01 March 2005
NAVAIR 01-1A-509-1
TM 1-1500-344-23-1
TO 1-1-689-1
CORROSION THEORY
occur easily at ambient temperatures. This type of
corrosion can be effectively treated by maintenance
personnel as discussed in this manual. Corrosion can
3-1.1. Maintenance of military aircraft and avionic
also occur in the absence of water but only at high
equipment requires knowledge of why metals corrode
temperatures, such as those found in gas turbine
and materials degrade. The theory lies in the definition
engines. However, the most common type of corrosion
and description of mechanisms that cause equipment
(and the one that can be most effectively treated by
to fail in field service. Corrosion is the chemical or
maintenance personnel) is electrochemical corrosion.
electrochemical deterioration of a material. This
types of corrosion, the frequent simultaneous presence
of several types of corrosion, and the design
3-5.1. ATOM. The smallest unit of an element, made
characteristics and maintenance/environmental factors
that make aircraft and avionic systems susceptible to
up of a positively charged nucleus surrounded by a
corrosion.
system of negatively charged electrons. There are over
100 elements, including metals (such as aluminum,
3-1.2. Corrosion can cause complete failure of
magnesium, gold, platinum, iron, nickel, titanium,
equipment or undesirable changes in electrical
cadmium, chromium, copper, silver, lead, beryllium,
characteristics. It is a process that is active on a 24 hour
zinc), and non-metals (such as carbon, boron, sulfur,
basis. Equipment does not necessarily have to be
installed, operated, or resident in a particularly harsh
3-5.2. ELECTRON. A negatively charged subatomic
environment. Some form of corrosion will take place
even in near ideal environments. All personnel should
particle. An electric current occurs when electrons are
recognize that corrosion is the natural continuing process
forced to move through metal conductors. Electrons
of materials returning to their normal state. Inadequate
flow through liquid solutions only in the presence of
corrosion prevention and control will ultimately affect
ions.
equipment life cycles, downtime, and overall system
3-5.3. ION. An atom or group of atoms or molecules
which has acquired a net electric charge by gaining
(negative ion) or losing (positive ion) electrons. When
provide maintenance personnel with the background
ions are forced to move through liquid solutions, an
knowledge necessary to understand the causes of
electric current can occur. Ions cannot move through
corrosion.
metal conductors.
3-3. SCOPE. This chapter is an introduction to corrosion
3-5.4. ELECTROLYTE. A liquid (usually water) solution
theory: the causes of corrosion and the factors which
containing ions. Sea water is an electrolyte: an aqueous
influence its development. The theory of corrosion and
(water-based) solution whose major components are
the factors influencing corrosion of aircraft metals are
sodium and chloride ions. Electrochemistry is the branch
described. The types of corrosion and how to recognize
of science concerned with chemical reactions at surfaces
them are discussed.
in contact with electrolytes.
to some extent in a natural environment. When a metal
properties due to its chemical reaction with the
corrodes, the atoms lose electrons and become positively
surrounding environment. This reaction occurs because
charged. In solution, the positively charged metal ions
of the tendency of metals to return to their naturally
can combine with negatively charged ions to form
occurring state, usually oxide or sulfide ores. For
corrosion products, such as metallic chlorides, oxides,
example, iron in the presence of moisture and air will
hydroxides, and sulfides.
return to its natural state, iron oxide or rust. Aluminum
and magnesium form corrosion products that are white
3-6.1. Four conditions (illustrated in Figure 3-1) must
oxides or hydroxides. When a water solution containing
exist before metal corrosion can occur.
soluble salts is present, corrosion of many alloys can