Figure 4-8.Surface corrosion on hydraulic linkage.
metal. This is caused by the force of expanding
corrosion products at the grain boundaries just below
the surface. This advanced attack is called exfoliation
(fig. 4-11). At this point, it can be seen by maintenance
personnel. Correction of such serious corrosion is vital
to aircraft safety. The insidious (sneaky) nature of such
an attack can seriously weaken structural members
before the volume of corrosion products accumulate
on the surface and the damage becomes apparent.
Metal that has been properly heat-treated is not
readily prone to intergranular attack. However,
localized overheating, such as could occur from
welding and fire damage, can make metal prone to
attack. If the intergranular attack has not penetrated so
far as to impair structural strength, correction as
outlined in the applicable structural repair manual
(SRM) can restore an aircraft to flight status.
Dissimilar Metal Corrosion
The terms galvanic or dissimilar metal corrosion
are applied when accelerated corrosion of metal is
caused by dissimilar metals being in contact in a
corrosive medium, such as salt spray or water.
Dissimilar metal corrosion is usually the result of a