Fire officials yesterday blamed the spectacular May fire that destroyed part of Sunoco's Marcus Hook refinery on a rusty 10-inch pipe that ruptured and caused a catastrophic natural-gas leak.

Alan L. Brown, chief deputy of the Delaware State Fire Marshal's office, called the blaze an accident. He said investigators concluded that a feed line to the refinery's ethylene unit failed and released gas that caught fire.

No one was injured in the May 17 inferno, which lit up the sky and destroyed the unit, situated on the Delaware side of the 781-acre refinery.

Sunoco Inc. spokesman Thomas P. Golembeski said yesterday that the Philadelphia company had stepped up efforts to examine piping systems at all its facilities to address immediate problems and had begun a process to revise its standard inspection practices to make them more rigorous.

"We have begun implementing a special-emphasis program for high-risk systems to be sure similar structures with similar conditions do not exist at any Sunoco facilities," Golembeski said in a statement.

Sunoco will have a third party review its inspection practices to identify other areas for improvement, he said.

Sunoco decided to not to reopen the ethylene unit because of depressed demand for the chemical, which is used to make plastics. As many as 50 jobs were eliminated, and the company took a $14 million after-tax charge on its second-quarter earnings to reflect the lost asset.

Golembeski said the corrosion occurred over many years and affected an area of less than one square foot. It was caused by moisture trapped between the pipe and a loose steel sleeve.

The corroded pipe was located on top of a concrete support about 18 feet above the ground, obscured from view, he said. The pipe maintained a constant temperature of about 60 degrees, which made the line sweat in humid weather, causing moisture to build up.

Sunoco said it was committed to sharing information about the pipe failure with other companies to prevent similar incidents.

Delaware fire officials conducted the investigation in conjunction with federal Bureau of Alcohol, Tobacco and Firearms.

Contact staff writer Andrew Maykuth at 215-854-2947 or amaykuth@phillynews.com.