The internal process environment that a pressure vessel is exposed to during operation can frequently cause the material to corrode. Therefore, a corrosion allowance must be added to the calculated minimum thicknesses required for each component of the vessel. The Code assigns to the owner/user of the vessel the responsibility for specifying the corrosion allowance. This is necessary to prevent corrosion from reducing the thicknesses below the required minimums during operation. Recommendations for determining corrosion allowances, based on operating experience, are discussed in Section 500.
An alternative to a corrosion allowance, especially when the process environment would result in a very high corrosion rate, is to employ a corrosion-resistant cladding or to apply a corrosion-resistant coating.
External corrosion is rarely significant. Pressure vessels usually either operate at high temperatures that prevent the condensation of moisture, or weather shielding is provided. Therefore, an external corrosion allowance is not usually necessary.
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