Stack design and materials selection are based on strength, oxidation resistance, and acid attack from condensing acids in the flue gas. Most stacks are built of carbon steel. They may have an internal refractory lining when flue gas temperatures are high. Some form of internal corrosion protection may be needed when stack temperatures are low enough to permit internal acid condensation.
Condensation of sulfuric or sulfurous acids can cause serious stack corrosion. It will occur if stack metal temperature drops below the flue gas dewpoint, which may be anywhere from 212°F to 350°F, depending on SOx content of the gas. If flue gas temperature to the stack is below about 600°F, external heat loss can result in the metal temperature of an uninsulated stack dropping below the dewpoint, causing significant corrosion. Additional information on recommended temperatures is covered in Section 3400.
Internal insulation is not necessarily a help. It lowers stack temperature below the acid dewpoint but is porous enough to allow flue gas to penetrate and condense on the steel. The major corrosion advantage of internal stack insulation is that it cools the metal to the point that a protective coating can survive. Coatings such as “Stalastic” are often used to prevent corrosion under insulation.
In special cases, where stacks operate under wet, acidic conditions, they have been constructed of stainless steel or fiberglass. Type 304L stacks have been installed as replacements for steel in two Company chemical plants. Type 316L stacks are used rather commonly in oilfield and refinery stack gas scrubbers. Use of a low-carbon or stabilized grade of stainless steel is essential in all of these services. Flue gas condensate will readily cause intergranular corrosion in weld areas of the regular grades of stainless steel (Types 304 or 316). The common low-carbon grades of stainless are 304L and 316L; the stabilized grades are Types 321 and 347.