90
Fundamentals of Corrosion
produce anodes and cathodes. It is a relatively complicated system consisting
of a metal, corrosion products, surface electrolyte, and the atmosphere.
For electrochemical reaction to proceed, two or more electrode reactions
must take place: the oxidation of the metal (anodic partial reaction) and the
reduction of an oxidizing agent (cathodic partial reaction). For the electrode
reactions to take place, it is necessary that an electrolyte be present.
Most metals, if exposed to the atmosphere at room temperature with virtually no humidity present, will form an oxide film. If the oxide film is stable,
the growth rate stops and the oxide film reaches a maximum thickness of 1
to 5 mm, protecting the metal.
Atmospheric corrosion falls into two categories: (1) “damp” atmospheric
corrosion, which takes place in the presence of water vapor and traces of
pollutants; and (2) “wet” atmospheric corrosion, which occurs when rain or
other forms of bulk water are present, with pollutants. For atmospheric corrosion to proceed, water of some sort must be present.
Damp atmospheric corrosion occurs when water is present on the surface
of the metal as an aqueous-phase layer, as caused by humidity, dew, or fog.
Wet atmospheric corrosion occurs when bulk water is present, such as rain.
4.3.1 Damp atmospheric Corrosion (adsorption Layers)
Aqueous-phase layers consist of water adsorbed on the metal surface. The
amount formed depends on the relative humidity of the atmosphere and the
chemical and physical properties of the corrosion products.
Water may be adsorbed from the atmosphere and wet the metal surface if
hygroscopic salts are deposited or formed by corrosion. This adsorption will
take place when the relative humidity exceeds the critical relative humidity.
The value of the critical relative humidity depends on the specific metal and
the specific contaminants. When the relative humidity exceeds the value at
which the salt starts to adsorb water and dissolve, the corrosion rate increases
sharply. This critical relative humidity corresponds to a vapor pressure above
a saturated solution of the salt present. Adsorption layers of electrolyte on
the surface of the metal may also be the result of capillary condensation.
The corrosion rate is directly influenced by the amount of water present on
the corroding surface. Laboratory studies have shown that the corrosion rate
above the critical value sharply increases with increasing relative humidity.
The amount of water present on a metal surface has been roughly estimated as
follows:
Conditions
Amount of Water
(g/m 2 )
Critical relative humidity
0.01
100% Relative humidity
1
Covered by dew
10
Wet from rain
100
Fundamentals of Corrosion
produce anodes and cathodes. It is a relatively complicated system consisting
of a metal, corrosion products, surface electrolyte, and the atmosphere.
For electrochemical reaction to proceed, two or more electrode reactions
must take place: the oxidation of the metal (anodic partial reaction) and the
reduction of an oxidizing agent (cathodic partial reaction). For the electrode
reactions to take place, it is necessary that an electrolyte be present.
Most metals, if exposed to the atmosphere at room temperature with virtually no humidity present, will form an oxide film. If the oxide film is stable,
the growth rate stops and the oxide film reaches a maximum thickness of 1
to 5 mm, protecting the metal.
Atmospheric corrosion falls into two categories: (1) “damp” atmospheric
corrosion, which takes place in the presence of water vapor and traces of
pollutants; and (2) “wet” atmospheric corrosion, which occurs when rain or
other forms of bulk water are present, with pollutants. For atmospheric corrosion to proceed, water of some sort must be present.
Damp atmospheric corrosion occurs when water is present on the surface
of the metal as an aqueous-phase layer, as caused by humidity, dew, or fog.
Wet atmospheric corrosion occurs when bulk water is present, such as rain.
4.3.1 Damp atmospheric Corrosion (adsorption Layers)
Aqueous-phase layers consist of water adsorbed on the metal surface. The
amount formed depends on the relative humidity of the atmosphere and the
chemical and physical properties of the corrosion products.
Water may be adsorbed from the atmosphere and wet the metal surface if
hygroscopic salts are deposited or formed by corrosion. This adsorption will
take place when the relative humidity exceeds the critical relative humidity.
The value of the critical relative humidity depends on the specific metal and
the specific contaminants. When the relative humidity exceeds the value at
which the salt starts to adsorb water and dissolve, the corrosion rate increases
sharply. This critical relative humidity corresponds to a vapor pressure above
a saturated solution of the salt present. Adsorption layers of electrolyte on
the surface of the metal may also be the result of capillary condensation.
The corrosion rate is directly influenced by the amount of water present on
the corroding surface. Laboratory studies have shown that the corrosion rate
above the critical value sharply increases with increasing relative humidity.
The amount of water present on a metal surface has been roughly estimated as
follows:
Conditions
Amount of Water
(g/m 2 )
Critical relative humidity
0.01
100% Relative humidity
1
Covered by dew
10
Wet from rain
100