An explosion occurred in a chemical site causing the cover of a 99.2% sulphuric acid tank (filled to 300 t of its total 1500 tonne capacity) to open partially. The cover opened along the circumference of the tank at the dome / shell fitting. The POI (internal emergency plan) was triggered. No emission or leak was reported except for the emission of fumarole at the opening. No environmental or human consequences were reported. Material damage was limited to the equipment located above the tank (acid supply pipes, air pipe going to the dryer, walkway, steam pipe, etc.). The presence of hydrogen (a few dozen grams), generated by the corrosion of steel by the weak acid was responsible for the explosion. The accident was further to a series of incidents in the unit manufacturing H2SO4 through the absorption of SO3 in packed columns: two successive piercing incidents of the water/acid heat exchangers of the transfer columns on the day before the accident caused an around 85% titer to flow into the acid storage tank (1st incident), followed by acid with a practically zero titer (2nd incident). Before the explosion the tank contained a heterogeneous mixture with a weaker surface titer. The explosion occurred 1 hour and 15 minutes after the tank had been agitated. The ignition of H2 (requires very low energy) was probably electrostatic. The two piercing incidents of the heat exchanger were due to a phenomenon of corrosion by pitting near the seam welds on the side of the cooling water. Modification of the anti-legionella biocide treatment since the previous year may have favoured the phenomena of corrosion by pitting. The use of liquid bleach instead of bromine greatly increased the oxidising power of water and thus lowered the starting temperature of corrosion pitting. Decreasing the speed of passage of water (on account of the drop in the workshop activity for a long period) contributed to increasing the temperature of the medium, is another aggravating factor for the accident. The measures taken include water treatment (temporary use of a non-oxidising biocide and study of a continuous chlorine-bromine treatment), detection of any abnormal arrival of water in the process (temperature detector with alarm, titrators), operating procedures (prohibition on adding weak acids to storage tanks).