External Corrosion of Oil and Natural Gas Pipelines

Abstract

36% were caused by external corrosion and 63% were caused by internal corrosion. For natural gas distribution pipeline accidents, only approximately 4% of the total accidents were caused by corrosion, and the majority of those were caused by external corrosion. The accidents reported in Table 1 are for major accidents that resulted in injury, fatality, or more than $50,000 in property damage. In addition to the reportable accidents, an average of 8000 corrosion leaks per year are repaired on natural gas transmission pipelines (Ref 7), and 1600 spills per year are repaired and cleaned up for liquid product pipelines. In a summary report for incidents between 1985 and 1994, corrosion accounted for 28.5% of pipeline incidents on natural gas transmission and gathering pipelines (Ref 8). In a summary report for incidents between 1986 and 1996, corrosion accounted for 25.1% of pipeline incidents on hazardous liquid pipelines (Ref 9). These values correspond very well to the statistics for 1994 to 1999 presented in Table 1. Given the implications of pipeline failures and the role that external corrosion plays in these failures, it is apparent that proper corrosion control can have a major impact on the safety, environmental preservation, and the economics of pipeline operation. The vast majority of underground pipelines are made of carbon steel, based on American Petroleum Institute API 5L specifications (Ref 10). Typically, maximum composition limits are specified for carbon, manganese, phosphorous, and sulfur. In some cases, other alloying elements are added to improve mechanical properties. Composition and tensile requirements for common line pipe steels are shown in Table 2. These steels have inadequate alloy additions to be considered corrosion resistant and undergo