Bolts rupture due to hydrogen embrittlement

The bolts on a flange, at a not further specified industrial facility failed after approximately 4000 hours in service (approximately 18 months in service). Their failure caused H2S leaks.
The bolts fractured as a result of a hydrogen embrittlement mechanism (Hydrogen Stress Cracking) in a H2S rich atmosphere and in presence of humidity.

The cause was a material/component failure due to hydrogen embrittlement (Sulphide Stress Cracking). The steel used for the bold belonged to a martensitic steel family which is not advised for use for continuous operation with wet H2S. The ROOT CAUSE could be attributed to a design failure.

Bolts

The fire mobilized about 70 firefighters; a safety perimeter of 800 m was set up; a cloud of mushroom-shaped black smoke rose more than 600 m above the site.

The specific hydrogen-related damage causing the rupture of the bolts is known as Sulphide Stress Cracking (SSC). In first instance, the steels react with hydrogen sulphide (H2S), forming metal sulphides (MeS) and atomic hydrogen as corrosion byproducts.
SSC is basically a hydrogen embrittlement phenomenon, in which atomic hydrogen penetrates in the steel cracking it. It is a mechanism susceptible to occur when 3 circumstances take place simultaneously: a susceptible material, a brittle atmosphere (H2S) and a sufficient maintained level of stresses. It is a mechanism that can happen under mechanical loads of remarkably lower magnitudes than the elastic limit of the material, being able to be sufficient the own residual stresses of manufacturing or assembly. Susceptibility to hydrogen embrittlement of steels is as much greater as it is its mechanical resistance or hardness, a reference value of 26 HRC being used for martensitic stainless steels with quenching and tempering thermal treatment (In this case, bolts had a hardness 48 HRC).