Abstract: Corrosion character on the keel beam lower chord of 737CL at BS535 area,which is prevalent on aging airplane,was investigated based on the corrosion signs,environment,stress,TEM and corrosion test in oceanic environment.The results showed that the main type of corrosion was EFC.The condensed water in air-condition bay provided the corrosive environment.The intrinsic factors of EFC include the elongated grains and the continuous PFZ along the grain boundary of the material.Compression stress accelerated EFC.