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.
Approximately 0.02 mm and 0.17 mm were milled from the corroded panel surface and surface plots were produced characterizing the extent of exfoliation corrosion penetrating to these depths.
After corrosion exposure, exfoliation corrosion was observed over the entire surface of the panel with the cladding removed.
As slivers of uncorroded aluminum are lifted from the substrate, additional grain boundaries are exposed to the corrosion environment and a new layer of exfoliation corrosion is initiated.
Uniform exfoliation corrosion was observed over the entire panel surface after corrosion exposure.
 "Standard Test Method for Exfoliation Corrosion Susceptibility 2XXX and 7XXX Series Aluminum Alloys (EXCO Test)," ASTM G34-01(2013), 2013.
* Visual Assessment of Exfoliation Corrosion
Susceptibility of 5XXX Series Aluminum Alloys (ASSET Test) (ASTM G66-99(2005)e1)
Idrissi, 'Use of acoustic emission technique for the early detection of aluminum alloys exfoliation corrosion
', NDT & E International, 35, (6), 385-392, 2002.
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.
Second, the backscatter electron image of Figure 11 showed significant corrosion under the paint adjacent to the fastener, including exfoliation corrosion
, as is often observed around fasteners.