Properties of Metals
Corrosion is a major factor in the selection of material for a reactor plant. The
material selected must resist the various types of corrosion discussed in the
Chemistry Fundamentals Handbook.
STATE the reason that galvanic corrosion is a concern in
design and material selection.
Corrosion is the deterioration of a material due to interaction with its environment. It is the
process in which metallic atoms leave the metal or form compounds in the presence of water and
gases. Metal atoms are removed from a structural element until it fails, or oxides build up inside
a pipe until it is plugged. All metals and alloys are subject to corrosion. Even the noble metals,
such as gold, are subject to corrosive attack in some environments.
The corrosion of metals is a natural process. Most metals are not thermodynamically stable in
the metallic form; they want to corrode and revert to the more stable forms that are normally
found in ores, such as oxides. Corrosion is of primary concern in nuclear reactor plants.
Corrosion occurs continuously throughout the reactor plant, and every metal is subject to it. Even
though this corrosion cannot be eliminated, it can be controlled.
General corrosion involving water and steel generally results from chemical action where the
steel surface oxidizes, forming iron oxide (rust). Many of the systems and components in the
plant are made from iron.
Some standard methods associated with material selection that protect against general corrosion
The use of corrosion-resistant materials such as stainless steel and nickel,
chromium, and molybdenum alloys.
(Keep in mind that the corrosion is
electrochemical by nature, and the corrosion resistance of the stainless steels
results from surface oxide films that interfere with the electrochemical process.)
The use of protective coatings such as paints and epoxies.
The application of metallic and nonmetallic coatings or linings to the surface
which protects against corrosion, but allows the material to retain its structural
strength (for example, a carbon steel pressure vessel with stainless steel cladding
as a liner).