The U.S. Chemical Safety and Hazard Investigation Board has released a preliminary report that explains how corrosion of a piping component led to the massive fire and explosions at the Philadelphia Energy Solutions Refinery on June 21. According to the report, the pipe elbow had corroded to about half the thickness of a credit card and appears to have ruptured in the refinery’s alkylation unit. The process fluid released from the alkylation unit included more than 5,000 pounds of hydrofluoric acid, or HF. The leaking fluid formed a large, ground-hugging vapor cloud that ignited minutes later to cause the fire and explosions. CSB’s report notes that the piping in the facility was susceptible to corrosion from HF in the process fluid and that the pipe elbow had corroded faster than the rest of the piping. The thickness of surrounding piping in the unit was periodically measured to monitor corrosion rates, but the failed pipe elbow was not monitored. CSB’s investigation has found that the pipe elbow had a high nickel and copper content. “Various industry publications have found that carbon steel with a higher percentage of nickel and copper corrodes at a faster rate than carbon steel with a lower percentage when used in a process with hydrofluoric acid,” CSB’s news release reads. This is the third major incident that CSB has investigated since 2015 at refineries that use HF for alkylation. Two similar incidents that occurred in Superior, Wis., and Torrance, Calif., did not result in an HF release. CSB Interim Executive Kristen Kulinowski stated during a news conference that the agency intends to more closely examine the use of HF in the refining process moving forward. CSB’s investigation is ongoing. For more information, download a PDF of the report from the agency's website.