Grantee Research Project Results

Final Report: Non-Chromate Conversion Coatings for Magnesium Alloys Used in Automotive Industry

EPA Contract Number: EPD07043
Title: Non-Chromate Conversion Coatings for Magnesium Alloys Used in Automotive Industry
Investigators: Woolsey, Earl R. , Gorman, William C
Small Business: Technology Applications Group, Inc.
EPA Contact: Richards, April
Phase: I
Project Period: March 1, 2007 through August 31, 2007
Project Amount: $69,620
RFA: Small Business Innovation Research (SBIR) - Phase I (2007) RFA Text |  Recipients Lists
Research Category: SBIR - Pollution Prevention , Small Business Innovation Research (SBIR)

Description:

This Small Business Innovation Research Phase I project evaluated the feasibility of using calcium, titanium, tungsten, or vanadium-based compounds as chemical conversion coatings for magnesium alloys. Magnesium alloys suffer from high corrosion rates and must be coated with either an anodized or chemical conversion coating to minimize corrosion. An effective conversion coating will promote paint adhesion, minimize general corrosion, interrupt or minimize the formation of galvanic couples between the base alloy and dissimilar metals, and persist on the surface to promote “healing” of scratches and minor damage to the finished article.

To succeed, new conversion coating systems need to reproduce the natural decontamination of the article surface offered by typical chromate treatments and form a viable protective coating without the use of toxic materials, and at comparable cost to the industry standards. The focus of this research is to show the feasibility of replacing chromate treatments with non-chrome, one- or two-step treatments that use low toxicity or very low toxicity components to decontaminate the article surface and form a persistent protective layer while remaining cost effective. Cheap, low toxicity coating of this type can facilitate increased use of low-weight magnesium alloy in environmentally aggressive situations in automobile and aircraft applications.

The research comprises: (1) preparing a series of calcium, titanium, tungsten, and vanadium-based formulations; (2) preparing the surface of commonly used magnesium alloys; (3) applying the formulations to commonly used magnesium alloys; and (4) measuring the salt spray corrosion resistance of the coated magnesium alloys. Acidified peroxy complexes of the base metal salts were applied to both bare magnesium alloy surfaces and to hydrated borate, phosphate, fluoride, or oxide films, which were formed on the magnesium alloy surfaces.

Summary/Accomplishments (Outputs/Outcomes):

The benchmark result was to formulate a coating system that performed as well or better than the widely used automotive chromate conversion coating NH35. Adequately decontaminating the alloy surface without using chromic acid proved to be the most difficult part of the research. Although chromic acid will decontaminate, passivate, and coat the surface in one heated step, the tested materials tended to be effective at only one of these at a time, requiring two or three steps to achieve a similar coating. The performance of these two- and three-step systems was low relative to the chromate systems, and did not significantly improve until a high etch rate sulfuric acid treatment was used to decontaminate the test articles. Salt spray of comparable samples showed one result of a peroxy complex of vanadium pentoxide and acetic acid applied to a sulfuric acid etched magnesium alloy surface to be superior to the NH35 treatment after
168 hours of exposure. However, commercialization would be difficult because the sulfuric acid roughened the surface and resulted in unacceptable levels of metal loss. Phase I feasibility was not adequately supported by the results.

Conclusions:

Used alone or as a posttreatment of a hydrated film, the peroxy-metal complex solutions tested did not decontaminate, passivate, and coat the surface sufficiently to outperform the chromate systems tested. The technical feasibility of using these methods was not demonstrated.

Supplemental Keywords:

small business, SBIR, EPA, magnesium alloys, anodized coatings, non-chromate conversion coatings, metal finishing products,, Sustainable Industry/Business, RFA, Scientific Discipline, Technology for Sustainable Environment, Sustainable Environment, Environmental Chemistry, Environmental Engineering, emission reduction, automotive components, chromium free surface finishing, alternative chromate conversion, automotive industry