Atmospheric Corrosion
89
H 2 O 2 is highly soluble in aqueous environments and is a powerful oxidizing agent.
However, the degree to which these pollutants will affect the corrosion
rate will be influenced by other factors that also have an influence on the
corrosion rate. The corrosive effect of these pollutants will depend on the
specific material with which they come into contact. Succeeding sections
deal with the mechanisms of corrosion of these pollutants and their effect
on specific materials.
4.3 Mechanisms of Atmospheric Corrosion of Metals
Atmospheric corrosion takes place by means of an electrochemical process
occurring in corrosion cells. A corrosion cell must have the following essential components:
1. A metal anode
2. A metal cathode
3. A metallic conductor between the anode and cathode
4. An electrolyte (water containing conductive salts) in contact with
the anode and cathode but not necessarily of the same composition
at the two locations
In addition, oxygen will usually be present as a depolarizing agent. For the
cell to function, there must be a difference in potential between the anode and
cathode. The difference in potential is mainly due to contact between dissimilar metallic conductors or differences in concentration of the solution, generally
related to dissolved oxygen in natural waters. Almost any lack of homogeneity
on the metal surface or in its environment may initiate attack by causing a differential in potentials that results in more or less localization of corrosion.
Atmospheric corrosion differs from the action that occurs in water or underground in that a plentiful supply of oxygen is always present. In this case, the
formation of insoluble films, and the presence of moisture and deposits from
the atmosphere, become the controlling factors. The presence of contaminants such as sulfur compounds and salt particles also affects the corrosion
rate. Nevertheless, atmospheric corrosion is mainly electrochemical, rather
than a direct chemical attack by the elements. The anodic and cathodic areas
are usually quite small and close together, so that corrosion is apparently
uniform rather than in the form of severe pitting as in soil or water. Anodes
and cathodes exist on all steel surfaces. Surface imperfections, grain orientation, lack of homogeneity of the metal, variation in the environment, localized
shear and torque during manufacture, mill scale, and existing iron rust will
89
H 2 O 2 is highly soluble in aqueous environments and is a powerful oxidizing agent.
However, the degree to which these pollutants will affect the corrosion
rate will be influenced by other factors that also have an influence on the
corrosion rate. The corrosive effect of these pollutants will depend on the
specific material with which they come into contact. Succeeding sections
deal with the mechanisms of corrosion of these pollutants and their effect
on specific materials.
4.3 Mechanisms of Atmospheric Corrosion of Metals
Atmospheric corrosion takes place by means of an electrochemical process
occurring in corrosion cells. A corrosion cell must have the following essential components:
1. A metal anode
2. A metal cathode
3. A metallic conductor between the anode and cathode
4. An electrolyte (water containing conductive salts) in contact with
the anode and cathode but not necessarily of the same composition
at the two locations
In addition, oxygen will usually be present as a depolarizing agent. For the
cell to function, there must be a difference in potential between the anode and
cathode. The difference in potential is mainly due to contact between dissimilar metallic conductors or differences in concentration of the solution, generally
related to dissolved oxygen in natural waters. Almost any lack of homogeneity
on the metal surface or in its environment may initiate attack by causing a differential in potentials that results in more or less localization of corrosion.
Atmospheric corrosion differs from the action that occurs in water or underground in that a plentiful supply of oxygen is always present. In this case, the
formation of insoluble films, and the presence of moisture and deposits from
the atmosphere, become the controlling factors. The presence of contaminants such as sulfur compounds and salt particles also affects the corrosion
rate. Nevertheless, atmospheric corrosion is mainly electrochemical, rather
than a direct chemical attack by the elements. The anodic and cathodic areas
are usually quite small and close together, so that corrosion is apparently
uniform rather than in the form of severe pitting as in soil or water. Anodes
and cathodes exist on all steel surfaces. Surface imperfections, grain orientation, lack of homogeneity of the metal, variation in the environment, localized
shear and torque during manufacture, mill scale, and existing iron rust will