and concentration of corrodent, then athin film lining of less than 20 mil can
be used. For general corrosion, this corrosionr ate is not considered severe.
However, if apinholeshould be present through the lining, aconcentration of
the corrosion current density occurs as aresult of the large ratio of cathode
area to anode area. The pitting corrosion rate will rapidly increase above the
20 mpy rate and through-wall penetrationcan occur in months.
When the substrate exhibits acorrosionrate in excess of 10–20 mpy,athick
film coating exceeding 20 mil in thickness is used. These thicknesses are less
susceptible to pinholes.
Thin linings are used for overall corrosion protection as wellascombating
localized corrosion suchaspitting and stress cracking of the substrate. Thin
fluoropolymer coatings are used to protect purity andt op rovide nonstick
surfaces for easy cleaning.
Among the materials available for thin coatings are those based on epoxy
and phenolic resins that are 0.15–0.30 mm (0.006–0.12 in.) thick. They are
either chemically cured or heat baked. Baked phenolic linings are used to
protect railroad tank cars transporting sulfonic acid. Tanks used to store
caustic soda (sodium hydroxide) have ap olyamidec ured epoxy coating.
Thin coatings of sprayeda nd baked FEP,P FA,a nd ETFE are also widely
used.T heya re applied to primed surfaces as sprayed water-borne
suspension or electrostatically charged powders sprayedo nahot surface.
Each coat is baked before the next is applied. Other fluoropolymers can also
be susceptible to delamination in applications where temperatures cycle
frequently betweena mbient and steam. Table4 .3 presents details of these
linings. Fluoropolymer thin coatings can be applied as thick coatings.
When the corrosion rate of the substrate exceeds 10-mpy-thickc oatings
exceeding 2.5 mil (0.025 in.) are recommended. One suchc oating is vinyl
ester reinforced with glass cloth or wovenr oving. Coatings greater than
125 mil( 0.125 in.) thick can be sprayedo rt roweled. Maximum service
temperature is 1708 F( 738 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 manya cidsa nd bases is
plasticized PVC.This has amaximum operating temperature of 1508 F(668 C).
Sprayed and bakede lectrostaticp owderc oatingo ffl uoropolymers,
described undert hin coatings, can also be applied as thick coatings. One
suchcoating is PVDFa nd glass or cotton fabric.
Manufacturers and/orother corrosionengineers should be consulted for
case histories of identical applications. Included in the case history should be
the nameofthe applicator whoapplied the coating, application conditions,
typeofequipment used, degree of application difficulty,and other required
special procedures. Al ining with superior chemical resistance will fail
rapidly if it cannotbeproperly applied, so it is advantageous to learnfrom
the experience of others.
To maximize sales, lining manufacturers formulate their products to meet
as broad ar ange as possible of chemical and solvent environments.
Consequently,at ank coating may be listed as suitable for more than 100
Corrosion of Linings and Coatings
78
CAT8247—CHAPTER 4—17/10/2006—14:33—SRIDHAR—XML MODEL B–pp. 69–174
be used. For general corrosion, this corrosionr ate is not considered severe.
However, if apinholeshould be present through the lining, aconcentration of
the corrosion current density occurs as aresult of the large ratio of cathode
area to anode area. The pitting corrosion rate will rapidly increase above the
20 mpy rate and through-wall penetrationcan occur in months.
When the substrate exhibits acorrosionrate in excess of 10–20 mpy,athick
film coating exceeding 20 mil in thickness is used. These thicknesses are less
susceptible to pinholes.
Thin linings are used for overall corrosion protection as wellascombating
localized corrosion suchaspitting and stress cracking of the substrate. Thin
fluoropolymer coatings are used to protect purity andt op rovide nonstick
surfaces for easy cleaning.
Among the materials available for thin coatings are those based on epoxy
and phenolic resins that are 0.15–0.30 mm (0.006–0.12 in.) thick. They are
either chemically cured or heat baked. Baked phenolic linings are used to
protect railroad tank cars transporting sulfonic acid. Tanks used to store
caustic soda (sodium hydroxide) have ap olyamidec ured epoxy coating.
Thin coatings of sprayeda nd baked FEP,P FA,a nd ETFE are also widely
used.T heya re applied to primed surfaces as sprayed water-borne
suspension or electrostatically charged powders sprayedo nahot surface.
Each coat is baked before the next is applied. Other fluoropolymers can also
be susceptible to delamination in applications where temperatures cycle
frequently betweena mbient and steam. Table4 .3 presents details of these
linings. Fluoropolymer thin coatings can be applied as thick coatings.
When the corrosion rate of the substrate exceeds 10-mpy-thickc oatings
exceeding 2.5 mil (0.025 in.) are recommended. One suchc oating is vinyl
ester reinforced with glass cloth or wovenr oving. Coatings greater than
125 mil( 0.125 in.) thick can be sprayedo rt roweled. Maximum service
temperature is 1708 F( 738 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 manya cidsa nd bases is
plasticized PVC.This has amaximum operating temperature of 1508 F(668 C).
Sprayed and bakede lectrostaticp owderc oatingo ffl uoropolymers,
described undert hin coatings, can also be applied as thick coatings. One
suchcoating is PVDFa nd glass or cotton fabric.
Manufacturers and/orother corrosionengineers should be consulted for
case histories of identical applications. Included in the case history should be
the nameofthe applicator whoapplied the coating, application conditions,
typeofequipment used, degree of application difficulty,and other required
special procedures. Al ining with superior chemical resistance will fail
rapidly if it cannotbeproperly applied, so it is advantageous to learnfrom
the experience of others.
To maximize sales, lining manufacturers formulate their products to meet
as broad ar ange as possible of chemical and solvent environments.
Consequently,at ank coating may be listed as suitable for more than 100
Corrosion of Linings and Coatings
78
CAT8247—CHAPTER 4—17/10/2006—14:33—SRIDHAR—XML MODEL B–pp. 69–174