Atmospheric Corrosion
119
The corrosion resistance of aluminum is reduced by most alloying constituents in clean or slightly polluted atmospheres. Urban or industrial atmospheres are those atmospheres that are high in CO and CO 2 , sulfates and
sulfites, and possibly various N x O gases. The corrosion rate is largely the
same for unalloyed aluminum as with most conventional aluminum alloys
(alloys with zinc, magnesium, silicon, and/or manganese). Aluminum alloys
with copper will exhibit corrosion rates 4 to 20 times higher. When the SO 2
concentration is less than 0.01% by volume, the atmospheric corrosion rate
even at a relative humidity of 98% is negligibly affected by the presence of
SO 2 . This is probably due to the low adsorption tendency for SO 2 on aluminum surfaces. When the SO 2 concentration exceeds 0.01% by volume, severe
corrosion effects occur.
Some highly urban areas, such as the greater Chicago area, have been
found to cause exfoliation, whereas other cities such as Los Angeles, which
have high Ν x Ο gases, are capable of causing increased susceptibility to stress
corrosion cracking. However, in general, most urban/industrial areas do not
cause exfoliation and the need for protection is to:
1. Retain aesthetic appearance
2. Prevent pitting formation of their parts
3. Avoid failure by stress corrosion cracking
The aesthetic problem of discoloration can be caused by excessive smoke
emissions. That was a major problem before regulations on smoke emissions
were enacted. The discoloration problem is not limited to aluminum; it also
occurs on other metals, masonry, and stone.
Practically all the aluminum alloys, at least with appropriate protective
measures, will provide satisfactory corrosion resistance in urban/industrial areas. Consequently, the choice of alloy can be based on other engineering considerations.
The most damaging natural atmosphere for aluminum is that of a seacoast,
with the severest corrosion taking place within 0.5 mi (0.8 km) of the shoreline. Corrosive effects will be greatly reduced at distances of 2 to 5 miles (3
to 8 km) from the shore. Prevailing wind directions and the roughness of the
surf determine the extent of the corrosive effect, and control how far inland
actual salt mist will be carried. To maintain the shiny metallic surface of
aluminum in a seacoast atmosphere, pure aluminum as well as some aluminum alloys must be protected with a coat of clear lacquer or an anodized
coating.
Alloys of the 2XXX and 7XXX with tempers that are susceptible to intergranular corrosion, exfoliation, or stress corrosion cracking should not be
used in seacoast atmospheres. Thin products should be afforded protection
against perforation even if they are only susceptible to pitting corrosion.
119
The corrosion resistance of aluminum is reduced by most alloying constituents in clean or slightly polluted atmospheres. Urban or industrial atmospheres are those atmospheres that are high in CO and CO 2 , sulfates and
sulfites, and possibly various N x O gases. The corrosion rate is largely the
same for unalloyed aluminum as with most conventional aluminum alloys
(alloys with zinc, magnesium, silicon, and/or manganese). Aluminum alloys
with copper will exhibit corrosion rates 4 to 20 times higher. When the SO 2
concentration is less than 0.01% by volume, the atmospheric corrosion rate
even at a relative humidity of 98% is negligibly affected by the presence of
SO 2 . This is probably due to the low adsorption tendency for SO 2 on aluminum surfaces. When the SO 2 concentration exceeds 0.01% by volume, severe
corrosion effects occur.
Some highly urban areas, such as the greater Chicago area, have been
found to cause exfoliation, whereas other cities such as Los Angeles, which
have high Ν x Ο gases, are capable of causing increased susceptibility to stress
corrosion cracking. However, in general, most urban/industrial areas do not
cause exfoliation and the need for protection is to:
1. Retain aesthetic appearance
2. Prevent pitting formation of their parts
3. Avoid failure by stress corrosion cracking
The aesthetic problem of discoloration can be caused by excessive smoke
emissions. That was a major problem before regulations on smoke emissions
were enacted. The discoloration problem is not limited to aluminum; it also
occurs on other metals, masonry, and stone.
Practically all the aluminum alloys, at least with appropriate protective
measures, will provide satisfactory corrosion resistance in urban/industrial areas. Consequently, the choice of alloy can be based on other engineering considerations.
The most damaging natural atmosphere for aluminum is that of a seacoast,
with the severest corrosion taking place within 0.5 mi (0.8 km) of the shoreline. Corrosive effects will be greatly reduced at distances of 2 to 5 miles (3
to 8 km) from the shore. Prevailing wind directions and the roughness of the
surf determine the extent of the corrosive effect, and control how far inland
actual salt mist will be carried. To maintain the shiny metallic surface of
aluminum in a seacoast atmosphere, pure aluminum as well as some aluminum alloys must be protected with a coat of clear lacquer or an anodized
coating.
Alloys of the 2XXX and 7XXX with tempers that are susceptible to intergranular corrosion, exfoliation, or stress corrosion cracking should not be
used in seacoast atmospheres. Thin products should be afforded protection
against perforation even if they are only susceptible to pitting corrosion.