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Fundamentals of Corrosion
doing, the protective film is removed, which permits corrosion to take place,
primarily in the form of pitting. There is also the possibility that corrosion
will promote cavitation damage by dissolving a matrix phase that holds hard
particles. Once these hard particles are loosened cavitation can take place.
Erosion corrosion. Erosion corrosion is similar to erosion but the mechanics are somewhat different. In erosion corrosion the fluid or moving particle
removes a corrosion product on the ceramic surface. When this occurs the
ceramic forms additional corrosion product, which would otherwise be protective, followed by removal of this protective product by erosion. As the
cycle repeats, localized ceramic loss occurs.
Ordinary erosion is the direct removal of a ceramic or component by
mechanical means with no resulting corrosion.
Galvanic corrosion. The general principles of galvanic corrosion of metals
apply to the galvanic corrosion of ceramics. Basic requirements for galvanic
corrosion to proceed are as follows:
1. There must be two electrically conductive but galvanically different
ceramics in electrical contact with each other.
2. There must be a corrosive electrolyte in contact with both materials.
As with metallic galvanic corrosion, the greater the galvanic difference
between the two ceramics, the greater will be the galvanic effect.
Selective leaching. Ceramics consisting of multiphases are subject to selective leaching. When a phase is significantly less resistant to a specific corrodent, the susceptible phase will be dissolved from the matrix while the other
phases remain unaffected.
Specific ions, rather than entire phases, may also be leached. What ions are
dissolved is specific to the corrosive and the ceramic.
Weatherability, hardness, and other aspects of performance are affected by
the leaching of small phases or specific ions.
Intergranular corrosion. Selective leaching and intergranular corrosion are
related but intergranular corrosion is unique in that it takes place along the
grain boundaries. Corrosion results from impurities and additives that tend
to segregate along the grain boundaries under attack.
Corrosion-assisted cracking. Stress corrosion cracking (SCC) of ceramics is
generally recognized as a type of subcritical crack growth. Relative humidity is an important factor in (SCC) crack growth for many ceramics. Water
generally increases SCC crack growth rates.
SCC occurs as a result of the combined action of internal or external stresses
and an aggressive environment.
Summary. As can be seen from the foregoing, ceramics are susceptible to
the same general forms of corrosion as metals, although in some cases the
mechanisms may be different. Atmospheric corrosion of ceramics can take
Fundamentals of Corrosion
doing, the protective film is removed, which permits corrosion to take place,
primarily in the form of pitting. There is also the possibility that corrosion
will promote cavitation damage by dissolving a matrix phase that holds hard
particles. Once these hard particles are loosened cavitation can take place.
Erosion corrosion. Erosion corrosion is similar to erosion but the mechanics are somewhat different. In erosion corrosion the fluid or moving particle
removes a corrosion product on the ceramic surface. When this occurs the
ceramic forms additional corrosion product, which would otherwise be protective, followed by removal of this protective product by erosion. As the
cycle repeats, localized ceramic loss occurs.
Ordinary erosion is the direct removal of a ceramic or component by
mechanical means with no resulting corrosion.
Galvanic corrosion. The general principles of galvanic corrosion of metals
apply to the galvanic corrosion of ceramics. Basic requirements for galvanic
corrosion to proceed are as follows:
1. There must be two electrically conductive but galvanically different
ceramics in electrical contact with each other.
2. There must be a corrosive electrolyte in contact with both materials.
As with metallic galvanic corrosion, the greater the galvanic difference
between the two ceramics, the greater will be the galvanic effect.
Selective leaching. Ceramics consisting of multiphases are subject to selective leaching. When a phase is significantly less resistant to a specific corrodent, the susceptible phase will be dissolved from the matrix while the other
phases remain unaffected.
Specific ions, rather than entire phases, may also be leached. What ions are
dissolved is specific to the corrosive and the ceramic.
Weatherability, hardness, and other aspects of performance are affected by
the leaching of small phases or specific ions.
Intergranular corrosion. Selective leaching and intergranular corrosion are
related but intergranular corrosion is unique in that it takes place along the
grain boundaries. Corrosion results from impurities and additives that tend
to segregate along the grain boundaries under attack.
Corrosion-assisted cracking. Stress corrosion cracking (SCC) of ceramics is
generally recognized as a type of subcritical crack growth. Relative humidity is an important factor in (SCC) crack growth for many ceramics. Water
generally increases SCC crack growth rates.
SCC occurs as a result of the combined action of internal or external stresses
and an aggressive environment.
Summary. As can be seen from the foregoing, ceramics are susceptible to
the same general forms of corrosion as metals, although in some cases the
mechanisms may be different. Atmospheric corrosion of ceramics can take