Corrosion of Metallic Coatings
289
Electroplated zinc-iron alloy layers containing more than 20% iron provide
a corrosion resistance 30% higher than zinc in industrial atmospheres. In
other atmospheres, the zinc-iron galvanized coatings provide as good a coating as coatings with an outer zinc layer. Sherardized coatings are superior to
electroplated coatings and equal to galvanized coatings of the same thickness. However, the structure of the alloy layer, and its composition, affects
the corrosion resistance.
If the zinc layer of a galvanized coating has weathered, or the zinc-iron
layer forms the top layer after galvanizing, brown staining can occur on sherardized or hot-dip galvanized coatings in atmospheric corrosion through
the oxidation of iron from the zinc-iron alloy layers or from the substrate.
Such staining is usually a dull brown rather than the bright red-brown of
uncontrolled rust. Usually, there is a substantial intact galvanized layer
underneath, leaving the life of the coating unchanged. Unless the aesthetic
appearance is undesirable, no action need be taken.
8.4.1.7 Aluminum Coatings
Aluminum coatings protect steel substrates by means of cathodic control, with an original barrier action of an air-formed film, which is chemically inert, and the rapid formation of oxide film by a self-healing ability.
Aluminum coatings are excellent in general corrosion resistance. However,
they do not act as a sacrificial anode in rural and industrial atmospheres,
but do in a chloride area, such as a marine environment. In a nonchloride environment, the formation of red rust occurs at sheared edges and
in other defects of an aluminum coating layer. However, the growth of red
rust is slow.
Aluminum coatings sealed with organic or composite layers such as etch
primer, zinc chromate, etc. will provide long service in seawater environments. The recommended coating thickness plus sealing for the splash zone
and submerged zone is 150 μm.
8.4.1.8 Cadmium Coatings
Cadmium coatings are produced almost exclusively by electrodeposition. A
cadmium coating on steel does not provide as much cathodic protection to
the steel as does a zinc coating because the potential between cadmium and
iron is not as great as between zinc and iron. Therefore, it becomes important
to minimize defects in the cadmium coating.
Unlike zinc, a cadmium coating will retain a bright metallic appearance.
It is more resistant to attack by salt spray and atmospheric condensate than
zinc. In aqueous solutions, cadmium will resist attack by strong alkali but
will be corroded by dilute acids and aqueous ammonia.
289
Electroplated zinc-iron alloy layers containing more than 20% iron provide
a corrosion resistance 30% higher than zinc in industrial atmospheres. In
other atmospheres, the zinc-iron galvanized coatings provide as good a coating as coatings with an outer zinc layer. Sherardized coatings are superior to
electroplated coatings and equal to galvanized coatings of the same thickness. However, the structure of the alloy layer, and its composition, affects
the corrosion resistance.
If the zinc layer of a galvanized coating has weathered, or the zinc-iron
layer forms the top layer after galvanizing, brown staining can occur on sherardized or hot-dip galvanized coatings in atmospheric corrosion through
the oxidation of iron from the zinc-iron alloy layers or from the substrate.
Such staining is usually a dull brown rather than the bright red-brown of
uncontrolled rust. Usually, there is a substantial intact galvanized layer
underneath, leaving the life of the coating unchanged. Unless the aesthetic
appearance is undesirable, no action need be taken.
8.4.1.7 Aluminum Coatings
Aluminum coatings protect steel substrates by means of cathodic control, with an original barrier action of an air-formed film, which is chemically inert, and the rapid formation of oxide film by a self-healing ability.
Aluminum coatings are excellent in general corrosion resistance. However,
they do not act as a sacrificial anode in rural and industrial atmospheres,
but do in a chloride area, such as a marine environment. In a nonchloride environment, the formation of red rust occurs at sheared edges and
in other defects of an aluminum coating layer. However, the growth of red
rust is slow.
Aluminum coatings sealed with organic or composite layers such as etch
primer, zinc chromate, etc. will provide long service in seawater environments. The recommended coating thickness plus sealing for the splash zone
and submerged zone is 150 μm.
8.4.1.8 Cadmium Coatings
Cadmium coatings are produced almost exclusively by electrodeposition. A
cadmium coating on steel does not provide as much cathodic protection to
the steel as does a zinc coating because the potential between cadmium and
iron is not as great as between zinc and iron. Therefore, it becomes important
to minimize defects in the cadmium coating.
Unlike zinc, a cadmium coating will retain a bright metallic appearance.
It is more resistant to attack by salt spray and atmospheric condensate than
zinc. In aqueous solutions, cadmium will resist attack by strong alkali but
will be corroded by dilute acids and aqueous ammonia.