Grantee Research Project Results
Final Report: New Environmentally Benign Heteropolymolybdate Conversion Coatings for Aluminum Alloys
EPA Contract Number: 68D00203Title: New Environmentally Benign Heteropolymolybdate Conversion Coatings for Aluminum Alloys
Investigators: Minevski, Zoran
Small Business: Lynntech Inc.
EPA Contact: Richards, April
Phase: II
Project Period: September 1, 2000 through September 1, 2002
Project Amount: $225,000
RFA: Small Business Innovation Research (SBIR) - Phase II (2000) Recipients Lists
Research Category: Nanotechnology , SBIR - Pollution Prevention , Pollution Prevention/Sustainable Development , Small Business Innovation Research (SBIR)
Description:
The use of chemical conversion coatings on aluminum alloys to achieve long-term corrosion resistance on painted spacecraft and aircraft structures has found widespread military and commercial applications. With increasing environmental concerns, the use of chemical conversion coatings that do not contain harmful chemicals is of particular interest to the Department of Defense, the National Aeronautics and Space Administration, and other federal agencies. Conventional baths currently in use for conversion coatings contain chromates, which are highly toxic and known carcinogens. Alternatives for chromate conversion coatings, which exhibit the same corrosion resistance as chromate coatings and are formulated from environmentally acceptable chemicals, are needed. Lynntech, Inc. has developed and tested, through Environmental Protection Agency funding, a new type of heteropolymolybdate-based conversion coating that provides both features.The use of chemical conversion coatings on aluminum alloys to achieve long-term corrosion resistance on painted spacecraft and aircraft structures has found widespread military and commercial applications. With increasing environmental concerns, the use of chemical conversion coatings that do not contain harmful chemicals is of particular interest to the Department of Defense, the National Aeronautics and Space Administration, and other federal agencies. Conventional baths currently in use for conversion coatings contain chromates, which are highly toxic and known carcinogens. Alternatives for chromate conversion coatings, which exhibit the same corrosion resistance as chromate coatings and are formulated from environmentally acceptable chemicals, are needed. Lynntech, Inc. has developed and tested, through Environmental Protection Agency funding, a new type of heteropolymolybdate-based conversion coating that provides both features.Summary/Accomplishments (Outputs/Outcomes):
Phase II tests demonstrated an exceptional corrosion resistance of the new coating prepared from formulations consisting of heteropolymolybdates and several important additives. Some coatings outperformed the chromate-based conversion coatings in electrochemical corrosion resistance tests and passed a standard 14-day salt fog test. The key to this technology is heteropolymolybdates, in which the primary effect of the hetero atom (Mn, V, Ce, Si) is an effective transformation of Mo (III) and Mo (IV) to stable Mo (V) and Mo (VI), thereby enhancing the formulation of conversion coatings on aluminum alloys.Key advantages of heteropolymolybdate conversion coatings are that they:
- Are environmentally friendly (coatings are inorganic and do not contain
chemicals or materials that are hazardous, toxic, or give rise to health and
safety concerns).
- Provide excellent protection (they protect 2024-T3, 6061-T6, and 7075-T6
aluminum alloys for at least 336 hours of exposure in a salt fog chamber according
to American Society for Testing and Materials B117, see Figure 1).
- Are versatile (can be applied by dipping, painting, or spraying methods).
- Can be applied easily (short treatment times, can be applied at room temperature,
and are compatible with existing cleaning and pretreatment procedures).
- Are inexpensive (only commercially available chemicals and materials are used in the conversion coating formulations. These chemicals can easily be adapted into existing application methods, and none of the chemicals require special storage provisions).
The cost of the heteropolymolybdate conversion coating process is attractive when compared to that of existing chromate-based conversion coatings because the need for toxic waste disposal, special handling, and human protection is greatly reduced. A major area of application is the aerospace industry, where coatings are needed for surface preparation to enhance paint bonding and corrosion resistance.
Conclusions:
Based on the research performed, several important conclusions can be drawn:
- The new heteropolymolybdate conversion coating, when prepared as shown in
Table 1, offers a superior corrosion resistance for the protection of aluminum
and aluminum alloys that is comparable to the standard chromate coatings.
- Heteropolymolybdates promote the formation of a hexavalent molybdenum oxide
film that enables self-healing and serves as a barrier to aggressive ions,
which matches the characteristics of chromate conversion coatings.
- Addition of a boehmite layer in the preconversion step can significantly
improve the quality of the conversion coating. X-ray photoelectron spectroscopy
and energy dispersive spectroscopy experiments have shown that boehmite minimizes
the copper redeposition and formation of copper intermetallics.
- The incorporation of calcium hydroxide at the surface of heteropolymolybdate
coatings demonstrated an exceptional improvement in corrosion resistance of
the coatings and was one of the key features responsible for successful performance
in salt fog testing.
- The heteropolymolybdate conversion process does not require the implementation
of new cleaning or conversion treatment procedures and is low cost, using
only commercially available chemicals.
- Heteropolymolybdate conversion coatings are environmentally friendly because they do not contain chemicals or materials that are hazardous, toxic, or give rise to safety concerns.
Table 1. Steps involved with Lynntech's heteropolymolybdate
conversion coating process.
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Cleaning Steps |
Degreasing
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Sonication in acetone
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Alkaline Cleaning
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10 min in Turco
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Deoxidizing
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10 min in Oakite
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Protection Steps |
Boehmite
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20 min in DI H2O
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Conversion Coating
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2 min in heteropolymolybdate
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Calcium Hydroxide Treatment
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20 min in calcium hydroxide
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Silicate Post Sealer
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2 min in Kasil
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Journal Articles:
No journal articles submitted with this report: View all 1 publications for this projectSupplemental Keywords:
small business, SBIR, EPA, pollution prevention, engineering, chemistry, heteropolymolybdate conversion coatings, corrosion protection, aluminum alloys, chromate free, Al 2024-T3, Al 6061-T6, Al 7075-T6, environmentally safe, self-healing, spacecraft, aircraft, RFA, Scientific Discipline, Toxics, Air, Sustainable Industry/Business, National Recommended Water Quality, Chemical Engineering, cleaner production/pollution prevention, Sustainable Environment, Environmental Chemistry, HAPS, Chemistry, Technology for Sustainable Environment, Chemistry and Materials Science, 33/50, Engineering, Chemistry, & Physics, Environmental Engineering, heteropolymolybdate conversion coatings, chromium & chromium compounds, cleaner production, Chromium, conversion coating, clean technology, Chromium Compounds, coatings, alternative coatings, pollution prevention, heteropolymolybdateSBIR Phase I:
New Environmentally Benign Heteropolymolybdate Conversion Coatings for Aluminum Alloys | Final ReportThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.