Government investigators said Wednesday a decades-old pipe that leaked and fed a massive fire at a Chevron refinery in Richmond was corroded, and the company knew it should have been replaced.

The report issued by the U.S. Chemical Safety Board and California Division of Occupational Safety and Health found the 8-inch steel pipe was installed in 1976 and ruptured due to corrosion.

The report confirmed previous findings by both the agencies and the company.

The fire created a large plume of black smoke, sending more than 15,000 nearby residents to hospitals complaining of eye irritation and trouble breathing. The cloud also engulfed 19 Chevron employees, who escaped severe injuries.

"The corroded pipe should have been replaced when opportunities arose years earlier," Rafael Moure-Eraso, board chairperson, said in a statement.

San Ramon-based Chevron said some of the report's findings were consistent with its own, and the company is inspecting every pipe susceptible to the same type of corrosion.

"While we do not agree with some of the characterizations in the (report), we are committed to discussing the findings from our investigation and our corrective actions with the investigating agencies," Sean Comey, a company spokesman, said in an email.

Cal-OSHA has fined Chevron nearly $1 million after finding willful, serious violations. Chevron is appealing those citations.

The report said there was wall thinning in the pipe because naturally occurring sulfur reacted with the steel, creating a specific type of corrosion.

The study found the oil processed in that unit of the refinery was high in sulfur and especially damaging to the pipe.

If the pipe had been replaced with one higher in silicon, which protects against sulfur-related corrosion, the leak would likely not have occurred, the report said.

The chemical board said its investigation into what caused the fire and the safety procedures involved is ongoing.