Forms of Metallic Corrosion
49
be taken so as not to induce a stress as a result of the installation. The design
should also avoid stagnant areas that could lead to pitting and the initiation
of stress concentration sites.
Operations such as cold-forming, rolling, deep drawing, thermal processing like welding, solidification of casting with large differences in section
thickness, or severe quenching operations are all capable of producing residual stresses of as high as 70% of the yield strength. Design stress raisers may
aggravate the situation. These stress raisers may be notches related to the
design, inclusions, welding strikes, rough machining marks, etc. They may
also result from localized corrosion attack such as pitting, intergranular corrosion, or selective leaching.
Thermal stresses of very high magnitude may be caused by exposing the
metal parts to high and low temperatures, which results in nonuniform heating rates and sharp thermal gradients. This is often found in heat exchangers.
When the environment on the cold side of the heat exchanger is conducive to
stress corrosion, cracking will be produced by these thermal stresses.
Corrosion products that accumulate in fissures or tightly joined parts may
produce a wedging action with sufficiently high stress to cause SCC.
Because stress corrosion environments usually are relatively mildly corrosive from the viewpoint of general corrosion, the surface of a stress-corroded
component will exhibit only faint signs of corrosion, while fine cracks penetrate deeply into the part. The cracks proceed in a direction perpendicular
to the stresses and can be either transgranular or intergranular.
One type of cracking is usually more prevalent in a specific alloy. For
example, austenitic stainless steels crack transgranularly in boiling magnesium chloride solution while carbon steels crack intergranularly in nitrate
solutions.
Depending on the environment, cold work, metallurgical conditions, or
alloy composition, it is possible to have a transition from one type of cracking to the other.
Stress corrosion cracks have the appearance of a brittle mechanical fracture, which is readily seen in a scanning electron microscope, whether transgranular or intergranular.
3.7.1 Preventive Measures
The most effective measure is to remove or prevent the application of tensile
stresses. By proper design and installation techniques, many tensile stresses
can be avoided.
When tensile stresses are present as a result of manufacturing/fabrication procedures, these stresses should be removed. Removal can be accomplished by annealing or by shot peening. When annealing is not practical, as
in the case of some stainless steels that sensitize and become susceptible to
intergranular attack, shot peening may be employed. However, shot peening
49
be taken so as not to induce a stress as a result of the installation. The design
should also avoid stagnant areas that could lead to pitting and the initiation
of stress concentration sites.
Operations such as cold-forming, rolling, deep drawing, thermal processing like welding, solidification of casting with large differences in section
thickness, or severe quenching operations are all capable of producing residual stresses of as high as 70% of the yield strength. Design stress raisers may
aggravate the situation. These stress raisers may be notches related to the
design, inclusions, welding strikes, rough machining marks, etc. They may
also result from localized corrosion attack such as pitting, intergranular corrosion, or selective leaching.
Thermal stresses of very high magnitude may be caused by exposing the
metal parts to high and low temperatures, which results in nonuniform heating rates and sharp thermal gradients. This is often found in heat exchangers.
When the environment on the cold side of the heat exchanger is conducive to
stress corrosion, cracking will be produced by these thermal stresses.
Corrosion products that accumulate in fissures or tightly joined parts may
produce a wedging action with sufficiently high stress to cause SCC.
Because stress corrosion environments usually are relatively mildly corrosive from the viewpoint of general corrosion, the surface of a stress-corroded
component will exhibit only faint signs of corrosion, while fine cracks penetrate deeply into the part. The cracks proceed in a direction perpendicular
to the stresses and can be either transgranular or intergranular.
One type of cracking is usually more prevalent in a specific alloy. For
example, austenitic stainless steels crack transgranularly in boiling magnesium chloride solution while carbon steels crack intergranularly in nitrate
solutions.
Depending on the environment, cold work, metallurgical conditions, or
alloy composition, it is possible to have a transition from one type of cracking to the other.
Stress corrosion cracks have the appearance of a brittle mechanical fracture, which is readily seen in a scanning electron microscope, whether transgranular or intergranular.
3.7.1 Preventive Measures
The most effective measure is to remove or prevent the application of tensile
stresses. By proper design and installation techniques, many tensile stresses
can be avoided.
When tensile stresses are present as a result of manufacturing/fabrication procedures, these stresses should be removed. Removal can be accomplished by annealing or by shot peening. When annealing is not practical, as
in the case of some stainless steels that sensitize and become susceptible to
intergranular attack, shot peening may be employed. However, shot peening