Atmospheric Corrosion
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combined with acidity resulting from sulfur gases, as would be found in
highly industrialized coastal areas.
Other pollutants also have an effect on the corrosion of galvanized surfaces. Deposits of soot or dust can be detrimental, both because of their
specific nature and because they have the potential to increase the risk of
condensation on the surface and hold more water in position. This is prevalent on upward-facing surfaces. Soot (carbon) absorbs large quantities of sulfur, which are released by rainwater.
In rural areas, over-manuring of agricultural land tends to increase the ammonia content in the air. The presence of normal atmospheric quantities of ammonia
does not accelerate zinc corrosion, and petrochemical plants where ammonium
salts are present show no accelerated attack on galvanized steel. However,
ammonia will react with atmospheric sulfur oxides, producing ammonium sulfate, which accelerates paint film corrosion as well as zinc corrosion.
When ammonium reacts with NO x compounds in the atmosphere, ammonium nitrite and nitrate are produced. Both compounds increase the rate of
zinc corrosion, but less so than SO 2 and SO 3 .
4.7.4.1 Zinc-Aluminum Hot Dip Coatings
Combinations of zinc and aluminum are the most important series of alloys.
These have been developed to improve the atmospheric corrosion resistance
of galvanized coatings. The two primary hot dip coatings are Galfan, which
is a zinc-5% aluminum alloy, and Galvalume, which is a zinc-55% aluminum-1.5% silicon alloy. Galfan coatings have a corrosion resistance up to
three times that of galvanized steel. The main difference between these two
coatings lies in the degree of cathodic protection they afford.
Galfan finds application in a mild urban-industrial atmosphere and in a
marine atmosphere. This latter application is particularly important because
the corrosion rate slows after about 4 yr, whereas conventional galvanized
steel shows rust in 5 yr. This slower rate of corrosion is evidence that the
zinc-5% aluminum coatings provide full cathodic protection to cut edges
over a longer period of time.
Galvalume provides good protection in urban-industrial atmospheres but
at the expense of reduced cathodic protection. The corrosion rate of these
coatings also tends to diminish after a year or two.
4.7.4.2 Zinc-Aluminum Thermally Sprayed Coatings
Zinc-15% aluminum coatings are available as thermally sprayed coatings.
These coatings have a two-phase structure consisting of a zinc-rich and
an aluminum-rich phase. The oxidation of the zinc-rich phase takes place
within the more inert aluminum-rich phase. The oxidation products formed
are encapsulated in the porous layer formed by the latter and do build up a
continuous surface layer as with pure zinc coatings. As a result, no thickness
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combined with acidity resulting from sulfur gases, as would be found in
highly industrialized coastal areas.
Other pollutants also have an effect on the corrosion of galvanized surfaces. Deposits of soot or dust can be detrimental, both because of their
specific nature and because they have the potential to increase the risk of
condensation on the surface and hold more water in position. This is prevalent on upward-facing surfaces. Soot (carbon) absorbs large quantities of sulfur, which are released by rainwater.
In rural areas, over-manuring of agricultural land tends to increase the ammonia content in the air. The presence of normal atmospheric quantities of ammonia
does not accelerate zinc corrosion, and petrochemical plants where ammonium
salts are present show no accelerated attack on galvanized steel. However,
ammonia will react with atmospheric sulfur oxides, producing ammonium sulfate, which accelerates paint film corrosion as well as zinc corrosion.
When ammonium reacts with NO x compounds in the atmosphere, ammonium nitrite and nitrate are produced. Both compounds increase the rate of
zinc corrosion, but less so than SO 2 and SO 3 .
4.7.4.1 Zinc-Aluminum Hot Dip Coatings
Combinations of zinc and aluminum are the most important series of alloys.
These have been developed to improve the atmospheric corrosion resistance
of galvanized coatings. The two primary hot dip coatings are Galfan, which
is a zinc-5% aluminum alloy, and Galvalume, which is a zinc-55% aluminum-1.5% silicon alloy. Galfan coatings have a corrosion resistance up to
three times that of galvanized steel. The main difference between these two
coatings lies in the degree of cathodic protection they afford.
Galfan finds application in a mild urban-industrial atmosphere and in a
marine atmosphere. This latter application is particularly important because
the corrosion rate slows after about 4 yr, whereas conventional galvanized
steel shows rust in 5 yr. This slower rate of corrosion is evidence that the
zinc-5% aluminum coatings provide full cathodic protection to cut edges
over a longer period of time.
Galvalume provides good protection in urban-industrial atmospheres but
at the expense of reduced cathodic protection. The corrosion rate of these
coatings also tends to diminish after a year or two.
4.7.4.2 Zinc-Aluminum Thermally Sprayed Coatings
Zinc-15% aluminum coatings are available as thermally sprayed coatings.
These coatings have a two-phase structure consisting of a zinc-rich and
an aluminum-rich phase. The oxidation of the zinc-rich phase takes place
within the more inert aluminum-rich phase. The oxidation products formed
are encapsulated in the porous layer formed by the latter and do build up a
continuous surface layer as with pure zinc coatings. As a result, no thickness