Thursday, March 03, 2011

Graphite Lubricants in Aircraft- The Corrosion Potential

Graphite Antiseize

"...shown conclusively that graphite in a resin-bonded solid film lubricant is deleterious from the point of view of corrosion protection provided by the lubricant... To use graphite is to invite corrosion difficulties in the presence of moisture."  Rock-Island-Arsenal-Lab, Technical Report, Dry-Lubricants and Corrosion, Prepared for Presentation at the Annual Meeting of the Society of Automotive Engineers, Detroit, Michigan 14-18 January 1963. Francis S. Meade and George P. Murphy, Jr.

Aircraft Spark Plug Graphite Antiseize

Graphite and water has been used and recommended for aircraft spark plugs for over 50 years.  Other anti-seize that contains metallic particles have been avoided because of the chance that the anti-seize particles may get into the combustion chamber, create a hot spot, and cause destructive pre-ignition.

Years ago mica-anti-seize was commonly used for spark plug anti-seize and it probably has better properties than graphite and water. However, there is a problem in changing anti-seize types; you change the torque tension relationship. Published torque values in aircraft spark plugs are based on using the manufacturer's recommended anti-seize. Use a different type of anti-seize and the required torque to achieve the proper amount of tension will change by an unknown amount.

Cleaning spark-plugs using glass-bead shot may remove the nickel plating from the threads. Now the steel threads are exposed to the water and graphite mixed anti-seize creating a corrosive environment.

When the steel threads rust, their surfaces expands and this causes the spark plug to seize in the threads.

Consideration should be given to using a different type of anti-seize in this special circumstance such as a mica based anti-seize.

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