Chevron shut one unit at its Richmond refinery following a fire in early August. Investigators with the U.S. Chemical Safety Board agency said refinery workers were working to stem a leak from an 8-inch line at one of the refinery units before the fire started.
Chevron, in a regular update on the incident, said it discovered that the type of metal in the pipe may have contributed to the failure.
"The contributing factor to the failure may be low silicon content of the individual component in the carbon steel pipe system," said Richmond refinery's General Manager Nigel Hearne in a statement.
"Individual low silicon carbon-steel components can corrode at an accelerated rate not detected by even multiple corrosion monitoring locations on a piping system."
Hearne said that some of the company's experts may have understood the impact of low silicon components but didn't effectively act upon that information.
The Richmond refinery is the third largest in California. It can process more than 242,000 barrels of oil per day.
Hearne offered no specifics on when full operations would return to the refinery.