryloxytrimethoxysilane.3. A rinse solution as defined in claim 1 wherein the zirconium ion concentration in the rinse solution is at least about 0.005% w/w and the organosilane is about 0.25 to 1.0% w/w γ-methacryloxytrimethoxysilane, with a pH about 2.5 to 4∅4. A rinse solution as defined in claim 1 wherein the zirconium ion concentration in the rinse solution is at least about 0.005% w/w and the organosilane is about 0.1 to 6.0% w/w 3-glycidoxypropyltrimethoxysilane.5. A rinse solution as defined in claim 1 wherein the zirconium ion concentration in the rinse solution is at least about 0.005% w/w and the organosilane is about 0.5 to 2.0% w/w

3-glycidoxypropyltrimethoxysilane, with a pH about 2.8 to 6∅6. A rinse solution as defined in claim 1 wherein the zirconium ion concentration in the rinse solution is at least about 0.005% w/w and the organosilane is about 0.25 to 6.0% w/w methyltrimethoxysilane, with a pH

about 2.5 to 8.8. 7. A rinse solution as defined in claim 1 wherein the zirconium ion is from a zirconium ion source selected from the group consisting of hexafluorozirconic acid, zirconium basic sulfate, zirconium hydroxychloride, zirconium basic carbonate, zirconium oxychloride, zirconium acetate, zirconium fluoride, zirconium hydroxide, zirconium orthosulfate, zirconium oxide, zirconium potassium carbonate and mixtures

thereof. 8. A rinse solution as defined in claim 1 wherein the zirconium

ion concentration is at least about 0.005% w/w. 9. A rinse solution as defined in claim 1 wherein the zirconium ion concentration in the rinse solution is at least about 0.005% w/w and the organosilane is about 0.1 to

2.0% w/w phenyltrimethoxysilane, with a pH about 2.0 to 6∅ 10. A rinse solution as defined in claim 1 wherein the zirconium ion concentration in the rinse solution is at least about 0.005% w/w and the organosilane is about 0.1 to 0.5% w/w phenyltrimethoxysilane, with a pH about 2.0 to 6∅

1. In a method for treating conversion-coated metal substrates for improving the adhesion and corrosion resistance of siccative coatings, wherein the improvement comprises:

providing an aqueous solution of zirconium ion and an organosilane in a concentration of about 0.1 to 6.0% w/w and selected from the group consisting of 3-glycidoxypropyltrimethoxysilane, methyltrimethoxysilane, γ-methacryloxytrimethoxysilane,phenyltrimethoxysilane, and mixtures thereof;

selecting the zirconium ion concentration to provide a pH of the solution of about 2.0 to 9.0; and

applying the solution to the substrate. 12. The method as defined in claim 11 wherein the zirconium ion concentration in the solution is at least about 0.005% w/w and the organosilane concentration in the solution is about 0.1 to 4.0% w/w γ-methacryloxytrimethoxysilane.13. The method as defined in claim 11 wherein the zirconium ion concentration in the solution is at least about 0.005% w/w and the organosilane concentration in the solution is about 0.25 to 1.0% w/w γ-methacryloxytrimethoxysilane, with a pH about 2.5 to 4∅14. The method as defined in claim 11 wherein the zirconium ion concentration in the solution is at least about 0.005% w/w and the organosilane concentration in the solution is about 0.1 to 6.0% w/w 3-glycidoxypropyltrimethoxysilane.15. The method as defined in claim 11 wherein the zirconium ion concentration in the solution is at least about 0.005% w/w and the organosilane concentration in the solution is about 0.5 to 2.0% w/w 3-glycidoxypropyltrimethoxysilane, with a pH about

2.8 to 6∅16. The method as defined in claim 11 wherein the zirconium ion concentration in the solution is at least about 0.005% w/w and the organosilane concentration in the solution is about 0.25 to 6.0% w/w

methyltrimethoxysilane, with a pH about 2.5 to 8.8. 17. The method as defined in claim 11 wherein the zirconium ion is from a zirconium ion source selected from the group consisting of hexafluorozirconic acid, zirconium basic sulfate, zirconium hydroxychloride, zirconium basic carbonate, zirconium oxychloride, zirconium acetate, zirconium fluoride, zirconium hydroxide, zirconium orthosulfate, zirconium oxide, zirconium

potassium carbonate and mixtures thereof. 18. The method as defined in claim 11 wherein the zirconium ion concentration is at least about 0.005%

w/w. 19. The method as defined in claim 11 wherein the zirconium ion concentration in the solution is at least about 0.005% w/w and the organosilane concentration in the solution is about 0.1 to 2.0% w/w

phenyltrimethoxysilane, with a pH about 2.0 to 6∅ 20. The method as defined in claim 11 wherein the zirconium ion concentration in the solution is at least about 0.005% w/w and the organosilane concentration in the solution is about 0.1 to 0.5% w/w phenyltrimethoxysilane, with a pH about 2.0 to 6∅