Eddy Current is an appropriate method for the corrosion detection of anodized and sealed Al Alloy 2024 for a large frequency bandwidth (from 10 to 100 kHz). It is well known that the depth of penetration is inversely proportional to the frequency. The higher the frequency the more the information about surface or near surface defects. By using lower frequencies hidden corrosion can be detected. Impedance given by specimens produced from exfoliation tests is increasing almost linearly with the degreasing of the specimen depth. Due to non-uniformity of the alternate immersion specimens the results gathered by the detection using Eddy Current are less straightforward. Whereas exfoliation tests produce inspectable corrosion damage in few hours (A5= 5 hrs of immersion), alternate immersion tests require several days (I90= 90 days of immersion) to produce damage detectable by Ultrasonics. Due to high attenuation of the signal passed through the exfoliation specimens a higher gain is required for those NDE tests. This is owed to the type of corrosion involved, which results in the formation of delaminated layers and small cracks (intergranular corrosion). On the contrary, lower gain of signal is required for the specimens generated by alternate immersion tests, where the type of corrosion resulting in pitting corrosion. The optimum frequency for the detection of damage is 5 MHz for through transmission tests and 2 MHz for the mirror tests.
Corrosion assessment in aircraft structures by NDE methods
Korrosionsbewertung in Luftfahrtstrukturen mit ZfP-Verfahren
1999
13 Seiten, 10 Bilder, 5 Tabellen, 7 Quellen
Aufsatz (Konferenz)
Englisch
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