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Fundamentals of Corrosion
its oxidation to a metallic ion. Combination with oxygen to form metallic
oxides, or scale, results in the loss of material in its useful engineering form;
scale ultimately flakes off to return to nature.
A metal resists corrosion by forming a passive film on the surface. This
film is formed naturally when the metal is exposed to the air for a period
of time. It can also be formed more quickly by chemical treatment. For
example, nitric acid, if applied to austenitic stainless steel, will form this
protective film. Such a film is actually corrosion but once formed, it prevents
further degradation of the metal, provided that the film remains intact. It
does not provide an overall resistance to corrosion because it may be subject to chemical attack. The immunity of the film to attack is a function of
the film composition, temperature, and the aggressiveness of the chemical.
Examples of such films are the patina formed on copper, the rusting of iron,
the tarnishing of silver, the fogging of nickel, and the high-temperature oxidation of metals.
There are two theories regarding the formation of this film. The first theory
states that the film formed is a metal oxide or other reaction compound. This is
known as the oxide film theory. The second theory states that oxygen is adsorbed
on the surface, forming a chemisorbed film. However, all chemisorbed films
react over a period of time with the underlying metal to form metal oxides.
Oxide films are formed at room temperature. Metal oxides can be classified as
network formers, intermediates, or modifiers. This division can be related to
thin oxide films on metals. The metals that fall into network-forming or intermediate classes tend to grow protective oxides that support anion or mixed
anion/cation movement. The network formers are noncrystalline, whereas the
intermediates tend to be macrocrystalline at low temperatures.
3.1.1 Passive Film on iron
Iron in iron oxides can assume a valence of two or three. The former acts as
a modifier and the latter as a network former. The iron is protected from the
corrosion environment by a thin oxide film 1 to 4 mm in thickness with a composition of Fe O /Fe O
2 3
3 4 . This is the same type of film formed by the reaction of clean iron with oxygen or dry air. The Fe O
2 3 layer is responsible for
the passivity, while the Fe 3 O 4 provides the basis for the formation of a higher
oxidation state. Iron is more difficult to passivate than nickel because with iron
it is not possible to go directly to the passivation species Fe O
2 3 . Instead, a
lower oxidation state of Fe 3 O 4 is required and the film is highly susceptible to
chemical dissolution. The Fe O
2 3 layer will not form until the Fe 3 O 4 phase
has existed on the surface for a reasonable period of time. During this time, the
Fe 3 O 4 layer continues to form.
Fundamentals of Corrosion
its oxidation to a metallic ion. Combination with oxygen to form metallic
oxides, or scale, results in the loss of material in its useful engineering form;
scale ultimately flakes off to return to nature.
A metal resists corrosion by forming a passive film on the surface. This
film is formed naturally when the metal is exposed to the air for a period
of time. It can also be formed more quickly by chemical treatment. For
example, nitric acid, if applied to austenitic stainless steel, will form this
protective film. Such a film is actually corrosion but once formed, it prevents
further degradation of the metal, provided that the film remains intact. It
does not provide an overall resistance to corrosion because it may be subject to chemical attack. The immunity of the film to attack is a function of
the film composition, temperature, and the aggressiveness of the chemical.
Examples of such films are the patina formed on copper, the rusting of iron,
the tarnishing of silver, the fogging of nickel, and the high-temperature oxidation of metals.
There are two theories regarding the formation of this film. The first theory
states that the film formed is a metal oxide or other reaction compound. This is
known as the oxide film theory. The second theory states that oxygen is adsorbed
on the surface, forming a chemisorbed film. However, all chemisorbed films
react over a period of time with the underlying metal to form metal oxides.
Oxide films are formed at room temperature. Metal oxides can be classified as
network formers, intermediates, or modifiers. This division can be related to
thin oxide films on metals. The metals that fall into network-forming or intermediate classes tend to grow protective oxides that support anion or mixed
anion/cation movement. The network formers are noncrystalline, whereas the
intermediates tend to be macrocrystalline at low temperatures.
3.1.1 Passive Film on iron
Iron in iron oxides can assume a valence of two or three. The former acts as
a modifier and the latter as a network former. The iron is protected from the
corrosion environment by a thin oxide film 1 to 4 mm in thickness with a composition of Fe O /Fe O
2 3
3 4 . This is the same type of film formed by the reaction of clean iron with oxygen or dry air. The Fe O
2 3 layer is responsible for
the passivity, while the Fe 3 O 4 provides the basis for the formation of a higher
oxidation state. Iron is more difficult to passivate than nickel because with iron
it is not possible to go directly to the passivation species Fe O
2 3 . Instead, a
lower oxidation state of Fe 3 O 4 is required and the film is highly susceptible to
chemical dissolution. The Fe O
2 3 layer will not form until the Fe 3 O 4 phase
has existed on the surface for a reasonable period of time. During this time, the
Fe 3 O 4 layer continues to form.