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Fundamentals of Corrosion
Cavitation damage can be minimized by the proper choice of materials.
Titanium alloys, austenitic stainless steels, nickel-chromium and nickelchromium-molybdenum alloys can be used in extremely severe conditions.
Hard facing of the surface with a resistant alloy is also beneficial. Cavitation
can also be combated by applying a resilient coating of rubber or other resilient elastomeric material to the surface. The softer coating reflects the shock
waves and reduces the damage on metallic parts.
3.7 Stress Corrosion Cracking (SCC)
SCC is defined as the delayed failure of alloys by cracking when exposed to
certain environments in the presence of static tensile stress. The importance
of a conjoint action of corrosion and stress is reflected in the definition; an
alternate application of stress and corrosive environment will not produce
SCC. The stress level at which failure occurs is well below the stress required
for a mechanical failure in the absence of corrosion. The minimum stress
below which SCC will occur is called the “threshold stress,” but this may
be as low as 10% of the yield stress in some systems. Corrosion alone in the
absence of stress does not cause SCC.
SCC occurs at points of stress. Usually, the metal or alloy is free of corrosion over most of its surface, yet fine cracks penetrate through the surface at the points of stress. Depending on the alloy system and corrodent
combination, the cracking can be intergranular or transgranular. The rate of
propagation can vary greatly and is affected by stress levels, temperature,
and concentration of the corrodent. This type of attack takes place in certain
media. All metals are potentially subject to SCC. The conditions necessary
for stress corrosion are:
1. Suitable environment
2. Tensile stress
3. Sensitive metal
4. Appropriate temperature and pH values
The first report of SCC took place in the beginning of the twentieth century. It was reported as “season cracking” of brass cartridge cases in ammonia-bearing atmospheres. Another classic example of SCC reported in the
early part of the twentieth century was that of “caustic embrittlement” of
riveted boiler plates in the early steam-driven locomotives. In this case, the
cause of SCC was the residual stress developed during the riveting operations and the presence of concentrated sodium hydroxide in the areas of the
residual stress.
Fundamentals of Corrosion
Cavitation damage can be minimized by the proper choice of materials.
Titanium alloys, austenitic stainless steels, nickel-chromium and nickelchromium-molybdenum alloys can be used in extremely severe conditions.
Hard facing of the surface with a resistant alloy is also beneficial. Cavitation
can also be combated by applying a resilient coating of rubber or other resilient elastomeric material to the surface. The softer coating reflects the shock
waves and reduces the damage on metallic parts.
3.7 Stress Corrosion Cracking (SCC)
SCC is defined as the delayed failure of alloys by cracking when exposed to
certain environments in the presence of static tensile stress. The importance
of a conjoint action of corrosion and stress is reflected in the definition; an
alternate application of stress and corrosive environment will not produce
SCC. The stress level at which failure occurs is well below the stress required
for a mechanical failure in the absence of corrosion. The minimum stress
below which SCC will occur is called the “threshold stress,” but this may
be as low as 10% of the yield stress in some systems. Corrosion alone in the
absence of stress does not cause SCC.
SCC occurs at points of stress. Usually, the metal or alloy is free of corrosion over most of its surface, yet fine cracks penetrate through the surface at the points of stress. Depending on the alloy system and corrodent
combination, the cracking can be intergranular or transgranular. The rate of
propagation can vary greatly and is affected by stress levels, temperature,
and concentration of the corrodent. This type of attack takes place in certain
media. All metals are potentially subject to SCC. The conditions necessary
for stress corrosion are:
1. Suitable environment
2. Tensile stress
3. Sensitive metal
4. Appropriate temperature and pH values
The first report of SCC took place in the beginning of the twentieth century. It was reported as “season cracking” of brass cartridge cases in ammonia-bearing atmospheres. Another classic example of SCC reported in the
early part of the twentieth century was that of “caustic embrittlement” of
riveted boiler plates in the early steam-driven locomotives. In this case, the
cause of SCC was the residual stress developed during the riveting operations and the presence of concentrated sodium hydroxide in the areas of the
residual stress.