External Corrosion Inspection
External corrosion of a pressure vessel is always attributable to some form of atmospheric corrosion. It is highly dependent on the natural atmospheric conditions prevailing at the geographic location. The natural atmosphere at humid sea coast locations can be expected to be more corrosive than at dry inland locations.
However, the chemical emissions at the plant site (or from other nearby industry) can considerably increase the corrosivity of the atmosphere. Locations where rain water can accumulate on pressure vessels for relatively long periods are especially prone to external attack. Crevices or pockets created by supports, rings, and other external attachments are typical examples.
Externally insulated vessels should be provided with weatherproofing to prevent rain water from seeping into the insulation where it can be trapped against the vessel shell. Soluble compounds in the insulation can be leached out of the insulation and contribute to the external corrosion, if the weatherproofing is not maintained in good condition.
Vessels that operate at temperatures above the dew point of water are less vulnerable to external corrosion, but the possibility should not be ignored. Insulated vessels that operate at low temperatures between the dew point and freezing point of water may be especially vulnerable to external corrosion.
Visual examination (VT) is the most appropriate procedure for detecting external corrosion. However, it is necessary to remove insulation and weatherproofing, at least at selected locations where the external corrosion is most likely to occur. External corrosion of the vessel supports, including anchor bolts, should not be overlooked.
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