A metallographic examination was performed on a lap joint section of a KC-135 tanker aircraft. Although the outside surface gave little indication of corrosion, extensive corrosion was found on the faying surface of the joint section. This paper describes a detailed examination of the different forms of corrosion found on the lap joint, with emphasis on the
intergranular attack under the exfoliation corrosion, The potential detrimental effect of exfoliation on the structural integrity of lap joints is discussed.