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Fundamentals of Corrosion
Thin linings are used for overall corrosion protection as well as for combating localized corrosion such as pitting and stress cracking of the substrate.
Thin fluoropolymer coatings are used to protect product purity and to provide nonstick surfaces for easy cleaning.
Among the materials available for thin coatings are those based on epoxy
and phenolic resins that are 0.15 to 0.30 mm (0.006 to 0.012 in.) thick. They
are either chemically cured or heat baked. Baked phenolic coatings are used
to protect railroad tank cars transporting sulfonic acid. Tanks used to store
caustic soda (sodium hydroxide) have a polyamide cured epoxy coating.
Thin coatings of sprayed and baked FEP, PFA, and ETFE are also widely
used. They are applied to primed surfaces as sprayed water-borne suspensions or electrostatically charged powders sprayed on a hot surface.
Each coat is baked before the next is applied. Other fluoropolymers can also
be applied as thin coatings. These coatings can be susceptible to delamination
in applications where temperatures cycle frequently between ambient and
steam. Fluoropolymer thin coatings can also be applied as thick coatings.
When the corrosion rate of the substrate exceeds 10 mpy, thick coatings
exceeding 25 mil (0.025 in.) are recommended. One such coating is vinyl ester
reinforced with glass cloth or woven roving. Coatings greater than 125 mil
(0.125 in.) thick can be sprayed or troweled. Maximum service temperature
is 170°F (73°C). These coatings can be applied in the field and are used in
service with acids and some organics.
Another thick coating material for service with many acids and bases is plasticized PVC. This has a maximum operating temperature of 150°F (66°C).
Sprayed and baked electrostatic powder coatings of fluoropolymers,
described under thin coatings, can be applied as thick coatings. One such
coating is PVDF and glass or carbon fabric.
Manufacturers and other corrosion engineers should be consulted for case
histories of identical applications. Included in the case history should be the
name of the applicator who applied the coating, application conditions, type
of equipment used, degree of application difficulty, and other special procedures required. A coating with superior chemical resistance will fail rapidly
if it cannot be properly applied, so it is advantageous to learn from the experience of others.
To maximize sales, coating manufacturers formulate their products to
meet as broad a range as possible of chemical and solvent environments.
Consequently, a tank coating may be listed as suitable for in excess of 100
products with varying degrees of compatibility. However, there is a potential for failure if the list is viewed only from the standpoint of the products
approved for service.
If more than one of these materials listed as being compatible with the
coating is to be used, consideration must be given to the sequence of use in
which the chemicals or solvents will be stored or carried in the tank. This
is particularly critical when the cargo is water miscible (e.g., methanol or
cellosolve) and is followed by a water blast. A sequence such as this creates
Fundamentals of Corrosion
Thin linings are used for overall corrosion protection as well as for combating localized corrosion such as pitting and stress cracking of the substrate.
Thin fluoropolymer coatings are used to protect product purity and to provide nonstick surfaces for easy cleaning.
Among the materials available for thin coatings are those based on epoxy
and phenolic resins that are 0.15 to 0.30 mm (0.006 to 0.012 in.) thick. They
are either chemically cured or heat baked. Baked phenolic coatings are used
to protect railroad tank cars transporting sulfonic acid. Tanks used to store
caustic soda (sodium hydroxide) have a polyamide cured epoxy coating.
Thin coatings of sprayed and baked FEP, PFA, and ETFE are also widely
used. They are applied to primed surfaces as sprayed water-borne suspensions or electrostatically charged powders sprayed on a hot surface.
Each coat is baked before the next is applied. Other fluoropolymers can also
be applied as thin coatings. These coatings can be susceptible to delamination
in applications where temperatures cycle frequently between ambient and
steam. Fluoropolymer thin coatings can also be applied as thick coatings.
When the corrosion rate of the substrate exceeds 10 mpy, thick coatings
exceeding 25 mil (0.025 in.) are recommended. One such coating is vinyl ester
reinforced with glass cloth or woven roving. Coatings greater than 125 mil
(0.125 in.) thick can be sprayed or troweled. Maximum service temperature
is 170°F (73°C). These coatings can be applied in the field and are used in
service with acids and some organics.
Another thick coating material for service with many acids and bases is plasticized PVC. This has a maximum operating temperature of 150°F (66°C).
Sprayed and baked electrostatic powder coatings of fluoropolymers,
described under thin coatings, can be applied as thick coatings. One such
coating is PVDF and glass or carbon fabric.
Manufacturers and other corrosion engineers should be consulted for case
histories of identical applications. Included in the case history should be the
name of the applicator who applied the coating, application conditions, type
of equipment used, degree of application difficulty, and other special procedures required. A coating with superior chemical resistance will fail rapidly
if it cannot be properly applied, so it is advantageous to learn from the experience of others.
To maximize sales, coating manufacturers formulate their products to
meet as broad a range as possible of chemical and solvent environments.
Consequently, a tank coating may be listed as suitable for in excess of 100
products with varying degrees of compatibility. However, there is a potential for failure if the list is viewed only from the standpoint of the products
approved for service.
If more than one of these materials listed as being compatible with the
coating is to be used, consideration must be given to the sequence of use in
which the chemicals or solvents will be stored or carried in the tank. This
is particularly critical when the cargo is water miscible (e.g., methanol or
cellosolve) and is followed by a water blast. A sequence such as this creates