A federal agency investigating a fatal fire at a Hoeganaes Corp. chemical plant in Tennessee said a corroded pipe leaking hydrogen gas caused an explosion that ignited combustible iron dust there. A total of four workers have died this year after being critically burned in fires at the Hoeganaes plant in Gallatin, with the latest one occurring last week. On Friday, the head of the U.S. Chemical Safety Board said the plant should be completely redesigned before the company restarts the manufacturing of metal powder for automotive and industrial uses. The CSB does not have the power to close the plant, which employs about 180 people, and can only make recommendations. This is the third serious accident at the Gallatin plant this year. Two workers died after a flash fire in January and another worker was injured in a March flash fire. The CSB investigation found both of those fires occurred when flammable dust became airborne and exploded. Last week''s explosion and fire occurred after hydrogen gas leaked from a corroded pipe, and the CSB plans to expand its investigation to include the maintenance of those pipes. The Tennessee Occupational Safety and Health Administration last week issued $42,900 in citations to Hoeganaes after an investigation into the January and March accidents found 12 serious violations. But neither TOSHA nor its federal counterpart, the U.S. Occupational Safety and Health Administration, have rules governing combustible dust.