Corrosion of carbon steel caustic header in the presence of chloride ions

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.