The Equipment Design Process 357
Environmental changes; inhibitors. In addition to changing the environment
by the use of inhibitors, changes can reduce corrosion by altering the tem-
perature and/or pressure in the process.
The economics of inhibitors can be favorable for a closed loop such as a
cooling circuit for a chemical process. In a closed loop, the concentration
may be 0.1% by weight or less of the inhibitor compared to the concentration
of the major corrodent. Even at this low concentration in a once-through cir-
cuit, inhibitors are generally too expensive to use. Inhibitors such as nitrites,
phosphates, and silicates are used singly and sometimes in combination. It
should be noted that an inhibitor for one material is often an accelerator of
corrosion for another material. An example is amine, which can be relatively
effective on steel and can cause stress corrosion cracking of copper-based
materials because of the ammonia complex.
Many companies specialize in the development and manufacture of inhib-
itors for particular metals in specic environments. These materials lend
themselves to certain production operations.
There is a class of inhibitors that works in the vapor phase, rather than in
the liquid phase, in preventing oxides and rust on common metals. These
vapor-phase inhibitors are generally organic in nature and have a relatively
high volatility compared to the crystalline solid material. Crystals of a vapor-
phase inhibitor can be placed in an open container in a closed vessel, and it
will be found that the interior surface of this vessel will not rust or oxidize
even if water vapor or oxygen enters the vessel. Vapor-phase inhibitors have
been used to maintain standby equipment such as boilers, turbines, and
spare tanks. This type of inhibitor can be applied by fogging a solution con-
taining the crystals into the container, where the solid inhibitor crystallizes
and puts a monomolecular lm on the surface of the container.
Coverings or liners. Protective coatings may prove to be an inexpensive sub-
strate compared to the material selection or the use of an inhibitor. The coat-
ing acts as a nonactive barrier between the environment and the substrate.
The coating must be impervious or almost impervious to the environment.
It must be continuous (free of voids or pinholes) and should be sufciently
durable to withstand the temperature and pressure conditions. There are
many committees of NACE, such as the Technical Practices Committee (T-6),
that have been involved in the special requirements for coatings for atmo-
spheric exposure and linings for immersion coatings. There are various ram-
ications to the application of liquid-applied linings, sheet linings, and brick
membrane linings. When these linings or combinations of linings are found
to be resistant to the environment, checks must be made of the difference in
the expansion coefcient of the lining and the substrate, especially if there
is a temperature gradient or a large change in temperature between the lin-
ing and the substrate. A material with superb corrosion resistance is not the
answer if it cracks because its expansion is greater than that of the vessel in
which it is installed.