Internal Corrosion Inspection

Internal corrosion is the most frequently encountered form of deterioration that can affect the integrity and reliability of a pressure vessel. Fortunately, it is usually also the easiest form of deterioration to detect during an in-service inspection, providing the correct locations inside the vessel are examined and the proper nondestructive examination procedures are used.

Many process environments are corrosive to the materials used for the construction of pressure vessels. For example, process environments that contain sulfur, chlorine, and organic or inorganic acids can be highly corrosive to carbon steels and low-alloy steels. The corrosion attack can be a general wastage of the material, or it can be highly localized. Deposits on internal surfaces or held in crevices can trap corrosive compounds in contact with the vessel shell and may cause severe localized corrosion.

Visual examination (VT) is usually adequate for detecting internal corrosion of pressure vessels. To detect some types of localized corrosion, the inspectors must look at locations that may be obscured by internal components or covered by deposits. Removal of the corrosion scale is necessary to determine the depth of localized corrosion and pitting. Ultrasonic examination (UT) using a longitudinal wave procedure from the O.D. surface of the vessel can also be used to detect internal corrosion and to determine the remaining thickness. It is helpful to mark locations of localized corrosion or pitting on the O.D. surface to guide the ultrasonic examination. Ultrasonic imaging techniques are available to “map” (discussed in Section 764) areas of localized corrosion or pitting and are especially useful for monitoring the progression of corrosion attack from one in-service inspection to the next.

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