CORROSION GUIDE 181108_new table content format ... - Reichhold

Alternate Materials
Phenolic Resins
Phenolic resins have been used for a long time. They
are highly crosslinked resins based on reaction between
phenol and formaldehyde. Advantages include very good
heat resistance as well as low smoke generation due to
ablative or carbonizing properties. The ratio of phenol to
formaldehyde primarily determines the properties. Novolac
resins are based on a defi ciency of formaldehyde and
are supplied as solid powders typically used in reactive
injection molding applications. They are then cured with
hexa methylene tetramine, which provides a formaldehyde
source. Resoles, on the other hand, are made with an
excess of formaldehyde and are normally supplied as
low viscosity liquids dissolved in water. They are normally
cured by application of heat and catalysis by an acid.
Composite applications employ the resole versions. A big
disadvantage to resole resins is the out-gassing of water
vapor which occurs during the cure. This leads to porosity
and voids as well as odor problems during processing.
These voids detract from composite properties including
corrosion resistance. Glass wet-out is another problem.
Quite often glass reinforcement commonly used in the
composites industry is not compatible with phenolic resin.
Since resoles are water soluble, corrosion resistance to
water or aqueous based solutions can be very poor if the
cure is not conducted properly. Care should also be taken
to avoid contact of phenolic composites with carbon steel
in the fi nal application. Over time, the acid catalyst can
leach out and severely corrode the steel.
Acid Resistant Brick and Refractories
Both castable and mortar block chemically resistant
refractories have been used extensively. A good example
is in chimney construction, to withstand sulfuric acid
dew point corrosion. Usually steel is used for structural
support along with appropriate buckstays. Installation
costs can be high. Castable products must be anchored
to the steel structure by studs or Y-anchors. Refractories
are not ductile and concerns involve thermal cycling and
cracking. Block must be skillfully placed with proper acid
resistant mortar. High weight is a factor as well as seismic
considerations. The biggest problems involve operation
of wet stacks in conjunction with fl ue gas desulfurization.
Moisture leads to absorption and swelling, which may
eventually induce leaning. It is also common practice with
wet stacks to employ pressurized membranes to prevent
condensation onto the cold external steel surface. This
also can be expensive.
Rubber and Elastomers
Rubber often displays good chemical resistance, especially
to sulfuric acid. It is sometimes used in FGD applications for
lining of steel piping and process equipment. Rubber liners
have also been used in various bleaching applications.
Apart from corrosion resistance, rubber can offer good
abrasion resistance.
In the case of rubber linings, skilled and specialized
installation is required, which tends to make them
expensive. Many of the linings are diffi cult, if not impossible,
to install around restrictive geometry. It is essential to
obtain good bonding between the rubber and steel since
any permeation or damage to the liner can cause the steel
to quickly corrode. The low glass transition temperature
of rubber restricts use to moderate temperatures. Some
rubbers and elastomers can become embrittled if subjected
to cyclic wet and dry conditions. Solvents present swelling
problems, and water permeation can also be an important
consideration.
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