|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|
Hexavalent chromium is a known carcinogen, with the major route of exposure through inhalation of vapors or dust. The primary health risk from exposure to Cr(VI) is an increased likelihood of developing lung cancer. Other potential health risks include asthma, nasal septa ulcerations and perforations, and dermatitis. In 2006, the Occupational Safety and Health Administration (OSHA) lowered the permissible exposure limit (PEL) ten-fold from 52 to 5 micrograms-per-cubic-meter, making it among the most stringently regulated materials used in manufacturing and maintenance operations.
|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.|
Despite its toxicity, Cr(VI) has a number of desirable properties, and it has been used in coatings and finishes for more than 50 years on materials such as aluminum alloys, steels, magnesium alloys, and others. When incorporated into coatings, Cr(VI) in the form of chromate compounds provides excellent corrosion protection to nearly all metals in a wide range of environments. When a coating is damaged, such as a scratch that exposes the base material, the solubility properties of the chromates allow them to migrate to the exposed area and inhibit corrosion. Chromates are used in a variety of military applications for metal finishing, including:
- Conversion coatings on aluminum, magnesium, and titanium alloys
- Corrosion inhibitors in primers
- Sealing of anodized coatings
- Post-treatment on sacrificial corrosion-resistant coatings such as cadmium
- Pre-treatments of steel surfaces prior to coating
- Adhesive bond primers
Many chromates are used to impart corrosion resistance in what are called coating “stack-ups.” As an example, most aircraft fuselages are manufactured from aluminum alloys, which are susceptible to corrosion. The coating stack-up might consist of a thin chromate conversion coating that imparts some corrosion resistance and enhances the adhesion of the subsequent coatings, followed by a primer that contains a chromate compound and a polyurethane topcoat.
Most potential exposures and releases associated with chromates occur during coatings removal processes, like bead blasting, sanding/grinding, and stripping of metal finishes and/or coatings. Additional exposures can occur during the application of surface treatments and coatings where vapors are emitted that contain Cr(VI).
In other applications, weapons systems components are placed in tanks containing chromic acid solutions, in which the chromium is in the hexavalent state, for either anodization or for hard chrome plating. These applications are more for wear resistance and, in the case of hard chrome plating, for restoring dimensional tolerances on many types of components. With these processes, there is no Cr(VI) in the final product, but vapors emitted by the tanks contain Cr(VI) and therefore significant effort must be made to ensure worker exposure is below the PEL.
One final avenue for Cr(VI) worker exposure is in high-temperature processing of chromium-containing materials, such as welding and brazing, which can generate fumes that contain Cr(VI).
The Department of Defense is committed to reducing the use of Cr(VI)-containing materials and processes. This was reflected in the issuance of a memorandum from the Under Secretary of Defense for Acquisition, Technology and Logistics on April 8, 2009 titled, “Minimizing the Use of Hexavalent Chromium.” In it, the Under Secretary stated, “Due to the serious human health and environmental risks related to Cr(VI) use, national and international restrictions and controls are increasing. These restrictions will continue to increase the regulatory burdens and life-cycle costs for DoD and decrease materiel availability.” He further stated, “This is an extraordinary situation that requires DoD to go beyond established hazardous materials management processes.” He directed the military departments to:
- Invest in appropriate research and development on substitutes
- Ensure funding for testing to qualify alternative materials and processes
- Approve the use of alternatives where they can perform adequately
- Update all relevant technical documents and specifications to authorize use of qualified alternatives
- Document system-specific Cr(VI) risks and efforts to qualify alternatives in the Programmatic Environment, Safety and Occupational Health Evaluation for the system
- Share knowledge derived from research, development, test and evaluation of alternatives
- Require the Program Executive Office (PEO) or equivalent level to certify there is no acceptable alternative when Cr(VI) is to be used on a new system
This policy memo will be formalized in a new Defense Federal Acquisition Regulation Supplement (DFARS) to be issued in 2011. The memorandum also called out SERDP’s role as a source for information on alternatives.
SERDP and ESTCP have made major investments over the past 15 years in the area of
Cr(VI). Given the wide range of applications and the longstanding use of Cr(VI), investments have been required that range from fundamental research through advanced development to test and evaluation for acceptance of alternatives. Fundamental research has focused on understanding the corrosion-inhibiting mechanisms of Cr(VI)-containing compounds and alternative inhibitors. Advanced development has built on fundamental research to develop new materials, new testing procedures, and new coating technologies. Demonstrations have collected validation data for alternative coatings on numerous weapons systems components. These investments have been guided by numerous workshops sponsored by SERDP and ESTCP and have led to widespread implementation of Cr(VI) alternatives across DoD’s depots and original equipment manufacturers.
As an example, in the late 1990s, ESTCP sponsored the Hard Chrome Alternatives Team (HCAT), which brought together DoD materials experts and end users, weapons systems manufacturers, and coatings vendors to demonstrate and validate high-velocity oxygen-fuel (HVOF) thermal spray ceramic/metallic coatings as an alternative to hard chrome plating, principally on aircraft components such as landing gear, hydraulic actuators, and gas turbine engine components. This resulted in the implementation of HVOF coatings on components on many types of military aircraft. The work of the HCAT, together with additional qualification studies conducted by industry, has led to the implementation of HVOF coatings on landing gear on all new commercial aircraft.
More recently, SERDP and ESTCP established Advanced Surface Engineering Technologies for a Sustainable Defense (ASETSDefense), an initiative that promotes alternatives to
Cr(VI) and other environmentally hazardous materials in surface engineering through periodic workshops and a web site. The ASETSDefense web site ( www.asetsdefense.org) includes a surface engineering data base that allows researchers and end users to access laboratory and field test data, authorizations, and implementations of alternatives to hazardous materials and processes, including Cr(VI).
DoD Policy Memo:
Minimizing the Use of Hexavalent Chromium (Cr6+)