In a chemical factory, grinding operations were scheduled on a tank having stored sulphuric acid. All operation procedures (neutralisation of residual sludge with carbonate, measurement of oxygen and inflammable gases at various points) were properly carried out. At the start of the operation, a deflagration occurred inside the tank. Casualties include one death and two cases of serious injury. The tank was partly destroyed. The explosion occurred due to the presence of hydrogen (100 g) in a dead area where no measurements were taken. The hydrogen resulted from the corrosion of the iron tank under the action of sulphuric acid.