NEW YORK (Reuters) - An old, degraded piece of metal pipe that had not been tested for corrosion led to the June fire and explosions at the Philadelphia Energy Solutions oil refinery, the U.S. Chemical Safety and Hazard Investigation Board said on Wednesday.

FILE PHOTO: The Philadelphia Energy Solutions oil refinery is shown following a recent fire that caused significant damage to the complex, in Philadelphia, Pennsylvania, U.S., June 26, 2019. REUTERS/Laila Kearney/File Photo

The pipe fitting gave way around 4:00 a.m. ET (0800 GMT) on June 21, releasing propane containing more than 3,200 pounds of highly toxic chemical hydrofluoric acid (HF) that escaped into the atmosphere, the CSB said in its first update on its investigation.

Three separate explosions then hurled pieces of the refinery across the nearby Schuylkill River and onto highways, causing a blaze that was visible for miles and destroying an alkylation unit that uses HF to produce components of high-octane gasoline.

No one was killed, and only five minor injuries were reported. However, Philadelphia Energy Solutions filed for bankruptcy a month later and shut down the 335,000 barrel-per day refinery, the largest on the U.S. East Coast.

“We need to focus on making sure that this type of an explosion at a refinery doesn’t happen anymore because it’s just a matter of time before the facts are just a little bit different and people die or are critically injured,” CSB Interim Director Kristen Kulinowski said at a news conference.

The report confirms what sources told Reuters in August, that the company was expected to say hydrofluoric acid was released during the incident. HF can burn the eyes, skin and lungs, and can be fatal.

A process operator at the plant quickly activated a fail-safe that dumped a large quantity of the acid into a protected vessel so more was not released, said CSB, a non-regulatory federal agency.

While CSB officials said they were unaware of health complaints tied to the leaked chemical, Reuters spoke to residents of the South Philadelphia neighborhood surrounding the refinery in the days after the fire who complained of burning eyes, itchy skin and other symptoms linked to HF exposure.

HF has come under criticism by the CSB and refinery worker unions for its use in U.S. refineries in populated areas.

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The ruptured pipe fitting, an elbow, was found to have a metal composition that included high levels of copper and nickel, failing to meet industry standards, the agency said.

“The elbow that ruptured corroded faster than the rest of the piping,” CSB investigator Lauren Grim said at a press conference.

After the blast, the thinnest portion of the pipe fitting was found to be half the thickness of a credit card, said CSB.

While the broader pipe system in the destroyed unit had been periodically measured for thickness to detect corrosion, that piece had not, the CSB said.

Pipe degraded by corrosion caused a 2012 explosion at Chevron Corp’s Richmond, California, refinery and a 2009 blast at the Silver Eagle refinery in Woods Cross, Utah, according to the CSB.

The board has no regulatory or enforcement power but is authorized under the U.S. Clean Air Act to determine the cause of chemical plant explosions and fires and make recommendations to prevent them in the future.

The investigation is ongoing and CSB will issue a final report with recommendations early next year, officials said.

Other local and federal agencies, including the Bureau of Alcohol, Tobacco, Firearms and Explosives as well as the Philadelphia fire department, are also investigating the incident.

“Once these investigations are complete, parties found responsible must be held accountable and appropriate actions must be taken to address this situation and the impact it has had on the workers and our region,” U.S. Representative Mary Gay Scanlon said in a statement.

Scanlon has criticized PES for how the company handled the closure of its refinery. “PES cannot simply get up and walk away from these workers and this community,” she said.

In September, Reuters reported that PES executives were paid $4.5 million in retention bonuses days after the fire. About 1,100 refinery workers were laid off without health benefits or severance pay.