Caustic soda produced in caustic cells at a petrochemical plant was loaded into a crude
tank by means of a caustic header line made of plain carbon steel. After a fraction of
expected service life, the header was severely corroded. Scanning electron microscopy
(SEM) coupled with energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction
(XRD) was used to identify the material and the corrosion product. Metallurgical evaluation
suggested that the mode of failure was localized attack by chlorine and/or hydrochloric
acid at pipe connections (flanges) where concentrated solutions could accumulate. Most
evidence pointed out that the failure resulted from corrosion attack due to condensation of
hydrochloric acid. Misalignment of the pipe at the flange led to cold work which is also
thought to accelerate the corrosion rate. It was recommended to consider a better pipe
alignment practice as well as the addition of an inhibitor as a short-term solution. Since
the piping system was used close to the critical operating conditions, it was recommended
as a long-term solution to replace the caustic header material by one of the higher grades
Ni–Cr–Mo alloys.