The DoD relies on multiple surface finishing processes to protect bare substrate from degradation to extend the service life of a weapon system. In many cases, a post treatment sealer is required to provide additional corrosion protection and fill in porosity of the surface treatment. Stainless steel passivation is a surface treatment to reduce the corrosion rate of stainless steels by removal of surface steel particles and forming a protective passive film. Many legacy sealers are based on chromic acid or other hexavalent chromium compounds like sodium dichromate or chromium trioxide. In an effort to eliminate hexavalent chromium, select applications have been transitioned to a trivalent chromium sealer.
In 2017, DoD depots reported using over 1500 pounds of hexavalent chromium sealers in processes across at least 8 depots. DoD has over 30,000 gallons of dedicated infrastructure for chrome sealer operations, including all plating and rinse baths. Sealers are used by the Air Force, Army and Navy. Passivation is performed at Army and Navy depots.
In 2018, the ACGIH published a revised chromium threshold limit value (TLV). The TLV set exposure limits for chromium, trivalent chromium, and hexavalent chromium, as well as a short term exposure limit for hexavalent chromium. The time weighted average (TWA) is nearly two orders of magnitude lower than the current hexavalent chromium OSHA permissible exposure limit (PEL) of 0.005 mg/m3 . The DA PAM 40-503 requires that the Army use ACGIH TLVs when they are more stringent than the OSHA regulations or when there is no PEL, and the other services are considering applying the TLV in select applications.
While chromium-free alternatives have been identified, demonstrated or implemented for select processes, it would be beneficial to identify a sealer (or family of sealers) that can be used for multiple applications. To date, chromium-free alternatives have not been adequately demonstrated to meet requirements for zinc plating.