The catastrophic explosion at Flixborough in June 1974 has been traced to the failure of a bypass
assembly introduced into a train of six cyclohexane oxidation reactors after one of the
reactors was removed owing to the development of a leak. The leaking reactor, like the
others, was constructed of 12.3 mm mild steel plate with 3 mm stainless steel bonded to it,
and it developed a vertical crack in the mild steel outer layer of the reactor from which
cyclohexane leaked leading to the removal of the reactor. One of the factors contributing to
the crack was stress corrosion, resulting from the presence of nitrates that had contaminated
river water being used to cool a leaking flange. Mild steel exposed to hot nitrate solution and
to stresses which in extreme cases may be well below the yield strength, and may remain in
the structure from the fabrication procedure or may derive from operating stresses, especially
if intensified by a defect, can develop intergranular cracks. Such stress corrosion cracking
results from the conjoint action of stress, of an appropriate magnitude, and a corrosive
environment, of specific composition, upon steel having a wide range of compositions or
structures. Different types of steel are not equally susceptible to cracking as measured by the
minimum stress to promote cracking or the concentration of the salt responsible for the
corrosion reactions that cause crack propagation