<title>NDI method to locate intergranular corrosion around fastener holes in aluminum wing skins</title>

Abstract

Contact between galvanically dissimilar metals, such as cadmium plated steel fasteners and aluminum wing skins are known to be a source of corrosion. There is a design requirement to fill the void between the contacting surfaces of steel fasteners with a wet sealant. However, if the contacting surface is damaged or a void exists between the fastener head and the aluminum skin, moisture can collect and intergranular corrosion may occur along aluminum grain boundaries, which run parallel to the surface of the wing skin. If intergranular corrosion is allowed to propagate, delamination of the thin layers of aluminum, known as exfoliation corrosion will occur. When this intergranular corrosion reaches an exfoliated state, extensive rework is involved in removing the corrosion. This paper discusses the results of a USAF E-3A Engineering Service Task 89-E3B3-16 to develop a nondestructive inspection procedure to detect intergranular corrosion in an incipient state before it reaches exfoliation. Eddy current and ultrasonic inspection techniques were evaluated. A novel ultrasonic pulse echo technique was developed which utilizes a focus transducer with a hand held fixture. Inspections were performed on test parts which were removed from the upper wing skin of a retired 707 which had varying degrees of intergranular and exfoliation corrosion. Inspection results are compared to the results from the mechanical rework of the wing skin and dissection of a wing skin fastener hole.