One type of corrosion you can easily spot around steel fasteners, as one example, is exfoliation corrosion
, which is an advanced form of intergranular corrosion.
Visual Assessment of Exfoliation Corrosion
Susceptibility of 5XXX Series Aluminum Alloys (ASSET Test) (ASTM G66-99(2005)e1)
This is characteristic of AE signals related to cracking phenomenon, as observed on metal hydride electrodes  and during aluminium exfoliation corrosion
 or hydrogen embrittlement process .
During a 7/56-day inspection on an SH-60B aboard USS Reuben James (FFG-57), Petty Officer Sadler prevented a possible mishap when he noticed serious exfoliation corrosion
on the hinge bracket of the APU door.