Corrosion of Metallic Coatings
285
Atmospheric chlorides will lead to the corrosion of zinc, but to a lesser
degree than the corrosion of steel, except in brackish water and flowing seawater. Any salt deposit should be removed by washing. The salt content of
the atmosphere will usually decrease rapidly inland further away from the
coast, but the change is more gradual and erratic because chloride is not the
primary pollutant affecting zinc corrosion. Chloride is most harmful when
combined with acidity resulting from sulfur gases.
Other pollutants also have an effect on the corrosion of galvanized surfaces.
Deposits of soot or dust can be detrimental because they have the potential
to increase the risk of condensation onto 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, overmanuring of agricultural land tends to increase the
ammonia content of the air. The presence of normal atmospheric quantities
of ammonia does not accelerate the corrosion of zinc, and petrochemical
plants where ammonium salts are present show no accelerated attack on galvanized steel. However, ammonia will react with atmospheric sulfur dioxides to produce 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 to a lesser extent than SO 2 or SO 3 .
Because of the Mears effect (wire corrodes faster per unit of area than more
massive materials), galvanized wire corrodes some 10 to 80% faster than galvanized sheet. However, the life of rope made from galvanized steel wires is
greater than the life of the individual wire. This is explained by the fact that
the parts of the wire that lie on the outside are corroded more rapidly and,
when the zinc film is penetrated in those regions, the uncorroded zinc inside
the rope provides cathodic protection for the outer regions.
8.4.1.3 Zinc-5% Aluminum Hot Dip Coatings
This zinc alloy coating is known as Galfan. 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. This increase in corrosion protection is evident in both a relatively
mild urban-industrial atmosphere and in a marine atmosphere, as can be seen
in Table 8.6. The latter is particularly significant because, unlike galvanizing,
the corrosion rate appears to slow down after about 4 years, and conventional
galvanized steel would show rust after 5 years (Figure 8.14). The slower rate
of corrosion also means that the zinc-5% aluminum coatings provide full
cathodic protection to cut edges over a longer period of time (see Table 8.7).
Because Galfan can be formed with much smaller cracks than can be
obtained in conventional galvanized coatings, it provides excellent protection at panel bulges. This reduced cracking means that less zinc is exposed to
285
Atmospheric chlorides will lead to the corrosion of zinc, but to a lesser
degree than the corrosion of steel, except in brackish water and flowing seawater. Any salt deposit should be removed by washing. The salt content of
the atmosphere will usually decrease rapidly inland further away from the
coast, but the change is more gradual and erratic because chloride is not the
primary pollutant affecting zinc corrosion. Chloride is most harmful when
combined with acidity resulting from sulfur gases.
Other pollutants also have an effect on the corrosion of galvanized surfaces.
Deposits of soot or dust can be detrimental because they have the potential
to increase the risk of condensation onto 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, overmanuring of agricultural land tends to increase the
ammonia content of the air. The presence of normal atmospheric quantities
of ammonia does not accelerate the corrosion of zinc, and petrochemical
plants where ammonium salts are present show no accelerated attack on galvanized steel. However, ammonia will react with atmospheric sulfur dioxides to produce 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 to a lesser extent than SO 2 or SO 3 .
Because of the Mears effect (wire corrodes faster per unit of area than more
massive materials), galvanized wire corrodes some 10 to 80% faster than galvanized sheet. However, the life of rope made from galvanized steel wires is
greater than the life of the individual wire. This is explained by the fact that
the parts of the wire that lie on the outside are corroded more rapidly and,
when the zinc film is penetrated in those regions, the uncorroded zinc inside
the rope provides cathodic protection for the outer regions.
8.4.1.3 Zinc-5% Aluminum Hot Dip Coatings
This zinc alloy coating is known as Galfan. 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. This increase in corrosion protection is evident in both a relatively
mild urban-industrial atmosphere and in a marine atmosphere, as can be seen
in Table 8.6. The latter is particularly significant because, unlike galvanizing,
the corrosion rate appears to slow down after about 4 years, and conventional
galvanized steel would show rust after 5 years (Figure 8.14). The slower rate
of corrosion also means that the zinc-5% aluminum coatings provide full
cathodic protection to cut edges over a longer period of time (see Table 8.7).
Because Galfan can be formed with much smaller cracks than can be
obtained in conventional galvanized coatings, it provides excellent protection at panel bulges. This reduced cracking means that less zinc is exposed to