Corrosion of Linings
183
the stain is removed and softening of the film is not apparent, failure has
not occurred.
Any of the above defects can result in undercutting. After the corrosive penetrates to the substrate, corrosion will proceed to extend under the film areas
that have not been penetrated or failed. Some coatings are more resistant than
others to undercutting or underfilm corrosion. Usually if the coating exhibits
good adhesive properties, and if the primer coat is chemically resistant to the
corrosive environment, underfilm corrosion will be greatly retarded.
In addition, a tank coating must not impart any impurities to the material
contained within it. The application is a failure if any taste, color, smell, or
other contamination is imparted to the product, even if the coating is intact.
Such contamination can be caused by the extraction of impurities from the
coating, leading to blistering between coats or to metal.
If the coating is unsuited for the service, complete failure may occur by
softening, dissolution, and finally complete disintegration of the coating.
This type of problem is prevalent between the interphase and bottom of the
tank. At the bottom of the tank and throughout the liquid phase, penetration
is of great concern.
The vapor phase of the tank is subject to corrosion from concentrated
vapors mixed with any oxygen present and can cause extensive corrosion.
6.2 Sheet Linings
Designers of tanks and process vessels face the problem of choosing the most
reliable material of construction at a reasonable cost. When handling corrosive materials, a choice must often be made between using an expensive
material of construction or using a relatively low-cost material from which
to fabricate the shell and then installing a corrosion-resistant lining. Carbon
steel has been, and still is, the material predominantly selected although
there has been a tendency over the past few years to use a fiberglass-reinforced plastic shell. This latter choice has the advantage of providing atmospheric corrosion protection to the shell exterior.
For many years, vessels have been successfully lined with various rubber
formulations, both natural and synthetic. Many such vessels have given over
20 years of reliable service.
With the development of newer elastomeric and plastic materials, the variety of lining materials has greatly increased.
The newer materials, particularly the fluorocarbons, have greatly increased
the ability of linings to protect substrates from corrosive chemicals. However,
for these linings to provide maximum protection, it is important that the vessel shell be properly designed and prepared and the lining properly installed.
Unless these precautions are taken, premature failure is likely to occur.
183
the stain is removed and softening of the film is not apparent, failure has
not occurred.
Any of the above defects can result in undercutting. After the corrosive penetrates to the substrate, corrosion will proceed to extend under the film areas
that have not been penetrated or failed. Some coatings are more resistant than
others to undercutting or underfilm corrosion. Usually if the coating exhibits
good adhesive properties, and if the primer coat is chemically resistant to the
corrosive environment, underfilm corrosion will be greatly retarded.
In addition, a tank coating must not impart any impurities to the material
contained within it. The application is a failure if any taste, color, smell, or
other contamination is imparted to the product, even if the coating is intact.
Such contamination can be caused by the extraction of impurities from the
coating, leading to blistering between coats or to metal.
If the coating is unsuited for the service, complete failure may occur by
softening, dissolution, and finally complete disintegration of the coating.
This type of problem is prevalent between the interphase and bottom of the
tank. At the bottom of the tank and throughout the liquid phase, penetration
is of great concern.
The vapor phase of the tank is subject to corrosion from concentrated
vapors mixed with any oxygen present and can cause extensive corrosion.
6.2 Sheet Linings
Designers of tanks and process vessels face the problem of choosing the most
reliable material of construction at a reasonable cost. When handling corrosive materials, a choice must often be made between using an expensive
material of construction or using a relatively low-cost material from which
to fabricate the shell and then installing a corrosion-resistant lining. Carbon
steel has been, and still is, the material predominantly selected although
there has been a tendency over the past few years to use a fiberglass-reinforced plastic shell. This latter choice has the advantage of providing atmospheric corrosion protection to the shell exterior.
For many years, vessels have been successfully lined with various rubber
formulations, both natural and synthetic. Many such vessels have given over
20 years of reliable service.
With the development of newer elastomeric and plastic materials, the variety of lining materials has greatly increased.
The newer materials, particularly the fluorocarbons, have greatly increased
the ability of linings to protect substrates from corrosive chemicals. However,
for these linings to provide maximum protection, it is important that the vessel shell be properly designed and prepared and the lining properly installed.
Unless these precautions are taken, premature failure is likely to occur.