Fuel Line Corrosion
Inspections and microstructural analysis revealed intergranular corrosion of 6061-T6 aluminum alloy aircraft fuel line beneath ferrules. The cause of the corrosion was traced to the fuel line marking process, which involved electrolytic labeling. Although subsequent rinsing of the fuel lines washed off most of the electrolyte, some was trapped between the 6061-T6 tubing and the ferrule. This condition made intergranular corrosion of the fuel lines inevitable. The attack caused grains to become dislodged, giving the appearance of pitting. Corrosion penetrated approximately 0.13 mm (0.005 in.) into the tubing. Experiments indicated that the corrosion products were inactive. It was recommended that another marking process be used that does not involve corrosive materials. The prevention of electrolyte from being trapped between the tubing and ferrules by using a MIL-S-8802 sealant was recommended.