Inspections can be focused at these locations. However, since not all corrosion problems can be anticipated, it is also critical to obtain as much inspection and corrosion monitoring data as possible related to the internal condition of the system. In the best case, several pipeline inspections may have been conducted over a period of time along with coupon or instrument monitoring. For onshore pipelines this information may be supplemented by external inspections using ultrasonic methods in areas of potentially severe corrosive attack. This means that they are not crack-like features, but are handled as local or general reductions in the wall thickness of the pipe.
General corrosion is the easiest type of corrosion to characterize.
Its effect on pipeline serviceability is generally related directly to the reduction in wall thickness and the increase in the associated net section stress. In the case of localized corrosion, however, this form of attack can be at least an order of magnitude greater in extent than the general corrosive attack found in immediately adjacent areas. As shown in this figure, not all forms of pitting can be properly assessed by surface measurements. Single isolated pits do not usually jeopardize the mechanical integrity of the pipeline. However, the major problem with this type of corrosion is that it is sometimes hard to find using NDT methods and can cause perforation of the pipeline resulting in leakage.