Mechanical failure modes
Corrosion can be defined as the disintegra-
tion of a material into its constituent atoms
due to chemical reactions with its surround-
ings. In the most common use of the word,
this means a loss of electrons of metals react-
ing with water and oxygen. Weakening of
iron due to oxidation of the iron atoms is a
well-known example of electrochemical cor-
rosion. This is commonly known as rusting.
This type of damage typically produces ox-
ide(s) and/or salt(s) of the original metal.
Corrosion can also refer to other materials
than metals, such as ceramics or polymers.
Although in this context, the term degrada-
tion is more common.
Most structural alloys corrode merely
from exposure to moisture in the air, but the
process can be strongly affected by exposure
to certain substances (see below). Corrosion
can be concentrated locally to form a pit or
crack, or it can extend across a wide area to
produce general deterioration. While some
efforts to reduce corrosion merely redirect
the damage into less visible, less predictable
forms, controlled corrosion treatments such
as passivation and chromate-conversion will
increase a material’s corrosion resistance.
Rust, the most familiar example of corrosion.
Corrosion on exposed metal.
Galvanic corrosion occurs when two different
metals electrically contact each other and are
immersed in an electrolyte. In order for gal-
vanic corrosion to occur, an electrically con-
ductive path and an ionically conductive path
are necessary. This effects a galvanic couple
where the more active metal corrodes at an
accelerated rate and the more noble metal
corrodes at a retarded rate. When immersed,
neither metal would normally corrode as
quickly without the electrically conductive
connection (usually via a wire or direct con-
tact). Galvanic corrosion is often utilised in
sacrificial anodes. What type of metal(s) to
use is readily determ