WO2004009714A1

WO2004009714A1 - Reduced chromate silane film

Info

Publication number: WO2004009714A1
Application number: PCT/US2003/023052
Authority: WO
Inventors: Wim Johan Van Ooij; Vignesh M. Palanivel
Original assignee: University Of Cincinnati
Priority date: 2002-07-24
Filing date: 2003-07-24
Publication date: 2004-01-29
Also published as: AU2003259218A1

Abstract

A silane film that can be used in a wide range of environments, on metals of engineering interest, as a standalone process or as a primer for a top-coating by common paint systems. The film generally comprises: a) at least one organofunctional silane; and., b) a leachable inhibitor. More particularly, the leachable inhibitor is present at a very minute level 100-4000 ppm as compared to prior art chromate coatings. Still further, the present invention teaches a silane composition that may be applied by dipping, wiping, spraying, brushing, or other conventional techniques, whereby the film composition provides a metal treatment that is water soluble and provides the availability of the coating to 'heal' whenever damage occurs from scrapes or scratches by way of the chromate inhibitor. A silane concentrate is likewise taught having a leachable inhibitor concentration of 2000 ppm to 80000 ppm.

Description

Title: REDUCED CHROMATE SILANE FILM

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/398,245, entitled "REDUCED CHROMATE SILANE FILM", filed on July 24, 2002, the disclosure of which is incorporated herein by reference.

BACKGROUND

Field of the Invention

[0002] The present invention relates to corrosion protection and increased adhesion between substrates and subsequently bonded materials. More specifically, the present invention relates to primers, manufactured from at least one organofunctional silane, having increased film thickness, chemical and scratch resistance as well as containing a leachable inhibitor and little to no VOCs. Industries such as the automotive, aerospace, building, galvanizing, and many others could benefit from this invention.

Background of the Invention

[0003] In recent years, organofunctional silanes have been shown to be powerful systems for protecting a wide range of metals against corrosion when applied as primers. The adhesion of paint is also drastically improved when organofunctional silanes are applied as a primer pretreatment. Adhesion and adhesion durability of metals to rubber compounds and structural adhesives have also been objectives of prior art organofunctional silanes systems. [0004] When used as a corrosion protection treatment without paint, prior art silane films have limitations in that the film thickness cannot be more than 0.3 μm. Such films provide remarkable protection, but are easily scratched or otherwise damaged. Also, it is very difficult to apply a thin silane film without pinholes or other defects. In addition, prior art silane films have been transparent and colorless, thereby providing little visual detection as to defects in the film. Consequently, prior art silane films applied alone provided only temporary protection of metals and therefore there is a need for a metal treatment system that is water soluble and provides superior corrosion resistance along with the availability of the coating to "heal" whenever damage occurs from scrapes or scratches.

SUMMARY OF THE INVENTION

[0005] The present invention provides a silane film that can be used in a wide range of environments, on metals of engineering interest, as a standalone process or as a primer for overlying layers of paint. The film generally comprises: a) at least one organofunctional silane; and, b) a water-soluble leachable inhibitor. More particularly, the leachable inhibitor is present at a very minute level as compared to prior art chromate coatings (e.g. 300 μg/m² Cr(VI) vs. 3 g/m² Cr(VI)).

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Fig. 1 is a visual comparison of corrosion exhibited by AA2024-T3 metal panels not coated or coated with prior art films versus films formulated in accordance with the present invention.

[0007] Fig. 2 is a data plot incorporating multiple data sets derived from the present invention embodying the corrosion inhibiting performance for A A 2024-T3 metal panels. [0008] Fig. 3 is a bar graph relating corrosion of various AA 2024-T3 metal panels versus the level of silane and K₂CrO₄ in the protective films tested.

[0009] Fig. 4 is another visual comparison of corrosion exhibited by AA 2024-T3 metal panels not coated or coated with prior art films versus films formulated in accordance with the present invention.

[0010] Fig. 5 is still a further visual comparison of corrosion exhibited by AA 2024-T3 metal panels not coated or coated with prior art films versus films formulated in accordance with the present invention.

[0011] Fig. 6 is a cross-sectional view of an exemplary testing structure incorporating a treated and untreated AA 2024-T3 metal panel.

[0012] Fig. 7 is a data plot incorporating multiple data sets embodying the corrosion inhibiting performance for AA 2024-T3 metal panels coated with the present invention

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0013] In recent years a series of benign surface treatments, developed in part by one of the inventors of the present invention, have been formulated to replace chromate-based metal treatments in a wide range of applications and industries.

[0014] A part of the driving force behind this work is the pressure by governments and environmental agencies in several countries to reduce and/or restrict the use of chromate ions (Cr(VI)) as well as reduce the level of hazardous air pollutants (HAPs) attributable to the solvents of corrosion coatings.

[0015] Although these effectively chromate-free silane compositions and their resulting protective films are effective and versatile corrosion-inhibiting metal pretreatments, they experience some difficulty in becoming accepted in some industries because of two perceived shortcomings as compared with chromate treatments:

1. Pure silane films are invisible, whereas chromate films are slightly yellow, so the chromate films can be detected on a production line, e.g., in a coil coating process; and

2 Chromate films provide a healing effect, i.e., they can protect the substrate metal even after a scribe or scratch in the chromate film. This is caused by the fact that chromate ions are very effective inhibitors for corrosionof many metals and by the very slight solubility in water of chromates such as ZnCrO₄ or SrCrO₄. This solubility is, however, high enough to cause migration of chromate ions to a corrosion site, but low enough to prevent paint blistering and delamination.

[0016] The first deficiency of silane films is the subject of a separate, co-pending patent application by one of the present inventors. The second deficiency, the absence of healing capabilities of silane films, is the subject of the present application. Silane films generally do not exhibit healing capabilities, as they form a highly insoluble siloxane network during drying of the film. In addition, silane molecules are not inhibitors. A silane film works well because it is an effective barrier to water diffusion through the film to the base metal. However, most silane films are not completely impervious to water. They can absorb a small amount of water and then swell slightly, but the process is reversible. The present invention capitalizes on this effect by incorporating water- soluble, highly efficient corrosion inhibitors of the base metal. While some organic inhibitors have been tested with some success, the best inhibitor appears to be chromate. Therefore, it may seem contradictory to incorporate a toxic ion that our silane treatment seeks to replace, but simple mathematics below show the enormous benefit of the present invention in reducing the amount of chromate required for efficient corrosion protection if used in conjunction with silanes. [0017] The effectively chromate-free films of the present invention may generally be applied as a diluted solution of an organofunctional silane to a cleaned metal substrate. Such organofuctional silanes include, preferably, the bis-silane type as these silanes have been discovered to protect metals better from corrosion than the more well-known mono- silanes. An exemplary group of silanes shown to be effective in the present invention are:

bis-[triethoxysilyl]methane (XO)₃ -Si- CH₂ -Si-(OX)₃; bis-[triethoxysilyl]ethane (XO)₃ -Si- (CH₂)₂ -Si-(OX)₃; bis-[triethoxysilyl]octane (XO)₃ -Si- (CH₂)₈ -Si-(OX)₃; bis-[triethoxysilylpropyl]amme (XO -Si-(CH₂ )₃ -NH-(CH₂ )₃ -Si-(OX)₃; bis-[triethoxysilylpropyl]ethylenediamine (XO)₃ -Si-(CH₂ )₃ -NH- (CH₂ )₂ -NH-(CH₂ )₃ -

Si-(OX)₃; bis-[triethoxysilylpropyl]disulfιde (XO)₃ -Si-(CH₂ )₃ -NH- S₂ -Si-(OX)₃; bis-[triethoxysilylpropyl]tetrasulfιde (XO)₃ -Si-(CH₂ )₃ -NH- S₄ -Si-(OX)₃; and, bis-[triethoxysilylpropyl]urea (XO -Si-(CH₂ )₃ -NH-CO-NH-(CH₂ )₃ -Si-(OX)₃, where:

X = CH₃ or C₂H₅ (methoxy or ethoxy) aminopropylsilsesquioxane; methylsilsesquioxane; vinylsilsesquioxane; ureidopropyltriakoxysilane; trimethoxysilylpropylamine; and vinyltriacetoxysilane.

[0018] About 100-4000 ppm of soluble Chromate (VI) ions or another leachable inhibitor are incorporated into the silane solutions, as well as a solvent, typically water. The resulting composition is water soluble and forms a film of approximately 0.3 μm in thickness that shows the same or even improved corrosion protection as a 3 μm thick chromate film. The forms of corrosion amenable to protection by silanes includes both uniform and localized corrosion, such as pitting corrosion, crevice corrosion, IGC (Intergranular Corrosion), SCC (Stress Corrosion Cracking), and CFC (Corrosion Fatigue Cracking). Among the metals successfully tested by application of silane films are carbon steel, stainless steel, galvanized steel, aluminum and many of its alloys, magnesium, copper and brass.

EXPERIMENT 1

[0019] Referencing Fig. l,the present invention was applied to panels of the aerospace alloy AA 2024-T3. This metal is precipitation-hardened and has a poor corrosion resistance in a salt environment without the present invention. It is known in the art that the metal corrodes rapidly when immersed in a salt solution with no protection. Panel A (having no film treatment), panel B (having only a 5% bis-amino/VTAS (2:1) film treatment), and panels C and D (having a 5% bis-amino/NTAS (2:1) + lOOmg/lOOmL of K₂CrO₄ film treatment) were scribed in an "X" fashion and immersed for 8 days in a 3.5% ΝaCl solution. Most of the corrosion results in pitting and a scribe in the metal forms many pits with large cathodic areas around these pits. It is readily apparent that panel A with no film treatment reflects the most corrosion, while panel B reflects substantial corrosion, but less corrosion than panel A. Thus, it has also been shown that water-soluble silane solutions alone (panel B) are not particularly effective for corrosion protection of AA 2024-T3 in a ΝaCl environment. The silane films (a mixture of a bis- aminosilane and vinyltriacetoxysilane) did reduce the pitting substantially, but there is still some uniform corrosion and the corrosion in the scribe cannot be prevented by the silane film alone because it exhibits no healing action.

[0020] The silane system of the present invention (panels C and D), however, has the advantage of being completely water-soluble and exhibits the healing action absent in a regular silane film as a result of the addition of a leachable inhibitor. It should be noted that there are other silanes that are more effective for the prevention of corrosion of AA 2024-T3 in a ΝaCl environment, but such silanes are not water-soluble and require a highly undesirable amount of alcohol in silane mixtures. [0021] Referencing Figs. 2 and 3, direct current polarization data verifies the visual indications of Fig. 1. In ascending order of corrosion protection, a first AA 2024-T3 panel having no film protection showed a relatively large degree of corrosion, while a second AA 2024-T3 having a silane only film alone showed substantial corrosion, but less than the first panel. However, the addition of a leachable inhibitor, chromate (VI) ions, to the silane films coated on the third, fourth, and fifth AA 2024-T3 panels showed a drastic decrease in corrosion. The highest concentration of chromate (VI) ions in the silane film, 1,500 ppm in the coating of the fifth panel, resulting in the best corrosion inhibiting performance. To reiterate, the present invention is a combination of a water soluble silane solution, preferable a bis-silane and/or a combination of silanes, in conjunction with a small amount of a soluble chromate inhibitor. Exemplary chromate inhibitors include potassium chromate (K₂CrO₄), potassium dichromate (K₂Cr₂O₇), and magnesium chromate (MgCrO₄). Since the resulting silane film is typically 0.3 μm thick (when spray applied in one coat), the amount of hexavalent chromate using the above formulation ration is on of the order of 300 μg/m², compared to 3.0 g/m² for prior art films. The present invention contains 1 x 10^"4 of the amount of hexavalent chromium in fully formed prior art chromate films. In sum, testing the AA 2024-T3 alloy with the leachable inhibitor subjected to a salty solution resulted in no uniform corrosion after eight days of immersion.

EXPERIMENT 2

[0022] Referencing Figs. 4 and 5, the present invention was applied to panels of the aerospace alloy A A 2024-T3. This metal is precipitation-hardened and has a poor corrosion resistance in a salt environment without the present invention. It is known in the art that the metal corrodes rapidly when immersed in a salt solution with no protection. Two sets of the AA 2024-T3 panels were prepared, including panel A (having no film treatment), panel B (having only a 5% bis-amino/NTAS (2:1) film treatment), panel C (having a 5% bis-amino/VTAS (2:1) + 150mg/100mL of K₂CrO₄ film treatment), and panel D (having a 5% bis-amino/NTAS (2:1) + 400mg/100mL of K CrO₄ film treatment). The first set of panels were scribed in an "X" fashionand immersed for 14 days (as shown in Fig. 4) in a 3.5% ΝaCl solution. The second set of panels were scribed in an "X" fashion and immersed for 41 days (as shown in Fig. 5) in a 3.5% ΝaCl solution. Most of the corrosion results in pitting and a scribe in the metal forms many pits with large cathodic areas around these pits. It is readily apparent in Figs. 4 and 5 that panel A with no film treatment reflects the most corrosion, while panel B reflects substantial corrosion, but less corrosion than panel A. Thus, it has also been shown that water-soluble silane solutions alone (panel B) are not particularly effective for corrosion protection of AA 2024-T3 in a ΝaCl environment. The silane films (a mixture of a bis-aminosilane and vinyltriacetoxysilane) did reduce the pitting substantially, but there is still some uniform corrosion and the corrosion in the scribe cannot be prevented by the silane film alone because it exhibits no healing action.

[0023] The silane system of the present invention (panels C and D) as shown in Figs. 4 and 5, however, has the advantage of being completely water-soluble and exhibits the healing action absent in a regular silane film. It is clear from Figs.4 and 5 that the leachable inhibitor teamed with a silane solution provides the best corrosion protection, with the corrosion protection increasing with the increased concentration of leachable inhibitor. As noted above, there are other silanes that are more effective for the prevention of corrosion of A A 2024-T3, but such silanes are not water-soluble and require a highly undesirable amount of alcohol in silane mixtures.

EXPERIMENT 3

[0024] Referencing Fig. 6, the present invention along with other silane only solutions were applied to panels of the aerospace alloy AA 2024-T3. A cross section of the test structure 10 includes an O-ring 12 positioned on top of a treated panel 14 and an area 16 bounded by the O-ring 12 being occupied by a 0.5 M solution of NaCl. An untreated panel 18 is positioned to effectively sandwich the O-ring and the NaCl solution between the panels 14, 18 to create a sealed structure. The sealed test structure 10 provides direct fluid communication for the NaCl solution between the treated panel 14 and the untreated panel 18. This direct fluid communication between the panels 14, 18 was maintained for variable durations (24-96 hrs) and from which the direct current polarization data as shown in Fig.7 was derived.

[0025] The treated panels 14 were coated with a 5% VTAS/bis-amino (1:1) solution having cerium nitrate (Ce(NO )₃) as the inhibitor at a concentration of 1,000 ppm. It is important to note that the corrosion data shown in Fig. 7 reflects the corrosion apparent on the untreated panels 18, thereby indicating that the leachable inhibitor, Ce (III) ions, is capable of providing leachable corrosion protection where the corrosion inhibitor travels from the protected substrate to the unprotected substrate through a fluid media.

[0026] Referencing Fig. 7, direct current polarization data taken from the AA 2024-T3 panel experiments above reflect decreased protection of the untreated panels as the duration was increased. Likewise, as the duration was increased, the corrosion protection also increased for a set concentration of cerium nitrate, 1,000 ppm. To reiterate, the present invention is a combination of a silane solution, preferable a bis-silane and/or a combination of silanes, in conjunction with a small amount of a leachable inhibitor, in this data set, cerium (III) ions.

[0027] It will be immediately obvious to one of ordinary skill in the art that applicants have disclosed an invention of immense industrial significance. The present invention utilizes a leachable inhibitor, particularly hexavalent chromate and cerium (III) ions, to its fullest potential, thereby providing silane films with a healing capability that appears to be as effective or nearly as effective as fully formed, prior art chromate films. Thus, the present invention teams the advantages of a water-soluble silane system with the healing capacity of a leachable inhibitor, while doing so in an exemplary embodiment by reducing the amount of chromate in the film by four orders of magnitude. [0028] It is also within the scope and spirit of the present invention to provide a 100% silane solution into which a leachable inhibitor is added. In the exemplary embodiments discussed above, a 5% silane solution was utilized, thereby resulting in leachable inhibitor concentrations 5% of the concentrated level; 2,000 ppm to 80,000 ppm.

[0029] Following from the above description and invention summaries, it should be apparent to those of ordinary skill in the art that, while the methods and apparatuses herein described constitute exemplary embodiments of the present invention, it is to be understood that the inventions contained herein are not limited to these precise embodiments and that changes may be made to them without departing from the scope of the inventions as defined by the claims. Additionally, it is to be understood that the invention is defined by the claims and it not intended that any limitations or elements describing the exemplary embodiments set forth herein are to be incorporated into the meanings of the claims unless such limitations or elements are explicitly listed in the claims. Likewise, it is to be understood that it is not necessary to meet any or all of the identified advantages or objects of the invention disclosed herein in order to fall within the scope of any claims, since the invention is defined by the claims and since inherent and/or unforeseen advantages of the present invention may exist even though they may not have been explicitly discussed herein.

[0030] What is claimed is:

Claims

1. A corrosion inhibiting composition amendable to dilution and application to a substrate, the composition comprising: a water soluble organofunctional silane; and a leachable inhibitor concentration ranging between about 2,000 parts per million and about 80,000 parts per million.

2. The corrosion inhibiting composition of claim 1, wherein the leachable inhibitor includes a leachable chromate inhibitor.

3. The corrosion inhibiting composition of claim 2, wherein the leachable chromate inhibitor comprises chromate (VI) ions.

4. The corrosion inhibiting composition of claim 1, wherein the leachable inhibitor includes a cerium inhibitor.

5. The corrosion inhibiting composition of claim 4, wherein the leachable cerium inhibitor comprises cerium (III) ions

6. The corrosion inhibiting composition of claim 1, wherein the water soluble organofunctional silane is a bis-silane.

7. The corrosion inhibiting composition of claim 1, wherein the water soluble organofunctional silane is a mixture of silanes.

8. The corrosion inhibiting composition of claim 1, wherein the water soluble organofunctional silane includes at least one of aminopropylsilsesquioxane, methylsilsesquioxane, vinylsilsesquioxane, ureidopropyltriakoxysilane, trimethoxysilylpropylamine, and vinyltriacetoxysilane.

9. The corrosion inhibiting composition of claim 1, wherein the leachable inhibitor concentration ranges between about 15,000 parts per million and about 50,000 parts per million.

10. The corrosion inhibiting composition of claim 1, wherein the leachable inhibitor concentration ranges between about 20,000 parts per million and about 40,000 parts per million.

11. A hydrophilic composition amendable to application to a substrate that upon curing results in a hydrophobic film inhibiting corrosion of the substrate, the composition comprising: a water soluble organofunctional silane; a water soluble solvent; and a leachable inhibitor concentration ranging between about 100 parts per million and about 4,000 parts per million.

12. The corrosion inhibiting composition of claim 11, wherein the leachable inhibitor includes a leachable chromate inhibitor.

13. The corrosion inhibiting composition of claim 12, wherein the leachable chromate inhibitor comprises chromate (VI) ions.

14. The corrosion inhibiting composition of claim 11, wherein the leachable inhibitor includes a cerium inhibitor.

15. The corrosion inhibiting composition of claim 14, wherein the leachable cerium inhibitor comprises cerium (III) ions

16. The corrosion inhibiting composition of claim 11, wherein the water soluble organofunctional silane includes at least two bis-silanes.

17. The corrosion inhibiting composition of claim 11 , wherein the water soluble organofunctional silane is a mixture of two or more orgamfunctional silanes.

18. The corrosion inhibiting composition of claim 11, wherein the water soluble organofunctional silane includes at least one of aminopropylsilsesquioxane, methylsilsesquioxane, vinylsilsesquioxane, ureidopropyltriakoxysilane, trimethoxysilylpropylamine, and vinyltriacetoxysilane.

19. The corrosion inhibiting composition of claim 11, wherein the leachable inhibitor concentration ranges between about 750 parts per million and about 2,500 parts per million.

20. The corrosion inhibiting composition of claim 11, wherein the leachable inhibitor concentration ranges between about 1,000 parts per million and about 2,000 parts per million.