7.9A luminum Coatings
Aluminum coatings protect steel substratesb ym eans 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 as elf-healing ability.A luminum
coatings are excellentingeneral corrosion resistance. However,they do not
act as as acrificial anode in rural and industrial atmospheres but do in a
chloride area such as amarineenvironment. In anonchloride environment,
the formation of red rust occurs at sheared edgesand in other defects of an
aluminum coating layer.However, the growth of red rust is slow.
Aluminumcoatings, sealed with organic or composite layers, such as etch
primer,z inc chromate, etc., will provide long service in seawatere nvironments. Recommendedcoating thickness plus sealing for the splash zone and
submerged zone is 150 m m.
7.10 Cadmium Coatings
Cadmium coatings are produced almost exclusively by electrodeposition.
Acadmium coating on steel does not provide as much cathodic protection to
the steel as does azinc coating because the potential betweencadmium and
iron is not as great as between zincand iron. Therefore, it becomes important
to minimize defects in the cadmium coating.
Unlike zinc, acadmium coating will retain abright metallic appearance. It
is more resistant to attackb ys alt spraya nd atmospheric condensatet han
zinc. In aqueous solutions, cadmiumwill resist attackbystrong alkalies, but
willb ec orroded by dilute acids and aqueous ammonia.
Cadmium coatings should not be allowed to come into contact with food
products because cadmium saltsare toxic. This coating is commonly used on
nuts and bolts, but because of its toxicity,u sage is declining.
7.11 ManganeseC oatings
Manganese is very active, havinga ne lectrode potential more negative
than zinc (Mn: K 1.5 m, Zn K 1.03 V, SCE). In anatural atmosphere, adense
corrosion layer buildso nt he surfaceo fm anganesed uring av ery short
period. However, defects in the coating accelerate the anodic dissolution of
manganese,thereby,shorteningthe life of the coating. Therefore, manganese
is combined with zinc to form ad uplexM n/Zn alloy coating. The types
of corrosion products found on thesec oatings were shown in Table 7.7.
The compound g -Mn 2 O 3 is effective for the formation of abarrier. The more
g -Mn 2 O 3 in the corrosion products, the denser the layer on aMn/Zn coating.
Corrosion of Linings and Coatings
452
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485
Aluminum coatings protect steel substratesb ym eans 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 as elf-healing ability.A luminum
coatings are excellentingeneral corrosion resistance. However,they do not
act as as acrificial anode in rural and industrial atmospheres but do in a
chloride area such as amarineenvironment. In anonchloride environment,
the formation of red rust occurs at sheared edgesand in other defects of an
aluminum coating layer.However, the growth of red rust is slow.
Aluminumcoatings, sealed with organic or composite layers, such as etch
primer,z inc chromate, etc., will provide long service in seawatere nvironments. Recommendedcoating thickness plus sealing for the splash zone and
submerged zone is 150 m m.
7.10 Cadmium Coatings
Cadmium coatings are produced almost exclusively by electrodeposition.
Acadmium coating on steel does not provide as much cathodic protection to
the steel as does azinc coating because the potential betweencadmium and
iron is not as great as between zincand iron. Therefore, it becomes important
to minimize defects in the cadmium coating.
Unlike zinc, acadmium coating will retain abright metallic appearance. It
is more resistant to attackb ys alt spraya nd atmospheric condensatet han
zinc. In aqueous solutions, cadmiumwill resist attackbystrong alkalies, but
willb ec orroded by dilute acids and aqueous ammonia.
Cadmium coatings should not be allowed to come into contact with food
products because cadmium saltsare toxic. This coating is commonly used on
nuts and bolts, but because of its toxicity,u sage is declining.
7.11 ManganeseC oatings
Manganese is very active, havinga ne lectrode potential more negative
than zinc (Mn: K 1.5 m, Zn K 1.03 V, SCE). In anatural atmosphere, adense
corrosion layer buildso nt he surfaceo fm anganesed uring av ery short
period. However, defects in the coating accelerate the anodic dissolution of
manganese,thereby,shorteningthe life of the coating. Therefore, manganese
is combined with zinc to form ad uplexM n/Zn alloy coating. The types
of corrosion products found on thesec oatings were shown in Table 7.7.
The compound g -Mn 2 O 3 is effective for the formation of abarrier. The more
g -Mn 2 O 3 in the corrosion products, the denser the layer on aMn/Zn coating.
Corrosion of Linings and Coatings
452
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485