The natural gas distribution system includes 2,785,000 km (1,730,00 mi) of relatively small diameter, low pressure piping, which is divided into 1,739,000 km (1,080,000 mi) of distribution main and 1,046,000 km (650,000 mi) of services. There are approximately 55 million services in the distribution system. A large percentage of the mains (57 percent) and services (46 percent) are made of steel, cast iron or copper, which are subject to corrosion. The total annual direct cost of corrosion was estimated at approximately $5.0 billion.
Several different materials have been used for distribution piping. Historically, distribution mains were primarily made of carbon steel pipe; however, since the 1970s a large portion of the gas distribution main lines have been made of plastic, mostly polyethylene (PE), but sometimes polyvinyl chloride (PVC). A large percentage of mains (57 percent) and services (46 percent) are made of metal (steel, cast iron, or copper). The methods for monitoring corrosion on the lines are the same as those used for transmission pipelines; however, leak detection is the most widely used technique.