What Is a Green Gun Barrel and Why Is the DoD Interested?
Green technologies, or green manufacturing, are technologies or processes that provide a more environmentally acceptable alternative to current practices. In order to be successful, these alternatives must be at least as effective or robust as the current practices. For example, the current electroplating process for gun barrels requires a caustic rinse, followed by acid polish and hard chrome plating and finally thermal treatment to alleviate hydrogen embrittlement. Chrome plating provides both wear and corrosion resistance, thus increasing the ballistic accuracy and diminishing the opportunities for mechanical fatigue failure. The hazards and wastes associated with the current practices are well documented.
Hexavalent chromium is toxic.
The term "hexavalent chromium" is a shorthand designation for chemical compounds that contain the element chromium in the positive 6 valence state [written as Cr(VI) or Cr6+]. Metallic chromium is in the zero valence state and naturally occurring chromium oxide contains chromium in the positive 3 valence state, also called trivalent chromium or Cr(III). It is important to make this distinction, since Cr(VI) is significantly more toxic than the other forms of chromium. The Permissible Exposure Limit (PEL) is the maximum concentration of a chemical to which a worker may be exposed and is usually based on an average value over the course of an 8-hour work shift. Due to the toxic nature, OSHA reduced the PEL on Cr (VI) from 52 to 5 micrograms per cubic meter in 2006.
In his April 8, 2009 memorandum, the honorable John J. Young, directed DoD Military Departments to minimize the use of hexavalent chromium and invest in alternative technologies. For the past decade or more, SERDP has invested in alternatives to hard chrome plating, chromium conversion coatings, and corrosion inhibition in a variety of areas of interest to the military. A particular application that has been targeted in recent years is the development of alternative technologies for the plating of gun barrels. Research projects WP-1074, WP-1425, and WP-1426 explored metal sputtering, vapor deposition processes, and explosive cladding methods. Tantalum coatings demonstrated the greatest promise in providing the best overall wear and corrosion performance, and functioned better than chrome in many cases. Side by side test firings were performed and the useful life of a tantalum clad barrel was estimated to be three times longer than that of the standard chromium plated barrel.
So, what are the next steps in providing Green Gun Barrels to the Military?
ESTCP projects WP-201111: Chromium Elimination Cannon Life Extension, and WP-201220: Chrome Replacement for Gun Barrels, were recently initiated to provide real world assessments of manufacturability and performance. These projects are on schedule to be completed in 2014 and 2015, respectively, and will hopefully be able to show improved performance at lower lifecycle costs with a significant environmental benefit.
Great strides are being made toward eliminating hexavalent chromium from DoD weapons systems and we can expect that the Army, Navy, Marines and Air Force will one day be using Green Guns to fire Green Munitions as they train and fight to protect our homeland.