In a surface treatment facility, fire broke out around 7 pm during maintenance works on an empty sulphuric acid tank. The technician immediately cut the gas and electricity supply lines feeding the equipment and notified the fire-fighters, who brought the fire under control. A chemical nickel-plating tank containing a mix of 1 tonne of potassium hydroxide and 1 tonne of nickel chloride, along with an empty H2SO4 tank and 4 empty rinse tanks were destroyed; the workshop (suspended ceiling, water pipes, etc.) was damaged and various polluted treatment baths rendered unusable. Fire extinction water and nickel solution flows were contained in a retention facility. The solid and liquid wastes were recovered and treated by a specialist firm. Property damage and operating losses amounted to an estimated €1 million each. Overheating of the H2SO4 bath tank heating system caused this accident. The investigation revealed that this system had been started up subsequent to an error committed by a subcontractor during a maintenance intervention (a protocol prohibited heating an empty tank). The bath level gauge malfunction in the tank prevented notifying the technician of his oversight; the typically immersed thermometric probe, also designed to avoid overheating, was found to be inoperable given the absence of liquid in the treatment tank. The Classified Facilities Inspectorate required the operator to install an efficient device to prevent overheating should liquid be absent in the tanks; verify the automatic opening systems for smoke exhaust hatches since some were not operable when the fire ignited; and determine, in conjunction with fire-fighters, the measures to implement to allow first responders to enter zones locked with keys. The operator also installed immersion heaters with fire protection systems and updated both operations and maintenance procedures.