This research examines the effectiveness of six non-chromate conversion coatings on aluminum armor alloys 5083,7039 and 2519. Evaluation included salt fog, cyclic salt spray, wet adhesion, and electrochemical impedance spectroscopy on both painted and unpainted test panels. Large differences in behavior were noted between the salt fog data obtained on unpainted panels and the cyclic salt spray data obtained on painted and scribed panels. How this data will affect the implementation of non-chromate pretreatment within the US Army is discussed. Additionally, these findings provide guidance for continued development of non-chromate conversion coatings and their eventual performance standards.
Avoiding corrosion of metallic components used in military equipment has always been a difficult task, due to the harsh nature of the battlefield and extreme environments encountered around the globe. Over the years, countless protective schemes have been devised. Of these, chemical treatments containing hexavalent chromium (Cr+6) have been one of the most effective and commonly implemented protective measures.
Unfortunately, over the last few decades it has been determined that Cr+6 poses a significant health risk. It is a known human carcinogen. The Environmental Protection Agency has continued to press for reductions in the use of hexavalent Cr and improved processing techniques to eliminate Cr+6 from the waste stream to reduce the serious threat to personnel and the environment. The U.S. Army, along with most of the commercial sector, is still using hexavalent chrome in its protection schemes for a wide variety of metallic materials. Due to these hazards, the Army is striving to reduce or even eliminate its dependency on the use of hexavalent chromium.
Chromate conversion coatings containing Cr+6 are used on aluminum alloys to enhance corrosion resistance and paint adhesion by “converting” the native aluminum oxide on the surface of the metal to a more stable, vitreous, and corrosion resistant structure. Significant amounts of hexavalent chromium can be introduced into waste streams via this process. Therefore, environmentally friendly alternatives to chromate pre-treatments need to be developed and exploited to further reduce the Army’s use of cr+6.
To accomplish this, the US Army Research Laboratory (ARL)- Weapons and Materials Research Directorate developed and executed a research program under the auspices of the Department of Defense Strategic Environmental Research and Development Program (SERDP) to examine commercially available non-chromate alternatives for protection of aluminum alloys. To evaluate these coatings properly it was necessary to use a variety of test techniques on both painted and unpainted test panels subjected to a variety of laboratory and exposure tests. These tests included salt fog, cyclic salt spray, adhesion, and electrochemical impedance spectroscopy.
This paper establishes the effectiveness of several non-chromate conversion coatings on aluminum armor alloys, and compares these results with similar work performed in parallel with the National Center for Manufacturing Sciences(NCMS), University of Michigan1. Our study focused on Aluminum armor alloys, while the cooperative NCMS study tested the more common commercial grades.
EXPERIMENTAL PROCEDURE
The selection of non-chromate conversion coatings to be tested was done in conjunction with the NCMS study partners.