When you touch an object you leave salt (chloride) residue on the surface. At high temperatures this salt can react with the metal and cause corrosion. Stainless steel (especially the common 300 series) does not like chlorides. Chlorides are found in salt water, road salt, fingerprints, and some cleaning solutions such as trichloroethane, and methylene chloride. (trichloroethane is often found in the cleaner portion of dye penetrate cleaners that are used in the aircraft industry to find cracks.) Some insulation material contain chlorides. The worst corrosion combination for stainless steel is low-oxygen and high chlorides as might be found in crevices. Here are several examples of chloride induced corrosion from the Australian Government, Civil Aviation Authority AAC1-13:
"An American turbine engine manufacturer recently published a service letter alerting operators that wrapping stainless steel tube assemblies with a chloride-based material, such as neoprene tubing and fiberglass tape to prevent chafing, has resulted in premature tube failure. A chloride-based material breaks down from the presence of high engine temperatures and attracts moisture, resulting in the formation of salts which are highly corrosive to stainless steel tubes. After a period of time, stress cracking develops resulting in failure of the tubes. Additional investigation along the same lines by a foreign engine manufacturer revealed that titanium is also affected by the chemical reaction between chloride-based materials when operating in temperatures in excess of 150 degrees C (302 degrees F).
A related problem is the use of chloride-based packaging material, such as PVC sheeting (plasticized polyvinyl chloride) as a packaging material. This can result in chloride-based residue being left on the component, possibly leading to the sort of failure described above.
In summary, operators are reminded to follow the engine manufacturer's publications in installing stainless steel engine air, oil and fuel tubes and warned against using chloride-based materials on any stainless steel or titanium components, whether installed on the engine or held in storage. "
Cleaning Fingerprints per FAA Advisory Circular AC43-206, condensed:
The salts and oils from fingerprints are highly corrosive. Apply a mixture of 1 part Isopropyl Alcohol and 1 part distilled water to the affected area with a clean cloth or brush, or cotton ball and wipe clean.
"Parts for installation in high temperature areas shall not be handled with bare hands because finger prints will cause carburization and pitting of surface when heated."
TO-1-1A Air Force Technical Manual, Engineering Series for Aircraft Repair Aerospace Metals, page 97.
