Localized Internal Corrosion
Areas of localized corrosion or pitting in pressure vessels can be permitted to have a remaining wall thickness that is less than the ASME Code minimum required thickness, within certain strictly defined limits. Local primary membrane stresses can exceed the maximum allowable design stress for the material of construction without significant risk of rupture, because of the “reinforcement” provided by the surrounding material.
The weld joint efficiency factor used for the design of a vessel need not be applied when calculating the minimum required thickness of the shell for the evaluation of localized corrosion or pits that are farther from the edge of any weld than twice the thickness of the shell. Weld joint efficiency factors are employed by the ASME Code to compensate for flaws that may exist in welds that do not receive full radiographic (RT) examination. However, flaws in the weld metal do not reduce the strength of the plate and forging materials used for construction of the vessel.
Wind and earthquake loadings need not be included in the minimum required thickness calculations, because highly localized reductions in shell thickness would not be expected to significantly affect the structural integrity of the vessel in the event of high winds or earthquakes.
API 510 limits the total area of localized corrosion or pits within any 8-inch diameter circle to 7 square-inches maximum. Furthermore, the remaining thickness of the vessel shell may not be less than one-half of the minimum required thickness at any point, and the sum of pit dimensions along any straight line within the circle may not exceed 2 inches.
Appendix 4 of ASME Code, Section VIII, Division 2, can also be used (for Division 1 or Division 2 vessels) to evaluate localized corrosion or pitting in a pressure vessel. Local primary membranes are permitted to reach 1.5 times the maximum allowable design of the material used for construction of the vessel. The area having a local primary membrane stress that exceeds the maximum allowable design stress may not exceed (Rt)1/2 (where R = the radius of the vessel shell, and t = the shell thickness); and individual areas of local high stress may not be closer to one another than 2.5(Rt)1/2. These criteria can provide for the acceptance of somewhat larger areas of localized corrosion than would be permitted by API 510, especially for larger diameter vessels with relatively thick shells.
Areas of local corrosion or pitting that do not meet the requirements of API 510 or Appendix 4 of the ASME Code, Section VIII, Division 2, must be repaired by weld buildup, or by removal of the corroded area and insertion of a butt-patch (see Section 800).
Note that highly localized corrosion and the growth of pits can progress at considerably faster rates than predicted by the corrosion rates for general wastage. Therefore, returning vessels that have exhibited localized corrosion or pitting to service may involve risk of failure unless measures are taken to stop the attack (see the Corrosion Prevention and Metallurgy Manual).
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