CSB Issues Update on Fatal Chemical Eruption in May 2024

The US Chemical Safety and Hazard Investigation Board (CSB) released an update into its ongoing investigation of the May 30, 2024, accidental release of a molten mixture of sodium hydroxide and sodium nitrate that fatally injured one employee and resulted in injuries to a second employee at the Techniques Surfaces USA (“TS USA”) Liquid Nitriding facility in Chattanooga, TN.

TS USA is a wholly owned subsidiary of HEF USA, headquartered in Springfield, OH, which is a wholly owned subsidiary of the HEF Group, headquartered in Andrézieux-Bouthéon, France.

The TS USA facility 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. The process involves submerging components in molten nitrogen-containing sodium and potassium salts to increase the hardness and durability of the treated materials.

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 still gathering facts and analyzing several key areas, including: