The Equipment Design Process
337
Some general design rules to be used to limit the effect of atmospheric corrosion are as follows:
1. Avoid crevices. Crevices provide areas where atmospheric pollutants
might collect and not be washed away by rain. The collected material will continue to react with the surface and corrode it.
2. Avoid mechanical stresses. Excess mechanical stresses and stress concentrations increase the potential for stress corrosion cracking.
3. Avoid galvanic contact. Same or similar metals should be used
throughout the structure. When dissimilar metals are in contact,
galvanic corrosion takes place. If the use of dissimilar metals cannot
be avoided, the corroding part should be designed in such a way that
it can be easily replaced. Fastening materials should be more noble.
4. Avoid sheltering from rain. Rain can be helpful in preventing or reducing atmospheric corrosion by washing away pollutants that have
accumulated during the dry period.
5. Provide corrosion allowance. The full life expectancy of a structure
may be ensured by increasing the mechanical design thickness of
the member by an amount equivalent to what will be lost as a result
of atmospheric corrosion during the lifetime of the structure.
11.3.1.2 Structures
Included under structures are many ancillary items such as ladders, grating, platforms, guardrails, and support structures. The general practice has
been to use steel, galvanized iron, and aluminum for these applications.
Corrosion of these items has always been a problem and has resulted in high
maintenance costs.
Much time has been spent over the years to provide materials that will
reduce or eliminate the problems of corrosion for these applications. These
efforts have resulted in the development of glass-reinforced polyester and
vinyl ester materials. At the present time there are available over 100 standard
structural shapes from which platforms, supports, structures, and decking
can be fabricated. These materials offer several advantages, including corrosion resistance, nonconductivity, high strength, light weight, and dimensional stability.
Three basic resins are used: (1) isophthalic polyester, which is the least
expensive and is used in most general applications for which it is not necessary for the material to have fire-retardant properties; (2) isophthalic polyester with a fire-retardant additive has the same general properties and area
of application as the basic isophthalic polyester resin; and (3) vinyl ester
resin, which is the most expensive but has the greatest corrosion resistance,
superior strength and physical properties, and is also fire retardant. These
materials are resistant to most corrodents found in the atmosphere and to a
337
Some general design rules to be used to limit the effect of atmospheric corrosion are as follows:
1. Avoid crevices. Crevices provide areas where atmospheric pollutants
might collect and not be washed away by rain. The collected material will continue to react with the surface and corrode it.
2. Avoid mechanical stresses. Excess mechanical stresses and stress concentrations increase the potential for stress corrosion cracking.
3. Avoid galvanic contact. Same or similar metals should be used
throughout the structure. When dissimilar metals are in contact,
galvanic corrosion takes place. If the use of dissimilar metals cannot
be avoided, the corroding part should be designed in such a way that
it can be easily replaced. Fastening materials should be more noble.
4. Avoid sheltering from rain. Rain can be helpful in preventing or reducing atmospheric corrosion by washing away pollutants that have
accumulated during the dry period.
5. Provide corrosion allowance. The full life expectancy of a structure
may be ensured by increasing the mechanical design thickness of
the member by an amount equivalent to what will be lost as a result
of atmospheric corrosion during the lifetime of the structure.
11.3.1.2 Structures
Included under structures are many ancillary items such as ladders, grating, platforms, guardrails, and support structures. The general practice has
been to use steel, galvanized iron, and aluminum for these applications.
Corrosion of these items has always been a problem and has resulted in high
maintenance costs.
Much time has been spent over the years to provide materials that will
reduce or eliminate the problems of corrosion for these applications. These
efforts have resulted in the development of glass-reinforced polyester and
vinyl ester materials. At the present time there are available over 100 standard
structural shapes from which platforms, supports, structures, and decking
can be fabricated. These materials offer several advantages, including corrosion resistance, nonconductivity, high strength, light weight, and dimensional stability.
Three basic resins are used: (1) isophthalic polyester, which is the least
expensive and is used in most general applications for which it is not necessary for the material to have fire-retardant properties; (2) isophthalic polyester with a fire-retardant additive has the same general properties and area
of application as the basic isophthalic polyester resin; and (3) vinyl ester
resin, which is the most expensive but has the greatest corrosion resistance,
superior strength and physical properties, and is also fire retardant. These
materials are resistant to most corrodents found in the atmosphere and to a