Atmospheric Corrosion 143
For corrosion to occur without input of energy from outside the reaction, it
must be thermodynamically and kinetically possible. It is possible to calcu-
late whether a specic corrosion reaction can occur spontaneously, providing
sufcient thermodynamic data are available. If on the basis of the available
thermodynamic data it is determined that a given corrosion reaction cannot
take place, then it is reasonable to assume that the particular corrosion reac-
tion is not a practical problem. If the thermodynamic data predict that a
given corrosion reaction can occur, it still may not be a practical problem if
the reaction is kinetically limited.
The primary problem in thermodynamically predicting corrosion lies in the
ability to nd adequate and complete thermodynamic data and considering all
the possible corrosion reactions that could be important for complex ceramic
compositions and structures, the use of thermodynamic data must be care-
fully applied and full consideration given to the impact of kinetic limitations.
4.8.4.1 Forms of Corrosion
Ceramics are subject to many of the same forms of corrosion as metals. However,
there are some important differences in the corrosion mechanisms.
Uniform corrosion. For uniform corrosion to take place in a metal, a galvanic
cell must be formed. This is not possible in a ceramic because ceramics are
insulators and no conductor is present — a requirement for a galvanic cell.
Uniform corrosion in a ceramic takes place as a result of chemical disso-
lution. The corrosion product formed is nonprotective, being either poorly
adherent, soluble, or a good transport medium for the corrodent to the ceramic
face. This is a predictable form of corrosion based on test data or experience.
Crevice corrosion. The mechanism of crevice corrosion for ceramics is simi-
lar to that of metals. Crevice corrosion develops in an occluded area of a
corroding material. Once corrosion has started within the crevice, corrosion
rates within the occluded area will increase with time as a result of either
(1) depletion of a passivating component within the crevice, or (2) increasing
acidity within the crevice, or (3) a combination of both.
However, there is a difference in the mechanism for ceramics vs. metals.
Increasing activity within the crevice area of a ceramic occurs only if the
ceramic is electrically conductive.
Pitting. Pitting is a form of highly localized corrosion. Pits are the result of
damage to a protective lm. The damage might be the result of a corrosive
attack of the protective lm, or the result of mechanical abrasion of the lm,
or from inclusions or pockets of a susceptible phase. In the latter cases, attack
takes place at the damaged site, which causes a hole to develop in the surface.
Pitting can be detrimental to the function of the ceramic component since it
may decrease the effective strength of the ceramic.
Cavitation. Cavitation, per se, is not a form of corrosion; however, it is closely
related. Cavitation is mechanical in nature. It is the mechanical removal of
material resulting from the implosion of vapor bubbles in the liquid. In so