Photo Courtesy PSHMSA
Pipe put in place in 1948 split along a 22-ft section near the weld seam.

ExxonMobil's Pegasus pipeline break, which dumped more than 5,000 barrels of crude oil in a suburban Little Rock, Ark., neighborhood, was caused by a 65-year-old manufacturing defect, a metallurgical testing report shows.

The recent report by Hurst Metallurgical Research Laboratory Inc., Euless, Texas, says "the presence of manufacturing defects," which resulted in "the reduction of the wall thickness in the upset zone of the Electric Resistance Weld (ERW) seam," caused the 22-ft crack in the pipeline on March 29 in Mayflower, Ark.

Hook cracks, which are related to some types of early electric-resistance welding processes, "were present in the ERW seam prior to the incident for an unknown period," the report says. "It is unclear, however, whether the hook cracks occurred immediately after manufacturing or during service."

The 850-mile-long Pegasus pipeline has transported, since 2006, Canadian heavy crude oil that opponents charge contains corrosive tar sands, but the testing showed "no internal corrosion areas."

Some corrosion was noted on the exterior wall, but it was not near the ruptured seam. The report blames the corrosion on a protective-outer-coating split that allowed contact with moisture.

The U.S. Pipeline and Hazardous Materials Safety Administration (PHMSA) is continuing to investigate the breach and has the pipeline shut down until it can be restarted safely, a spokesman said.

ExxonMobil is doing its own testing in cooperation with PHMSA, an ExxonMobil spokesman said.

The ruptured pipeline section was manufactured in 1947 and 1948 by the now-closed Youngstown Sheet & Tube Co., Youngstown, Ohio. The pipe was put into service in 1948.

"At a minimum, they ought to replace the pre-1970 pipe," said Jim E. Hall, former National Transportation Safety Board chairman. "That would be the responsible thing to do."

Pipe produced before 1970 was made of lower-quality steel with lower-frequency welds, Hall said.