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Fundamentals of Corrosion
6.2.3 Design Considerations
In addition to selecting a lining material that is resistant to the corrodent
being handled, there are other factors to consider in the design: permeation,
absorption, and environmental stress cracking. Permeation and absorption
can cause:
1. Bond failure and blistering, resulting from the accumulation of
fluids at the bond when the substrate is less permeable than the
liner or from corrosion 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 lining as the result of the
eventual failure of the substrate. In unbonded linings, it is important
that the space between the liner and the 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.
6.2.3.1 Permeation
All materials are somewhat permeable to chemical molecules but plastic
materials tend to be an order of magnitude greater than metals in their permeability. Polymers can be permeated by gases, liquids, or vapors. Permeation
is strictly a physical phenomenon; there is no chemical attack on the polymer. It is a molecular migration through either microvoids in the polymer (if
the structure is more or less porous) or between polymer molecules.
Permeation is a function of two variables: one relating to diffusion between
molecular chains and the other to the solubility of the permeant in the polymer. The driving forces of diffusion are the concentration gradients in liquids and the partial pressure gradient for gases. Solubility is a function of
the affinity of the permeant for the polymer.
Material passing through cracks and voids is not related to permeation. These are two distinct happenings. They are not related in any way.
Permeation is affected by the following factors:
1. Temperature and pressure
2. Permeant concentration
3. Thickness of the polymer
An increase in temperature will increase the permeation rate because
the solubility of the permeant in the polymer will increase, and as the temperature rises, the polymer chain movement is stimulated, permitting more
permeants to diffuse among the chain more easily. For many gases, the
Fundamentals of Corrosion
6.2.3 Design Considerations
In addition to selecting a lining material that is resistant to the corrodent
being handled, there are other factors to consider in the design: permeation,
absorption, and environmental stress cracking. Permeation and absorption
can cause:
1. Bond failure and blistering, resulting from the accumulation of
fluids at the bond when the substrate is less permeable than the
liner or from corrosion 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 lining as the result of the
eventual failure of the substrate. In unbonded linings, it is important
that the space between the liner and the 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.
6.2.3.1 Permeation
All materials are somewhat permeable to chemical molecules but plastic
materials tend to be an order of magnitude greater than metals in their permeability. Polymers can be permeated by gases, liquids, or vapors. Permeation
is strictly a physical phenomenon; there is no chemical attack on the polymer. It is a molecular migration through either microvoids in the polymer (if
the structure is more or less porous) or between polymer molecules.
Permeation is a function of two variables: one relating to diffusion between
molecular chains and the other to the solubility of the permeant in the polymer. The driving forces of diffusion are the concentration gradients in liquids and the partial pressure gradient for gases. Solubility is a function of
the affinity of the permeant for the polymer.
Material passing through cracks and voids is not related to permeation. These are two distinct happenings. They are not related in any way.
Permeation is affected by the following factors:
1. Temperature and pressure
2. Permeant concentration
3. Thickness of the polymer
An increase in temperature will increase the permeation rate because
the solubility of the permeant in the polymer will increase, and as the temperature rises, the polymer chain movement is stimulated, permitting more
permeants to diffuse among the chain more easily. For many gases, the