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TS USA Molten Salt Eruption

Overview

On May 30, 2024, an explosion and molten salt eruption occurred at the TS USA liquid nitriding facility in Chattanooga, Tennessee. One employee was fatally injured. Fires and property damage occurred. The CSB released its final report on June 3, 2025.

The CSB found that water trapped in a hollow roller part contacted molten salt during reintroduction to the oxidizing bath. Rapid expansion occurred. Overpressure developed. A steam explosion and molten salt eruption followed. The final report identified deficiencies in hazard analysis, operating procedures, training, incident investigation, and corporate knowledge management.

Incident Snapshot

Field Value
Facility / Company TS USA (Techniques Surfaces USA), a subsidiary of HEF USA / HEF Groupe
Location Chattanooga, Tennessee, USA
Incident Date 2024-05-30
Investigation Status The CSB's investigation was released on June 3, 2025.
Accident Type Explosion, molten salt eruption, fatal injury
Final Report Release Date 2025-06-03

What Happened

  • On April 5, 2024, TS USA received six rollers for treatment from a customer.
  • The customer requested that TS USA treat one roller as a prototype to verify its properties after undergoing the liquid nitriding process.
  • Before the end of April 2024, TS USA finished the liquid nitriding process for the single roller and informed the customer that the roller took longer to drain than expected.
  • On May 13, 2024, the customer confirmed that the roller met the required specifications and approved TS USA to treat the remaining five rollers.
  • On May 24, 2024, at approximately 11:36 a.m., the remaining five rollers began the nitriding process.
  • By the end of the day, the rollers had been processed through the nitriding bath and were placed in the rinse bath.
  • On May 28, 2024, at approximately 9:56 a.m., the rollers were removed from the rinse baths to drain.
  • At approximately 10:48 a.m., the remaining five rollers were moved in a single fixture to the polishing area of the Chattanooga facility to be polished.
  • The polishing operators noted that the rollers retained water and salt in their cavities.
  • The operators tipped the rollers over to allow water to drain from the cavities and used compressed air in an attempt to remove any remaining materials, such as water or salt, from the rollers’ cavities.
  • The rollers completed the polishing process on the morning of May 29, 2024, and were ready for further processing.
  • Less than two hours later, at approximately 10 a.m., the TS USA line operator connected the fixture containing the rollers to an overhead crane and began the re-oxidation process.
  • By approximately 3:06 p.m., the rollers reached the hot water rinse bath step in the process, where they remained overnight.
  • On May 29, 2024, when the five rollers were submerged in the oxidizer salt bath, the molten salts entered the cavities through the 5-millimeter holes on the top and bottom.
  • After the oxidizing salt bath treatment was completed, the line operator pulled the rollers out and let the molten salt drain out of the holes at the bottom of the rollers.
  • The molten salt was unable to fully drain out of one of the rollers before it cooled and solidified, likely as a thick disk or plug.
  • The rollers were then fully submerged in the hot water rinse tank to remove residual salt from the parts.
  • The water from the hot water rinse tank entered the rollers’ cavities through the five-millimeter drain holes on the top of the roller.
  • The water was unable to freely flow through the one roller because of the solidified salt plug.
  • On May 30, 2024, at 7:09 a.m., a TS USA line operator removed the rollers from the rinse bath containing 160–180℉ water.
  • The rollers were suspended over the rinse tank to allow water to drain from the parts and then moved to the end of the production line to cool down.
  • Approximately 15 minutes later, the plant manager moved the rollers to the end of the process line and placed them on the floor to cool down.
  • After the rollers had cooled for over an hour, the plant manager and supervisor observed that at least one roller was too hot to continuously touch.
  • The drain hole appeared to be clogged, which the plant manager and supervisor believed to be due to oxidizer salt.
  • Before the incident, the plant manager and supervisor attempted to clear the obstruction from the roller using a piece of wire, but they were unsuccessful.
  • The plant manager believed that the part only contained solidified oxidizer salt and no water.
  • At 8:46 a.m., the plant manager and supervisor instructed the line operator to reintroduce the rollers into the oxidizing bath, as the process engineer had recommended.
  • All five rollers were reintroduced to the oxidizing bath.
  • At 8:54 a.m., the rollers were submerged in the oxidizing tank.
  • Approximately two minutes later, steam began to vent from one of the submerged rollers and intensified as the line operator raised the rollers from the oxidizer bath.
  • At 8:58 a.m., the chemical mixture inside the oxidizing tank erupted, releasing hot molten salt into the process area and engulfing the line operator.
  • The line operator was treated on-site by Chattanooga Fire Department personnel and then transported by Hamilton County Emergency Medical Services personnel to a nearby hospital.
  • He died later that day at the hospital.
  • Three other TS USA employees suffered minor burns and were treated on-site.
  • The hot molten salt likely reached or exceeded the autoignition temperature of combustible materials within the building, triggering multiple fires in the facility.
  • The fires were extinguished by TS USA employees and Chattanooga Fire Department personnel.
  • Following the incident, the plugged roller was found outside of the oxidizer vessel, indicating that it was propelled from the vessel due to the release of the steam.
  • Damage to the overhead crane indicated that the roller had contacted the equipment approximately 20 feet above.

Facility and Process Context

  • TS USA operates a liquid nitriding facility in Chattanooga, Tennessee.
  • The facility began operations in September 2017 and was designed for the surface treatment of large and heavy metal parts.
  • The facility operated in two 8-hour shifts and the incident occurred during the first shift.
  • Liquid nitriding is a process whereby components are submerged in molten nitrogen-containing sodium and potassium salts to achieve surface enhancement.
  • The process includes a pre-heating step to remove excess moisture, a nitriding bath, an oxidizer bath containing molten sodium hydroxide and sodium nitrate, and quenching/rinse baths.
  • The only process engineer who supports the Chattanooga facility is located in southeast Georgia and reports to the HEF USA CEO.
  • The Chattanooga facility did not process or store materials covered by OSHA PSM or EPA RMP and was not subject to those requirements.
  • Due to the sodium hydroxide used in the oxidizer bath, the facility was subject to CERCLA and the HAZWOPER standard.

Consequences

  • Fatalities: 1 TS USA employee fatally injured. The line operator died later that day at the hospital.
  • Injuries: Three other TS USA employees suffered minor burns and were treated on-site. The line operator suffered second- and third-degree chemical and thermal burns over 95 percent of his body.
  • Environmental release: An estimated 4,500 pounds of molten salt mixture was released from the oxidizer bath.
  • Facility damage: Approximately $1.3 million in property damage. Multiple fires occurred throughout the facility. Damage occurred to the overhead crane.
  • Operational impact: The facility was shut down for approximately eight months until it reopened in February 2025.

Key Findings

Immediate Causes

  • The introduction of water contained in the roller cavity to the 800°F oxidizing salt bath.
  • The hot salt caused the water to expand and boil in the cavity of a roller, which resulted in an overpressure, a steam explosion, and a molten salt eruption.

Contributing Factors

  • TS USA’s lack of awareness of the accumulation hazards associated with parts containing cavities.
  • These cavities presented accumulation hazards when processed in the nitriding line.
  • The drain hole appeared to be clogged, which the plant manager and supervisor believed to be due to oxidizer salt.
  • The plant manager and supervisor attempted to clear the obstruction from the roller using a piece of wire, but they were unsuccessful.
  • The plant manager believed that the part only contained solidified oxidizer salt and no water.

Organizational and Systemic Factors

  • TS USA did not have an adequate process safety management system in place at its Chattanooga facility.
  • TS USA lacked procedures for reprocessing parts in the oxidizer bath.
  • TS USA did not have training on the hazards present in the nitriding line.
  • TS USA lacked hazard analyses on processing new parts, on the liquid nitriding line, and for reprocessing parts.
  • TS USA did not perform any safety-based risk assessments on the liquid nitriding process.
  • TS USA did not perform an additional hazard analysis following the initial trial run.
  • TS USA did not perform a hazard analysis on the non-routine task for reworking the rollers.
  • TS USA’s training program did not provide information or guidance on the process hazards.
  • TS USA did not have a training program on how to identify hazards in the process or when evaluating new parts.
  • The operating manual used by TS USA employees did not provide sufficient information to ensure that employees were aware of the potential hazards in the nitriding operation.
  • The oxidizing salt mixture safety data sheet (SDS) does not provide sufficient information on hazards for materials in the process.
  • The review process for new parts used by TS USA and HEF USA did not adequately assess the potential accumulation hazards or hazards due to modifications to new parts.
  • HEF Groupe developed hazard analyses for the nitriding process, but the details of these analyses and the safeguards were not communicated to HEF Groupe’s subsidiary companies, including TS USA.
  • HEF Groupe did not ensure that its subsidiaries were following the company’s guidance, such as the risk assessments and safety alert letters, developed at the corporate level.
  • HEF Groupe did not ensure that the safety knowledge maintained at the corporate level was communicated to its subsidiary companies.
  • TS USA and HEF Groupe did not ensure that safety incidents were investigated or that lessons learned from events were shared and incorporated across the organization.
  • HEF Groupe did not have an adequate process safety management system.
  • HEF USA did not have safety personnel.
  • HEF Groupe did not manage the safety knowledge at the corporate level.
  • HEF Groupe did not provide the resources necessary to ensure that safety requirements and management systems were in place at its facilities.

Failed Safeguards or Barrier Breakdowns

  • The operating manual used by TS USA employees did not provide sufficient information to ensure that employees were aware of the potential hazards in the nitriding operation.
  • The oxidizing salt mixture safety data sheet (SDS) does not provide sufficient information on hazards for materials in the process.
  • The additional passive engineering controls in place at the Chattanooga facility were insufficient to protect the line operator due to the severity of the eruption of molten salt.
  • The protective barrier at the oxidizer bath did not protect the line operator from the eruption.
  • The viewing window melted due to exposure to the molten salts.
  • TS USA did not perform any safety-based risk assessments on the liquid nitriding process.
  • TS USA did not perform an additional hazard analysis following the initial trial run.
  • TS USA did not perform a hazard analysis on the non-routine task for reworking the rollers.
  • TS USA’s training program did not provide information or guidance on the process hazards.
  • TS USA did not have a training program on how to identify hazards in the process or when evaluating new parts.
  • The review process for new parts used by TS USA and HEF USA did not adequately assess the potential accumulation hazards or hazards due to modifications to new parts.
  • The risk analyses were not communicated to the TS USA facilities.
  • HEF Groupe did not communicate the risk assessments performed by HEF Groupe on the nitriding process.
  • HEF Groupe did not sufficiently share critical process safety knowledge with its subsidiaries, particularly by not communicating and explaining the hazards of accumulations in parts with cavities.
  • HEF Groupe did not manage safety knowledge throughout the company.
  • HEF Groupe did not ensure that its subsidiary companies were provided with all the safety information that had been developed or that safety knowledge had been transferred and managed at its facilities.
  • HEF Groupe did not ensure that the corporation’s subsidiary facilities were aware of the prohibition on processing hollow parts, what parts could be considered “hollow” (and therefore prohibited), and why the prohibition on processing these parts exists in the nitriding process.

Recommendations

  1. 2024-01-I-TN-R1 | Recipient: TS USA | Status: Open | Summary: Implement physical, protective barriers around the molten salt baths that isolate employees from hazardous releases at all locations that perform liquid nitriding.
  2. 2024-01-I-TN-R2 | Recipient: TS USA | Status: Open | Summary: Develop a safety management system that incorporates industry guidance and includes, but is not limited to: a. A hazard analysis program for assessing the nitriding process. The program shall apply to new and existing parts, assess parts for accumulation hazards and sealed cavities, and include non-routine tasks such as reprocessing unsatisfactory parts. b. Written operating procedures for the nitriding process. c. A training program, including written materials, for the employees involved in the nitriding process. d. An incident investigation program including causal analysis, written reports, and communication of findings and corrective actions throughout the entire TS USA organization.
  3. 2024-01-I-TN-R3 | Recipient: TS USA | Status: Closed – Acceptable Action | Summary: For each TS USA facility, establish a position with specific professional expertise and experience in safety management systems, such as risk-based process safety. This position shall be responsible for TS USA’s safety management system, ensuring that HEF Groupe’s safety information is incorporated at the site level, and implementing regulatory and industry safety guidance.
  4. 2024-01-I-TN-R4 | Recipient: HEF Groupe | Status: Open – Acceptable Response or Alternate Response | Summary: Include physical, protective barriers as part of the standard design for liquid nitriding processes. These protective barriers shall be intended to isolate employees from molten salt releases.
  5. 2024-01-I-TN-R5 | Recipient: HEF Groupe | Status: Open – Acceptable Response or Alternate Response | Summary: Develop a safety management system that incorporates industry guidance and includes, but is not limited to: a. A hazard analysis program for assessing the nitriding process. The program shall apply to new and existing parts, assess parts for accumulation hazards and sealed cavities, and include non-routine tasks such as reprocessing unsatisfactory parts. b. Written operating procedures for the nitriding process. c. A training program, including written materials, for the employees involved in the nitriding process. d. An incident investigation program including causal analysis, written reports, and communication of findings and corrective actions throughout the entire TS USA organization.
  6. 2024-01-I-TN-R6 | Recipient: HEF Groupe | Status: Open – Acceptable Response or Alternate Response | Summary: Develop and implement an effective and comprehensive Knowledge Management program for sharing knowledge throughout the HEF Groupe organization. Knowledge shall include all information from audits, hazard analyses, and incident investigations, including causal analyses and corrective actions recommended and taken.
  7. 2024-01-I-TN-R7 | Recipient: HEF Groupe | Status: Open – Acceptable Response or Alternate Response | Summary: Develop and implement a comprehensive and effective Corporate Governance program. This program shall include regular audits of subordinate facilities throughout the organization, with tracking and accountability for implementation of all recommendations and corrective actions identified in the audits.

Key Engineering Lessons

  • Parts with cavities, sealed cavities, or hollow sections require explicit hazard analysis before liquid nitriding or reprocessing, because trapped water can rapidly expand when exposed to molten salt.
  • Preheating and drying steps are critical controls for removing moisture before immersion in molten salt baths, and non-routine rework should not bypass those controls without a documented hazard review.
  • Passive barriers alone may be insufficient to protect workers from a severe molten salt eruption; process design should isolate employees from the release hazard.
  • Operating manuals, SDS information, and training materials must clearly address accumulation hazards and prohibited part geometries so that operators can recognize when a part is unsafe to process.
  • Corporate-level hazard analyses and safety knowledge must be transferred to site-level operations and applied to new parts, existing parts, and non-routine tasks.

Source Notes

  • Priority 1 final report used as the primary authority for incident facts, causal findings, consequences, and recommendations.
  • Priority 3 recommendation status summaries used to update recommendation statuses for R3, R1/R2/R4/R5/R6/R7 where provided.
  • Priority 4 supporting document used only to supplement process context and early investigation details where consistent with the final report.
  • Where source documents differed, the higher-priority final report was used.

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