My coworker told me it is known as "poultice corrosion". Here is an excerpt from this site discussing it:
http://ammtiac.alionscience.com/pdf/2005MaterialEASE31.pdf
Crevice Corrosion
Crevice corrosion occurs as a result of water or other liquids getting
trapped in localized stagnant areas creating an enclosed corrosive
environment. This commonly occurs under fasteners,
gaskets, washers and in joints or in other components with small
gaps. Crevice corrosion can also occur under debris built-up on
surfaces, sometimes referred to as “poultice corrosion.” Poultice
corrosion can be quite severe, due to a gradually increasing acidity
in the crevice area.
Several factors including crevice gap width, depth, and the
surface ratios of materials affect the severity or rate of crevice
corrosion. Tighter gaps, for example, have been known to
increase the rate of crevice corrosion of stainless steels in chloride
environments. The larger crevice depth and greater surface
area of metals will generally increase the rate of corrosion.
Materials typically susceptible to crevice corrosion include
aluminum alloys and stainless steels. Titanium alloys normally
have good resistance to crevice corrosion. However, they may
become susceptible in elevated temperature and acidic environments
containing chlorides. Copper alloys can also experience
crevice corrosion in seawater environments.
To protect against problems with crevice corrosion, systems
should be designed to minimize areas likely to trap moisture, other
liquids, or debris. For example, welded joints can be used instead
of fastened joints to eliminate a possible crevice. Where crevices
are unavoidable, metals with a greater resistance to crevice corrosion
in the intended environment should be selected. Avoid the use
of hydrophilic materials (strong affinity for water) in fastening
systems and gaskets. Crevice areas should be sealed to prevent the
ingress of water. Also, a regular cleaning schedule should be
implemented to remove any debris build up. Table 3 provides a
brief list of guidelines that can help minimize crevice corrosion.
