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Fundamentals of Corrosion
For alloys whose tempers are resistant to exfoliation and stress corrosion
cracking, the need for protection depends on two factors:
1. The desire to retain a pleasing appearance
2. Whether the application is fatigue-critical (if so, the depth of pitting
corrosion might cause sufficient stress to initiate a crack)
Structural products about 12 mm or more in thickness that are used for
nonfatigue-critical structural applications will darken and have the surface
stained as a result of weathering. Pitting corrosion will roughen the surface
to a maximum depth of approximately 0.15 mm. However, the loss in residual tensile strength and load-carrying ability will be negligible.
The higher strength alloys such as 5XXX and 6XXX do not require any preventative coating because they have sufficient inherent corrosion resistance.
These alloys will darken and their surface will undergo a slight roughening but they are capable of surviving 20 or more years of exposure without
appreciable degradation.
Until recently, rural atmospheres were considered noncorrosive to aluminum alloys. However, airborne pollution from coal-burning power plants
located upwind have produced an acid rain that falls over a widespread area.
This has resulted in staining and darkening of the aluminum surfaces but
has not resulted in more severe forms of corrosion. Acid rainfalls in rural
areas have caused greater corrosion problems on steel than on aluminum.
4.7.7.1 Protective Measures
Proper design of the product along with the selection of the proper alloy are
the first steps that should be taken toward protection of the finished product.
When necessary, there are various coatings that can be applied.
Anodic coatings. It is possible to apply an anodic coating to aluminum alloy
products. The coating can be a spray coating of zinc, pure aluminum, or a
more anodic aluminum alloy.
Alclad coatings. Alclad coatings consist of a metallurgical bond between two
pure aluminum sheets forming a sandwich on either side of an aluminum
alloy sheet, which is referred to as the core. The cladding process provides a
barrier layer of a more resistant material than the core. Should the barrier layer
become damaged by inadvertent dings or scratches, or accidentally penetrated
by drilling or sawing, or if natural pitting corrosion of the cladding reaches
the core, the electrochemical nature of the cladding takes over. Any further
corrosion spreads laterally, confining itself to the cladding rather than penetrating the core. Cladding is one of the few surface protection methods that
provides effective protection even when a break or flaw exists in the coating.
Barrier coatings. Barrier coatings are of two types. The first consists of a
thin surface layer of a more resistant metal. This can be accomplished by
Fundamentals of Corrosion
For alloys whose tempers are resistant to exfoliation and stress corrosion
cracking, the need for protection depends on two factors:
1. The desire to retain a pleasing appearance
2. Whether the application is fatigue-critical (if so, the depth of pitting
corrosion might cause sufficient stress to initiate a crack)
Structural products about 12 mm or more in thickness that are used for
nonfatigue-critical structural applications will darken and have the surface
stained as a result of weathering. Pitting corrosion will roughen the surface
to a maximum depth of approximately 0.15 mm. However, the loss in residual tensile strength and load-carrying ability will be negligible.
The higher strength alloys such as 5XXX and 6XXX do not require any preventative coating because they have sufficient inherent corrosion resistance.
These alloys will darken and their surface will undergo a slight roughening but they are capable of surviving 20 or more years of exposure without
appreciable degradation.
Until recently, rural atmospheres were considered noncorrosive to aluminum alloys. However, airborne pollution from coal-burning power plants
located upwind have produced an acid rain that falls over a widespread area.
This has resulted in staining and darkening of the aluminum surfaces but
has not resulted in more severe forms of corrosion. Acid rainfalls in rural
areas have caused greater corrosion problems on steel than on aluminum.
4.7.7.1 Protective Measures
Proper design of the product along with the selection of the proper alloy are
the first steps that should be taken toward protection of the finished product.
When necessary, there are various coatings that can be applied.
Anodic coatings. It is possible to apply an anodic coating to aluminum alloy
products. The coating can be a spray coating of zinc, pure aluminum, or a
more anodic aluminum alloy.
Alclad coatings. Alclad coatings consist of a metallurgical bond between two
pure aluminum sheets forming a sandwich on either side of an aluminum
alloy sheet, which is referred to as the core. The cladding process provides a
barrier layer of a more resistant material than the core. Should the barrier layer
become damaged by inadvertent dings or scratches, or accidentally penetrated
by drilling or sawing, or if natural pitting corrosion of the cladding reaches
the core, the electrochemical nature of the cladding takes over. Any further
corrosion spreads laterally, confining itself to the cladding rather than penetrating the core. Cladding is one of the few surface protection methods that
provides effective protection even when a break or flaw exists in the coating.
Barrier coatings. Barrier coatings are of two types. The first consists of a
thin surface layer of a more resistant metal. This can be accomplished by