The magnitude of anyofthe effects willbeafunction of the combination
of polymerand permeant in actual service.
6.3.2A bsorption
Polymers have the potential to absorb varying amounts of corrodents that
they come into contactw ith, particularly organic liquids.T his can result in
swelling, cracking, and penetrationt ot he substrate. Swelling can cause
softening of the polymer,i ntroduce highs tresses, and precipitate failure of
the bond. If the polymerh as ah igh absorption rate, permeation will
probably take place. An approximation of the expected permeationand/or
absorption of apolymer can be based on the absorption of water.This data is
usually available. Table 6.1 provides the water absorption rates for the more
commonpolymers used for linings.
The failure because of absorption can best be understood by considering
the “steam cycle”t est described in the ASTM standards for lined pipe.
Asection of lined pipe is subjected to thermal and pressurefluctuations. This
is repeated for 100 cycles. The steam creates at emperature and pressure
gradient through the liner,c ausing absorption of as mallq uantity of steam
that condenses to water within the inner wall.Uponpressurerelease, or on
reintroduction of steam, the entrapped water can expand to vapor causing
an original micropore. The repeated pressurea nd thermal cycling enlarges
the micropores, ultimately producing visible water-filled blisters within
the liner.
In an actual process, the polymer may absorb process fluids, and repeated
temperature or pressurecycling can cause blisters. Eventually,the corrodent
may find its way to the substrate.
Related effects can occur when process chemicals are absorbedt hat may
later react, decompose, or solidify within the structure of the plastic.
Prolonged retention of the chemicals may lead to theirdecomposition within
TABLE 6.1
Water Absorption Rates for Common Polymers
Polymer
Water Absorption 24 hat73 8 F/238 C(%)
PVC
0.05
CPVC
0.03
PP (Homo)
0.02
PP (Co)
0.03
EHMW PE
! 0.01
ECTFE
! 0.1
PVDF
! 0.04
PEA
! 0.03
ETFE
0.029
PTFE
! 0.01
FEP
! 0.01
Corrosion of Linings and Coatings
292
CAT8247—CHAPTER 6—6/10/2006—12:28—SRIDHAR—XML MODEL B–pp. 287–402
of polymerand permeant in actual service.
6.3.2A bsorption
Polymers have the potential to absorb varying amounts of corrodents that
they come into contactw ith, particularly organic liquids.T his can result in
swelling, cracking, and penetrationt ot he substrate. Swelling can cause
softening of the polymer,i ntroduce highs tresses, and precipitate failure of
the bond. If the polymerh as ah igh absorption rate, permeation will
probably take place. An approximation of the expected permeationand/or
absorption of apolymer can be based on the absorption of water.This data is
usually available. Table 6.1 provides the water absorption rates for the more
commonpolymers used for linings.
The failure because of absorption can best be understood by considering
the “steam cycle”t est described in the ASTM standards for lined pipe.
Asection of lined pipe is subjected to thermal and pressurefluctuations. This
is repeated for 100 cycles. The steam creates at emperature and pressure
gradient through the liner,c ausing absorption of as mallq uantity of steam
that condenses to water within the inner wall.Uponpressurerelease, or on
reintroduction of steam, the entrapped water can expand to vapor causing
an original micropore. The repeated pressurea nd thermal cycling enlarges
the micropores, ultimately producing visible water-filled blisters within
the liner.
In an actual process, the polymer may absorb process fluids, and repeated
temperature or pressurecycling can cause blisters. Eventually,the corrodent
may find its way to the substrate.
Related effects can occur when process chemicals are absorbedt hat may
later react, decompose, or solidify within the structure of the plastic.
Prolonged retention of the chemicals may lead to theirdecomposition within
TABLE 6.1
Water Absorption Rates for Common Polymers
Polymer
Water Absorption 24 hat73 8 F/238 C(%)
PVC
0.05
CPVC
0.03
PP (Homo)
0.02
PP (Co)
0.03
EHMW PE
! 0.01
ECTFE
! 0.1
PVDF
! 0.04
PEA
! 0.03
ETFE
0.029
PTFE
! 0.01
FEP
! 0.01
Corrosion of Linings and Coatings
292
CAT8247—CHAPTER 6—6/10/2006—12:28—SRIDHAR—XML MODEL B–pp. 287–402