Most aerospace aluminum structure is protected with chrome in two ways. First, the aluminum is treated with a chromate conversion coating. The conversion coating provides some corrosion protection and improves subsequent paint adhesion. The structure is then coated with a chromated primer. The chromated primer provides the majority of the corrosion protection.
The current OSHA Permissible Exposure Limit (PEL) for Cr+6 is 52 mg/m3 (ceiling limit). OSHA is currently considering a PEL in the range of 0.5 mg/m3 to 5 mg/m3 with a possible final rule date of Jan. 18, 2006.
OSHA's expanded standard could include enhanced training, medical surveillance, enhanced personal protective equipment and designated changing areas. Finding a suitable, environmentally friendly replacement for chromium would be a major effort for the coatings industry.
The word "suitable" is critical. No one wants to sacrifice corrosion resistance when implementing a nonchrome protective coating.
The Boeing Co. has worked hard to develop nonchrome corrosion inhibitors suitable for protecting aluminum aircraft structures and has tested more than 100 nonchrome primers over the past few years. Getting a nonchrome primer to successfully perform on all the various aluminum alloys used on aircraft is a significant challenge. While laboratory-accelerated corrosion methods are used in the industry, operational testing is needed as a real life method of determining nonchrome primers' effectiveness.
Boeing had the opportunity to lead a nonchrome primer evaluation program sponsored by the Joint Group for Pollution Prevention (JG-PP) beginning in 1995. The charter of this effort called for evaluating existing state-of-the-art nonchrome primer technology, and performing an extensive field evaluation of the leading candidate coatings. Multiple nonchrome primers were tested, and the best primers were found to be approximately 80% as good as the chromate control in corrosion protection. Even though the nonchrome primer did not meet the laboratory testing requirements of chromated primer, the stakeholders chose to proceed with operational testing.
The plan was to evaluate performance for two years, but the project was extended since no discernable differences were noted after two years. Nonchrome primers were test flown on multiple military aircraft for up to six years with no discernible corrosion performance deficiencies.
Additional military aircraft painted in 2000 and 2001 continue to be monitored. To minimize risk, the primers were only applied to external surfaces that can be easily inspected for corrosion. Internal structure, where most corrosion occurs, will someday need to be field-evaluated with nonchrome primers. The latest field evaluation of the JG-PP aircraft shows the nonchrome primers to be performing as good as the chromated primers when used with a chromate conversion coating pretreatment.
It may take multiple nonchrome corrosion-inhibiting compounds to provide corrosion protection comparable to chromium. Chrome has been used for many years, yet its mechanism of corrosion protection has only recently been understood. Understanding the mechanism of the nonchrome corrosion inhibitors is also needed to aid further development efforts.
Finding a replacement for chromium is a daunting challenge, but progress continues to be made. As the clock ticks down to OSHA's impending expanded chromium standard, industry continues to search for a safe and effective replacement for chromium.