122
Fundamentals of Corrosion
The shiny metal appearance will gradually disappear and the surface will
roughen under the formation of a gray patina of corrosion products. If soot is
present in the atmosphere, it will be absorbed by the corrosion products and
give the patina a dark color. If the surface has been anodized, the shiny metal
appearance will be retained. Except in highly polluted atmospheres, the low
penetration rate and shallow pitting do not usually affect the mechanical
properties of the aluminum.
Dust and other pollutants may collect in rain-sheltered positions and
accelerate corrosion by disturbing the formation of a protective oxide coating. These rain-sheltered areas will often show staining as a result of the
corrosion. When possible, these rain-sheltered areas should be eliminated
during the design stage.
Aluminum and its alloys are also suitable for use in marine atmospheres.
Care must be taken in the design because under these conditions, the aluminum is susceptible to bimetallic corrosion when in contact with copper
or carbon steel. If the chloride concentration is high, the aluminum may be
susceptible to crevice corrosion.
4.7.8 Stainless Steel
Stainless steel is probably the most widely known material that is used for
corrosion resistance. However, stainless steel is not a singular material as the
name might imply, but rather a broad group of alloys each of which exhibits
its own physical and corrosion-resistant properties.
Stainless steels are alloys of iron to which a minimum of 11% chromium
has been added to provide a passive film to resist rusting when the material
is exposed to the weather. The film is self-forming and self-healing in environments where the stainless steel is resistant. Improved corrosion resistance
is achieved as more chromium is added to the alloy. Consequently, there are
stainless steels with chromium contents of 15%, 17%, 20%, and higher. Other
ingredients are added to further improve corrosion resistance and mechanical strength.
As a result of these alloying possibilities, more than 70 stainless steels
are available. They can be divided into four major categories depending on
their microstructure:
1. Austenitic
2. Ferritic
3. Martensitic
4. Duplex
Resistance to atmospheric corrosion was one of the earliest characteristics of the chromium-bearing alloys. When compared to iron and steel, their
resistance prompted the name “stainless.”
Fundamentals of Corrosion
The shiny metal appearance will gradually disappear and the surface will
roughen under the formation of a gray patina of corrosion products. If soot is
present in the atmosphere, it will be absorbed by the corrosion products and
give the patina a dark color. If the surface has been anodized, the shiny metal
appearance will be retained. Except in highly polluted atmospheres, the low
penetration rate and shallow pitting do not usually affect the mechanical
properties of the aluminum.
Dust and other pollutants may collect in rain-sheltered positions and
accelerate corrosion by disturbing the formation of a protective oxide coating. These rain-sheltered areas will often show staining as a result of the
corrosion. When possible, these rain-sheltered areas should be eliminated
during the design stage.
Aluminum and its alloys are also suitable for use in marine atmospheres.
Care must be taken in the design because under these conditions, the aluminum is susceptible to bimetallic corrosion when in contact with copper
or carbon steel. If the chloride concentration is high, the aluminum may be
susceptible to crevice corrosion.
4.7.8 Stainless Steel
Stainless steel is probably the most widely known material that is used for
corrosion resistance. However, stainless steel is not a singular material as the
name might imply, but rather a broad group of alloys each of which exhibits
its own physical and corrosion-resistant properties.
Stainless steels are alloys of iron to which a minimum of 11% chromium
has been added to provide a passive film to resist rusting when the material
is exposed to the weather. The film is self-forming and self-healing in environments where the stainless steel is resistant. Improved corrosion resistance
is achieved as more chromium is added to the alloy. Consequently, there are
stainless steels with chromium contents of 15%, 17%, 20%, and higher. Other
ingredients are added to further improve corrosion resistance and mechanical strength.
As a result of these alloying possibilities, more than 70 stainless steels
are available. They can be divided into four major categories depending on
their microstructure:
1. Austenitic
2. Ferritic
3. Martensitic
4. Duplex
Resistance to atmospheric corrosion was one of the earliest characteristics of the chromium-bearing alloys. When compared to iron and steel, their
resistance prompted the name “stainless.”