Key to the operability and preservation of aluminum has been the use of chromated systems (conversion coatings, primers and hard chromium plated).  Hard chrome plating on components is done through an electrochemical process; the electrochemically adhered chrome provides barrier protection to the substrate by forming a dense self-healing oxide layer on the surface.  The electroplated chromium is chemically resistant to most compounds and offers excellent corrosion protection.  Hard chrome plating also confers increased wear resistance and is most frequently used on landing gears, actuators, gearboxes, rotor heads and other high impact/wear components.  Conversely, with applied coatings, the high corrosion resistance offered by chromated films is attributed to the presence of both hexavalent and trivalent chromium in the coating.  The trivalent chromium is present as an insoluble hydrated oxide, whereas the hexavalent chromium imparts a “self-healing” character to the coating during oxidative (corrosive) attack.  Hexavalent chrome coatings also play a critical role in supporting and enhancing the adhesion of the primer coating to the substrate.

Occupational Safety and Health Administration (OSHA) studies have determined that hexavalent chromium poses significant medical risks to users.  Hexavalent chromium is considered a potential lung carcinogen.  Studies of workers in the chromate production, plating, and pigment industries consistently show increased rates of lung cancer.  It has also been shown that direct eye contact with chromic acid or chromate dusts can cause permanent eye damage.  Hexavalent chromium can irritate the nose, throat, and lungs and repeated or prolonged exposure can damage the mucous membranes of the nasal passages and result in ulcers.  In severe cases, exposure causes perforation of the septum (the wall separating the nasal passages).  It has also been proven  that prolonged skin contact can result in dermatitis and skin ulcers. Some workers develop an allergic sensitization to chromium and kidney damage has been linked to high levels of dermal exposures.

While chromated systems (applied coatings and plated) have set the bar for treatment and protection of aluminum, it is now known that hexavalent chromium is carcinogenic and poses significant risk to human health.  On February 28, 2006 OSHA lowered the 8 hour time-weighted average (TWA) Permissible Exposure Limit (PEL) for hexavalent chromium from 52 ΅g/m3 (micrograms per cubic meter of air) to 5 ΅g/m3.  OSHA issued a deadline of May 31, 2010 for employers to come into compliance with this PEL through the implementation of engineering controls.   The final rule also includes provisions for employee protection such as preferred methods for controlling exposure, respiratory protection, protective work clothing and equipment, hygiene areas and practices, medical surveillance, hazard communication and record keeping.  In the interest of worker safety, as well as the cost and operational implications of new and pending environmental, safety and health regulations, both NASA and the US Department of Defense (DoD) continue to search for alternatives to hexavalent chrome in applied coatings and plating applications that meet  performance requirements in corrosion protection, cost, operability, and health and safety; while underlining that performance must be equal to or greater than existing systems.

Objective
Evaluate and test a fully non-chromated coating systems (pretreatment, primer and topcoat) as replacements for hexavalent chrome coatings in aircraft and aerospace applications.

Period of Performance

• October 2005 to May 2009

Stakeholders
NASA Centers (Kennedy Space Center, Marshall Space Flight Center); Space Shuttle Elements/contractors (Boeing-Orbiter, Reusable Solid Rocket Motor, United Space Alliance); Shuttle Environmental Assurance initiative, and U.S. Air Force (Hill Air Force Base, Air Force Materials & Manufacturing Directorate at Wright Patterson Air Force Base).

Benefits

• Meets environmental and safety regulatory requirements
• Reduces need to monitor for chromium exposure due to new regulations
• Decreased risk of environmental, worker and public exposure
• Reduced maintenance cost and government liability 
• Addresses NASA and Air Force requirements on aluminum alloys 2219, 2195, 6061, 2024 Bare, 2024 Clad, and 7075

• Draft Joint Test Protocol and Joint Test Report will follow on into Phase II Reporting.

Milestones

• Completed Filiform Corrosion tests  - February 2007
• Completed Dissimilar Metals (Sandwich Corrosion) – March 2007
• Completed SAS box corrosion tests are complete United States Air Force/ Air Force Research Laboratory – May 2007
• Completed Cyclic Corrosion – June 2007
• Completed Salt Fog Corrosion Testing (2,000 hours) – October 2007 
• Completed Adhesion Testing – October 2007
• Identified Three Systems for Down-Selection to Atmospheric Testing (Phase II) – November 2007
• Received all test reports except Air Force – SAS Box Test Report – September 2008
• Completed Statistical Analysis of Cyclic Corrosion Testing – December 2008
• NASA FY2006 funds allocated to support testing work being performed at Hill Air Force Base, Kennedy Space Center, Boeing Huntington Beach and Marshall Space Flight Center and Wright Patterson Air Force Base.
• Hill Air Force Base cut and coated all test panels.  They were then shipped to facilities for testing:
  • Filiform Corrosion Testing  (1,000 hour test started January, 16, 2007) - Kennedy Space Center
  • Salt Spray Corrosion Test (3,000 hour test started December 20, 2006) - Hill Air Force Base
  • Adhesion Testing (will begin in  April, 2007) – Hill Air Force Base
  • Cyclic Corrosion Testing (3-phase 1,000 hour test began Jan 21, 2007) – Marshall Space Flight Center
  • Simulated Aircraft Structure (SAS) Box Corrosion Testing (run-to-failure test began October 30, 2006) - Wright Patterson Air Force Base
  • Dissimilar Metals (Sandwich) Corrosion Test (2,000 hour test began Jan 8, 2007) – Boeing, Huntington Beach
• Carried best performers into next phase of testing (2008)
• Completed individual test reports and submit for International Traffic in Arms Regulations approved - June 2009
• Summary report of all testing and results – June 2009

   ·      + Dissimilar Metals Corrosion Final.pdf document - (M&P-3-1854.pdf, 5MB, 42 pages, June 11, 2007)

+ Reports