116
Fundamentals of Corrosion
Alloys containing 15% or less zinc resist de-alloying and are generally more
resistant to stress corrosion cracking than the high-zinc alloys. Red brass is
a typical alloy of this group; it contains 15% zinc. Dissolved air, oxidizing
materials, and compounds that form copper complexes (e.g., ammonia) are
corrosive to the low-zinc brasses.
4.7.6.2 Bronzes
Copper-tin alloys are known as tin bronzes or phosphor bronzes. Phosphorus
is always present in small amounts even though tin is the principal alloying
ingredient. Phosphorus is used as an oxidizer and generally has a concentration of less than 0.5%
These alloys date back to the Bronze Age and probably are the oldest alloys
known. Many of the artifacts produced during the Bronze Age are still in
existence. Items such as bells, statues, vases, and swords have survived hundreds of years of exposure to a wide variety of environments, proving the
corrosion resistance of these materials.
Copper-aluminum alloys are known as aluminum bronzes. They are complex alloys containing iron, nickel, silicon, manganese, and other elements
as well as aluminum.
As with copper, the aluminum bronzes find many architectural applications. A special aluminum bronze alloy designated C-61500 has been specifically developed for architectural use. It is reported to have a film resistance
20 times that of copper C-11000. These are suitable for marine atmospheres.
4.7.6.3 Copper-Nickel Alloys
The copper-nickel alloys are referred to as cupronickels and have nickel as
the principal alloying ingredient. Of all the copper alloys, the cupronickels
are the most resistant to stress corrosion cracking in ammonia and ammoniacal environments. These alloys do not find many applications to resist
atmospheric corrosion because of their cost. They would be most suitable for
marine atmospheres.
4.7.7 aluminum alloys
The resistance to atmospheric corrosion of aluminum is due to the rapid formation of a thin, compact, adherent oxide film over the surface that limits corrosion. This surface film, when formed in air, is approximately 10 to 20 mm
thick. It is thicker when formed in the presence of water or water vapor. This
oxide film is stable (insoluble) in the pH range of approximately 4 to 9, which
includes many atmospheric environments.
Pourbaix potential-pH diagrams can be used to predict the regions of
oxide stability and of uniform corrosion in atmospheric types, as shown in
Figure 4.3. Note that aluminum is thermodynamically stable only at low
Fundamentals of Corrosion
Alloys containing 15% or less zinc resist de-alloying and are generally more
resistant to stress corrosion cracking than the high-zinc alloys. Red brass is
a typical alloy of this group; it contains 15% zinc. Dissolved air, oxidizing
materials, and compounds that form copper complexes (e.g., ammonia) are
corrosive to the low-zinc brasses.
4.7.6.2 Bronzes
Copper-tin alloys are known as tin bronzes or phosphor bronzes. Phosphorus
is always present in small amounts even though tin is the principal alloying
ingredient. Phosphorus is used as an oxidizer and generally has a concentration of less than 0.5%
These alloys date back to the Bronze Age and probably are the oldest alloys
known. Many of the artifacts produced during the Bronze Age are still in
existence. Items such as bells, statues, vases, and swords have survived hundreds of years of exposure to a wide variety of environments, proving the
corrosion resistance of these materials.
Copper-aluminum alloys are known as aluminum bronzes. They are complex alloys containing iron, nickel, silicon, manganese, and other elements
as well as aluminum.
As with copper, the aluminum bronzes find many architectural applications. A special aluminum bronze alloy designated C-61500 has been specifically developed for architectural use. It is reported to have a film resistance
20 times that of copper C-11000. These are suitable for marine atmospheres.
4.7.6.3 Copper-Nickel Alloys
The copper-nickel alloys are referred to as cupronickels and have nickel as
the principal alloying ingredient. Of all the copper alloys, the cupronickels
are the most resistant to stress corrosion cracking in ammonia and ammoniacal environments. These alloys do not find many applications to resist
atmospheric corrosion because of their cost. They would be most suitable for
marine atmospheres.
4.7.7 aluminum alloys
The resistance to atmospheric corrosion of aluminum is due to the rapid formation of a thin, compact, adherent oxide film over the surface that limits corrosion. This surface film, when formed in air, is approximately 10 to 20 mm
thick. It is thicker when formed in the presence of water or water vapor. This
oxide film is stable (insoluble) in the pH range of approximately 4 to 9, which
includes many atmospheric environments.
Pourbaix potential-pH diagrams can be used to predict the regions of
oxide stability and of uniform corrosion in atmospheric types, as shown in
Figure 4.3. Note that aluminum is thermodynamically stable only at low