Following the loss of containment of a boiler on 18/03/16 (ARIA 47992), the 3 remaining boilers in a particularly high-production steel plant had experienced several pierced tubes in the following months and notably during the month of July. These perforations lead to a series of alternating boiler shutdowns. Several simultaneous boiler shutdowns in July required that the blast furnaces be shut down and restarted several times. On 20/07, during operations to restart a blast furnace, a gas discharge occurred on the bleeders for 11 hours at a standard of about 30 minutes. The volume released is estimated at 700,000 Nm³. On 23/07, the restart of the second blast furnace resulted in a gas discharge of 1,250,000 Nm³. The economic toll of the incidents was estimated at a production loss of 2 weeks or 160,000 t of product lost.

Metallographic analysis of the tubes and sleeves of the boilers revealed the following:

• large deposits on the inside, often related to thinning of the wall;
• intergranular microcracking near leaks or in thinning areas. The morphology of these microcracks and their location in a thinned area is typical of hydrogen embrittlement;
• the microstructure was not degraded and exhibited a non-ageing ferrite-perlite structure;
• many internal deposits were observed in the sleeves and could reach a maximum thickness of 1.5 mm. These deposits contain many corrosive elements such as S, P and Cl, as well as Cu, Mn, Mg, Na and other trace elements. The most likely internal source of this amount of copper is the condenser.

Hydrogen embrittlement could result from poor management of the chemical quality of the water used by the operator. The sudden and massive arrival of seawater, following the leak in the condensers on 18 March, resulted in a large amount of chlorine and oxygen being injected into the system which accelerated the corrosion phenomenon of the tubes which had already been weakened by a degraded water quality.

Several recommendations were made following the expert assessment:

• ultrasonic mapping of the tubes to determine the corroded sections and to replace those less than 2 mm thick;
• chemical cleaning on those tubes having a residual thickness that allows it;
• improvement of chemical water management. The operator is currently working on this point. It has invested in online water quality monitoring equipment and a complementary demineralised water production unit.