With the answers to these questions, an appropriate selection of liner and
shell can be made.
6.3D esignC onsiderations
In addition to selectingaliningm aterial that is resistant to the corrodent
beingh andled, there are three other factors to be consideredi nt he design:
permeation, absorption, and environmental stress cracking.Permeation and
absorption can cause
1. Bondf ailure and blistering, resultingf rom the accumulation
of fluids at the bondw hent he substrate is less permeable than
the liner or from corrosion/reaction products if the substrate is
attacked by the permeant.
2. Failure of the substrate from corrosive attack.
3. Loss of contents through the substrate and liner as ar esulto f
the eventual failure of the substrate. In unbondedl inings, it is
importantthat the space between the liner and substrate be vented
to the atmosphere, not only to allow minute quantities of permeant
vapor to escape but also to prevent expansion of entrapped air
from collapsing the liner.
6.3.1P ermeation
All materials are somewhat permeable to chemical molecules,b ut plastic
materials tend to be an order of magnitude greater in theirp ermeability
rates than metals.P olymers can be permeated by gases, vapors,o rl iquids.
Permeationi ss trictly ap hysical phenomenon;t here is no chemical attack
on the polymer. It is am olecular migration eithert hrough microvoids
in the polymer (if the structurei sm ore or less porous) or between
polymer molecules.
Permeation is af unction of two variables: one relating to diffusion
between molecular chainsand the other to the solubility of the permeant in
the polymer.The driving forces of diffusion are the concentration gradients
in liquidsand the partial pressuregradient for gases. Solubility is afunction
of the affinity of the permeant for the polymer.
Material passingt hrough cracksa nd voids is not related to permeation.
These are two distinct happenings.Theya re notrelated in any way.
Permeation is affected by the following factors:
1. Temperature and pressure
2. Permeant concentration
3. Thickness of the polymer
Corrosion of Linings and Coatings
290
CAT8247—CHAPTER 6—6/10/2006—12:28—SRIDHAR—XML MODEL B–pp. 287–402
shell can be made.
6.3D esignC onsiderations
In addition to selectingaliningm aterial that is resistant to the corrodent
beingh andled, there are three other factors to be consideredi nt he design:
permeation, absorption, and environmental stress cracking.Permeation and
absorption can cause
1. Bondf ailure and blistering, resultingf rom the accumulation
of fluids at the bondw hent he substrate is less permeable than
the liner or from corrosion/reaction products if the substrate is
attacked by the permeant.
2. Failure of the substrate from corrosive attack.
3. Loss of contents through the substrate and liner as ar esulto f
the eventual failure of the substrate. In unbondedl inings, it is
importantthat the space between the liner and substrate be vented
to the atmosphere, not only to allow minute quantities of permeant
vapor to escape but also to prevent expansion of entrapped air
from collapsing the liner.
6.3.1P ermeation
All materials are somewhat permeable to chemical molecules,b ut plastic
materials tend to be an order of magnitude greater in theirp ermeability
rates than metals.P olymers can be permeated by gases, vapors,o rl iquids.
Permeationi ss trictly ap hysical phenomenon;t here is no chemical attack
on the polymer. It is am olecular migration eithert hrough microvoids
in the polymer (if the structurei sm ore or less porous) or between
polymer molecules.
Permeation is af unction of two variables: one relating to diffusion
between molecular chainsand the other to the solubility of the permeant in
the polymer.The driving forces of diffusion are the concentration gradients
in liquidsand the partial pressuregradient for gases. Solubility is afunction
of the affinity of the permeant for the polymer.
Material passingt hrough cracksa nd voids is not related to permeation.
These are two distinct happenings.Theya re notrelated in any way.
Permeation is affected by the following factors:
1. Temperature and pressure
2. Permeant concentration
3. Thickness of the polymer
Corrosion of Linings and Coatings
290
CAT8247—CHAPTER 6—6/10/2006—12:28—SRIDHAR—XML MODEL B–pp. 287–402