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Extramural Research

Final Report: Zeolite Coatings by In-Situ Crystallization as an Environmentally Benign Alternative to Chromate Conversion and Anodization Coatings

EPA Grant Number: R828134
Title: Zeolite Coatings by In-Situ Crystallization as an Environmentally Benign Alternative to Chromate Conversion and Anodization Coatings
Investigators: Yan, Yushan
Institution: University of California - Riverside
EPA Project Officer: Richards, April
Project Period: August 1, 2000 through July 31, 2003 (Extended to July 31, 2004)
Project Amount: $250,316
RFA: Technology for a Sustainable Environment (1999)
Research Category: Nanotechnology , Pollution Prevention/Sustainable Development

Description:

Objective:

The objective of this research project was to develop a chromium-free zeolite coating that has comparable thickness to chromate conversion and anodization coatings and equivalent or superior performance in coating adhesion, corrosion protection, abrasion resistance, and paint adhesion. An intrinsically inexpensive, safe, and non-polluting in situ crystallization process that is capable of coating large surfaces with complex shape and in confined spaces also was developed.

Zeolites are microporous crystalline silicate materials and have been exploited for their microporosity (< 15Å) as catalysts and separation media. Many high silica zeolites, however, are non-porous in their as-synthesized state because of the organic molecules occluded in their pores during crystallization. High-silica zeolites also are known for their thermal and chemical stability and high mechanical strength. The goal of this project was to explore these dense polycrystalline high silica or pure silica zeolite films in their as-synthesized state for corrosion protection.

Summary/Accomplishments (Outputs/Outcomes):

Accomplishment 1: Corrosion Resistance of ZSM-5 Coatings on Aluminum Alloys

We have demonstrated for the first time that as-synthesized high-silica zeolite ZSM-5 coatings can be non-porous and extremely corrosion resistant (Cheng, et al., 2001; Mitra, et al., 2002b). Syntheses of high-silica zeolites usually involve using an organic template (or structure-directing agent) to crystallize the desired structure. These template molecules (e.g., tetrapropylammonium) are bulky and eventually trapped inside the zeolite structure. To free up the pore spaces for catalysis and separation applications, these trapped molecules are usually burned-out by high temperature calcinations (e.g., 500ºC) in air or oxygen overnight. Therefore, as-synthesized zeolite coatings (without going through calcinations) can be non-porous.

We have shown that high-silica zeolite ZSM-5 coatings on aluminum alloy 2024-T3 (AA-2024-T3) and other metals offer remarkably better corrosion resistance than chromate conversion coating and anodization coating in strong acids and bases (0.5 M NaOH), and pitting aggressive environments (Cheng, et. al., 2001). For example, there is no decrease of corrosion resistance after the zeolite coated AA-2024-T3 sample is immersed in 0.5 M H2SO4 for 10 days. The zeolite coating is chemically bonded to the aluminum alloys or other metals through condensation of surface hydroxyl groups (Yan, et al., 1995). Zeolite coating also has good thermal stability (e.g., 240ºC), good thermal shock stability (e.g., fast thermal cycling between 240ºC and -70ºC), and good adhesion under mechanical stresses (cutting, impact, and bending). It has excellent paint compatibility and is compatible to polyurethane type paints. Zeolite coatings also have better abrasion resistance than anodization coatings. The coating thickness can be readily controlled between 0.5 to 50 µm according to specific needs.

Accomplishment 2: Zeolite Coating by In Situ Crystallization

The high-silica ZSM-5 coatings discussed above were prepared by in situ crystallization process. In situ crystallization refers to a coating process in which the zeolite crystals making up the eventual polycrystalline coating are crystallized directly at the solid-liquid interface from a synthesis solution during the coating formation process (Yan, et al., 1995). The synthesis solution used is a clear, dilute aqueous solution containing primarily molecular species of silicon and aluminum. The term in situ crystallization originates from the fact that there are no preformed zeolite particles in the synthesis solution. The in situ crystallization coating process is an intrinsically simple (one-step) low temperature (60-180ºC) hydrothermal process. The feature of low temperature deposition is important because it does not affect the mechanical properties of the alloys. Briefly, the substrate to be coated is immersed in the synthesis solution and any surface that is in contact with the synthesis solution during crystallization process receives a uniform coating. Because the dilute aqueous solution has very low viscosity (similar to water) and consequently is able to penetrate confined non-line-of-sight spaces, in situ crystallization can selectively coat surfaces of complex geometry and in confined spaces (Cheng, et al., 2001; Yan and Beving, 2004). The in situ crystallization process is fast; the shortest deposition time is about 2 hours using conventional oven heating and 5-15 minutes with microwave heating (Yan and Beving, 2004). For conventional oven heating, the reactor is loaded into an oven preheated at the crystallization temperature to start the crystallization. Thus, a significant portion of the 2 hour heating goes to bring the temperature of the reaction mixture from ambient to the desired reaction temperature.

Accomplishment 3: Synthesis and Corrosion Resistance of High-Silica Zeolite MTW, BEA, and MFI Coatings on Steel and Aluminum

We have successfully synthesized highly corrosion resistant high-silica zeolite MTW, BEA, and MFI coatings with excellent adhesion on Al-2024-T3, Al-6061-T4, and SS-304 (Mitra, et al., 2002b). The zeolite coatings can protect the metals from corrosion in both acidic and basic environments. Based on the corrosion resistance data of high-silica zeolite BEA, MTW, and MFI coatings, it is reasonable to conclude that high silica zeolite coatings, in general, can be used as corrosion resistant coatings. The corrosion resistance of zeolite coatings does not seem to depend on the thickness of the coatings, and a coating of a few hundred nanometers of thickness is sufficient to protect metals and metal alloys from corrosion.

We have demonstrated that zeolite coatings could potentially become a general surface finish for combating metal corrosion by showing that in addition to high-silica MFI, other as-synthesized high-silica zeolite coatings can be corrosion resistant. High-silica zeolites were preferred over low-silica ones because of their high thermal and chemical stability. The three high-silica zeolites investigated (MTW, BEA, and MFI) had different pore dimensionality, microporosity, and framework density (Table 1).

Table 1. High-Silica Zeolite Coatings Synthesized for Corrosion Protection

Coating Type

Template Used

Channel System1

Framework Density

Density2

MTW

TEAOH3

[010] 12 5.6 x 6.0*

19.39 T/1000Å3

1.9343 g/cm3

MFI

TPAOH4

{[100] 10 5.1 x 5.5 <-> [010] 10 5.3 x 5.6}***

17.97 T/1000Å3

1.7926 g/cm3

BEA

TEAOH

<100> 12 6.6 x 6.7** <-> [001] 12 5.6 x 5.6*

15.6 T/1000Å3

1.5562 g/cm3

1 The number of asterisks in the notation indicates whether the channel system is one-, two-, or three-dimensional
2 Density of zeolite was calculated from framework density data assuming negligible intercrystal void space for siliceous end product.
3 TEAOH = tetraethylammonium hydroxide
4 TPAOH-tetrapropylammonium hydroxide

Accomplishment 4: Extension of ZSM-5 Coating to Other Aluminum Alloys

We have successfully extended ZSM-5 coating from AA 2024-T3 to AA-6061-T6, AA7075-T6, and steel SS1008 using 1x3 inch coupons. A universal solution composition and a universal deposition procedure were developed that produce high quality coatings on all of the aluminum alloys tested (2000, 5000, 6000, and 7000 series). Universal solution and deposition procedures are important as these eliminate the need to adjust the solution composition and deposition procedure to suit an individual alloy, which means significant cost savings. The molar composition of the synthesis solution is TPAOH/0.16:NaOH/0.64:TEOS/1:H2O/92:Al/0.0018. Here TPAOH is tetrapropylammonium hydroxide and TEOS is tetraethylorthosilicate. This is a fairly dilute aqueous solution and the deposition process produces uniform coating on the substrate with little bulk crystallization. The deposition was carried out in a convection oven at 175°C for 12-16 hours. The coatings are polycrystalline, continuous, and well adhered to the substrate. Thickness varies slightly from 7-10 μm. The corrosion resistance of the ZSM-5 coatings on these aluminum alloys is measured in acid, base, and pitting aggressive media. Good corrosion resistance was obtained on all of these coatings and in all of the media tested.

Accomplishment 5: Scale-Up of the Deposition Process to 3x6 Panels From 1x3 Coupons

We have successfully scaled up the coating process from 1x3 inch coupons to 3x6 inch panels (ZSM-5 on AA-2024-T3). This allows the performance of many ASTM and Mil-spec testing. Here we focused on the details of deposition. The same chemical solution (TPAOH/0.16:NaOH/0.64:TEOS/1:H2O/92:Al/0.0018) was used for the large 3x6 inch panels as for the small 1x3 inch coupons. The deposition also was carried out in a convection oven at 175°C for 16-24 hours. The reactor used was a simple standard steel autoclave from Parr Instruments. Teflon insert was used to occupy space so that a minimum amount of solution is used for coating deposition. The substrate was hung in the small slit. It appears that the scale up has little impact on deposition chemistry. The coatings are extremely uniform, have excellent adhesion, and are very flexible.

We initially aimed to produce coatings as thin as possible to offer the best mechanical properties such as flexibility and we have been successful in achieving the thin coatings (< 2 μm thick). By adjusting the solution, the surface area ratio, and the crystallization time, we now can produce uniform 3x6 inch panels of 0.5, 2, 4.5, 7.5, 11, and 13 μm thick.

Accomplishment 6: Electrical Insulation Systems (EIS) Testing of the Large Panels

For small 1x3 inch coupons, we have been using epoxy to seal off the edges before the EIS testing. This has been effective for coupons, but this means the large panel has to be cut into small pieces for EIS tests. To save the sample and to be able to characterize the same panel with both EIS and other tests such as salt-fog, we have developed a new test procedure that only requires a small scratch on one side of the panel for electrical contact for the EIS test. This small cut can be sealed easily and put on the backside in the salt-fog tests. This allows us to correlate the EIS results and salt-fog results. We can use this quick and almost nondestructive method to quality control the coatings. We also can use this setup to check the uniformity of the coating by testing several spots on the same panel. Thus far, it has been shown that our panels have uniform corrosion resistance.

Accomplishment 7: Quantitative/Qualitative Mechanical Testing of ZSM-5 Coating on Aluminum and Steel Substrates

ASTM D3359-02 Adhesion Test. We have carried out the adhesion test according to ASTM D3395-02 on ZSM-5 coating on AA-2024-T3, AA-5052-H32, AA-6061-T4, and AA-7075-T6. This is a dry adhesion test involving cross-cutting through the coating to the substrate with a multi-blade knife followed by adhesive taping and peeling. The cut areas were examined using a low magnification glass (x6). The ASTM D3359-02 test kit and the rating protocol were used to assess damages; 5B is the highest rating under this protocol. ZSM-5 coatings on all of the aluminum alloys tested received a rating of 5B. Although the ASTM D3395-02 protocol does not require a high magnification microscope, we examined the cuts under a high magnification microscope (x2000), and extremely clean cuts were revealed. There was no chipping or cracking in the coating after the cutting operation. This is a clear indication that the coatings are strongly adhered to the substrate. High temperature calcination also was performed to see if the coating would behave differently. A 5B rating was retained for all of the samples after calcination at 400°C for 2 hours.

Mechanical Cutting Tests. To test the properties of the zeolite coating under mechanical stress, zeolite coated panels were cut using a standard lab shear. The cut area, which was examined by SEM,. was very clean, with no crack formation and propagation into the coating.

Mechanical Hole Punching. The ZSM-5 coated AA-2024-T3 3x6 inch panels were punched using a standard punch in a machine shop. Very clean holes were generated. Similar results were obtained on ZSM-5 coating on carbon steel S-1008. The edge of the hole was examined by SEM, and no crack was formed along the cutting edge, showing excellent adhesion and mechanical properties of zeolite coating.

ASTM D-2794-93 Impact Test. Impact tests according to ASTM D-2794-93 were performed on ZSM-5 coated AA-2024-T3 panels. The coating was about 2 μm thick. When the impact is not enough to break the panel, it generates a dent that shows no cracking of zeolite coatings. Once the impact is large enough, both the coating and the panel were broken. This is similar to the punching tests in that these tests again show clean cuts.

ASTM D 522-93a Bending Test. To examine the flexibility and adhesion of the zeolite coating, ZSM-5 coated AA-2024-T3 panels were tested according to ASTM D 522-93a. Basically, the zeolite-coated panels were bent around a metal cone and the coating experiences a different degree of bending at different height along the cone. No cracking of the coating was observed along the whole height of the cone.

Accomplishment 8: ASTM B117 Salt-Fog Test

Aluminum 2024-T3 panels that we had coated with zeolite of approximately 7.5 μm were delivered to the Navy China Lake facility for salt-fog testing. Based on the previous testing results, it appears that a 7.5 μm-thick coating had sufficient corrosion resistance. Therefore, we decided to focus on 7.5 μm coatings. We produced 15 zeolite coated panels grouped in three series of five panels each. The S series have been examined on the back by EIS using the almost non-destructive method and shown to have excellent corrosion resistance. Series B are the non-tested (by EIS) twins (panels are synthesized in pairs and each is referred to as a twin of the other because of very similar corrosion performance) of the S samples. Series A are the non-tested twins of five panels that showed medium corrosion resistance (0.856M NaCl, 10-4 A/cm2).

All zeolite panels have passed 1,000 hours of salt spray exposure. Discoloration could be seen on the bottom corners of several panels, which is the result of color transfer of the wood that is used to hold the panels at the required 15° angle.In summary, the zeolite coating offers excellent corrosion protection on 2024-T3 aluminum in the accelerated weather tests. The reproducibility of the deposition process is excellent. If the coating is breached, corrosion remains localized to the damaged area. The zeolite does not blister or delaminate from any damaged areas. It appears from the most recent test results that a consistent zeolite application has been achieved. After 1,000 hours of salt fog exposure there is no visual difference between a rough or smooth zeolite surface.

Accomplishment 9: High-Aluminum Zeolite Coatings on Corrodible Metal Surfaces

We have successfully prepared high-aluminum zeolite A and Y coatings on aluminum alloys and have filed a U.S. Patent (Yan and Beving, 2004) entitled: “High Aluminum Zeolite Coatings on Corrodible Metal Surfaces.” The process demonstrates a novel three-step synthesis method that allows coating of aluminum substrates with low-silica (high-aluminum) zeolite. Prior to this discovery, zeolite coatings synthesized by in situ crystallization on aluminum and its alloys included pure silica (no aluminum) and high silica (low aluminum) zeolite coatings.

The ability to coat aluminum alloys with low-silica zeolite shows great utility for adsorption applications, such as in air conditioning and air separation devices. It is known that high conductivity metals, such as aluminum and aluminum alloys are preferred as heat exchanger parts (e.g., hot coils and chilled coils, and fins) in an air conditioning system. One way of improving the performance of an air conditioning device is to increase the heat transfer efficiency for both the cooling coil and heat rejection coil by coating the heat exchanging surfaces (i.e., aluminum or aluminum alloys) with a coating that can selectively adsorb moisture from air. It is generally known that high aluminum zeolites including X, Y, A, and many others are useful as adsorbents for moisture adsorption. In addition to enhancing the efficiency of air conditioning applications, zeolite coatings also may be used in space applications for removing contaminants and for producing oxygen enriched air. For these applications as well as many others, aluminum or aluminum alloy substrates are preferred for their better thermal conductivity and lighter weight. Likewise, for these applications, high aluminum zeolites are preferred for their better adsorption properties.

Pure or high silica zeolites are hydrophobic, or not significantly hydrophilic, whereas, their aluminosilicate counterparts are hydrophilic. The pH of the synthesis solutions for many of the pure and high silica types of coatings is mild or neutral, allowing a zeolite coating to form on the aluminum before corrosion occurs. High aluminum zeolite coatings such as zeolite X, zeolite Y, and zeolite A are characterized by high pH synthesis solutions, which upon immersion of the aluminum substrate, begin to oxidize and possibly totally corrode the substrate before a zeolite coating can form . To date, there have been no reported high-aluminum zeolite coatings on aluminum and its alloys.


Journal Articles on this Report : 18 Displayed | Download in RIS Format

Other project views: All 36 publications 19 publications in selected types All 18 journal articles

Type Citation Project Document Sources
Journal Article Cheng X, Wang Z, Yan Y. Corrosion-resistant zeolite coatings by in situ crystallization. Electrochemical and Solid-State Letters 2001;4(5):B23-B26.
abstract available  
R828134 (2001)
R828134 (2002)
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R828134 (Final)
  • Abstract: ECS - Abstract
    Exit
  • Journal Article Huang L, Wang H, Hayashi CY, Tian B, Zhao D, Yan Y. Single-strand spider silk templating for the formation of hierarchically ordered mesoporous silica fibers. Journal of Materials Chemistry 2003;13(4):666-668.
    abstract available   full text available
    R828134 (2003)
    R828134 (Final)
  • Abstract: RSC - Abstract
    Exit
  • Journal Article Li S, Demmelmaier C, Itkis M, Liu Z, Haddon R, Yan Y. Micropatterned oriented zeolite monolayer films by direct in situ crystallization. Chemistry of Materials 2003;15(14):2687-2689. R828134 (2003)
    R828134 (Final)
  • Other: ACS - Citation
    Exit
  • Journal Article Li S, Wang X, Beving D, Chen Z, Yan Y. Molecular sieving in a nanoporous b-oriented pure-silica-zeolite MFI monocrystal film. Journal of the American Chemical Society 2004;126(13):4122-4123.
    abstract available   full text available
    R828134 (Final)
  • Abstract from PubMed
  • Abstract: ACS - Abstract
    Exit
  • Journal Article Li S, Li Z, Bozhilov KN, Chen Z, Yan Y. TEM investigation of formation mechanism of monocrystal-thick b-oriented zeolite MFI film. Journal of the American Chemical Society 2004;126(5):10732-10737.
    abstract available   full text available
    R828134 (Final)
  • Abstract from PubMed
  • Abstract: ACS - Abstract
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  • Journal Article Li S, Sun J, Li Z, Peng H, Gidley D, Ryan ET, Yan Y. Evaluation of pore structure in pure silica zeolite MFI low-k thin films using positronium annihilation lifetime spectroscopy. Journal of Physical Chemistry B 2004;108(31):11689-11692.
    abstract available   full text available
    R828134 (Final)
  • Abstract: ACS - Abstract
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  • Journal Article Li Z, Li S, Yan Y. Effects of crystallinity in spin-on pure-silica-zeolite MFI low-dielectric-constant films. Advanced Functional Materials 2004;14(10):1019-1024.
    abstract available  
    R828134 (Final)
  • Abstract: Wiley - Abstract
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  • Journal Article McDonnell AMP, Beving D, Wang A, Chen W, Yan Y. Hydrophilic and antimicrobial zeolite coatings for gravity-independent water separation. Advanced Functional Materials 2005;15(2):336-340.
    abstract available  
    R828134 (Final)
  • Abstract: Wiley Online - Abstract
    Exit
  • Journal Article Mitra A, Kirby CW, Wang Z, Huang L, Wang H, Huang Y, Yan Y. Synthesis of pure-silica MTW powder and supported films. Microporous and Mesoporous Materials 2002;54(1-2):175-186.
    abstract available   full text available
    R828134 (Final)
  • Abstract: Science Direct - Abstract
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  • Journal Article Mitra A, Wang Z, Cao T, Wang H, Huang L, Yan Y. Synthesis and corrosion resistance of high-silica zeolite MTW, BEA and MFI coatings on steel and aluminum. Journal of the Electrochemical Society 2002;149(10):B472-B478.
    abstract available  
    R828134 (2002)
    R828134 (2003)
    R828134 (Final)
  • Abstract: ECS - Abstract
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  • Journal Article Mitra A, Cao T, Wang H, Wang Z, Huang L, Li S, Li Z, Yan Y. Synthesis and evaluation of pure-silica-zeolite BEA as low dielectric constant material for microprocessors. Industrial and Engineering Chemistry Research 2004;43(12):2946-2949.
    abstract available   full text available
    R828134 (Final)
  • Abstract: ACS - Abstract
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  • Other: ACS - Citations Page
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  • Journal Article Wang H, Wang Z, Huang L, Mitra A, Yan Y. Surface patterned porous films by convection-assisted dynamic self-assembly of zeolite nanoparticles. Langmuir 2001;17(9):2572-2574.
    abstract available   full text available
    R828134 (Final)
  • Abstract: ACS - Abstract
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  • Other: ACS - Citation Information
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  • Journal Article Wang H, Huang L, Wang Z, Mitra A, Yan Y. Hierarchical zeolite structures with designed shape by gel-casting of colloidal nanocrystal suspensions. Chemical Communications 2001;(15):1364-1365.
    abstract available   full text available
    R828134 (Final)
  • Abstract: RSC - Abstract
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  • Journal Article Wang H, Wang Z, Huang L, Mitra A, Holmberg B, Yan Y. High-surface-area zeolitic silica with mesoporosity. Journal of Materials Chemistry 2001;11(9):2307-2310.
    abstract available   full text available
    R828134 (Final)
  • Abstract: RSC - Abstract
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  • Journal Article Wang H, Huang L, Holmberg B, Yan Y. Nanostructured zeolite 4A molecular sieving air separation membranes. Chemical Communications 2002;(16):1708-1709.
    abstract available   full text available
    R828134 (Final)
  • Abstract from PubMed
  • Abstract: RSC - Abstract
    Exit
  • Journal Article Wang H, Holmberg BA, Yan Y. Homogeneous polymer-zeolite nanocomposite membranes by incorporating dispersible template-removed zeolite nanocrystals. Journal of Materials Chemistry 2002;12(12):3640-3643.
    abstract available   full text available
    R828134 (Final)
  • Abstract: RSC - Abstract
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  • Journal Article Wang H, Holmberg BA, Yan Y. Synthesis of template-free zeolite nanocrystals by using in situ thermoreversible polymer hydrogels. Journal of the American Chemical Society 2003;125(33):9928-9929.
    abstract available   full text available
    R828134 (Final)
  • Abstract from PubMed
  • Abstract: ACS - Abstract
    Exit
  • Journal Article Yan Y, Li S, Li Z. Pure-silica-zeolite low dielectric constant films: present and the future. Zeolite News Letters 2003;20(3):111-119.
    abstract available  
    R828134 (Final)
  • Other: Author's Web Page - Citation
    Exit
  • Supplemental Keywords:

    corrosion, zeolite, coating, thin film, aluminum, aluminum alloys, steel, chromium, anodization, conversion coating, alternative materials, green chemistry, innovative technology, metal plating industry, microelectronics,, RFA, Scientific Discipline, Toxics, Water, Sustainable Industry/Business, cleaner production/pollution prevention, Wastewater, Environmental Chemistry, Sustainable Environment, Technology for Sustainable Environment, 33/50, Environmental Engineering, hexavalent chromium, in situ crystallization, chromium & chromium compounds, cleaner production, zeolites, environmentally conscious manufacturing, environmentally benign solvents, alternative materials, metal plating industry, anodization coatings, coating processes, carcinogenicity, innovative technology, microelectronics, pollution prevention, water treatment, corrsion protection, green chemistry

    Progress and Final Reports:
    Original Abstract
    2001 Progress Report
    2002 Progress Report
    2003 Progress Report

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