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Concept PaperRecent Alternatives for Chromate Conversion Coatings(revised and excerpted, 12/10/98) ProblemChromate conversion coatings are applied to aluminum as a corrosion inhibitor and to provide an adhesion base for paints. Particularly for the high strength copper-bearing aluminum alloys used extensively in airframes, there is currently no generally accepted alternative. All conversion coatings which currently qualify as Class 1A coatings under MIL-C-81706 contain chromate. As a known human carcinogen, chromate is subject to strict health and environmental regulation, and its use in manufacturing involves an increasing burden of expense and potential liability. The need to find alternatives to chromate has been recognized as an important priority, both in defense maintenance facilities and in the private sector. Previous WorkIn 1995, NCMS released a widely circulated report of a study which provided, for the first time, a side-by-side comparison of 29 chromate free alternative coatings, together with four standard chromate coatings. The coatings were applied by their manufacturers on test coupons of five representative aluminum alloys and were submitted to NCMS project participants for testing. Salt fog corrosion, electrical conductivity, and wet and dry paint adhesion testing was carried out on all samples. The detailed test results were included in the report, together with an environmental impact study of the alternatives, compared to the standard coatings. The report indicated that, while suitable alternatives were available for a large number of applications, it was still difficult to find substitutes for the most difficult alloys under the most demanding conditions. This was particularly problematic for aerospace applications. Since the appearance of the report, several manufacturers have released products which may offer significantly improved performance compared to those available in 1994, when the original testing was carried out. Several recent candidates are tabulated below. Proposed ProjectIt is proposed that a project be organized under the NCMS Commercial Technologies for Maintenance Activities (CTMA) program to organize a test series for recently developed chromate-free alternatives for conversion coating of aluminum to determine the extent to which the new alternatives can substitute for existing applications.
Table of Potential Alternative Coatings Recently Developed(Note: This table lists several recently developed coatings that had been identified during the formation phase of the project. Coatings are included here as examples. This is not intended to represent a definitive list, and is not meant to imply that coatings listed here will be included in the study.)
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