32
Fundamentals of Corrosion
It is common practice for the prevention of uniform corrosion to make use
of such measures as the application of protective coatings or linings, the use
of inhibitors, and the use of cathodic or anodic protection.
3.2 Intergranular Corrosion
This is a specialized type of attack that takes place at the grain boundaries
of a metal. Little or no attack is observed on the main body of the grain. The
grain boundary material, which is a limited area, acts as an anode, and the
larger area of grains acts as cathodes. This results in the flow of energy from
the small anode area to the large cathode area, which causes rapid attack
penetrating deeply into the metal.
The grain boundary region is an area of crystallographic mismatch between
the orderly structures within the adjacent grains. Because of this, it is slightly
chemically more active than the grain area. Under certain conditions, the
grain boundaries remain very reactive; and under corrosive conditions, the
attack along the grain boundaries results in intergranular corrosion (IGC).
The following factors contribute to the increased reactivity of the grain
boundary areas:
1. Segregation of specific elements or compounds, as in aluminum
alloys or nickel-chromium alloys
2. Enrichment of one of the alloying elements at the grain boundary,
as in brass
3. Depletion of the corrosion-resistant element at the grain boundary,
as in stainless steels
All the factors that lead to intergranular corrosion are the result of the
thermal exposure of the metals, such as in welding, stress relief, and other
heat treatments.
3.2.1 austenitic Stainless Steels
When austenitic stainless steels are heated or cooled through the temperature range of about 800 to 1650°F (427 to 899°C), the chromium along the
grain boundaries tends to combine with carbon to form chromium carbides.
Called sensitization, or carbide precipitation, the effect is a depletion of chromium and the lowering of corrosion resistance in the areas adjacent to the
grain boundary. This is a time–temperature phenomenon, as in Figure 3.1.
Slow cooling from annealing temperature, stress relieving in the sensitization range, or welding may cause carbide precipitation. The sensitization of
Fundamentals of Corrosion
It is common practice for the prevention of uniform corrosion to make use
of such measures as the application of protective coatings or linings, the use
of inhibitors, and the use of cathodic or anodic protection.
3.2 Intergranular Corrosion
This is a specialized type of attack that takes place at the grain boundaries
of a metal. Little or no attack is observed on the main body of the grain. The
grain boundary material, which is a limited area, acts as an anode, and the
larger area of grains acts as cathodes. This results in the flow of energy from
the small anode area to the large cathode area, which causes rapid attack
penetrating deeply into the metal.
The grain boundary region is an area of crystallographic mismatch between
the orderly structures within the adjacent grains. Because of this, it is slightly
chemically more active than the grain area. Under certain conditions, the
grain boundaries remain very reactive; and under corrosive conditions, the
attack along the grain boundaries results in intergranular corrosion (IGC).
The following factors contribute to the increased reactivity of the grain
boundary areas:
1. Segregation of specific elements or compounds, as in aluminum
alloys or nickel-chromium alloys
2. Enrichment of one of the alloying elements at the grain boundary,
as in brass
3. Depletion of the corrosion-resistant element at the grain boundary,
as in stainless steels
All the factors that lead to intergranular corrosion are the result of the
thermal exposure of the metals, such as in welding, stress relief, and other
heat treatments.
3.2.1 austenitic Stainless Steels
When austenitic stainless steels are heated or cooled through the temperature range of about 800 to 1650°F (427 to 899°C), the chromium along the
grain boundaries tends to combine with carbon to form chromium carbides.
Called sensitization, or carbide precipitation, the effect is a depletion of chromium and the lowering of corrosion resistance in the areas adjacent to the
grain boundary. This is a time–temperature phenomenon, as in Figure 3.1.
Slow cooling from annealing temperature, stress relieving in the sensitization range, or welding may cause carbide precipitation. The sensitization of