SAE AIR5479

Description / Abstract: This SAE Aerospace Information Report (AIR) describes the performance of plating’s and coatings for landing gear that potentially provide environmental compliance benefits versus the current baseline processes. The hazardous systems addressed in this version of the document include cadmium plating, chromated primers, and high VOC (volatile organic compounds) topcoats.

The AIR applies to landing gear structures and mechanisms for all types of civil and military aircraft. The potential replacements apply to both Original Equipment Manufacturer (OEM) hardware and overhaul of in-service landing gears.

Background

This document addresses a number of materials and processes used in landing gear manufacturing and overhaul including cadmium, chromium, and Volatile Organic Compounds (VOC). Environmentally compliant requirements throughout the world are constantly changing and related materials and processes must be reviewed prior to usage. In the United States (US), these substances are controlled in three primary areas: (1) the Occupational Safety and Health Administration (OSHA) oversees exposure in the workplace, (2) the Environmental Protection Agency (EPA) oversees airborne emissions via the Clean Air Act, and (3) the EPA also oversees waterborne effluent via the Clean Water Act. The US Department of Defense (DoD) Emerging Contaminants Program works with the EPA and Environmental Council of States and has published the action list (see Table 1) for chemicals and materials that have pathways to enter the environment and present unacceptable risks to humans or the environment. In Europe, the European Union Reach (Registration, Evaluation and Authorization of Chemicals) requires companies to register data on +30000 chemicals with the European Chemicals Agency (ECHA) in Helsinki, Finland, seek authorization for the use of substances of very high concern (SVHC) and substitute these chemicals with safer alternatives for human health and the environment. SVHCs need to be authorized for specific uses if they appear in Annex XIV of the legislation.

Cadmium is widely used as a sacrificial coating for corrosion protection of steel alloys. However, cadmium is easily removed during paint stripping operations, resulting in contamination of stripping media and large volumes of hazardous waste. Because low embrittlement cadmium is a soft, easily removable coating, it contaminates virtually everything it physically contacts (e.g., wiping rags, hydraulic oil, masking materials). The 1992 OSHA Expanded Standard for Cadmium established a Permissible Exposure Limit (PEL) of 5 μg/m³ and reduced the Action Level (AL) to 2.5 μg/m³. This standard went into effect in 1996 and put nearly all overhaul operations at risk of violating AL and/or PEL, requiring the use of continually more stringent overhaul process control, worker medical surveillance and record keeping. Cadmium levels in effluent are regulated by the Clean Water Act, but the needs and capabilities of the local waste water treatment authority require their involvement in establishing individual discharge standards. The European Union (EU) has already banned the use of cadmium for various applications but exempted flight safety critical parts. European airlines expect the exemption to be phased out and are requesting (a) harmonization of maintenance procedures and processes in which cadmium is involved, (b) a policy commitment to replace cadmium for aircraft, engines and components and c) consensus on selection of cadmium alternatives.

Hexavalent chromium is a corrosion inhibitor used in chromated conversion coatings and chromated primers. Hexavalent chromium is a known carcinogen and a suspected human mutagen. It is the most dangerous form of chromium compounds. Once in metallic form, such as hard chrome plate, chromium is not considered a significant environmental problem. Other forms of chromium: metallic as in plated hard chrome, or as an ingredient in corrosion-resistant steels; trivalent, as in trivalent chrome conversion coatings or solutions, are not considered a significant health, safety, or environmental problem.

The OSHA Cr⁶⁺ PEL issued on February 28, 2006 lowered the upper limit for exposure of workers to hexavalent chromium (as Cr) to 5 μg m^-3 from its previous 52 μg m^-3, measured as 8-h time-weighted averages. This drastic reduction affected any industrial process that could generate Cr⁶⁺ air emissions, including hard chrome plating, chromic acid anodizing, priming and painting, chromate conversion processes, welding, and rework of materials containing chromium. The limit applies to all forms of Cr⁶⁺, including chromium trioxide, chromic acid and chromates.

In addition to reducing the PEL for Cr⁶⁺, the OSHA rule places a number of burdens on employers, who are now required to:

Monitor employee exposure to Cr⁶⁺

Establish separate regulated areas when Cr⁶⁺ levels are expected to exceed the PEL

Provide respirators for workers exposed above the PEL

Provide other Personal Protective Equipment (PPE) as necessary for eye and skin protection, together with change rooms and wash facilities

Institute housekeeping activities to control spills and releases of Cr⁶⁺

Provide medical surveillance for employees who are exposed above the PEL, show signs or symptoms of Cr⁶⁺ exposure, or are exposed in an emergency

Train workers about Cr⁶⁺ hazards, and use signs and labels to communicate the hazards

Keep records of exposure, surveillance and training.

The PEL action level, which is the threshold that determines when ongoing monitoring is necessary, is 50% of the PEL or 2.5 μg m^-3. However, if Cr⁶⁺ concentrations are shown to be 0.5 μg m^-3 under all expected conditions, then the OSHA rule does not apply at all.

There has been a more recent EPA final ruling on chrome emissions issued in 2012 for the national emission standards for hazardous air pollutant emissions: hard and decorative chromium electroplating and chromium anodizing tanks. The final rule reduced the emissions for hard chrome for the following: 1) small existing sources: from 0.030 mg/dscm to 0.015 mg/dscm, 2) small new sources: from 0.015 mg/dscm to 0.006 mg/dscm, 3) large existing sources: from 0.015 mg/dscm to 0.011 mg/dscm, and 4) large new sources: 0.015 mg/dscm to 0.006 mg/dscm, US EPA. (EPA, 2012), Note: mg/DSCM stands for milligrams per dry standard cubic meter.

The European Union REACH program has noted that effective on September 21, 2017 the EU will enact legislation banning the use of hexavalent chrome solutions. This will impact finishers in Europe along with all who export finished goods into the EU using hexavalent chromium finishes. Exemptions can be given if it is required for defense products or if safety is affected.

The EPA has finalized a National Emission Standard for Hazardous Air Pollutants (NESHAP) that includes coating operations at aerospace manufacturing and rework facilities. The standard requires existing and new major sources to control emissions to the level achievable by the Maximum Achievable Control Technology (MACT) consistent with section 112(d) of the Clean Air Act. The Hazardous Air Pollutants (HAPs) covered by this final rule include chromium, cadmium, methylene chloride, toluene, xylene, methyl ethyl ketone, ethylene glycol and glycol ethers. For existing sources, the requirements of this rule became effective September 1998. The NESHAP limits on VOC are 420 g/L for topcoats and 350 g/L for primers.