An explosion occurred in a facility dedicated to the renovation and maintenance of boilers during repair work on one such boiler. A technician had previously de-scaled the device using hydrochloric acid before proceeding with the neutralisation step with soda and draining the passive acid introduced. The significant scaling on the boiler required a second acid injection, followed by welding work, but this time without draining the boiler. Hydrogen emitted by a metal + acid reaction was the cause of this accident. An employee wound up in hospital in a coma. Labour inspection authorities noted that operating conditions were not favourable towards environmental protection, i.e. failure to control discharge pH, absence of written procedures.