TO 35-1-3 - Robins Air Force Base

CHAPTER 1
CORROSION THEORY, IDENTIFICATION, CAUSES AND EFFECTS
TO 35-1-3
1.1 PURPOSE.
The purpose of this chapter is to provide SE maintenance personnel
with the basic knowledge necessary to understand the
causes of corrosion, identify the different types of corrosion
that will in-turn help minimize corrosion damage through prevention
and early detection and treatment. To help prevent
corrosion, SE technicians must first need to understand corrosion
causes and effects and be able to recognize there are
several types of corrosion with different preventive measures.
This chapter is an introduction to corrosion theory, the causes
of corrosion and the factors that influence its development.
The various forms of corrosion and the effect of corrosive
environments normally on SE are also described in this chapter.
1.2 DEFINITION OF CORROSION.
Corrosion is the electrochemical deterioration of a metal
because of its chemical reaction with the surrounding environment.
This reaction occurs because of the tendency of metals
to return to their naturally occurring states, usually oxide or
sulfide ores. For example, iron in the presence of moisture and
air will return to its natural state, iron oxide or rust. Aluminum
and magnesium form corrosion products that are white oxides
or hydroxides. When corrosion occurs, water is usually
present in some form (e.g., humidity, moisture, condensation,
rain, salt spray, etc.), acting as an electrolyte and reacting
chemically with metal surfaces.
1.3 CHEMICAL DEFINITIONS.
1.3.1 Atom. The smallest unit of an element. There are more
than 100 elements, including metals (such as aluminum, magnesium,
iron, nickel, titanium, cadmium, chromium, copper,
and carbon) and non-metals (such as hydrogen, oxygen, sulfur,
and chlorine).
1.3.2 Electron. A negatively charged particle much smaller
than an atom. An electrical current occurs when electrons are
forced to move through metal conductors. Electrons also flow
through water solutions, but only in the presence of ions.
1.3.3 Ions. Atoms or groups of atoms bound together that is
either positively or negatively charged. An electrical current
occurs when ions are forced to move through water solutions.
Ions cannot move through metal conductors.
1.4 THEORY OF CORROSION.
A corrosion cell is much like a battery. When a metal corrodes,
the metal atoms lose electrons and become metal ions in
an electrolyte solution. The positively charged metal ions can
combine with negatively charged ions to form corrosion products,
such as metallic chlorides, oxides, hydroxides, and sulfides.
Four conditions must exist before this type of corrosion
can occur.
1.4.1 Anode. A metal must be present that has a tendency to
corrode. The corroding metal is known as the anode.
1.4.2 Cathode. A dissimilar conductive material (the cathode)
that has less of a tendency to corrode than the anode must
be present. Examples include a different metal, a protected
part of the same metal, or conductive plastics/composite.
1.4.3 Electrolyte. A conductive liquid (electrolyte) must
connect the anode and cathode so that ions can carry electrical
current between them.
1.4.4 Electrical Path. Electrical contact between the anode
and cathode (usually in the form of metal-to-metal contact)
must exist so that electrons can move from the anode, where
they are released, to the cathode. Eliminating any one of these
four conditions, illustrated in Figure 1-1, will stop corrosion.
For example, a paint film on a metal surface will prevent the
conducting liquid (electrolyte) from connecting the anode and
cathode, thereby stopping the electric current. Another example
is when two connected dissimilar metal parts placed in
pure water corrode very slowly because of the lack of ions to
conduct the electric current. In seawater, the corrosion reaction
is accelerated by a factor of 1,000 or more.
1.5 DEVELOPMENT OF CORROSION.
1.5.1 Corrosion Origination Locations. All corrosive
attacks begin on the surface of metals whether it is the inside
of a bolt hole, border of a metal crystal, hand rail interior or
bottom of a frame. If allowed to progress, corrosion can penetrate
into and through the metal. When corrosion products
form, they often precipitate onto the corroding surface as a
powdery or scaled deposit. Other evidence of corrosion is the
bulging/blistering of the metal surfaces. Thin film corrosion
forms on electrical contact points and pins may appear as a
tarnish, or powdery deposit on the metal surface.
1.3.4 Electrolyte. A liquid solution (usually water) containing
ions. Salt water is an electrolyte, an aqueous (i.e., water)
solution of sodium ions and chloride ions.
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