Corrosion in aerospace metals

STUDY OVERVIEW

A leading aviation maintenance, repair and overhaul company provided dolphitech with a corroded aluminium alloy fastener strip from an aircraft. The nominal thickness was 3mm, with general corrosion present on the interior surface.

SOLUTION 

The sample was inspected using a dolphicam2 and a TRM-CI-5MHz transducer. This TRM has an 8mm Aqualene delay line, the flexibility of which provides conformance to surface unevenness. Water was used as a couplant, delivered via a spray bottle. The sample was inspected from the exterior (non-corroded) surface using a manual stitching technique, whereby the probe is moved sequentially over the surface of the specimen and data acquired at each location.

FINDINGS

The fastener strip was successfully inspected, with the corroded area readily resolved in both the amplitude and the Time of Flight (ToF) C-scan maps. In the amplitude C-scan, it presented as a reduction in the signal amplitude. This occurs since the inner surface is no longer parallel to the inspection surface, thus less sound energy is reflected directly back to the transducer. In the ToF C-scan, the corrosion presents as a change of color from blue, through green and yellow to orange. This enabled the location of minimum remaining wall thickness to be easily identified and measured with the dolphicam2’s point measurement tool. Additional corroded regions that were concealed to the eye, as they still had a flaking aluminium layer over them, were also detected.

CONCLUSION

A TRM-CI-5MHz is ideally suited to inspection of a wide range of metallic and composite aerospace materials. This versatile transducer can map out C-scans with ease, and quickly reveal minimum remaining wall thickness locations in corroded metals.