Grantee Research Project Results

Final Report: New Environmentally Benign Heteropolymolybdate Conversion Coatings for Aluminum Alloys

EPA Contract Number: 68D00203
Title: 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: SBIR - Pollution Prevention , Nanotechnology , 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.
Cleaning Steps	Degreasing	Sonication in acetone
	Alkaline Cleaning	10 min in Turco
	Deoxidizing	10 min in Oakite
Protection Steps	Boehmite	20 min in DI H2O
	Conversion Coating	2 min in heteropolymolybdate
	Calcium Hydroxide Treatment	20 min in calcium hydroxide
	Silicate Post Sealer	2 min in Kasil

Journal Articles:

No journal articles submitted with this report: View all 1 publications for this project

Supplemental 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, Sustainable Industry/Business, RFA, Air, Scientific Discipline, Toxics, National Recommended Water Quality, Technology for Sustainable Environment, Chemical Engineering, Chemistry, Sustainable Environment, HAPS, Environmental Chemistry, 33/50, Engineering, Chemistry, & Physics, Chemistry and Materials Science, cleaner production/pollution prevention, Environmental Engineering, chromium & chromium compounds, clean technology, Chromium, pollution prevention, coatings, heteropolymolybdate, heteropolymolybdate conversion coatings, Chromium Compounds, conversion coating, cleaner production, alternative coatings

SBIR Phase I:

New Environmentally Benign Heteropolymolybdate Conversion Coatings for Aluminum Alloys  | Final Report