FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 6. Corrosion, Inspection, and Protection | 2. Types of Corrosion | 6-15. Active-passive Cells
AC 43.13-1B CHG 1
9/27/01
6-14. CONCENTRATION CELL COR-
ROSION. Concentration cell corrosion, (also
known as Crevice Corrosion) is corrosion of
metals in a metal-to-metal joint, corrosion at
the edge of a joint even though the joined met
als are identical, or corrosion of a spot on the
metal surface covered by a foreign material.
Metal ion concentration cells and oxygen con
centration cells are the two general types of
concentration cell corrosion. (See figure 6-6.)
a. Metal Ion Concentration Cells. The
solution may consist of water and ions of the
metal which is in contact with water. A high
concentration of the metal ions will normally
exist under faying surfaces where the solution
is stagnant, and a low concentration of metal
ions will exist adjacent to the crevice which is
created by the faying surface. An electrical
potential will exist between the two points; the
area of the metal in contact with the low con
centration of metal ions will be anodic and
corrode, and the area in contact with the high
metal ion concentration will be cathodic and
not show signs of corrosion.
b. Oxygen Concentration Cells. The
solution in contact with the metal surface will
normally contain dissolved oxygen. An oxy
gen cell can develop at any point where the
oxygen in the air is not allowed to diffuse into
the solution, thereby creating a difference in
oxygen concentration between two points.
Typical locations of oxygen concentration
cells are under gaskets, wood, rubber, and
other materials in contact with the metal sur
face. Corrosion will occur at the area of low
oxygen concentration (anode). Alloys such as
stainless steel are particularly susceptible to
this type of crevice corrosion.
6-15. ACTIVE-PASSIVE CELLS. Metals
which depend on a tightly adhering passive
film, usually an oxide, for corrosion protection
are prone to rapid corrosive attack by active-
passive cells. Active-passive cells are often re
ferred to as a type of concentration cell corro
sion. However, the active-passive cell is actu
ally two forms of corrosion working in con
junction. The corrosive action usually starts as
an oxygen concentration cell. As an example,
salt deposits on the metal surface in the pres
ence of water containing oxygen can create the
oxygen cell. The passive film will be broken
beneath the salt crystals. Once the passive
film is broken, the active metal beneath the
film will be exposed to corrosive attack. (See
figure 6-7.) Rapid pitting of the active metal
will result. This reaction can become locally
intense due to several factors. First the reac
tion is augmented by the affected area, since
the proportion of the exposed base metal is
small compared to the surrounding non
reactive metal. This effectively concentrates
the focal point of the reaction, often resulting
in deep pits in a short time and a greater rate of
corrosion.
FIGURE 6-6. Concentration cell corrosion.
Page 6-6
Par 6-14