CSB Releases Investigative Update into Fatal TS USA Chemical Eruption in Chattanooga, Tennessee

The U.S. Chemical Safety and Hazard Investigation Board (CSB) released an update into its ongoing investigation of a May 30, 2024, accidental release of a molten mixture of sodium hydroxide and sodium nitrate which fatally injured one employee and resulted in injuries to a second employee at the Techniques Surfaces USA (“TS USA”) Liquid Nitriding facility in Chattanooga, Tennessee. TS USA is a wholly owned subsidiary of HEF USA, headquartered in Springfield, Ohio, which in turn is a wholly owned subsidiary of the HEF Group, headquartered in Andrézieux-Bouthéon, France.

Chairperson Steve Owens said, “The CSB is issuing this update to keep the public informed on the agency’s ongoing investigation into the tragic incident at TS USA. Moving forward, the CSB will continue its analysis and of the cause of this incident in order to prevent similar events from occurring.” 

The TS USA facility located in Chattanooga, Tennessee began operations in September 2017 and was designed for processing large and heavy parts requiring nitriding. Liquid nitrating is a surface hardening process that enhances the wear resistance of iron and steel by diffusing nitrogen into the material. Liquid nitriding involves submerging components in molten nitrogen-containing sodium and potassium salts to increase the hardness and durability of the treated materials. The liquid nitriding process is particularly beneficial for industries requiring durable components, such as automotive, aerospace, and manufacturing, where parts are subject to high stress and wear.

On the day before the incident, TS USA had completed a liquid nitriding process for five 316 stainless steel rollers. The rollers were left in a hot water rinse bath overnight, a common practice at the facility. The next morning, the plant manager found one roller still leaking water and suspected that it was clogged with solidified oxidizer salt. After consulting with a process engineer who was working remotely, the manager decided to reintroduce the rollers directly into the oxidizing bath without preheating. At 8:58 a.m., an eruption of molten salt from the oxidizing bath occurred, fatally injuring one employee and igniting combustible materials in the area, leading to multiple fires.

The CSB is continuing to gather facts and analyze several key areas, including:

• The cause of the molten salt eruption
• Hazard Analyses of Liquid Nitriding Processes
• TS USA’s Hazard Identification Program
• TS USA’s Safety Management Systems
• HEF and TS USA’s Corporate Governance of Safety Programs
• Regulatory and Industry Guidance for Liquid Nitriding Facilities

Source: CSB