Atmospheric Corrosion
105
to prevent detrimental staining, effects such as coloring the masonry brown,
so that any staining will not be obvious. The ground area exposed to staining
can be covered with an easily exchangeable material, such as gravel.
The corrosion mechanism for weathering steels is similar to that of unalloyed carbon steels. The rust forms a more dense and compact layer on the
weathering steels than on unalloyed carbon steels. The rust layer more effectively screens the steel surface from the corrosive environments of the atmosphere. The corrosion process may be affected in several ways by this rust
layer. The cathodic reaction may be affected by the low diffusion rate of oxygen, whereas the anodic reaction may be retarded by limiting the supply of
water and corrosion-stimulating ions that can reach the surface of the steel.
In addition, the increased electrolyte resistance may also decrease the corrosion rate.
Each of the alloying ingredients reacts in a different manner to improve
the resistance of the weathering steels to atmospheric corrosion.
Copper has the most pronounced effect of any of the individual elements
in decreasing the corrosion rate. An increase in the copper content from 0.01
to 0.4% decreases the corrosion rate by up to 70%. Only a slight improvement in the corrosion resistance results from an increase in copper in the
range of 0.2 to 0.5%. Several theories have been proposed regarding the
mechanism by which copper improves the corrosion resistance. One theory
is that the beneficial effect is due to the formation of a surface coating of
metallic copper that acts as either protection in itself or promotes anodic
passivation by supporting the cathodic reaction. Another theory is that copper ions dissolved from the base metal are able to precipitate sulfide ions
originating either from sulfide inclusions in the steel or from atmospheric
pollution, and thus eliminate their detrimental effect. The most probable
theory is that copper forms basic sulfates with low solubility, which precipitate within the pores of the rust layer, thereby decreasing their porosity.
Weathering steels usually contain 0.2 to 0.5% copper.
When added in combination with copper, chromium and nickel further
increase the resistance of weathering steels. Chromium is usually added in a
content of 0.4 to 1%, whereas nickel is usually added up to 0.065%. Chromium
appears to be more effective than nickel. The mechanical properties of the
steel are improved by both elements.
Chromium is enriched in the inner rust layer together with copper and
phosphorus. They promote the formation of a dense layer of amorphous
FeOOH next to the steel surface. This layer acts as a barrier to the transport
of water, oxygen, and pollutants. Nickel is supposed to act by forming insoluble basic sulfates in pores of the rust layer.
Phosphorous also helps to improve the corrosion resistance of weathering steels. By increasing the phosphorous content from less than 0.01% to
0.1%, a 20 to 30% improvement in the corrosion resistance of copper-bearing steels is realized. Phosphorus may form layers of insoluble phosphates
in the rust, acting as transportation barriers in the same manner as the
105
to prevent detrimental staining, effects such as coloring the masonry brown,
so that any staining will not be obvious. The ground area exposed to staining
can be covered with an easily exchangeable material, such as gravel.
The corrosion mechanism for weathering steels is similar to that of unalloyed carbon steels. The rust forms a more dense and compact layer on the
weathering steels than on unalloyed carbon steels. The rust layer more effectively screens the steel surface from the corrosive environments of the atmosphere. The corrosion process may be affected in several ways by this rust
layer. The cathodic reaction may be affected by the low diffusion rate of oxygen, whereas the anodic reaction may be retarded by limiting the supply of
water and corrosion-stimulating ions that can reach the surface of the steel.
In addition, the increased electrolyte resistance may also decrease the corrosion rate.
Each of the alloying ingredients reacts in a different manner to improve
the resistance of the weathering steels to atmospheric corrosion.
Copper has the most pronounced effect of any of the individual elements
in decreasing the corrosion rate. An increase in the copper content from 0.01
to 0.4% decreases the corrosion rate by up to 70%. Only a slight improvement in the corrosion resistance results from an increase in copper in the
range of 0.2 to 0.5%. Several theories have been proposed regarding the
mechanism by which copper improves the corrosion resistance. One theory
is that the beneficial effect is due to the formation of a surface coating of
metallic copper that acts as either protection in itself or promotes anodic
passivation by supporting the cathodic reaction. Another theory is that copper ions dissolved from the base metal are able to precipitate sulfide ions
originating either from sulfide inclusions in the steel or from atmospheric
pollution, and thus eliminate their detrimental effect. The most probable
theory is that copper forms basic sulfates with low solubility, which precipitate within the pores of the rust layer, thereby decreasing their porosity.
Weathering steels usually contain 0.2 to 0.5% copper.
When added in combination with copper, chromium and nickel further
increase the resistance of weathering steels. Chromium is usually added in a
content of 0.4 to 1%, whereas nickel is usually added up to 0.065%. Chromium
appears to be more effective than nickel. The mechanical properties of the
steel are improved by both elements.
Chromium is enriched in the inner rust layer together with copper and
phosphorus. They promote the formation of a dense layer of amorphous
FeOOH next to the steel surface. This layer acts as a barrier to the transport
of water, oxygen, and pollutants. Nickel is supposed to act by forming insoluble basic sulfates in pores of the rust layer.
Phosphorous also helps to improve the corrosion resistance of weathering steels. By increasing the phosphorous content from less than 0.01% to
0.1%, a 20 to 30% improvement in the corrosion resistance of copper-bearing steels is realized. Phosphorus may form layers of insoluble phosphates
in the rust, acting as transportation barriers in the same manner as the