Chevron closed parts of its Richmond oil refinery after an explosion and fire in August. Federal investigators said refinery workers were trying to stem a leak from an 8-inch line at a refinery unit before the fire started.
An investigation by the U.S. Chemical Safety Board and the California Division of Occupational Safety and Health said the pipe, installed in 1976, showed signs of "severe" corrosion.
Chevron had said it discovered that the type of metal in the pipe may have contributed to the failure. It added that it was replacing piping in the fire-damaged area with a chrome alloy pipe that was resistant to corrosion.
California OSHA Chief Ellen Widess said in a statement that Chevron failed to act soon enough to address the faulty pipe.
"This reports confirms what Chevron already knew -- that the pipe was severely corroded and should have been replaced," she said.
The CSB-OSHA report said low levels of a silicon-based corrosion inhibitor were inside the walls of the faulty pipe.
Chevron said the report is consistent with its own preliminary observations. It stated it was inspecting pipes at the Richmond unit and would replace those unsuitable for service.